A Paranigral VTA Nociceptin Circuit that Constrains Motivation for Reward

Nociceptin and its receptor are widely distributed throughout the brain in regions associated with reward behavior, yet how and when they act is unknown. Here, we dissected the role of a nociceptin peptide circuit in reward seeking. We generated a prepronociceptin (Pnoc)-Cre mouse line that revealed a unique subpopulation of paranigral ventral tegmental area (pnVTA) neurons enriched in prepronociceptin. Fiber photometry recordings during progressive ratio operant behavior revealed pnVTAPnoc neurons become most active when mice stop seeking natural rewards. Selective pnVTAPnoc neuron ablation, inhibition, and conditional VTA nociceptin receptor (NOPR) deletion increased operant responding, revealing that the pnVTAPnoc nucleus and VTA NOPR signaling are necessary for regulating reward motivation. Additionally, optogenetic and chemogenetic activation of this pnVTAPnoc nucleus caused avoidance and decreased motivation for rewards. These findings provide insight into neuromodulatory circuits that regulate motivated behaviors through identification of a previously unknown neuropeptide-containing pnVTA nucleus that limits motivation for rewards.

Nociceptin and the NOP receptor in aversive learning in mice

The endogenous neuropeptide nociceptin (N/OFQ), which mediates its actions via the nociceptin receptor (NOP), is implicated in multiple behavioural and physiological functions. This study examined the effects of the NOP agonists N/OFQ and the synthetic agonist Ro 64-6198, the antagonists NNN and NalBzoH, as well as deletion of the Pronociceptin gene on emotional memory in mice. The animals were tested in the passive avoidance (PA) task, dependent on hippocampal and amygdala functions. N/OFQ injected intraventricularly (i.c.v.) prior to training produced a biphasic effect on PA retention; facilitation at a low dose and impairment at higher doses. Ro 64-6198 also displayed a biphasic effect with memory facilitation at lower doses and impairment at a high dose. None of the agonists influenced PA training latencies. NNN did not significantly modulate retention in the PA task but antagonized the inhibitory effects of N/OFQ. NalBzoH facilitated memory retention in a dose-dependent manner and blocked the impairing effects of N/OFQ. However, neither NNN nor NalBzoH blocked the memory-impairing effects of Ro 64-6198. Finally, the Pnoc knockout mice exhibited enhanced PA retention latencies compared to the wild type mice. The biphasic effect of the natural ligand and Ro 64-6198 and the failure of the antagonists to block the action of Ro 64-6198 indicate complexity in ligand-receptor interaction. These results indicate that brain nociceptin and its NOP has a subtle role in regulation of mechanisms of relevance for treatment of disorders with processing disturbances of aversive events e.g. Alzheimer's disease, anxiety, depression and PTSD.

Novel role of the nociceptin system as a regulator of glutamate transporter expression in developing astrocytes

Our previous results showed that oligodendrocyte development is regulated by both nociceptin and its G-protein coupled receptor, the nociceptin/orphanin FQ receptor (NOR). The present in vitro and in vivo findings show that nociceptin plays a crucial conserved role regulating the levels of the glutamate/aspartate transporter GLAST/EAAT1 in both human and rodent brain astrocytes. This nociceptin-mediated response takes place during a critical developmental window that coincides with the early stages of astrocyte maturation. GLAST/EAAT1 upregulation by nociceptin is mediated by NOR and the downstream participation of a complex signaling cascade that involves the interaction of several kinase systems, including PI-3K/AKT, mTOR, and JAK. Because GLAST is the main glutamate transporter during brain maturation, these novel findings suggest that nociceptin plays a crucial role in regulating the function of early astrocytes and their capacity to support glutamate homeostasis in the developing brain.

The anxiolytic-like profile of the nociceptin receptor agonist, endo-8-[bis(2-chlorophenyl)methyl]-3-phenyl-8-azabicyclo[3.2.1]octane-3-carboxamide (SCH 655842): comparison of efficacy and side effects across rodent species

The endogenous opioid-like peptide, nociceptin, produces anxiolytic-like effects that are mediated via the nociceptin (NOP) receptor. Similarly, synthetic, non-peptide NOP agonists produce robust anxiolytic-like effects although these effects are limited by marked side effects. In the present studies, the effects of a novel NOP receptor agonist, SCH 655842, were examined in rodent models sensitive to anxiolytic drugs and tests measuring potential adverse affects. Oral administration of SCH 655842 produced robust, anxiolytic-like effects in three species, i.e., rat, guinea pig, and mouse. Specifically, SCH 655842 was effective in rat conditioned lick suppression (3-10 mg/kg) and fear-potentiated startle (3-10 mg/kg) tests, a guinea pig pup vocalization test (1-3 mg/kg), as well as in mouse Geller-Seifter (30 mg/kg) and marble burying (30 mg/kg) tests. The anxiolytic-like effect of SCH 655842 in the conditioned lick suppression test was attenuated by the NOP antagonist, J-113397. In mice, SCH 655842 reduced locomotor activity and body temperature at doses similar to the anxiolytic-like dose and these effects were absent in NOP receptor knockout mice. In rats, SCH 655842 did not produce adverse behavioral effects up to doses of 70-100 mg/kg. Pharmacokinetic studies in the rat confirmed dose-related increases in plasma and brain levels of SCH 655842 across a wide oral dose range. Taken together, SCH 655842 may represent a NOP receptor agonist with improved tolerability compared to other members of this class although further studies are necessary to establish whether this extends to higher species.

