Nociceptin/orphanin FQ acts as a functional antagonist of corticotropin-releasing factor to inhibit its anorectic effect

Nociceptin/orphanin FQ (N/OFQ), the endogenous ligand of the NOP opioid receptor (previously referred to as ORL1 or OP4 receptor), exerts a variety of behavioral effects. N/OFQ as well as the synthetic NOP receptor agonist Ro 64-6198 have been reported to possess antistress properties and to elicit a pronounced hyperphagic effect in freely feeding rats. These findings have raised our interest to investigate possible interactions in the control of ingestive behavior between N/OFQ and corticotropin-releasing factor (CRF), which is well known to be a major mediator of stress and to possess anorectic properties. These studies have shown that intracerebroventricular injections of N/OFQ or of Ro 64-6198 reverse the anorectic action evoked by intracerebroventricular administration of CRF. The anti-anorectic effect of N/OFQ or Ro 64-6198 is antagonized by the selective NOP receptor antagonist [Nphe1]N/OFQ1-13NH2, providing evidence that it is mediated by this receptor. The effect occurs at doses that are not hyperphagic per se and is clearly selective versus the anorectic action of CRF since N/OFQ or Ro 64-6198 do not influence the anorectic effect of Escherichia coli lipopolysaccharide (LPS). Neither N/OFQ nor Ro 64-6198 shows affinity for CRF receptors, suggesting that NOP receptor agonists might act as functional antagonists of CRF with regard to its anorectic action. Microinjection studies have revealed that the bed nucleus of the stria terminalis (BNST) is highly sensitive to the anorectic action of CRF, as well as to the anti-anorectic action of N/OFQ; pretreatment with 0.025-0.25 microg/site of N/OFQ into the BNST blocked the anorectic action of 0.1 microg/site of CRF given in the same area. On the other hand, intra-BNST microinjection of 0.025-0.25 microg/site of N/OFQ did not modify basal food intake. Thus, the BNST may be the site where the functional antagonism between N/OFQ and CRF takes place. These findings raise interest for the N/OFQ-NOP receptor system as a pharmacological target to block the anorectic effect of CRF. In comparison to CRF receptor antagonists, NOP receptor agonists may have the advantage of not inhibiting the hypothalamic-pituitary-adrenal (HPA) axis.     

The nociceptin receptor antagonist [Nphe1,Arg14,Lys15]nociceptin/orphanin FQ-NH2 blocks the stimulatory effects of nociceptin/orphanin FQ on the HPA axis in rats

Nociceptin/orphanin FQ (N/OFQ) is an opioid-related peptide that stimulates corticosterone release after i.c.v. administration in non-stressed rats. We employed in situ hybridization histochemistry to investigate N/OFQ-stimulated activation of the HPA axis at the hypothalamic and pituitary level. We have demonstrated that N/OFQ-induced activation of the HPA axis is mediated via the central N/OFQ peptide receptor (NOP) using the recently described selective NOP antagonist [Nphe(1),Arg(14),Lys(15)]nociceptin/orphanin FQ-NH(2) (UFP-101).We found that, at 30 min post-i.c.v. injection, N/OFQ dose-dependently increased plasma adrenocorticotrophin hormone and corticosterone compared with the vehicle-injected controls. N/OFQ (1.0 microg) significantly increased CRF mRNA but not AVP mRNA within the parvocellular hypothalamic paraventricular nucleus compared with the control group, and significantly increased pro-opiomelanocortin (POMC) mRNA in the anterior pituitary. While UFP-101 (1.0 microg) alone had no significant effect on plasma corticosterone concentration it blocked the effect of N/OFQ (1.0 microg) on plasma corticosterone levels when compared with N/OFQ administered alone. UFP-101 also blocked the N/OFQ-induced increase in CRF mRNA and POMC mRNA. These results demonstrate that centrally administered N/OFQ activates the HPA axis via up-regulation of CRF and POMC mRNA and stimulation of corticosterone release in rats. Further, we have demonstrated for the first time that the selective NOP receptor antagonist UFP-101 blocks these effects indicating that N/OFQ-induced HPA axis activation is mediated via central NOP receptors.     

