fibromyalgia is a chronic pain syndrome of unknown origin , estimated to affect 25% in all populations studied ( raspe , 1992 ; wolfe et al . , 1995 ;
fibromyalgia is characterized by widespread pain , fatigue , poor sleep , and dyscognition ( wolfe et al . , 2010 ) and is often also associated with mood disorders such as depressive episodes and anxiety .
although the underlying pathophysiology is not entirely understood , fibromyalgia has been associated with augmented central nervous system ( cns ) processing of nociceptive stimuli using both quantitative sensory testing ( qst ) and functional neuroimaging .
( gracely et al . , 2002 ; desmeules et al . , 2003 ; petzke et al . , 2003 ; smith et al . ,
context the notion of dysfunctional endogenous pain inhibition has been proposed to play a pivotal role in the genesis of chronic widespread pain .
this is supported by studies demonstrating impaired or absent conditioned pain modulation ( cpm ) in these patients ( lautenbacher and rollman , 1997 ; vierck et al . , 2001 ; julien et al . , 2005 ; chalaye et al . ,
functional ( gracely et al . , 2002 ; giesecke et al . , 2004 ; jensen et al . ,
2012 ) and structural ( kuchinad et al . , 2007 ; schmidt - wilcke et al . , 2007 ;
2013 ) brain imaging studies have shed some light on possible central mechanisms that might play a role in the genesis of chronic pain in fibromyalgia .
one such approach is the investigation of fluctuations in blood oxygenation level dependent ( bold ) signals at rest , termed resting state functional connectivity ( rs - fc ) .
only recently have changes in rs - fc been demonstrated in fibromyalgia , such as a hyper - connectivity between the default mode network ( dmn ) , a constellation of brain regions involved in self - referential thought , and the insular cortex ( ic ) , a brain region known to play a pivotal role in pain perception ( napadow et al . , 2010 ) .
interestingly this hyper - connectivity may be a marker for chronic pain intensity as two independent trials have shown that decreases in connectivity between the dmn and ic following treatment were associated with reductions in clinical pain ( napadow et al . , 2012 ; harris et al . , 2013 ) .
treatment of fibromyalgia in terms of a clinically relevant reduction in widespread pain is often challenging and both pharmacological and non - pharmacological approaches ( bernardy et al . , 2013 ) .
food and drug administration for the treatment of pain in fibromyalgia : pregabalin , a compound that binds to the 2 subunit of a voltage dependent presynaptic calcium channel , and two selective serotonin ( 5-ht ) and norepinephrine ( ne ) reuptake inhibitors , milnacipran ( mln ) and duloxetine ( goldenberg et al . , 2010 ; schmidt - wilcke and clauw , 2010 ; schmidt - wilcke and clauw , 2011 ) .
other drugs , such as tricyclic compounds ( tca , e.g. amitryptiline ) , viewed as non - selective 5-ht and ne reuptake inhibitors , have also repeatedly been shown to be efficacious in the treatment of fibromyalgia and are frequently used in pharmacological treatment regimens .
the way 5-ht and ne reuptake inhibitors act to reduce pain is still a matter of debate as spinal , subcortical , and cortical mechanisms have all been proposed .
however the overall effect of any of these individual treatments has been modest ( huser et al . , 2013 ) .
moreover a key limitation in the treatment of fibromyalgia , and chronic pain in general , is that there are no reliable tools to guide treatment assignment for individual patients . as such
recently our group has shown that chemical and functional imaging in fibromyalgia can be used to predict treatment response to pregabalin in fibromyalgia ( harris et al . , 2013 ) .
here we sought to identify rs - fc patterns that might predict treatment response in fibromyalgia to the selective 5-ht and ne reuptake inhibitor mln .
since preclinical studies have indicated that dual reuptake inhibitors are thought to have a favorable effect on endogenous pain inhibition which is believed to be dysfunctional in fibromyalgia ( lautenbacher and rollman , 1997 ; julien et al . , 2005 )
, we specifically focused on rs - fc to brain regions involved in antinociception and pain modulation such as : the periaqueductal gray ( pag ) , the rostral part of the anterior cingulate cortex ( acc ) , the dorsolateral prefrontal cortex ( dlpfc ) and the amygdala .
