Stress-induced Analgesia Research Articles

Hypothalamic Pomc expression restricted to GABAergic neurons suppresses Npy overexpression and restores food intake in obese mice

ObjectiveHypothalamic arcuate proopiomelanocortin (Arc-POMC) neurons are involved in different physiological processes such as the regulation of energy balance, glucose homeostasis, and stress-induced analgesia. Since these neurons heterogeneously express different biological markers and project to many hypothalamic and extrahypothalamic areas, it is proposed that Arc-POMC neurons could be classified into different subpopulations having diverse physiological roles. The aim of the present study was to characterize the contribution of the subpopulation of Arc-POMC neurons cosecreting gamma-aminobutyric acid (GABA) neurotransmitter in the control of energy balance. MethodsArc-Pomc expression restricted to GABAergic-POMC neurons was achieved by crossing a reversible Pomc-deficient mouse line (arcPomc−) with a tamoxifen-inducible Gad2-CreER transgenic line. Pomc expression was rescued in the compound arcPomc−/−:Gad2-CreER female and male mice by tamoxifen treatment at postnatal days 25 (P25) or 60 (P60), and body weight, daily food intake, fasting glycemia, and fasting-induced hyperphagia were measured. POMC recovery was quantified by immunohistochemistry and semiquantitative RT-PCR. Neuropeptide Y (NPY) and GABAergic neurons were identified by in situ hybridization. Arc-POMC neurons projecting to the dorsomedial hypothalamic nucleus (DMH) were studied by stereotactic intracerebral injection of fluorescent retrobeads into the DMH. ResultsTamoxifen treatment of arcPomc−/−:Gad2-CreER mice at P60 resulted in Pomc expression in ∼23–25% of Arc-POMC neurons and ∼15–23% of Pomc mRNA levels, compared to Gad2-CreER control mice. Pomc rescue in GABAergic-POMC neurons at P60 normalized food intake, glycemia, and fasting-induced hyperphagia, while significantly reducing body weight. Energy balance was also improved in arcPomc−/−:Gad2-CreER mice treated with tamoxifen at P25. Distribution analysis of rescued POMC immunoreactive fibers revealed that the DMH is a major target site of GABAergic-POMC neurons. Further, the expression of the orexigenic neuropeptide Y (NPY) in the DMH was increased in arcPomc−/− obese mice but was completely restored after Pomc rescue in arcPomc−/−:Gad2-CreER mice. Finally, we found that ∼75% of Arc-POMC neurons projecting to the DMH are GABAergic. ConclusionsIn the present study, we show that the expression of Pomc in the subpopulation of Arc-GABAergic-POMC neurons is sufficient to maintain normal food intake. In addition, we found that DMH-NPY expression is negatively correlated with Pomc expression in GABAergic-POMC neurons, suggesting that food intake may be regulated by an Arc-GABAergic-POMC → DMH-NPY pathway.

Pain and stress: functional evidence that supra-spinal mechanisms involved in pain-induced analgesia mediate stress-induced analgesia.

Analgesia induced by stressful and painful stimuli is an adaptive response during life-threatening situations. There is no evidence linking the mechanisms underlying them, while the former depends on the activation of stress-related brain pathways, the second depends on opioidergic mechanisms in the nucleus accumbens and on nicotinic cholinergic mechanisms in the rostral ventromedial medulla. In this study, we hypothesized that stress-induced analgesia is also dependent on opioidergic mechanisms in the nucleus accumbens and on nicotinic cholinergic mechanisms in the rostral ventromedial medulla. We used immobilization, a classical procedure to induce acute stress, and evaluated its ability to decrease the nociceptive responses induced either by carrageenan or by formalin in rats. Immobilization stress significantly decreased either carrageenan-induced hyperalgesia or formalin-induced tonic nociception in a time-dependent manner. This stress-induced analgesia is similar to pain-induced analgesia, as revealed by contrasting the antinociceptive effect induced by immobilization and by a forepaw injection of capsaicin. The administration of a µ-opioid receptor antagonist (CTOP, 0.5 µg) into the nucleus accumbens, as well as that of a nicotinic cholinergic receptor antagonist (mecamylamine, 0.6 µg) into the rostral ventromedial medulla, blocked immobilization stress-induced analgesia in both pain models. These results demonstrate that supraspinal mechanisms which are known to mediate pain-induced analgesia also mediate stress-induced analgesia. Therefore both forms of analgesia have overlapping mechanisms, probably recruited in response to the perception of danger.

