The distribution of urotensin-II-immunoreactivity (irU-II) was studied in the rat brainstem and spinal cord with the use of an antiserum against the human urotensin II (U-II) peptide. A population of ventral horn neurons in the spinal cord, hypoglossal nucleus, dorsal motor nucleus of the vagus, facial motor nucleus, nucleus ambiguus, abducens nucleus and trigeminal motor nucleus exhibited irU-II of varying intensities. The number of irU-II motor neurons was higher in the lumbar segments as compared to that of cervical, thoracic and sacral segments. Double-labeling the sections with U-II- and choline acetyltransferase (ChAT)-antisera revealed that nearly all irU-II ventral horn and brainstem neurons were ChAT-positive. The result provides the first immunohistochemical evidence of the presence of irU-II in cholinergic motoneurons of the rat spinal cord and brainstem.

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The binding of [³H]opioid ligands to homogenates prepared from the spinal cords of rat and other species has been studied. Similar numbers of sites were seen in all areas of the cord when measured in a rostrocaudal direction. There was found to be approximately 2 x higher density of sites in the dorsal half of the cord compared with the ventral half. Binding studies suggested a similar relative distribution of mu, delta and kappa sites in all areas of the cord. The results are discussed in relation to the reported distribution of opioid peptides. In the above study the kappa binding site was defined as the binding of [³H] unselective opioids in the presence of cold ligands to suppress binding to mu- and delta-sites. Competitive binding assays, however, suggested this site did not have the properties of a single homogeneous group. Approximately 50% of the apparent kappa binding was consistent with a classical kappa site. Saturated binding assays afforded Bmax values which suggested lower 'true' kappa site numbers than previously supposed, values which were confirmed using the kappa peptide' [³H]Dynorphin A-(1-9), and the kappa selective [³H]U-69593. Heterogeneity was also seen in other central nervous system tissues. The heterogeneous nature of the kappa site may be due to different sites, due to interactions at a non-opioid site or may represent different conformations of the same site. The second possibility was discounted since observed binding followed the cellular distribution of the plasma marker Na+/K+-ATPase was stereoselective for levorphanol over dextrorphan, and fully displaceable by naloxone. The third possibility was investigated by studying the role of Na+ and MG2+ ions, which are reported to affect receptor conformation in binding assays employing brain tissues. None of the results obtained suggested that conformational changes were responsible for the observed effects, although the experiments were not exhaustive.