inclusion criteria were : ( 1 ) meeting 1990 american college of rheumatology criteria for fm with chronic widespread pain for at least 6  months ; ( 2 ) 1870  years of age ; ( 3 ) non - lactating and non - pregnant ; ( 4 ) right handed ; ( 5 ) score between 40 and 90  mm ( inclusive ) on a 100  mm pain visual analogue scale ( vas ) ; ( 6 ) willing to withdraw from cns - active therapies marketed as antidepressants ( monoamine oxidase inhibitors , tricyclics , tetracyclics , selective serotonin reuptake inhibitors , norepinephrine reuptake inhibitors , and snris ) ; ( 7 ) willing to withdraw from stimulant medications such as those used to treat attention deficit disorder and attention deficit hyperactivity disorder ( e.g. mixed amphetamine salts , methylphenidate , dextroamphetamine ) or fatigue associated sleep apnea or shift work ( e.g. modafinil ) ; ( 8) willing to withdraw from anorectic agents such as diethylpropion , sibutramine , and phentermine ; and ( 9 ) if currently taking pregabalin and/or gabapentin , to remain on a stable dosage throughout the duration of the study .
major exclusion criteria were : ( 1 ) significant risk of suicide ; ( 2 ) medical conditions including cardiac diseases , glaucoma , autoimmune disease , systemic infections ( e.g. human immunodeficiency virus , hepatitis ) , active cancer , pulmonary disease or dysfunction , unstable endocrine disease ( must be stable at least 3  months prior to study enrollment ) , unstable diabetes , unstable thyroid disease ; ( 3 ) pregnant or lactating ; ( 4 ) any other severe , acute , or chronic medical or psychiatric conditions that could increase risk or interfere with trial results ; ( 5 ) body mass index greater than 36 ; ( 6 ) treatment with any experimental agent , including mln , within 30  days before screening ; and ( 7 ) contraindications with mri procedures .
all study participants gave written informed consent . the study protocol and informed consent documents were approved by the university of michigan institutional review board ( ann arbor , michigan ) and forest laboratories ( new york , ny ) .
all imaging data were stored , validated , analyzed , and assessed for quality at the university of michigan independent of forest personnel .
patient demographics , medications , and identification of inclusion for analysis are listed in table 1 .
all patients were randomized in a double - blind , two - period crossover study of mln versus placebo ( fig .
potential participants underwent an initial visit , prior to the first neuroimaging session , wherein they were evaluated for study criteria .
after meeting inclusion / exclusion criteria , consenting patients were randomized to either mln first or placebo first for period 1 , and which followed 14  week washout period to withdraw from all excluded medications that could interfere with efficacy and neuroimaging measures .
this washout period included a 1-week single - blind placebo run - in period to reduce the possibility of placebo effects during the first double - blind treatment period .
following the placebo run - in period , all participants underwent their first neuroimaging scan ( pretreatment for period 1 ) which involved functional connectivity magnetic resonance imaging ( fcmri ) . following this initial scan ,
subjects randomized to receive mln in the first period , underwent dose escalation of mln up to 200  mg / day over the course of 2  weeks , with a maintained fixed dose for 4  weeks , at which time an identical post - treatment fcmri session was conducted .
those subjects randomized to placebo for period 1 took matching placebo pills over the course of 6  weeks before undergoing an identical post - treatment fcmri session .
all participants then entered a 1  week taper and 2  weeks of a placebo washout , during which time a placebo sugar pill was consumed daily .
once the washout period was completed , all patients crossed over to the other study drug for period 2 ( i.e. those who had mln for period 1 received placebo for period 2 and vice versa . ) .
placebo data included in all analyses came from either period 1 or period 2 ( depending on treatment order ) , and were included for all subjects regardless of treatment order .
all subjects were informed that they would be dosed with mln or placebo at various times throughout the study , but were not told when they were transferred from one treatment to the other .