Attenuation of fear-conditioned analgesia in rats by monoacylglycerol lipase inhibition in the anterior cingulate cortex: Potential role for CB2 receptors.

Improved understanding of brain mechanisms regulating endogenous analgesia is important from a fundamental physiological perspective and for identification of novel therapeutic strategies for pain. The endocannabinoid system plays a key role in stress-induced analgesia, including fear-conditioned analgesia (FCA), a potent form of endogenous analgesia. Here, we studied the role of the endocannabinoid 2-arachidonoyl glycerol (2-AG) within the anterior cingulate cortex (ACC; a brain region implicated in the affective component of pain) in FCA in rats. FCA was modelled in male Lister-hooded rats by assessing formalin-evoked nociceptive behaviour in an arena previously paired with footshock. The effects of intra-ACC administration of MJN110 (inhibitor of monoacylglycerol lipase [MGL], the primary enzyme catabolizing 2-AG), AM630 (CB2 receptor antagonist), AM251 (CB1 receptor antagonist) or MJN110 + AM630 on FCA were assessed. MJN110 attenuated FCA when microinjected into the ACC, an effect associated with increased levels of 2-AG in the ACC. This effect of MJN110 on FCA was unaltered by co-administration of AM251 but was blocked by AM630, which alone reduced nociceptive behaviour in non-fear-conditioned rats. RT-qPCR confirmed that mRNA encoding CB1 and CB2 receptors was detectable in the ACC of formalin-injected rats and unchanged in those expressing FCA. These results suggest that an MGL substrate in the ACC, likely 2-AG, modulates FCA and that within the ACC, 2-AG-CB2 receptor signalling may suppress this form of endogenous analgesia. These results may facilitate increased understanding and improved treatment of pain- and fear-related disorders and their co-morbidity.

Neuropeptide S-initiated sequential cascade mediated by OX1, NK1, mGlu5 and CB1 receptors: a pivotal role in stress-induced analgesia

BackgroundStress-induced analgesia (SIA) is an evolutionarily conserved phenomenon during stress. Neuropeptide S (NPS), orexins, substance P, glutamate and endocannabinoids are known to be involved in stress and/or SIA, however their causal links remain unclear. Here, we reveal an unprecedented sequential cascade involving these mediators in the lateral hypothalamus (LH) and ventrolateral periaqueductal gray (vlPAG) using a restraint stress-induced SIA model.MethodsMale C57BL/6 mice of 8–12 week-old were subjected to intra-cerebroventricular (i.c.v.) and/or intra-vlPAG (i.pag.) microinjection of NPS, orexin-A or substance P alone or in combination with selective antagonists of NPS receptors (NPSRs), OX1 receptors (OX1Rs), NK1 receptors (NK1Rs), mGlu5 receptors (mGlu5Rs) and CB1 receptors (CB1Rs), respectively. Antinociceptive effects of these mediators were evaluated via the hot-plate test. SIA in mice was induced by a 30-min restraint stress. NPS levels in the LH and substance P levels in vlPAG homogenates were compared in restrained and unrestrained mice.ResultsNPS (i.c.v., but not i.pag.) induced antinociception. This effect was prevented by i.c.v. blockade of NPSRs. Substance P (i.pag.) and orexin-A (i.pag.) also induced antinociception. Substance P (i.pag.)-induced antinociception was prevented by i.pag. Blockade of NK1Rs, mGlu5Rs or CB1Rs. Orexin-A (i.pag.)-induced antinociception has been shown previously to be prevented by i.pag. blockade of OX1Rs or CB1Rs, and here was prevented by NK1R or mGlu5R antagonist (i.pag.). NPS (i.c.v.)-induced antinociception was prevented by i.pag. blockade of OX1Rs, NK1Rs, mGlu5Rs or CB1Rs. SIA has been previously shown to be prevented by i.pag. blockade of OX1Rs or CB1Rs. Here, we found that SIA was also prevented by i.c.v. blockade of NPSRs or i.pag. blockade of NK1Rs or mGlu5Rs. Restrained mice had higher levels of NPS in the LH and substance P in the vlPAG than unrestrained mice.ConclusionsThese results suggest that, during stress, NPS is released and activates LH orexin neurons via NPSRs, releasing orexins in the vlPAG. Orexins then activate OX1Rs on substance P-containing neurons in the vlPAG to release substance P that subsequently. Activates NK1Rs on glutamatergic neurons to release glutamate. Glutamate then activates perisynaptic mGlu5Rs to initiate the endocannabinoid retrograde inhibition of GABAergic transmission in the vlPAG, leading to analgesia.

