Mu, delta, and kappa opioid receptor mRNA expression in the rat CNS: an in situ hybridization study

mu-Opioid and delta-opioid receptors are expressed in brainstem antinociceptive circuits: studies using immunocytochemistry and retrograde tract-tracing

Opioid-produced antinociception in mammals seems to be mediated in part by pathways originating in the periaqueductal gray (PAG) and the rostroventral medulla (RVM), and these pathways may include serotonergic neurons. In the present study, we examined the relationship of the cloned mu- and delta-receptors (MOR1 and DOR1, respectively) to PAG neurons projecting to the RVM, and RVM neurons projecting to the dorsal spinal cord. This was carried out by combining immunocytochemical staining for MOR1, DOR1, and serotonin with fluorescent retrograde tract-tracing. Of 133 retrogradely labeled cells in the RVM, 31% were immunoreactive for MOR1. Of the double-labeled cells, 41% also were immunoreactive for 5HT. Fifty-three percent of retrogradely labeled cells were apposed by DOR1-ir varicosities; 29% of the apposed cells were immunoreactive for 5HT. In the mesencephalon, cells retrogradely labeled from the RVM were usually surrounded by MOR1-ir structures; however, retrogradely labeled cells were never observed to be immunoreactive for MOR1. Similarly, retrogradely labeled cells in the caudal midbrain were seldom, if ever, labeled for DOR1; however, they frequently were apposed by DOR1-ir varicosities. Of 156 retrogradely labeled profiles from three rats, 52 (33%) were apposed by DOR1-ir varicosities. We conclude that both mu- and delta-opioid receptors could be involved in the antinociception mediated by the PAG-RVM-spinal cord circuit. In addition, opioids seem likely to have both direct and indirect effects on spinally projecting RVM cells in general, and on serotonergic RVM cells in particular.

Cellular and subcellular localization of delta opioid receptor immunoreactivity in the rat dentate gyrus

To study a potential locus of action of opioids in the rat dentate gyrus, we examined the localization of the delta opioid receptor (DOR) by immunocytochemistry. Two antisera raised to unique, non-overlapping peptide sequences located within the extracellular N-terminal sequence of DOR were tested. By light microscopy, numerous neurons in the central hilar region were intensely labeled for DOR, while the granule cell layer contained light DOR immunoreactivity. To further characterize hilar neuron cell types which contained DOR, sections through the dentate gyrus were double labeled using immunofluorescence with antisera to DOR and either gamma-aminobutyric acid (GABA), neuropeptide Y (NPY), or somatostatin-28 antisera. Most DOR-labeled perikarya also contained GABA and NPY, while a subpopulation contained somatostatin. Electron microscopic examination of sections labeled for DOR revealed that the immunoreactivity was common in profiles which exhibited the morphological characteristics of granule cells, as well as those of non-granule cells. DOR immunoreactivity was located at postsynaptic sites within neuronal perikarya (2%), dendrites (27%), and dendritic spines (22%); as well as in presynaptic axon terminals (25%) and glia (23%) (n = 279). In dendrites and dendritic spines, DOR immunoreactivity was most often associated with the plasmalemmal surface near asymmetric synapses. In axon terminals, DOR immunoreactivity primarily surrounded small, clear vesicles, and was less consistently found on the plasmalemmal surface. The distribution of DOR-labeled profiles overlapped with, but was not restricted to regions known to contain enkephalin. These data suggest that opiates acting at the DOR can modulate both hilar neurons and granule cells both pre- and postsynaptically.

Delta-Opioid receptor modulation of the release of substance P-like immunoreactivity in the dorsal horn of the rat following mechanical or thermal noxious stimulation

