Low‐ and high‐frequency transcutaneous electrical acupoint stimulation induces different effects on cerebral μ‐opioid receptor availability in rhesus monkeys

Although systematic studies have demonstrated that acupuncture or electroacupuncture (EA) analgesia is based on their accelerating endogenous opioid release to activate opioid receptors and that EA of different frequencies is mediated by different opioid receptors in specific areas of the central nervous system, there is little direct, real‐time evidence to confirm this in vivo. The present study was designed to investigate the effects of transcutaneous electrical acupoint stimulation (TEAS), an analogue of EA, at low and high frequencies on μ‐opioid receptor (MOR) availability in the brain of rhesus monkeys. Monkeys underwent 95‐min positron emission tomography (PET) with ¹¹C‐carfentanil three times randomly while receiving 0, 2, or 100 Hz TEAS, respectively. Each TEAS was administered in the middle 30 min during the 95‐min PET scan, and each session of PET and TEAS was separated by at least 2 weeks. The results revealed that 2 Hz but not 100 Hz TEAS evoked a significant increase in MOR binding potential in the anterior cingulate cortex, the caudate nucleus, the putamen, the temporal lobe, the somatosensory cortex, and the amygdala compared with 0 Hz TEAS. The effect remained after the end of TEAS in the anterior cingulate cortex and the temporal lobe. The selective increase in MOR availability in multiple brain regions related to pain and sensory processes may play a role in mediating low‐frequency TEAS efficacy. © 2014 Wiley Periodicals, Inc.     

Genetics of adult attachment and the endogenous opioid system

Since the pioneering work by Panksepp et al, the neurobiological bases of attachment behavior have been closely linked with opioid neurotransmission. Candidate gene studies of adult individuals have shown that variation in the mu-opioid receptor gene (OPRM1) influences attachment behavior. Early maternal care and the A/A genotype of the A118G polymorphism interact in modulating levels of fearful attachment. Compared to their counterparts carrying the A/A genotype, individuals expressing the minor 118G allele show lower levels of avoidant attachment and experience more pleasure in social situations. Brain imaging research has strengthened the biological plausibility of candidate gene studies. The avoidance dimension of attachment correlates negatively with mu-opioid receptor availability in the thalamus and anterior cingulate cortex, as well as the frontal cortex, amygdala, and insula. Overall, findings from human studies combined with those from animal models suggest that research on the genetic bases of attachment should include the endogenous opioid system among the investigated variables.     

Attachment avoidance modulates neural response to masked facial emotion

According to recent models of individual differences in attachment organization, a basic dimension of adult attachment is avoidance. Attachment‐related avoidance corresponds to tendencies to withdraw from close relationships and to an unwillingness to rely on others. In the formation of attachment orientation during infancy facial emotional interaction plays a central role. There exists an inborn very rapid decoding capacity for facial emotional expression. In this study, functional magnetic resonance imaging was used to examine differences in automatic brain reactivity to facial emotions as a function of attachment avoidance in a sample of 51 healthy adults. Pictures of sad and happy faces (which are approach‐related interpersonal signals) were presented masked by neutral faces. The Relationship Scales Questionnaire (RSQ) was used to assess the attachment avoidance. Masked sad faces activated the amygdala, the insula, occipito‐temporal areas, and the somatosensory cortices. Independently from trait anxiety, depressivity, and detection performance, attachment avoidance was found to be inversely related to responses of the primary somatosensory cortex (BA 3) to masked sad faces. A low spontaneous responsivity of the primary somatosensory cortex to negative faces could be a correlate of the habitual unwillingness to deal with partners' distress and needs for proximity. The somatosensory cortices are known to be critically involved in the processes of emotional mimicry and simulation which have the potential to increase social affiliation. Our data are consistent with the idea that people who withdraw from close relationships respond spontaneously to a lesser extent to negative interpersonal emotional signals than securely attached individuals. Hum Brain Mapp, 2009. © 2009 Wiley‐Liss, Inc.     