The role of the NOP receptor in regulating food intake, meal pattern, and the excitability of proopiomelanocortin neurons

We evaluated the role of the nociceptin/orphanin FQ (NOP) receptor in regulating food intake, meal pattern and the activity of hypothalamic arcuate (ARC) neurons. The microstructural analysis of food intake and meal pattern was performed under both food-deprived and ad libitum conditions. Whole-cell patch clamp recordings were obtained using the in vitro hypothalamic slice preparation and biocytin-filled electrodes. NOP receptor knockout mice exhibited significantly reduced body weight. Fasting-induced hyperphagia was diminished for the first 2h of a 6-h re-feeding period, and was associated with decreased meal duration and size, as well as a biphasic effect on meal frequency. The genotype effect observed under ad libitum conditions was comparatively unremarkable. Orphanin FQ/nociceptin (OFQ/N) was able to decrease evoked excitatory postsynaptic current amplitude, increase the S(2):S(1) ratio via the paired-pulse paradigm, and decrease miniature excitatory postsynaptic current frequency in ARC neurons from wild type animals but not NOP receptor knockouts. In addition OFQ/N activated a reversible outward current that was antagonized by the G-protein activated, inwardly-rectifying K(+) (GIRK) channel blocker tertiapin in wild type but not NOP knockout animals. Both the presynaptic and postsynaptic actions of OFQ/N were observed in ARC neurons subsequently determined to be immunopositive for characteristic phenotypic markers of anorexigenic proopiomelanocortin (POMC) neurons. Taken together, these results demonstrate the contribution of the NOP receptor in controlling food intake and meal pattern, as well as glutamate release and GIRK1 channel activity at POMC synapses.

MPTP treatment increases expression of pre-pro-nociceptin/orphanin FQ mRNA in a subset of substantia nigra reticulata neurons

Antagonists selectively inhibiting activation of the nociceptin/orphanin FQ (N/OFQ) receptor reduce motor symptoms in experimental models of Parkinson's disease, and genetic deletion of the ppN/OFQ gene offers partial protection of mid-brain dopamine neurons against the neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). MPTP increased ppN/OFQ mRNA expression in the substantia nigra (SN). We have evaluated the temporal relationship of dopamine cell loss to increased ppN/OFQ mRNA expression in the substantia nigra after MPTP treatment, and characterized the cellular locations in which increased ppN/OFQ mRNA expression was observed after MPTP treatment. MPTP increased by about 5-fold the number of neurons expressing ppN/OFQ mRNA in the pars reticulata of SN (SNr) by 24 h after treatment and the elevation remained significant for at least 7 days. This period coincided with the timing of the loss of dopamine neurons from the pars compacta of substantia nigra (SNc) after MPTP. The increased expression of ppN/OFQ mRNA co-localized with a neuronal marker in the SNr. MPTP treatment resulted in a small increase in the numbers of neurons expressing ppN/OFQ in the SNc in mice from one mouse colony but the increase did not reach statistical significance in mice from another colony. No changes in ppN/OFQ-mRNA expression were observed in the ventral tegmental area (VTA), the caudate-putamen, the subthalamic nucleus, or in two other brains areas. These results demonstrate that increased N/OFQ expression in the SNr is closely associated with the MPTP-induced loss of dopamine neurons in the SNc in a widely used animal model of Parkinson's disease.

Endogenous nociceptin (orphanin FQ) suppresses basal hedonic state and acute reward responses to methamphetamine and ethanol, but facilitates chronic responses

The opioid peptide nociceptin (orphanin FQ) suppresses drug reward, drug self-administration, and impedes some of the processes believed to underlie the transition to addiction. As virtually all previous studies have used administration of nociceptin receptor agonists to evaluate the role of nociceptin on addiction-like behavior, the current study used a pharmacological (nociceptin receptor antagonist) and genetic (nociceptin receptor knockout mice) approach to elucidate the role of endogenous nociceptin. The nociceptin receptor antagonist UFP-101 induced a modest place preference, and enhanced the conditioned place preference induced by methamphetamine. In agreement with this, nociceptin receptor knockout mice had slightly enhanced methamphetamine and ethanol conditioned place preferences compared to wild-type mice. This effect did not appear to depend on differences in learning ability, as nociceptin receptor knockout mice had slightly weaker-conditioned place aversions to lithium chloride, the kappa-opioid receptor agonist, U50488H, and the general opiate antagonist, naloxone. The development of behavioral sensitization to methamphetamine was lower in nociceptin receptor knockout mice, and attenuated by UFP-101 administration to wild-type mice. Additionally, ethanol consumption and preference in a two-bottle choice test was lower in nociceptin receptor knockout mice, though ethanol-stimulated locomotion was stronger. Whereas the rewarding effect of methamphetamine and ethanol following chronic treatment, as measured by place conditioning, strengthened in wild-type mice, this effect was absent in nociceptin receptor knockout mice. These results suggest that endogenous N/OFQ suppresses basal and drug-stimulated increases in hedonic state, and plays either a permissive or facilitatory role in the development of addiction.

Effects of constitutive deletion of opioid receptors on the basal densities of Fas and Fas-associated protein with death domain (FADD) in the mouse brain: a delta-opioid tone inhibits FADD

The acute effects of opiate drugs and opiate addiction have been associated with modulation of Fas/FADD (Fas-Associated protein with Death Domain) signaling complex in the rat brain. This study investigated the possible existence of endogenous opioid tones regulating the basal activities of Fas receptor forms and FADD in the brain, using gene-targeted mice lacking mu-, delta- or kappa-opioid peptide receptors (KO mice). In mu-KO mice, but not in delta- or kappa-KO mice, the basal immunodensity of native Fas (35 kDa monomeric form) was decreased in the cerebral cortex (33%) when compared with WT littermates. In delta-KO mice, but not in mu- or kappa-KO mice, the basal content of 120 kDa Fas aggregates (complexes of monomers relevant in Fas signaling) was markedly increased in the cortex (93%). In contrast, no differences between genotypes were observed in the basal expression of glycosylated Fas (51/48/45 kDa forms). Notably, the basal content of FADD (the adaptor protein that couples Fas to caspases and transmits the death signal) was increased in the cerebral cortex of delta-KO mice (48%), but not in mu- or kappa-KO mice. In addition, the basal content of phosphorylated FADD at Ser191 (the relevant species of FADD implicated in nonapoptotic signals) was also upregulated in the cortices of delta-opioid receptor KO mice (6.5-11.0-fold). The results suggest that mu-receptors tonically stimulate (through endogenous opioid peptides) the activation of native Fas, whereas delta-receptors tonically inhibit the expression of Fas aggregates and that of FADD and phosphorylated FADD (Ser191) in the mouse brain. These data are in line with the acute opposite modulation of Fas and FADD induced by mu- and delta-opiate agonists, and strongly support the notion of an anti-apoptotic delta-opioid tone that restrains Fas signaling.

