Evidence of the synthesis of opioid receptor like 1 receptor in nociceptinergic neurons in rat brain suggests the existence of autoreceptor: a confocal double staining study

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.

Nociceptin/orphanin FQ decreases serotonin efflux in the rat brain but in contrast to a kappa-opioid has no antagonistic effect on mu-opioid-induced increases in serotonin efflux

Similar to kappa-opioids, nociceptin/orphanin FQ (OFQ) exerts anti-mu-opioid actions. This may involve interactions within the circuitry controlling 5-HT neurons in the dorsal raphe nucleus (DRN) that project to the nucleus accumbens (NAcc). To test this hypothesis, we compared the effects of OFQ and kappa-opioids on 5-HT efflux in the CNS of freely behaving rats. First, OFQ (30-300 microM) infused into the DRN for 120 min dose-dependently decreased 5-HT efflux in the DRN. The opioid receptor-like 1 (ORL-1) antagonist [Nphe(1)]nociceptin(1-13)NH(2) blocked this effect. Using dual-probe microdialysis we observed that OFQ (300 microM) infused into the DRN for 120 min produced parallel decreases in 5-HT efflux in the DRN and NAcc, suggesting that ORL-1 receptors in the DRN inhibit serotonergic neurons projecting to the NAcc. Also, 5-HT efflux in the NAcc was dose-dependently decreased during OFQ (30-300 microM) infusion into the NAcc. This suggests that OFQ can reduce 5-HT efflux in the NAcc both by inhibiting serotonergic neurons in the DRN and by stimulating ORL-1 receptors in the NAcc. Similar to OFQ, the kappa-opioids U-50,488 (300 microM) and dynorphin A(1-13) (300 microM) infused into the DRN for 120 min decreased 5-HT efflux in the DRN. This effect was blocked only by the kappa-opioid receptor antagonist nor-BNI. Lastly, we compared the ability of OFQ and U-50,488 to block mu-opioid-induced increases in 5-HT. The kappa-opioid U-50,488 (1000 microM) attenuated the increase in 5-HT induced by the mu-opioid agonist endomorphin-1 (300 microM) in the DRN. In contrast, OFQ (300-1000 microM) did not alter mu-opioid-induced increases in 5-HT efflux. In summary, kappa-opioids and OFQ both decreased 5-HT efflux in the CNS. However, in contrast to kappa-opioids, which reversed mu-opioid-induced increases in 5-HT efflux, the anti-mu-opioid effects of OFQ apparently do not involve changes in 5-HT transmission under our experimental conditions.

The existence of opioid receptors in the cochlea of guinea pigs

Several independent investigations have demonstrated the presence of opioid peptides in the inner ear organ of Corti and in particular in the efferent nerve fibers innervating the cochlear hair cells. However, the precise innervation pattern of opioid fibers remains to be investigated. In the present study the expression of opioid receptors and their peptides is demonstrated in young adult guinea pig cochlea. Opioid receptors are mainly expressed in hair cells of the organ of Corti and in inner and outer spiral bundles with different characteristics for each type of receptor. Co-localization studies were employed to compare the distribution of mu-, delta- and kappa-opioid receptors and their respective peptides, beta-endorphin, leu-enkephalin and dynorphin. Additionally, immunostaining of synaptophysin was used in this study to identify the presynaptic site. Immunoreactivity for enkephalin and dynorphin was found in the organ of Corti. Leu-enkephalin was co-localized with synaptophysin prominently in the inner spiral bundle (ISB). Dynorphin was co-localized with synaptophysin in both inner and outer spiral bundles. Delta-opioid receptor was most prominently co-localized with its peptide in the ISB bundle. Kappa-opioid receptor was seemingly present with dynorphin in both inner and outer spiral bundles. The co-staining of both peptides and receptors with synaptophysin in the same areas suggests that some of the opioid receptors may act as auto-receptors. The results provide further evidence that opioids may function as neurotransmitters or neuromodulators in the cochlea establishing the basis for further electrophysiological and pharmacological investigations to understand better the roles of the opioid system in auditory function.

