Morphine-induced kinase activation and localization in the periaqueductal gray of male and female mice

Morphine is a potent opioid analgesic with high propensity for the development of antinociceptive tolerance. Morphine antinociception and tolerance are partially regulated by the midbrain ventrolateral periaqueductal gray (vlPAG). However, the majority of research evaluating mu-opioid receptor signaling has focused on males. Here, we investigate kinase activation and localization patterns in the vlPAG following acute and chronic morphine treatment in both sexes. Male and female mice developed rapid antinociceptive tolerance to morphine (10 mg/kg i.p.) on the hot plate assay, but tolerance did not develop in males on the tail flick assay. Quantitative fluorescence immunohistochemistry was used to map and evaluate the activation of extracellular signal-regulated kinase 1/2 (ERK 1/2), protein kinase-C (PKC), and protein kinase-A (PKA). We observed significantly greater phosphorylated ERK 1/2 in the vlPAG of chronic morphine-treated animals which co-localized with the endosomal marker, Eea1. We note that pPKC is significantly elevated in the vlPAG of both sexes following chronic morphine treatment. We also observed that although PKA activity is elevated following chronic morphine treatment in both sexes, there is a significant reduction in the nuclear translocation of its phosphorylated substrate. Taken together, this study demonstrates increased activation of ERK 1/2, PKC, and PKA in response to repeated morphine treatment. The study opens avenues to explore the impact of chronic morphine treatment on G-protein signaling and kinase nuclear transport.

Donepezil prevents morphine tolerance by regulating N-methyl-d-aspartate receptor, protein kinase C and CaM-dependent kinase II expression in rats

Current studies have indicated that donepezil as a cholinesterase inhibitor can attenuate morphine-induced tolerance. The present study aimed to evaluate the possible role of N-methyl-d-aspartate receptors (NMDARs), protein kinase C (PKC) and CaM-dependent kinase II (CaMKII) pathways in this effect. Female Wistar rats received daily morphine (10 mg/kg, i.p.) alone or in combination with donepezil (1.5 or 2 mg/kg, gavaged) for 14 days. The analgesic effect was assessed by Von-frey, hotplate and tail flick test. On the 15th day, the periaqueductal gray (PAG) and lumbar spinal cord of rats were dissected. Then, protein levels of NMDAR-NR1, NR2B, PKCγ and CaMKIIα were tested using Western blot method. The results showed that morphine tolerance was seen after 8-10 days of injection compared with control group, while daily co-administration of donepezil with morphine prolonged the occurrence of analgesic tolerance. Western blot showed that morphine significantly increased NR1, PKCγ and CaMKIIα expressions in PAG, and significantly increased PKCγ and CaMKIIα in spinal cord. In contrast, donepezil downregulated NR1 and PKCγ in PAG, and downregulated PKCγ and CaMKIIα in spinal cord. Moreover, donepezil alone activates NR1 and NR2B in spinal cord, which needs to be further studied. Thus, the present results suggest that the attenuation effects of donepezil on morphine tolerance are possibly mediated by preventing morphine-induced upregulations in NR1, PKCγ and CaMKIIα expressions.

Intravenous doxapram administration as a potential model of panic attacks in rats

Panic disorder can be categorized into the nonrespiratory or the respiratory subtypes, the latter comprising dyspnea, shortness of breath, chest pain, feelings of suffocation, and paresthesias. Doxapram is an analeptic capable of inducing panic attacks with respiratory symptoms in individuals diagnosed with the disorder; however, its neuroanatomical targets and its effects on experimental animals remain uncharacterized. One of the brain regions proposed to trigger panic attacks is the midbrain periaqueductal gray (PAG). Therefore, in this study, we evaluated the effects of doxapram in Fos (c-Fos) protein expression in the PAG and characterized its cardiorespiratory and behavioral effects on the elevated T maze and in the conditioned place aversion (CPA) paradigms. Doxapram increased Fos expression in different columns of the PAG, increased respiratory frequency, decreased heart rate, and increased arterial pressure when injected via intravenous route. Alprazolam, a panicolytic benzodiazepine, injected via intraperitoneal route, decreased respiratory frequency, whereas URB597, an anandamide hydrolysis inhibitor injected via intraperitoneal route, was ineffective. Doxapram injected via intraperitoneal route induced an anxiogenic-like effect in the elevated T-maze model; however, it failed to induce CPA. This study suggests that the cardiorespiratory and behavioral effects of doxapram in rodents serve as an experimental model that can provide insights into the neurobiology of panic attacks.

Electrical stimulation of the posterior insula induces mechanical analgesia in a rodent model of neuropathic pain by modulating GABAergic signaling and activity in the pain circuitry

The insula has emerged as a critical target for electrical stimulation since it influences pathological pain states. We investigated the effects of repetitive electrical stimulation of the insular cortex (ESI) on mechanical nociception, and general locomotor activity in rats subjected to chronic constriction injury (CCI) of the sciatic nerve. We also studied neuroplastic changes in central pain areas and the involvement of GABAergic signaling on ESI effects. CCI rats had electrodes implanted in the left agranular posterior insular cortex (pIC), and mechanical sensitivity was evaluated before and after one or five daily consecutive ESIs (15 min each, 60 Hz, 210 μs, 1 V). Five ESIs (repetitive ESI) induced sustained mechanical antinociception from the first to the last behavioral assessment without interfering with locomotor activity. A marked increase in Fos immunoreactivity in pIC and a decrease in the anterior and mid-cingulate cortex, periaqueductal gray and hippocampus were noticed after five ESIs. The intrathecal administration of the GABAA receptor antagonist bicuculline methiodide reversed the stimulation-induced antinociception after five ESIs. ESI increased GAD65 levels in pIC but did not interfere with GABA, glutamate or glycine levels. No changes in GFAP immunoreactivity were found in this work. Altogether, the results indicate the efficacy of repetitive ESI for the treatment of experimental neuropathic pain and suggest a potential influence of pIC in regulating pain pathways partially through modulating GABAergic signaling.

Role of 5-HT1A and 5-HT2C receptors of the dorsal periaqueductal gray in the anxiety- and panic-modulating effects of antidepressants in rats

Antidepressant drugs are first-line treatment for panic disorder. Facilitation of 5-HT_1A receptor-mediated neurotransmission in the dorsal periaqueductal gray (dPAG), a key panic-associated area, has been implicated in the panicolytic effect of the selective serotonin reuptake inhibitor fluoxetine. However, it is still unknown whether this mechanism accounts for the antipanic effect of other classes of antidepressants drugs (ADs) and whether the 5-HT interaction with 5-HT_2C receptors in this midbrain area (which increases anxiety) is implicated in the anxiogenic effect caused by short-term treatment with ADs. The results showed that previous injection of the 5-HT_1A receptor antagonist WAY-100635 in the dPAG blocked the panicolytic-like effect caused by chronic systemic administration of the tricyclic AD imipramine in male Wistar rats tested in the elevated T-maze. Neither chronic treatment with imipramine nor fluoxetine changed the expression of 5-HT_1A receptors in the dPAG. Treatment with these ADs also failed to significantly change ERK1/2 (extracellular-signal regulated kinase) phosphorylation level in this midbrain area. Blockade of 5-HT_2C receptors in the dPAG with the 5-HT_2C receptor antagonist SB-242084 did not change the anxiogenic effect caused by a single acute injection of fluoxetine or imipramine in the Vogel conflict test. These results reinforce the view that the facilitation of 5-HT_1A receptor-mediated neurotransmission in the dPAG is a common mechanism involved in the panicolytic effect caused by chronic administration of ADs. On the other hand, the anxiogenic effect observed after short-term treatment with these drugs does not depend on 5-HT_2C receptors located in the dPAG.

Neuropeptide W, an endogenous NPBW1 and NPBW2 ligand, produced an analgesic effect via activation of the descending pain modulatory system during a rat formalin test

Neuropeptide W (NPW) messenger ribonucleic acid (mRNA) and NPBW1 and/or NPBW2 mRNA are expressed in the descending pain inhibitory system. In the present study, we examined whether NPW microinjected into the descending pain inhibitory system, such as the periaqueductal gray (PAG), locus coeruleus (LC), and rostral ventromedial medulla (RVM), produces an analgesic effect using a rat formalin test. Microinjections of NPW into the PAG ipsilateral and contralateral to the formalin-injected side, LC ipsilateral and contralateral to the formalin-injected side, and RVM produced an analgesic effect. In the RVM study, the analgesic effect was antagonized by WAY100135, a 5-HT1A antagonist, and enhanced by prazosin, an α1 antagonist, and SB269970, a 5-HT7 antagonist. Naloxone, an opioid antagonist, also antagonized the effect of NPW in the RVM study. In the ipsilateral LC study, the analgesic effect was antagonized by WAY100135, idazoxan, an α2 antagonist, and naloxone and was enhanced by prazosin and SB269970. In the contralateral LC study, the analgesic effect was antagonized by prazosin, idazoxan, SB269970, and naloxone. The analgesic effect was antagonized by WAY100135, SB269970, idazoxan, and naloxone in the ipsilateral and contralateral PAG studies. These findings strongly suggest that NPBW1/W2 activation by NPW microinjection into the RVM, LC, and PAG affect the descending pain modulatory system and produce anti-nociceptive and pro-nociceptive effects in the rat formalin test.

Topiramate inhibits offensive aggression through targeting ventrolateral periaqueductal gray

Topiramate is an approved antiepileptic drug clinically used to treat epilepsy and prevent migraines. Currently, topiramate has been found to be effective in treating aggressive symptoms in neuropsychiatric patients. In preclinical studies, however, the effects and mechanisms of topiramate on offensive aggression are still largely uninvestigated. Our previous studies indicated that glutamatergic transmission in the ventrolateral periaqueductal gray (vlPAG) plays a crucial role in regulating elements of offensive aggressive behaviors. In the present work, we investigated the actions of topiramate on vlPAG glutamatergic transmission and aggressive behaviors in group-housed (GH) and socially isolated (SI) rats. The results suggested that a single injection of topiramate systemically and dose-dependently inhibited elements of offensive aggressive behaviors of both GH and SI rats in the resident-intruder test (RIT), with long-lasting effective time profiles in SI rats. Moreover, systemic single administration of topiramate reduced the frequency and amplitude of miniature excitatory postsynaptic currents (mEPSCs) in the vlPAG. Bath perfusion of topiramate directly decreased the frequency and amplitude of mEPSCs and shortened the amplitude of evoked excitatory postsynaptic currents (EPSCs) in the vlPAG. Furthermore, intra-vlPAG single microinjection of topiramate dose-dependently inhibited offensive aggressive behaviors in GH and SI rats in a time-dependent manner. Additionally, both systemic and local topiramate inhibited offensive aggressive behaviors in a (2R,6R)-hydroxynorketamine (HNK)-dependent rat model. In conclusion, the present results suggest that topiramate exerts anti-aggressive roles through its inhibitory actions on glutamatergic activities in the vlPAG. These preclinical results support topiramate as a candidate drug to treat patients with heightened offensive aggression.