The present study was undertaken to investigate the effects of the opioid peptide Met-enkephalin (met-enk) on the release of substance P-like immunoreactivity (SPLI) in the lumbar dorsal horn during the application of a noxious mechanical or thermal stimulus to the ipsilateral hind paw and lower limb of the rat. A push-pull cannula was introduced to the lumbar dorsal horn in non-anesthetized decerebrate/spinal transected rats. The dorsal horn was perfused with artificial CSF and the collected perfusates were assayed for SPLI using radioimmunoassay. A noxious mechanical or thermal stimulus was applied to different areas of the ipsilateral hind paw and lower limb. Met-enk (500 nM) applied to the dorsal horn through the perfusate reduced the basal release of SPLI by 29 +/- 9% and prevented the increase in the release of SPLI evoked by the noxious mechanical or thermal stimulus. The effect of met-enk was blocked by the selective delta-opioid receptor antagonist naltrindole (500 nM). Naltrindole (NTD) alone elicited a 75 +/- 30% increase in the basal release of SPLI. These data show that met-enk inhibits the thermally or mechanically evoked release of SPLI in the dorsal horn by activating the delta opioid receptors. These receptors are also involved in the tonic spinal regulation of the release of SPLI.

Chronic morphine administration enhances the expression of Kv1.5 and Kv1.6 voltage-gated K+ channels in rat spinal cord

Prolonged opiate administration leads to the development of tolerance and dependence. These phenomena are accompanied by selective regulation of distant cellular proteins and mRNAs, including ionic channels. Acute opiate administration differentially affects voltage-dependent K+ currents. Whereas, opiate activation of K+ channels is well established opioid-induced inhibition of K+ conductance has also been studied. In this study, we focused on the effect of chronic morphine exposure on voltage-dependent Shaker-related Kv1.5 and Kv1.6 K+ channel gene expression and on Kv1.5 protein levels in the rat spinal cord. Several experimental approaches including in-situ hybridization, RNAse protection, reverse transcriptase-polymerase chain reaction (RT-PCR), Western blotting and immunohistochemistry were employed. We found that motor neurons are highly enriched in Kv1.5 and Kv1.6 mRNA and in Kv1.5 channel protein. Moreover, we found significant increases in the amount of mRNA encoding for these two K+ channels and in Kv1.5 channel protein in the spinal cord of morphine-treated rats, compared with controls. For example, quantitative in-situ hybridization, revealed a 2.1 +/- 0.15- and 2.3 +/- 0.5-fold increase in Kv1.5 and Kv1.6 channel mRNA levels, respectively. Similar results were obtained by semiquantitative RT-PCR analyses. Kv1.5 protein level was increased by 1.9-fold in the spinal cord or morphine-treated rats. Our results suggest that Kv1.5 and Kv1.6 Shaker K+ channels play an important role in regulating motor activity that increases in mRNA and protein levels of the spinal cord K+ channels after chronic morphine exposure could be viewed as a cellular adaptation which compensates for a persistent opioid-induced inhibition of K+ channel activity. These alterations may account, in part, for the cellular events leading to opiate tolerance and dependence.

Opposite changes in the expression of enkephalin in the amygdala and hypothalamus after lesions of the bed nucleus of the stria terminalis in the rat

The expression of enkephalin in neurons of the rat forebrain was studied by in situ hybridization and immunohistochemistry after unilateral injections of ibotenic acid into the bed nucleus of the stria terminalis. Initially, we observed that the destruction of nerve cell bodies in this nucleus resulted in a prominent bilateral increase in the number of neuronal perikarya immunoreactive for [Met]enkephalin in the lateral/basolateral amygdaloid complex-especially in the anterior division of the latter nucleus-as compared with NaCl-injected rats. In a separate set of experiments, this effect was associated with a significant (two times) enhancement of the number of nerve cell bodies containing preproenkephalin A messenger RNAs in the same amygdaloid nucleus ipsilateral to the injection, as compared with controls. In the hypothalamus of both experimental and control rats, the nerve cell bodies immunoreactive for [Met]enkephalin were few since the animals were not pretreated with colchicine, and the effects of the lesion were difficult to appreciate. However, using in situ hybridization, numerous nerve cell bodies containing preproenkephalin A messenger RNAs were detected bilaterally in the perifornical area, the paraventricular (parvocellular division) and the ventromedial nuclei of the hypothalamus. In the latter nucleus, the lesion of the bed nucleus of the stria terminalis resulted in a strong decrease (about two times) in the number of labelled cell bodies as compared with the controls, whereas no significant changes were found bilaterally in the paraventricular nucleus. In agreement with some data of the literature, our results indicate that the bed nucleus of the stria terminalis plays an important role in the regulation of neuropeptide genes expression in certain regions of the limbic system. Such a role is often exerted by nerve fibres afferents to the nerve cell bodies considered. However, from numerous neuroanatomical data of the literature, it appears more probable that the induction or inhibition of the expression of enkephalin in presynaptic neurons is due to the disappearance of their postsynaptic target in the bed nucleus of the stria terminalis.

