INTRODUCTION
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MicroRNAs (miRNAs) are non-coding, endogenous \~23 nucleotide (nt) RNAs that regulate target-gene expression by either translation inhibition or mRNA degradation ([@B13]). They were first discovered in *C. elegans* where *lin-4* plays a critical role in the developmental orchestration ([@B112]). Since then, the number of identified miRNAs has reached a thousand in many species. Owing to their uncommon targeting properties, a single miRNA species can bind and regulate multiple targets, and it is now thought that the expression of most, if not all, genes involves modulation by miRNAs. Thus, the "macro switch" function of miRNAs in neuronal development is now well documented ([@B118]). More recently, accruing evidence established miRNAs as essential regulators of proper neuronal function. In particular, it was demonstrated that miRNAs regulate many proteins involved in neuronal adaptation to network activity. miRNAs take part in the morphological and functional changes sustaining neuronal plasticity. For instance, miR-134 is expressed in hippocampal neurons where it regulates the LIM-domain kinase 1, which represses cofilin, an actin depolymerization factor, thus modifying spine morphology ([@B168]). Another example is miR-284, which regulates the expression of the glutamate receptor GluA2 at the neuromuscular junction in *Drosophila* ([@B88]).

As an actor in neuronal plasticity, miRNAs are likely to play a role in the onset and maintenance of neurological diseases. Indeed screening strategies have identified altered expression of specific miRNAs in Alzheimer's, Parkinson's, Huntington's disease, and Tourette syndrome patients ([@B86]; [@B165]). While these results raise hope for new treatments, more work is needed to fully differentiate the miRNA changes that trigger pathological mechanisms from those resulting from the disease. A tempting hypothesis is that long-term alteration of miRNA pathways normally regulating basal neuron function could lead to disease. Long-lasting modifications of gene expression are thought to result in chronic disease such as neuropathic pain. Therefore, miRNA deregulation could play a key role in chronic pain.

Here, we briefly examine current knowledge on miRNA biogenesis, target recognition, and regulation. Then, we review the literature to highlight miRNAs that are involved in neuronal plasticity, with a special focus on chronic pain pathologies.

miRNA BIOGENESIS
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To date, miRbase, a specialized miRNA database, comprises 1872 entries for the human genome ([@B64]; [@B99]). miRNA genes are present on all chromosomes, with the exception of the human Y chromosome. Nearly 50% of miRNA-coding genes are situated within the intergenic space and possess their own regulatory elements ([@B105]; [@B39]). In contrast, 40% of miRNAs genes are positioned within introns ([@B158]; [@B175]), and 10% are located within exon terminals. As a consequence, the expression of half of the miRNA genes depends on the regulation of their host gene, so they may be involved in the control of genetic networks related to the expected function of the host gene product ([@B141]). An interesting feature is that many miRNA genes are grouped within clusters, with an intergenic distance ranging from 0.1 to 50 kb, and exhibit a similar expression pattern ([@B15]). In addition, miRNAs within clusters are often, but not always, related to each other, while miRNAs from the same family are occasionally clustered ([@B105]; [@B110]).

Transcription of miRNAs is performed by either RNA Polymerase II or RNA Polymerase III, producing primary miRNA (pri-miRNA), a large stem-loop structure (3--4 kb in length) with a 5′ cap and a poly(A) tail ([@B28]; [@B114]). Then, this pri-miRNA structure is recognized and processed by the microprocessor complex which is composed of the ribonuclease Drosha (RNase III enzyme), the RNA-binding protein (RBP) DGCR8 (DiGeorge syndrome critical region gene 8; [@B113]; [@B42]; [@B63]) and other auxiliary factors ([@B68]), generating \~60- to 70-nt-long stem-loop precursor miRNAs (pre-miRNA; [@B134]). Processing of pri-miRNAs into pre-miRNAs occurs concomitantly with transcriptional events, thus rapidly constituting a nuclear pool of pre-miRNAs ([@B134]). The next step to functional miRNAs is the export of pre-miRNAs from the nucleus to the cytoplasm by the karyopherin protein family member, Exportin-5, in a GTP-dependent manner ([@B201]; [@B22]; [@B125]). Once in the cytoplasm, the pre-miRNA is loaded into the RNA-induced silencing complex (RISC) loading complex, where it is further processed into a \~21-nt-long miRNA duplex by Dicer, a type-III ribonuclease ([@B19]; [@B78]; [@B89]; [@B95]). At this stage, only one strand is finally incorporated into the RISC, the "guide" strand, whereas the other strand called "the passenger" is likely degraded. The criteria defining which of the two strands is loaded into the RISC still need to be clarified but it appears that, in most of the cases, it is the one whose 5′ end is less tightly paired ([@B90]; [@B169]).

Another processing pathway independent of the Drosha/DGCR8 microprocessor machinery and previously discovered in *Drosophila* and *C. elegans* ([@B142]; [@B159]) has been recently identified in mammals. The so-called mirtons are directly spliced from the introns of mRNA coding genes, directly exported to the cytoplasm and processed by Dicer ([@B142]; [@B159]).

TARGET RECOGNITION AND TRANSLATION INHIBITION
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BASICS OF miRNA:mRNA INTERACTIONS
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Loading of the "guide" strand into the Argonaute protein of the RISC makes it functional and ready to generate mRNA inhibition ([@B79]; [@B135]). The mechanisms and the consequent magnitude of this regulation are dependent on the characteristics of the miRNA:target mRNA binding. Thus, in metazoans, extensive base pairing has been demonstrated to induce mRNA cleavage ([@B79]; [@B157]; [@B200]), whereas imperfect binding leads to target repression through translational inhibition and/or mRNA destabilization ([@B112]; [@B196]; [@B119]). In addition, the location of miRNA binding sites in the target mRNA plays a predominant role on the efficacy of the regulation. Although binding of miRNA has been demonstrated in the 5′ untranslated region (UTR) and the open reading frame (ORF) of mRNA ([@B47]; [@B126]; [@B37]; [@B115]), these sites are less effective than those located in the 3′UTR ([@B94]; [@B53]; [@B116]; [@B119]; [@B65]; [@B160]; [@B6]). This might reflect the fact that in 5′UTR and ORF, the translation machinery has a higher affinity than the RISC, thus displacing the silencing complex ([@B13]).