Social Novelty Investigation in the Juvenile Rat: Modulation by the μ‐Opioid System

The drive to approach and explore novel conspecifics is inherent to social animals and may promote optimal social functioning. Juvenile animals seek out interactions with novel peers more frequently and find these interactions to be more rewarding than their adult counterparts. In the present study, we aimed to establish a behavioural paradigm to measure social novelty‐seeking in juvenile rats and to determine the involvement of the opioid, dopamine, oxytocin and vasopressin systems in this behaviour. To this end, we developed the social novelty preference test to assess the preference of a juvenile rat to investigate a novel over a familiar (cage mate) conspecific. We show that across the juvenile period both male and female rats spend more time investigating a novel conspecific than a cage mate, independent of subject sex or repeated exposure to the test. We hypothesised that brain systems subserving social information processing and social motivation/reward (i.e. the opioid, dopamine, oxytocin, vasopressin systems) might support social novelty preference. To test this, receptor antagonists of each of these systems were administered i.c.v. prior to exposure to the social novelty preference test and, subsequently, to the social preference test, to examine the specificity of these effects. We find that μ‐opioid receptor antagonism reduces novel social investigation in both the social novelty preference and social preference tests while leaving the investigation of a cage mate (social novelty preference test) or an object (social preference test) unaffected. In contrast, central blockade of dopamine D₂ receptors (with eticlopride), oxytocin receptors (with des‐Gly‐NH₂,d(CH₂)₅[Tyr(Me)²,Thr₄]OVT) or vasopressin V1a receptors [with (CH₂)₅Tyr(Me²)AVP] failed to alter social novelty preference or social preference. Overall, we have established a new behavioural test to study social novelty‐seeking behaviour in the juvenile rat and show that the μ‐opioid system facilitates this behaviour, possibly by reducing risk avoidance and enhancing the hedonic and/or motivational value of social novelty.     

Aerobic exercise modulates anticipatory reward processing via the μ‐opioid receptor system

Physical exercise modulates food reward and helps control body weight. The endogenous µ‐opioid receptor (MOR) system is involved in rewarding aspects of both food and physical exercise, yet interaction between endogenous opioid release following exercise and anticipatory food reward remains unresolved. Here we tested whether exercise‐induced opioid release correlates with increased anticipatory reward processing in humans. We scanned 24 healthy lean men after rest and after a 1 h session of aerobic exercise with positron emission tomography (PET) using MOR‐selective radioligand [¹¹C]carfentanil. After both PET scans, the subjects underwent a functional magnetic resonance imaging (fMRI) experiment where they viewed pictures of palatable versus nonpalatable foods to trigger anticipatory food reward responses. Exercise‐induced changes in MOR binding in key regions of reward circuit (amygdala, thalamus, ventral and dorsal striatum, and orbitofrontal and cingulate cortices) were used to predict the changes in anticipatory reward responses in fMRI. Exercise‐induced changes in MOR binding correlated negatively with the exercise‐induced changes in neural anticipatory food reward responses in orbitofrontal and cingulate cortices, insula, ventral striatum, amygdala, and thalamus: higher exercise‐induced opioid release predicted higher brain responses to palatable versus nonpalatable foods. We conclude that MOR activation following exercise may contribute to the considerable interindividual variation in food craving and consumption after exercise, which might promote compensatory eating and compromise weight control.     

The effect of increased NaCl intake on rat brain endogenous μ‐opioid receptor signalling

Numerous studies demonstrate the significant role of central β‐endorphin and its receptor, the μ‐opioid receptor (MOR), in sodium intake regulation. The present study aimed to investigate the possible relationship between chronic high‐NaCl intake and brain endogenous MOR functioning. We examined whether short‐term (4 days) obligatory salt intake (2% NaCl solution) in rats induces changes in MOR mRNA expression, G‐protein activity and MOR binding capacity in brain regions involved in salt intake regulation. Plasma osmolality and electrolyte concentrations after sodium overload and the initial and final body weight of the animals were also examined. After 4 days of obligatory hypertonic sodium chloride intake, there was clearly no difference in MOR mRNA expression and G‐protein activity in the median preoptic nucleus (MnPO). In the brainstem, MOR binding capacity also remained unaltered, although the maximal efficacy of MOR G‐protein significantly increased. Finally, no significant alterations were observed in plasma osmolality and electrolyte concentrations. Interestingly, animals that received sodium gained significantly less weight than control animals. In conclusion, we found no significant alterations in the MnPO and brainstem in the number of available cell surface MORs or de novo syntheses of MOR after hypertonic sodium intake. The increased MOR G‐protein activity following acute sodium overconsumption may participate in the maintenance of normal blood pressure levels and/or in enhancing sodium taste aversion and sodium overload‐induced anorexia.     