Immunolabeling of Mu opioid receptors in the rat nucleus of the solitary tract: extrasynaptic plasmalemmal localization and association with Leu5-enkephalin

Activation of the mu opioid receptor (MOR) by morphine within the caudal nucleus of the solitary tract (NTS) is known to mediate both cardiorespiratory and gastrointestinal responses. Leu5-enkephalin (LE), a potential endogenous ligand for MOR, is also present within neurons in this region. To determine the cellular sites for the visceral effects of MOR ligands, including LE, we used immunogold-silver and immunoperoxidase methods for light and electron microscopic localization of antisera against MOR (carboxyl terminal domain) and LE in the caudal NTS of rat brain. Light microscopy of coronal sections through the NTS at the level of the area postrema showed MOR-like immunoreactivity (MOR-LI) and LE labeling in punctate processes located within the subpostremal, dorsomedial and medial subnuclei. Electron microscopy of sections through the medial NTS at this level showed gold-silver particles identifying MOR-LI prominently distributed to the cytoplasmic side of the plasma membranes of axons and terminals. MOR labeled terminals formed mostly symmetric (inhibitory-type) synapses but sometimes showed multiple asymmetric junctions, characteristic of excitatory visceral afferents. MOR-LI was also present along extrasynaptic plasma membranes of dendrites receiving afferent input from unlabeled and LE-labeled terminals. We conclude that MOR ligands, possibly including LE, can act at extrasynaptic MORs on the plasma membranes of axons and dendrites in the caudal NTS to modulate the presynaptic release and postsynaptic responses of neurons. These are likely to include local inhibitory neurons and both gastric and cardiorespiratory afferents known to terminate in the subnuclei with the most intense MOR-LI.

Naloxone blockade of (-)pentazocine-induced changes in auditory function

OBJECTIVE

In a previous report, we found that intravenous (i.v.) (-)pentazocine improved auditory sensitivity and significantly altered compound action potential (CAP) amplitudes. Its sigma (sigma)-receptor-selective optical isomer (+)pentazocine administered at the same dose was without effect, suggesting that the observed auditory neural effects might be mediated by an opioid receptor. To directly test this hypothesis, in the present investigation we attempted to antagonize the auditory neural effects of (-)pentazocine using the pure, nonspecific drug antagonist naloxone.

DESIGN

In 25 normal-hearing, male, pigmented chinchillas, amplitude and latency changes in the click-evoked auditory nerve CAP (N1) and cochlear microphonic (CM) were tracked at six stimulus intensities during a baseline period and after the postbaseline administration of the opioid drug agonist (-)pentazocine (16 mg/kg; i.v.). In separate groups of chinchillas, (-)pentazocine was given alone or administered in combination with the standard opioid receptor antagonist naloxone administered at two doses.

RESULTS

Robust changes in CAP amplitudes after (-)pentazocine occurred in the absence of measurable alterations in CAP response latencies, CM amplitudes, or blood chemistries and were significantly antagonized when naloxone (5 mg/kg) was added to the i.v. infusion.

CONCLUSIONS

The observed blockade clearly indicates that the agonist effects of (-)pentazocine are opioid receptor-mediated and suggests a connection between opioid receptors and auditory neural function. Mechanisms of action and the connection between an opioid modulation of auditory function and stress, hyperacusis, and tinnitus are discussed.