UFP-101 antagonizes the spinal antinociceptive effects of nociceptin/orphanin FQ: behavioral and electrophysiological studies in mice

Nociceptin/orphanin FQ (N/OFQ) modulates various biological functions, including nociception, via selective stimulation of the N/OFQ peptide receptor (NOP). Here we used the NOP selective antagonist UFP-101 to characterize the receptor involved in the spinal antinociceptive effects of N/OFQ evaluated in the mouse tail withdrawal assay and to investigate the mechanism underlying this action by assessing excitatory postsynaptic currents (EPSC) in laminas I and II of the mouse spinal cord dorsal horn with patch-clamp techniques. Intrathecal (i.t.) injection of N/OFQ in the range of 0.1-10 nmol produced a dose dependent antinociceptive effect, which was prevented by UFP-101, but not by naloxone. In contrast the antinociceptive effect of the mu-opioid peptide receptor agonist endomorphin-1 was blocked by naloxone but not by UFP-101. Moreover, N/OFQ and endomorphin-1 induced a significant antinociceptive effect in wild type mice while in mice knockout for the NOP receptor gene only endomorphin-1 was found to be active. In mouse spinal cord slices 1 microM N/OFQ reduced EPSC to 60+/-4% of control values. This inhibitory effect was reversed in a concentration dependent manner by UFP-101 (pA2 value 6.44). The present results demonstrate that N/OFQ-induced spinal antinociception in vivo and inhibition of spinal excitatory transmission in vitro are mediated by receptors of the NOP type.

Dissociation of analgesic and hormonal responses to forced swim stress using opioid receptor knockout mice

Exposure to stress triggers hormonal and behavioral responses. It has been shown that the endogenous opioid system plays a role in some physiological reactions to stress. The opioid system was described to mediate analgesia induced by mild stressors and to modulate the activation of the hypothalamic-pituitary-adrenal axis. Our study assessed the contribution of opioid receptors in stress-induced analgesia and adrenocorticotropic hormone (ACTH) and corticosterone release by a genetic approach. We performed a parallel analysis of mice deficient in mu, delta, or kappa opioid receptors, as well as of triple opioid receptor knockout mice, following exposure to a mild stress (3-min swim at 32 degrees C). In wild-type mice, stress elicited an increase in jumping latency on the hot plate, which was influenced by gender and genetic background. This analgesic response was reversed both by naloxone and by the triple mutation, and decreased in mu and delta opioid receptor knockout females. In wild-type females, stress also delayed front- and hindpaw behaviors in the hot plate test and increased tail-flick latency in the tail immersion test. Opioid receptor deletion however did not affect these stress responses. In addition, stress produced an increase in ACTH and corticosterone plasma levels. This endocrine response remained unchanged in all mutant strains. Therefore our data indicate that, under our stress conditions, the endogenous opioid system is recruited to produce some analgesia whereas it does not influence hypothalamic-pituitary-adrenal axis activity. This implies that brain circuits mediating analgesic and hormonal responses to stress can be dissociated.

The opioid peptide nociceptin/orphanin FQ mediates prostaglandin E2-induced allodynia, tactile pain associated with nerve injury

Pain often outlasts its usefulness as warning and aid in wound healing, and becomes chronic and intractable after tissue damage and nerve injury. Many molecules have been implicated as mediators and modulators in persistent pain such as hyperalgesia and tactile pain (allodynia). We previously showed that prostaglandin (PG) E(2), PGF(2alpha) or the neuropeptide nociceptin, also called orphanin FQ (N/OFQ) administered intrathecally (i.t.) produced allodynia in conscious mice. In the present study, we examined the relationship of pain responses between PGs and N/OFQ using the N/OFQ receptor (NOP) antagonist, N-(4-amino-2-methylquinolin-6-yl)-2-(4-ethylphenoxy-methyl)benzamide monohydrochloride (JTC-801), and in mice lacking the N/OFQ prepropeptide (ppN/OFQ(-/-)) and the NOP receptor (NOP(-/-)). JTC-801 dose-dependently blocked the N/OFQ- and PGE(2)-induced allodynia, but not the PGF(2alpha)-induced one. Neither N/OFQ nor PGE(2) induced allodynia in NOP(-/-) mice. By contrast, the N/OFQ-induced allodynia was not affected by inhibition of PG production by a 60-min pretreatment with the non-steroidal anti-inflammatory drug, indomethacin. Among PGE receptor (EP) subtype-selective agonists, the EP4 agonist, AE1-329, markedly stimulated the release of N/OFQ from spinal slices and induced allodynia. AE1-329 also increased nitric oxide production in spinal slices using fluorescent nitric oxide detection, which was blocked by pretreatment with JTC-801. Conversely, PGE(2)-induced allodynia was not observed in ppN/OFQ(-/-) mice. N/OFQ immunoreactive puncta were colocalized with EP4. Taken together, these results demonstrate that PGE(2) induced allodynia by stimulation of N/OFQ release in the spinal cord via EP4 receptor subtypes.

Methamphetamine sensitization in nociceptin receptor knockout mice: locomotor and c-fos expression

The role of endogenous nociceptin in the development and expression of sensitization to repeated methamphetamine administration in a novel environment was studied in nociceptin receptor knockout mice. No differences in acute or sensitized locomotor responses were found in nociceptin receptor knockout mice. However, analysis of c-fos expression revealed significant interactions between chronic methamphetamine treatment and genotype in the nucleus accumbens and lateral septum. This was due to increased c-fos expression in chronically methamphetamine-treated nociceptin receptor knockout mice contrasted with reduced c-fos expression in chronically vehicle-treated nociceptin receptor knockout mice. Two further regions (nucleus accumbens core and ventromedial caudate putamen) showed significant interactions between genotype, chronic, and acute methamphetamine treatment due to accentuated c-fos expression in nociceptin receptor knockout mice sensitized and challenged with methamphetamine. These findings suggest endogenous nociceptin modulates the response of the central nervous system to repeated psychostimulant administration, although this is little reflected in locomotion.