Identification of a novel “almost neutral” μ‐opioid receptor antagonist in CHO cells expressing the cloned human μ‐opioid receptor

The basal (constitutive) activity of G protein‐coupled receptors allows for the measurement of inverse agonist activity. Some competitive antagonists turn into inverse agonists under conditions where receptors are constitutively active. In contrast, neutral antagonists have no inverse agonist activity, and they block both agonist and inverse agonist activity. The μ‐opioid receptor (MOR) demonstrates detectable constitutive activity only after a state of dependence is produced by chronic treatment with a MOR agonist. We therefore sought to identify novel MOR inverse agonists and novel neutral MOR antagonists in both untreated and agonist‐treated MOR cells. CHO cells expressing the cloned human mu receptor (hMOR‐CHO cells) were incubated for 20 h with medium (control) or 10 μM (2S,4aR,6aR,7R,9S,10aS,10bR)‐9‐(benzoyloxy)‐2‐(3‐furanyl)dodecahydro‐6a,10b‐dimethyl‐4,10‐dioxo‐2H‐naphtho‐[2,1‐c]pyran‐7‐carboxylic acid methyl ester (herkinorin, HERK). HERK treatment generates a high degree of basal signaling and enhances the ability to detect inverse agonists. [³⁵S]‐GTP‐γ‐S assays were conducted using established methods. We screened 21 MOR “antagonists” using membranes prepared from HERK‐treated hMOR‐CHO cells. All antagonists, including CTAP and 6β‐naltrexol, were inverse agonists. However, LTC‐274 ((?)‐3‐cyclopropylmethyl‐2,3,4,4α,5,6,7,7α‐octahydro‐1H‐benzofuro[3,2‐e]isoquinolin‐9‐ol)) showed the lowest efficacy as an inverse agonist, and, at concentrations less than 5 nM, had minimal effects on basal [³⁵S]‐GTP‐γ‐S binding. Other efforts in this study identified KC‐2‐009 ((+)‐3‐((1R,5S)‐2‐((Z)‐3‐phenylallyl)‐2‐azabicyclo[3.3.1]nonan‐5‐yl)phenol hydrochloride) as an inverse agonist at untreated MOR cells. In HERK‐treated cells, KC‐2‐009 had the highest efficacy as an inverse agonist. In summary, we identified a novel and selective MOR inverse agonist (KC‐2‐009) and a novel MOR antagonist (LTC‐274) that shows the least inverse agonist activity among 21 MOR antagonists. LTC‐274 is a promising lead compound for developing a true MOR neutral antagonist. Synapse 64:280–288, 2010. © 2009 Wiley‐Liss, Inc.     

Ultralow concentrations of bupivacaine exert anti‐inflammatory effects on inflammation‐reactive astrocytes