Feeding and locomotion responses to centrally injected nociceptin/orphanin FQ in chicks

Nociceptin/orphanin FQ (N/OFQ), the endogenous ligand of the opioid receptor-like receptor or nociceptin receptor (NOP), has been shown to induce feeding, locomotion, anti-stress and anxiolytic effects in rodents after central nervous system injection. In this study, the effect of intracerebroventricular (icv) injection of N/OFQ on feeding and locomotion behavior was evaluated in male broiler-type chickens. The icv injection of N/OFQ caused a moderate but significant increase in feed intake similar to the classical opioid peptides in rats. It also increased feed pecking frequency and feeding time 1 h after injection. Stepping, wing flapping and preening were not affected by N/OFQ. These results suggest that N/OFQ can act within the central nervous system of chickens to increase feed intake.     

Interaction between nociceptin/orphanin FQ (N/OFQ) and GABA in response to feeding

Nociceptin/orphanin FQ (N/OFQ) and gamma aminobutyric acid (GABA) agonists have been shown to increase feed intake in mammals and birds. In this study, the effect of intracerebroventricular (icv) injection of the potent NOP receptor agonists Nociceptin (1-13) NH(2), the GABA(A) receptor antagonist bicuculline, and the GABA(A) receptor agonist muscimol on feed intake in cockerels was investigated. The icv injection of N/OFQ and muscimol increases feed intake. The effect of N/OFQ on feed intake was strongly blocked by the injection of bicuculline whereas the effect of muscimol was stimulated by N/OFQ. These results suggest that N/OFQ may act at GABA(A) receptors or increases overflow of GABA in the brain of chickens to stimulate feeding.     

Bradycardia elicited by microinjections of nociceptin/orphanin FQ into the intermediolateral cell column at T1-T2 in the rat

Microinjections (30 nl) of nociceptin/orphanin FQ (N/OFQ) into the intermediolateral cell column (IML) at T1 and T2 levels of the spinal cord elicited bradycardia. The decreases in HR were 12.3+/-2.9, 17.3+/-2.7, 26.7+/-3.1, and 18.6+/-3.4 beats/min in response to 0.075, 0.15, 0.62, and 1.25 mM concentrations, respectively. Maximally effective concentration of N/OFQ was 0.62 mM. No changes in BP were elicited by microinjections of N/OFQ into the IML at T1-T2. The bradycardic responses were completely blocked by prior microinjections of a N/OFQ receptor (NOP receptor) antagonist ([N-phe(1)]-nociceptin-(1-13)-NH(2), 9 mM) into the IML at T1-T2. Blockade of myocardial beta-1 adrenergic receptors also abolished the bradycardic responses elicited by microinjections of N/OFQ into the IML. It was concluded that activation of NOP receptors in right IML at T1-T2 by N/OFQ elicited bradycardic responses which were mediated via the sympathetic nervous system.     

Endogenous nociceptin/orphanin FQ (N/OFQ) contributes to haloperidol-induced changes of nigral amino acid transmission and parkinsonism: a combined microdialysis and behavioral study in naïve and nociceptin/orphanin FQ receptor knockout mice

