However, of great importance is not only the sensory discrimina- tive, but also the emotion al or aff ective component of pain, and the fact that it has a uniquely unpleasant and stre ssful quality and the potential to cause great suff ering. As soon as a child is old enough for self-report, researchers and clinicians attempt to score the unpleasantness as well as the intensity of pain experience in order to better understand and alleviate the pain. Th e key to the matura- tion of this complete pain experience lies in neural activation of cen tral subcortical and cortical regions of the brain and neurosci- entists aim to map the noc iceptive output from the spinal cord as it projects through ascending pathways, brainstem, and subcortical structures to the cerebral cortex and to investigate how diff erent regions of the brain concerned with sensory and emotional aspects of pain become activated at diff erent stages of infant and child development. Another key determinant of pain experience is the activation of endogenous control mechanis ms within the CNS that can modu- late nociceptive activity through the release of endogenou s opioids and a powerful system of pathways descending from the brainstem to the spinal cor d. Th ese descending and endogenous pain modula- tory pathways are themselves controlled by subcortical and cortical networks and are the route by which factors such as attention and distraction, suggestion and expectation, stress and anxiety, context and past experience, can infl uence pain experience. Understanding the neural basis of developing endogenous des cending pain control is as important as understanding ascending pain pathways, as this will infl uence a child \200\231s ability to cope with pain and the extent to which it may be m anipulated. Introduction Pain has many dimensions and is processed at multiple diff erent lev- els of t he nervous system. When tissue is injured in young individu- als, it is essential that noci ceptive pathways in the peripheral and central nervous system (CNS) are activated to stimul ate protective behavior and alert the attention of caregivers. Th us noxious stimu- lation stimulates refl ex motor responses through circuits in the spinal cord and brainstem to ens ure that the aff ected body region is pro- tected from harm. In addition, autonomic circuit s in the brainstem and hypothalamus are activated, to produce appropriate changes in the ca rdiovascular, respiratory, and endocrine systems and main- tain homeostatic control of the body. It is these reactions that form the basis of pediatric clinical pain scores that meas ure movement, facial expression, cry, and other physiological indicators. Finally, the plasticity of these CNS pain pathways in infants and children is of great clin ical importance. Th e formation of correct connections in sensory regions of the brain is h ighly dependent on an appropriate balance of sensory input from the periphery, and excessiv e noxious stimulation in childhood may alter the formation of cortical pain connections thr ough neural activity-dependent mechanisms. Importantly, it may also change the normal balan ce of descending endogenous pain control, leaving an individual less able to cope with pain in the future. CHAPTER 8 central nociceptive pathways and descending modulation 75 In this chapter, the current state of knowledge of the develop- ment of central nociceptive pathways and descending controls is reviewed. Information has been gathered from laborator y studies in animal models and clinical and experimental investigations in infants and chil dren. Th e picture is far from complete but this is a fast moving and important area of res earch. inputs and although WDR and NS cells responding to nociceptive input can be recorded in the newborn rat dorsal horn, numbers are low in the fi rst week and build up slowly (see Table 8.1). Activity in dorsal horn networks ensure that spinal projection neurons transmit information to higher centers that is already sub- stantially altered from the original aff erent noci ceptive input. Th us noxious information may have already been dampened down or amplifi ed in magnitude and duration, through local interneuronal activity and central sensitization m echanisms both of which dif- fer in young and adult animals (see Walker and Baccei, Chapter 6, this volume). It may also be modulated by descending inhibitory or excitatory infl uenc es arising in the brainstem that also change through postnatal development (see \200\230Th e development of descend- ing brainstem modulation of nociception \200\231 section). Th e developmental anatomy and physiology of central pain pathways Th e development and mat uration of