Bupivacaine is a widely used, local anesthetic agent that blocks voltage‐gated Na⁺ channels when used for neuro‐axial blockades. Much lower concentrations of bupivacaine than in normal clinical use, < 10^?8 m , evoked Ca²⁺ transients in astrocytes from rat cerebral cortex, that were inositol trisphosphate receptor‐dependent. We investigated whether bupivacaine exerts an influence on the Ca²⁺ signaling and interleukin‐1β (IL‐1β) secretion in inflammation‐reactive astrocytes when used at ultralow concentrations, < 10^?8 m . Furthermore, we wanted to determine if bupivacaine interacts with the opioid‐, 5‐hydroxytryptamine‐ (5‐HT) and glutamate‐receptor systems. With respect to the μ‐opioid‐ and 5‐HT‐receptor systems, bupivacaine restored the inflammation‐reactive astrocytes to their normal non‐inflammatory levels. With respect to the glutamate‐receptor system, bupivacaine, in combination with an ultralow concentration of the μ‐opioid receptor antagonist naloxone and μ‐opioid receptor agonists, restored the inflammation‐reactive astrocytes to their normal non‐inflammatory levels. Ultralow concentrations of bupivacaine attenuated the inflammation‐induced upregulation of IL‐1β secretion. The results indicate that bupivacaine interacts with the opioid‐, 5‐HT‐ and glutamate‐receptor systems by affecting Ca²⁺ signaling and IL‐1β release in inflammation‐reactive astrocytes. These results suggest that bupivacaine may be used at ultralow concentrations as an anti‐inflammatory drug, either alone or in combination with opioid agonists and ultralow concentrations of an opioid antagonist.     

Effects of (+)‐pentazocine on the antinociceptive effects of (–)‐pentazocine in mice

Previous studies have shown that sigma‐1 receptor chaperone (Sig‐1R) ligands can regulate pain‐related behaviors, and Sig‐1R itself can regulate μ‐opioid receptor functions as well as signal transduction. Even though (±)‐pentazocine has been used clinically for the treatment of pain through opioid receptors, (+)‐pentazocine is known to be a selective Sig‐1R agonist. To the best of our knowledge, there is no information available regarding the involvement of Sig‐1R agonistic action in the antinociceptive effects of (±)‐pentazocine. Therefore, the present study was designed to investigate the effects of (+)‐pentazocine on the antinociceptive effects of (–)‐pentazocine in mice. Both and (–)‐pentazocine induced biphasic antinociceptive effects as measured by the warm‐plate test. The early phase, but not the delayed phase, of the antinociceptive effects induced by (–)‐pentazocine, which are mediated by the activation of μ‐opioid receptors, were suppressed by pretreatment with (+)‐pentazocine. These results suggest that the innate antinociceptive action of (±)‐pentazocine could be marginally reduced by the effects of (+)‐pentazocine, but (+)‐pentazocine can suppress the antinociceptive effects of (–)‐pentazocine at certain time points. Synapse 69:166–171, 2015. © 2015 Wiley Periodicals, Inc.     

Involvement of κ‐opioid receptors in visceral nociception in mice

Abstract It has been shown that the behavioural responses to chemically evoked visceral nociception are increased in transgenic mice lacking the κ‐opioid receptor (KOR). The aim of the present study was to evaluate the contribution of KOR in mechanically evoked visceral pain by performing colorectal distension (CRD) and monitoring the subsequent visceromotor response (VMR) in control mice (KOR^+/+) and in mice lacking KOR (KOR^?/?). Pseudo‐affective visceral pain responses were evoked in conscious mice using increasing (10–80 mmHg) and repeated (12 × 55 mmHg) phasic CRD paradigms. The resulting VMR was determined by monitoring the electromyographic activity of the abdominal muscle. The increasing and repeated CRD paradigms, respectively, evoked similar responses in both KOR^+/+ and KOR^?/? mice. The selective KOR‐agonists U‐69593 (5 and 25 mg kg^?1, s.c.) and asimadoline (25 mg kg^?1, s.c.) significantly decreased the VMR in KOR^+/+ mice, while having no effect in KOR^?/? mice. In contrast, the selective μ‐opioid receptor agonist fentanyl significantly reduced the VMR in both types of mice and appeared more efficacious in KOR^?/? mice. The opioid receptor antagonist naloxone (0.3–30 mg kg^?1 s.c.) did not affect the response to CRD in C57BL/6 mice at any dose tested. In conclusion, the data confirm that the KOR agonists used in this study inhibit the VMR to CRD in mice by acting via KOR receptors. In addition, the data suggest that the endogenous opioid system is not likely to modulate the VMR to mechanically evoked visceral pain in mice.     

