Effects of nociceptin/orphanin FQ and endomorphin-1 on glutamate and GABA release, intracellular [Ca2+] and cell excitability in primary cultures of rat cortical neurons

Nociceptin/Orphanin FQ Receptor Structure, Signaling, Ligands, Functions, and Interactions with Opioid Systems

The NOP receptor (nociceptin/orphanin FQ opioid peptide receptor) is the most recently discovered member of the opioid receptor family and, together with its endogenous ligand, N/OFQ, make up the fourth members of the opioid receptor and opioid peptide family. Because of its more recent discovery, an understanding of the cellular and behavioral actions induced by NOP receptor activation are less well developed than for the other members of the opioid receptor family. All of these factors are important because NOP receptor activation has a clear modulatory role on mu opioid receptor-mediated actions and thereby affects opioid analgesia, tolerance development, and reward. In addition to opioid modulatory actions, NOP receptor activation has important effects on motor function and other physiologic processes. This review discusses how NOP pharmacology intersects, contrasts, and interacts with the mu opioid receptor in terms of tertiary structure and mechanism of receptor activation; location of receptors in the central nervous system; mechanisms of desensitization and downregulation; cellular actions; intracellular signal transduction pathways; and behavioral actions with respect to analgesia, tolerance, dependence, and reward. This is followed by a discussion of the agonists and antagonists that have most contributed to our current knowledge. Because NOP receptors are highly expressed in brain and spinal cord and NOP receptor activation sometimes synergizes with mu receptor-mediated actions and sometimes opposes them, an understanding of NOP receptor pharmacology in the context of these interactions with the opioid receptors will be crucial to the development of novel therapeutics that engage the NOP receptor.

Gonadal steroids differentially modulate the actions of orphanin FQ/nociceptin at a physiologically relevant circuit controlling female sexual receptivity

Orphanin FQ/nociceptin (OFQ/N) inhibits the activity of pro-opiomelanocortin (POMC) neurones located in the hypothalamic arcuate nucleus (ARH) that regulate female sexual behaviour and energy balance. We tested the hypothesis that gonadal steroids differentially modulate the ability of OFQ/N to inhibit these cells via presynaptic inhibition of transmitter release and postsynaptic activation of G protein-gated, inwardly-rectifying K(+) (GIRK)-1 channels. Whole-cell patch clamp recordings were performed in hypothalamic slices prepared from ovariectomised rats. OFQ/N (1 μm) decreased the frequency of miniature excitatory postsynaptic currents (mEPSCs) and miniature inhibitory postsynaptic currents (mIPSCs), and also caused a robust outward current in the presence of tetrodotoxin, in ARH neurones from vehicle-treated animals. A priming dose of oestradiol benzoate (EB; 2 μg) increased basal mEPSC frequency, markedly diminished both the OFQ/N-induced decrease in mEPSC frequency and the activation of GIRK-1 currents, and potentiated the OFQ/N-induced decrease in mIPSC frequency. Steroid treatment regimens that facilitate sexual receptivity reinstate the basal mEPSC frequency, the OFQ/N-induced decrease in mEPSC frequency and the activation of GIRK-1 currents to levels observed in vehicle-treated controls, and largely abolish the ability of OFQ/N to decrease mIPSC frequency. These effects were observed in an appreciable population of identified POMC neurones, almost one-half of which projected to the medial preoptic nucleus. Taken together, these data reveal that gonadal steroids influence the pleiotropic actions of OFQ/N on ARH neurones, including POMC neurones, in a disparate manner. These temporal changes in OFQ/N responsiveness further implicate this neuropeptide system as a critical mediator of the gonadal steroid regulation of reproductive behaviour.

The role of orphanin FQ/nociceptin in neuroplasticity: relationship to stress, anxiety and neuroinflammation