Evodiamine via targeting nNOS and AMPA receptor GluA1 inhibits nitroglycerin-induced migraine-like response

ETHNOPHARMACOLOGICAL RELEVANCE

Evodiamine (EVO) is a natural compound derived from Tetradium ruticarpum (A.Juss.) T.G.Hartley used to treat pain and migraine in traditional Chinese medicine. EVO is the primary active ingredient of Tetradium ruticarpum. However, the preventive effect of EVO against migraine remains unexplored.

AIM OF THE STUDY

To investigate the preventive effect of EVO against nitroglycerin (NTG)-induced acute migraine in rats.

MATERIALS AND METHODS

Male Sprague-Dawley rats were intragastrically administered EVO (45 or 90 mg/kg) for nine days. To establish an acute migraine model, we subcutaneously injected rats with a 10 mg/kg NTG solution. The migraine-like behavior of the rats was evaluated via the formalin test and the warm water tail-withdrawal assay. The periaqueductal gray (PAG) and serum samples were collected from the rats and used to determine the effect of EVO on the levels of serum nitric oxide (NO), CGRP, c-Fos, neuronal nitric oxide synthase (nNOS), inducible nitric oxide synthase (iNOS) and the α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptor GluA1.

RESULTS

The formalin test and the warm water tail-withdrawal assay showed that EVO inhibited the licking foot/shaking response and reversed the shortened tail-withdrawal latency in NTG-treated rats. Additionally, EVO suppressed serum NO levels and reduced the mRNA/protein expression of c-Fos and nNOS, but not iNOS, in the PAG. Furthermore, EVO suppressed total protein expression of the AMPA receptor GluA1 and its phosphorylation at Ser831 and Ser845.

CONCLUSIONS

This study showed that EVO inhibits the migraine-like pain response and that this beneficial effect might be attributed to the regulation of nNOS and suppression of the AMPA receptor GluA1. We suggest that EVO has the potential to treat migraine as a lead compound of natural origin.

Cannabinoids in the descending pain modulatory circuit: Role in inflammation

The legalization of cannabis in some states has intensified interest in the potential for cannabis and its constituents to lead to novel therapeutics for pain. Our understanding of the cellular mechanisms underlying cannabinoid actions in the brain have lagged behind opioids; however, the current opioid epidemic has also increased attention on the use of cannabinoids as alternatives to opioids for pain, especially chronic pain that requires long-term use. Endogenous cannabinoids are lipid signaling molecules that have complex roles in modulating neuronal function throughout the brain. In this review, we discuss cannabinoid functions in the descending pain modulatory pathway, a brain circuit that integrates cognitive and emotional processing of pain to modulate incoming sensory inputs. In addition, we highlight areas where further studies are necessary to understand cannabinoid regulation of descending pain modulation.

Tibetan medicine Ru-yi-Zhen-bao Pills exhibits anti-migraine effect through mediating PAG anti-nociceptive channel

ETHNOPHARMACOLOGICAL RELEVANCE

Migraine is a disabling neurovascular disorder, which increases risk of cardiovascular events and is a social burden worldwide. The present first-line anti-migraine medications can cause overwhelming side-effects, of which one includes the onset of cardiovascular disease. As one of the marketed Tibetan drugs, Ru-yi-Zhen-bao Pills (RYZBP) have been clinically used to treat cardiovascular disorders and as anti-migraine medication. However, there is currently no research exploring the anti-migraine actions of RYZBP.

AIM OF THE STUDY

The current research was designed to assess the anti-migraine roles of RYZBP and explore the underlying mechanisms in a nitroglycerin (NTG)-induced migraine rat model trial.

MATERIALS AND METHODS

120 rats were randomly divided into the following six groups of 20 rats each: normal control group, model control group, positive control group, and RYZBP high/medium/low-dose groups (Ru-yi-Zhen-bao Pills; TH 1.00 g/kg, TM 0.50 g/kg and TL 0.25 g/kg). All rats were administered intragastrically for 7 consecutive days, which were subcutaneously injected with the NTG (10 mg/kg) after the last gavage (except in the normal control group). 3min after NTG treatment, 30 rats (5 rats from each group) were anesthetized and devoted to electroencephalogram(EEG) testing, which was used to evaluate the analgesic effect of RYZBP. One hour after NTG treatment, the rest of the 90 rats (15 rats from each group) were anesthetized and midbrain tissue sample was dissected. The dissection was then washed with physiological saline and collected. The histopathological changes in the periaqueductal gray(PAG) of 5 tissue samples were determined by aematoxylin-eosin (H&E) staining, as well as an estimation of substance P (SP) and neurokinin 1 receptor (NK1R) expression through immunohistochemically staining(IHC). Another 5 midbrain preparations were carried out to evaluate calcitonin gene-related peptide (CGRP), proenkephalin (PENK), SP, and cholecystokinin (CCK) expressions by real-time quantitative polymerase chain reaction (RT-qPCR). The rest of the 5 brainstem tissues were then used to measure CCK, CGRP, and opioid peptide receptor (DORR) levels by western blotting(WB).

RESULTS

In the EEG test, RYZBP (TM 0.50 g / kg) treatment transformed the EEG pain-wave of the NTG-induced migraine model rats in different time period. In the mechanism assay, compared with the model control group, RYZBP pretreatment reduced inflammatory cell infiltration, fibrosis and vacuolation of neuronal cells of PAG tissue seen by HE staining. IHC experiments further showed that RYZBPTM up-regulated SP expression levels and enhanced NK1R levels in the NTG-induced migraine rats (P < 0.05). Therapeutic administration of RYZBP also increased PENK mRNA expression and DORR protein level. Both RT-qPCR and western blotting trials indicated that RYZBP treatment significantly decreased CCK and CGRP expression levels (P < 0.01 or P < 0.05) in the NTG-induced migraine rats.

CONCLUSIONS

RYZBP has the potential to be an effective anti-migraine treatment through suppressing the EEG pain-wave, increasing the levels of SP, PENK, DORR and reducing expression of CCK and CGRP. Mediating the PAG anti-nociceptive channel and inhibiting central sensitization were the two potential mechanisms, which offers further evidence for clinical therapy.

Orexinergic modulation of serotonin neurons in the dorsal raphe of a diurnal rodent, Arvicanthis niloticus

The hypothalamic neuropeptide, orexin (or hypocretin), is implicated in numerous physiology and behavioral functions, including affective states such as depression and anxiety. The underlying mechanisms and neural circuits through which orexin modulates affective responses remain unclear. The objective of the present study was to test the hypothesis that the serotonin (5-HT) system of the dorsal raphe nucleus (DRN) is a downstream target through which orexin potentially manifests its role in affective states. Using a diurnal rodent, the Nile grass rat (Arvicanthis niloticus), we first characterized the expression of the orexin receptors OX1R and OX2R in the DRN using in situ hybridization. The results revealed distinct distributions of OX1R and OX2R mRNAs, with OX1R predominantly expressed in the dorsal and lateral wings of the DRN that are involved in affective processes, while OX2R was mostly found in the ventral DRN that is more involved in sensory-motor function. We next examined how the orexin-OX1R pathway regulates 5-HT in the DRN and some of its projection sites using a selective OX1R antagonist SB-334867 (10 mg/kg, i.p.). A single injection of SB-334867 decreased 5-HT-ir fibers within the anterior cingulate cortex (aCgC); five once-daily administrations of SB-334867 decreased 5-HT-ir not only in the aCgC but also in the DRN, oval bed nucleus of the stria terminalis (ovBNST), nucleus accumbens shell (NAcSh), and periaqueductal gray (PAG). HPLC analysis revealed that five once-daily administrations of SB-334867 did not affect 5-HT turnover to any of the five sites, although it increased the levels of both 5-HT and 5-HIAA in the NAcSh. These results together suggest that orexinergic modulation of DRN 5-HT neurons via OX1Rs may be one pathway through which orexin regulates mood and anxiety, as well as perhaps other neurobiological processes.

Augmented fear bradycardia in rats with heart failure

In congestive heart failure (CHF), while resting parasympathetic activity becomes reduced, parasympathetically-mediated responses to stressors have not been described. This study aimed to (1) elucidate the effect of CHF on fear bradycardia, a parasympathetically-mediated response, and (2) examine if brain oxidative stress of CHF mediates fear bradycardia. White noise sound (WNS) exposure to conscious rats induced freezing behavior and elicited bradycardia. WNS exposure-elicited bradycardia was greater in rats with CHF than in controls. Superoxide dismutase mimetics administered in the lateral/ventrolateral midbrain periaqueductal gray (l/vlPAG), a region that contributes to the generation of fear bradycardia, had no effect on the bradycardia response in control and CHF rats. Dihydroethidium staining in situ showed that superoxide generation in the l/vlPAG of CHF rats was increased as compared to controls. These results demonstrate that CHF leads to the augmentation of fear bradycardia. Moreover, oxidative stress in the l/vlPAG of CHF unlikely mediates the augmented fear bradycardia.

