5-HT3 receptor binding sites are on capsaicin-sensitive fibres in the rat spinal cord.

Abstract Specific binding sites for [ 3 H]zacopride were found in the dorsal part of the rat spinal cord, particularly in the superficial layers of the dorsal horn. These binding sites had the same pharmacological profile as 5-HT 3 receptors in membranes from the rat entorhinal cortex or from NG 108-15 neuroblastoma-glioma cells. Administration of capsaicin (50 mg/kg s.c.) to neonatal rats to induce degeneration of unmyelinated primary sensory fibres resulted in a significant decrease in [ 3 H]zacopride specific binding (−50%) in the dorsal zone of the spinal cord of 4 month-old rats. This decrease was as pronounced as the decrease in [ 3 H]bremazocine and [ 3 H]naloxone binding to opiate receptors. These data support the presynaptic location of 5-HT 3 receptors, at least in part, on capsaicin-sensitive primary afferent fibres in the rat spinal cord.