The neuropeptide, orphanin FQ/nociceptin (OFQ/N or simply, nociceptin), is expressed in both neuronal and non-neuronal tissue, including the immune system. In the brain, OFQ/N has been investigated in relation to stress, anxiety, learning and memory, and addiction. More recently, it has also been found that OFQ/N influences glial cell functions, including oligodendrocytes, astrocytes, and microglial cells. However, this latter research is relatively small, but potentially important, when observations regarding the relationship of OFQ/N to stress and emotional functions is taken into consideration and integrated with the growing evidence for its involvement in cells that mediate inflammatory events. This review will first provide an overview and understanding of how OFQ/N has been implicated in the HPA axis response to stress, followed by an understanding of its influence on natural and learned anxiety-like behavior. What emerges from an examination of the literature is a neuropeptide that appears to counteract anxiogenic influences, but paradoxically, without attenuating HPA axis responses generated in response to stress. Studies utilized both central administration of OFQ/N, which was shown to activate the HPA axis, as well as antagonism of NOP-R, the OFQ/N receptor. In contrast, antagonist or transgenic OFQ/N or NOP-R knockout studies, showed augmentation of HPA axis responses to stress, suggesting that OFQ/N may be needed to control the magnitude of the HPA axis response to stress. Investigations of behavior in standard exploratory tests of anxiogenic behavior (eg., elevated plus maze) or learned fear responses have suggested that OFQ/N is needed to attenuate fear or anxiety-like behavior. However, some discrepant observations, in particular, those that involve appetitive behaviors, suggest a failure of NOP-R deletion to increase anxiety. However, it is also suggested that OFQ/N may operate in an anxiolytic manner when initial anxiogenic triggers (eg., the neuropeptide CRH) are initiated. Finally, the regulatory functions of OFQ/N in relation to emotion-related behaviors may serve to counteract potential neuroinflammatory events in the brain. This appears to be evident within the glial cell environment of the brain, since OFQ/N has been shown to reduce the production of proinflammatory cellular and cytokine events. Given that both OFQ/N and glial cells are activated in response to stress, it is possible that there is a possible convergence of these two systems that has important repercussions for behavior and neuroplasticity.

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.

Swim stress enhances nociceptin/orphanin FQ-induced inhibition of rat dorsal raphe nucleus activity in vivo and in vitro: role of corticotropin releasing factor

The effects of nociceptin/orphanin FQ on putative serotonin (5HT) neurons of the dorsal raphe nucleus (DRN), known to modulate the behavioral responses to stress, were investigated in vivo and in vitro. In DRN slices from unstressed rats, nociceptin/orphanin FQ concentration-dependently inhibited the firing rate of putative 5HT neurons (EC(50) = 21.6 +/- 1.21 nM) and the selective NOP receptor antagonist UFP-101 shifted the concentration-response curve to the right (estimated pA(2) 6.86). Nociceptin/orphanin FQ potency was enhanced in slices prepared from rats previously subjected to a 15 min swim stress (EC(50) = 1.98 +/- 0.11 nM). Swim stress did not change the number or affinity of NOP receptors in DRN. Stress-elicited potentiation involved corticotropin-releasing factor (CRF)(1) receptors, GABA signaling and protein synthesis, being attenuated by pre-treatment with antalarmin (20 mg/kg, i.p.), diazepam (2.4 mg/kg, i.p.) and cycloheximide (2.5 mg/kg, i.p.), respectively. In anesthetized unstressed rats, locally applied nociceptin/orphanin FQ (0.03 and 0.1 ng/30 nl) inhibited the firing rate of DRN neurons (to 80 +/- 7 and 54 +/- 10% of baseline, respectively). Nociceptin/orphanin FQ inhibition was potentiated both 24 h after swim stress and 1 h after CRF (30 ng/30 nl intra-DRN). Stress-induced potentiation was prevented by the selective CRF(1) receptor antagonist, NBI 30755 (20 mg/kg, i.p.). In contrast, the inhibitory response of DRN neurons to the 5HT(1A) agonist, 8OH-DPAT (1 microg/1 microl, intra-DRN) was not potentiated by swim stress, ruling out a non-specific enhanced permeability of GIRK channel. Together, these findings suggest that CRF and the nociceptin/orphanin FQ/NOP system interact in the DRN during stress to control 5HT transmission; this may play a role in stress-related neuropsychopathologies.

Inhibition of serotonin outflow by nociceptin/orphaninFQ in dorsal raphe nucleus slices from normal and stressed rats: Role of corticotropin releasing factor