Oxytocin in the periaqueductal gray mainly comes form the hypothalamic supraoptic nucleus to participate in pain modulation

Oxytocin (OXT) that effects the nociception process is mainly synthesized and secreted in the hypothalamic supraoptic nucleus (SON). Although the periaqueductal gray (PAG) hardly synthesizes OXT, OXT in PAG also plays a role in pain regulation. The communication investigates whether OXT in the PAG comes from SON to influence pain modulation. RT-PCR was used to analyze OXT mRNA expression and radioimmunoassay to measure OXT concentration. The results showed that (1) pain stimulation enhanced OXT mRNA expression in the SON at 10 min (268.1 ± 39.2%, p < 0.001) and 20 min (135.4±37.9%, p < 0.05) treatment and did not change in the PAG; (2) OXT level increase in SON perfusion liquid during pain stimulation [236.7±22.1% at 10 min (p < 0.001), 223.1±12.4% at 20 min (p<0.001), 56.1 ± 15.7% at 30 min (p < 0.01) and 11.2±14.2% at 40 min] was earlier than that in PAG perfusion liquid [17.8±9.7% at 10 min, 375.6±35.1% at 20 min (p <  0.001), 123.2±17.7% at 30 min (p <  0.001) and 52.7±22.4% at 40 min (p < 0.05)]; (3) SON excitation (L-glutamate sodium microinjection) induced OXT level increase in PAG perfusion liquid in a dose-dependent manner; (4) the bilateral SON cauterization completely controlled and the right SON cauterization partly reversed the pain stimulation induced-OXT concentration increase in PAG perfusion liquid. The data suggested that OXT in PAG came from SON, which might influence the pain process.

Early postpartum resting-state functional connectivity for mothers receiving buprenorphine treatment for opioid use disorder: A pilot study

Between 1999 and 2014, the prevalence of opioid use disorder (OUD) among pregnant women quadrupled in the USA. The standard treatment for peripartum women with OUD is buprenorphine. However, the maternal behavior neurocircuit that regulates maternal behavior and mother-infant bonding has not been previously studied for human mothers receiving buprenorphine treatment for OUD (BT). Rodent research shows opioid effects on reciprocal inhibition between maternal care and defence maternal brain subsystems: the hypothalamus and periaqueductal gray, respectively. We conducted a longitudinal functional magnetic resonance imaging (fMRI) pilot study in humans to specifically examine resting-state functional connectivity (rs-FC) between the periaqueductal gray and hypothalamus, as well as to explore associations with maternal bonding for BT. We studied 32 mothers who completed fMRI scans at 1 month (T1) and 4 months postpartum (T2), including seven mothers receiving buprenorphine for OUD and 25 non-OUD mothers as a comparison group (CG). The participants underwent a 6-minute resting-state fMRI scan at each time point. We measured potential bonding impairments using the Postpartum Bonding Questionnaire to explore how rs-FC with periaqueductal gray is associated with bonding impairments. Compared to CG, BT mothers differed in periaqueductal gray-dependent rs-FC with the hypothalamus, amygdala, insular cortex and other brain regions at T1, with many of these differences disappearing at T2, suggesting potential therapeutic effects of continuing buprenorphine treatment. In contrast, the "rejection and pathological anger" subscale of the Postpartum Bonding Questionnaire at T1 and T2 was associated with the T1-to-T2 increases in periaqueductal gray-dependent rs-FC with the hypothalamus and amygdala. Preliminary evidence links maternal bonding problems for mothers with OUD early in the postpartum to connectivity between specific care and defence maternal brain circuits, which may be mitigated by buprenorphine treatment. This exploratory study supports a potential mechanism for investigating both the therapeutic benefits and risks of opioids for maternal care and bonding with infants.

Intra-periaqueductal gray matter administration of orexin-A exaggerates pulpitis-induced anxiogenic responses and c-fos expression mainly through the interaction with orexin 1 and cannabinoid 1 receptors in rats

Different types of trigeminal pains are frequently associated with psychophysiological concerns. Orexin-A and orexin 1 receptor (OX1R) are involved in modulation of both trigeminal pain and anxiety responses. Ventrolateral periaqueductal gray matter (vlPAG), a controlling site for nociception and emotion, receives orexinergic inputs. Here, the role of vlPAG OX1Rs and their interaction with cannabinoid 1 (CB1) receptor was evaluated in anxiety-like behavior following capsaicin-induced dental pulp pain. Rats were cannulated in the vlPAG and orexin-A was injected at the doses of 0.17, 0.35 and 0.51 μg/rat prior to the induction of pain. The elevated plus maze (EPM) and open field (OF) tests were used for assessing the anxiety responses. In addition, the induction of c-fos, in the vlPAG, was investigated using immunofluorescence microscopy. Capsaicin-treated rats displayed significantly higher anxiogenic behavior on EPM and OF tests. Pretreatment with orexin-A (0.51 μg/rat) attenuated capsaicin-mediated nociception, while exaggerated anxiogenic responses (p < 0.05). In addition, orexin-A effects were diminished by the administration of OX1R (SB-334867, 12 μg/rat) and cannabinoid 1 (AM251, 4 μg/rat) receptor antagonists. Intradental capsaicin induced a significant increase in c-fos expression in the vlPAG that was exaggerated by orexin-A (0.51 μg/rat). Blockage of OX1R and CB1 receptors attenuated the effect of orexin-A on c-fos expression in capsaicin-treated rats. In conclusion, the data suggest that manipulation of OX1R and CB1 receptors in the vlPAG alters capsaicin-evoked anxiety like behaviors and c-fos induction in rats.

Differential effects of cholecystokinin (CCK-8) microinjection into the ventrolateral and dorsolateral periaqueductal gray on anxiety models in Wistar rats

Cholecystokinin (CCK) is one of the main neurohormone peptide systems in the brain, and a major anxiogenic mediator. The periaqueductal gray (PAG) is a key midbrain structure for defensive behaviors, which could include anxiety, fear, or even panic. The CCK system has wide distribution in the PAG, where the dorsolateral region (DL) participates in active defensive behavior and the ventrolateral region (VL) in passive defensive behavior. The aim of this study was to assess the effect of CCK-8 microinjection into DL-PAG or VL-PAG on anxiety-like behavior through two tests: elevated plus maze (EPM) and defensive burying behavior (DBB). CCK-8 (0.5 and 1.0 μg/0.5 μL) presently microinjected into the DL-PAG produced an anxiogenic-like effect on the EPM evidenced by decreasing the time spent/number of entries in open arms compared to vehicle group. Additionally, the latency to burying decreased and burying time increased on the DBB test. Contrarily, CCK-8 microinjected into the VL-PAG resulted in greater open-arm time and more open-arm entries compared to the vehicle-microinjected group. The results on the DBB test confirmed an anxiolytic-like response of CCK-8 into the VL-PAG. In conclusion, CCK-8 microinjected into DL-PAG produced anxiety-like behavior on EPM, and for first time reported on DBB. Contrarily, CCK-8 microinjected into the VL-PAG reduced anxiety-like behavior also for first time reported using both behavioral models EPM and DBB.

[Microinjection of endomorphin-1 in the ventrolateral periaqueductal gray induces descending inhibition of cardiac nociception by activating μ-opioid receptor in rats]

OBJECTIVE

To observe descending inhibition of cardiac nociception induced by microinjection of endomorphin-1 (EM1) in the ventrolateral periaqueductal gray (VLPAG) in rats effect and explore the role of μ-opioid receptor in mediating this effect.

METHODS

Male SD rats were randomized into electromyography (EMG) group and c-Fos group, both of which were further divided into 5 subgroups, namely 0.9% NaCl group, bradykinin (BK) group, BK+EM1 group, BK+CTOP group, and BK+CTOP+EM1 group. Rat models of cardiac nociception were established by intrapericardial injection of BK. The changes of cardiaosomatic motor reflex induced by BK were observed by assessing EMG responses of the dorsal spinotrapezius muscle; c-Fos expression in the spinal dorsal horn at levels T₃-T₅ was tested.

RESULTS

Compared with 0.9% NaCl, intrapericardial BK injection induced obvious EMG activities and significantly increased c-Fos expression in the spinal dorsal horn at T₃-T₅ (P &lt; 0.05). Compared with BK injection, microinjection of EM1 in the VLPAG dose-dependently inhibited EMG activities and significantly decreased c-Fos expression (P &lt; 0.05); microinjection of CTOP in the VLPAG produced no significant effect on EMG or c-Fos expression (P &gt; 0.05). Microinjection of CTOP obviously reversed EM1-induced inhibition of EMG activities and c-Fos expression (P &lt; 0.05).

CONCLUSIONS

Microinjection of EM1 in the VLPAG produces descending inhibition of cardiac nociception in rats by activating μ-opioid receptor.

Analgesic and anxiolytic effects of [Leu31,Pro34]-neuropeptide Y microinjected into the periaqueductal gray in rats

Several reports have demonstrated that neuropeptide Y (NPY) is involved in food intake, epilepsy, circadian rhythms, drug seeking, pain and anxiety, and other physiological or pathological conditions. On the other hand, periaqueductal gray (PAG) is a key brain center for modulating pain, anxiety and fear. It is the main structure implicated in integrated defensive behaviors. One such behavior, tonic immobility (TI), resembles fear and is able to induce analgesia. After microinjection of [Leu³¹,Pro³⁴]-Neuropeptide Y ([Leu³¹,Pro³⁴]-NPY) into the PAG dorsal (D) or ventrolateral (VL) of adult male Wistar rats, the following parameters were assessed: i) the analgesic effect by means of the tail-flick test (TF), ii) the duration of TI as a passive defensive behavioral response and as an anxiety/fear model (considering both TF and TI as single behaviors), iii) TI-induced analgesia by the combination of TF/TI, and iv) the anxious-like state through the elevated plus maze (EPM), and defensive burying behavior (DBB). The results show that the microinjection of [Leu³¹,Pro³⁴]-NPY into the PAG produced an analgesic effect (increasing the TF latency); overall decreased the TI duration, which might represent an important anti-fear effect. Moreover, [Leu³¹,Pro³⁴]-NPY microinjected into the PAG allows for a TI-induced analgesic effect, as well as, a substantial anxiolytic effect (evidenced by the EPM and DBB models). Hence, [Leu³¹,Pro³⁴]-NPY microinjected into the PAG, especially at 0.47nmol/0.5μL produces both analgesic and anxiolytic effects, in a higher magnitude within ventrolateral area.

