Synergism between two BLA-to-BNST pathways for appropriate expression of anxiety-like behaviors in male mice.

Understanding how distinct functional circuits are coordinated to fine-tune mood and behavior is of fundamental importance. Here, we observe that within the dense projections from basolateral amygdala (BLA) to bed nucleus of stria terminalis (BNST), there are two functionally opposing pathways orchestrated to enable contextually appropriate expression of anxiety-like behaviors in male mice. Specifically, the anterior BLA neurons predominantly innervate the anterodorsal BNST (adBNST), while their posterior counterparts send massive fibers to oval BNST (ovBNST) with moderate to adBNST. Optogenetic activation of the anterior and posterior BLA inputs oppositely regulated the activity of adBNST neurons and anxiety-like behaviors, via disengaging and engaging the inhibitory ovBNST-to-adBNST microcircuit, respectively. Importantly, the two pathways exhibited synchronized but opposite responses to both anxiolytic and anxiogenic stimuli, partially due to their mutual inhibition within BLA and the different inputs they receive. These findings reveal synergistic interactions between two BLA-to-BNST pathways for appropriate anxiety expression with ongoing environmental demands.

The active fragments of ghrelin cross the blood-brain barrier and enter the brain to produce antinociceptive effects after systemic administration.

G (1-5)-NH2, G (1-7)-NH2, and G (1-9) are the active fragments of ghrelin. The aim of this study was to investigate the antinociceptive effects, their ability to cross the blood-brain barrier, and the receptor mechanism(s) of these fragments using the tail withdrawal test in male Kunming mice. The antinociceptive effects of these fragments (2, 6, 20, and 60 nmol/mouse) were tested at 5, 10, 20, 30, 40, 50, and 60 min after intravenous (i.v.) injection. These fragments induced dose- and time-related antinociceptive effects relative to saline. Using the near infrared fluorescence imaging experiments, our results showed that these fragments could cross the brain-blood barrier and enter the brain. The antinociceptive effects of these fragments were completely antagonized by naloxone (intracerebroventricular, i.c.v.); however, naloxone methiodide (intraperitoneal, i.p.), which is the peripheral restricted opioid receptor antagonist, did not antagonize these antinociceptive effects. Furthermore, the GHS-R1α antagonist [D-Lys3]-GHRP-6 (i.c.v.) completely antagonized these antinociceptive effects, too. These results suggested that these fragments induced antinociceptive effects through central opioid receptors and GHS-R1α. In conclusion, our studies indicated that these active fragments of ghrelin could cross the brain-blood barrier and enter the brain and induce antinociceptive effects through central opioid receptors and GHS-R1α after intravenous injection.

Role of lateral amygdala calstabin2 in regulation of fear memory.

Calstabin2, also named FK506 binding protein 12.6 (FKBP12.6), is a subunit of ryanodine receptor subtype 2 (RyR2) macromolecular complex, an intracellular calcium channel. Studies from our and other's lab have shown that hippocampal calstabin2 regulates spatial memory. Calstabin2 and RyR2 are widely distributed in the brain, including the amygdala, a key brain area involved in the regulation of emotion including fear. Little is known about the role of calstabin2 in fear memory. Here, we found that genetic deletion of calstabin2 impaired long-term memory in cued fear conditioning test. Knockdown calstabin2 in the lateral amygdala (LA) by viral vector also impaired long-term cued fear memory expression. Furthermore, calstabin2 knockout reduced long-term potentiation (LTP) at both cortical and thalamic inputs to the LA. In conclusion, our present data indicate that calstabin2 in the LA plays a crucial role in the regulating of emotional memory.

Nociceptin impairs acquisition of novel object recognition memory in perirhinal cortex.