Стресс-вызванная анальгезия: роль гормонов гипоталамо-гипофизарно-адренокортикальной системы

One of adaptive consequences of the stress response is a short-term decrease in pain sensitivity in humans and animals known as “stress-induced analgesia” (SIA). The HPA axis is a key system that activates during stress and provides organism adaptation. One of the mechanisms of SIA may be associated with the HPA hormones, in particular with corticotropin-releasing factor (CRF), the major hormone of the HPA axis, and glucocorticoids, whose analgesic properties are well known and used in medical practice. Based on the results of our research and literature review, the aim of the paper was to analyze SIA mechanisms related to CRF and glucocorticoid hormones mediated by CRF receptors 1 and 2 as well as glucocorticoid and opioid receptors.

Analgesia mediated by stress: Possible antalgic mechanism of the dry needling technique

Abstract Introduction and Aim One of the possible mechanisms of action of dry needling is stress-induced analgesia, produced by the activation of the sympathetic nervous system (SNS). Cortisol is a steroid hormone, which is considered to be the most representative indicator of physiological states that are altered after a stressful stimulus. Although studies of other invasive techniques exist which measure the cortisol levels, no studies on dry needling have been found which gather this stress indicator. Thus, the aim of this study was to determine the changes produced in the levels of cortisol concentration in saliva after the application of the dry needling technique. Material and Methods Sixty-two healthy volunteer subjects participated in this randomized controlled single blinded trial performed at the University of Alcalá. Participants were randomly distributed into two intervention groups: deep dry needling (n = 33) and placebo needling (control) (n = 32). The saliva samples were gathered prior to the intervention and 3 minutes after finalization of the same. The level of cortisol in saliva was determined in a laboratory by performing a solid phase enzyme immunoassay using the ELISA® kit. Statistical significance was set at p < 0.05. The differences between both groups were compared based on the percentage of change between the initial level and at three minutes via the Student's t-test for independent samples, estimating the effect size via the Cohen's d test. Results An increase of the levels of cortisol in saliva of 11.27% ± 4.76 was observed after the deep dry needling; in the group of placebo needling, an increase of 1.51% ± 3.08 was observed. However, when comparing both groups, no statistically significant differences were obtained (p = 0.920), with an effect size of d = 0.42. Conclusions The dry needling technique leads to an increase in the release of cortisol, one of the hormones involved in the sympathetic-excitatory relationship. This could be related to one of the possible effects of this invasive procedure with mechanisms of stress-induced analgesia. However, further studies are required to deepen our knowledge on the plausible physiological effects which support the application of the dry needling technique.

A Review of Pain Assessment Methods in Laboratory Rodents.

Ensuring that laboratory rodent pain is well managed underpins the ethical acceptability of working with these animals in research. Appropriate treatment of pain in laboratory rodents requires accurate assessments of the presence or absence of pain to the extent possible. This can be challenging some situations because laboratory rodents are prey species that may show subtle signs of pain. Although a number of standard algesiometry assays have been used to assess evoked pain responses in rodents for many decades, these methods likely represent an oversimplification of pain assessment and many require animal handling during testing, which can result in stress-induced analgesia. More recent pain assessment methods, such as the use of ethograms, facial grimace scoring, burrowing, and nest-building, focus on evaluating changes in spontaneous behaviors or activities of rodents in their home environments. Many of these assessment methods are time-consuming to conduct. While many of these newer tests show promise for providing a more accurate assessment of pain, most require more study to determine their reliability and sensitivity across a broad range of experimental conditions, as well as between species and strains of animals. Regular observation of laboratory rodents before and after painful procedures with consistent use of 2 or more assessment methods is likely to improve pain detection and lead to improved treatment and care-a primary goal for improving overall animal welfare.