Localization of mu-opioid receptor-like immunoreactivity in the central components of the vagus nerve: a light and electron microscope study in the rat

mu-Opioid receptor, the opioid receptor that shows the highest affinity for morphine, appears to induce a variety of side-effects, at least partly, directly through the mu-opioid receptor on neurons constituting the autonomic part of the vagus nerve. Thus, in the present study, location of mu-opioid receptor-like immunoreactivity in the central components of the autonomic part of the vagus nerve was examined in the rat. The intense immunoreactivity was observed light microscopically in the neuropil of the commissural subnucleus and the dorsal part of the medial subnucleus of the nucleus of the solitary tract, and in the neuropil of the rostral half of the ambiguus nucleus. The immunoreactivity was moderate in the neuropil of the rostral and lateral subnuclei and ventral part of the medial subnucleus of the nucleus of the solitary tract, and weak in the neuropil of the dorsal motor nucleus of the vagus nerve. In the nodose ganglion, many neurons of various sizes (17-48 microns in soma diameter) showed moderate immunoreactivity. After unilateral vagotomy at a level proximal to the nodose ganglion, the immunoreactivity in the ipsilateral ambiguus nucleus was apparently reduced within 48 h of the operation, and completely disappeared by the seventh day after the operation. In the nucleus of the solitary tract and dorsal motor nucleus of the vagus nerve, the reduction of immunoreactivity after the ganglionectomy was detectable on the fourth day after the operation, and became readily apparent by the seventh day after the operation; the immunoreactivity, none the less, still remained on the 10th day after the operation. Electron microscopically, the immunoreactivity in the ambiguus nucleus was seen mainly on dendritic profiles and additionally on somatic ones; no immunoreactivity was detected in axonal profiles. The immunoreactivity in the dorsal motor nucleus of the vagus nerve was observed only on dendritic profiles. The immunoreactivity in the nucleus of the solitary tract was seen on axonal and dendritic profiles, but not on somatic profiles. The immunoreactive axon terminals in the nucleus of the solitary tract were filled with spherical synaptic vesicles and made asymmetric synapses with dendritic profiles. The results indicate that the mu-opioid receptor in the central components of the autonomic part of the vagus nerve is located on dendrites and cell bodies of efferent neurons in the ambiguus, on dendrites of efferent neurons in the dorsal motor nucleus, and on axons which arise from nodose ganglion neurons and terminate in the nucleus of the solitary tract. The receptors on these structures may constitute the targets of enkephalin-containing and beta-endorphin-containing afferent axons arising from brainstem neurons. The receptors on the axon terminals of nodose ganglion neurons may be involved in regulation of the release of neurotransmitters and/or neuromodulators.

Immunohistochemical localization of mu-opioid receptors in the central nervous system of the rat

Of the three major types of opioid receptors ( mu, delta, kappa) in the nervous system, mu-opioid receptor shows the highest affinity for morphine that exerts powerful effects on nociceptive, autonomic, and psychological functions. So far, at least two isoforms of mu-opioid receptors have been cloned from rat brain. The present study attempted to examine immunohistochemically the distribution of mu-opioid receptors in the rat central nervous system with two kinds of antibodies to recently cloned mu-opioid receptors (MOR1 and MOR1B). One antibody recognized a specific site for MOR1, and the other bound to a common site for MOR1 and MOR1B. Intense MOR1-like immunoreactivity (LI) was seen in the 'patch' areas and subcallosal streak in the striatum, medial habenular nucleus, medial terminal nucleus of the accessory optic tract, interpeduncular nucleus, median raphe nucleus, parabrachial nuclei, locus coeruleus, ambiguous nucleus, nucleus of the solitary tract, and laminae I and II of the medullary and spinal dorsal horns. Many other regions, including the cerebral cortex, amygdala, thalamus, and hypothalamus, also contained many neuronal elements with MOR1-LI. The distribution pattern of the immunoreactivity revealed with the antibody to the common site for MOR1 and MOR1B (MOR1/1B-LI) was almost the same as that of MOR1-LI. Both MOR1-LI and MOR1/1B-LI were primarily located in neuronal cell bodies and dendrites. However, the immunoreactivities were observed in the accessory optic tract, fasciculus retroflexus, solitary tract, and primary afferent fibers in the superficial layers of the medullary and spinal dorsal horns. The presynaptic location of MOR1-LI and MOR1/1B-LI was confirmed by lesion experiments: Enucleation, placing a lesion in the medial habenular nucleus, removal of the nodose ganglion, or dorsal rhizotomy resulted in a clear reduction of the immunoreactivities, respectively, in the nuclei of the accessory optic tract, some subnuclei of the interpeduncular nucleus, nucleus of the solitary tract, or laminae I and II of the spinal dorsal horn. The results indicate that the mu-opioid receptors are widely distributed in the brain and spinal cord, mainly postsynaptically and occasionally presynaptically. Opioids, including morphine, may inhibit the excitation of neurons via the postsynaptic mu-opioid receptors, and also suppress the release of neurotransmitters and/or neuromodulators from axon terminals through the presynaptic mu-opioid receptors.