The nucleotide sequence in the target mRNA that is engaged in miRNA binding is called the miRNA recognition element (MRE; [@B174]) or "seed region" ([@B12]). miRNA target recognition and seed region classification have been extensively reviewed previously (see [@B13]). The composition of this seed region varies but always involves a sequence with conserved Watson--Crick pairing (i.e., the hydrogen bonding that pairs guanine--cytosine and adenine--thymine) to the 5′ region of the miRNA centered on nucleotides 2--7 ([@B13]). Several types of seed regions exist depending on the length and composition of the sequence involved in miRNA:mRNA binding, resulting in different affinities and inhibition properties. These include canonical sites with a 6- to 8-nt match between miRNA and mRNA, and non-canonical sites that include additional pairing at the 3′ end of the miRNA. Additional factors have been shown to affect miRNA seed efficacy such as AU-rich sites, which have been shown to be more effective ([@B116]; [@B65]; [@B137]; [@B147]).

Other factors include the position of the seed within the 3′UTR of the target-mRNA ([@B65]), with sites at the extremities of the 3′UTR being more accessible than those in the middle because long UTRs may form occlusive interactions, thus greatly reducing miRNA site accessibility.

Another important feature is site multiplicity, where the presence of multiple sites for the same or different miRNAs on the target-mRNA increases the repression response. Multiple sites might contribute to target mRNA inhibition in either a non-cooperative manner or cooperatively. miRNA cooperation results in a stronger repression than the sum of the individual and independent sites. Cooperative regulation implies that seed sequences are within a 40-nt region but with a minimum 8 nt gap between them ([@B43]; [@B65]; [@B137]; [@B163]).

Given the large and growing number of miRNA species, the enormous number of putative target mRNAs and the length of their 3′UTR sequences, the manual prediction of miRNA:mRNA interactions would be impossible. Therefore, computational analysis using algorithms and prediction databases is the usual method to identify miRNA targets. Hence, target prediction programs are based on seed region recognition tools that basically list all the sites with miRNA-binding properties and rank them accordingly to the kind of seed, conservation across evolution and some of the above-mentioned factors known to modulate site efficiency (for review, see [@B156]).

However, despite refinements of prediction tools, the percentage of predicted targets that pass the experimental validation stage is still sub-optimal. Therefore, it often constitutes a time-consuming bottleneck in miRNA studies that involve complex experimental paradigms. In line with this, recent data suggest that miRNA:mRNA interactions sometimes involve the 3′ end of miRNAs with little evidence for 5′ contacts, and some of these interactions have been shown to be functional ([@B71]). In addition, other parameters not taken into account in these algorithms could affect the accuracy of target prediction tools. For instance, there is the importance of the secondary structure of RNA and its association with RBPs, which influence target site accessibility, and thereby its regulation ([@B27]).

MECHANISMS OF miRNA-MEDIATED INHIBITION OF TRANSLATION
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miRNA binding to its target mRNA is the starting point of a post-transcriptional regulation of gene expression occurring in the cytoplasm, mostly by translational repression or mRNA degradation ([@B188]; [@B83]; [@B138]; [@B152]; [@B177]). In fact, microRNA-induced silencing complex (miRISC)-mediated gene inhibition has been demonstrated to arise from three putative mechanisms: (i) site-specific cleavage, (ii) enhanced mRNA decay, and (iii) translational inhibition. Thanks to recent studies combining cutting-edge transcriptomic and proteomic analyses, a genome-wide view of the changes elicited by miRNA down or up-regulation on the protein output is now available ([@B6]; [@B171]; [@B72]; [@B66]). These experimental data are critical for validating the prediction tools for miRNA targeting as well as for comparing the respective roles of the different modes of action driven by miRNAs.

### Site-specific cleavage

This Ago2-mediated process requires full complementarity between miRNA and its target ([@B67]; [@B79]), which seems to be very limited in mammals ([@B200]; [@B75]). Loading of the miRNA into the Ago2 protein involves conformational changes which should enhance Watson--Crick pairing to the target mRNA. In contrast to this rare event, translational inhibition or mRNA degradation, which both result from partial binding of miRNA to mRNA, seem to be the most widely occurring mechanisms in mammals (for review, see [@B52]).

### Translational repression

Protein translation repression by miRNAs may result from various mechanisms: inhibition of translation initiation, inhibition of translation elongation; protein degradation while translation is being processed, premature termination of the translation (also known as ribosome drop-off); or a combination of some of these mechanisms ([@B7]; [@B151]; [@B140]; [@B149]).

At the translation initiation step, miRNAs can inhibit protein production by disrupting the interaction between the 5′-cap and the 3′-poly(A) tail of the mRNA. The mechanisms involved are not fully understood but it is thought that Ago could compete with eukaryotic translation initiation factor 4 for mRNA-cap binding ([@B77]; [@B92]). Furthermore, miRNAs could prevent translation by inhibiting the association of the ribosomal subunits ([@B35]; [@B183]).

At the post-initiation stage, translation inhibition by miRNAs could be mediated by premature termination and ribosome drop-off ([@B149]), stalling of the elongation by slowing down the ribosome process ([@B161]; [@B133]), or co-translational degradation of nascent polypeptides ([@B140]). However, experimental evidence for these mechanisms and explaining their interdependence is still only partial. As summarized by [@B56], the inhibition of translation at the initiation or elongation phase is perhaps not exclusive and it is likely that initiation is always inhibited. However, ribosomes queuing on the mRNA when elongation is also inhibited may hide this obligate mechanism ([@B56]).