neural connections in the brain are commonly mapped in the laboratory animals us ing ana- tomical, electrophysiological and behavioral techniques, and more recently, optoge netic stimulation of genetically controlled light-sensitive neurons. Knowledge gained from the laboratory can be combined with human data, from traditional autopsy studies and more r ecently, neurophysiological recording and imaging of the living human brain using structura l and functional magnetic resonance imaging (fMRI) to provide an overall picture of the mat - uration of a particular pathway. Ascending spinal nociceptive projection pathways and cortical pain processing in adults Th e cell bodies of dorsal horn projection neurons are found in superfi cial lamina I and deep er laminae VI to V and their long axons travel up the spinal cord in the contralateral spin othalamic tract (STT). Since the STT arises from WDR and NS cells, this tract does not excl usively transmit nociceptive information but also low-threshold tactile information and tem perature sense. Th e thalamus is a major sensory integration center and the informa- tion t hat thalamic neurons receive and send on to the cerebral cor- tex through thalamocortical t racts is essential for the awareness of pain. Nevertheless, it should be remembered that on ly 10% of STT axons directly terminate in the thalamus. Th us, while the clas- sic terminat ion site of STT axons in the ventral posterior nucleus (VPn) and other sensory nuclei in th e thalamus (collectively called the ventrobasal complex) are commonly emphasized, most STT fi bres also project to the brainstem reticular formation including the periaqueductal gray (PAG), the parabrachial nucleus (PBn), the locus coeruleus, and the hypothalamus (Gauriau and Bernard, 2004; Todd 2010). Changing output from the spinal dorsal horn Nociceptive information from the periphery is f i rst integrated in the dorsal horn of the spinal cord (for the body) and trigeminal nuclei (for the head and neck). Multiple sensory aff erent nerve fi bres, car- rying information from the skin, muscle, joints, deep tissues, and organs synapse onto single dorsal horn neu rons resulting in spatial and modality convergence. Receptive fi elds of single dorsal horn neu- rons are larger and more overlapping than those of primary aff erents, and multiple s ensory modalities, such as touch and pinch, are proc- essed by single \200\230wide dynamic range \200\231 (WDR) cells. However, there are also a smaller number of \200\230nocicepti ve-specifi c \200\231 (NS) cells in the dorsal horn that receive information only from noci ceptive aff erents. Th e vast majority of dorsal horn neurons are interneurons, connecting locally with other neurons in the same segment of the spinal cord, but a small percentage ( c. 5%), both WDR and NS, are \200\230projection \200\231 cells, that have long axons that travel up the spinal cord to higher cen- tres in the CNS. Th us nociceptive information is transmitted partly by so-called \200\230labeled lines \200\231 devoted solely to nocicept ion, and partly by multimodal pathways, where nociceptive information is coded by spatial a nd temporal patterns of neuronal activity. Th e importance of these diff erent pathways lies in the informa- tion that they subserve. Th us it appears that there is an anatomical basis for separating the sensory or nociceptiv e aspects of pain from its emotional or unpleasant aspects. Recent research has focused upo n the projections of NS neurons in lamina I. Direct projections to thalamus are thought to mediate the sensory discriminative aspects of pain, while projections to the PAG and to the PBn, which in turn project to the hypothalamus and amygdala, are thought to mediate the em otional and autonomic aspects of pain. Importantly, the proportion of WDR and NS neurons changes substantially with postnatal age (Fitzgerald and Jennings, 1999). Th e neonatal dorsal horn is dominated by low-threshold, t actile Table 8.1 Postnatal change in the proportion of rat dorsal horn cell responding to cutaneou s noxious pinch, non-noxious brush or both (P = postnatal days). Age Dorsal horn cell response Brush Pinch Brush and pinch P3 (n = 22) 20 (91%) 1 (4.5%) 1 (4.5%) P6 (n = 65) 54 (83%) 7 (11%) 4 (16%) P10 (n = 53) 22 (42%) 12 (22%) 19 (36%) P21 (n = 35) 10 (29%) 5 (14%) 20 (57%) Reproduced with permission from Fitzgerald, M. and Jennings, E., Th e postnatal development of spinal sensory processing, Proceedings of the National Academy of Sciences USA , Volume 96, Issue 14, pp. 7719 \200\2237722, Copyright National Academy of Sciences 1999. SECTION 2 biological basis of pediatric pain 76 Th e development of thalamocortical connections \200\224rodents