Mesocorticolimbic hemodynamic response in Parkinson's disease patients with compulsive behaviors

BACKGROUND

PD patients treated with dopamine therapy can develop maladaptive impulsive and compulsive behaviors, manifesting as repetitive participation in reward-driven activities. This behavioral phenotype implicates aberrant mesocorticolimbic network function, a concept supported by past literature. However, no study has investigated the acute hemodynamic response to dopamine agonists in this subpopulation.

OBJECTIVES

We tested the hypothesis that dopamine agonists differentially alter mesocortical and mesolimbic network activity in patients with impulsive-compulsive behaviors.

METHODS

Dopamine agonist effects on neuronal metabolism were quantified using arterial-spin-labeling MRI measures of cerebral blood flow in the on-dopamine agonist and off-dopamine states. The within-subject design included 34 PD patients, 17 with active impulsive compulsive behavior symptoms, matched for age, sex, disease duration, and PD severity.

RESULTS

Patients with impulsive-compulsive behaviors have a significant increase in ventral striatal cerebral blood flow in response to dopamine agonists. Across all patients, ventral striatal cerebral blood flow on-dopamine agonist is significantly correlated with impulsive-compulsive behavior severity (Questionnaire for Impulsive Compulsive Disorders in Parkinson's Disease- Rating Scale). Voxel-wise analysis of dopamine agonist-induced cerebral blood flow revealed group differences in mesocortical (ventromedial prefrontal cortex; insular cortex), mesolimbic (ventral striatum), and midbrain (SN; periaqueductal gray) regions.

CONCLUSIONS

These results indicate that dopamine agonist therapy can augment mesocorticolimbic and striato-nigro-striatal network activity in patients susceptible to impulsive-compulsive behaviors. Our findings reinforce a wider literature linking studies of maladaptive behaviors to mesocorticolimbic networks and extend our understanding of biological mechanisms of impulsive compulsive behaviors in PD. © 2017 International Parkinson and Movement Disorder Society.

Angiotensin AT1 receptors modulate the anxiogenic effects of angiotensin (5-8) injected into the rat ventrolateral periaqueductal gray

Losartan and PD 123,319 are non-peptide angiotensin (Ang) receptor antagonists for the AT1 and AT2 subtypes of Ang II receptors, respectively. The tetrapeptide Ang (5-8) is the smallest Ang-peptide that elicits anxiogenic effects on unconditioned and conditioned experimental models upon injection into the ventrolateral column of the periaqueductal gray (vlPAG), and Ang (5-8) can be synthesized (from Ang II or Ang III) and inactivated in this mesencephalic structure. The vlPAG is also known to play a central role in mechanisms of fear and anxiety. We therefore utilized male Wistar rats to examine the effects of losartan and PD 123,319 injections, selective antagonists of the AT1 and AT2 receptors, respectively, into the vlPAG in the elevated plus-maze, a classic rat model of anxiety, and against the anxiogenic effect of Ang (5-8) (0.4 nmol/0.25μL) upon injection into the same region. The anxiolytic profile was dependent on the dose of intra-vlPAG losartan, whereas no effects on experimental anxiety were observed in the plus-maze following PD 123,319 injection. The anxiogenic effect of Ang (5-8) injection into the vlPAG remained unchanged in the PD 123,319-pretreated rats, but the effect did not occur in losartan-pretreated rats. The results led us to suggest that the anxiogenic effect of Ang (5-8) injection into the vlPAG may depend on the local activation of AT1, but not AT2 receptors. Activation of AT1 receptors in structures nearby vlPAG may be tonically involved in fear and experimental anxiety.

Activation of extrasynaptic δ-GABAA receptors globally or within the posterior-VTA has estrous-dependent effects on consumption of alcohol and estrous-independent effects on locomotion

Recent reports support higher than expected rates of binge alcohol consumption among women and girls. Unfortunately, few studies have assessed the mechanisms underlying this pattern of intake in females. Studies in males suggest that alcohol concentrations relevant to the beginning stages of binge intoxication may selectively target tonic GABAergic inhibition mediated by GABA_A receptor subtypes expressing the δ-subunit protein (δ-GABA_ARs). Indeed, administration of agonists that interact with these δ-GABA_ARs prior to alcohol access can abolish binge drinking behavior in male mice. These δ-GABA_ARs have also been shown to exhibit estrous-dependent plasticity in regions relevant to drug taking behavior, like the hippocampus and periaqueductal gray. The present experiments were designed to determine whether the estrous cycle would alter binge drinking, or our ability to modulate this pattern of alcohol use with THIP, an agonist with high selectivity and efficacy at δ-GABA_ARs. Using the Drinking-in-the-Dark (DID) binge-drinking model, regularly cycling female mice were given 2h of daily access to alcohol (20%v/v). Vaginal cytology or vaginal impedance was assessed after drinking sessions to track estrous status. There was no fluctuation in binge drinking associated with the estrous cycle. Both Intra-posterior-VTA administration of THIP and systemic administration of the drug was also associated with an estrous cycle dependent reduction in drinking behavior. Pre-treatment with finasteride to inhibit synthesis of 5α-reduced neurosteroids did not disrupt THIP's effects. Analysis of δ-subunit mRNA from posterior-VTA enriched tissue samples revealed that expression of this GABA_A receptor subunit is elevated during diestrus in this region. Taken together, these studies demonstrate that δGABA_ARs in the VTA are an important target for binge drinking in females and confirm that the estrous cycle is an important moderator of the pharmacology of this GABA_A receptor subtype.

Effect of melatonin injection into the periaqueductal gray on antinociception and tonic immobility in male rats

Melatonin (MLT) is a neurohormone with significant involvement in several biological functions, of which antinociception and tonic immobility (TI) may be the key neurobehavioral components to survive in adverse conditions such as a predator attack. TI-induced antinociception can be elicited, facilitated, or increased through opioid and γ-aminobutyric acid (GABA) among other chemical mediators at several levels of the central nervous system, mainly in the periaqueductal gray (PAG). The aim of this study was to assess the effect of the microinjection of MLT into the main PAG regions that are related to different integrated defensive responses, namely dorsal (D) and ventrolateral (VL), on both antinociception through the tail-flick (TF) test and TI duration as single behavioral response and on combined behavioral responses (TF/TI). We found that the microinjection of MLT into the main PAG areas produced antinociception but did not affect the TI duration. The microinjection of MLT into the D-PAG decreased TF latency during TI in the combined trial (TF/TI), which implies that TI-induced antinociception was blocked. The microinjection of MLT into the VL-PAG maintained the antinociceptive capability of the TI without addition or increase in the antinociceptive effects, implying a permissive effect by MLT on the TI-induced antinociception. MLT administration into the D-PAG decreased the TI duration on the TF/TI, whereas MLT administration into the VL-PAG had the opposite effect of significantly increasing TI duration with the TF/TI trial.

Nanoemulsion Thermoreversible Pluronic F127-Based Hydrogel Containing Hyptis pectinata (Lamiaceae) Leaf Essential Oil Produced a Lasting Anti-hyperalgesic Effect in Chronic Noninflammatory Widespread Pain in Mice

We evaluated if a nanostructured thermoreversible Pluronic F127-based hydrogel incorporated with Hyptis pectinata leaf essential oil (NE-EOH) produces a long-lasting anti-hyperalgesic effect on chronic muscle pain in an animal model. We induced chronic muscle pain by injecting the gastrocnemius with saline injections. Paw and muscle withdrawal thresholds and motor performance were evaluated after treatment and compared with morphine, diazepam, or vehicle. Naloxone and methysergide administration tested the involvement of opioid and serotonin receptors, respectively. Sites of action in the central nervous system for the NE-EOH were examined by measuring substance P (SP) levels in the spinal cord and Fos protein in the brainstem. NE-EOH increased paw and muscle withdrawal thresholds when compared with vehicle but had no effect on motor function. This analgesic effect was reversed by both naloxone and methysergide. NE-EOH decreased elevated substance P levels and reduced Fos-labeled neurons in the spinal cord and increased the number of Fos-labeled neurons in the periaqueductal gray (PAG), nucleus raphe magnus (NRM), and locus coeruleus (LC). NE-EOH was shown to produce a lasting anti-hyperalgesic effect. It uses opioid and serotonin receptors, activates brainstem inhibitory pathways, and reduces the release of excitatory neurotransmitters in the spinal cord and is a substance with potential to be used in the treatment of noninflammatory pain conditions. Graphical Abstract.

Neuropeptide glutamic acid-isoleucine (NEI)-induced paradoxical sleep in rats

Neuropeptideglutamic acid-isoleucine (NEI) as well as melanin concentrating hormone (MCH) is cleaved from the 165 amino acid protein, prepro-melanin concentrating hormone (prepro-MCH). Among many physiological roles of MCH, we demonstrated that intracerebroventricular (icv) injection of MCH induced increases in REM sleep episodes as well as in non REM sleep episodes. However, there are no studies on the effect of NEI on the sleep-wake cycle. As for the sites of action of MCH for induction of REM sleep, the ventrolateral periaqueductal gray (vlPAG) has been reported to be one of its site of action. Although MCH neurons contain NEI, GABA, MCH, and other neuropeptides, we do not know which transmitter(s) might induce REM sleep by acting on the vlPAG. Thus, we first examined the effect of icv injection of NEI on the sleep-wake cycle, and investigated how microinjection of either NEI, MCH, or GABA into the vlPAG affected REM sleep in rats. Icv injection of NEI (0.61μg/5μl: n=7) significantly increased the time spent in REM episodes compared to control (saline: 5μl; n=6). Microinjection of either NEI (61ng/0.2μl: n=7), MCH (100ng/0.2μl: n=6) or GABA (250mM/0.2μl: n=7) into the vlPAG significantly increased the time spent in REM episodes and the AUC. Precise hourly analysis of REM sleep also revealed that after those microinjections, NEI and MCH increased REM episodes at the latter phase, compared to GABA which increased REM episodes at the earlier phase. This result suggests that NEI and MCH may induce sustained REM sleep, while GABA may initiate REM sleep. In conclusion, our findings demonstrate that NEI, a cleaved peptide from the same precursor, prepro-MCH, as MCH, induce REM sleep at least in part through acting on the vlPAG.