### Enhanced mRNA decay

Large-scale analyses of the expression of miRNAs and their predicted mRNA targets have shown an inverse correlation, strongly suggesting that mRNA degradation is a hallmark of miRNA-mediated inhibition of transcripts ([@B34]). A likely scenario relies on the deadenylation of the mRNA target, which would prevent circularization of the transcript. Circularization is an essential mechanism for mRNA translation that also prevents mRNA degradation, since linear transcripts are more prone to degradation ([@B192]). Nevertheless, whether deadenylation precedes ([@B192]; [@B82]) or follows ([@B51]; [@B203]) translational inhibition is still an unresolved issue.

Beyond the question of the mechanisms responsible for mRNA translation inhibition, there is the issue of the sub-cellular localization of this regulation. The most popular hypothesis posits that mRNAs under miRNA-mediated inhibition are stored ([@B26]; [@B20]) and degraded ([@B50]; [@B146]) into cytoplasmic particles called processing bodies or glycine-tryptophan (GW) bodies. These cytoplasmic foci are free of translational machinery but are enriched in proteins that are required for efficient mRNA inhibition ([@B84]; [@B121]; [@B155]; [@B18]).

miRNAs ARE REGULATORS OF NEURONAL ACTIVITY
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Although the way miRNAs regulate their targets is still imperfectly understood, it is widely accepted that they play a major role in gene expression regulation. As such, miRNA have a crucial function in neuronal development where they seem to act as a master switch of the genome (for review, see [@B180]). In the mature nervous system, increasing evidence suggests that miRNA are important for normal neuronal function ([@B153]; [@B49]; [@B38]; [@B185]; [@B45]; [@B76]; [@B111]; [@B162]).

The role of miRNA in neuronal function intermingles (i) miRNA that regulates neuronal activity with (ii) neuronal activity that modulates miRNA expression, which could in turn regulate targets with an impact on neuronal activity (feed-forward loop). In addition, neuronal activity should not be considered as a whole but as the output of thousands of synaptic inputs. Upon stimulation, single synapses or sets of synapses can go through intense and selective modifications in an independent manner for long periods of time ([@B128]; [@B136]; [@B195]). Although short-term modifications of synapses could mainly be the result of post-translational events, long-term changes require regulation of gene expression at the transcriptional and post-transcriptional levels ([@B128]; [@B195]). Thus, local post-transcriptional regulation regulated by activity is certainly a key element for the maintenance and plasticity of neural connections ([@B4]; [@B181]; [@B25]). Indeed, it has been demonstrated that specific mRNAs and components of the translational machinery, including ribosomes and other non-coding RNAs, are localized to dendritic regions of neurons, where they are likely to serve local translation ([@B178]; [@B41]; [@B186]; [@B187]; [@B109]; [@B179]). Localization of mRNAs into dendrites is an active phenomenon involving the 3′UTR sequence of messenger where RBPs bind to initiate transport (for review, see [@B46]; [@B61]). In particular, some mRNAs are located at the post-synaptic density (PSD) as polyribosome structures ([@B194]). Some of these mRNAs encode proteins such as kinases and translational control factors, which are attractive candidates to mediate synaptic changes ([@B48]; [@B179]). Interestingly, the number of polyribosome-containing spines increases in response to the long-term potentialization (LTP) protocol ([@B145]), and their PSD area enlarges when compared to polyribosome-free spines. This indicates that the presence of polyribosomes produces these structural changes, which were previously ascribed to changes in synaptic strength ([@B128]). It has been suggested that this local translation mechanism could play a major role in synaptic plasticity and therefore contribute to the molecular basis of learning and memory ([@B87]; [@B29]; [@B91]).

Recently, miRNAs have been proposed as mediators regulating local protein synthesis at the synaptic level. miRNAs are enriched in the brain ([@B106]; [@B100]; [@B91]; [@B172]; [@B98]), where they have been shown to be differently expressed not only in distinct areas ([@B108]; [@B9]; [@B143]), but also at the synaptic level ([@B150]). Moreover, studies on synaptoneurosomes revealed the abundance of several components of the miRNAs biogenesis pathway and their silencing complex machinery at PSDs ([@B124]; [@B150]). Thus, dendritic mRNA translation can be modulated by miRNAs which have been selectively transported as mature or pre-miRNAs. In response to synaptic activity, this miRNA-driven regulation can either increase or decrease local mRNA translation through different mechanisms (**Figure [1](#F1){ref-type="fig"}**). Interestingly, not all miRNAs have the same regulation upon stimulus. However, it is not clear whether the involvement of these different types of regulation depends on the type of synaptic activity or on the type of stimulus.

![**MicroRNAs can modulate local translation in response to changes in synaptic activity.** After transport from the soma to the dendrites, some mRNAs are locally translated into proteins. This local translation mechanism can be modulated in response to synaptic activity changes by dendritic miRNAs that have also been transported from the soma. This miRNA-driven mechanism can either decrease **(A)** or increase **(B)** local translation. **(A)** Decrease of local translation could be the result of an activity-related miRNA loading into miRISC and specific binding to target mRNAs. **(B)** Increase of translation may be the consequence of an unbinding of miRNA from target-mRNAs which could then escape P-body and undergo local translation.](fncel-08-00031-g001){#F1}