The contribution of endogenous nociceptin/orphanin FQ (N/OFQ) to neuroleptic-induced parkinsonism has been evaluated in haloperidol-treated mice. Pharmacological blockade of N/OFQ receptors (NOP) via systemic administration of 1-[(3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H benzimidazol-2-one (J-113397, 0.01–10 mg/kg i.p.) or central injection of [Nphe¹,Arg¹⁴,Lys¹⁵]N/OFQ-NH₂ (UFP-101, 10 nmol i.c.v.) attenuated (0.8 mg/kg) haloperidol-induced motor deficits as evaluated by a battery of behavioral tests providing complementary information on motor parameters: the bar, drag and rotarod tests. A combined neurochemical and behavioral approach was then used to investigate whether the substantia nigra reticulata could be involved in antiakinetic actions of J-113397. Microdialysis combined to the bar test revealed that haloperidol (0.3 and 0.8 mg/kg i.p.) caused a dose-dependent and prolonged elevation of immobility time (i.e. akinesia) which was associated with an increase in nigral glutamate and a reduction in GABA release. Conversely, J-113397 (1 mg/kg) alone reduced glutamate and elevated nigral GABA release, and when challenged against haloperidol, counteracted its behavioral and neurochemical effects. Microdialysis coupled to behavioral testing also demonstrated that NOP receptor knockout mice were resistant to haloperidol (0.3 mg/kg) compared to wild-type mice, lack of response being associated with a reversal of glutamate release facilitation into inhibition and no change in nigral GABA release. This study provides pharmacological and genetic evidence that endogenous N/OFQ contributes to haloperidol-induced akinesia and changes of amino acid transmission in mice. Moreover, it confirms the view that NOP receptor antagonists are capable of reversing akinesia across species and genotypes and may prove effective in relieving neuroleptic-induced parkinsonism.     

Autoradiographic localization of (125)i[Tyr(14)] nociceptin/orphanin FQ binding sites in macaque primate CNS

Nociceptin/orphanin FQ (N/OFQ) is a recently identified neuropeptide that has been implicated in a multitude of CNS functions. These include nociception, feeding, cognition, locomotion, stress and neuroendocrine control. The endogenous receptor for this ligand is the nociceptin/orphanin FQ peptide (NOP) receptor. The distribution of NOP in rodent has been widely reported by the use of in situ hybridization, immunohistochemistry and autoradiographic radioligand binding but less is known of its localization in higher species. We have therefore sought to optimize and determine the distribution of (125)I[Tyr(14)]N/OFQ binding sites in macaque primate brain and spinal cord. Highest levels of binding were observed in neocortical areas, hippocampus, amygdala, caudate nucleus and putamen, medial thalamic nuclei and superficial laminae of the superior colliculus. These novel data present for the first time, the distribution of N/OFQ receptors in non-human primate CNS and, by comparison with localization in the rat, reveal that species differences may exist in the distribution of this neuropeptide receptor. These data have important implications regarding the roles of N/OFQ across species and may have ramifications in the interpretation of preclinical pharmacological studies.     

Role of amygdaloid nuclei in the anxiolytic-like effect of nociceptin/orphanin FQ in rats

We investigated the mechanism underlying the anxiolytic actions of the neuropeptide nociceptin/orphanin FQ (N/OFQ) using the elevated plus-maze test and T-maze test. Microinfusions of N/OFQ (10 or 32pmol) into the central amygdala (ACE) increased the time spent in the open arms of the elevated plus-maze (anxiolytic-like effects), whereas microinfusions of N/OFQ (10, 32 or 100 pmol) into the basolateral amygdala (ABL) did not affect the time spent in the open arms. Moreover, microinfusions of N/OFQ (32 pmol) into the ACE impaired escape performance from the open arms of the elevated T-maze (anxiolytic-like effects), but did not change inhibitory avoidance of the open arms. A non-peptidyl N/OFQ receptor (NOP) antagonist, J-113397(1-[(3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H-benzimidazol-2-one) (10 mg/kg, s.c.), blocked the anxiolytic-like effects induced by N/OFQ. These results indicate that the anxiolytic-like effects of N/OFQ might be due to impaired escape performance from the open arms and it implicates the N/OFQ system within the ACE in the mediation of panic action.     