Blockade of nociceptin/orphanin FQ receptor signaling in rat substantia nigra pars reticulata stimulates nigrostriatal dopaminergic transmission and motor behavior

A multidisciplinary approach was followed to investigate whether the opioid-like peptide nociceptin/orphanin FQ (N/OFQ) regulates the nigrostriatal dopaminergic pathway and motor behavior. Nigrostriatal dopaminergic cells, which express N/OFQ peptide (NOP) receptors, are located in the substantia nigra pars compacta and extend their dendrites in the substantia nigra pars reticulata, thereby modulating the basal ganglia output neurons. In vitro electrophysiological recordings demonstrated that N/OFQ hyperpolarized the dopaminergic cells of the substantia nigra pars compacta and inhibited their firing activity. In vivo dual-probe microdialysis showed that N/OFQ perfused in the substantia nigra pars reticulata reduced dopamine release in the ipsilateral striatum, whereas UFP-101 ([Nphe1,Arg14,Lys15]N/OFQ(1-13)-NH2) (a selective NOP receptor peptide antagonist) stimulated it. N/OFQ microinjected in the substantia nigra pars reticulata impaired rat performance on a rotarod apparatus, whereas UFP-101 enhanced it. Electromyography revealed that N/OFQ and UFP-101 oppositely affected muscle tone, inducing relaxation and contraction of triceps, respectively. The selective NOP receptor nonpeptide antagonist J-113397 (1-[3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H benzimidazol-2-one), either injected intranigrally or given systemically, also elevated striatal dopamine release and facilitated motor activity, confirming that these effects were caused by blockade of endogenous N/OFQ signaling. The inhibitory role played by endogenous N/OFQ on motor activity was additionally strengthened by the finding that mice lacking the NOP receptor gene outperformed wild-type mice on the rotarod. We conclude that NOP receptors in the substantia nigra pars reticulata, activated by endogenous N/OFQ, drive a physiologically inhibitory control on motor behavior, possibly via modulation of the nigrostriatal dopaminergic pathway.

Opposite alterations of NPFF1 and NPFF2 neuropeptide FF receptor density in the triple MOR/DOR/KOR-opioid receptor knockout mouse brains

Mice lacking the mu-delta-kappa-opioid receptor (MOR/DOR/KOR) genes and their corresponding wild-type littermates have been used to quantify NPFF(1) and NPFF(2) (neuropeptide FF) receptors by in vitro autoradiography in the central nervous tissues. Adjacent coronal sections were labelled with [125I]YVP ([125I]YVPNLPQRF-NH(2)) and [125I]EYF ([125I]EYWSLAAPQRF-NH(2)) as specific radioligands for NPFF(1) and NPFF(2) receptors, respectively. NPFF(2) receptors are predominantly expressed in both genotypes, but their density increases significantly in non cortical regions of mutant mice: 64% in the amygdaloid area, 89, 308, 1214 and 49% in the nucleus of the vertical limb of the diagonal band, substantia nigra, the vestibular nucleus and the spinal cord, respectively. In contrast, the density of the NPFF(1) subtype is lower than NPFF(2) in both genotypes and significantly decreased in some brain areas of mutant mice: -99, -90 and -90% in the nucleus of the vertical limb of the diagonal band, substantia nigra and the spinal cord, respectively. This study shows that mice lacking opioid receptors have brain region-dependent increases (NPFF(2)) and decreases (NPFF(1)) in NPFF receptors densities and suggests a different functional participation of each NPFF receptor subtype in the actions of opioids.

Lack of nociceptin receptor alters body temperature during resting period in mice

The role of nociceptin (NOC) receptor on body core temperature (Tcore) control was examined using NOC receptor knockout mice. In homozygote NOC receptor-knockout, wild-type, and control C57BL/6J and 129/SV mice, Tcore was continuously recorded under 12:12 h light:dark (LD) and conditions of constant darkness (DD). The Tcore values during the resting period were higher in the NOC receptor-knockout mice than in both wild-type and control mice under both LD and DD conditions. Spontaneous activity during the resting period and plasma cortisol levels were not different between the NOC receptor-knockout and control mice. The findings herein indicate that the NOC receptor is involved in the control of Tcore during the resting period and is independent of light, physical activity and/or cortisol regulation.

Pharmacological profile of nociceptin/orphanin FQ receptors regulating 5-hydroxytryptamine release in the mouse neocortex

A synaptosomal preparation was employed to pharmacologically characterize the role of presynaptic nociceptin/orphanin FQ (N/OFQ) receptors (NOP receptors) in the regulation of 5-hydroxytryptamine release in the Swiss mouse neocortex. In the present study, the NOP receptor ligands N/OFQ, Ac-RYYRWK-NH(2) and [Phe(1)psi(CH(2)-NH)Gly(2)]N/OFQ(1-13)-NH(2) inhibited the K(+)-induced [(3)H]-5-HT overflow with similar maximal effects ( approximately -35%) but different potencies (pEC(50) of 8.56, 8.35 and 7.23, respectively). The novel agonist [Arg(14),Lys(15)]N/OFQ also inhibited [(3)H]-5-HT overflow, but the concentration-response curve was biphasic and the efficacy higher ( approximately -45%). Receptor selectivity of NOP receptor agonists was demonstrated by showing that synaptosomes from NOP receptor knockout mice were unresponsive to N/OFQ, [Arg(14),Lys(15)]N/OFQ and [Phe(1)psi(CH(2)-NH)Gly(2)]N/OFQ(1-13)-NH(2) but maintained full responsiveness to endomorphin-1. Moreover, the inhibitory effect of N/OFQ was prevented by peptide ([Nphe(1)]N/OFQ(1-13)-NH(2) and UFP-101) and nonpeptide (J-113397 and JTC-801) NOP receptor selective antagonists. Desensitization occurred under perfusion with high (3 and 10 microm) N/OFQ concentrations. This phenomenon was prevented by the protein kinase C inhibitor, bisindolylmaleimide. Moreover, N/OFQ-induced desensitization did not affect mu opioid receptor responsiveness. Finally, it was observed in a similar preparation of rat cerebrocortical synaptosomes, although it was induced by higher N/OFQ concentrations than that used in the mouse. Together, these findings indicate that presynaptic NOP receptors inhibit 5-hydroxytryptamine release in the mouse neocortex. Based on present and previous studies, we conclude that NOP receptors in the mouse are subtly different from the homologous receptor population in the rat, strengthening the view that there exist species differences in the pharmacology of central NOP receptors.