we assessed treatment response for both clinical pain as well as evoked experimental pain in all participants prior to and following each study period .
clinical pain was assessed with the short form of the brief pain inventory ( bpi ) which captures both pain severity ( bpi sev ) and interference due to pain ( bpi int ) ( cleeland and ryan , 1994 ) measured over the course of the previous week .
changes in these components ( post minus pre ) were used as measures of treatment response for clinical pain .
changes in pain both following mln and placebo were assessed in spss , version 20 , by performing paired t - tests .
further , we specifically investigated whether changes following mln treatment differed significantly from changes following placebo treatment by performing paired samples t - tests of the change scores for all behavioral measures , also in spss , version 20
. when assessing for responders compared to non - responders to a treatment period , we defined a patient to be a responder if she had 30% improvement for a behavioral measure after a treatment period .
patients with < 30% improvement after a treatment period were categorized as non - responders . evoked experimental pain data was also collected in and outside the mri scanner before and after each treatment period .
pressure - pain was administered to the left thumbnail with three distinct conditions using the multi - modal automated sensory testing ( mast ) system ( harte et al . , 2013 ) :
1 ) an equal pressure condition of 1.5  kg / cm , 2 ) the amount of pressure required to elicit pain50 on the 0100 nrs , and 3 ) a faint touch rest condition .
both behavioral and bold fmri response data will be analyzed and reported in a subsequent manuscript . in this investigation
we were primarily interested in identifying baseline neuroimaging parameters that could predict changes in clinical and evoked pain specifically following treatment with mln . resting state fmri data
were acquired using a t2 * -weighted spiral sequence ( tr  =  2.0  s , te  =  30  ms , fa  =  90 , matrix size 64    64 with 43 slices , fov  =  20  cm and 3.12    3.12    3  mm voxels ) , using a 3  tesla general electric , signa scanner 9.0 , vh3 with 16 rod birdcage transmit  receive radio frequency coil . during the 6  min resting state fmri acquisition period ( 180 scans ) ,
subjects were asked to remain awake with their eyes open and to stare at a motionless cross presented on the screen .
minimal cognitive tasks such as staring at a cross are thought not to disrupt resting state networks ( greicius et al . , 2003 ) .
the first 6 images of the resting state scan were discarded from the data set and not analyzed in order to avoid equilibration effects . a t-1 weighted structural gradient echo data set ( tr 1400  ms , te 1.8  ms , flip angle 15 , fov 256    256 , yielding 124 sagittal slices with a defined voxel size of 1    1    1.2  mm ) was also acquired for each subject .
data were pre - processed and analyzed using fsl ( http://www.fmrib.ox.ac.uk/fsl ) and statistical parametric mapping software packages ( spm , version 8 , functional imaging laboratories , london , uk ) , as well as the functional connectivity toolbox conn ( cognitive and affective neuroscience laboratory , massachusetts institute of technology , cambridge , usa ) running under matlab 7.5b ( mathworks , sherborn , ma , usa ) . upon collection of the functional data , cardiorespiratory artifacts were corrected for using the retroicor ( hu et al .
pre - processing steps included motion correction ( realignment to the first image of the time series ) , normalization to the standard spm  epi template ( generating 2    2    2  mm resolution images ) and smoothing ( convolution with an 8  mm fwhm gaussian kernel ) .
subject head motion was assessed by evaluating three translations and three rotations for each scan .
translational thresholds were set to 2  mm , while rotational thresholds were limited to 1. a subject was to be excluded from the analysis if head motion exceeded either of the thresholds in one of the six dimensions .
as snris are thought to augment antinociceptive mechanisms , we were particularly interested in rs - fc to brain regions known to be involved in the descending antinociceptive / pain modulatory system .
the following regions were chosen based on the current literature suggesting involvement in antinociception / pain modulation ( bingel , 2010 ) : three seeds covering the rostral acc , the ventral ( vacc ) , pre - genual ( pgacc ) and subgenual ( sgacc ) regions , and bilateral seeds in : the dorsolateral prefrontal cortex ( dlpfc ) , the amygdala , and the periaqueductal gray ( pag ) ; seed regions were created using the spm extension tool marsbar .