Periaqueductal Grey EP3 Receptors Facilitate Spinal Nociception in Arthritic Secondary Hypersensitivity

Descending controls on spinal nociceptive processing play a pivotal role in shaping the pain experience after tissue injury. Secondary hypersensitivity develops within undamaged tissue adjacent and distant to damaged sites. Spinal neuronal pools innervating regions of secondary hypersensitivity are dominated by descending facilitation that amplifies spinal inputs from unsensitized peripheral nociceptors. Cyclooxygenase-prostaglandin (PG) E2 signaling within the ventrolateral periaqueductal gray (vlPAG) is pronociceptive in naive and acutely inflamed animals, but its contributions in more prolonged inflammation and, importantly, secondary hypersensitivity remain unknown. In naive rats, PG EP3 receptor (EP3R) antagonism in vlPAG modulated noxious withdrawal reflex (EMG) thresholds to preferential C-nociceptor, but not A-nociceptor, activation and raised thermal withdrawal thresholds in awake animals. In rats with inflammatory arthritis, secondary mechanical and thermal hypersensitivity of the hindpaw developed and was associated with spinal sensitization to A-nociceptor inputs alone. In arthritic rats, blockade of vlPAG EP3R raised EMG thresholds to C-nociceptor activation in the area of secondary hypersensitivity to a degree equivalent to that evoked by the same manipulation in naive rats. Importantly, vlPAG EP3R blockade also affected responses to A-nociceptor activation, but only in arthritic animals. We conclude that vlPAG EP3R activity exerts an equivalent facilitation on the spinal processing of C-nociceptor inputs in naive and arthritic animals, but gains in effects on spinal A-nociceptor processing from a region of secondary hypersensitivity. Therefore, the spinal sensitization to A-nociceptor inputs associated with secondary hypersensitivity is likely to be at least partly dependent on descending prostanergic facilitation from the vlPAG.

SIGNIFICANCE STATEMENT

After tissue damage, sensitivity to painful stimulation develops in undamaged areas (secondary hypersensitivity). This is found in many painful conditions, particularly arthritis. The periaqueductal gray (PAG) is an important center that controls spinal nociceptive processing, on which secondary hypersensitivity depends. Prostaglandins (PGs) are mediators of inflammation with pronociceptive actions within the PAG under normal conditions. We find that secondary hindpaw hypersensitivity in arthritic rats results from spinal sensitization to peripheral A-nociceptor inputs. In the PAG of arthritic, but not naive, rats, there is enhanced control of spinal A-nociceptor processing through PG EP3 receptors. The descending facilitatory actions of intra-PAG PGs play a direct and central role in the maintenance of inflammatory secondary hypersensitivity, particularly relating to the processing of A-fiber nociceptive information.

Acquisition and expression of fear memories are distinctly modulated along the dorsolateral periaqueductal gray axis of rats exposed to predator odor

The dorsolateral region of the midbrain periaqueductal gray (dlPAG) modulates both innate and conditioned fear responses. However, the contribution of the rostrocaudal portions of the dlPAG to defense reactions and aversive memories remains unclear. Here, we sought to investigate the effects of N-methyl-d-aspartate (NMDA) receptor blockade within rostral or caudal dlPAG of rats exposed to innate and learned fear to cat odor. For this, adult male Wistar rats were microinjected with the NMDA antagonist D-2-amino-5-phosphono-pentanoate (AP5; 3 or 6nmol/0.2μl) into the rostral or caudal dlPAG before and after the exposure to the cat odor or to the context paired with the predator odor. The results demonstrated that cat odor exposure induced unconditioned defensive behaviors as well as contextual fear. AP5 microinjected in the rostral dlPAG reduced the defensive responses to cat odor and impaired the acquisition, but not consolidation of contextual fear. On the other hand, AP5 infused within the caudal dlPAG promoted long-lasting reduction of contextual fear expression. Altogether, our data suggest that NMDA receptors mediate a functional dichotomy in the rostrocaudal axis of dlPAG regulating unconditioned and conditioned defensive reactions to predatory cues.

Role played by periaqueductal gray neurons in parasympathetically mediated fear bradycardia in conscious rats

Freezing, a characteristic pattern of defensive behavior elicited by fear, is associated with a decrease in the heart rate. Central mechanisms underlying fear bradycardia are poorly understood. The periaqueductal gray (PAG) in the midbrain is known to contribute to autonomic cardiovascular adjustments associated with various emotional behaviors observed during active or passive defense reactions. The purpose of this study was to elucidate the role played by PAG neurons in eliciting fear bradycardia. White noise sound (WNS) exposure at 90 dB induced freezing behavior and elicited bradycardia in conscious rats. The WNS exposure-elicited bradycardia was mediated parasympathetically because intravenous administration of atropine abolished the bradycardia (P < 0.05). Moreover, WNS exposure-elicited bradycardia was mediated by neuronal activation of the lateral/ventrolateral PAG (l/vlPAG) because bilateral microinjection of muscimol, a GABAA agonist, into the l/vlPAG significantly suppressed the bradycardia. It is noted that muscimol microinjected bilaterally into the dorsolateral PAG had no effect on WNS exposure-elicited bradycardia. Furthermore, retrograde neuronal tracing experiments combined with immunohistochemistry demonstrated that a number of l/vlPAG neurons that send direct projections to the nucleus ambiguus (NA) in the medulla, a major origin of parasympathetic preganglionic neurons to the heart, were activated by WNS exposure. Based on these findings, we propose that the l/vlPAG-NA monosynaptic pathway transmits fear-driven central signals, which elicit bradycardia through parasympathetic outflow.

Stress induces analgesia via orexin 1 receptor-initiated endocannabinoid/CB1 signaling in the mouse periaqueductal gray

The orexin system consists of orexin A/hypocretin 1 and orexin B/hypocretin 2, and OX1 and OX2 receptors. Our previous electrophysiological study showed that orexin A in the rat ventrolateral periaqueductal gray (vlPAG) induced antinociception via an OX1 receptor-initiated and endocannabinoid-mediated disinhibition mechanism. Here, we further characterized antinociceptive effects of orexins in the mouse vlPAG and investigated whether this mechanism in the vlPAG can contribute to stress-induced analgesia (SIA) in mice. Intra-vlPAG (i.pag.) microinjection of orexin A in the mouse vlPAG increased the hot-plate latency. This effect was mimicked by i.pag. injection of WIN 55,212-2, a CB1 agonist, and antagonized by i.pag. injection of the antagonist of OX1 (SB 334867) or CB1 (AM 251), but not OX2 (TCS-OX2-29) or opioid (naloxone), receptors. [Ala(11), D-Leu(15)]-orexin B (i.pag.), an OX2 selective agonist, also induced antinociception in a manner blocked by i.pag. injection of TCS-OX2-29, but not SB 334867 or AM 251. Mice receiving restraint stress for 30 min showed significantly longer hot-plate latency, more c-Fos-expressing orexin neurons in the lateral hypothalamus and higher orexin levels in the vlPAG than unrestrained mice. Restraint SIA in mice was prevented by i.pag. or intraperitoneal injection of SB 334867 or AM 251, but not TCS-OX2-29 or naloxone. These results suggest that during stress, hypothalamic orexin neurons are activated, releasing orexins into the vlPAG to induce analgesia, possibly via the OX1 receptor-initiated, endocannabinoid-mediated disinhibition mechanism previously reported. Although activating either OX1 or OX2 receptors in the vlPAG can lead to antinociception, only OX1 receptor-initiated antinociception is endocannabinoid-dependent.

Distinct pathways for norepinephrine- and opioid-triggered antinociception from the amygdala

BACKGROUND

The amygdala has an important role in pain and pain modulation. We showed previously in animal studies that α2 -adrenoreceptor activation in the central nucleus of the amygdala (CeA) mediates hypoalgesia produced by restraint stress, and that direct application of an α2 -agonist in this region produces analgesia.

AIMS

In the present animal experiments, we investigated the pathways through which α2 -sensitive systems in the CeA produce behavioural analgesia. The CeA has dense connections to a descending pain modulatory network, centred in the midbrain periaqueductal grey (PAG) and the rostral ventromedial medulla (RVM), which is implicated in various forms of stress-related hypoalgesia and which mediates the antinociceptive effect of morphine applied in the basolateral amygdala. We investigated whether this circuit mediates the hypoalgesic effects of α2 -adrenergic agonist administration into the CeA as well as the contribution of endogenous opioids and cannabinoids. We also tested the possibility that activation of α2 -receptors in the CeA produces antinociception by recruitment of noradrenergic pathways projecting to the spinal cord.

RESULTS

Hypoalgesia resulting from bilateral application of the α2 -adrenergic agonist clonidine in the CeA was not reversed by chemical inactivation of the RVM or by systemic injections of naloxone (μ-opioid antagonist) or rimonabant (CB1 antagonist). By contrast, spinal α2 -receptor blockade (intrathecal idazoxan) completely prevented the hypoalgesic effect of clonidine in the CeA, and unmasked a small but significant hyperalgesia.

CONCLUSION

In rats, adrenergic actions in the CeA mediating hypoalgesia require spinal adrenergic neurotransmission but not the PAG-RVM pain modulatory network, or opiate or cannabinoid systems.

Pharmacologically inhibiting GluR2 internalization alleviates neuropathic pain

Neuropathic pain is of serious clinical concern and only about half of patients achieve partial relief with currently-available treatments, so it is critical to find new drugs for this condition. Recently, the cellsurface trafficking of pain-related receptors has been suggested as an important mechanism underlying persistent neuropathic pain. Here, we used the short peptide GluA2-3y, which specifically inhibits the GluA2-dependent endocytosis of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors, and tested its anti-nociceptive effect in the periaqueductal grey (PAG) of intact rats and rats with neuropathic pain. Intra-PAG injection of 0.15, 1.5, 7.5, and 15 pmol of GluA2-3y induced dose-dependent increases in hindpaw withdrawal latencies to noxious thermal and mechanical stimuli in intact rats, suggesting that GluA2 cell-surface trafficking in the PAG is involved in pain modulation. Furthermore, GluA2-3y had much stronger anti-nociceptive effects in rats with neuropathic pain induced by sciatic nerve ligation. Interestingly, the intra-PAG injection of 15 pmol GluA2-3y had an analgesic effect similar to 10 μg (35 nmol) morphine in rats with neuropathic pain. Taken together, our results suggested that GluA2 trafficking in the PAG plays a critical role in pain modulation, and inhibiting GluA2 endocytosis with GluA2-3y has potent analgesic effects in rats with neuropathic pain. These findings strongly support the recent hypothesis that targeting receptor trafficking could be a new strategy for the treatment of neuropathic pain.