Cannabinoid receptor and WIN 55 212-2-stimulated [35S]-GTPgammaS binding in the brain of mu-, delta- and kappa-opioid receptor knockout mice

Numerous studies have shown the existence of functional links between the endogenous cannabinoid and opioid systems. However, extensive research is still needed to elucidate the biochemical mechanisms involved in this cannabinoid-opioid interaction. Mice lacking mu- (MOR), delta- (DOR) and kappa- (KOR) opioid receptors have been generated and some specific pharmacological effects induced by cannabinoids have been reported to be modified in these animals. In order to clarify further the possible mechanisms involved in this modification of cannabinoid responses we have now evaluated the expression and functional activity of cannabinoid receptors in different brain structures in these mutant animals. For this purpose, we have performed quantitative receptor autoradiography of CB1 cannabinoid receptors and activation of GTP-binding proteins by CB1 agonists in the brain of wild-type and homozygous MOR, DOR and KOR knockout mice. There were no significant differences in the levels of CB1 receptors in the brain of MOR mutant mice. In contrast, the efficacy of CB1 receptor activation by the cannabinoid agonist WIN 55 212-2 was dramatically reduced in the caudate-putamen of MOR knockout animals. The density of CB1 receptors as well as the stimulation of GTP-binding proteins by WIN 55 212-2 were significantly increased in the substantia nigra of mice deficient in DOR. Finally, there were no major changes in the levels and functional activity of CB1 cannabinoid receptors in any brain region in KOR knockout mice. Taken together, these results indicate that deletion of MOR and DOR causes alterations in cannabinoid receptor levels and functional activity in specific brain structures, which could explain some of the functional interactions observed between these two neuronal systems.

Buprenorphine-induced antinociception is mediated by mu-opioid receptors and compromised by concomitant activation of opioid receptor-like receptors

Buprenorphine is a mixed opioid receptor agonist-antagonist used clinically for maintenance therapy in opiate addicts and pain management. Dose-response curves for buprenorphine-induced antinociception display ceiling effects or are bell shaped, which have been attributed to the partial agonist activity of buprenorphine at opioid receptors. Recently, buprenorphine has been shown to activate opioid receptor-like (ORL-1) receptors, also known as OP4 receptors. Here we demonstrate that buprenorphine, but not morphine, activates mitogen-activated protein kinase and Akt via ORL-1 receptors. Because the ORL-1 receptor agonist orphanin FQ/nociceptin blocks opioid-induced antinociception, we tested the hypothesis that buprenorphine-induced antinociception might be compromised by concomitant activation of ORL-1 receptors. In support of this hypothesis, the antinociceptive effect of buprenorphine, but not morphine, was markedly enhanced in mice lacking ORL-1 receptors using the tail-flick assay. Additional support for a modulatory role for ORL-1 receptors in buprenorphine-induced antinociception was that coadministration of J-113397, an ORL-1 receptor antagonist, enhanced the antinociceptive efficacy of buprenorphine in wild-type mice but not in mice lacking ORL-1 receptors. The ORL-1 antagonist also eliminated the bell-shaped dose-response curve for buprenorphine-induced antinociception in wild-type mice. Although buprenorphine has been shown to interact with multiple opioid receptors, mice lacking micro-opioid receptors failed to exhibit antinociception after buprenorphine administration. Our results indicate that the antinociceptive effect of buprenorphine in mice is micro-opioid receptor-mediated yet severely compromised by concomitant activation of ORL-1 receptors.

Nociceptin/orphanin FQ knockout mice display up-regulation of the opioid receptor-like 1 receptor and alterations in opioid receptor expression in the brain

The opioid receptor-like 1 receptor is a novel member of the opioid receptor family and its endogenous peptide ligand has been termed nociceptin and orphanin FQ. Activation of the opioid receptor-like 1 receptor by nociceptin/orphanin FQ in vivo produces hyperalgesia when this peptide is given supraspinally but analgesia at the spinal level. Nociceptin/orphanin FQ also reverses stress-induced analgesia, suggesting that the peptide has anti-opioid properties. Nociceptin/orphanin FQ knockout mice show alterations in pain sensitivity and stress responses and display increased morphine dependence, suggesting an interaction of the nociceptin/orphanin FQ system with classical opioid receptor function. To determine if the behavioural phenotype of nociceptin/orphanin FQ knockout mice reflects changes in either opioid receptor-like 1 or classical opioid receptor expression, we have carried out quantitative autoradiography of the opioid receptor-like 1, mu-, delta- and kappa-opioid receptors in the brains of these animals. Receptor density was measured on coronal sections from wild-type, heterozygous and homozygous mice using [(3)H]nociceptin, [(3)H][D-Ala(2)-N-methyl-Phe(4)-Gly(5) ol] enkephalin, [(3)H]deltorphin-I, or [(3)H](-)-N-methyl-N-[7-(1-pyrrodinyl)-1-oxospiro[4,5]dec-8-yl]-4-benzofuranacetamide to label opioid receptor-like 1, mu-, delta- and kappa-receptors, respectively. A region-specific up-regulation of the opioid receptor-like 1 receptor (up to 135%) was seen in brains from homozygous mice. Mu-Receptors also showed significant differences between genotypes whilst changes in delta- and kappa- receptors were minor. In conclusion the region-specific up-regulation of the opioid receptor-like 1 receptor indicates a tonic role for nociceptin/orphanin FQ in some brain structures and may suggest the peptide regulates the receptor expression in these regions. The changes in the opioid receptor-like 1 receptor may relate to the anxiogenic phenotype of these animals but the observed change in mu-receptors does not correlate with altered morphine responses.