In addition to their role in physiological conditions, accruing evidence suggests that miRNAs play an important role in the pathological mechanisms of neurodegenerative diseases (reviewed in [@B86]; [@B165]). Hence, alterations of specific miRNAs have been associated with Alzheimer's, Parkinson's, Huntington's disease, and Tourette syndrome. Furthermore, in other debilitating diseases of the nervous system, miRNA deregulation has been correlated with altered neuronal activity, as in chronic pain, which is characterized by increased excitability within the pain circuitry. One of the first demonstrations of the role of miRNAs in chronic pain came from a study by [@B205] where a conditional knockout of Dicer was targeted to sodium channel Nav1.8-positive nociceptor neurons in the dorsal root ganglia (DRG). This impairment of miRNA function induced a strong attenuation of inflammatory pain, suggesting that miRNAs are necessary to enhance excitability of this subpopulation of neurons in response to inflammation mediators. Here, we review specific miRNAs that have been shown to play a role in the regulation of neuronal activity and highlight their possible association with pain processes (summarized in **Table [1](#T1){ref-type="table"}**). In some cases, the causal role of miRNA in pain mechanisms is strongly suggested by targeted-mRNAs identification and functional studies. On the other hand, we conducted a retrospective analysis of miRNA expression data published for different pain models and speculated about their potential implication based on their known targets in normal neuronal activity.

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MicroRNAs involved in plasticity and pain mechanisms.

  miRNA       Involvement in plasticity mechanisms     Involvement in pain mechanisms                                                                                                        
  ----------- ---------------------------------------- -------------------------------- -------------------------------------------- ------------------------------------------ ------------ ----------------------------------
  miR-134     BDNF exposure                            LIMK1                            [@B168]                                      Inflammation/CCI                           ND           [@B8], [@B60]
  miR-132     Bicuculline/KCl/monocular deprivation    P250GAP /MeCP2                   [@B190], [@B93], [@B193], [@B130], [@B185]   CRPS/CCI                                   ND           [@B144], [@B3]
  miR-188     Long-term-potentiation                   Nrp-2                            [@B111]                                      CCI                                        ND           [@B3]
  miR-29a/b   Psychoactive drugs                       Arpc3                            [@B120]                                      Inflammation/CRPS/ CCI                     ND           [@B8], [@B144], [@B60]
  miR-181a    Psychoactive drugs                       GluA2                            [@B167], [@B32], [@B162]                     Neuropathic/ zymosan-induced pelvic pain   GABA~Aα-1~   [@B117], [@B173]
  miR-219     Disturbance of NMDA signaling            CaMKIIγ                          [@B96]                                       Traumatic spinal cord injury/CCI           ND           [@B122], [@B60]
  miR-125     Fragile X syndrome                       NR2A                             [@B49]                                       CCI                                        ND           [@B3]
  miR-124     Neuronal differentiation/axon guidance   Sema3A/RhoG/ Zif268              [@B17], [@B58], [@B199]                      Inflammation/nerve injury                  MeCP2        [@B8], [@B198], [@B197], [@B104]

CCI, chronic constriction injury; CRPS, complex regional pain syndrome; ND, not determined.

FUNCTIONAL IMPLICATION OF miRNA IN NEURONAL PLASTICITY AND PAIN SENSITIZATION
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One of the first studies reporting the control of local translation by miRNA at the synapse upon synaptic activity showed that miR-134 negatively regulates LIMK1 in an activity-dependent manner ([@B168]). LIMK1 is a protein kinase that controls actin filament dynamics through inhibition of actin depolymerizing factor (ADF)/cofilin ([@B10]). This regulation could be critical for synaptic transmission, synaptic integration, and plasticity since the majority of the excitatory synapses are formed on dendritic spines, which are actin-rich protrusions from the dendritic shaft ([@B73]; [@B23]; [@B176]). The study demonstrated the compartmentalization of miR-134 and its target LIMK1 mRNA at the synapto-dendritic area. It has been proposed that the miR-134 association with LIMK1 mRNA keeps the LIMK1 mRNA in a dormant state while it is being transported within dendrites to synaptic sites. Over-expression of miR-134 inhibits LIMK1 mRNA local translation, resulting in a negative regulation of the size of the dendritic spines. Exposure of neurons to external stimuli like the neurotrophic factor brain-derived neurotrophic factor (BDNF) reduces miR-134 inhibition on LIMK1 mRNA translation ([@B168]). Recently, the same group reported another mechanism by which pre-miR-134 is transported to dendritic sites. Replacing the loop sequence of pre-miR-134 with the loop of a non-dendritic pre-miRNA abolished dendritic accumulation of pre-miR-134. Mutagenesis analysis revealed that the five central loop nucleotides are critical because of an interaction with the Asp-Glu-Ala-His-box (DEAH-box) helicase DEAH box protein 36 (DHX36). DHX36 is known to compete with Dicer *in vitro*, and Argonaute complexes that contain DHX36 are devoid of Dicer activity ([@B74]). Indeed, knockdown of DHX36 displayed a significant reduction in the percentage of dendritic pre-miR-134, without affecting the global levels of pre-miR-134 or mature miR-134. Consistently with the role of miR-134 on spine morphology, functional experiments revealed that DHX36 negatively regulates dendritic spine morphogenesis in hippocampal neurons ([@B21]).