The endocannabinoid, endovanilloid and nitrergic systems could interact in the rat dorsolateral periaqueductal gray matter to control anxiety-like behaviors

Cannabinoid compounds usually produce biphasic effects in the modulation of emotional responses. Low doses of the endocannabinoid anandamide (AEA) injected into the dorsolateral periaqueductal gray matter (dlPAG) induce anxiolytic-like effects via CB1 receptors activation. However, at higher doses the drug loses this effect, in part by activating Transient Receptor Potential Vanilloid Type 1 (TRPV1). Activation of these latter receptors could induce the formation of nitric oxide (NO). Thus, the present study tested the hypothesis that at high doses AEA loses it anxiolytic-like effect by facilitating, probably via TRPV1 receptor activation, the formation of NO. Male Wistar rats received combined injections into the dlPAG of vehicle, the TRPV1 receptor antagonist 6-iodo-nordihydrocapsaicin or the NO scavenger carboxy-PTIO (c-PTIO), followed by vehicle or AEA, and were submitted to the elevated plus maze (EPM) or the Vogel conflict test (VCT). A low dose (5pmol) of AEA produced an anxiolytic-like effect that disappeared at higher doses (50 and 200pmol). The anxiolytic-like effects of these latter doses, however, were restored after pre-treatment with a low and ineffective dose of c-PTIO in both animal models. In addition, the combined administration of ineffective doses of 6-iodo-nordihydrocapsaicin (1nmol) and c-PTIO (0.3nmol) produced an anxiolytic-like response. Therefore, these results support the hypothesis that intra-dlPAG injections of high doses of AEA lose their anxiolytic effects by favoring TRPV1 receptors activity and consequent NO formation, which in turn could facilitate defensive responses.

DAMGO depresses inhibitory synaptic transmission via different downstream pathways of μ opioid receptors in ventral tegmental area and periaqueductal gray

Opioid-induced rewarding and motorstimulant effects are mediated by an increased activity of the ventral tegmental area (VTA) dopamine (DA) neurons. The excitatory mechanism of opioids on VTA-DA neurons has been proposed to be due to the depression of GABAergic synaptic transmission in VTA-DA neurons. However, how opioids depress GABAergic synaptic transmission in VTA-DA neurons remain to be studied. In the present study, we explored the mechanism of the inhibitory effect of [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (DAMGO) on GABAergic synaptic transmission in VTA-DA neurons using multiple approaches and techniques. Our results showed that (1) DAMGO inhibits GABAergic inputs in VTA-DA neurons at presynaptic sites; (2) effect of DAMGO on GABAergic inputs in VTA-DA neurons is inhibited by potassium channel blocker 4-aminopyridine (4-AP) and Gi protein inhibitor N-ethylmaleimide (NEM); (3) phospholipase A2 (PLA2) does not mediate the effect of DAMGO on GABAergic inputs in VTA-DA neurons, but mediates it in the periaqueductal gray (PAG); (4) multiple downstream signaling molecules of μ receptors do not mediate the effect of DAMGO on GABAergic inputs in VTA-DA neurons. These results suggest that DAMGO depresses inhibitory synaptic transmission via μ receptor-Gi protein-Kv channel pathway in VTA-DA neurons, but via μ receptor-PLA2 pathway in PAG neurons.

Endogenous cholinergic neurotransmission contributes to behavioral sensitization to morphine

Neuroplasticity in the mesolimbic dopaminergic system is critical for behavioral adaptations associated with opioid reward and addiction. These processes may be influenced by cholinergic transmission arising from the laterodorsal tegmental nucleus (LDTg), a main source of acetylcholine to mesolimbic dopaminergic neurons. To examine this possibility we asked if chronic systemic morphine administration affects expression of genes in ventral and ventrolateral periaqueductal gray at the level of the LDTg using rtPCR. Specifically, we examined gene expression changes in the area of interest using Neurotransmitters and Receptors PCR array between chronic morphine and saline control groups. Analysis suggested that chronic morphine administration led to changes in expression of genes associated, in part, with cholinergic neurotransmission. Furthermore, using a quantitative immunofluorescent technique, we found that chronic morphine treatment produced a significant increase in immunolabeling of the cholinergic marker (vesicular acetylcholine transporter) in neurons of the LDTg. Finally, systemic administration of the nonselective and noncompetitive neuronal nicotinic antagonist mecamylamine (0.5 or 2 mg/kg) dose-dependently blocked the expression, and to a lesser extent the development, of locomotor sensitization. The same treatment had no effect on acute morphine antinociception, antinociceptive tolerance or dependence to chronic morphine. Taken together, the results suggest that endogenous nicotinic cholinergic neurotransmission selectively contributes to behavioral sensitization to morphine and this process may, in part, involve cholinergic neurons within the LDTg.

Contributions of purinergic P2X3 receptors within the midbrain periaqueductal gray to diabetes-induced neuropathic pain

Hyperalgesia and allodynia are commonly observed in patients with diabetic neuropathy. The mechanisms responsible for neuropathic pain are not well understood. Thus, in this study, we examined the role played by purinergic P2X3 receptors of the midbrain periaqueductal gray (PAG) in modulating diabetes-induced neuropathic pain because this brain region is an important component of the descending inhibitory system to control central pain transmission. Our results showed that mechanical withdrawal thresholds were significantly increased by stimulation of P2X3 receptors in the dorsolateral PAG of rats (n = 12, P < 0.05 vs. vehicle control) using α,β-methylene-ATP (α,β-meATP, a P2X3 receptor agonist). In addition, diabetes was induced by an intraperitoneal injection of streptozotocin (STZ) in rats, and mechanical allodynia was observed 3 weeks after STZ administration. Notably, the excitatory effects of P2X3 stimulation on mechanical withdrawal thresholds were significantly blunted in STZ-induced diabetic rats (n = 12, P < 0.05 vs. control animals) as compared with control rats (n = 12). Furthermore, the protein expression of P2X3 receptors in the plasma membrane of the dorsolateral PAG of STZ-treated rats was significantly decreased (n = 10, P < 0.05 vs. control animals) compared to that in control rats (n = 8), whereas the total expression of P2X3 receptors was not significantly altered. Overall, data of our current study suggest that a decrease in the membrane expression of P2X3 receptors in the PAG of diabetic rats is likely to impair the descending inhibitory system in modulating pain transmission and thereby contributes to the development of mechanical allodynia in diabetes.

Nociceptin/orphanin FQ peptide receptor antagonist JTC-801 reverses pain and anxiety symptoms in a rat model of post-traumatic stress disorder

BACKGROUND AND PURPOSE

Single-prolonged stress (SPS), a rat model of post-traumatic stress disorder (PTSD), also induces long-lasting hyperalgesia associated with hypocortisolism and elevated nociceptin/orphanin FQ (N/OFQ) levels in serum and CSF. Here, we determined the effect of JTC-801 (N-(4-amino-2-methylquinolin-6-yl)-2-(4-ethylphenoxymethyl) benzamide monohydrochloride), a nociceptin/orphanin FQ peptide (NOP) receptor antagonist, on symptoms of pain and anxiety in rats after SPS exposure, and examined N/OFQ-NOP receptor system changes.

EXPERIMENTAL APPROACH

Male Sprague Dawley rats received JTC-801 (6 mg kg(-1) i.p., once daily) during days 7-21 of SPS. The ability of JTC-801 to inhibit N/OFQ-stimulated [(35) S]-GTPγS binding was confirmed in rat brain membranes. Anxiety-like behaviour and pain sensitivity were monitored by changes in elevated plus maze performance and withdrawal responses to thermal and mechanical stimuli. Serum corticosterone and N/OFQ content in CSF, serum and brain tissues were determined by radioimmunoassay; NOP receptor protein and gene expression in amygdala, hippocampus and periaqueductal grey (PAG) were examined by immunoblotting and real-time PCR respectively.

KEY RESULTS

JTC-801 treatment reversed SPS-induced mechanical allodynia, thermal hyperalgesia, anxiety-like behaviour and hypocortisolism. Elevated N/OFQ levels in serum, CSF, PAG and hippocampus at day 21 of SPS were blocked by JTC-801; daily JTC-801 treatment also reversed NOP receptor protein and mRNA up-regulation in amygdala and PAG.

CONCLUSION AND IMPLICATIONS

JTC-801 reversed SPS-induced anxiety- and pain-like behaviours, and NOP receptor system up-regulation. These findings suggest that N/OFQ plays an important role in hyperalgesia and allodynia maintenance after SPS. NOP receptor antagonists may provide effective treatment for co-morbid PTSD and pain.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Functionally selective signaling for morphine and fentanyl antinociception and tolerance mediated by the rat periaqueductal gray

Functionally selective signaling appears to contribute to the variability in mechanisms that underlie tolerance to the antinociceptive effects of opioids. The present study tested this hypothesis by examining the contribution of G protein-coupled receptor kinase (GRK)/Protein kinase C (PKC) and C-Jun N-terminal kinase (JNK) activation on both the expression and development of tolerance to morphine and fentanyl microinjected into the ventrolateral periaqueductal gray of the rat. Microinjection of morphine or fentanyl into the periaqueductal gray produced a dose-dependent increase in hot plate latency. Microinjection of the non-specific GRK/PKC inhibitor Ro 32-0432 into the periaqueductal gray to block mu-opioid receptor phosphorylation enhanced the antinociceptive effect of morphine but had no effect on fentanyl antinociception. Microinjection of the JNK inhibitor SP600125 had no effect on morphine or fentanyl antinociception, but blocked the expression of tolerance to repeated morphine microinjections. In contrast, a microinjection of Ro 32-0432 blocked the expression of fentanyl, but not morphine tolerance. Repeated microinjections of Ro 32-0432 blocked the development of morphine tolerance and inhibited fentanyl antinociception whether rats were tolerant or not. Repeated microinjections of SP600125 into the periaqueductal gray blocked the development of tolerance to both morphine and fentanyl microinjections. These data demonstrate that the signaling molecules that contribute to tolerance vary depending on the opioid and methodology used to assess tolerance (expression vs. development of tolerance). This signaling difference is especially clear for the expression of tolerance in which JNK contributes to morphine tolerance and GRK/PKC contributes to fentanyl tolerance.

