Kappa 1 receptor mRNA distribution in the rat CNS: comparison to kappa receptor binding and prodynorphin mRNA

Distribution of opioid peptide gene expression in the limbic system of Fawn-Hooded (alcohol-preferring) and Wistar-Kyoto (alcohol-non-preferring) rats

Preprodynorphin and preproenkephalin mRNA expression was examined in the CNS of two rat strains, the alcohol-preferring Fawn-Hooded (FH) and the alcohol-non-preferring Wistar-Kyoto (WKY), using in situ hybridisation histochemistry. Relative to the WKY, the FH showed significantly lower levels of preproenkephalin mRNA in the striatum and nucleus accumbens (-24% and -17% respectively), but a higher level of preprodynorphin mRNA in the hippocampus (+33%). The depressed level of preproenkephalin mRNA in the nucleus accumbens may be implicated in alcohol-seeking behaviour.

Mu and kappa opioid receptors in periaqueductal gray and rostral ventromedial medulla

The periaqueductal gray (PAG) and rostral ventromedial medulla (RVM) are important brain stem pain modulating regions. Recent evidence suggests that kappa opioids antagonize the effects of mu opioids in the RVM. However, the anatomical relationship between mu and kappa opioid receptors in PAG and RVM is not well characterized. This study examined relationships between mu and kappa opioid receptor immunoreactivity (IR) and mRNA in PAG and RVM. Brain slices were processed for either immunocytochemistry or in situ hybridization. We found considerable anatomical overlap of mu and kappa opioid IR and mRNA in the RVM and PAG. These results provide an anatomical basis for recent behavioral and electrophysiological findings in RVM, and suggest modulatory interactions between mu and kappa opioids in PAG.

Effects of kappa and delta opioid agonists on activity and thermosensitivity of rat hypothalamic neurons

Extracellular recordings were made from 161 warm-sensitive, six cold-sensitive and 153 temperature-insensitive neurons in slices of the preoptic area/anterior hypothalamus (PO/AH) of rats, to investigate the effects of the kappa-receptor opioid agonist dynorphin A1-17 and the delta-receptor opioid agonist DPDPE on neuronal response characteristics. While 61% of the neurons exhibited kappa-receptors, delta-receptors were only present in 37% of the neurons. No co-localization was observed between kappa- and delta-receptors, whereas mu-receptors could be co-localized with kappa- as well as delta-receptors. Antagonistic effects on tonic activity were induced by different concentrations of the kappa-agonist dynorphin A1-17. At 0.5 nM, the excitatory effect was predominant, while 50% of the neurons were already inhibited at 5 nM and inhibition was the major effect at 100 nM. A significant increase in temperature sensitivity was observed in warm-sensitive neurons during administration of 0.5 nM dynorphin A1-17; in contrast, the temperature sensitivity was significantly decreased at the high dose of 100 nM. In most of the neurons responding to the delta-receptor agonist DPDPE (0.5-100 nM) the firing rate was decreased. The temperature sensitivity was only affected in warm-sensitive neurons, and was increased in the majority of neurons at 0.5 and 5 nM, but predominantly decreased at higher concentrations. The effects of low concentrations of dynorphin A1-17 and DPDPE were prevented by pre- and co-perfusion of the appropriate antagonists. The present results suggest that changes of the temperature sensitivity of warm-sensitive PO/AH neurons are an important mechanism for the effect of low doses of opioids on body temperature.

Down-regulation of mu-opioid receptors in rat and monkey dorsal root ganglion neurons and spinal cord after peripheral axotomy

