Indirect evidence for presynaptic location of opiate receptors on chemosensitive primary sensory neurones.

1. Rats were pretreated with 50 mg/kg s.c. capsaicin or solvent on the second day of life; 5 months later 3H-diprenorphine binding to homogenates of the whole spinal cord or of the upper dorsal horn of the spinal cord was investigated. 2. Capsaicin pretreatment resulted in a 17% decrease of opiate binding sites in the whole spinal cord and a 37% decrease in the upper dorsal horn with no change in their affinity. 3. Since neonatal capsaicin pretreatment causes degeneration of chemosensitive primary sensory neurones it is concluded that some opiate receptors are located presynaptically on the central terminals of these neurones.