all seed regions were created as spheres . for further information including size and location ,
, seed regions ' time - series were extracted ; white matter , cerebrospinal fluid , and realignment parameters were entered into the analysis as covariates of no interest . a band - pass filter ( frequency window : 0.010.1  hz )
first level analyses were performed correlating seed region signal with voxel signal throughout the whole brain , thereby creating seed region to voxel connectivity maps ( one map per seed per individual ) . for group analyses we focused on pre- mln rs - fc as a predictor of subsequent change in pain resulting from the drug .
our primary approach was to enter pre - treatment connectivity maps in a multiple regression analysis with changes in pain ( post - mln minus pre - mln ) in spm .
results were deemed significant using a family wise error ( fwe ) cluster corrected threshold of p  <  0.05 . within a priori brain regions ,
including : the ic , posterior cingulate cortex ( pcc ) , precuneus , inferior parietal lobule ( ipl ) , and dlpfc , a small volume correction using a sphere with a radius of 5  mm was performed , and results were deemed significant at a ( fwe ) cluster level corrected threshold of p  <  0.05 .
these regions had been selected as a priori regions because we were specifically interested in rs - fc changes to regions known to be involved in pain processing , which allowed for small volume correction analysis based on previously published results .
significant results were then extracted and entered into spss , version 20 , to assess for outliers . to determine if these results were specific to mln , identical analyses were performed for the placebo period , using the significant regions identified as predicting response to mln . in order to rule out chance differences by treatment period at baseline , we compared the mln and placebo correlations for the two cross - over periods separately ( i.e. those that had mln first and those that had placebo first ) .
correlations were performed in spss , version 20 , and results were found to be significant at p  <  0.05 .
finally , we sought to incorporate multiple functional connectivity measures in a regression analysis to explore for collinearity amongst our rs - fc outcomes and to further improve prediction of treatment response , while simultaneously controlling for pre - treatment pain levels ( harris et al . ,
a multiple linear regression model was created with pre - treatment pain measures and rs - fc correlation values serving as independent variables and post - treatment pain as the dependent variable .
a model was first constructed with each rs - fc measure individually and then in combination ( when multiple rs - fc values predicted the same pain outcome ) by way of forward selection .
eight were excluded from the analysis for the following reasons : six withdrew prior to completing all neuroimaging sessions ( two prematurely stopped taking mln , two had side effects from the drug that prevented participation in the trial , and two terminated due to personal reasons ) , and two additional participants were excluded because they did not reach the pre - specified dose of mln before imaging .
the drop - out rate due to adverse events of the medication was 9% ( 2 of 23 participants ) .
thus fifteen subjects ( mean age : 40.7    10.2 ) were included in the present analysis .
multiple dimensions of clinical pain were found to show significant decreases and trends towards decreased pain after treatment with mln but not placebo ( bpi sev change ; mln : mean  =  0.88    1.8 , p  =  0.076 ; pbo : mean  =  0.17    2.3 , p  =  0.78 ; bpi int change ; mln : mean  =  1.1    1.7 , p  =  0.03 ; pbo : mean  =  0.56    2.1 , p  =  0.31 ) . when comparing the effects of treatment periods to each other , no significant differences between mln and placebo treatment were observed ( bpi sev : p  =  0.39 , bpi int : p  =  0.50 ) . according to our a priori definition of responders : 5 of 15 patients ( 33% ) were responders to mln and 3 of 15 patients ( 20% ) were responders to placebo treatment for bpi sev , whereas 7 of 15 patients ( 47% ) were responders to mln treatment and 4 of 15 patients ( 27% ) were responders to placebo for bpi int scores ( table 2 ) .
we found strong associations between baseline rs - fc values ( i.e. rs - fc correlation values between our pre - specified antinociceptive brain regions and other brain regions involved in pain processing / modulation ) and changes in clinical and experimental pain measures after treatment periods ( table 3 ) .
a significant association was found between rs - fc of the right pag seed and the right mid - ic , and subsequent reduction in clinical pain severity ( bpi sev ; mln : r  =  0.885 , p  <  0.001 ; placebo : r  =  0.216 , p  =  0.440 ; table 3 and fig .
less rs - fc activity was associated with greater reductions in clinical pain following mln treatment but not placebo .
we also observed a significant association between connectivity of the right dlpfc and the left ipl , and changes in bpi sev during mln but not placebo ( mln : r  =  0.873 , p  <  0.001 ; placebo : r  =  0.030 , p  =  0.917 ; table 3 and fig .