Opioid receptor-like 1 (NOP) receptors in the rat dorsal raphe nucleus: evidence for localization on serotoninergic neurons and functional adaptation after 5,7-dihydroxytryptamine lesion

A high density of opioid receptor-like 1 (ORL1) receptor (also referred to as NOP receptor) is found in limbic areas and in regions containing monoamines, which are implicated in emotional activity and physiopathology of depression and anxiety. We aimed at defining precisely the localization of ORL1 receptors in dorsal raphe nucleus, by means of a lesion strategy and autoradiographic studies. In control rats, [3H]nociceptin and nociceptin-stimulated [35S]GTPgammaS bindings were found to be correlated in several brain regions. We performed in rats a selective destruction of serotoninergic neurons by surgical stereotaxic injection of 5,7-dihydroxytryptamine (5,7-DHT) in dorsal raphe nucleus. This led to a marked decrease in serotonin contents in striata and frontal cortices (about -60%) and in autoradiographic [3H]citalopram binding in posterior regions. In dorsal raphe nucleus, [3H]nociceptin binding was decreased to the same extent as [3H]citalopram binding, whereas it was unchanged in the other regions studied. Nevertheless, in the dorsal raphe, nociceptin-stimulated [35S]GTPgammaS binding was decreased to a lesser extent than [3H]nociceptin binding in 5,7-DHT-lesioned rats. The ratio between nociceptin-stimulated [35S]GTPgammaS binding and [3H]nociceptin binding was significantly increased in 5,7-DHT-lesioned rats compared with controls in this region. These data demonstrate 1) that ORL1 receptors are located on serotoninergic neurons in the dorsal raphe nucleus and 2) that, after a lesion, the functionality of remaining ORL1 receptors appears to be up-regulated, which could correspond to a compensatory mechanism.

Mechanism of cardiovascular effects of nociceptin microinjected into the nucleus tractus solitarius of the rat

Microinjections (100 nl) of 0.15, 0.31, 0.62, and 1.25 mmol/l of nociceptin into the medial nucleus tractus solitarius (mNTS) elicited decreases in mean arterial pressure (11 +/- 1.8, 20 +/- 2.1, 21.5 +/- 3.1, and 15.5 +/- 1.9 mmHg, respectively) and heart rate (14 +/- 2.7, 29 +/- 5.5, 39 +/- 5.2, and 17.5 +/- 3.1 beats/min, respectively). Because maximal responses were elicited by microinjections of 0.62 mmol/l nociceptin, this concentration was used for other experiments. Repeated microinjections of nociceptin (0.62 mmol/l) into the mNTS, at 20-min intervals, did not elicit tachyphylaxis. Bradycardia induced by microinjections of nociceptin into the mNTS was abolished by bilateral vagotomy. The decreases in mean arterial pressure and heart rate elicited by nociceptin into the mNTS were blocked by prior microinjections of the specific ORL1-receptor antagonist [N-Phe(1)]-nociceptin-(1-13)-NH(2) (9 mmol/l). Microinjections of the ORL1-receptor antagonist alone did not elicit a response. Prior combined microinjections of GABA(A) and GABA(B) receptor antagonists (2 mmol/l gabazine and 100 mmol/l 2-hydroxysaclofen, respectively) into the mNTS blocked the responses to microinjections of nociceptin at the same site. Prior microinjections of ionotropic glutamate receptor antagonists (2 mmol/l NBQX and 5 mmol/l d-AP7) also blocked responses to nociceptin microinjections into the mNTS. These results were confirmed by direct neuronal recordings. It was concluded that 1) nociceptin inhibits GABAergic neurons in the mNTS, 2) GABAergic neurons may normally inhibit the release of glutamate from the terminals of peripheral afferents in the mNTS, and 3) inhibition of GABAergic neurons by nociceptin results in an increase in the release of glutamate in the mNTS, which in turn elicits depressor and bradycardic responses via activation of ionotropic glutamate receptors on secondary mNTS neurons.