Blockade of nociceptin/orphanin FQ-NOP receptor signalling produces antidepressant-like effects: pharmacological and genetic evidences from the mouse forced swimming test

Nociceptin/orphanin FQ (N/OFQ), the endogenous ligand of the NOP receptor, regulates several central functions such as pain transmission, learning and memory, fear and anxiety and feeding and locomotor activity. It has been recently reported that NOP receptor antagonists induce antidepressant-like effects in the mouse forced swimming test (FST), i.e. reduce immobility time. This assay was used in the present study for further investigating the involvement of the NOP receptor in depression states. In male Swiss mice, intracerebroventricular injection (i.c.v) of the novel NOP receptor antagonist, UFP-101 (1-10 nmol) dose-dependently reduced the immobility time (control 192 +/- 14 s, UFP-101 91 +/- 15 s). The effect of 3 or 10 nmol UFP-101 was fully or partially reversed, respectively, by the coadministration of 1 nmol N/OFQ, which was inactive per se. NOP receptor knockout mice showed a reduced immobility time compared with their wild-type littermates (wild-type 215 +/- 10 s, knockout 143 +/- 12 s). Moreover, i.c.v. injected UFP-101 (10 nmol) significantly reduced immobility time in wild-type mice but not in NOP receptor knockout animals. In conclusion, these results, obtained using a combined pharmacological and genetic approach, indicate that blockade of the N/OFQ-NOP receptor signalling in the brain produces antidepressant-like effects in the mouse FST. These findings support the NOP receptor as a candidate target for the development of innovative antidepressant drugs.

A subset of nociceptin/orphanin FQ receptor-expressing neurons in the anterior hypothalamic area, as revealed in mice with lacZ reporter gene

Nociceptin/orphanin FQ (N/OFQ) is an endogenous peptide agonist for the opioid receptor homolog, N/OFQ receptor, and serves for the central control of autonomic functions. Morphological details including the cell types that may account for such N/OFQ functions, however, remain unclear. By using X-gal histochemistry for the detection of receptor-expressing cells at both light and electron microscopic levels, we examined the hypothalamus from the receptor-deficient mice bearing a lacZ insertional mutation in the N/OFQ receptor gene. The N/OFQ receptor reflected by lacZ expression was seen at high levels in the anterior hypothalamic area. With electron microscopy, lacZ expression was observed in a subset of neurons showing large cell size and indented nucleus.

Morphine tolerance and dependence in the nociceptin receptor knockout mice

Here we report the involvement of nociceptin receptor in tolerance to morphine-induced antinociception and in morphine dependence. There was no different nociceptive perception and antinociceptive effects of morphine between wild-type and the nociceptin receptor knockout mice. Tolerance to morphine (10 mg/kg)-induced antinociception was developed in both wild-type and the nociceptin receptor knockout mice after administration of morphine (10 mg/kg) twice a day for 5 days. When naloxone (5 mg/kg) was administered to mice treated with morphine repeatedly on the 6th day, morphine withdrawal syndrome was observed in both wild-type and the nociceptin receptor knockout mice, which were accompanied by the elevation of cyclic AMP levels. While naloxone benzoylhydrazone (1 mg/kg), a putative antagonist for nociceptin receptor/naloxone benzoylhydrazone-sensitive sites, also induced the morphine withdrawal signs in both wild-type and the nociceptin receptor knockout mice, the jumping signs in the nociceptin receptor knockout mice were less severe than those in wild-type mice. Treatment with naloxone benzoylhydrazone in morphine-dependent wild-type mice caused a significant increase in cyclic AMP levels in the thalamus while it had no effect in the nociceptin receptor knockout mice. The analysis of opioid mu-receptor binding showed no difference between wild-type and the nociceptin receptor knockout mice. These results suggest that the nociceptin receptor/naloxone benzoylhydrazone-sensitive sites contribute to the induction of morphine withdrawal syndrome in part. Furthermore, it is demonstrated that morphine withdrawal syndrome excepting jumping can be induced by naloxone benzoylhydrazone without any changes in the cyclic AMP levels in the thalamus.

A combined pharmacological and genetic approach to investigate the role of orphanin FQ in learning and memory

Using a combination of the selective opioid receptor-like1 (ORL1) receptor agonist, Ro 64-6198, and orphanin FQ/nociceptin (OFQ/N) peptide knockout (KO) mice, the influence of OFQ/N on cognition has been studied in the rodent. In wild type, C57BL/6J mice, Ro 64-6198 (0.3-1 mg/kg i.p.) impaired the acquisition of spatial learning in the Morris water maze, although a mild neurological impairment was evident which complicated precise interpretation. In Lister hooded rats, Ro 64-6198 (6 mg/kg i.p.) produced delay dependent impairments in rats performing either a delayed matching or a delayed nonmatching to position task with only a modest (< 20%) effect on omissions - an effect consistent with a short-term memory impairment. Electrophysiological studies demonstrated an inhibitory effect of OFQ/N on LTP recorded from the CA1 region of wild type mice, but not in ORL1 receptor knockout mice. In contrast to the ORL1 agonist, mice deficient in the OFQ/N peptide showed some evidence of improved spatial learning, fear conditioning and passive avoidance retention. However, CA1 LTP was similar between OFQ/N peptide KO mice and wild type controls. Subsequent receptor radioautography studies demonstrated the presence of ORL1 receptors within various regions of the medial temporal lobe system: i.e. CA1, dentate gyrus molecular layer, subiculum, perirhinal cortex. Taken together, these results suggest a bi-directional effect of OFQ/N containing systems on aspects of cognitive behaviour, particularly those elements associated with hippocampal function. This is consistent with a likely modulatory role of OFQ/N on hippocampal and associated cortical circuitry.

Studies on the antinociceptive effect of [Nphe1]nociceptin(1-13)NH2 in mice

Nociceptin/orphanin FQ (NC) and its receptor (OP(4)) have been implicated in the regulation of various functions including nociception. [Nphe(1)]NC(1-13)NH(2) (Nphe) is a selective OP(4) antagonist which prevents the pronociceptive effects of supraspinal NC and causes per se a naloxone-insensitive antinociceptive effect. In the present study, we tested Nphe in wild type (WT) and OP(4) receptor knock out mice and found that a clear antinociceptive effect of the antagonist was evident only in WT mice. Moreover, we evaluated, over 5 days of treatment, the antinociceptive effects of Nphe in comparison with those of DAMGO and found that tolerance develops to the effects of the opioid receptor agonist but not to Nphe. These data demonstrate that the antinociceptive action of Nphe is due to the block of OP(4) receptors and that no tolerance develops to this kind of antinociception.