One of the first studies on a possible role of miRNAs in pain showed that miR-134 is modulated in the trigeminal ganglion in response to inflammatory pain ([@B8]). These authors induced inflammatory pain by injecting complete Freund's adjuvant (CFA) into the rat masseter muscle, a paradigm known to produce transient inflammation accompanied by allodynia. Interestingly, miR-134 was down-regulated at the onset of pain, 30 min after CFA injection, returned to control level 1 day later, showed an over-expression rebound at day 4 and finally returned to control level at day 12. This phasic deregulation of miR-134 expression suggests that it could be an adaptive response to the early phase of inflammation. Another study in the chronic constriction injury (CCI) model showed that miR-134 was up-regulated in the dorsal horn of the spinal cord at day 14 after surgery ([@B60]). In this case, miR-134 deregulation occurred later after the onset of the painful behavior and could have been a consequence of the pain mechanisms. In contrast, our own results show that miR-134 was down-regulated 7 days after injury in the dorsal horn of a rat model of sciatic nerve ligation. This was accompanied by an up-regulation of LIMK1, the miR-134 target (A. Salam et al., unpublished data). Therefore, it will be important to undertake functional studies to elucidate the role of miR-134 in chronic pain.

miR-132 is another example of activity-modulated miRNA. Experiments where neuronal activity is increased through a bicuculline-mediated blockade of GABA~A~ inhibitory tone showed a rapid increase in the expression of miR-132 precursor and mature miR-132 ([@B193]). Interestingly, this regulation was attenuated by pretreatment with the selective *N*-methyl-[D]{.smallcaps}-aspartate receptor (NMDAR) antagonist amino-5-phosphonovaleric acid (APV). In addition, KCl treatment increased transcription of the miR-132 precursor, suggesting that different modalities of neuronal activity stimulation also trigger a similar miR-132 increase. The explanation lies in the pathway leading to miR-132 over-expression. Indeed, inhibitors for CaM kinase, MEK--ERK (mitogen-activated protein kinase kinase--extracellular signal-regulated kinase) or CREB (cAMP response element-binding protein), all prevent miR-132 up-regulation upon activity enhancement, suggesting that miR-132 is regulated predominantly by the CREB pathway. miR-132 directly targets p250GAP, a protein known to inhibit Rho family GTPases ([@B182]; [@B204]). The final outcome of miR-132 over-expression is an increase in dendrite morphogenesis, presumably as a response to Rac activity. Indeed, the same group recently proposed that miR-132 regulates dendritic spinogenesis through the Rac1-Pak actin remodeling pathway ([@B80]). Another target of miR-132 is methyl CpG-binding protein 2 (MeCP2), which is involved in dendritic development and synaptogenesis and whose modulation can affect synaptic plasticity ([@B93]). Additional evidence of miR-132 having an impact on neuronal morphology came from transgenic experiments where the genomic locus containing miR-132 (and miR-212 as well) was deleted, leading to a dramatic decrease in dendritic length, arborization, and spine density ([@B127]). The impact of miR-132 regulation on spinogenesis is revealed in synaptic plasticity paradigms. For instance, *in vitro* short-term plasticity is affected by miR-132 over-expression without altering basal synaptic transmission ([@B107]). *In vivo*, short-term recognition memory is impaired by miR-132 over-expression in the perirhinal cortex. This results from a deficit in both long-term depression and potentiation ([@B170]). In the visual cortex, two recent studies demonstrated that miR-132 is a key component of experience-dependent plasticity. [@B185] showed that monocular deprivation decreased miR-132 expression in the cortex contralateral to the deprived eye. Counterbalancing this miR-132 decrease with miR-132 mimic supplementation completely blocked ocular dominance plasticity. At the same time, [@B130] identified miR-132 as the miRNA whose expression is the most affected by dark rearing and/or monocular deprivation. In addition, miR-132 inhibition occluded ocular dominance plasticity after monocular deprivation, which is associated with an altered maturation of dendritic spines. Finally, a recent study on olfactory bulb neurons born in the neonatal subventricular zone revealed that miR-132 is essential for normal dendritic complexity and spine morphology ([@B148]). In conclusion, and as previously proposed ([@B184]), finely regulated miR-132 expression seems to be essential for plasticity. Thus, too high an expression would make dendritic spines too stable. Too low an expression would lead to very unstable spines, thus impeding synaptic plasticity.

[@B190] demonstrated that BDNF triggered miR-132 up-regulation in cortical neurons. Interestingly, BDNF was recently identified as a modulator of nociception. BDNF is up-regulated in the DRG and the spinal cord in inflammatory and neuropathic pain models (reviewed in [@B132] and [@B189], respectively), where it is associated with an increased excitatory synaptic drive. In addition, in most cases, intrathecal BDNF injections exhibit pro-nociceptive effects ([@B132]). Thus, one could hypothesize that the effect of BDNF on nociceptive behavior could partially be the result of a BDNF-driven miR-132 over-expression. Thus, BDNF would induce an up-regulation of miR-132, which in turn would increase dendrite morphogenesis and arborization in the nociceptive pathway, resulting in an increased transmission of the pain signal. In patients with complex regional pain syndrome (CRPS), a disabling chronic neuropathic pain affecting one or more extremities, miR-132 blood levels were significantly altered ([@B144]). Apart from the direct nociceptive pathway, other nervous structures like the limbic system may influence pain perception. Indeed, the hippocampus is thought to be part of the descending anti-nociceptive system, potentially exerting an inhibitory role in neuropathic pain ([@B14]). [@B3] screened miRNA expression in the hippocampus of animals submitted to the CCI paradigm of neuropathic pain. miR-132 was down-regulated early after the onset of the neuropathic behavior and stayed under-expressed until the latest time-point analyzed. Thus, it is tempting to hypothesize that this modulation of miR-132 could lower the excitatory drive of the hippocampus and therefore lower the anti-nociceptive effect on the spinal pain pathway.

miR-188 is another activity-regulated miRNA with a proven synaptic plasticity tuning function. [@B111] found that miR-188 was up-regulated in hippocampal slices in response to the LTP protocol. Neuropilin-2 (Nrp-2), one of the predicted targets of miR-188, is down-regulated after the LTP procedure and luciferase experiments confirmed the interaction. Nrp-2 is a receptor of semaphorin 3F, which induces the repulsion of neuronal growth cones expressing Nrp-2 ([@B97]; [@B103]). In addition, miR-188 treatment can rescue the decreased miniature excitatory postsynaptic current (mEPSC) frequency and reduction of spine density induced by Nrp-2 over-expression, suggesting that miR-188 plays a role in synaptic plasticity by buffering Nrp-2 expression.