Nociceptin/Orphanin FQ (N/OFQ) plays an important role in the regulation of spatial, fear and recognition memories. N/OFQ receptors are highly distributed in the perirhinal cortex, which is a key brain area involved in modulating novel object recognition (NOR) memory. However, the role of N/OFQ in NOR memory in the perirhinal cortex was still unknown. Moreover, the effects of N/OFQ on different stages of NOR memory were still unclear. In NOR task, we found that pre-training intracerebroventricular (icv) injection of N/OFQ (0.3 and 1 nmol) impaired long-term memory in a dose-dependent manner. However, icv infusion of N/OFQ immediately after training did not affect NOR memory consolidation even at a high dose of 3 nmol. Pre-test icv injection of N/OFQ (1 nmol) also did not influence NOR memory retrieval. These data indicate that N/OFQ negatively modulates long-term NOR memory during the acquisition phase. Furthermore, the amnesia effect of N/OFQ (1 nmol, icv) could be antagonist by pre-treatment with the selective N/OFQ receptor antagonist [Nphe1]N/OFQ(1-13)NH2 (10 nmol, icv), indicating pharmacological specificity. Then, we found that pre-training infusion of N/OFQ (0.1 and 0.3 nmol/side) into the bilateral perirhinal cortex impaired long-term NOR memory, suggesting the perirhinal cortex is a critical brain structure in mediating the amnesic effect of N/OFQ in NOR task. In conclusion, our data, for the first time, indicate that N/OFQ in the perirhinal cortex impairs NOR memory acquisition through the NOP receptors.

Delta Subunit-Containing Gamma-Aminobutyric Acid A Receptor Disinhibits Lateral Amygdala and Facilitates Fear Expression in Mice.

BACKGROUND: Maintaining gamma-aminobutyric acidergic (GABAergic) inhibition in the amygdala within a physiological range is critical for the appropriate expression of emotions such as fear and anxiety. The synaptic GABA type A receptor (GABAAR) is generally known to mediate the primary component of amygdala inhibition and prevent inappropriate expression of fear. However, little is known about the contribution of the extrasynaptic GABAAR to amygdala inhibition and fear. METHODS: By using mice expressing green fluorescent protein in interneurons (INs) and lacking the δ subunit-containing GABAAR (GABAA(δ)R), which is exclusively situated in the extrasynaptic membrane, we systematically investigated the role of GABAA(δ)R in regulating inhibition in the lateral amygdala (LA) and fear learning using the combined approaches of immunohistochemistry, electrophysiology, and behavior. RESULTS: In sharp contrast to the established role of synaptic GABAAR in mediating LA inhibition, we found that either pharmacological or physiological recruitment of GABAA(δ)R resulted in the weakening of GABAergic transmission onto projection neurons in LA while leaving the glutamatergic transmission unaltered, suggesting disinhibition by GABAA(δ)R. The disinhibition arose from IN-specific expression of GABAA(δ)R with its activation decreasing the input resistance of local INs and suppressing their activation. Genetic deletion of GABAA(δ)R attenuated its role in suppressing LA INs and disinhibiting LA. Importantly, the GABAA(δ)R facilitated long-term potentiation in sensory afferents to LA and permitted the expression of learned fear. CONCLUSIONS: Our findings suggest that GABAA(δ)R serves as a brake rather than a mediator of GABAergic inhibition in LA. The disinhibition by GABAA(δ)R may help to prevent excessive suppression of amygdala activity and thus ensure the expression of emotion.

Endomorphin-1 attenuates Aß42 induced impairment of novel object and object location recognition tasks in mice.

A growing body of evidence suggests that the agglomeration of amyloid-ß (Aß) may be a trigger for Alzheimer׳s disease (AD). Central infusion of Aß42 can lead to memory impairment in mice. Inhibiting the aggregation of Aß has been considered a therapeutic strategy for AD. Endomorphin-1 (EM-1), an endogenous agonist of µ-opioid receptors, has been shown to inhibit the aggregation of Aß in vitro. In the present study, we investigated whether EM-1 could alleviate the memory-impairing effects of Aß42 in mice using novel object recognition (NOR) and object location recognition (OLR) tasks. We showed that co-administration of EM-1 was able to ameliorate Aß42-induced amnesia in the lateral ventricle and the hippocampus, and these effects could not be inhibited by naloxone, an antagonist of µ-opioid receptors. Infusion of EM-1 or naloxone separately into the lateral ventricle had no influence on memory in the tasks. These results suggested that EM-1 might be effective as a drug for AD preventative treatment by inhibiting Aß aggregation directly as a molecular modifier.

