Distinct distribution of opioid receptor types in rat lumbar spinal cord.

The distribution of opiate binding sites was studied in sections of rat lumbar spinal cord under conditions selective for mu, delta and kappa receptors. While the levels of mu binding sites were highest in the substantia gelatinosa, elevated levels were also observed in laminae III, IV, V and VIII. In contrast, delta binding was notable only in lamina I. The levels of typical kappa sites were low, and were concentrated in the substantia gelatinosa. An additional, atypical site was detected using 3H-diprenorphine in the presence of mu, delta and kappa receptor blocking agents, and this site was also concentrated in the substantia gelatinosa.