While nociceptive activity in the spinal cord, brainstem, and sub- cortical midbrain structures are suffi cient to gen erate refl ex behav- iours, autonomic and hormonal responses, they are not suffi cient to s upport pain perception and awareness. Understanding the matu- ration of pain perception req uires the study of supraspinal brain centers especially at the cortical level during early life. Th e output from the thalamus also appears to divide into two projections, a lateral and me dial stream. While the lateral stream emanates from the thalamic ventroposterolateral nucle us (VPL) and projects directly to the primary somatosensory cortex (SI), a key cor- tical r egion for the sensory aspects of pain experience, the medial stream arises in the medial do rsal thalamic nuclei and projects to the secondary somatosensory cortex (SII) and the anter ior cingulate and may contribute to the emotional aspects of pain (Willis et al., 2002). Im portantly, there is no \200\230pain \200\231 cortex, analogous to the visual or auditory c ortex, rather noxious stimulation evokes a characteristic pattern of activity in many brain areas, including somatosensory, insular, and cingulate areas, as well as frontal and parie tal areas. Coordinated activity in a collection of brain regions, none of which are unique to pain, are proposed to result in the multidimen- sional perception of pain. Other brain r egions may be recruited to increase or reduce the intensity and unpleasantness of the pain in certain circumstances or in certain individuals (Apkarian et al., 2005; Tracey and Manty h, 2007; Tracey and Johns, 2010). Th is is a rapidly changing area of pain research, howeve r, with some contro- versy (LeGrain et al., 2011). A key step in the cortical processing of noxious stimuli is the development of thalamocorti cal projections. In rodents, thalamic axons leave the thalamus at E13.5 and begin to grow t owards the subplate (SP) neurons, which are among the fi rst born cortical neurons and resi de at the white matter/cortical plate boundary. Th e SP zone, marginal zone, and cortical p late represent the early structures from which the layers of the cerebral cortex develop; t he SP is largely transient, and the marginal zone forms layer I and cortical plate neurons form layers II to VI. Th alamocortical projections appear to exit the thalamus by E16 and f orm their fi rst functional synaptic connections in the SP region around E18 to E19. By bir th, thalamic axons are extending into layers IV, V, and VI of the developing cortex in a to pographically precise manner and thalamocortical aff erents and cortical sensory areas sett le into place in the fi rst week aft er birth. Th e refi nement of topographical projection s from the thalamus to the cortex is thought to arise through an activity-dependent process in which thalamic axons compete for cortical targets. If activity is altered during a crit ical period in early development, normal connectivity is disrupted. Synaptic connections ar e strengthened during development by correlated pre- and postsynaptic activity, and a likel y mechanism for this process is N-methyl-D-aspartate receptor-dependent long-term potentiat ion (Li and Crair 2011). Cortical synaptic circuitry develops rapidly in the second postnat al week, simul- taneous with experience-dependent turnover of dendritic spines. Patterned n euronal activity, such as spindle bursts in the neonatal cortex, promote the maturation of cortical synapses and neuronal circuits. Selective removal of SP neurons in the limb region of the somatosensory S1 cortex abolishes endogenous and sensory- evoked spindle burst acti vity and weakens thalamocortical inputs to layer 4 neurons. Th us, SP neurons are crucially involved in the generation of early network activity in the neonatal cortex, which is an i mportant feature of cortical development (Tolner et al., 2012). Th e development of ascending spinal nociceptive projection pathways to brainstem and thala mus STT neurons develop before birth in rodents: embryonic-day (E)-18, 2-day-old, and 1-wee k-old mice do not diff er from adults in the number or distribution of STT neurons back-lab eled from the thalamus although they signifi cantly increase in size over the fi rst postna tal week. Arborizations and boutons of STT neurons can also be observed within the ventroba sal complex of neonatal thalamus, suggesting that synaptic connections are formed with thal amic cells at this very early age (Davidson et al., 2010). However, while anatomical connections to the thalamus may be present early in development, the functional maturation of nocic- eptive inputs to these regions is slower. Th e neural a ctivity in