Highly specific radioligands and quantitative autoradiography reveal strikingly different neuroanatomical patterns for the ,u, 8, and K opioid receptors of rat brain. The ,I receptors are most densely localized in patches in the striatum, layers I and III of the cortex, the pyramidal cell layer of the hippocampal formation, specific nuclei of the thalamus, the pars reticulata of the substantia nigra, the interpeduncular nucleus, and the locus coeruleus. In contrast, 8 receptors are highly confined, exhibiting selective localization in layers I, H, and VIa of the neocortex, a diffuse pattern in the striatum, and moderate concentration in the pars reticulata of the substantia nigra and in the interpeduncular nucleus. 8 receptors are absent in most other brain structures. This distribution is unexpected in that the enkephalins, the putative endogenous ligands of the 8 receptor, occur essentially throughout the brain. The K receptors of rat brain exhibit a third pattern distinct from that of the ,u and 8 receptors. K receptors occur at low density in patches in the striatum and at particularly high density in the nucleus accumbens, along the pyramidal and molecular layers of the hippocampus, in the granular cell layer of the dentate gyrus, specific midline nuclei of the thalamus, and hindbrain regions. K receptors appear to be uniformly distributed across regions in the neocortex with the exception of layer m, which revealed only trace levels of binding. An important conclusion of the present study is that 8 receptors occur at high density only in the forebrain and in two midbrain structures, whereas ,u and K receptors exhibit discrete patterns in most major brain regions.

K opioid receptors (K receptors) have been characterized in homogenates of guinea pig and rat brain under in vitro binding conditions. K receptors were labeled by using the tritiated prototypic K opioid ethylketocyclazocine under conditions in which ,u and 8 opioid binding was suppressed. In the case of guinea pig brain membranes, a single population of high-affinity K opioid receptor sites (K sites; Kd = 0.66 nM, BMMX = 80 fmol/mg of protein) was observed. In contrast, in the case ofrat brain, two populations of K sites were observedhigh-affinity sites at low density (Kd = 1.0 nM, Bm. = 16 fmol/mg of protein) and low-affinity sites at high density (Kd = 13 nM, Bin" = 111 fmol/mg of protein). To test the hypothesis that the high-and low-affinity K sites represent two distinct K receptor subtypes, a series of opioids were tested for their abilities to compete for binding to the two sites. U-69,593 and Cambridge 20 selectively displaced the high-affinity K site in both guinea pig and rat tissue, but were inactive at the rat-brain low-affinity site. Other K opioid drugs, including U-50,488, ethylketocyclazocine, bremazocine, cyclazocine, and dynorphin (1-17), competed for binding to both sites, but with different rank orders of potency. Quantitative light microscopy in vitro autoradiography was used to visualize the neuroanatomical pattern of K receptors in rat and guinea pig brain. The distribution patterns of the two K receptor subtypes of rat brain were clearly different. The pattern of rat high-affmiity K sites paralleled that of guinea pig in the caudate-putamen, midbrain, central gray substance of cerebrum, and substantia nigra; interspecies differences were apparent throughout most of the rest of the brain. Collectively, these data provide direct evidence for the presence of two K receptor subtypes; the U-69,593-sensitive, high-affinity K, site predominates in guinea pig brain, and the U-69,593-insensitive, low-affinity K2 site predominates in rat brain. Pharmacological studies have established that ketocyclazocine-like opioids produce their antinociceptive and unique sedative actions through an interaction with K receptors (2). These drugs effect a more pronounced sedation than do other opioids and have been evaluated as anesthetic agents. K opioid drugs neither suppress morphine abstinence nor induce abstinence in morphine-dependent monkeys (3). The endogenous opioid peptide dynorphin also interacts with high selectivity at K receptors.Evidence for a separate K receptor distinct from the morphine (A) and enkephalin (Enk; 8) receptors has been provided by pharmacological (2, 4), electrophysiological (5, 6), binding (4,7,8), and solubilization and purification (9)(10)(11) studies. In vitro autoradiography was used to visualize K receptors in rat (12) and guinea pig brain (13)

No abstract

Light microscopy autoradiography has been used to visualize neuroanatomical patterns of brain opiate receptor upregulation in response to chronic naltrexone administration. Slide-mounted brain sections of frozen rat brain were labeled in vitro with dihydro[3H]morphine, a relatively selective ,u opioid ligand. The greatest relative increases in opiate receptor density were observed in the nucleus accumbens, the amygdala, striatal patches, nuclei of the thalamus and hypothalamus, layers I and III of neocortex, substantia nigra compacta, midbrain periaqueductal gray regions, and the parabrachial nuclei of the brainstem. The substantia nigra reticulata, surrounding areas of striatal patches, and the locus ceruleus, were not affected by this drug treatment. These findings demonstrate that chronically administered naltrexone differentially regulates opiate receptors throughout the brain. In particular, three brain systems appear to be target areas of receptor upregulation: (i) the dopamine A9/AlO systems, (ii) the limbic system, and (iii) structures that receive input from afferent sensory pathways. Two possible mechanisms to account for this finding are (i) that the drug does not have uniform effects throughout the brain or (it) that the receptors themselves may be associated with different functional systems. Receptor density changes are paralleled by increases in methionine-enkephalin content in the striatum, nucleus accumbens, periaqueductal gray, and hypothalamic areas of chronic naltrexone-treated rats relative to control rats. Thus opiate receptors and opioid peptides appear to be subject to regulatory mechanisms similar to those that modulate other neurotransmitters and their receptors. These results document in a visual manner brain patterns of opiate receptor upregulation.Opiate narcotic analgesics produce tolerance and dependence in vivo and desensitization in vitro. The observation of these phenomena has raised the question as to whether opiate receptors undergo up-or downregulation in vivo, in response to long-term administration of opiate drugs. Opiate receptor downregulation has been difficult to document. Several groups have reported that chronic administration of narcotic agonists in vivo does not produce any change in either receptor number or affinity (1-7). On the other hand, receptor downregulation has been observed in neurotumor cell lines after long-term exposure to enkephalin (8-10) but not to alkaloid agonists (8). Possible interpretations of the latter findings are (i) that opioid peptides bind differently than do opioid narcotic agonists to the same receptor or (ii) that only a subpopulation of opioid receptors can be so modulated.By contrast, upregulation of brain opiate receptors in vivo after drug-induced denervation (11) or long-term narcotic antagonist administration (12) is well established (7,13,14). Chronic blockade of opiate receptors results in a nearly 2-fold increase in the density of both ,u and 8 receptors (12). The newly synthesized or unmasked receptors are more sensi...

No abstract