3b ) . with respect to the limbic system , connectivity between the left amygdala and the precuneus / posterior cingulate cortex ( pcc ) was found to have a negative correlation with change in bpi sev in response to mln but not placebo ( mln : r  =  0.916 , p  <  0.001 ; placebo : r  =  0.389 , p  =  0.152 ; table 3 and fig .
low levels of connectivity between the pgacc and the right posterior ic were found to be associated with a greater reduction in clinical pain interference ( bpi int ) following mln but not placebo ( mln : r  =  0.900 , p  <  0.001 ; placebo : r  =  0.082 , p  =  0.771 ; table 3 and fig .
4a ) . for placebo , lower levels of connectivity between the pgacc and the left dlpfc
were associated with greater pain reduction , i.e. patients with less connectivity of these two structures at baseline would showed a greater response in clinical pain interference ( bpi int ) during placebo treatment ( placebo : r  =  0.869 , p  <  0.001 , mln : r  =  0.050 , p  =  0.859 ; table 3 , fig . 5 ) .
when assessing each treatment period separately , it was found that order of treatment ( mln or placebo administered in the first period ) did not impact the results .
two linear regression models predicting changes in bpi sev during the mln period were constructed .
the first model for this measure included pre - mln bpi sev values ( adjusted r square  =  0.34 , p  =  0.014 ) , right pag to right mid - ic connectivity ( adjusted r square  =  0.50 , p  <  0.001 ) , and right dlpfc to left ipl connectivity ( adjusted r square  =  0.10 , p  =  0.001 ) as independent predictors and explained 94% of the variance of post - mln pain severity .
a second model explained 95% of the variance of post - mln bpi sev scores which included pre - mln bpi sev values ( adjusted r square  =  0.34 , p  =  0.014 ) , right pag to right mid - ic connectivity ( adjusted r square  =  0.51 , p  <  0.001 ) , and left amygdala to right pcc / precuneus ( adjusted r square  =  0.10 , p  <  0.001 ) as independent predictors ( table 4 ) .
independent predictors including pre - mln bpi int scores ( adjusted r square  =  0.58 , p  =  0.001 ) and pre - mln connectivity between the pgacc and right posterior ic ( adjusted r square  =  0.32 , p  <  0.001 ) explained 90% of the variance of post - mln pain interference ( table 4 ) .
we investigated rs - fc of cortical and subcortical structures involved in pain modulation to determine parameters that would predict treatment response to treatment with mln in patients with fibromyalgia .
importantly , we find that acc  ic as well as pag  ic connectivity at baseline were predictive of treatment response : patients that displayed lower pgacc  ic connectivity or pag  ic connectivity , respectively , showed greater reductions in clinical pain following mln treatment .
other rs - fc measures were also predictive of clinical response to mln treatment , such as dlpfc  ipl and amygdala  precuneus / pcc connectivity , while less pgacc  dlpfc connectivity was predictive of placebo response .
we hypothesize that a subgroup of fibromyalgia patients with poor connectivity between pro- and antinociceptive brain regions , profits from snri treatment , and this pattern reflects a dysfunctional endogenous antinociceptive system or can be viewed as a biomarker thereof .
a dysfunctional endogenous antinociceptive system has been suggested to be a contributor to the genesis of pain in fm ( lautenbacher and rollman , 1997 ; vierck et al .
there is indeed evidence that cns levels of the two key neurotransmitters within the antinociceptive system , 5-ht and ne , are lowered in fm as indicated by decreased levels of the corresponding metabolites in the cerebrospinal fluid ( russell et al . , 1992 ;
snris are thought to support the antinociceptive system by increasing synaptic 5-ht and ne levels that in turn reduce the nociceptive input to the brain .