sub- cortical brain areas has been mapped following formalin-induced injury to t he paw using the neural activity marker, fos, in the brain of fetal and infant rats. By day 3, formalin evoked fos expression in the ventral lateral medulla (the site of origin of de scending control pathways to the spinal cord), but not in subcortical areas respon- sible f or signaling sensory and emotional information (namely the paraventricular and medial dorsa l nuclei of the thalamus, the hypothalamus, and the PAG) which were all clearly labeled by 14 days of age (Barr, 2011). Th is developmental pattern may be related not only to the mat uration of pain perception but also to development of autonomic and defensive reactions to pain in the infant (Chan et al., 2011). Th e maturation of transmission from thalamocortical connec- tions takes place over a prolo nged postnatal period. Th us a clear somatosensory-evoked potential from stimulation of the forepaw is not recorded until the end of the fi rst week of life in rats and does not acqu ire a recognizably adult pattern until postnatal day (P) 12 (Th airu, 1971). An investigati on of single cells in 7-day-old rat primary somatosensory cortex revealed low levels of spo ntaneous activity compared to adults and diminished excitability and longer latencies to cu taneous stimulation than in adults. In addition, recep- tive fi elds were considerably larg er at 7 days, which reduces spatial discrimination. In contrast to adult cells, the immatur e cells com- monly responded cyclically, with alternating phases of increased and decreased fi ring rate for periods of up to 3 s following punctate stimulation (Armstrong-James, 197 5). Little is known about the development of the ascending STTs to the brainstem and thalamus i n man although their presence is inferred from cortical activity in preterm infants (see la ter). Changes in facial expression in response to noxious stimulation are observed in extre mely preterm infants, even in the presence of cerebral injury, suggesting direct functional nociceptive connections from the spinal cord to the brainstem at a very early age (Oberlan der et al., 2002). Non-invasive diff usion tensor MRI and tractography in 1-to 4-month-old healthy infants show the spinothalamic tract and the corticospinal tract to be among the mo st mature bundles, while the most immature tracts are the internal capsule and cingu- lum ( Dubois et al., 2008). More complex functions in the cortex require linked activity in several areas, such as the pain matrix, carried out by large-scale CHAPTER 8 central nociceptive pathways and descending modulation 77 neuronal networks integrating several cortical areas. Little is known about the functional development of these networks and the maturational processes by which distant networks beco me functionally connected. Recordings across the whole cortical sur- face together with int racortical electrodes, reveals that sensory evoked cortical responses mature continuously t hroughout the fi rst 3 weeks with the strongest developmental changes occurring in a very s hort time around postnatal day 13 (P13) (Quairiaux et al., 2011). Th is period, at the end of the second postnatal week coincides with the onset of mature pain behavior (Fitzgerald, 2005), as well as major synaptic and functional changes within the S1 cortex. Distinct and separate noxious evoked cortical activity Spontaneous bursts of cortical activity evoked by all sensory stimulation Thalamocortical a \200erents innervating cortical plate Th e neural circuitry of the medial prefrontal cortex (mPFC), an area of the brain that is key to assessing the threat of sensory stimuli and generating defensive responses, progress ively develops more capacities as the animal matures. When rats are exposed to a threat- en ing stimulus at P14, the mPFC is neither active nor responsive, but becomes responsive in p rocessing aversive sensory stimulation at P26, and only fi nally regulates freezing in adol escence at P38 to P42 days (Chan et al., 2011). Th e development of thalamocortical connections \200\224humans Th e growth of thalamocortic al aff erents has been well documented in the human brain (Kostovic and Judas, 2010). In in fants younger than 24 gestational weeks (GW), thalamocortical aff erents are \200\230waiti ng \200\231 in a transient SP zone. Th ese axons then penetrate the cor- tical plate betwee n 24 and 26 GW and form functional synapses with layer IV cortical neurons beginning at 29 GW. Th e SP region gradually dissolves between 33 to 35 GW and aft er 36 GW there is a subs tantial growth and maturation of associative and callosal con- nections in the cortex (see Figure 8.1). 