Despite its potentially crucial role in neuronal function, evidence for the involvement of miR-188 in pain mechanisms is very scarce. However, one study reported an altered expression under pain conditions. [@B3] quantified a down-regulation of miR-188 in the hippocampus of CCI animals but the significance of this modification remains to be elucidated.

The role of miRNAs in dendritic spine remodeling as a substrate for synaptic plasticity has also been evidenced *in vivo*. [@B120] exposed adult mice to psychoactive drugs like nicotine, cocaine, or amphetamine and quantified miRNA expression in different brain regions. Many miRNAs were regulated in response to these neuroadaptation paradigms, and especially the miR-29a/b locus was consistently over-expressed. *In vitro* experiments confirmed that these miRNAs were up-regulated in an activity-dependent manner. Arpc3 was identified as a target of miR-29a/b. This protein is part of the ARP2/3 actin nucleation complex which plays a critical role in actin branching, a mechanism involved in dendritic spine maturation. Therefore, this work strongly suggests that miR-29a/b regulates the activity-dependent structural plasticity associated with psycho-stimulant exposure.

Several studies identified a deregulation of miR-29a/b in pain conditions. The first evidence came from [@B8] who reported a down-regulation of this miRNA in the trigeminal ganglion neurons after inflammatory muscle pain. It was later confirmed in two other pain conditions, the CRPS ([@B144]) and the CCI model ([@B60]). Nevertheless, these studies did not investigate the subsequent regulation of Arpc3 or any other target of miR-29a/b, so the role of miR-29a/b in pain processing still needs to be elucidated.

In the context of chronic exposure to drugs of abuse, [@B162] focused their attention on the regulation of miRNAs in the nucleus accumbens. This is one of the most important brain regions in behavioral responses to drugs of abuse and is elicited by dopamine release. Synaptoneurosome preparations revealed a strong enrichment of miR-181a in response to treatment. A previous study showed that miR-181a expression in the striatum was regulated by Ago2 in cocaine addiction ([@B167]). This miRNA was predicted to target GluA2 and was confirmed in luciferase experiments ([@B162]). In addition, miR-181a over-expression in hippocampal cultures reduced AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor surface clusters, spine volume, and mEPSC frequency. Finally, the association between miR-181a and drugs of abuse has been highlighted ([@B32]). In that study, lentiviral over-expression of miR-181a in the nucleus accumbens enhanced cocaine-induced conditioned place preference. Two studies performed on the miRNA expression in the dorsal horn of spinal cord in neuropathic pain models quantified a down-regulation of miR-181a ([@B117]) or other members of the miR-181 family, namely miR-181b/c/d ([@B191]). Thus, this miR-181 down-regulation could lead to an up-regulation of the excitatory receptor GluA2 in the nociceptive pathway.

Involvement of AMPA receptors in neuropathic pain mechanisms has already been suggested (for review, see [@B59]). Unfortunately, the expression of GluA2 was not assessed in these two miRNA studies. Interestingly, another study on a different model, chronic pelvic pain, revealed a role of miR-181a through the regulation of a different target, the inhibitory receptor GABA~Aα-1~ ([@B173]). Pelvic pain was experimentally elicited by intra-vesicular injections of zymosan (a carbohydrate extracted from yeast and known to induce inflammation) into the bladder at postnatal day 14. This induced long-lasting neuroanatomical and neurophysiological changes in the nociceptive system still present at postnatal day 60. At that stage, zymosan-treated animals were characterized by visceral hypersensitivity, an up-regulation of miR-181a and a down-regulation of GABA~Aα-1~ expression in the lombo-sacral segment of the spinal cord. Bioinformatics analyses identified multiple sites for miR-181a/b in the 3′UTR of GABA~Aα-1~ and luciferase experiments proved the inhibitory effect of miR-181a on GABA~Aα-1~ translation. These results strongly suggest that this miR-181a-induced down-regulation of GABA~A~ receptors results in a loss of inhibition in the spinal cord sustaining long-lasting visceral hypersensitivity. They highlight the importance of target identification in miRNA studies.

*N*-methyl-[D]{.smallcaps}-aspartate receptors are another kind of glutamate receptor known to play a crucial role in synaptic transmission and plasticity. Altered NMDA-mediated signaling is associated with behavioral impairments in psychiatric disorders. [@B96] demonstrated that miR-219 is down-regulated in the prefrontal cortex in response to pharmacological or genetic disturbances of NMDA signaling. In addition, they showed that miR-219 targets calcium/calmodulin-dependent protein kinase II (CaMKII) subunit γ, a downstream component of the NMDA signaling cascade. Thus, miR-219 down-regulation upon NMDA inhibition appears to be a compensatory mechanism that attenuates behavioral disorders associated with acute receptor antagonism.

Two miRNA screening studies suggest that miR-219 may be involved in pain mechanisms. The first evidence came from a model of traumatic spinal cord injury where miR-219 was significantly down-regulated 7 days after contusion ([@B122]). On the contrary, sciatic nerve ligation (CCI model) was demonstrated to induce an up-regulation of this miRNA ([@B60]). However, in both cases the investigation of targeted mRNA still needs to be conducted to shed light on the pathological role of miR-219 in pain pathways. In addition, the group of M. Sheng demonstrated that the expression of the NR2A subunit of NMDARs was regulated by miR-125b in hippocampal neurons ([@B49]). Indeed, endogenous miR-125b affected NMDAR composition and therefore induced functional changes in channel kinetics. As a consequence, miR-125b expression is likely to be critical for synaptic plasticity. Interestingly, a CCI pain model induced the down-regulation of miR-125b in the hippocampus ([@B3]), a structure that is part of the descending anti-nociceptive system.