To understand the role of opioids and their receptors in chronic pain following peripheral nerve injury, we have studied the mu-opioid receptor in rat and monkey lumbar 4 and 5 dorsal root ganglion neurons and the superficial dorsal horn of the spinal cord under normal circumstances and after peripheral axotomy. Our results show that many small neurons in rat and monkey dorsal root ganglia, and some medium-sized and large neurons in rat dorsal root ganglia, express mu-opioid receptor-like immunoreactivity. Most of these neurons contain calcitonin gene-related peptide. The mu-opioid receptor was closely associated with the somatic plasmalemma of the dorsal root ganglion neurons. Both mu-opioid receptor-immunoreactive nerve fibers and cell bodies were observed in lamina II of the dorsal horn. The highest intensity of mu-opioid receptor-like immunoreactivity was observed in the deep part of lamina II. Most mu-opioid receptor-like immunoreactivity in the dorsal horn originated from spinal neurons. A few mu-opioid receptor-positive peripheral afferent terminals in the rat and monkey dorsal horn were calcitonin gene-related peptide-immunoreactive. In addition to pre- and post-junctional receptors in rat and monkey dorsal horn neurons, mu-opioid receptors were localized on the presynaptic membrane of some synapses of primary afferent terminals in the monkey dorsal horn. Peripheral axotomy caused a reduction in the number and intensity of mu-opioid receptor-positive neurons in the rat and monkey dorsal root ganglia, and of mu-opioid receptor-like immunoreactivity in the dorsal horn of the spinal cord. The decrease in mu-opioid receptor-like immunoreactivity was more pronounced in the monkey than in the rat dorsal root ganglia and spinal cord. It is probable that there was a parallel trans-synaptic down-regulation of mu-opioid-like immunoreactivity in local dorsal horn neurons of the monkey. These data suggest that one factor underlying the well known insensitivity of neuropathic pain to opioid analgesics could be due to a marked reduction in the number of mu-opioid receptors in the axotomized sensory neurons and in interneurons in the dorsal horn of the spinal cord.

Preprodynorphin mRNA-expressing neurones in the rat parabrachial nucleus: subnuclear localization, hypothalamic projections and colocalization with noxious-evoked fos-like immunoreactivity

The dorsal lateral subnucleus of the rat pontine parabrachial nucleus is a major target for ascending nociceptive information from the spinal cord. With in situ hybridization histochemistry, using a radiolabelled cRNA probe, we demonstrate that neurones in and near the dorsal lateral subnucleus express preprodynorphin mRNA. The cRNA probe was constructed from a PCR product amplified from rat genomic DNA. Sequencing of the PCR product revealed that it corresponded to the sequence 466-1101 of the rat preprodynorphin gene exon 4. Tract tracing experiments, using injection of cholera toxin subunit B into the hypothalamic median preoptic nucleus, showed a retrograde labelling pattern of neurones in the parabrachial nucleus that was almost identical to that of the preprodynorphin mRNA expressing neurones. Double-labelling, combining immunohistochemical detection of tracer and in situ hybridization, revealed that the retrogradely labelled neurones expressed preprodynorphin mRNA. A similar double-labelling, combining in situ hybridization with immunohistochemical detection of noxious-evoked fos following formalin injection into one hindpaw of awake animals, showed that almost all fos-immunoreactive neurones in the dorsal lateral parabrachial subnucleus also expressed preprodynorphin mRNA. Quantitative analysis suggested that the evoked fos immunoreactivity was accompanied by an increased preprodynorphin mRNA expression. The findings provide evidence that neurones in the dorsal lateral subnucleus produce dynorphin and project to the median preoptic nucleus, and that noxious stimulation in awake animals synaptically activates the dynorphinergic neurones in this subnucleus. These observations are consistent with the idea of a functional and chemical heterogeneity among different parabrachial subnuclei that serves to produce specific homeostatic responses to stimuli that changes the physiological status of the organism, including tissue damage.

Kappa2 opioid receptors in limbic areas of the human brain are upregulated by cocaine in fatal overdose victims

Cocaine is thought to be addictive because chronic use leads to molecular adaptations within the mesolimbic dopamine (DA) circuitry that affect motivated behavior and emotion. Although the reinforcing effects of cocaine are mediated primarily by blocking DA reuptake into the presynaptic nerve terminal, reciprocal signaling between DA and endogenous opioids has important implications for cocaine dependence. The present study used the opioid antagonist 6 beta-[125iodo]-3,14-dihydroxy-17-cyclopropylmethyl-4,5 alpha-epoxymorphinan ([125I]IOXY) after pretreatment with the site-directed acylating agents 2-(p-ethoxybenzyl)-1-diethylaminoethyl-5-isothiocyanatobenzimid iazole -HCl (mu-selective) and N-phenyl-N-[1-(2-(4-isothiocyanato)-phenethyl)-4-piperidinyl]-p ropana mide-HCl (delta-selective) to examine the effect of cocaine exposure on the distribution and density of kappa2 receptors in autopsy studies of human cocaine fatalities. The selective labeling of the kappa2 receptor subtype was demonstrated by competition binding studies, which gave a pharmacological signature (IOXY >/= (+)-bremazocine >> U50,488 >/= U69,593) distinct from either the kappa1 or kappa3 receptor subtypes. Visualization of [125I]IOXY labeling revealed that kappa2 receptors localize to mesocortical and subcortical limbic areas, including the cingulate, entorhinal, insular, and orbitofrontal cortices and the nucleus accumbens and amygdala. The number of kappa2 receptors in the nucleus accumbens and other limbic brain regions from cocaine fatalities was increased twofold as compared with age-matched and drug-free control subjects. Cocaine overdose victims, who experienced paranoia and marked agitation before death, also had elevated densities of kappa2 receptors in the amygdala. These findings demonstrate for the first time that kappa2 receptor numbers are upregulated by cocaine exposure. The molecular adaptation of kappa2 receptor numbers may play a role in the motivational incentive associated with episodes of binge cocaine use and in the dysphoria that follows abrupt cocaine withdrawal.