24.0 26.0 28.0 30.0 32.0 34.0 36.0 38.0 40.0 Gestational age (weeks) Figure 8.1 Th e development of thalamocortical input and the changing pattern of somatosens ory evoked activity in the human infant cortex over 24 to 40 weeks \200\231 gestation. resting state networks in the adult; therefore fcMRI in this age group should be interprete d with care (Colonnese and Khasipov, 2012; see also Poorun and Slater, Chapter 39, this vol ume). Results from a systematic fMRI study on mainly sedated pre- term infants are consistent wit h the anatomical data and demon- strate well-localized blood oxygen level-dependent (BOLD) signal activity in the SI in response to balloon infl ation of the hand in infants aged 25. 5 to 34 GW. In this preterm group, positive BOLD activity to this proprioceptive and tactil e stimulus was seen in the contralateral hemisphere only, while in preterm infants studied at term-corrected gestational age (36.5 \200\22341.5 weeks GA), the spatial distribution of the response was more complex with bilateral activ- ity, and additional adjacent areas of negative BOLD signal change in the contralateral primary somatosensory and motor cortices. Th e results suggest that thalamocortical synapses carrying tactile information are functio nal by 25.5 weeks but more complex intra- and interhemispheric projections are not active u ntil aft er 34 weeks GA (Arichi et al., 2010). Th e development of cortical pain processing Advances in our knowledge of the onset of cort ical pain processing in human infants and children have come through the application of ima ging and electroencephalographic (EEG) measurements during clinical noxious procedures or i n the presence of chronic pain. Measurements of cerebral oxygenation have demonstrated that brief noxious stimulation (in t he form of a heel lance for routine blood sampling) evokes a clear response in the contrala teral som- atosensory cortex of infants from 25 weeks postmenstrual age. Importantly, this cortical response is modality-specifi c, as non- noxious stimulation of the heel failed to evoke the response even when accompanied by a withdrawal refl ex (Slater et al., 2006). Nox ious inputs of longer duration may result in bilateral activa- tion of the somatosensory co rtex (Bartocci et al., 2006). Th ese haemodynamic responses, which refl ect a localized inc rease in total hemoglobin concentration, are caused by increased neuro- nal activity within the same area, as time-locked EEG recordings have demonstrated that the heel lance also ev okes a clear nocic- eptive-specifi c potential in the infant cortex (Slater et al., 2010a). Furthermore EEG recording of noxious-evoked activity in infants Functional connectivity magnetic resonance imaging (fcMRI) utilizes spontaneous, low-freque ncy (<0.1 Hz), coherent fl uctua- tions in the BOLD signal to identify networks of function al cerebral connections and promises to be useful for mapping the develop- ment of key conn ections in the brain. Such mapping indicates that cortical hubs of connectivity in the newb orn infant brain are largely targeted in the primary sensorimotor, auditory, and visual sys tems and only to a small extent in higher-order frontal, associative cortex (Fransson et al ., 2011). However, the immature brain displays spon- taneous patterns of electrical activit y not seen in adults which may not be precursors of the spontaneous neuronal activity that forms SECTION 2 biological basis of pediatric pain 78 Th e development of descending brainstem modulation of nociception It has long been appreci ated that the activation of descending path- ways from brainstem regions, such as rostroven tral medulla (RVM), can exert both inhibitory and facilitatory eff ects on spinal excitabil ity and pain sensitivity in the adult (Heinricher et al., 2009). Th e RVM has strong inputs from the PAG which in turn is driven from the amygdala and other parts of the limbic syste m (see Figure 8.1). Th is is therefore the pathway that mediates top-down control of nocic- eptive input and pain, as has been elegantly demonstrated in man in imaging studies (Eippe rt et al., 2009; Tracey and Mantyh, 2007). However, laboratory studies have demonstrated th at the maturation of this descending control is relatively late in development. Hence, in y oung animals, in the resting state, descending facilitation of spinal nociceptive processin g dominates during the fi rst weeks of life, with the typical biphasic descending facilitat ion and inhibition observed in adults not apparent until P28 (Hathway et al., 2009, 2012). Th e RVM, which is the main output nucleus for brainstem descend- ing control, undergoes a remarkable maturational switch aft er P21: both lesioning and electrical