Quantitative autoradiographic mapping of the ORL1, mu-, delta- and kappa-receptors in the brains of knockout mice lacking the ORL1 receptor gene

Until recently the opioid receptor family was thought to consist of only the mu-, delta- and kappa-receptors. The cloning of opioid receptor like receptor (ORL1) and its endogenous ligand nociceptin/orphanin FQ, which displayed anti-opioid properties, has raised the issue of functional co-operativity of this system with the classical opioid system. ORL1 receptor knockout mice have been successfully developed by homologous recombination to allow the issue of potential heterogeneity of this receptor and also of compensatory changes in mu-, delta- or kappa-receptors in the absence of ORL1 to be addressed. We have carried out quantitative autoradiographic mapping of these receptors in the brains of mice that are wild-type, heterozygous and homozygous for the deletion of the ORL1 receptor. ORL1, mu-, delta- and kappa-receptors were labelled with [(3)H] leucyl-nociceptin (0.4 nM), [(3)H] DAMGO (4 nM), [(3)H] deltorphin-I (7 nM), and [(3)H] CI-977 (2.5 nM) respectively. An approximately 50% decrease in [(3)H] leucyl-nociceptin binding was seen in heterozygous ORL1 mutant mice and there was a complete absence of binding in homozygous brains indicating the single gene encodes for the ORL1 receptor and any putative subtypes. No significant gross changes in the binding to other opioid receptors were seen across genotypes in the ORL1 mutant mice demonstrating a lack of major compensation of classical opioid receptors in the absence of ORL1. There were a small number of region specific changes in the expression of classical opioid receptors that may relate to interdependent function with ORL1.

Modulation of synaptic transmission by nociceptin/orphanin FQ and nocistatin in the spinal cord dorsal horn of mutant mice lacking the nociceptin/orphanin FQ receptor

Nociceptin/orphanin FQ (N/OFQ) and nocistatin (NST) are two neuropeptides derived from the same precursor protein that exhibit opposing effects on spinal neurotransmission and nociception. Here, we have used whole-cell, patch-clamp recordings from visually identified neurons in spinal cord dorsal horn slices of genetically modified mice to investigate the role of the N/OFQ receptor (N/OFQ-R) in the modulatory action of both peptides on excitatory glutamatergic and inhibitory glycinergic and gamma-aminobutyric acid (GABA)-ergic synaptic transmission. In wild-type mice, N/OFQ selectively suppressed excitatory transmission in a concentration-dependent manner but left inhibitory synaptic transmission unaffected. In contrast, NST reduced only inhibitory but not alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor-mediated excitatory synaptic transmission. N/OFQ-mediated inhibition of excitatory transmission was completely absent in N/OFQ-R receptor-deficient (N/OFQ-R(-/-)) mice and significantly reduced in heterozygous (N/OFQ-R(+/-)) mice, whereas the action of NST on inhibitory neurotransmission was completely retained. To test for the relevance of these results for spinal nociception, we investigated the effects of intrathecally injected N/OFQ in the mouse formalin test, an animal model of tonic pain. N/OFQ (3 nmol/mouse) induced significant antinociception in wild-type mice, but had no antinociceptive effects in N/OFQ-R(-/-) mice. These results indicate that the inhibitory action of N/OFQ on excitatory glutamatergic synaptic transmission and its spinal antinociceptive action are mediated via the N/OFQ receptor, whereas the action of NST is independent of this receptor.

Improgan, a cimetidine analog, induces morphine-like antinociception in opioid receptor-knockout mice

Improgan is an analog of the H(2) antagonist cimetidine that does not act on known histamine receptors, but induces highly effective analgesia in rodents following intracerebroventricular (icv) administration. Since the mechanism of action of this compound remains unknown, improgan analgesia was characterized presently with the tail immersion nociceptive test in mutant mice lacking either the mu (exon 1 of MOR-1), delta (exon 2 of DOR-1) or kappa (exon 3 of KOR-1) opioid receptor. Improgan (30 microg, icv) induced reversible, maximal analgesia in both sexes of all three genotypes (+/+, +/- and -/-) of MOR-1 mutant mice 10 and 20 min after administration, whereas morphine analgesia was reduced (+/-) or abolished (-/-) in these subjects. In DOR-1 mutant mice, improgan was equally effective in all three genotypes, despite the reduction (+/-) or complete loss (-/-) of delta opioid receptor (3H-[D-Pen(2), D-Pen(5)]enkephalin, DPDPE) binding. Similarly, improgan analgesia was equivalent in all three genotypes of KOR-1 mutant mice, whereas kappa-mediated analgesia (U50,488) and kappa opioid (3H-U69,593) binding were abolished in the homozygous (-/-) mice. These studies demonstrate that improgan analgesia does not require intact MOR-1, DOR-1, or KOR-1 genes, and support the hypothesis that improgan-like analgesics act in the CNS by non-opioid mechanisms.

Distribution of nociceptin/orphanin FQ precursor protein and receptor in brain and spinal cord: a study using in situ hybridization and X-gal histochemistry in receptor-deficient mice

Nociceptin/orphanin FQ (N/OFQ) is an opioid-like heptadecapeptide agonist for the opioid receptor homolog, N/OFQ receptor. To explore the precise distribution of the peptide-receptor system, the authors examined the brain and spinal cord from receptor-deficient mice bearing the targeted mutation (morc(m1)), a lacZ insertional mutation in the N/OFQ receptor gene. Precursor protein N/OFQ (preproN/OFQ) mRNA was detected by using in situ hybridization, and the N/OFQ receptor was detected by using X-gal histochemistry. The N/OFQ receptor reflected by lacZ expression was observed at high levels in the dentate gyrus, lateral septum, subparafascicular thalamic nucleus, medial preoptic area, median preoptic nucleus, ventromedial preoptic nucleus, anterior hypothalamic area, paraventricular hypothalamic nucleus, ventromedial hypothalamic nucleus, auditory brainstem nuclei, pontine dorsal tegmentum, and nucleus of the solitary tract. In situ detection of the N/OFQ receptor mRNA by digoxigenin-labeled riboprobes coupled with tyramide signal amplification in normal and wild-type mice resulted in the regional distribution paralleling the lacZ expression in these regions. PreproN/OFQ mRNA was expressed at high levels in the subparafascicular thalamic nucleus, central gray, central tegmental field, auditory brainstem nuclei, caudal spinal trigeminal nucleus, and spinal dorsal horn. Furthermore, variable levels of expression of the peptide and receptor were seen in distinct sites of the brain and spinal cord. These data indicate a correspondence of the peptide and the receptor in local distribution at limbic, hypothalamic, and brainstem sites. Together with concurrent physiologic and behavioral studies in mutant mice, the results suggest functional roles for the N/OFQ system, including the central regulation of learning and memory, hearing ability, water balance, food intake, and blood pressure.