miR-124 is one of the most abundant miRNAs in the brain and its role in the development of the mammalian brain is now well established. Thus, miR-124 is a neuronal differentiation promoter that also acts as an inhibitor of neuronal stem cells ([@B36]; [@B202]; [@B1]). During *X. laevis* development, miR-124 plays a crucial role in timing the axon pathfinding of retinal ganglion cells (RGCs; [@B17]). Thus, a key event in the correct navigation of RGC axons is a change in growth cone sensitivity to guidance cues. [@B17] demonstrated that miR-124 regulates the initiation of growth cone responsiveness to semaphorin Sema3A. In addition, miR-124 was shown to regulate neuronal process complexity by modulating the small GTPase RhoG ([@B58]). Indeed, RhoG is a critical modulator of the actin cytoskeleton. Consequently, RhoG inhibits axonal branching so miR-124-mediated inhibition of RhoG stimulates axonal and dendritic complexity. [@B199] demonstrated that miR-124 is a direct regulator of the transcription factor Zif268, which is essential for activity-related modulation of synaptic transmission and cognition.

In the context of chronic pain, miR-124 down-regulation in DRG neurons was reported in inflammatory muscle pain ([@B8]), as well as in sciatic nerve after peripheral nerve crush ([@B198]). Moreover, intrathecal injection of miR-124 had an anti-nociceptive effect in both inflammatory and nerve injury-induced chronic pain models ([@B197]). Finally, Niederberger's group showed that intravenous injection of miR-124 alleviates the nociceptive response to the formalin test and identified the target involved as MeCP2 ([@B104]). Thus, in addition to miR-132 ([@B93]), miR-124a also regulates MeCP2 translation. MeCP2 is an epigenetic modulator of BDNF, one of the major players in inflammatory pain mechanisms ([@B132]).

One of the interesting characteristics of miRNAs that has been conserved through evolution is that a given miRNA has the potential to regulate multiple targets owing to the permissive hybridization to target mRNAs. This mechanism of action is now well accepted and it can have an important impact on gene expression regulation. Thus, one or few miRNAs could regulate the expression of multiple genes that participate in the same function or which constitute the different subunits of a macromolecular complex ([@B55]; [@B70]).

In the context of chronic pain, we demonstrated such a multiple targeting by one miRNA. Indeed, we showed that a single miRNA, miR-103, simultaneously regulates the expression of the three subunits of the Cav1.2-comprising L-type calcium channel (Cav1.2-LTC) in an integrative regulation. Sensitization to pain involves the activation of various types of voltage-gated calcium channels. Some control synaptic transmission ([@B129]; [@B24]; [@B2]) while others regulate intrinsic membrane properties. Among the latter, Cav1.2-LTC regulates gene expression underlying long-term plastic changes ([@B44]; [@B57]). miR-103-mediated modulation of Cav1.2 function is bidirectional since knocking-down or over-expressing miR-103 respectively up- or down-regulate the level of Cav1.2-LTC translation. Functionally, we showed that miR-103 knockdown in naive rats results in hypersensitivity to pain. Moreover, miR-103 was down-regulated in neuropathic animals and miR-103 intrathecal applications successfully relieved pain, thus identifying miR-103 as a possible novel therapeutic target in neuropathic chronic pain ([@B54]).

ACTIVITY-INDUCED MODULATION OF miRNA LEVELS
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A set of evidence is growing to suggest that miRNAs can regulate neuronal activity by modulating the expression of ion channels, the morphology of dendritic spines and the molecular pathways downstream from receptor activation. However, the mechanisms that induce an activity-related modulation of miRNA-mediated inhibition of target genes remain poorly understood. Recent reports mostly in non-neuronal cells showed that it involves regulation of both the metabolism and function of miRNAs, and some of these mechanisms were also found to play a crucial role in neurons (for review, see [@B102]). Below, we summarize the known mechanisms that modulate miRNA levels as a result of activity variations in neurons.

First, modulation of miRNA levels in response to activity could result from transcription regulation as for mRNAs. Indeed, [@B139] found that the transcription of miR-184, a brain-specific miRNA, is repressed by the binding of MeCP2. miR-184 is an imprinted gene exclusively expressed from the paternal allele. Interestingly, enhancement of neuronal activity by high-KCl treatment induced MeCP2 phosphorylation and the consequent release of phosphorylated MeCP2 from the miR-184 promoter region. Thus, the increase in miR-184 levels in response to neuronal activity involves at least an augmented transcription of miR-184.

CREB is a transcription factor that plays a crucial role in nervous system development and plasticity (reviewed in [@B123]). A genome-wide screen for CREB-binding sites identified hundreds of locations that are associated with non-coding RNAs and miRNAs, including miR-132 ([@B190]). DNase I footprinting assay confirmed the CREB binding at two consensus cAMP regulatory elements (CREs) in the close 5′ region of the miR-212/-132 locus. CREB is known to be activated by neurotrophins such as BDNF, and the authors demonstrated that miR-132 transcription was enhanced in response to BDNF. In addition, CREB inhibition partially blocked the BDNF-induced increase of miR-132, strongly suggesting that this mechanism involved the CREB pathway. [@B193] demonstrated that miR-132 is regulated by CREB in an activity-dependent manner. Hence, in hippocampal neuronal cultures, inhibition of GABA~A~ inhibitory tone with bicuculline increased synaptic activity and the levels of miR-132 precursor and mature forms. Interestingly, blockade of CREB-dependent transcription abolished the miR-132 up-regulation in response to synaptic activity.

Second, the function of miRNA can be modulated by regulating miRISC components. Hence, MOV10, the mammalian ortholog of the SDE3 helicase Armitage in *Drosophila* ([@B5]), is present at synapses and undergoes rapid proteolysis in response to NMDAR-mediated activity ([@B11]). As a consequence, it relieves miRNA-mediated translation inhibition of key proteins at the synapse like CaMKII or LIMK1 ([@B5]; [@B11]).