Periaqueductal grey cyclooxygenase-dependent facilitation of C-nociceptive drive and encoding in dorsal horn neurons in the rat

The experience of pain is strongly affected by descending control systems originating in the brainstem ventrolateral periaqueductal grey (VL-PAG), which control the spinal processing of nociceptive information. A- and C-fibre nociceptors detect noxious stimulation, and have distinct and independent contributions to both the perception of pain quality (fast and slow pain, respectively) and the development of chronic pain. Evidence suggests a separation in the central processing of information arising from A- vs. C-nociceptors; for example, inhibition of the cyclooxygenase-1 (COX-1)-prostaglandin system within the VL-PAG alters spinal nociceptive reflexes evoked by C-nociceptor input in vivo via descending pathways, leaving A-nociceptor-evoked reflexes largely unaffected. As the spinal neuronal mechanisms underlying these different responses remain unknown, we determined the effect of inhibition of VL-PAG COX-1 on dorsal horn wide dynamic-range neurons evoked by C- vs. A-nociceptor activation. Inhibition of VL-PAG COX-1 in anaesthetised rats increased firing thresholds of lamina IV-V wide dynamic-range dorsal horn neurons in response to both A- and C-nociceptor stimulation. Importantly, wide dynamic-range dorsal horn neurons continued to faithfully encode A-nociceptive information, even after VL-PAG COX-1 inhibition, whereas the encoding of C-nociceptor information by wide dynamic-range spinal neurons was significantly disrupted. Dorsal horn neurons with stronger C-nociceptor input were affected by COX-1 inhibition to a greater extent than those with weak C-fibre input. These data show that the gain and contrast of C-nociceptive information processed in individual wide dynamic-range dorsal horn neurons is modulated by prostanergic descending control mechanisms in the VL-PAG.

Amygdalar neuronal activity mediates the cardiovascular responses evoked from the dorsolateral periaqueductal gray in conscious rats

There is ample evidence that both lateral/dorsolateral periaqueductal gray (l/dlPAG) and basolateral amygdala (BLA) are essential for the regulation of the autonomic responses evoked during innate reactions to threatening stimuli. However, it is not well established to what extent the BLA regulates the upstream functional connection from the l/dlPAG. Here we evaluated the role of the BLA and its glutamatergic receptors in the cardiovascular responses induced by l/dlPAG stimulation in rats. We examined the influence of acute inhibition of the BLA, unilaterally, by injecting muscimol on the cardiovascular responses evoked by the injection of N-methyl D-aspartate (NMDA) into the l/dlPAG. We also evaluated the role of BLA ionotropic glutamate receptors in these responses by injecting antagonists of NMDA and AMPA/kainate receptor subtypes into the BLA. Our results show that the microinjection of NMDA in the BLA increased the mean arterial pressure (MAP) and heart rate (HR). Injection of NMDA into the l/dlPAG caused similar increases in these variables, which was prevented by the prior injection of muscimol, a GABAA agonist, into the BLA. Moreover, injection of glutamatergic antagonists (2-amino-5-phosphonopentanoate (AP5) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)) into the BLA reduced the increase in MAP and HR induced by l/dlPAG activation. Finally, the inhibition of the central amygdala neurons failed to reduce the cardiovascular changes induced by l/dlPAG activation. These results indicate that physiological responses elicited by l/dlPAG activation require the neuronal activity in the BLA. This ascending excitatory pathway from the l/dlPAG to the BLA might ensure the expression of the autonomic component of the defense reaction.

The midbrain periaqueductal grey has no role in the generation of the respiratory motor pattern, but provides command function for the modulation of respiratory activity

It has previously been shown that stimulation of cell-columns in the periaqueductal grey (PAG) triggers site-specific cardiorespiratory effects. These are believed to facilitate changes in behaviour through coordinated changes in autonomic outflow. Here, we investigated whether PAG-evoked respiratory commands can be studied in situ using the decerebrate perfused brainstem preparation. Phrenic, vagus and abdominal iliohypogastric nerves were recorded before and after microinjection of L-glutamate (30-50 nl, 10 mM) or isoguvacine (GABA-receptor agonist, 30-50 nl, 10 mM) into the PAG. L-glutamate microinjection triggered a range of site-specific respiratory modulations (n = 17 preparations). Subsequent microinjection of isoguvacine into the same PAG sites had no effect on the baseline respiratory motor pattern or rhythm. We conclude that while the PAG has no function in respiratory pattern generation, PAG-evoked respiratory modulations can be evoked in situ in the absence of higher brain centres and while homeostatic parameters that may affect respiratory drive are held static.

Visceral analgesic effect of 5-HT(4) receptor agonist in rats involves the rostroventral medulla (RVM)

The 5-HT(4) receptor agonist tegaserod (TEG) has been reported to modulate visceral pain. However, the underlying mechanism remains unknown. The objective of the present study was to examine the analgesic mechanism and site of action of TEG. In male rats, visceral pain was assessed by measuring visceromotor response (VMR) to colorectal distension (CRD). Inflammation was induced by intracolonic injection of tri-nitrobenzene sulfonic acid (TNBS). The effect of TEG on the VMR was tested by injecting intraperitoneal (i.p.), intrathecal (i.t.), intracerebroventricular (i.c.v) or in the rostroventral medulla (RVM). The effect of the drug was also tested on responses of CRD-sensitive pelvic nerve afferents (PNA) and lumbo-sacral (LS) spinal neurons. Systemic injection of TEG attenuated VMR in naive and TNBS-treated rats. Similarly, supraspinal, but not spinal, injection of TEG attenuated the VMR. While GR113808, (selective 5-HT(4) antagonist) blocked the effect, naloxone (NLX) an opioid receptor antagonist reversed the effect of TEG. Although i.t. NLX did not block the inhibitory effect of TEG in VMR study, i.t. injection of α2-adrenergic receptor antagonist yohimbine blocked the effect of TEG when given systemically. While TEG had no effect on the responses of CRD-sensitive PNA, it inhibited the responses of CRD-sensitive LS neurons in spinal intact condition. This inhibition was blocked by GR113808, NLX and β-funaltrexamine (β-FNA) when injected into the RVM. Results indicate that TEG produces analgesia via activation of supraspinal 5-HT(4) receptors which triggers the release of opioids at supraspinal site, which activates descending noradrenergic pathways to the spinal cord to produce analgesia.

1-Oleoyl lysophosphatidic acid: a new mediator of emotional behavior in rats

The role of lysophosphatidic acid (LPA) in the control of emotional behavior remains to be determined. We analyzed the effects of the central administration of 1-oleoyl-LPA (LPA 18∶1) in rats tested for food consumption and anxiety-like and depression-like behaviors. For this purpose, the elevated plus-maze, open field, Y maze, forced swimming and food intake tests were performed. In addition, c-Fos expression in the dorsal periaqueductal gray matter (DPAG) was also determined. The results revealed that the administration of LPA 18∶1 reduced the time in the open arms of the elevated plus-maze and induced hypolocomotion in the open field, suggesting an anxiogenic-like phenotype. Interestingly, these effects were present following LPA 18∶1 infusion under conditions of novelty but not under habituation conditions. In the forced swimming test, the administration of LPA 18∶1 dose-dependently increased depression-like behavior, as evaluated according to immobility time. LPA treatment induced no effects on feeding. However, the immunohistochemical analysis revealed that LPA 18∶1 increased c-Fos expression in the DPAG. The abundant expression of the LPA₁ receptor, one of the main targets for LPA 18∶1, was detected in this brain area, which participates in the control of emotional behavior, using immunocytochemistry. These findings indicate that LPA is a relevant transmitter potentially involved in normal and pathological emotional responses, including anxiety and depression.

Impaired fear inhibitory properties of GABAA and μ opioid receptors of the dorsal periaqueductal grey in alcohol-withdrawn rats

The dorsal periaqueductal grey (DPAG) is a midbrain region that plays a fundamental role on the expression of the anxiety and fear-related responses. Increased fear and anxiety levels are the most frequent symptoms present in the alcohol withdrawal syndrome. This study aims to assess whether GABA and opioid receptors of the DPAG could be implicated in the expression of the conditioned fear state elicited in alcohol withdrawn-rats. For this purpose, we used the fear-potentiated startle (FPS) procedure. Drugs used were the selective GABAA agonist muscimol (1 nmol/0.2 μL) and the predominantly mu opiate receptors agonist morphine (10 nmol/0.2 μL). Exposure to aversive cues of the elevated-plus-maze (EPM) was used in order to validate the influence of alcohol withdrawal on emotionality. Data from FPS pointed out to an anxiogenic-like profile of withdrawal, a result sustained by the data collected from the EPM test. Muscimol and morphine showed their well-known anxiolytic-like profile, decreasing the fear-potentiated startle (FPS) amplitude in control subjects. However, the drugs had no effect on the levels of fear evoked in rats pre-treated with and withdrawn from alcohol. It is suggested that this lack of effect was possibly due to the desensitization of the GABAA receptors, as well as by the decrease on the responsiveness of the functions of μ opioid receptors, resulting from chronic administration of ethyl alcohol. These findings shed light on some aspects of the anxiety-like behavior elicited during alcohol withdrawal bringing new information on the influence of GABA and opioid receptors of the DPAG on the expression of unconditioned and conditioned fear responses.