The expression of mRNA for a kappa opioid receptor in the substantia nigra of Parkinson's disease brain

We molecularly cloned the kappa opioid receptor from a human substantia nigra cDNA library. When expressed in HEK293 cells, the cloned receptor had similar pharmacological characteristics to the rat kappa opioid receptor. Northern blot analysis showed the presence of a single transcript of about 6 kb in size for mRNA prepared from the substantia nigra. Using in situ hybridization histochemistry, we studied the expression of this receptor in postmortem human brains from control and Parkinson's disease subjects. Kappa opioid receptor mRNA was present in melanized (possibly dopaminergic) neurons of the substantia nigra and the nucleus paranigralis. On the other hand, Parkinson's disease brains had markedly fewer melanized neurons, as expected, and correspondingly very low or background levels of mRNA for the kappa opioid receptor. However, in some cases, remaining melanized neurons still expressed the receptor mRNA. From these results we suggest that dopaminergic neurons in the human substantia nigra and the nucleus paranigralis synthesize kappa opioid receptors and express them in their perikarya and their terminal regions. The kappa opioid receptor expressed in the melanized neurons may play a role in the normal function of dopaminergic systems and possibly in the etiology of Parkinson's disease.

Beta-endorphin inhibition of endogenous norepinephrine release from the A2 noradrenergic nucleus in vitro: role of mu opiate receptors and Na+ ion permeability

An in vitro approach was used to determine the opioid receptor subtype mediating beta-endorphin inhibition of endogenous norepinephrine release from the A2 nucleus in the caudal dorsomedial medulla of rats. The voltage-sensitive Na+ channel blocker tetrodotoxin was used to investigate the role of Na+-dependent action potentials in beta-endorphin inhibition of K+-evoked norepinephrine release. Human beta-endorphin(1-31) inhibited K+-evoked norepinephrine release in a concentration-dependent fashion. Activation of delta- and kappa-opioid receptors had no effect on endogenous norepinephrine release. The inhibitory effect of beta-endorphin was blocked in a concentration-dependent manner by the mu-opioid receptor antagonist CTOP (Cys2, Tyr3, Orn5, Pen7 amide). Tetrodotoxin (TTX) inhibited norepinephrine release evoked by 25 mM K+ in a concentration-dependent manner and blocked the inhibitory effects of beta-endorphin. These results indicate that beta-endorphin acts on mu-opioid receptors to inhibit K+-evoked norepinephrine release from A2 neurons and suggest that the receptors involved are not located on noradrenergic nerve terminals.

Brain substrates activated by electroacupuncture of different frequencies (I): Comparative study on the expression of oncogene c-fos and genes coding for three opioid peptides

Low and high frequency electroacupuncture (EA)-produced analgesia have been shown to be mediated by different brain substrates and different opioid peptides. In this study, Fos-like immunoreactivity (FLI) and in situ hybridization of the three opioid mRNAs were used to examine the effect of low (2 Hz) and high (100 Hz) frequency EA on neuronal activities, and the expression of opioid genes. 2 Hz and 100 Hz EA induced a markedly different spatial patterns of Fos expression in the rat brain, suggesting there are distinct neuronal pathways underlying EA of different frequencies. Likewise, 2 Hz and 100 Hz EA exert differential effects on opioid gene expression: while 2 Hz EA induced a more extensive and intensive preproenkephalin (PPE) mRNA expression than 100 Hz EA, it had no effect on preprodynorphin (PPD) mRNA expression which was significantly increased by 100 Hz EA stimulation. In contrast, EA of both frequencies did not affect POMC mRNA expression.