Immunohistochemical localization of the cloned kappa 1 receptor in the rat CNS and pituitary

Several lines of evidence have demonstrated the presence of three opioid receptor types in the CNS and periphery. These receptors are referred to as mu, delta and kappa, and have been implicated in a wide variety of functions. The present study examines the localization of the kappa 1 receptor, a region of the receptor that has little homology with mu and delta receptors. Immunohistochemical studies in Zamboni-fixed rat tissue demonstrate immunoreactive perikarya and/or fibers in such regions as the deep layers of the parietal, temporal and occipital cortex, parasubiculum, central and medial amygdala, bed nucleus stria terminalis, nucleus accumbens, olfactory tubercle, endopiriform nucleus, claustrum, hypothalamic nuclei, median eminence, midline thalamic nuclei, zona incerta, central gray, caudal linear and dorsal raphe, substantia nigra, pars reticulata, ventral tegmental area, parabrachial nucleus, spinal trigeminal nucleus, nucleus of the solitary tract, spinal cord and the dorsal root ganglia. Specific kappa 1 receptor-like immunohistochemical staining is also observed in the pituitary, where immunoreactive perikarya and fibers are localized in the neural and intermediate lobes. Transfection and preabsorption controls suggest that the antibody is selective for the cloned kappa 1 receptor, and does not recognize mu or delta. This immunohistochemical localization corresponds well to previously described kappa 1 receptor mRNA and binding distributions and provides new insights into the cellular localization and pre- and postsynaptic organization of the kappa 1 receptor-like proteins in the rat brain and pituitary. The functional implications of these results are discussed in light of the kappa 1 receptors play in hormonal regulation, antinociception and reward.

Intraseptal microinjection of beta-funaltrexamine blocked a microwave-induced decrease of hippocampal cholinergic activity in the rat

Acute (45 min) exposure to pulsed (2 microseconds pulse width, 500 pulses per second) 2450-MHz microwaves at a power density of 1 mW/cm2 (whole body specific absorption rate 0.6 W/kg) microwaves caused a decrease in cholinergic activity in the hippocampus of the rat as measured by the sodium-dependent high-affinity choline uptake. Microinjection of beta-funaltrexamine (1 microgram) into the septum before microwave exposure blocked this effect. These data indicate that mu-opioid receptors in the septum mediate a microwave-induced decrease in cholinergic activity in the hippocampus and support our hypothesis that microwaves at a whole body SAR of 0.6 W/kg can activate endogenous opioids in the brain.

Alterations in opiate receptor binding in the hippocampus of aged Long-Evans rats

Quantitative in vitro autoradiography was used to examine [3H]D-Ala2, MePhe4, Gly-015 enkephalin (DAGO) (mu-agonist) and [3H]diprenorphine (general opiate antagonist) binding sites in the hippocampal formation of young (6-8 months) and aged (25-28 months) Long-Evans rats. Age-related changes in these binding sites were restricted to specific regions but were not generally dependent on the ligand used. No reliable age-related changes in opiate binding were observed in the CA1 field or subicular region. In contrast, a decrease in the density of binding was found in both dorsal and ventral hippocampus within the CA3 field of aged brains. An age-related decrease in opiate binding within the dentate gyrus differed in its topography at dorsal and ventral levels of the hippocampus. A uniform decrease of opiate receptor binding was found throughout the dorsal dentate gyrus, while a more localized decrease of these sites occurred in hilar and granular layers of the ventral dentate gyrus. These results indicate that a decrease of opiate binding in the hippocampal formation is largely localized to the CA3 region and dentate gyrus of aged rats. These findings are discussed with reference to age-related changes in hippocampal pathways containing opioid peptides. The implications for hippocampal opioid function in learning and age-related cognitive decline are also considered.