Endogenous cannabinoid modulation of restraint stress-induced analgesia in thermal nociception.

It is thought that endogenous cannabinoids have a role in the analgesia induced by specific forms of stress. We examined if the role of endogenous cannabinoids is also dependent upon the mode of nociception, and whether this could be altered by drugs which block their enzymatic degradation. In C57BL/6 mice, restraint stress produced analgesia in the hot-plate and plantar tests, two thermal pain assays that engage distinct supraspinal and spinal nociceptive pathways. Stress-induced analgesia in the hot-plate test was abolished by pre-treatment with the opioid receptor antagonist naltrexone but was unaffected by the cannabinoid receptor antagonist 1-(2,4-Dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-4-morpholinyl-1H-pyrazole-3-carboxamide(AM281). By contrast, stress-induced analgesia in the plantar test was abolished by pre-treatment with naltrexone plusAM281, but not by either antagonist individually. Remarkably, inhibiting the breakdown of endocannabinoids, with the dual fatty acid amide hydrolase and monoacylglycerol lipase inhibitor JZL195, rescued stress-induced analgesia in the hotplate test when endogenous opioid signalling was blocked by naltrexone. Furthermore, JZL195 recruited analgesia induced by sub-threshold restraint stress in both thermal pain assays. These findings indicate the role of endocannabinoids in stress-induced analgesia differs with the type of thermal pain behaviour. However, by inhibiting their breakdown, endocannabinoids can be recruited to substitute for endogenous opioid signalling when their activity is blocked, indicating a degree of redundancy between opioid and cannabinoid systems. Together these data suggest targeting endocannabinoid breakdown could provide an alternative, or adjuvant to mainstream analgesics such as opioids.

Orexin-initiated endocannabinoid signaling in pain and reward: Stress-induced analgesia, stress-induced cocaine relapse & acupuncture analgesia

Orexin-initiated endocannabinoid signaling in pain and reward: Stress-induced analgesia, stress-induced cocaine relapse & acupuncture analgesia

Divergent Response to Cannabinoid Receptor Stimulation in High and Low Stress-Induced Analgesia Mouse Lines Is Associated with Differential G-Protein Activation

Bidirectional selection of mice for high (HA) and low (LA) swim stress-induced analgesia (SSIA) is associated with a divergent response to opioids. In the current study, we investigated whether the genetic divergence in opioid system activity between HA and LA mice also affects cannabinoid sensitivity. Additionally, we also investigated whether the endocannabinoid system mediates SSIA in these lines. Numerous reports support the existence of pharmacological and molecular interactions between the opioid and cannabinoid systems along the pain pathways, as both systems utilize the same G-protein subtype for signal transduction. Mice from both lines were treated with a non-selective CB1/CB2 agonist, WIN55,212–2 and their behavior was evaluated according to the tetrad paradigm assessing antinociception, catalepsy, hypothermia and locomotor activity. Surprisingly, the engagement of CB1 receptors in SSIA was not confirmed. G-protein activation was studied in different brain regions and the spinal cord in the [35S]GTPγS assay. It was shown that WIN55,212–2 produced more potent antinociception in HA than in LA mice. Also, HA mice displayed stronger cannabinoid-induced catalepsy in the bar test. However, LA mice were more sensitive to the hypothermic effect of WIN55,212–2. The intensity of behavioral responses to WIN55,212–2 was correlated with increased G-protein activation in the periaqueductal gray matter, frontal cortex, striatum and thalamus in HA mice. A weak response to WIN55,212–2 in LA mice could depend on impaired CB2 receptor signaling. In conclusion, differences in both opioid and cannabinoid sensitivity between HA and LA mice could stem from alterations in intracellular second messenger mechanisms involving G-protein activation.