Dynorphin regulates D1 dopamine receptor-mediated responses in the striatum: relative contributions of pre- and postsynaptic mechanisms in dorsal and ventral striatum demonstrated by altered immediate-early gene induction

Dynorphin, an endogenous kappa opioid receptor ligand, acts in the striatum to regulate the response of striatonigral neurons to D1 dopamine receptor stimulation. We investigated the relative contributions of both presynaptic kappa receptors on dopamine terminals and postsynaptic kappa receptors on striatal neurons by analyzing opioid regulation of D1 effects in the absence of presynaptic kappa receptors, after 6-hydroxydopamine depletion of striatal dopamine. D1-receptor-mediated immediate-early gene induction was measured by using in situ hybridization histochemistry. First, repeated treatment with the D1-receptor agonist SKF-38393 (2 mg/kg/day, 3-14 days) was used to increase dynorphin levels in rats with dopamine depletions. In the nucleus accumbens, increased dynorphin expression was accompanied by reduced induction of the immediate-early genes c-fos and zif 268 by SKF-38393. In contrast, in dorsal/lateral aspects of the dopamine-depleted striatum, this D1 response was sustained despite a large increase in dynorphin expression. These results are consistent with a requirement of dopamine terminals (presynaptic kappa receptors) for the inhibitory action of dynorphin in the dorsal/lateral striatum, but not in the ventral striatum. Second, the kappa receptor agonist spiradoline (1-10 mg/kg) reduced c-fos and zif 268 induction by SKF-39393 (2.5 mg/kg) preferentially in ventral parts of the dopamine-depleted striatum, which contain higher levels of kappa receptor mRNA and binding. These results also indicate that postsynaptic kappa receptors contribute to the inhibition of the D1 response at least in the ventral striatum. Together, these results indicate that dynorphin in the striatum functions to regulate dopamine input to striatonigral neurons, acting at both pre- and postsynaptic sites, and that the relative contributions of these mechanisms differ between dorsal and ventral striatal regions.

Autoradiographical and immunohistochemical analysis of receptor localization in the central nervous system

Quantitative receptor autoradiographic methods have been widely used over the past two decades. Some of the advantages and limitations of these techniques are reviewed here. Comparison with immunohistochemical and in situ hybridization methods is also highlighted, as well as the use of these approaches to study receptor gene over-expression in cell lines. Together, data obtained using these various methodologies can provide unique information on the potential physiological roles of a given receptor protein and/or binding sites in various tissues.

Involvement of the opioid system in the anxiolytic effect of diazepam in mice

In the present study, the anticonflict effect of diazepam was significantly abolished by pretreatment with naloxone, beta-funaltrexamine or nor-binaltorphimine but not naltrindole, using a Vogel-type conflict paradigm in mice. However, naloxone alone had a significant proconflict effect, and beta-funaltrexamine alone tended to produce a proconflict effect. Spontaneous drinking behavior was not affected by treatment with diazepam and nor-binaltorphimine. In addition, nor-binaltorphimine had no effect on diazepam-induced motor incoordination, hypothermia or anticonvulsant action, respectively. Moreover, the stable dynorphin analog E2078 ([N-methyl-Tyr1, N-alpha-methyl-Arg7-D-Leu8]dynorphin A-(1-8) ethylamide) and the highly selective kappa-opioid receptor agonist U50,488H (trans-3,4-dichloro-N-(2-(1-pyrrolidinyl)cyclohexyl)benzenacetamide++ + methanesulfonate hydrochloride) produced a significant anticonflict effect, which was completely antagonized by pretreatment with nor-binaltorphimine. These findings suggested that the kappa-opioid system may play an important role in the anxiolytic effect of benzodiazepine and the regulation of anxiety.