Another possible mechanism could be the activity-dependent regulation of dendritic P-bodies. [@B40] demonstrated that ribonucleoprotein particles (RNPs) with a P-body-like structure are present in dendrites and that they exhibit motorized movements upon synaptic activation to relocate to distant sites ([@B40]). In addition, neuronal activation induced a loss of AGO2 from these P-body-like structures, suggesting that it may regulate local translation. Thus, inhibited messenger RNPs (mRNPs) may be stored in these particles at resting state and then be released in response to synaptic activity in order to be translated.

Besides regulation of miRISC components, recent evidence suggests that synergistic pathways may contribute to miRNA function and could therefore be activity-regulated. Hence, FMRP (fragile X mental retardation protein) is a protein with multiple RNA-binding domains that acts as a translational suppressor of particular mRNAs ([@B16]). Interestingly, FMRP interacts with RISC proteins and miRNAs, although it is not necessary for miRNA-mediated inhibition ([@B31]; [@B81]; [@B85]). [@B49] recently showed that both FMRP and miR-125b bound NR2A NMDAR subunit mRNA, thus leading to translation inhibition. Moreover, the inhibition of NR2A by FMRP is enhanced by miR-125b and FMRP depletion reversed miR-125b effects, strongly suggesting a synergistic effect.

Finally, obtaining a net change in miRNA levels without affecting miRNA transcription could be the result of miRNA decay alterations. Indeed, recent evidence suggests that miRNA turnover is fast in neuronal cells owing to rapid decay ([@B101]). In addition, the authors demonstrated that the rate of miRNA decay is activity-dependent. Thus, blocking neuronal activity with either tetrodotoxin (TTX; inhibiting action potentials) or 6-nitro-sulfamoyl-benzo-quinoxaline-dione/3-carboxy- piperazin-propyl phosphonic acid (NBQX/CPP; inhibiting glutamate transmission) reduced miRNA decay. Some elements of the enzymatic machinery involved in miRNA degradation have recently been discovered ([@B154]; [@B33]). However, the factors responsible for the rapid and activity-dependent miRNA decay in neurons are still unknown and constitute a new challenge. The most recent breakthrough in our understanding of miRNA regulation in neurons came from an underestimated RNA class, circular RNAs (circRNAs). circRNAs were first discovered in plants ([@B166]) and then in unicellular organisms ([@B62]), but the best-known circRNA is the sex-determining region Y RNA (Sry) which is highly expressed in testes ([@B30]). Recently, two groups identified a brain-expressed circRNA \[(named CDR1as (cerebellar degeneration-related protein 1 antisense) or circular RNA sponge for miR-7 (ciRS-7)\] that contains roughly 70 target sites for miR-7 ([@B69]; [@B131]). This circRNA acts as a natural miRNA sponge since it binds several miR-7 copies at a time, thus reducing miR-7 inhibition on mRNA targets. Interestingly, a very recent study identified miR-7 as a regulator of neuronal activity in chronic pain conditions. Hence, [@B164] showed that miR-7a was strongly down-regulated in the DRG of neuropathic pain models in animals. They demonstrated that miR-7a targets the β2-subunit of voltage-gated sodium channel, thus increasing the excitability of nociceptive neurons. In addition, miR-7a exogenous over-expression in DRG of injured animals alleviated neuropathic pain.

CONCLUSION
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How miRNAs regulate neuronal function in response to activity and whether deregulation of these mechanisms triggers pathological states are still poorly understood. For instance, are the miRNA pathways that control neuronal fate during development also involved in brain homeostasis? What are the mechanisms that could modulate miRNA expression in response to neuronal activity? Do the pathways involved in miRNA regulation depend on the kind of stimulus or on the miRNA species? What is the spatial resolution of miRNA control on local translation? What is the therapeutic potential of targeting activity-regulated miRNA?

The analysis of miRNA regulation in challenging pathological conditions such as chronic pain may help to highlight crucial miRNA mechanisms that are discreet in basal conditions. In the past few years, many studies have screened for miRNA expression in various animal models and in cohorts of patients with diverse neurodegenerative diseases. Although highlighting biomarker miRNAs with a possible value for therapeutic approaches, this kind of approach does not improve our understanding of miRNA targeting and function in pathological mechanisms. Indeed, it would be risky to evaluate targets of candidate miRNAs on algorithm-based prediction databases only, since the number of putative targets is very large and confirmation of targeting sometimes difficult to achieve. Therefore, it will be important to improve miRNA target prediction tools maybe by considering recent experimental data suggesting that "seed" region may not be the only signature of miRNA targeting ([@B71]). Moreover, it would be interesting to compare miRNA and mRNA expression data as often as possible in the light of experimentally confirmed miRNA:mRNA interactions.

Nevertheless, the various experimental paradigms with their sometimes conflicting data could hinder progress toward a better understanding of miRNA function. Therefore, the use of unified experimental parameters could further clarify the role of miRNA in neuronal plasticity. The large body of pre-existing data provides solid evidence for the involvement of miRNA regulation in both physiological and pathological conditions. Thus, miRNA is an attractive potential therapeutic target.

Conflict of Interest Statement
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The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

This work received funding from the Centre National de la Recherche Scientifique, Agence Nationale pour la Recherche (grant MirPAIN), Association pour la Recherche sur le Cancer (ARC), Conseil Régional Aquitaine. We acknowledge Y. Le Feuvre and C. Baudet for helpful comments on the manuscript.

[^1]: Edited by: *Alessandro Cellerino, Scuola Normale Superiore, Italy*

[^2]: Reviewed by: *Marco Martina, Northwestern University, USA; Daniel Kaganovich, Hebrew University of Jerusalem, Israel*