stimulation of RVM at diff erent post- natal ages reveal that RVM control over spinal nociceptive circuits sw itches from being entirely facilitatory before P21 to inhibitory at older ages. Between P25 and P35, descending inhibition begins to dominate, but it is not as powerful as in the adu lt until P40. Th us, the infl uence of the RVM over spinal nociceptive refl exes and dorsal horn neuronal activity gradually changes over this critical develop- mental period (Hathwa y et al., 2009). Th e delayed maturation of these inhibitory pathways from the RVM could ex plain why activat- ing the PAG region (which sends extensive projections to the RVM) does n ot produce analgesia until P21 (van Praag and Frenk, 1991) or why stimulating the dorsolate ral funiculus is unable to inhibit the fi ring of dorsal horn neurons or c-fos activation u ntil at least P10 (Boucher et al., 1998; Fitzgerald and Koltzenburg, 1986). of diff erent gestational ages has revealed how the brain begins to distinguish noxious inf ormation from innocuous tactile stimula- tion in early life (Fabrizi et al., 2011). Recordi ng the brain neu- ronal activity in response to time-locked touches and clinically essentia l noxious lances of the heel in infants aged 28 to 45 weeks \200\231 gestation reveals a tr ansition in brain response following tactile and noxious stimulation from non-specifi c, ev enly dispersed neuronal bursts to modality-specifi c, localized, evoked potentials (see Fig ure 8.1). Th ese recordings suggest that the haemodynamic recordings in very young infants refl ect an increase in spontane- ous burst responses, which is greater with noxious than t actile stimulation, but that the specifi c neural circuits necessary to dis- tinguish the q ualitative diff erence between touch and nocicep- tion emerge from 35 to 37 weeks \200\231 gestation in the human brain (Fabrizi et al., 2011). Th e measurement of noxious-evoked brain activity in human infants has provided a valuable opportunity to determine the extent to which the clinical tools used to assess pediatric pa in are correlated to the level of cortical activation following noxious stimulation. For ex ample, facial expression correlates better with cortical haemodynamic activity than physiol ogical responses such as heart rate, but cortical pain responses were observed in some infa nts in the absence of any change in facial expression follow- ing heel lance (Slater et al. , 2008). In a double-blind, randomized controlled trial of oral sucrose, commonly used to r educe the behavioral response to painful procedures in neonatal intensive care, EEG and ele ctromyographic recording revealed that sucrose does not signifi cantly aff ect activity in neonatal brain or spinal cord nociceptive circuits (Slater et al., 2010a). Th is study sugg ests that the ability of sucrose to reduce clinical observational scores aft er noxious eve nts in newborn infants is mediated at brainstem level. Little information is available about the development of corti- cal pain processing in olde r infants and children. Childhood is a time of great change in the cerebral cortex: the hig hest number of dendritic spines in the dorsal prefrontal cortex is reached at 2.5 to 7 year s and dendritic spine density in childhood exceeds adult values by two- to threefold and be gins to decrease during puberty (Petanjek et al., 2011), but very little is known about how this infl uences the pain experience. Th e explosion of brain imag- ing studies examining the functional anatomy of pain processing in adults is gradually being extended to the pedi atric popula- tion. A fi rst, exploratory fMRI study of nociceptive processing in the cereb ral cortex of thirteen 11- to 16-year-olds to tonic mild to moderately painful heat showed signifi cant activation in somatosensory cortex, the anterior insula, the premotor cor- tex , and the prefrontal cortex (Hohmeister et al., 2010), all brain regions known to be activa ted by pain in adults (Apkarian, 2005). Activation of the anterior insula has been associat ed with pain aff ect while the prefrontal cortex activation has been related to cognitive a spects of pain processing, such as anticipation, atten- tion, stimulus evaluation, and memo ry. Prefrontal cortex activa- tion can modulate the pain experience and the premotor cortex has been linked to pain-evoked motor planning. Interestingly, these children did not show activation in the anterior cingu- late, a region primarily involved in aff ective pain proc essing and modulation in adults (Hohmeister et al., 2010; see also Hermann, Chapter 40, thi s volume). Th e mechanism of change is likely to lie in the maturation of RVM and PAG circuitry, rathe r than in lack of