Differential messenger RNA expression of prodynorphin and proenkephalin in the human brain

The opiate system is involved in a wide variety of neural functions including pain perception, neuroendocrine regulation, memory, drug reward, and tolerance. Such functions imply that endogenous opioid peptides should have anatomical interactions with limbic brain structures believed to be involved in the experience and expression of emotion. Using in situ hybridization histochemistry, the messenger RNA expression of the opioid precursors, prodynorphin and proenkephalin, was studied in whole hemisphere human brain tissue. Different components of the limbic system were found to be characterized by a high gene expression of either prodynorphin or proenkephalin messenger RNA. Brain regions traditionally included within the limbic system (e.g. amygdala, hippocampus, entorhinal cortex and cingulate cortex) as well as limbic-associated regions including the ventromedial prefrontal cortex and patch compartment of the neostriatum showed high prodynorphin messenger RNA expression. In contrast, high levels of proenkephalin messenger RNA were more widely expressed in the hypothalamus, periaqueductal gray, various mesencephalic nuclei, bed nucleus of the stria terminalis, and ventral pallidum; brain regions associated with endocrine-reticular-motor continuum of the limbic system. The marked anatomical dissociation between the expression of these two opioid peptide genes, seen clearly in whole hemisphere sections, indicates that distinct functions must be subserved by the prodynorphin and proenkephalin systems in the human brain.

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.

Effect of morphine on dynorphin B and GABA release in the basal ganglia of rats

In vivo microdialysis was used to study the effects of systemic, as well as intracerebral administration of morphine and naloxone on dynorphin B release in neostriatum and substantia nigra of rats. The release of dopamine (DA), gamma-aminobutyric acid (GABA), glutamate (Glu) and aspartate (Asp) was also investigated. Systemic injection of morphine (1 mg/kg s.c.) induced long-lasting increases in extracellular dynorphin B and GABA levels in the substantia nigra, whereas DA, Glu and Asp levels, measured in the same region, were not significantly affected. No effect on striatal neurotransmitter levels was observed following systemic morphine administration. Local perfusion of the substantia nigra with morphine (100 microM) through the microdialysis probe also increased nigral dynorphin B and GABA levels. Perfusion of the neostriatum with morphine (100 microM) significantly increased GABA and dynorphin B levels in the ipsilateral substantia nigra, but no effect was observed locally. Naloxone blocked the effect of systemic morphine administration on nigral dynorphin B and GABA release, already at a dose of 0.2 mg/kg s.c. Naloxone alone, given either systemically (0.2-4 mg/kg s.c.) or intracerebrally (1-100 microM), did not affect dynorphin B or amino acid levels, either in neostriatum or in substantia nigra. However, naloxone produced a concentration-dependent increase in DA levels. The present results indicate that systemic morphine administration stimulates the release of dynorphin B in the substantia nigra, probably by activating the mu-subtype of opioid receptor, since the effect of morphine on nigral dynorphin B and GABA was antagonized by a low dose of naloxone. The increase in extracellular DA levels produced by high concentrations of naloxone, both in neostriatum and substantia nigra, indicates a disinhibitory effect of this drug on DA release, probably via a non-mu subtype of opioid receptors located on nigro-striatal DA neurones.

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.

Assessment of delta opioid antinociception and receptor mRNA levels in mouse after chronic naltrexone treatment

The antinociceptive potency of the delta opioid receptor (DOR) agonist [D-Ala2]Deltorphin II and the levels of DOR mRNA were measured in mice chronically treated with naltrexone. ED50 determinations for [D-Ala2]Deltorphin II, using the tail-flick test with mice that had been treated with naltrexone for 7 days followed by a 24 h naltrexone free period (study day 8), revealed a 7.7-fold increase in antinociceptive potency, indicating functional supersensitivity. Utilization of a micro-dissection technique followed by quantitative solution hybridization measurements of RNA extracts from mouse CNS revealed levels of DOR mRNA ranging from 2.8 pg/microgram RNA in the caudate-putamen to 0.3 pg/microgram RNA in cerebellum. However, despite the functional increase in DOR sensitivity, the DOR mRNA levels in selected brain areas and spinal cord of naltrexone-treated and control mice did not differ. Assessment of DOR mRNA levels in whole brain and selected CNS regions after shorter treatment intervals (1, 6 and 12 h and 2 and 7 days) in naltrexone-treated and control mice revealed a similar pattern of results. Northern blot analysis of mouse whole brain RNA extracts after 7 days of naltrexone treatment did not show any alteration in the size of the DOR transcript. These data demonstrate that DOR mRNA levels are not altered during and after chronic naltrexone treatment and therefore are not associated with opioid-induced DOR up-regulation or DOR functional supersensitivity.

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.