[Opioid receptor knockout mice]

mu, delta, kappa opioid receptors are target molecules for analgesia, reward and many physiological functions of opiates. Opioid receptor knockout mice generated by gene-targeting technology which can introduce mutation into specified locus provide invaluable animal models to elucidate the in vivo function of opiates and develop new therapeutic drugs. The disruptions of mu receptor expression decreases the nociceptive threshold to thermal stimuli and increases the threshold to visceral chemical stimuli paradoxically. Analgesia, reward, respiratory depression, constipation, immunosuppression and physical dependence induced by morphine are absent in mice lacking the mu receptor. These data show that the mu receptor is a molecular target for most effects of morphine, both therapeutic and side effects. mu Receptor expression is required for most delta receptor-mediated and some kappa receptor analgesic effects. These results support substantial roles for mu receptor in the analgesic properties of delta, kappa receptors. Cocaine and ethanol reward require mu receptor systems' intactness. Mice lacking the mu receptor will be a useful tool to study complex interactions between endogenous opiate and dopamine systems.

[Behavioral pharmacological characterization of mice lacking the nociceptin receptor]

Nociceptin and nociceptin receptor, which show structural similarities to opioid peptides and opioid receptors, respectively, have been recently found to constitute a novel neuromodulatory system. In the central nervous system, however, the physiological role of modulation via the nociceptin receptor is still unclear. Here, we report the behavioral pharmacological characterization of mice lacking the nociceptin receptor. Nociceptin produced hyperalgesia and hypolocomotion, whereas the nociceptin receptor-knockout mice showed no significant abnormalities in nociceptive thresholds (tail-flick, hot-plate, electric, and acetic acid-induced writhing tests) and locomotion. In the learning and memory tests, nociceptin induced impairment of learning and memory in wild-type mice. Nociceptin receptor-knockout mice possessed greater learning ability and had better memory than wild-type mice. These results suggest that the nociceptin system plays a role in regulation of nociception or locomotion and seems to play negative roles in learning and memory. Next, we compared nociceptive responses induced by various opioids between the nociceptin receptor-knockout and wild-type mice. As previously reported, morphine (mu-opioid receptor agonist), U-50,488 H (kappa 1-opioid receptor agonist), and naloxone benzoylhydrazone (NalBzoH; kappa 3-opioid receptor agonist) induced antinociceptive effects in wild-type mice. Surprisingly, knockout mice lacked the antinociceptive effect induced by NalBzoH, but not by morphine and U-50,488H. Further, NalBzoH completely inhibited nociceptin-induced hyperalgesia and hypolocomotion in wild-type mice. Experiments on the cultured cells transfected with the nociceptin receptor cDNA showed that NalBzoH competed in [3H]-nociceptin binding and attenuated the nociceptin-induced inhibition of cyclic AMP accumulation induced by forskolin. These results clearly suggest that NalBzoH acts as a potent antagonist for the nociceptin receptor. Our studies suggest that the nociceptive system and/or learning and memory processes could be modulated by ligands to the nociceptin receptor, and further that the antagonists are worth testing for the alleviation of pain and memory disorders.

Facilitation of long-term potentiation and memory in mice lacking nociceptin receptors

The peptide nociceptin (also named orphanin FQ) acts in the brain to produce various pharmacological effects, including hyperalgesia and hypolocomotion. The nociceptin receptor uses guanine-nucleotide-binding proteins to mediate the inhibition of adenylyl cyclase, the activation of potassium channels and inhibition of calcium channels. It has been shown using knock-out mice that the nociceptin receptor is not required for regulation of nociceptive responses or locomotion activity, but modulates the auditory function. Here we show that mice lacking the nociceptin receptor possess greater learning ability and have better memory than control mice. Histological analysis revealed the expression of both the nociceptin precursor and the nociceptin receptor in the hippocampus, thought to take part in aspects of learning and memory. Moreover, the receptor-deficient mice showed larger long-term potentiation in the hippocampal CA1 region than control mice, without apparent changes in presynaptic or postsynaptic electrophysiological properties. These results show that the loss of the nociceptin receptor results in a gain-of-function mutation in both the memory process and the long-term potentiation mechanism in CA1, perhaps as a result of altered intracellular signal transduction systems in neurons.

Opioid receptors: pinning down the opiate targets

Recent studies of knockout mice conclusively show that the mu opioid receptor mediates the analgesic effects of morphine; they further suggest that the mu opioid receptor also has a role in mediating the effects of other opioid agonists, and provide insights into the non-analgesic effects of mu opioids.

C57BL/6J-bgJ (beige) mice: differential sensitivity in the tail flick test to centrally administered mu- and delta-opioid receptor agonists

The antinociceptive effects of two mu-opioid receptor agonists, morphine and [D-Ala2, MePhe4, Gly-ol5]enkephalin (DAGO), and a selective delta-receptor agonist, [D-Pen2, L-Pen5]enkephalin (DPLPE), were determined in C57BL/6J-bgJ (beige) and control mice (CRS-CDl and C57BL/6By) using a standard tail-flick assay. The antinociceptive response of C57BL/6J-bgJ mice to intracerebro-ventricularly administered morphine and DAGO was significantly reduced compared to controls, but there was no difference in the antinociceptive response to DPLPE. These results suggest that there is a genetic deficit of mu-opioid receptor number or a genetically-induced alteration in receptor function in regions of C57BL/6J-bgJ brains involved in antinociception, that delta-opioid receptors can mediate antinociception in mice, and that the C57BL/6J-bgJ strain may offer a practical new animal model for studying the function of opioid receptor subtypes.