POSSIBLE MECHANISMS OF THYMUS PEPTIDES ANALGESIC ENDPOINT

Thymus polypeptides participate in stress-limiting system work. Considering that electropain stimulation is the main stress factor in many behavioral models, it is logical to assume that thymus polypeptides have analgesic activity. The objective of this research is study of opioid and serotoninergic systems role of thymus polypeptides analgesic endpoints implementation. The rats presented with tactivin and thymulin thymus peptides analgesic activity during tail withdrawal test, which is totally blocked by naloxone, and thus is based on opioid system activation. This effect is associated with appeared serotoninergic component of stress-induced analgesia. The possible mechanism of analgesic action and the role of serotoninergic system of thymus peptides were offered based on the results of the research conducted.

Abstract # 3146 Increased pain sensitivity and decreased opioid analgesia in t cell-deficient mice and implications for sex differences

The processing of pain in the central nervous system is now known to have an important immune component, including T cells. T cells have been shown to release endogenous opioids, and although it is well known that opioids have effects on T cell populations, very little attention has been given to the converse: how T cells may affect opioid regulation. We find here that in addition to displaying significantly increased baseline pain sensitivity across various pain modalities, T cell deficient mice exhibit pronounced deficiencies in morphine inhibition of thermal or inflammatory pain. Nude mice are also deficient in endogenous opioid-mediated analgesia, exhibiting no stress-induced analgesia from restraint. The relevant T cell subpopulation appears to be CD4 + T cells, since adoptive transfer of them but not CD8 + cells into nude mice rescues both the pain and morphine analgesia phenotypes. As previously reported, we also observe a sex difference in CD-1 mice, with females requiring 2–3-fold more morphine than males to produce equal analgesia. Nude mice display no sex differences in morphine analgesia, and the sex difference is restored in nude mice of either sex receiving CD4 + T cells from CD-1 donor male or female mice. These results suggest that CD4 + T cells play an as yet unappreciated role in opioid analgesia, and may be a driver of sex differences therein.

PC-FACS

PC-FACS

High activity of endogenous opioid system protects against gastric damage development in mouse models of gastric mucosal injury.

Gastric mucosal injury appears when acid and pepsin production, simultaneously with inadequate mucosal response, overwhelms protective mechanism in stomach. Here we aimed to explore the linkage between gastric lesion formation and endogenous opioid system activity. Two mouse lines bidirectionally selected for high (HA) and low (LA) swim stress-induced analgesia associated with high and low endogenous opioid system activity were used. Gastric mucosal injury was induced by ethanol (EtOH) and chronic mild stress. To investigate the anti-inflammatory effect of the endogenous opioid system macroscopic score, myeloperoxidase (MPO) activity, the expression of inflammatory molecules as well as oxidative stress markers were determined. Moreover, expression of opioid receptors μ (MOR), κ (KOR) and δ (DOR) at mRNA levels were determined in gastric tissue. High activity of the endogenous opioid system alleviated gastric lesions development in the EtOH-and chronic mild stress-induced mouse gastric mucosal injury models, as demonstrated by decreased macroscopic score in HA line compared to LA. Additionally, antioxidative stress defence mechanisms were positively modulated in both models of gastric mucosal injury. MOR and partially KOR receptors may be responsible for the gastroprotective effect. To our knowledge this is the first study to show that increased activity of the endogenous opioid system prevents from gastric lesion formation by influencing - among others - the anti-inflammatory and anti-oxidant mechanisms in the mice stomach. Hence, we suggest that opioids may play an important role in gastric mucosal injury prevention.

Ethanol consumption following mild traumatic brain injury is related to blood-brain barrier permeability.

Several preclinical and clinical studies that deal with the neuropathological consequences of mild traumatic brain injury (mTBI) have focused on unraveling its effect on ethanol drinking behavior. Previous reports describe changes in ethanol consumption, both in animal models of mTBI as well as in patients, after concussive brain injury. However, the neurobiological mechanisms underlying this phenomenon are still poorly understood. In the present study, we used a unique model of mouse lines divergently selected for high (HA) or low (LA) swim stress-induced analgesia to examine the effect of mTBI on ethanol drinking behavior. In comparison with LA mice, their HA counterparts exhibited increased blood-brain barrier (BBB) permeability, lower basal alcohol preference, and lower level of stress-induced ethanol intake. Here, we showed that mTBI attenuates voluntary ethanol intake in LA, but not in HA mice. Interestingly, BBB disruption after mannitol infusion also decreases the level of ethanol drinking behavior in this line. We conclude that in alcohol-preferring LA mice, BBB disruption as a consequence of mTBI attenuates ethanol consumption. Our results suggest that the innate level of BBB integrity plays a pivotal role in regulation of ethanol consumption in mice showing differential endogenous opioid system activity.