The role of the opioid receptor-like (ORL1) receptor in motor stimulatory and rewarding actions of buprenorphine and morphine

We have previously shown that the ability of buprenorphine to activate the opioid receptor-like (ORL1) receptor compromises its antinociceptive effect. Furthermore, morphine has been shown to alter the level of orphanin FQ/nociceptin (OFQ/N), the endogenous ligand of the ORL1 receptor, raising the possibility that the endogenous OFQ/N/ORL1 receptor system may be involved in the actions of these opioids. Thus, using mice lacking the ORL1 receptor and their wild-type littermates, the present study assessed the role of the ORL1 receptor in psychomotor stimulant and rewarding actions of buprenorphine and morphine. Morphine (5, 10 mg/kg) dose-dependently increased motor activity and induced conditioned place preference. However, the magnitude of each response was comparable for the mutant mice and their wild-type littermates. In contrast, buprenorphine (1 mg/kg) induced greater motor stimulation in ORL1 receptor knockout mice as compared with their wild-type littermates. Further, single conditioning with buprenorphine (3 mg/kg) induced place preference in mutant mice but not in their wild-type littermates. The results of binding assay showed that buprenorphine concentration-dependently (0-1000 nM) displaced specific binding of [(3)H]-OFQ/N in brain membrane of wild-type mice. Together, the present results suggest that the ability of buprenorphine to interact with the ORL1 receptor modulates its acute motor stimulatory and rewarding effects.     

Effects of nociceptin/orphanin FQ in the acquisition of contextual and tone fear conditioning in rats

Nociceptin, or orphanin FQ (N/OFQ), the endogenous ligand of NOP receptors, is known to regulate learning and memory processes. To verify the role of N/OFQ in the acquisition of contextual (CFC) and tone fear conditioning (TFC), Wistar male rats received intracerebroventricular injections of N/OFQ (0.1-5.0 nmol) before training, and were tested 24 and 48 hr later to access the freezing response to context and tone, respectively. The intermediate doses (1.0 and 2.5 nmol) impaired the CFC test, sparing TFC. The highest dose (5.0 nmol) reduced freezing during both tests, a result that may be due to nonspecific effects. The posttraining injection of N/OFQ (1 or 5 nmol) did not interfere with CFC and TFC, suggesting a specific effect of the peptide in acquisition processes. Moreover, the impairment observed with N/OFQ (1 nmol) in CFC cannot be attributed to a state-dependent learning because it was not reversed by its pretest administration. The data support the negative role of N/OFQ in the acquisition of aversively motivated tasks, which encompass a spatial component and depend on the hippocampus.     

GIRK channel-mediated inhibition of melanin-concentrating hormone neurons by nociceptin/orphanin FQ

Targeting the melanin-concentrating hormone (MCH) system has been suggested as a potential treatment for obesity, anxiety disorders, as well as addiction. Despite the therapeutic potential of MCH agonists and antagonists, the endogenous factors regulating MCH activity, in particular those implicated in anxiety and reward, are ill-defined. The present study investigated the cellular effects of nociceptin/orphanin FQ (N/OFQ), an endogenous opioid with anxiolytic and antireward properties, on MCH neurons. We found that N/OFQ induced a concentration-dependent reversible outward current in MCH neurons (EC(50) = 50.7 nM), an effect that was blocked by the competitive antagonist of the nociceptin opioid peptide (NOP) receptor UFP-101. N/OFQ-induced outward currents persisted in TTX, reversed near the potassium equilibrium potential, and displayed inward rectification, suggesting direct postsynaptic potassium channel activation. Tertiapin-Q completely abolished the N/OFQ effect, whereas glibenclamide did not, implicating protein G-dependent inwardly rectifying potassium (GIRK) and not ATP-sensitive potassium (K(ATP)) channels as the effector ion channel. The N/OFQ-induced outward current desensitized during repeated applications and occluded the inhibitory effect of dynorphin, suggesting that dynorphin and N/OFQ activate the same pathway. N/OFQ also reversibly inhibited voltage-gated calcium currents in MCH neurons. In conclusion, our study indicates N/OFQ as a robust endogenous regulator of MCH neurons, which may play a role in anxiety and drug addiction.     