Mapping neurotransmitter networks with PET: An example on serotonin and opioid systems

All functions of the human brain are consequences of altered activity of specific neural pathways and neurotransmitter systems. Although the knowledge of “system level” connectivity in the brain is increasing rapidly, we lack “molecular level” information on brain networks and connectivity patterns. We introduce novel voxel‐based positron emission tomography (PET) methods for studying internal neurotransmitter network structure and intercorrelations of different neurotransmitter systems in the human brain. We chose serotonin transporter and μ‐opioid receptor for this analysis because of their functional interaction at the cellular level and similar regional distribution in the brain. Twenty‐one healthy subjects underwent two consecutive PET scans using [¹¹C]MADAM, a serotonin transporter tracer, and [¹¹C]carfentanil, a μ‐opioid receptor tracer. First, voxel‐by‐voxel “intracorrelations” (hub and seed analyses) were used to study the internal structure of opioid and serotonin systems. Second, voxel‐level opioid–serotonin intercorrelations (between neurotransmitters) were computed. Regional μ‐opioid receptor binding potentials were uniformly correlated throughout the brain. However, our analyses revealed nonuniformity in the serotonin transporter intracorrelations and identified a highly connected local network (midbrain–striatum–thalamus–amygdala). Regionally specific intercorrelations between the opioid and serotonin tracers were found in anteromedial thalamus, amygdala, anterior cingulate cortex, dorsolateral prefrontal cortex, and left parietal cortex, i.e., in areas relevant for several neuropsychiatric disorders, especially affective disorders. This methodology enables in vivo mapping of connectivity patterns within and between neurotransmitter systems. Quantification of functional neurotransmitter balances may be a useful approach in etiological studies of neuropsychiatric disorders and also in drug development as a biomarker‐based rationale for targeted modulation of neurotransmitter networks. Hum Brain Mapp 35:1875–1884, 2014. © 2013 Wiley Periodicals, Inc.     

Enhanced dendritic availability of μ‐opioid receptors in inhibitory neurons of the extended amygdala in mice deficient in the corticotropin‐releasing factor‐1 receptor

Activation of the corticotropin‐releasing factor‐1 (CRF‐1) receptor in the anterolateral BNST (BSTal), a key subdivision of the extended amygdala, elicits opiate‐seeking behavior exacerbated by stress. However, it is unknown whether the presence of CRF‐1 affects expression of the μ‐opioid receptor (μ‐OR) in the many GABAergic BSTal neurons implicated in the stress response. We hypothesized that deletion of the CRF‐1 receptor gene would alter the density and/or subcellular distribution of μ‐ORs in GABAergic neurons of the BSTal. We used electron microscopy to quantitatively examine μ‐OR immunogold and γ‐aminobutyric acid (GABA) immunoperoxidase labeling in the BSTal of CRFr‐1 knockout (KO) compared to wild‐type (WT) mice. To assess regional specificity, we examined μ‐OR distribution in dorsal striatum. The μ‐ORs in each region were predominantly localized in dendrites, many of which were GABA‐immunoreactive. Significantly, more cytoplasmic μ‐OR gold particles per dendritic area were observed selectively in GABA‐containing dendrites of the BSTal, but not of the dorsal striatum, in KO compared to WT mice. In both regions, however, significantly fewer GABA‐immunoreactive axon terminals were present in KO compared to WT mice. Our results suggest that the absence of CRF‐1 results in enhanced expression and/or dendritic trafficking of μ‐ORs in inhibitory BSTal neurons. They also suggest that the expression of CRF‐1 is a critical determinant of the availability of GABA in functionally diverse brain regions. These findings underscore the complex interplay between CRF, opioid, and GABA systems in limbic and striatal regions and have implications for the role of CRF‐1 in influencing the pharmacological effects of opiates active at μ‐ORs. Synapse 65:8–20, 2011. © 2010 Wiley‐Liss, Inc.     