The role of apelin-13 in novel object recognition memory.

Apelin and its receptor APJ (apelin receptor) are prominently expressed in brain regions involved in learning and memory. However, the role of apelin in cognition was largely unclear. Here, the role of apelin-13 in memory processes was investigated in mice novel object recognition task. Post-training injection of apelin-13 (0.3 and 1 nmol) dose-dependently impaired short-term memory (STM), however, pre-training infusion of apelin-13 (1 nmol) did not affect STM, suggesting apelin-13 blocks formation but not acquisition of STM. Apelin-13 (1 nmol) administered immediately, 30, 60 or 120 min post-training impaired long-term memory (LTM) in a time-dependent manner (30 min), however, both pre-training and pre-test infusion of apelin-13 (1 nmol) did not affect LTM, suggesting apelin-13 impaired consolidation but not acquisition and recall of LTM. Taken together, for the first time, our results indicate that apelin-13 blocks STM formation and LTM consolidation in novel object recognition task.

Neuropeptide S interacts with the basolateral amygdala noradrenergic system in facilitating object recognition memory consolidation.

The noradrenergic activity in the basolateral amygdala (BLA) was reported to be involved in the regulation of object recognition memory. As the BLA expresses high density of receptors for Neuropeptide S (NPS), we investigated whether the BLA is involved in mediating NPS's effects on object recognition memory consolidation and whether such effects require noradrenergic activity. Intracerebroventricular infusion of NPS (1nmol) post training facilitated 24-h memory in a mouse novel object recognition task. The memory-enhancing effect of NPS could be blocked by the ß-adrenoceptor antagonist propranolol. Furthermore, post-training intra-BLA infusions of NPS (0.5nmol/side) improved 24-h memory for objects, which was impaired by co-administration of propranolol (0.5µg/side). Taken together, these results indicate that NPS interacts with the BLA noradrenergic system in improving object recognition memory during consolidation.

Neuropeptide S enhances memory and mitigates memory impairment induced by MK801, scopolamine or Aß1₋42 in mice novel object and object location recognition tasks.

Neuropeptide S (NPS), the endogenous ligand of NPSR, has been shown to promote arousal and anxiolytic-like effects. According to the predominant distribution of NPSR in brain tissues associated with learning and memory, NPS has been reported to modulate cognitive function in rodents. Here, we investigated the role of NPS in memory formation, and determined whether NPS could mitigate memory impairment induced by selective N-methyl-D-aspartate receptor antagonist MK801, muscarinic cholinergic receptor antagonist scopolamine or Aß1₋42 in mice, using novel object and object location recognition tasks. Intracerebroventricular (i.c.v.) injection of 1 nmol NPS 5 min after training not only facilitated object recognition memory formation, but also prolonged memory retention in both tasks. The improvement of object recognition memory induced by NPS could be blocked by the selective NPSR antagonist SHA 68, indicating pharmacological specificity. Then, we found that i.c.v. injection of NPS reversed memory disruption induced by MK801, scopolamine or Aß1₋42 in both tasks. In summary, our results indicate that NPS facilitates memory formation and prolongs the retention of memory through activation of the NPSR, and mitigates amnesia induced by blockage of glutamatergic or cholinergic system or by Aß1₋42, suggesting that NPS/NPSR system may be a new target for enhancing memory and treating amnesia.

Reversal of scopolamine-induced spatial and recognition memory deficits in mice by novel multifunctional dimers bis-cognitins.