吗啡诱导的CPP复燃大鼠前额叶皮层和海马区EAAT3蛋白表达的变化

目的: 观察吗啡诱导的条件性位置偏爱(CPP)复燃大鼠前额叶皮层和海马区兴奋性氨基酸转运蛋白3(EAAT3)的表达变化,探讨前脑皮层及海马区EAAT3在阿片类药物复吸过程中的作用。方法: 成年雄性SD大鼠40只,随机分为对照组(control)、CPP建立(Es)、消退(Ex)、复燃2 h(Re2)、复燃4 h(Re4)组,每组8只。腹腔注射吗啡(10 mg/kg)连续10 d,建立CPP模型;停止给药使CPP逐渐消退;单次腹腔注射吗啡 2.5 mg/kg诱导已消退的CPP复燃。Western blotting检测各组大鼠前额叶皮层和海马区EAAT3蛋白表达变化。 结果: (1)腹腔注射恒定剂量的吗啡10 mg/kg, Es组大鼠在伴药箱的停留时间比control组明显延长(P<0.05),成功建立CPP模型;待CPP消退后,吗啡2.5 mg/kg腹腔注射诱发Re2和Re4组大鼠在伴药箱的停留时间再次比control组明显延长(P<0.05),CPP复燃。(2)Es组前额叶皮层EAAT3比control组表达减少(P<0.05),CPP消退的Ex组表达回升,在Re4组表达再次减少(P<0.05)。(3)海马区EAAT3在各组表达水平未见明显变化(P>0.05);而Es、Ex组海马CA1区EAAT3比control组表达明显升高(P<0.05)。 结论: 吗啡诱导CPP复燃时,前额叶皮层EAAT3的蛋白表达水平降低,重现CPP建立时的变化,提示阿片类药物复吸行为的形成可能与前脑皮层EAAT3的表达减少有关。     

A 24-h access I.V. self-administration schedule of morphine reinforcement and the estimation of recidivism: Pharmacological modification by arecoline

Central cholinergic neurons are known to play a role in the pharmacological actions of opiates. The purpose of the study was to determine whether the muscarinic receptor agonist arecoline, administered during morphine self-administration, would mitigate the subsequent return to self-administration behavior. Rats self-administered increasing concentrations of morphine in operant chambers according to a schedule that permitted unlimited access to lever-activated i.v. infusions on a continuous 24 h basis from 10 to 14 days. Abstinence was induced by discontinuation of the morphine solution and mild withdrawal symptoms were evident from 14 to 74 h. Thereafter the rats remained in their home cages for a 6-week period of protracted abstinence. They were then returned to the operant chambers where lever responding had no reward consequence. The cholinergic muscarinic agonist arecoline was administered twice daily (0.25 or 1 mg/kg, s.c.) throughout the self-administration schedule of morphine. Arecoline treatment partly decreased the self-administration of morphine, it prevented the abstinence-induced decrease in body weight, and it reduced lever responding after protracted withdrawal (by 56%). In animals already dependent on morphine, arecoline failed to alter ongoing self-administration behavior, but responding induced by lever reinstatement 6 weeks after withdrawal was significantly reduced (by 33%). There was a significant relationship between the degree of self-administration activity and the degree of lever responding during reinstatement after protracted abstinence. The results of this study support the role of cholinergic systems in self-administration behavior and context-induced post-withdrawal drug seeking.     

