Kappa opiate receptors localized by autoradiography to deep layers of cerebral cortex: relation to sedative effects.

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Abstract
.kappa. opiate drugs differ from other opiates in their unique sedative actions and lack of cross-tolerance. .kappa. opiate receptors were visualized by in vitro autoradiography using the .kappa. drugs [3H]ethylketazocine ([3H]EKC) and [3H]bremazocine. Though these ligands also label .mu. and .delta. opiate receptors, their binding is rendered .kappa. specific by coincubation with morphine and [D-Ala2, D-Leu5]enkephalin (DADL-Enk) to displace .mu. and .delta. interactions, respectively. Labeling patterns with [3H]EKC and [3H]bremazocine are the same and differ markedly from localizations of .mu. and .delta. opiate receptors visualized with [3H]dihydromorphine and [3H]DADL-Enk, respectively. The highest density and most selective localization of putative .kappa. receptors occurs in layers V and VI of the cerebral cortex. In these layers cells are localized which project to the thalamus regulating sensory input to the cortex. Receptors in these layers could account for the unique sedative and possibly analgesic effects of .kappa. opiates.
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