Our previous reports indicated that bis(propyl)-cognitin (B3C) and bis(heptyl)-cognitin (B7C), as novel dimers derived from tacrine, may be potential multifunctional drugs for treating Alzheimer's disease. There is little knowledge on the cognitive function of B3C while B7C appeared to reverse learning and memory impairments. In this study, for the first time, we evaluated the anti-amnesic effects of B3C and B7C on learning and memory deficits induced by scopolamine using both Morris water maze and novel object recognition tasks in mice. Under the same experimental condition, the anti-amnesic effect of tacrine was also compared. Briefly, in both tasks, scopolamine (0.1-0.6 mg/kg, ip) dose-dependently impaired learning and memory functions. B3C (1.5-2.5 µmol/kg), B7C (0.4-0.6 µmol/kg) or tacrine (8-12 µmol/kg), each administered ip, dose-dependently mitigated scopolamine-induced learning and memory impairments in both tasks. Our present results show, for the first time, that B3C and B7C reverse cognitive impairment resulted from scopolamine in both water maze and object recognition tasks; and under the same condition, the relative potency of B3C and B7C to improve cognitive capacity was 5-20 folds over that of tacrine. These novel in vivo findings further demonstrate that both B3C and B7C may potentially be developed as Alzheimer's therapeutic drugs for different severities of neurodegenerations.

Central Neuropeptide S inhibits food intake in mice through activation of Neuropeptide S receptor.

Neuropeptide S (NPS), the endogenous ligand of NPS receptor (NPSR), can regulate a variety of biological functions, including arousal, anxiety, locomotion, memory and drug abuse. Previous studies have shown that central NPS inhibited food intake in rats and chicks. In the present study, we investigated the role of central NPS on food intake in fasted mice, and detected the underlying mechanism(s) by using NPSR antagonist [D-Val(5)]NPS and Corticotropin-Releasing Factor 1 (CRF1) Receptor antagonist NBI-27914. The present results indicated that intracerebroventricular injection of NPS (0.001-0.1 nmol) dose-dependently inhibited food intake in fasted mice. The anorectic effect of NPS reached the maximum at the dose of 0.1 nmol, which could be antagonized by co-injection of 10 nmol NPSR antagonist [D-Val(5)]NPS. Furthermore, CRF1 receptor antagonist NBI-27914 at the dose of 2 µg antagonized the hyperlocomotor action of NPS, but did not affect the role of NPS on food intake. In conclusion, our results demonstrated central NPS inhibited food intake in fasted mice, mediated by its cognate NPSR, but not by CRF1 receptor.

Effects of central neuropeptide S in the mouse formalin test.

Neuropeptide S (NPS), a recently discovered bioactive peptide, was reported to regulate arousal, anxiety, locomotion, feeding behaviors, memory, and drug addiction. NPS receptor (NPSR) mRNA was found in several brain regions related to descending control system of pain, including the periaqueductal gray (PAG). Our previous study had shown that NPS could produce antinociception in mice. The present study was designed to evaluate whether NPS may produce antinociceptive effect observed in the mouse formalin test, a model of inflammatory pain. NPS (0.1-100 pmol) administrated intracerebroventricularly (i.c.v.) dose-dependently attenuated both first-phase and second-phase nociceptive behaviors induced by paw formalin injection. NPS (10 pmol, i.c.v.)-elicited antinociceptive effect was counteracted by co-injection with 1000 and 10,000 pmol [D-Val(5)]NPS, which alone induced neither hyperalgesia nor antinociception. The antinociception induced by NPS (10 pmol, i.c.v.) was not affected by naloxone (i.p., 10 mg/kg) and naloxone alone had no effect in the formalin test. In addition, compared to the saline (i.c.v.) treated group, NPS (10 pmol, i.c.v.) treated group increased c-Fos protein expression in nearly all subdivisions of the PAG in the formalin-injected mice. The above results revealed that NPS could produce antinociception in the formalin test through NPSR, which may be involved in the activation of PAG, suggesting that NPS-NPSR system may be a potential target for developing new analgesic drugs.

Central Neuropeptide S inhibits distal colonic transit through activation of central Neuropeptide S receptor in mice.