Increased pain sensitivity and decreased opioid analgesia in T-cell-deficient mice and implications for sex differences.

The processing of pain in the central nervous system is now known to have an important immune component, including T cells of the adaptive immune system. T cells have been shown to release endogenous opioids, and although it is well known that opioids have effects on T-cell populations, very little attention has been given to the converse: how T cells may affect opioid regulation. We find here that, in addition to displaying significantly increased baseline pain sensitivity across various pain modalities, T-cell-deficient mice (CD-1 nude, Rag1 null mutant, and Cd4 null mutant) exhibit pronounced deficiencies in morphine inhibition of thermal or inflammatory pain. Nude mice are also deficient in endogenous opioid-mediated analgesia, exhibiting no stress-induced analgesia from restraint. The relevant T-cell subpopulation seems to be CD4 T cells because adoptive transfer of them but not CD8 cells into nude mice rescues both the pain and morphine analgesia phenotypes. As previously reported, we also observe a sex difference in CD-1 mice, with females requiring 2- to 3-fold more morphine than males to produce equal analgesia. Nude mice display no sex differences in morphine analgesia, and the sex difference is restored in nude mice of either sex receiving CD4 T cells from CD-1 donor male or female mice. These results suggest that CD4 T cells play an as yet unappreciated role in opioid analgesia and may be a driver of sex differences therein.

Bidirectional selection for high and low stress-induced analgesia affects G-protein activity

Mice selected for high (HA) and low (LA) swim stress-induced analgesia (SSIA) are a unique model for studying the genetic background of this phenomenon. HA and LA miceshow substantial differences in the magnitude of the antinociceptive response to stress and when treated with exogenous opioids. However, the direct cause underplaying this distinctive feature has not yet been identified. The current study was designed to investigate the possibility that disturbances in G-protein signaling could explain the divergent response to opioid agonists. Supraspinal and spinal opioid sensitivity was assessed in vivo with intraperitoneal morphine and subsequent thermal stimulus exposure. The level of opioid receptor-mediated G-protein activation was investigated by means of DAMGO and morphine-stimulated [35S]GTPγS assay in the brain and spinal cord homogenates from HA and LA mice. Morphine (3–249 μmol/kg, i.p) was over 6 - and 3 - times more potent in HA than LA mice in the hot plate and tail-flick assays, respectively. Additionally, HA mice showed elevated β - endorphin levels in the brain. Enhanced efficacy of agonist-stimulated [35S]GTPγS binding was detected in opioid receptor-rich limbic regions of HA mice like the hypothalamus and hippocampus. Increased G-protein activity also emerged in the thalamus, periaqueductal gray matter and prefrontal cortex. In conclusion, the magnitude of the antinociceptive response to opioids in HA and LA mice is correlated with alterations in G-protein activation in brain regions responsible for integration and descending modulation of nociceptive information as well as at sites governing the emotional response to stressful stimuli.

Brain corticotropin-releasing factor signaling: Involvement in acute stress-induced visceral analgesia in male rats.