descending con- nections, neurotransmitter levels, or receptor expression in the spinal cord. Interestingly, the postnatal development of inhibitory GABAergic neuro ns in the rat PAG matter mirrors the develop- ment of descending controls: very few such ne urons are found from P0 to P5, with a small increase from P5 to P10, a marked matura- tion beginning at P14 and the adult pattern of GABA immunore- activity only established between P20 to P30 (Barbaresi, 2010). Th e maturation of brainstem opioidergic signaling is also an important factor contributing to this developmental switch, as the switch also occurs when -opioid receptor agonists are microinjected into the RVM suggesting that the descending facilitation of spinal nocicep- tion in young animals is mediated by -opioid receptor pat hways within the RVM. Furthermore, the developmental transition from descending facilitatio n to inhibition of pain is determined by activ- ity in central opioid networks at a critica l period of periadolescence (Hathway et al., 2012). Blockade of tonic opioidergic activity from P21 to P28, but not at earlier or later ages, prevents the normal development of desce nding RVM inhibitory control of spinal noci- ceptive refl exes, while enhancing opioidergic activity with chronic morphine over P7 to P14 accelerates this development. Th is change i n control of supraspinal centers over spinal nociceptive refl exes and dorsal horn neuronal activity over a critical periadolescent CHAPTER 8 central nociceptive pathways and descending modulation 79 developmental period is likely to be of key importance in setting future pain sensitivity i n adulthood. unit can aff ect early brain development in preterm infants. Infants born very preterm (24 \200\22332 weeks GA) have been followed prospec- tively from birth and the number of skin-b reaking events quantifi ed to search for structural changes in their brain scans. Aft er ad justing for clinical confounders such as illness severity, morphine exposure, brain injury, and surgery, the results show that greater neonatal pro- cedural pain is associated with r educed measures of white matter and of subcortical gray matter. Reduced white matter was pr edicted by early pain, whereas gray matter changes were predicted by pain exposure througho ut the neonatal course, suggesting a primary early eff ect on subcortical structures, such as the thalamus, followed by secondary white matter reductions (Brummelte et al., 2012; see also Grunau, Chapter 4, this volume). Plasticity of developing central nociceptive pathways Evidence in rodent brain While it is well known that the development of central sensory path- ways, such as the visual and audit ory systems, is dependent upon a balance of sensory activity arising in the periphery, the absence of a clear \200\230pain \200\231 cortex and the complexity of regions activated du ring pain experience has meant that the infl uence of peripheral nocicep- tive activity in shaping the maturation of central pain pathways has been harder to establish, but evidence is rapidly emerging. Th us, rat pups that have received skin incisions in early life, not o nly have greater pain sensitivity to a repeat injury when they reach adult- hood but also s how diff erences in reward-related neural activity. Following exposure to a novelty-induced hypophagia paradigm (a test of motivation), fos activity in lateral hypothalamic orexin ne urons, part of a specifi c reward seeking pathway in the brain, in acutely injured rats was the same as in uninjured naive animals but in those rats with a \200\230neonatal pain his tory \200\231, orexinergic activity was signifi cantly increased and highly correlated with their moti- vational behavior. Th us neonatal pain experience can cause long- term changes in brain motivational orexinergic pathways, known to modulate mesolimbic dopaminergic rewa rd circuitry (Low et al., 2012). Infants who are born prematurely and who have experienced at least 40 days of intensive or special care also have increased brain neuronal responses to noxious stimuli compared to he althy new- borns at the same postmenstrual age. Evoked potentials generated by noxious clin ically-essential heel lances are signifi cantly larger in prematurely-born infants, while r esponses to non-noxious touch stimuli are unaff ected, suggesting a functional change in pa in processing in the brain (Slater et al., 2010b). FMRI of cerebral pain responses in a sim ilar cohort of preterm infants when they have reached school-age (11 \200\22316 years) show that aside from more gen- eral neurodevelopmental consequences, repeated neonatal nocic- e ptive input may result in continued increased sensitivity in central pain