Neuropeptide S (NPS), the endogenous ligand of NPS receptor (NPSR), regulates many biological functions, including arousal, anxiety, locomotion and food intake. NPSR mRNA is expressed in several regions of central autonomic network through which the brain controls visceromotor and other responses essential for survival. However, the role of NPS/NPSR system in regulating gastrointestinal motor is still unknown. Here, we studied the effects of NPS on distal colonic transit in mice. Intracerebroventricular (i.c.v.) injection of NPS (1-1000 pmol) inhibited fecal pellet output and bead expulsion in a dose-dependent manner. However, intraperitoneal injection of NPS (1000 and 10000 pmol) did not affect fecal pellet output and bead expulsion. In vitro, NPS (0.1-10 microM) also did not modulate distal colonic contractions. Furthermore, i.c.v. co-administration of [D-Val(5)]NPS, a pure and potent NPSR antagonist, dose-dependently antagonized the inhibitory effects of NPS on fecal pellet output and bead expulsion. In conclusion, our results firstly indicate that central NPS inhibits distal colonic transit through the activation of central NPSR, which implicate that NPS/NPSR system might be a new target to treat function disorder of distal colon.

Neuropeptide S facilitates spatial memory and mitigates spatial memory impairment induced by N-methyl-D-aspartate receptor antagonist in mice.

Neuropeptide S (NPS) is a recently discovered peptide shown to be involved in regulating arousal and anxiety. NPS receptor (NPSR) mRNA is expressed significantly in the major input and output regions of hippocampal formation, which are critical in the modulation of learning and memory. However, the role of NPS/NPSR system in regulating of learning and memory is still unknown. Here, we use the Morris water maze (MWM) to determine the effects of NPS on spatial learning and memory following intracerebroventricular (i.c.v.) injection in mice. Our data show that i.c.v. injection of NPS facilitates spatial memory in the MWM without significant alteration of latency to the target and swimming speed. Furthermore, NPS (i.c.v.) mitigates spatial memory impairment induced by the selective N-methyl-d-aspartate receptor antagonist MK801. Taken together, our results firstly demonstrate that NPS facilitates spatial memory and mitigates MK801-induced spatial memory impairment in mice.

Involvement of NMDA receptor in nociceptive effects elicited by intrathecal [Tyr6] gamma2-MSH(6-12), and the interaction with nociceptin/orphanin FQ in pain modulation in mice.

The mas-related genes (Mrgs, also known as sensory neuron-specific receptors, SNSRs) are specifically expressed in small diameter sensory neurons in the trigeminal and dorsal root ganglia, suggesting an important role of the receptors in pain transmission. The present study aimed to investigate the underlying mechanism of the nociceptive effects after activation of MrgC, and the interaction between MrgC and N/OFQ-NOP receptor system in modulation of nociception in mice. Intrathecal (i.t.) administration of [Tyr(6)] gamma2-MSH(6-12), the most potent agonist for MrgC receptor, produced a significant hyperalgesic response as assayed by tail withdrawal test and a series of characteristic nociceptive responses, including biting, licking and scratching, in a dose-dependent manner (0.01-10 pmol and 0.01-10 nmol, respectively) in mice. These pronociceptive effects induced by [Tyr(6)] gamma2-MSH(6-12) were inhibited dose-dependently by co-injection of competitive NMDA receptor antagonist D-APV, non-competitive NMDA receptor antagonist MK-801, and nitric oxide (NO) synthase inhibitor L-NAME. However, the tachykinin NK(1) receptor antagonist L-703,606, and tachykinin NK(2) receptor antagonist MEN-10,376, had no influence on pronociceptive effects elicited by [Tyr(6)] gamma2-MSH(6-12). In other groups, [Tyr(6)] gamma2-MSH(6-12)-induced nociceptive responses were bidirectionally regulated by the co-injection of N/OFQ. N/OFQ inhibited nociceptive responses at high doses (0.01-1 nmol), but potentiated the behaviors at low doses (1 fmol-3 pmol). Furthermore, both hyperalgesia and nociceptive responses were enhanced after the co-administration with NOP receptor antagonist [Nphe(1)]N/OFQ(1-13)-NH(2). These results suggest that intrathecal [Tyr(6)] gamma2-MSH(6-12)-induced pronociceptive effects may be mediated through NMDA receptor-NO system in the spinal cord, and demonstrate the interaction between MrgC and N/OFQ-NOP receptor system in pain transmission.