The kappa-opioid receptor is primarily postsynaptic: combined immunohistochemical localization of the receptor and endogenous opioids

Antisera were raised against a synthetic peptide corresponding to the carboxyl terminus of the kappa-opioid receptor (KOR1). Specificity of the antisera was verified by staining of COS-7 cells transfected with KOR1 and epitope-tagged KOR1 cDNAs, by recognition by the antisera of proteins on Western blots of both transfected cells and brain tissue, by the absence of staining of both brain tissue and transfected cells after preabsorption of the antisera with the cognate peptide, and on the strong correlation between the distribution of KOR1 immunoreactivity and that of earlier ligand binding and in situ hybridization studies. Results indicate that KOR1 in neurons is targeted into both the axonal and somatodendritic compartments, but the majority of immunostaining was seen in the somatodendritic compartment. In sections from rat and guinea pig brain, prominent KOR1 staining was seen in the ventral forebrain, hypothalamus, thalamus, posterior pituitary, and midbrain. While the staining pattern was similar in both species, distinct differences were also observed. The distribution of preprodynorphin and KOR1 immunoreactivity was complementary in many brain regions, suggesting that KOR1 is poised to mediate the physiological actions of dynorphin. However, the distribution of KOR1 and enkephalin immunoreactivity was complementary in some regions as well. These results suggest that the KOR1 protein is primarily, but not exclusively, deployed to postsynaptic membranes where it mediates the effects of products of preprodynorphin and possibly preproenkephalin.

Distribution of neuropeptide receptors. New views of peptidergic neurotransmission made possible by antibodies to opioid receptors

The cloning of receptors for neuropeptides made possible studies that identified the neurons that utilize these receptors. In situ hybridization can detect transcripts that encode receptors and thereby identify the cells responsible for their expression, whereas immunocytochemistry enables one to determine the region of the plasma membrane where the receptor is located. We produced antibodies to portions of the predicted amino acid sequences of delta, mu, and kappa opioid receptors and used them in combination with antibodies to a variety of neurotransmitters in multicolor immunofluorescence studies visualized by confocal microscopy. Several findings are notable: First, the cloned delta opioid receptor appears to be distributed primarily in axons, and therefore most likely functions in a presynaptic manner. Second, the cloned mu and kappa opioid receptors are found associated with neuronal plasma membranes of dendrites and cell bodies and therefore most likely function in a postsynaptic manner. However, in certain, discrete populations of neurons, mu and kappa opioid receptors appear to be distributed in axons. Third, enkephalin-containing terminals are often found in close proximity (although not necessarily synaptically linked) to membranes containing either the delta or mu opioid receptors, whereas dynorphin-containing terminals are often found in proximity to kappa opioid receptors. Finally, a substantial mismatch between opioid receptors and their endogenous ligands was observed in some brain regions. However, this mismatch was characterized by complementary zones of receptor and ligand, suggesting underlying principles of organization that underlie long-distance, nonsynaptic neurotransmission.

Immunohistochemical localization of the cloned mu opioid receptor in the rat CNS

Three opioid receptor types have recently been cloned that correspond to the pharmacologically defined mu, delta and kappa 1 receptors. In situ hybridization studies suggest that the opioid receptor mRNAs that encode these receptors have distinct distributions in the central nervous system that correlate well with their known functions. In the present study polyclonal antibodies were generated to the C terminal 63 amino acids of the cloned mu receptor (335-398) to examine the distribution of the mu receptor-like protein with immunohistochemical techniques. mu receptor-like immunoreactivity is widely distributed in the rat central nervous system with immunoreactive fibers and/or perikarya in such regions as the neocortex, the striatal patches and subcallosal streak, nucleus accumbens, lateral and medial septum, endopiriform nucleus, globus pallidus and ventral pallidum, amygdala, hippocampus, presubiculum, thalamic and hypothalamic nuclei, superior and inferior colliculi, central grey, substantia nigra, ventral tegmental area, interpeduncular nucleus, medial terminal nucleus of the accessory optic tract, raphe nuclei, nucleus of the solitary tract, spinal trigeminal nucleus, dorsal motor nucleus of vagus, the spinal cord and dorsal root ganglia. In addition, two major neuronal pathways, the fasciculus retroflexus and the stria terminalis, exhibit densely stained axonal fibers. While this distribution is in excellent agreement with the known mu receptor binding localization, a few regions, such as neocortex and cingulate cortex, basolateral amygdala, medial geniculate nucleus and the medial preoptic area fail to show a good correspondence. Several explanations are provided to interpret these results, and the anatomical and functional implications of these findings are discussed.

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.