Expression of ORL1 mRNA in some brain nuclei in neuropathic pain rats

The present study was designed to investigate changes of opioid receptor like 1 receptor (ORL(1), NOP) mRNA expression in some pain-related brain nuclei of neuropathic pain rats using in situ hybridization technique. Nociceptin/orphanin FQ (N/OFQ), the endogenous ligand of ORL(1), plays an important role in neuropathic pain through its receptor. There are ORL(1) mRNA expression in the nucleus of raphe magnus (NRM), ventrolateral periaqueductal gray (vlPAG) and dorsal raphe nucleus (DRN) of rat mesencephalon. In the sciatic nerve chronic constriction injury (CCI)-induced neuropathic pain model, a significant increase of ORL(1) mRNA expression was observed in these three regions on the 7th day after operation, and the changes lasted for 2 weeks. The result indicated that ORL(1) synthesis was increased in NRM, vlPAG and DRN of neuropathic pain rats, suggesting that ORL(1) was involved in nociceptive transmission of neuropathic pain.

Cardiovascular responses to microinjections of nociceptin into a midline area in the commissural subnucleus of the nucleus tractus solitarius of the rat

Immunoreactivity for nociceptin, an endogenous ligand for the ORL1 opioid receptors, has been reported in the nucleus tractus solitarius (nTS). A midline area in the commissural subnucleus (nCom) of nTS is the site of peripheral chemoreceptor projections. This investigation was carried out in urethane-anesthetized, artificially ventilated, adult male Wistar rats, to study the cardiovascular effects of the activation of ORL1 receptors in a midline area of the nCom. Microinjections (30 nl) of nociceptin (0.15-0.62 mM) into the nCom elicited depressor and bradycardic responses. Prior microinjections of [N-Phe(1)]-nociceptin-(1-13)-NH(2) (4.5 mM), a specific antagonist for ORL1 opioid receptors, into the nCom blocked the effects of nociceptin (0.31 mM, the maximally effective concentration), but not endomorphin-2 (0.6 mM; an endogenous ligand for micro -opioid receptors). On of other hand, naloxone (0.125 mM; an antagonist for classical opioid receptors) did not block the effects of nociceptin, while it did block the effects of endomorphin-2. The blockade of nociceptin effects by [N-Phe(1)]-nociceptin-(1-13)-NH(2) and endomorphin-2 by naloxone, was not due to some nonspecific effects because the responses to L-Glu (5 mM) remained unaltered after the microinjection of these antagonists. These results indicate that activation of ORL1 receptors in the nCom may play a role in the regulation of cardiovascular function.

Distribution of nociceptin-like immunoreactivity in the central nervous system of the Mongolian gerbil: an immunohistochemical study

This study is designed to demonstrate the distribution of nociceptin, endogenous ORL1 receptor ligand, in the central nervous system of the Mongolian gerbil. To intensify the nociceptin-like immunoreactivity (NOC-LI), colchicine was administered into the lateral ventricle, at 48 h prior to the transcardiac perfusion. In the group without colchicine treatment, NOC-LI was observed in the fibres of the spinal dorsal horn, specifically in the superficial layers. However, the NOC-LI in the superficial layers disappeared after the administration of colchicine. In the brain, NOC-LI was prominent in the hypothalamus, hippocampus, cerebral peduncle, substantia nigra, dorsal raphe, periaqueductal grey, locus coeruleus and trapezoid nucleus. Colchicine treatment markedly intensified the NOC-LI in the somata of the central nervous system, whereas the untreated sections were too weak to observe and analyse. The distribution of NOC-LI provides informative data for studies of the neuronal circuit that nociceptin may be involved in.