Neuropeptide S produces antinociceptive effects at the supraspinal level in mice.

Neuropeptide S (NPS), a recently identified bioactive peptide through reverse pharmacology approach, was reported to regulate arousal, anxiety, locomotor activity, feeding behaviors and drug reward. NPS receptor (NPSR) mRNA was found in the area related to the descending control system of pain, such as the periaqueductal gray (PAG), raphe nuclei, and lateral parabrachial nucleus (PBN), suggesting a possible role of the NPS-NPSR system in the regulation of pain transmission. In the present study, we evaluated the effects of NPS in pain modulation at the supraspinal level for the first time, using the tail withdrawal test and hot-plate test in mice. NPS (mouse, 0.01-1 nmol) injected intracerebroventricularly (i.c.v.) caused a significant increase of tail withdrawal latency and paw-licking/jumping latency in the tail withdrawal test and the hot-plate test, respectively. Antinociceptive effect elicited by NPS (0.1 nmol, i.c.v.) was not affected by naloxone (i.c.v., 10 nmol co-injection or i.p., 10 mg/kg, 10 min prior to NPS) in both tail withdrawal test and hot-plate test. However, at the doses, naloxone significantly inhibited the antinociceptive effect induced by morphine (i.c.v., 3 nmol). NPS (0.1 nmol, i.c.v.)-induced antinociception was inhibited by co-injection with 10 nmol, but not 3 nmol [D-Cys(tBu)(5)]NPS, a peptidergic antagonist identified more recently, while [D-Cys(tBu)(5)]NPS (3 and 10 nmol) alone induced neither hyperalgesia nor antinociception. These results revealed that NPS could produce antinociception through NPS receptor, but not opioid receptor, and NPS-NPSR system could be a potential target for developing new analgesic drugs.

Neuropeptide S inhibits the acquisition and the expression of conditioned place preference to morphine in mice.

Neuropeptide S (NPS), a recently identified bioactive peptide, was reported to regulate arousal, anxiety, motoring and feeding behaviors. NPS precursor and NPS receptor mRNA were found in the amygdala, the ventral tegmental area (VTA) and the substantia nigra, the area thought to modulate rewarding properties of drugs. In the present study, we examined the influence of NPS on the rewarding action of morphine, using the unbiased conditioned place preference (CPP) paradigm. Morphine (1, 3 and 6 nmol, i.c.v.) induced a significant place preference. For testing the effect of NPS on the acquisition of morphine CPP, mice were given the combination of NPS and morphine on the conditioning days, and without drug treatment on the followed test day. To study the effect of NPS on the expression of morphine CPP, mice received the treatment of saline/morphine on the conditioning days, and NPS on the test day, 15 min before the placement in the CPP apparatus. Our results showed that NPS (0.3-10 nmol) alone neither induced place preference nor aversion, however, NPS (1 and 3 nmol) blocked the acquisition of CPP induced by 3 nmol morphine, and acquisition of 6 nmol morphine-induced CPP was also reduced by NPS (6 and 10 nmol). Moreover, the expression of CPP induced by 6 nmol morphine was also inhibited by NPS (0.1, 1 and 10 nmol). These results revealed the involvement of NPS in rewarding activities of morphine, and demonstrated the interaction between NPS system and opioid system for the first time.

Novel potent agonist [(pF)Phe4,Aib7,Aib11,Arg14,Lys15]N/OFQ-NH2 and antagonist [Nphe1,(pF)Phe4,Aib7,Aib11,Arg14,Lys15]N/OFQ-NH2 of nociceptin/orphanin FQ receptor.