snris are well characterized with respect to their molecular mechanisms , targeting presynaptic transporter proteins ( e.g. sert ) , thereby raising synaptic 5-ht and ne levels . however , since 5-ht and ne can bind to different receptor subtypes with different effects and concentrations at different cns sites , the overall effect of 5-ht and ne reuptake inhibition is highly complex . in the context of pain and pain modulation however predominantly spinal mechanisms
are discussed to account for the analgesic effects ( yoshimura and furue , 2006 ; burnham and dickenson , 2013 ) . for 5-ht
this is via binding to 5-ht1a , 5-ht1b and 5-ht1d receptors that lead to pre- and post - synaptic hyperpolarization of pain transmitting neurons .
furthermore 5-ht increases inhibitory transmitter release ( e.g. gaba ) from interneurons via 5-ht3 and possibly 5-ht2 .
ne , on the other hand is thought to act via presynaptic 1 receptors leading to a suppression of glutamate release from both a and c afferent fibers , as well as via 2 receptors increasing inhibitory transmitter release from interneurons . importantly these mechanisms seem to play a beneficial role in a variety of chronic pain conditions , such as diabetic neuropathy ( boyle et al . , 2012 ) , osteoarthritis ( chappell et al . , 2009 ) as well as fibromyalgia ( goldenberg et al . , 2010 ; huser et al
, 2012b ) and are not thought to be specific to one particular pain condition .
interestingly for fibromyalgia a recently published study investigating spinal effects of mln via the assessment of the nociceptive flexion reflex ( r iii ) , came to the conclusion that mln must also have supraspinal effects , based on the observation that there was a dose dependent analgesic effect in the absence of an mln associated modulation of the nociceptive flexion reflex ( matthey et al . , 2013 ) .
the only study to date that we are aware of to directly test whether diminished descending analgesic activity is predictive of treatment response with these classes of drugs is a study by yarnitsky and colleagues showing that those neuropathic pain patients with impaired descending activity at baseline on qst were more likely to respond to duloxetine ( yarnitsky et al . , 2012 ) .
our choice of seed regions for the rs - fc connectivity analyses was based on the current literature highlighting the role of some key structures in pain inhibition and pain modulation , i.e. the pag , the rostral acc , the dlpfc and the amygdala ( petrovic et al .
, 2002 ; wager et al . , 2004 ; bingel et al . , 2007 ) .
our strongest findings , which are also well in line with our a priori hypotheses is , that acc 
ic connectivity and pag  ic connectivity were predictive of treatment response . within the endogenous antinociceptive system the pag plays a central role coordinating via the rostroventromedial medulla ( rvm ) ( moreau and fields , 1986 ; heinricher et al . ,
2009 ) activity in the descending 5-ht and ne pathways that project to the spinal cord to decrease nociceptive routing from the periphery .
the pag itself receives input from both spinal nociceptive neurons a variety of cortical structures .
it has been hypothesized that there are two cortical systems that mediate cortical top down modulation ; one pathway involves descending input from the rostral acc ( to the prefrontal cortex and then to the pag ) ; a second pathway arrives at the pag from the ic via the amygdala ( schweinhardt and bushnell , 2010 ) . that said , it needs to be acknowledged that most evidence stems from animal research ( fields , 2004 ) and that the precise mapping of these pathways in humans is currently unknown .
however , the notion of the rostral acc  pag connectivity ( hardy and leichnetz , 1981 ) , interacting in this regard , has recently been supported by both functional ( petrovic et al . , 2002 ; bingel , 2010 ; kong et al . , 2010 ) and structural ( stein et al . , 2012 ) imaging studies .
the interaction of the rostral acc and ic in antinociception and pain modulation on the other hand remains to be further elucidated .
both structures are part of a cortical opioidergic network ( petrovic et al . , 2002 )
, as such the rostral acc might exert direct  antinociceptive activity  on the ic cortex , or both structures modulate the pag either interactively or independently , and rs - fc reflects coordinated activity of the two systems .