Endogenous opioid regulation of hippocampal function

Endogenous opioid peptides modulate neural transmission in the hippocampus. Procnkephalin-derived peptides have been demonstrated to act at mu and delta opioid receptors to inhibit GABA release from inhibitory interneurons, resulting in increased excitability of hippocampal pyramidal cells and dentate gyrus granule cells. Prodynorphin-derived peptides primarily act at presynaptic kappa opioid receptors to inhibit excitatory amino acid release from perforant path and mossy fiber terminals. Opioid receptors reduce membrane excitability by modulating ion conductances, and in this way they may decrease voltage-dependent calcium influx and transmitter release. Synaptic plasticity in the hippocampus also is modulated by endogenous opioids. Enkephalins facilitate long-term potentiation, whereas dynorphins inhibit the induction of this type of neuroplasticity. Further, opioids may play important roles in hippocampal epilepsy. Recurrent seizures induce changes in the expression of opioid peptides and receptors. Also, enkephalins have proconvulsant effects in the epileptic hippocampus, whereas dynorphins may function as endogenous anticonvulsants.

Presynaptic localization of mu-opioid receptor-like immunoreactivity in retinal axon terminals within the terminal nuclei of the accessory optic tract: a light and electron microscope study in the rat

Neuropil within the terminal nuclei of the accessory optic tract of the rat showed intense to moderate mu-opioid receptor-like immunoreactivity (MOR-LI). After unilateral enucleation, MOR-LI within the terminal nuclei almost disappeared or was markedly reduced on the side contralateral to the operation. Electron microscopy revealed that MOR-LI axon terminals within the terminal nuclei were filled with round synaptic vesicles and in asymmetric synaptic contact mainly with dendritic profiles, and occasionally with somatic profiles.

Molecular biology of the opioid receptors: structures, functions and distributions

Opiates like morphine and endogenous opioid peptides exert their pharmacological and physiological effects through binding to their endogenous receptors, opioid receptors. The opioid receptors are classified into at least three types, mu-, delta- and kappa-types. Recently, cDNAs of the opioid receptors have been cloned and have greatly advanced our understanding of their structure, function and expression. This review focuses on the recent advances in the studies on opioid receptors using the cloned cDNAs. We describe the molecular cloning of the opioid receptor gene family and studies of the structure-function relationships, modes of coupling to second messenger systems, pharmacological effects of antisense oligonucleotide and anatomical distributions of opioid receptors.

Quantitative distribution of the delta opioid receptor mRNA in the mouse and rat CNS

We have used a sensitive solution hybridization assay that employs a riboprobe obtained from the mouse delta opioid receptor (DOR) coding sequence to quantitate the relative abundance of DOR mRNA in the central nervous system (CNS) of the adult mouse and rat. In brain Poly A+ RNA extracts this riboprobe hybridized to a single 10 kb transcript from mouse and two transcripts, one of 12 and the other of 4.5 kb in size from rat. In mouse CNS the highest levels of DOR mRNA were found in the caudate-putamen at 3.3 +/- 0.1 (SEM) pg/micrograms RNA. DOR mRNA levels in the range from 2.6 to 2.1 pg/micrograms RNA were measured in frontal cortex, nucleus accumbens, whole brain and olfactory tubercle. Spinal cord, periaqueductal gray and hippocampus had DOR mRNA levels in the range from 1.8 to 1.5 pg/micrograms RNA, while medial thalamus and cerebellum had the lowest levels (0.5 pg/micrograms RNA). These results correlate with the reported relative distribution of DOR mRNA in mouse using an in situ hybridization technique. In rat CNS, the highest levels of DOR mRNA were measured in caudate-putamen at 2.3 +/- 0.1 pg equivalents/micrograms RNA. Whole brain, cerebral cortex, olfactory bulb and brain stem had levels in the range from 1.5 to 0.9 pg equivalents/micrograms RNA while the lowest DOR mRNA levels were measured at 0.5 pg equivalents/micrograms RNA or less in thalamus, hippocampus, substantia nigra and cerebellum. This study demonstrates the ability of solution hybridization assays to quantitate homologous (mouse) as well as similar but heterologous (rat) DOR mRNA levels.