Two novel ligands for the nociceptin/orphanin FQ (N/OFQ) receptor (NOP), [(pF)Phe4,Aib7, Aib11,Arg14,Lys15]N/OFQ-NH2 (peptide-1) and [Nphe1,(pF)Phe4,Aib7,Aib11,Arg14,Lys15]N/OFQ-NH2 (peptide-2), have been generated by combining different modifications of N/OFQ sequence. In the present study, we investigated the actions of two analogues and compared them with those of N/OFQ in four assays. Peptide-1 mimicked N/OFQ effects in mouse vas deferens and mouse colon and showed similar maximal effects but higher potency relative to N/OFQ. The effects of peptide-1 were sensitive to NOP receptor selective antagonist ([Nphe1]N/OFQ(1-13)-NH2) but not to naloxone in vitro. Peptide-1 (25 pmol, i.c.v.) mimicked the pronociceptive action of N/OFQ (2.5 nmol, i.c.v.) in mouse tail withdrawal assay, displaying higher potency and longer lasting effects. In anesthetized rats, peptide-1 (1 nmol/kg, i.v.) produced a marked decrease in mean arterial pressure, which was comparable to that evoked by i.v. N/OFQ (100 nmol/kg). Peptide-2 did not produce any effect per se but antagonized N/OFQ actions in mouse vas deferens and mouse colon assays. Peptide-2 is active in vivo where it prevented the pronociceptive effect induced by 2.5 nmol N/OFQ i.c.v. in the mouse tail withdrawal assay. Furthermore, peptide-2 at 5 nmol produced alone a robust and long lasting antinociceptive effect. Moreover, peptide-2 (10 and 40 nmol/kg i.v.) didn't produce any effect per se but antagonized hypotensive actions produced by i.v. administration of N/OFQ. Collectively, these findings demonstrate that [(pF)Phe4,Aib7,Aib11, Arg14,Lys15]N/OFQ-NH2 behaves as a highly potent NOP receptor agonist which produces long lasting effects in vivo and [Nphe1,(pF)Phe4,Aib7,Aib11,Arg14,Lys15]N/OFQ-NH2 acts as a pure and competitive antagonist of the NOP receptor.

Structure-activity studies on different modifications of nociceptin/orphanin FQ: identification of highly potent agonists and antagonists of its receptor.

Nociceptin/orphanin FQ (N/OFQ) and its receptor system modulate a variety of biological functions and further understandings of physiological and pathological roles of this system require new potent agonists and antagonists of its receptor. Two series of N/OFQ related analogues were synthesized to investigate the relationship of different modifications. We combined modifications including: (a) Phe(4)-->(pF)Phe(4); (b) Ala(7), Ala(11)-->Aib(7), Aib(11); (c) Leu(14), Ala(15)-->Arg(14), Lys(15). Compared with the first series, N-terminus of the second series was changed from Phe(1) to Nphe(1). All the analogues were amidated at C-terminus. These compounds were tested in binding studies on rat brain membranes and mouse vas deferens assay. Results indicated that the compounds of the first series showed higher affinity and potency than N/OFQ (pK(i)=9.33; pEC(50)=7.50). In particular, [(pF)Phe(4), Aib(7), Aib(11), Arg(14), Lys(15)] N/OFQ-NH(2) was found to be a highly potent agonist with pK(i)=10.78 in binding studies and pEC(50)=9.37 in mouse vas deferens assay. The second series all competitively antagonized the effects of N/OFQ in mouse vas deferens assay. [Nphe(1), (pF)Phe(4), Aib(7), Aib(11), Arg(14), Lys(15)] N/OFQ-NH(2) was the best antagonist with pA(2)=8.39 and showed high binding affinity with pK(i)=9.99. Thus modifications which increase the potency of agonist have synergistic effect on biological activity and a replacement of N-terminus leads to shift of analogues from agonist to antagonist.