Seasonal Variation in the Brain μ-Opioid Receptor Availability

Seasonal rhythms influence mood and sociability. The brain μ-opioid receptor (MOR) system modulates a multitude of seasonally varying socioemotional functions, but its seasonal variation remains elusive with no previously reported in vivo evidence. Here, we first conducted a cross-sectional study with previously acquired human [¹¹C]carfentanil PET imaging data (132 male and 72 female healthy subjects) to test whether there is seasonal variation in MOR availability. We then investigated experimentally whether seasonal variation in daylength causally influences brain MOR availability in rats. Rats (six male and three female rats) underwent daylength cycle simulating seasonal changes; control animals (two male and one female rats) were kept under constant daylength. Animals were scanned repeatedly with [¹¹C]carfentanil PET imaging. Seasonally varying daylength had an inverted U-shaped functional relationship with brain MOR availability in humans. Brain regions sensitive to daylength spanned the socioemotional brain circuits, where MOR availability peaked during spring. In rats, MOR availabilities in the brain neocortex, thalamus, and striatum peaked at intermediate daylength. Varying daylength also affected the weight gain and stress hormone levels. We conclude that cerebral MOR availability in humans and rats shows significant seasonal variation, which is predominately associated with seasonal photoperiodic variation. Given the intimate links between MOR signaling and socioemotional behavior, these results suggest that the MOR system might underlie seasonal variation in human mood and social behavior.SIGNIFICANCE STATEMENT Seasonal rhythms influence emotion and sociability. The central μ-opioid receptor (MOR) system modulates numerous seasonally varying socioemotional functions, but its seasonal variation remains elusive. Here we used positron emission tomography to show that MOR levels in both human and rat brains show daylength-dependent seasonal variation. The highest MOR availability was observed at intermediate daylengths. Given the intimate links between MOR signaling and socioemotional behavior, these results suggest that the MOR system might underlie seasonal variation in human mood and social behavior.     

Neural substrates of approach‐avoidance conflict decision‐making

Animal approach‐avoidance conflict paradigms have been used extensively to operationalize anxiety, quantify the effects of anxiolytic agents, and probe the neural basis of fear and anxiety. Results from human neuroimaging studies support that a frontal–striatal–amygdala neural circuitry is important for approach‐avoidance learning. However, the neural basis of decision‐making is much less clear in this context. Thus, we combined a recently developed human approach‐avoidance paradigm with functional magnetic resonance imaging (fMRI) to identify neural substrates underlying approach‐avoidance conflict decision‐making. Fifteen healthy adults completed the approach‐avoidance conflict (AAC) paradigm during fMRI. Analyses of variance were used to compare conflict to nonconflict (avoid‐threat and approach‐reward) conditions and to compare level of reward points offered during the decision phase. Trial‐by‐trial amplitude modulation analyses were used to delineate brain areas underlying decision‐making in the context of approach/avoidance behavior. Conflict trials as compared to the nonconflict trials elicited greater activation within bilateral anterior cingulate cortex, anterior insula, and caudate, as well as right dorsolateral prefrontal cortex (PFC). Right caudate and lateral PFC activation was modulated by level of reward offered. Individuals who showed greater caudate activation exhibited less approach behavior. On a trial‐by‐trial basis, greater right lateral PFC activation related to less approach behavior. Taken together, results suggest that the degree of activation within prefrontal‐striatal‐insula circuitry determines the degree of approach versus avoidance decision‐making. Moreover, the degree of caudate and lateral PFC activation related to individual differences in approach‐avoidance decision‐making. Therefore, the approach‐avoidance conflict paradigm is ideally suited to probe anxiety‐related processing differences during approach‐avoidance decision‐making. Hum Brain Mapp 36:449–462, 2015. © 2014 Wiley Periodicals, Inc.     