when looking at resting state networks the bilateral ic together with the dorsal acc , mcc and supplementary motor area make up the so called salience network and connectivity within this network influences perceptual decisions on pain .
the vacc , pgacc and sgacc as such are not part of the salience network , but rather belong to the executive control network ( vacc ) and the dmn ( pgacc and sgacc ) ( beckmann et al . ,
as such one would not expect highly correlated bold activity between the rostral acc and ic even in healthy subjects ; intriguingly a negative correlation in resting state activity between these two regions is predictive of mln response . in a recently performed study we found an increased rostral acc  ic rs - fc in young patients with temporomandibular disorder that we interpreted as an early compensatory mechanism in a group of young patients developing a chronic pain condition ( ichesco et al . , 2012 ) .
 effort  , it is tempting to hypothesize that a break down in this mechanism is then associated with further pain chronification , based on an increasingly dysfunctional antinociceptive system .
the other rs - fc patterns that predicted response to mln treatment were the right dlpfc and the left ipl as well as amygdala and the posterior cingulate cortex .
the ipl as well as the posterior cingulate cortex are both part of the dmn .
the role of the dmn in chronic pain is beginning to be investigated , as in our previous work in fibromyalgia suggesting that ic connectivity to the dmn is associated with increased pain ( napadow et al . , 2010 ) .
interestingly in this present work , we show that lowered dmn connectivity to antinociceptive regions , such as the dlpfc , are predictive of response to mln .
other dmn regions such as the pcc have also been highlighted as being involved in pain in other functional and structural imaging studies ( erpelding et al . ,
another interesting finding of our study is that pgacc  left dlpfc connectivity predicted response to placebo treatment .
fibromyalgia patients with a lower pgacc  left dlpfc connectivity showed greater pain reductions while undergoing placebo treatment .
the placebo effect in fibromyalgia patients has recently been investigated in meta - analyses based on randomized , placebo - controlled trials . in these studies , 1830% of patients
have been shown to be placebo responders ; as such the magnitude of placebo responders in drug trials of fibromyalgia is similar to that seen in other chronic pain conditions ( huser et al . , 2011
the neural mechanisms underlying the clinical observation that cognitive factors such as beliefs and expectations can modulate pain perception have been investigated using various brain imaging methods ( petrovic et al . , 2002 ; bingel , 2010 ) , mostly using short term interventions .
the role of the dlpfc cortex in placebo research has been underlined by both fmri studies ( wager et al . , 2004 ;
wager et al . , 2011 ) as well as transcranial magnetic stimulation ( tms ) studies describing a significant impairment of placebo analgesia associated with tms induced functional lesions of the left dlpfc ( krummenacher et al . , 2010 ) .
as such the dlpfc is viewed as key region for initiating placebo related changes in pain perception , the implementation of which then requires the interaction of other cortical and subcortical brain regions , such as the rostral acc and the pag . while increases in the rostral acc activity and acc  pag connectivity during placebo analgesia are most likely to be mediated by opioidergic transmission ( petrovic et al . , 2002 )
dlpfc playing a key role in placebo analgesia , they also extend it to a clinical setting , where baseline rs - fc predicts placebo effects visible 6  weeks after treatment initiation .
despite the significant progress that has been made in understanding the pathophysiology of fibromyalgia , these advances have not yet been translated into pharmacological treatment .
it is generally thought that the underlying pathophysiology of fibromyalgia is heterogeneous leading to a similar phenotype of chronic widespread pain , with only a subgroup of patients with diminished synaptic 5-ht and ne levels responding to drugs that augment this activity . despite our small sample size
the rates of both drug and placebo responders are well in line with larger , randomized studies .
consistent with this idea , the effect of any one of the united states food and drug administration 's approved drugs examined in isolation is modest with a 30% improvement in pain occurring in only 3040% of patients ( huser et al . , 2012b ) , which is also the case in other chronic pain states ; nsaids and opioids for example have modest efficacy in conditions such as osteoarthritis or chronic low back pain ( clauw , 2010 ) .