Neuronal adaptation to amphetamine and dopamine: molecular mechanisms of prodynorphin gene regulation in rat striatum

Induction of prodynorphin gene expression by psychostimulant drugs may represent a compensatory adaptation to excessive dopamine stimulation and may contribute to the aversive aspects of withdrawal. We therefore investigated the molecular mechanisms by which dopamine psychostimulant drugs induce prodynorphin gene expression in vivo and in rat primary striatal cultures. We demonstrate that three recently described cAMP response elements (CREs), rather than a previously reported noncanonical AP-1 site, are critical for dopamine induction of the prodynorphin gene in striatal neurons. CRE-binding protein (CREB) binds to these CREs in striatal cell extracts and is phosphorylated on Ser-133 after dopamine stimulation in a D1 dopamine receptor-dependent manner. Surprisingly, following chronic administration of amphetamine, levels of phosphorylated CREB are increased above basal in rat striatum in vivo, whereas c-fos mRNA is suppressed below basal levels. D1 receptor-mediated CREB phosphorylation appears to mediate adaptations to psychostimulant drugs in the striatum.

Dynorphin opioid inhibition of cocaine-induced, D1 dopamine receptor-mediated immediate-early gene expression in the striatum

Neurons in the striatum that project to the substantia nigra contain the opioid peptide dynorphin. Stimulation of D1 dopamine receptors results in increased expression of mRNA encoding dynorphin as well as expression of immediate-early genes such as c-fos in these neurons. Levels of dynorphin vary in different regions of the normal rat striatum, being highest in ventral and medial striatum. In a prior study, we have shown that both regional and temporal patterns of c-fos induction following treatment with the indirect dopamine receptor agonist cocaine are inversely related to those of dynorphin expression. These results suggested that dynorphin is involved in regulating the responsiveness of these neurons to dopamine input. In the present experiments, we examined such a potential role for dynorphin by analyzing the influence of the dynorphin (kappa opioid receptor) agonist spiradoline on immediate-early gene induction by cocaine, and we determined that this immediate-early gene response is mediated by D1 dopamine receptors located in the striatum. As a marker of neuron activation, expression of c-fos and zif 268 immediate-early genes was assessed with quantitative in situ hybridization histochemistry. Results showed that 1) intrastriatal infusion of the D1 dopamine receptor antagonist SCH-23390 (2.5-250 pmol) resulted in a dose-dependent blockade of immediate-early gene induction by cocaine (30 mg/kg); 2 systemic administration of the kappa opioid receptor agonist spiradoline (0.5-10.0 mg/kg) decreased cocaine-induced expression of c-fos and zif 268 mRNAs in striatum in a dose-dependent manner; 3) intrastriatal infusion of spiradoline (1-50 nmol) also suppressed immediate-early gene induction by cocaine, demonstrating that kappa opioid receptors located in the striatum mediate such an effect; and 4) systemic and intrastriatal administration of spiradoline also affected immediate-early gene expression in cortex. These results demonstrate that, in striatum, immediate-early gene induction by cocaine is a D1 dopamine receptor-mediated process that is inhibited by activation of kappa opioid receptors. Therefore, these findings suggest that the striatal dynorphin opioid system acts directly and/or indirectly to inhibit dopamine input to striatonigral neurons through kappa opioid receptor-mediated processes in the striatum.

Molecular pharmacology of the opioid receptors

Opioid receptors are the primary sites of actions of opiates and endogenous opioid peptides, which have a wide variety of pharmacological and physiological effects. The opioid receptors are classified into at least three subtypes, mu, delta, and kappa, and their cDNAs have been cloned. In this review, we describe the molecular cloning of opioid receptor gene family and studies of the structure-function relationships, modes of coupling to second messenger systems, pharmacological effects of antisense oligonucleotides, and anatomical distribution of opioid receptor mRNAs.

Opioid-receptor mRNA expression in the rat CNS: anatomical and functional implications

The cloning of the opioid receptors has profoundly affected our understanding of opioid-receptor expression, regulation and function. This review focuses on the impact that cloning has had on our understanding of opioid-receptor anatomy, and provides broad anatomical maps of the three opioid-receptor mRNAs in relation to their binding sites. In addition, three model anatomical systems, the nigrostriatal and mesolimbic dopamine systems, the hypothalamic neuroendocrine axes, and the ascending and descending pain pathways, have been highlighted to discuss issues of receptor transport, trafficking and pre- versus postsynaptic localization.