In the dorsal raphe nucleus (DRN) many inputs converge and interact to modulate serotonergic neuronal activity and the behavioral responses to stress. The effects exerted by two stress-related neuropeptides, corticotropin releasing factor (CRF) and nociceptin/orphaninFQ (N/OFQ), on the outflow of [(3)H]5- hydroxytryptamine were investigated in superfused rat dorsal raphe nucleus slices. Electrical stimulation (100 mA, 1 ms for 2 min) evoked a frequency-dependent peak of [(3)H]5- hydroxytryptamine outflow, which was sodium and calcium-dependent. Corticotropin releasing factor (1-100 nM), concentration-dependently inhibited the stimulation (3 Hz)-evoked [(3)H]5-hydroxytryptamine outflow; the inhibition by 30 nM corticotropin releasing factor (to 68 +/- 5.7%) was prevented both by the non selective CRF receptor antagonist alpha-helicalCRF(9-41) (alpha-HEL) (300 nM) and by the CRF(1) receptor antagonist antalarmin (ANT) (100 nM). The CRF(2) agonist urocortin II (10 nM) did not modify [(3)H]5- hydroxytryptamine outflow, ruling out the involvement of CRF(2) receptors. Bicuculline (BIC), a GABAA antagonist (10 microM), prevented the inhibitory effect of corticotropin releasing factor (30 nM), supporting the hypothesis that the inhibition was mediated by increased gamma-aminobutyric acid (GABA) release. Nociceptin/ orphaninFQ (1 nM-1 microM) exerted an antalarmin- and bicuculline-insensitive inhibition on [(3)H]5- hydroxytryptamine outflow, with the maximum at 100 nM (to 63+/- 4.2%), antagonized by the NOP receptor antagonist UFP-101 (1 microM). Dorsal raphe nucleus slices prepared from rats exposed to 15 min of forced swim stress displayed a reduced [(3)H]5-hydroxytryptamine outflow, in part reversed by antalarmin and further inhibited by nociceptin/orphaninFQ. These findings indicate that (i) both corticotropin releasing factor and nociceptin/orphaninFQ exert an inhibitory control on dorsal raphe nucleus serotonergic neurons; (ii) the inhibition by corticotropin releasing factor involves gamma-aminobutyric acid neurons; (iii) nociceptin/ orphaninFQ inhibits dorsal raphe nucleus serotonin system in a corticotropin releasing factor- and gamma-aminobutyric acid-independent manner; (iv) nociceptin/orphaninFQ modulation is still operant in slices prepared from stressed rats. The nociceptin/orphaninFQ-NOP receptor system could represent a new target for drugs effective in stress-related disorders.

Nociceptin/orphanin FQ modulates motor behavior and primary motor cortex output through receptors located in substantia nigra reticulata

This study was set to investigate whether motor effects of nociceptin/orphanin FQ (N/OFQ) can be related to changes in primary motor cortex output. N/OFQ injected i.c.v. biphasically modulated motor performance, low doses being facilitating and higher ones inhibitory. These effects were counteracted by the N/OFQ receptor antagonist [Nphe(1) Arg(14),Lys(15)]N/OFQ-NH(2) (UFP-101) confirming the specificity of N/OFQ action. However, UFP-101 alone facilitated motor performance, suggesting that endogenous N/OFQ inhibits motor function. N/OFQ and UFP-101 injected into the substantia nigra reticulata but not motor cortex replicated these effects, suggesting motor responses were mediated by subcortical circuits involving the basal ganglia. Intracortical microstimulation technique showed that i.c.v. N/OFQ also biphasically modulated motor cortex excitability and movement representation. Low N/OFQ doses caused a leftward shift of threshold distribution curve in the forelimb area without affecting the number of effective sites. Conversely, high N/OFQ doses increased unresponsive and reduced excitable (movement) sites in vibrissa but not forelimb area. However, increased threshold currents and rightward shift of threshold distribution curve were observed in both areas, suggesting an overall inhibitory effect on cortical motor output. UFP-101 alone evoked effects similar to low N/OFQ doses, suggesting tonic inhibitory control over forelimb movement by endogenous N/OFQ. As shown in behavioral experiments, these effects were replicated by intranigral, but not intracortical, N/OFQ or UFP-101 injections. We conclude that N/OFQ receptors located in the substantia nigra reticulata mediate N/OFQ biphasic control over motor behavior, possibly through changes of primary motor cortex output.

GABA(A) signalling is involved in N/OFQ anxiolytic-like effects but not in nocistatin anxiogenic-like action as evaluated in the mouse elevated plus maze

Nociceptin/orphanin FQ (N/OFQ) and nocistatin are two neuropeptides originated from the same precursor prepronociceptin/orphanin FQ (ppN/OFQ). N/OFQ is the endogenous ligand of the NOP receptor, while the target of action of nocistatin is still unknown. N/OFQ modulates various biological functions, including anxiety. Conversely, nocistatin either behaves as a functional N/OFQ antagonist or evokes per se effects opposite to those of N/OFQ. Here we investigated the interaction between the anxiolytic-like effects of N/OFQ and the anxiogenic-like action of nocistatin with those evoked by GABA(A) receptor ligands in the mouse elevated plus maze. The anxiogenic-like effects of the GABA(A) receptor antagonist pentylenetetrazol (20mg/kg; intraperitoneal, i.p.) were abolished by the co-treatment with N/OFQ (10pmol; intracerebroventricular, i.c.v.) while potentiated by the administration of nocistatin (0.01pmol; i.c.v.). The anxiolytic-like effects of the benzodiazepine receptor agonist diazepam (0.75mg/kg, i.p.) were reversed by nocistatin (0.1pmol; i.c.v.), whereas signs of sedation were observed when mice were co-treated with diazepam and N/OFQ (3pmol). Interesting enough, the i.p. treatment with flumazenil (1mg/kg) blocked the anxiolytic-like effects of N/OFQ (10pmol; i.c.v.), but not the anxiogenic effect elicited by nocistatin. Collectively, our findings suggest that the effects on anxiety elicited by pentylenetetrazol and diazepam can be counteracted or potentiated in the presence of N/OFQ and nocistatin. In addition, the effects on anxiety of N/OFQ, but not nocistatin, appear to be dependent on the benzodiazepine site of the GABA(A) receptor.