Blockade of Toll-like receptor 4 attenuates morphine tolerance and facilitates the pain relieving properties of morphine

The ventrolateral periaqueductal gray (vlPAG) is an integral locus for morphine action. Although it is clear that glia contribute to the development of morphine tolerance, to date, the investigation of their role has been limited to spinal and medullary loci. Opioids induce a neuroinflammatory response that opposes acute and long-term analgesia, thereby limiting their efficacy as therapeutic agents. Recent data suggest that the innate immune receptor Toll-like receptor 4 (TLR4), along with its coreceptor myeloid differentiation factor-2 (MD-2), mediates these effects. To date, the brain loci through which TLR4 modulates morphine tolerance have not been identified. We have previously demonstrated that chronic subcutaneous morphine results in tolerance that is accompanied by increases in vlPAG glial cell activity. Using in vivo pharmacological manipulations of vlPAG glia and TLR4 in the adult male rat, we show that intra-vlPAG administration of the general glial cell metabolic inhibitor propentofylline or the astrocyte activity inhibitor fluorocitrate attenuate tolerance to morphine. Characterization of MD-2 expression within the PAG revealed dense MD-2 expression throughout the vlPAG. Further, antagonizing vlPAG TLR4 dose dependently prevented the development of morphine tolerance, and vlPAG microinjections of TLR4 agonists dose dependently produced a "naive" tolerance to subsequent challenge doses of morphine. Finally, using a model of persistent inflammatory pain and pharmacological manipulation of TLR4 we demonstrate that systemic antagonism of TLR4 potentiated acute morphine antihyperalgesia. These results, together, indicate that vlPAG glia regulate morphine tolerance development via TLR4 signaling, and implicate TLR4 as a potential therapeutic target for the treatment of pain.

Endocannabinoids in the brainstem modulate dural trigeminovascular nociceptive traffic via CB1 and "triptan" receptors: implications in migraine

Activation and sensitization of trigeminovascular nociceptive pathways is believed to contribute to the neural substrate of the severe and throbbing nature of pain in migraine. Endocannabinoids, as well as being physiologically analgesic, are known to inhibit dural trigeminovascular nociceptive responses. They are also involved in the descending modulation of cutaneous-evoked C-fiber spinal nociceptive responses from the brainstem. The purpose of this study was to determine whether endocannabinoids are involved in the descending modulation of dural and/or cutaneous facial trigeminovascular nociceptive responses, from the brainstem ventrolateral periaqueductal gray (vlPAG). CB1 receptor activation in the vlPAG attenuated dural-evoked Aδ-fiber neurons (maximally by 19%) and basal spontaneous activity (maximally by 33%) in the rat trigeminocervical complex, but there was no effect on cutaneous facial receptive field responses. This inhibitory vlPAG-mediated modulation was inhibited by specific CB1 receptor antagonism, given via the vlPAG, and with a 5-HT1B/1D receptor antagonist, given either locally in the vlPAG or systemically. These findings demonstrate for the first time that brainstem endocannabinoids provide descending modulation of both basal trigeminovascular neuronal tone and Aδ-fiber dural-nociceptive responses, which differs from the way the brainstem modulates spinal nociceptive transmission. Furthermore, our data demonstrate a novel interaction between serotonergic and endocannabinoid systems in the processing of somatosensory nociceptive information, suggesting that some of the therapeutic action of triptans may be via endocannabinoid containing neurons in the vlPAG.

Alcohol effects on synaptic transmission in periaqueductal gray dopamine neurons

The role of dopamine (DA) signaling in regulating the rewarding properties of drugs, including alcohol, has been widely studied. The majority of these studies, however, have focused on the DA neurons located in the ventral tegmental area (VTA), and their projections to the nucleus accumbens. DA neurons within the ventral periaqueductal gray (vPAG) have been shown to regulate reward but little is known about the functional properties of these neurons, or how they are modified by drugs of abuse. This lack of knowledge is likely due to the highly heterogeneous cell composition of the vPAG, with both γ-aminobutyric acid (GABA) and glutamate neurons present in addition to DA neurons. In this study, we performed whole-cell recordings in a TH-eGFP transgenic mouse line to evaluate the properties of vPAG-DA neurons. Following this initial characterization, we examined how both acute and chronic alcohol exposure modify synaptic transmission onto vPAG-DA neurons. We found minimal effects of acute alcohol exposure on GABA transmission, but a robust enhancement of glutamatergic synaptic transmission in vPAG-DA. Consistent with this effect on excitatory transmission, we also found that alcohol caused an increase in firing rate. These data were in contrast to the effects of chronic intermittent alcohol exposure, which had no significant impact on either inhibitory or excitatory synaptic transmission on the vPAG-DA neurons. These data add to a growing body of literature that points to alcohol having both region-dependent and cell-type dependent effects on function.

Anxiogenic-like effect induced by TRPV1 receptor activation within the dorsal periaqueductal gray matter in mice

Pharmacological manipulation of TRPV1 (Transient Receptor Potential Vanilloid type-1) receptors has been emerging as a novel target in the investigation of anxiety states. Here, we attempt to show the role played by the TRPV1 receptors within the dorsal periaqueductal gray matter (dPAG), a midbrain structure strongly involved in the modulation of anxiety. Anxiety was assessed by recording spatiotemporal [percent open arm entries (%OE) and percent open arm time (%OT)] and ethological [e.g., head dipping (HD), stretched-attend postures (SAP)] measures in mice exposed to the elevated plus-maze (EPM). Mice received an intra-dPAG injection of the TRPV1 agonist capsaicin (0, 0.01, 0.1 or 1.0nmol/0.2μL; Experiment 1) or antagonist capsazepine (0, 10, 30 or 60nmol/0.2μL; Experiment 2), or combined injections of capsazepine (30nmol) and capsaicin (1.0nmol) (Experiment 3), and were exposed to the EPM to record spatiotemporal and ethological measures. While capsaicin produced an anxiogenic-like effect (it reduced %OE and %OT and frequency of SAP and HD in the open arms), capsazepine did not change any behavior in the EPM. However, when injected before capsaicin (1.0nmol), intra-dPAG capsazepine (30nmol-a dose devoid of intrinsic effects) antagonized completely the anxiogenic-like effect of the TRPV1 agonist. These results suggest that the anxiogenic-like effect produced by capsaicin is primarily due to TRPV1 activation within the dPAG in mice, but that dPAG TRPV1 receptors do not exert a tonic control over defensive behavior in mice exposed to the EPM.

Conditioned fear is modulated by CRF mechanisms in the periaqueductal gray columns

The periaqueductal gray (PAG) columns have been implicated in controlling stress responses through corticotropin-releasing factor (CRF), which is a neuropeptide with a prominent role in the etiology of fear- and anxiety-related psychopathologies. Several studies have investigated the involvement of dorsal PAG (dPAG) CRF mechanisms in models of unconditioned fear. However, less is known about the role of this neurotransmission in the expression of conditioned fear memories in the dPAG and ventrolateral PAG (vlPAG) columns. We assessed the effects of ovine CRF (oCRF 0.25 and 1.0 μg/0.2 μL) locally administered into the dPAG and vlPAG on behavioral (fear-potentiated startle and freezing) and autonomic (arterial pressure and heart rate) responses in rats subjected to contextual fear conditioning. The lower dose injected into the columns promoted proaversive effects, enhanced contextual freezing, increased the blood pressure and heart rate and decreased tail temperature. The lower dose of oCRF into the vlPAG, but not into the dPAG, produced a pronounced enhancement of the fear-potentiated startle response. The results imply that the PAG is a heterogeneous structure that is involved in the coordination of distinct behaviors and autonomic control, suggest PAG involvement in the expression of contextual fear memory as well as implicate the CRF as an important modulator of the neural substrates of fear in the PAG.

Evidence for the involvement of descending pain-inhibitory mechanisms in the antinociceptive effect of hecogenin acetate

Hecogenin is a sapogenin present in the leaves of species from the Agave genus, with a wide spectrum of reported pharmacological activities. The present study was undertaken to evaluate whether hecogenin acetate (1) has antinociceptive properties and to determine its mechanism of action. The nociceptive threshold was evaluated using the tail flick test in mice. Mice motor performance was evaluated in a Rotarod test. By using Fos expression as a marker of neural activation, the involvement of the periaqueductal gray in 1-induced antinociception was evaluated. Intraperitoneal administration of 1 (5-40 mg/kg) produced a dose-dependent increase in the tail flick latency time compared to vehicle-treated group (p < 0.01). Mice treated with 1 (40 mg/kg) did not show motor performance alterations. The antinociception of 1 (40 mg/kg) was prevented by naloxone (nonselective opioid receptor antagonist; 5 mg/kg), CTOP (μ-opioid receptor antagonist; 1 mg/kg), nor-BNI (κ-opioid receptor antagonist; 0.5 mg/kg), naltrindole (δ-opioid receptor antagonist; 3 mg/kg), or glibenclamide (ATP-sensitive K(+) channel blocker; 2 mg/kg). Systemic administration of 1 (5-40 mg/kg) increased the number of Fos positive cells in the periaqueductal gray. The present study has demonstrated for the first time that 1 produces consistent antinociception mediated by opioid receptors and endogenous analgesic mechanisms.

Periaqueductal gray neuroplasticity following chronic morphine varies with age: role of oxidative stress

The development of tolerance to the antinociceptive effects of morphine has been associated with networks within ventrolateral periaqueductal gray (vlPAG) and separately, nitric oxide signaling. Furthermore, it is known that the mechanisms that underlie tolerance differ with age. In this study, we used a rat model of antinociceptive tolerance to morphine at two ages, postnatal day (PD) 7 and adult, to determine if changes in the vlPAG related to nitric oxide signaling produced by chronic morphine exposure were age-dependent. Three pharmacological groups were analyzed: control, acute morphine, and chronic morphine group. Either morphine (10mg/kg) or equal volume of normal saline was given subcutaneously twice daily for 6½ days. Animals were analyzed for morphine dose-response using Hot Plate test. The expression of several genes associated with nitric oxide metabolism was evaluated using rtPCR. In addition, the effect of morphine exposure on immunohistochemistry for Fos, and nNOS as well as nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) reaction at the vlPAG were measured. In both age groups acute morphine activated Fos in the vlPAG, and this effect was attenuated by chronic morphine, specifically in the vlPAG at the level of the laterodorsal tegmental nucleus (LDTg). In adults, but not PD7 rats, chronic morphine administration was associated with activation of nitric oxide function. In contrast, changes in the gene expression of PD7 rats suggested superoxide and peroxide metabolisms may be engaged. These data indicate that there is supraspinal neuroplasticity following morphine administration as early as PD7. Furthermore, oxidative stress pathways associated with chronic morphine exposure appear age-specific.