Dopaminergic and GABA‐ergic markers of impulsivity in rats: evidence for anatomical localisation in ventral striatum and prefrontal cortex

Accumulating evidence indicates that impulsivity, in its multiple forms, involves cortical and subcortical mechanisms and abnormal dopamine (DA) transmission. Although decreased DA D2/D3 receptor availability in the nucleus accumbens (NAcb) predicts trait‐like impulsivity in rats it is unclear whether this neurochemical marker extends to both the NAcb core (NAcbC) and shell (NAcbS) and whether markers for other neurotransmitter systems implicated in impulsivity such as serotonin (5‐HT), endogenous opioids and γ‐amino‐butyric acid (GABA) are likewise altered in impulsive rats. We therefore used autoradiography to investigate DA transporter (DAT), 5‐HT transporter (5‐HTT) and D1, D2/D3, μ‐opioid and GABA(A) receptor binding in selected regions of the prefrontal cortex and striatum in rats expressing low and high impulsive behaviour on the five‐choice serial reaction‐time task. High‐impulsive (HI) rats exhibited significantly lower binding for DAT and D2/D3 receptors in the NAcbS and for D1 receptors in the NAcbC compared with low‐impulsive (LI) rats. HI rats also showed significantly lower GABA(A) receptor binding in the anterior cingulate cortex. For all regions where receptor binding was altered in HI rats, binding was inversely correlated with impulsive responding on task. There were no significant differences in binding for 5‐HTT or μ‐opioid receptors in any of the regions investigated. These results indicate that altered D2/D3 receptor binding is localised to the NAcbS of trait‐like impulsive rats and is accompanied by reduced binding for DAT. Alterations in binding for D1 receptors in the NAcbC and GABA(A) receptors in the anterior cingulate cortex demonstrate additional markers and putative mechanisms underlying the expression of behavioural impulsivity.     

Evidence for modulation of opioidergic activity in central vestibular processing: A [18F] diprenorphine PET study

Animal and functional imaging studies had identified cortical structures such as the parieto‐insular vestibular cortex, the retro‐insular cortex, or the anterior cingulate cortex belonging to a vestibular cortical network. Basic animal studies revealed that endorphins might be important transmitters involved in cerebral vestibular processing. The aim of the present study was therefore to analyse whether the opioid system is involved in vestibular neurotransmission of humans or not. Changes in opioid receptor availability during caloric air stimulation of the right ear were studied with [¹⁸F] Fluoroethyl‐diprenorphine ([¹⁸F]FEDPN) PET scans in 10 right‐handed healthy volunteers and compared to a control condition. Decrease in receptor availability to [¹⁸F]FEDPN during vestibular stimulation in comparison to the control condition was significant at the right posterior insular cortex and the postcentral region indicating more endogenous opioidergic binding in these regions during stimulation. These data give evidence that the opioidergic system plays a role in the right hemispheric dominance of the vestibular cortical system in right‐handers. Hum Brain Mapp, 2010. © 2009 Wiley‐Liss, Inc.     

Src family kinases mediate the inhibition of substance P release in the rat spinal cord by μ‐opioid receptors and GABAB receptors,but not α2 adrenergic receptors

GABA_B, μ‐opioid and adrenergic α₂ receptors inhibit substance P release from primary afferent terminals in the dorsal horn. Studies in cell expression systems suggest that μ‐opioid and GABA_B receptors inhibit transmitter release from primary afferents by activating Src family kinases (SFKs), which then phosphorylate and inhibit voltage‐gated calcium channels. This study investigated whether SFKs mediate the inhibition of substance P release by these three receptors. Substance P release was measured as neurokinin 1 receptor (NK1R) internalization in spinal cord slices and in vivo. In slices, NK1R internalization induced by high‐frequency dorsal root stimulation was inhibited by the μ‐opioid agonist DAMGO and the GABA_B agonist baclofen. This inhibition was reversed by the SFK inhibitor PP1. NK1R internalization induced by low‐frequency stimulation was also inhibited by DAMGO, but PP1 did not reverse this effect. In vivo, NK1R internalization induced by noxious mechanical stimulation of the hind paw was inhibited by intrathecal DAMGO and baclofen. This inhibition was reversed by intrathecal PP1, but not by the inactive PP1 analog PP3. PP1 produced no effect by itself. The α₂ adrenergic agonists medetomidine and guanfacine produced a small but statistically significant inhibition of NK1R internalization induced by low‐frequency dorsal root stimulation. PP1 did not reverse the inhibition by guanfacine. These results show that SFKs mediate the inhibition of substance P release by μ‐opioid and GABA_B receptors, but not by α₂ receptors, which is probably mediated by the binding of G protein βγ subunits to calcium channels.     