pathways into ado lescence. Tonic heat stimuli (of individually adjusted moderate pain intensity) applied to the ex-preterm chil- dren causes signifi cant activation in the thalamus, anterior cingu- l ate cortex, cerebellum, basal ganglia, and PAG that is not observed in full-term hospitaliz ed children or controls. Th e preterms also show signifi cantly higher activation in the S1 , anterior cingulate cortex, and insula than controls. Th is exaggerated brain response is pain specifi c and is not observed during non-painful warmth stimulation (Hohmeister et al. , 2010) Neonatal pain has also been shown to infl uence the generation of new neurons in the dentat e gyrus region of the hippocampus. Rat pups receiving intraplantar injections of the painfu l infl am- matory agent complete Freund \200\231s adjuvant, on P8, had more BrdU- labeled c ells and a higher density of cells expressing doublecortin, both measures of newborn neuron s, in the subgranular zone of the dentate gyrus, thought to be involved in memory formation (Leslie et al., 2011). Th ere is also evidence that neonatal intensive care can alter endog- enous descending cont rol systems. Children, aged between 7 and 11 years, divided into term, preterm exposed to n umerous painful interventions, or preterm exposed to few painful interventions, have compar able pain thresholds but diff er in their responses to condi- tioning cold stimulation. Th us while cold stimulation signifi cantly increases heart rate and decreases thermal pain se nsitivity of term- born children and \200\230preterm less pain \200\231 children, changes in heart rate and pain sensitivity in response to conditioning cold stimulation were not ob served in \200\230preterm more pain \200\231 children. Th ese results suggest that endogen ous pain modulatory mechanisms are not as well developed as those of children not exposed t o noxious insult at birth (Goff aux et al., 2008; Wollgarten-Hadamek et al., 2011) Descending pain control systems in the brainstem can also be infl uenced by early pain expe rience. Infl ammatory pain induced by intraplantar carrageenan at birth induces signifi can t changes in the brainstem endogenous opioid system into adulthood, namely increased beta-e ndorphin and met/leu-enkephalin protein levels and decreased opioid receptor expression in the PAG. Th ese long- lasting alterations in endogenous opioid tone are likely to aff ect l ater pain sensitivity (Laprairie and Murphy, 2009). Th is is supported by the observation t hat focal electrical stimulation of the RVM in adult animals treated neonatally with carrag eenan produced sig- nifi cantly greater descending inhibition of nociceptive responses to n oxious thermal stimuli applied to the hindpaws and the tail, indicating that neonatal tissu e injury and infl ammation produces lasting alterations in descending modulatory systems th at modify nociceptive processing (Zhang et al., 2010). Evidence in human brain \200\224pediatric chronic pain Evidence suggests that central chang es in the brain can occur in older children who develop chronic pain syndromes. Children wi th migraine when undertaking an experimental attentional task, display EEG activity associa ted with an automatic attentional bias towards painful and potentially painful somatosensor y stimuli, perhaps due to alterations in connectivity within the limbic system (Zohsel et a l., 2008). Evidence in human brain \200\224e \200 ects of neonatal intensive care Th e developmental plasticity observed in laboratory animals is sup- ported by structural and functional evide nce of changes in the human infant brain following repeated noxious stimulation. Structural MRI studies suggest that procedural pain in the neonatal intensive care CNS activation to sensory stimulation in pediatric patients (9 \200\22318 years) with compl ex regional pain syndrome (CRPS) aff ecting SECTION 2 biological basis of pediatric pain 80 the lower extremity also reveals changes. In an fMRI study each patient underwent two scann ing sessions: once during an active period of pain (CRPS(+)), and once aft er symptomatic r ecovery (CRPS( \210\222 ). In each session, mechanical (brush) and thermal (cold) stimuli were applied to the aff ected region of the involved limb and the corresponding mirror regi on of the unaff ected limb. Several interesting observations were made in this study but no tably in the CRPS( \210\222 ) state, signifi cant activation diff erences persist despite nearly complete elimination of evoked pain suggesting prolonged changes in CNS pain circuit ry in pediatric patients with CRPS that outlast the illness itself (Lebel et al., 2008). 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