Orphanin FQ and glutamate connection in the regulation of gonadotropin-releasing hormone secretion in the preoptic area of conscious male rats

Utilizing the method of push-pull perfusion and radioimmunoassay (RIA), the secretory profile of gonadotropin-releasing hormone (GnRH) in the preoptic area (POA) and serum-luteinizing hormone (LH) levels were examined in conscious male rats after administration of [Nphe(1)]NC(1-13)NH(2), a competitive antagonists of the opioid receptor-like 1 receptor (ORL1 receptor) which is endogenous receptor for Orphanin FQ (OFQ). Glutamate release in the POA was also measured by high-performance liquid chromatography (HPLC) after perfusion of [Nphe(1)]NC(1-13)NH(2), i.e. NC13. The results showed that GnRH secretion from the POA and serum LH levels was increased significantly 40 min and 60 min, respectively after perfusion of 2 and 20 mmol/L NC13 in freely moving male rats (p<0.05). Pretreatment with a glutamate, N-methyl-D-aspartate (NMDA) receptor antagonist (MK-801, s.c., 0.2 mg/kg) abolished the increase of GnRH release in the POA induced by 2 mmol/L NC13. Additionally, 20 mmol/L NC13 significantly enhanced glutamate release in the POA at 40 min post-perfusion in a dose-dependent manner. These findings suggest that hypothalamic OFQ/ORL1 receptor system plays a role in the physiological inhibitory control of GnRH secretion in the POA of male rats, and provide evidence for involvement of an OFQ and glutamate pathway in the control of GnRH secretion.     

The nociceptin/orphanin FQ antagonist UFP-101 differentially modulates the glucocorticoid response to restraint stress in rats during the peak and nadir phases of the hypothalamo-pituitary-adrenal axis circadian rhythm

The involvement of nociceptin (N/OFQ) and the nociceptin/orphanin FQ peptide (NOP) receptor in behavior associated with stress and anxiety has been established but their role in the regulation of the hypothalamo-pituitary-adrenal (HPA) axis under conditions of stress has not been fully investigated. We used the selective NOP receptor antagonist UFP-101 to examine the contribution of endogenous N/OFQ to HPA axis control under conditions of restraint stress in the morning and the evening. We found that in the morning during the HPA axis circadian nadir rats exposed to restraint stress in both the presence and absence of UFP-101 exhibited significantly elevated plasma corticosterone at 30 min post-i.c.v. injection compared to the home cage control group. Additionally, rats treated with UFP-101 and exposed to restraint had significantly elevated corticosterone levels at 60 min post-i.c.v. injection compared to all other treatment groups. Interestingly, while there was a significant increase in the expression of CRF mRNA in the paraventricular nucleus (PVN) of rats exposed to restraint stress only, there was no comparable increase in those co-treated with UFP-101. There was no change in the expression of AVP or POMC mRNA in any of the treatment groups. In contrast, when carried out in the evening we observed significantly elevated plasma corticosterone in the vehicle-treated restraint group only at 30 min post-i.c.v. injection. There was no significant difference between the UFP-101-treated restraint group and either of the home cage control groups or the vehicle-treated restraint group. Additionally, in contrast to the morning study, UFP-101 did not prolong glucocorticoid release at the 60 min time-point. These results demonstrate for the first time a differential effect of UFP-101 on restraint stress-induced HPA axis activity characterized by significant prolongation of stress-induced activity in the morning but no significant effect on the response to restraint in the evening.     

Spinal analgesic activity of orphanin FQ/nociceptin and its fragments

Previous work reveals that orphanin FQ/nociceptin (OFQ/N) administered supraspinally produces an initial hyperalgesic response followed by analgesia. Spinally, OFQ/N elicits a rapidly appearing, naltrexone-reversible, dose-dependent analgesia in the tailflick assay without any indication of hyperalgesia. Two OFQ/N fragments, OFQ/N (1–7) and OFQ/N (1–11), are active, but far weaker. Blockade of sigma receptors with haloperidol enhances the analgesic potency of spinal OFQ/N, OFQ/N (1–7) and OFQ/N (1–11), but not as dramatically as supraspinal OFQ. Antisense probes targeting the second and third coding exons, but not the first exon, of the cloned mouse OFQ/N receptor (KOR-3) partially block OFQ/N analgesia.     