this stresses the need to develop tools that predict treatment response , in order to then design individually tailored therapies ( woolf , 2010 ) . only very recently have studies indicated that brain imaging might be suited to perform such predictions ( harris et al . , 2013 ;
resting state fmri might be a particularly interesting tool , as it is easy and safe to perform with only little demands on patients ' cognition and cooperation .
first of all our study is based on a rather small study sample , i.e. our findings might not be representative of the larger fibromyalgia population and may only apply to a subgroup of fibromyalgia patients .
also while our drop - out rates , as well as treatment response to mln and placebo , were comparable to those seen in larger randomized , placebo - controlled trials , our results need to be reproduced in a larger study sample .
as rs - fc analyses allow no assumptions on causality , or on the directedness of influence , it is conceivable that connectivity between two regions could be driven by a third region , not identified in the analysis .
furthermore there is no necessity for a direct causal relationship between brain connectivity patterns identified in this study and clinical response .
dual reuptake inhibitors are likely to act first and foremost on the spinal level ( in the context of pain inhibition ) , and the connectivity measures of forebrain structures might thus relate only indirectly to the site of action . in this scenario connectivity measures would need to be viewed as surrogate markers , however this would not detract from their potential clinical usefulness . finally , there are four snris used in clinical practice , including venlafaxine , desvenlafaxine , duloxetine and mln .
mln blocks 5-ht and ne reuptake to an equal extent whereas greater selectivity at 5-ht sites has been described for venlafaxine and duloxetine . in this regard
it is uncertain whether our results can be generalized to other snris or even tcas . besides confirmation by studies with larger sample sizes further research
is needed to extent our findings to other snri and non - selective reuptake inhibitors ( e.g. tcas . ) .
overall we were able to show that rs - fc patterns of brain structures involved in antinociception and pain modulation might be useful parameters for the prediction of treatment response to the snri mln in fibromyalgia patients .
as in clinical practice only a subset of patients respond to pharmacological treatment , such approaches might turn out useful tools to identify subgroups of patients likely to respond to one or the other approach moving towards an individualized medicine .
 fibromyalgia is a chronic pain syndrome characterized by widespread pain , fatigue , and memory and mood disturbances . despite advances in our understanding of the underlying pathophysiology 
 , treatment is often challenging . 
 new research indicates that changes in functional connectivity between brain regions , as can be measured by magnetic resonance imaging ( fcmri ) of the resting state , may underlie the pathogenesis of this and other chronic pain states . as such , this parameter may be able to be used to monitor changes in brain function associated with pharmacological treatment , and might also be able to predict treatment response.we performed a resting state fcmri trial using a randomized , placebo - controlled , cross - over design to investigate mechanisms of action of milnacipran ( mln ) , a selective serotonin and norepinephrine reuptake inhibitor ( snri ) , in fibromyalgia patients . 
 our aim was to identify functional connectivity patterns at baseline that would differentially predict treatment response to mln as compared to placebo . since preclinical studies of mln suggest that this medication works by augmenting antinociceptive processes , we specifically investigated brain regions known to be involved in pain inhibition.15 fibromyalgia patients completed the study , consisting of 6  weeks of drug and placebo intake ( order counterbalanced ) with an interspersed 2  week wash out period . as a main finding 
 we report that reductions in clinical pain scores during mln were associated with decreased functional connectivity between pro - nociceptive regions and antinociceptive pain regions at baseline , specifically between the rostral part of the anterior cingulate cortex ( acc ) and the insular cortex ( ic ) , as well as between the periaqueductal gray ( pag ) and the ic : patients with lower preexisting functional connectivity had the greatest reduction in clinical pain . 
 this pattern was not observed for the placebo period . 
 however a more robust placebo response was associated with lower baseline functional connectivity between the acc and the dorsolateral prefrontal cortex.this study indicates that acc  ic connectivity might play a role in the mechanism of action of mln , and perhaps more importantly fcmri might be a useful tool to predict pharmacological treatment response . 
Introduction
Material and methods
Results
Discussion