Water avoidance stress (WAS) induces a naloxone-independent visceral analgesia in male rats under non-invasive conditions of monitoring. The objective of the study was to examine the role of brain CRF signaling in acute stress-induced visceral analgesia (SIVA). Adult male Sprague-Dawley rats were chronically implanted with an intracerebroventricular (ICV) cannula. The visceromotor response (VMR) to graded phasic colorectal distension (CRD: 10, 20, 40, 60mmHg, 20seconds, 4minutes intervals) was monitored using manometry. The VMR to a first CRD (baseline) was recorded 5minutes after an ICV saline injection, followed 1hour later by ICV injection of either CRF (30, 100, or 300ng and 1, 3, or 5μg/rat) or saline and a second CRD, 5minutes later. Receptor antagonists against CRF1 /CRF2 (astressin-B, 30μg/rat), CRF2 (astressin2 -B, 10μg/rat), oxytocin (tocinoic acid, 20μg/rat), or vehicle were injected ICV 5minutes before CRF (300ng/rat, ICV) or 15minutes before WAS (1hour). ICV CRF (100 and 300ng) reduced the VMR to CRD at 60mmHg by -36.6%±6.8% and -48.7%±11.7%, respectively, vs baseline (P<0.001), while other doses had no effect and IP CRF (10µg/kg) induced visceral hyperalgesia. Astressin-B and tocinoic acid injected ICV induced hyperalgesia and prevented the analgesic effect of ICV CRF (300ng/rat) and WAS, while astressin2 -B only blocked WAS-induced SIVA. These data support a role for brain CRF signaling via CRF2 in SIVA in a model of WAS and CRD likely mediated by the activation of brain oxytocin pathway.

Yin-and-yang bifurcation of opioidergic circuits for descending analgesia at the midbrain of the mouse

In the descending analgesia pathway, opioids are known to disinhibit the projections from the periaqueductal gray (PAG) to the rostral ventromedial medulla (RVM), leading to suppression of pain signals at the spinal cord level. The locus coeruleus (LC) has been proposed to engage in the descending pathway through noradrenergic inputs to the spinal cord. Nevertheless, how the LC is integrated in the descending analgesia circuit has remained unknown. Here, we show that the opioidergic analgesia pathway is bifurcated in structure and function at the PAG. A knockout as well as a PAG-specific knockdown of phospholipase C β4 (PLCβ4), a signaling molecule for G protein-coupled receptors, enhanced swim stress-induced and morphine-induced analgesia in mice. Immunostaining after simultaneous retrograde labeling from the RVM and the LC revealed two mutually exclusive neuronal populations at the PAG, each projecting either to the LC or the RVM, with PLCβ4 expression only in the PAG-LC projecting cells that provide a direct synaptic input to LC-spinal cord (SC) projection neurons. The PAG-LC projection neurons in wild-type mice turned quiescent in response to opiates, but remained active in the PLCβ4 mutant, suggesting a possibility that an increased adrenergic function induced by the persistent PAG-LC activity underlies the enhanced opioid analgesia in the mutant. Indeed, the enhanced analgesia in the mutant was reversed by blocking α2-noradrenergic receptors. These findings indicate that opioids suppress descending analgesia through the PAG-LC pathway, while enhancing it through the PAG-RVM pathway, i.e., two distinct pathways with opposing effects on opioid analgesia. These results point to a therapeutic target in pain control.

Pharmacological rescue of nociceptive hypersensitivity and oxytocin analgesia impairment in a rat model of neonatal maternal separation.

Oxytocin (OT), known for its neurohormonal effects around birth, has recently been suggested for being a critical determinant in neurodevelopmental disorders. This hypothalamic neuropeptide exerts a potent analgesic effect through an action on the nociceptive system. This endogenous control of pain has an important adaptive value but might be altered by early life stress, possibly contributing to its long-term consequences on pain responses and associated comorbidities. We tested this hypothesis using a rat model of neonatal maternal separation (NMS) known to induce long-term consequences on several brain functions including chronic stress, anxiety, altered social behavior, and visceral hypersensitivity. We found that adult rats with a history of NMS were hypersensitive to noxious mechanical/thermal hot stimuli and to inflammatory pain. We failed to observe OT receptor-mediated stress-induced analgesia and OT antihyperalgesia after carrageenan inflammation. These alterations were partially rescued if NMS pups were treated by intraperitoneal daily injection during NMS with OT or its downstream second messenger allopregnanolone. The involvement of epigenetic changes in these alterations was confirmed since neonatal treatment with the histone deacetylase inhibitor SAHA, not only normalized nociceptive sensitivities but also restored OT receptor-mediated stress-induced analgesia and the endogenous antihyperalgesia in inflamed NMS rats. There is growing evidence in the literature that early life stress might impair the nociceptive system ontogeny and function. This study suggests that these alterations might be restored while stimulating OT receptor signaling or histone deacetylase inhibitors, using molecules that are currently available or part of clinical trials for other pathologies.