Manipulation of dopamine d1-like receptor activation in the rat medial prefrontal cortex alters stress- and cocaine-induced reinstatement of conditioned place preference behavior

These studies examined the ability of the dopamine D1-like agonist SKF 81297 and D1-like antagonist SCH 23390 in the medial prefrontal cortex to alter the reinstatement of cocaine-induced conditioned place preference behavior. Male Sprague-Dawley rats were fitted with bilateral cannulae over the medial prefrontal cortex and subsequently trained in a conditioned place preference task. Animals were trained in this task using four pairings of cocaine (12 mg/kg, i.p.). Conditioned place preference was demonstrated in all animals, and this behavior was then extinguished over a 5-10-day period before testing for reinstatement. Just prior to reinstatement by immobilization stress or a cocaine priming injection (5 mg/kg, i.p.), a microinjection of the D1-like receptor antagonist SCH 23390 (0.01, 0.1 or 1.0 microg/side), or the D1-like receptor agonist SKF 81297 (0.1, 0.3 or 1.0 microg/side) was given into the medial prefrontal cortex. SCH 23390 blocked both stress- and cocaine-induced reinstatement of conditioned place preference after the two higher doses were administered into the medial prefrontal cortex. The highest dose of SKF 81297 (1.0 microg/side) prevented immobilization stress- but not cocaine-induced reinstatement. The highest dose of these drugs given in the absence of stress or cocaine did not produce reinstatement. The results indicate that immobilization stress given within the place-preference chamber is capable of producing reinstatement of cocaine-seeking behavior. The microinjection studies suggest that D1-like receptor antagonism within the prefrontal cortex is sufficient to block reinstatement by stress and cocaine. Furthermore, the results from D1-like receptor activation in the medial prefrontal cortex point to utilization of different neural pathways for stress- and cocaine-induced reinstatement.     

Injection of orphanin FQ/nociceptin into the periaqueductal gray suppresses the forebrain-elicited vocalization in the guinea pig

Electrical stimulation of the caudoventral part of the anterior cingulate cortex (AC) evokes a species-specific vocalization in the guinea pig. After injections of glutamate receptor antagonists into the periaqueductal gray (PAG) regions where the AC projected, the forebrain-elicited vocalization (FEV) was suppressed completely. This result indicated that the FEV was induced at least partly by glutamatergic activation of PAG neurons. Injection of orphanin FQ/nociceptin (OFQ/N) into the PAG profoundly suppressed the FEV. This inhibitory effect of OFQ/N was not antagonized by naloxone, an opioid receptor antagonist. Since OFQ/N was reported to modify the glutamatergic transmission, the present results suggest that OFQ/N exerts influences upon the vocalization by regulating the glutamatergic inputs into the PAG.     

The ethanol self-administration context as a reinstatement cue: acute effects of naltrexone

A major difficulty in treating alcohol addiction is the high rate of relapse even after prolonged abstinence. Relapse can be triggered by several factors, including stress, re-exposure to the drug, conditioned discrete stimuli and exposure to the context in which alcohol consumption occurred. The present study investigated the role played by the environmental context on ethanol relapse using an extinction/reinstatement animal model: rats were trained to self-administer ethanol in a distinctive context, and extinction occurred in a setting that differed by visual, tactile and olfactory properties; reinstatement was tested by placing the animals into the ethanol-associated context in the absence of ethanol. We found that re-exposure to the ethanol-associated context significantly increased responses on the ethanol-paired lever. The increase in responding required the presence of the complete configuration of the multimodal context. The non-selective opioid receptor antagonist naltrexone (0.3 mg/kg) administered 20 min prior to the reinstatement test significantly attenuated context-induced reinstatement of lever press responding, compared with saline-treated subjects. These data indicate that the environmental context associated with ethanol availability influences ethanol-seeking behavior in the rat, and that endogenous opioids are involved in this process. Our findings are in accordance with clinical reports demonstrating naltrexone efficacy in the treatment of alcohol relapse in humans, and indicate that the context-induced reinstatement model described here may be useful to investigate the biological mechanisms underlying alcohol relapse.