Effects of defeat stress on behavioral flexibility in males and females: modulation by the mu‐opioid receptor

Behavioral flexibility is a component of executive functioning that allows individuals to adapt to changing environmental conditions. Independent lines of research indicate that the mu opioid receptor (MOR) is an important mediator of behavioral flexibility and responses to psychosocial stress. The current study bridges these two lines of research and tests the extent to which social defeat and MOR affect behavioral flexibility and whether sex moderates these effects in California mice (Peromyscus californicus). Males and females assigned to social defeat or control conditions were tested in a Barnes maze. In males, defeat impaired behavioral flexibility but not acquisition. Female performance was unaffected by defeat. MOR binding in defeated and control mice in the orbitofrontal cortex (OFC), striatum and hippocampus was examined via autoradiography. Stressed males had reduced MOR binding in the OFC whereas females were unaffected. The MOR antagonist beta‐funaltrexamine (1 mg/kg) impaired performance in males naïve to defeat during the reversal phase but had no effect on females. Finally, we examined the effects of the MOR agonist morphine (2.5 and 5 mg/kg) on stressed mice. As expected, morphine improved behavioral flexibility in stressed males. The stress‐induced deficits in behavioral flexibility in males are consistent with a proactive coping strategy, including previous observations that stressed male California mice exhibit strong social approach and aggression. Our pharmacological data suggest that a down‐regulation of MOR signaling in males may contribute to sex differences in behavioral flexibility following stress. This is discussed in the framework of coping strategies for individuals with mood disorders.     

Attachment style,affective loss and gray matter volume: A voxel‐based morphometry study

Early patterns of infant attachment have been shown to be an important influence on adult social behavior. Animal studies suggest that patterns of early attachment influence brain development, contributing to permanent alterations in neural structure; however, there are no previous studies investigating whether differences in attachment style are associated with differences in brain structure in humans. In this study, we used Magnetic Resonance Imaging (MRI) and voxel‐based morphometry (VBM) to examine for the first time the association between attachment style, affective loss (for example, death of a loved one) and gray matter volume in a healthy sample of adults (n = 32). Attachment style was assessed on two dimensions (anxious and avoidant) using the ECR‐Revised questionnaire. High attachment‐related anxiety was associated with decreased gray matter in the anterior temporal pole and increased gray matter in the left lateral orbital gyrus. A greater number of affective losses was associated with increased gray matter volume in the cerebellum; in this region, however, the impact of affective losses was significantly moderated by the level of attachment‐related avoidance. These findings indicate that differences in attachment style are associated with differences in the neural structure of regions implicated in emotion regulation. It is hypothesized that early attachment experience may contribute to structural brain differences associated with attachment style in adulthood; furthermore, these findings point to a neuronal mechanism through which attachment style may mediate individual differences in responses to affective loss. Hum Brain Mapp, 2010. © 2010 Wiley‐Liss, Inc.     

Do neural responses to rejection depend on attachment style? An fMRI study

Social bonds fulfill the basic human need to belong. Being rejected thwarts this basic need, putting bonds with others at risk. Attachment theory suggests that people satisfy their need to belong through different means. Whereas anxious attachment is associated with craving acceptance and showing vigilance to cues that signal possible rejection, avoidant attachment is associated with discomfort with closeness and using avoidant strategies to regulate one's relationships. Given these different styles by which people satisfy their need to belong (that can operate simultaneously within the same individual), responses to social rejection may differ according to these individual differences in attachment anxiety and avoidance. To test this hypothesis, we used neuroimaging techniques to examine how the degree to which people display each of the two attachment dimensions (anxiety and avoidance) uniquely correlated with their neural activity during a simulated experience of social exclusion. Anxious attachment related to heightened activity in the dorsal anterior cingulate cortex (dACC) and anterior insula, regions previously associated with rejection-related distress. In contrast, avoidant attachment related to less activity in these regions. Findings are discussed in terms of the strategies that individuals with varying attachment styles might use to promote maintenance of social bonds.