Opioid receptor binding in parahippocampus of patients with temporal lobe epilepsy: its association with the antiepileptic effects of subacute electrical stimulation

Opioid receptor binding was evaluated in parahippocampal cortex (PHC) obtained from patients with intractable mesial temporal lobe epilepsy (MTLE) with and without subacute high frequency electrical stimulation (HFS) in this brain area. Mu, delta and nociceptin receptor binding was determined by autoradiography in PHC of five patients (ESAE group) with MTLE history of 14.8 +/- 2.5 years and seizure frequency of 11 +/- 2.9 per month, two of them (40%) with mesial sclerosis. This group demonstrated antiepileptic effects following subacute HFS (130 Hz, 450 micros, 200-400 microA), applied continuously during 16-20 days in PHC. Values were compared with those obtained from patients with severe MTLE (history of 21.7 +/- 2.8 years and seizure frequency of 28.2 +/- 14 per month) in whom electrical stimulation did not induce antiepileptic effects (ESWAE group, n = 4), patients with MTLE in whom no electrical stimulation was applied (MTLE group, n = 4) and autopsy material acquired from subjects without epilepsy (n = 4 obtained from three subjects). Enhanced 3H-DAMGO (MTLE, 755%; ESAE, 375%; ESWAE, 693%), 3H-DPDPE (MTLE, 242%; ESAE, 80%; ESWAE, 346%) and 3H-nociceptin (MTLE, 424%; ESAE, 217%; ESWAE, 451%) binding was detected in the PHC of all epileptic groups. However, tissue obtained from ESAE group demonstrated lower opioid receptor binding (3H-DAMGO, 44.5%, p < 0.05; 3H-DPDPE, 47%, p < 0.05; 3H-nociceptin, 39.3%, p < 0.5) when compared with MTLE group. The present results indicate that a high effectiveness to the antiepileptic effects induced by HFS is associated with reduced opioid peptide binding.

Serotonin modulation of cell excitability and of [3H]GABA and [3H]D-aspartate efflux in primary cultures of rat cortical neurons

The effects of 5-hydroxytryptamine (5-HT) on neuronal excitability, evaluated as depolarization-induced firing rate, and on amino acid release, measured as electrically-evoked [(3)H]GABA and [(3)H]d-aspartate efflux, were investigated in rat primary cortical neuronal cultures. 5-HT displayed a concentration-dependent, bimodal effect on neuronal excitability: at 3-10microM it increased excitability through 5-HT(2A) receptors, and was blocked by the selective 5-HT(2A) antagonist MDL 100907, whereas at 30-100microM it reduced excitability through 5-HT(1A) receptors, and was, in turn, blocked by the selective 5-HT(1A) antagonist WAY 100135. The electrically-evoked [(3)H]GABA efflux was concentration-dependently inhibited by 5-HT (pEC(50)=4.74) and such inhibition was prevented by WAY 100135, but not by GR 55562, a selective 5-HT(1D/B) receptor antagonist. Conversely, 5-HT concentration-dependently increased stimulus-evoked [(3)H]d-aspartate efflux (pEC(50)=4.71). The increase was facilitated by methiothepin and was reversed into inhibition by ICS 205930, a selective 5-HT(3) receptor antagonist. In the presence of ICS 205930, the inhibition induced by 5-HT was prevented by the selective 5-HT(1D/B) receptor antagonist GR 55562, but not by WAY 100135. These findings suggest that 5-HT inhibits GABA release through 5-HT(1A) receptors and exerts a dual modulation on glutamate release, mostly facilitatory (through 5-HT(3) receptors) but also inhibitory (through 5-HT(1D/B) receptors), leading to a prevalently positive modulation of the excitatory signal by amino acid neurotransmitter containing neurons.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.