Distribution and binding sites of substance P and calcitonin gene-related peptide and their capsaicin-sensitivity in the spinal cord of rats and chicken: a comparative study.

In a comparative study, the distribution and binding sites of substance P (SP) and calcitonin gene-related peptide (CGRP) in the spinal cord, and their susceptibility towards capsaicin pretreatment were studied in rats and chicken. Rats: In accordance with the SP immunohistochemistry, specific binding sites for 125I-Bolton-Hunter-SP were highest in laminae I-III. Binding sites for 125I-0Tyr-rat-CGRP were found to be dense around the central canal, moderate in the dorsal and weak in the ventral horn. Neonatal capsaicin pretreatment, that reduced SP and CGRP immunoreactivities, increased SP specific binding sites in laminae I-III and X by 20 and 100%, respectively. An increase in CGRP binding density was detected in laminae IV, V and in the lumbar ventral horn. Displacement studies revealed a significant decrease of EC50-values for SP. Chicken: SP and CGRP immunoreactivities and SP specific binding sites were distributed similarly as in rats. Binding sites for radiolabelled CGRP, however, were highest in lamina X and in the ventral horn. Capsaicin (800 mg/kg) injected into the eggs 9 days before hatching had no influence on growth rate, nociception, peptide immunoreactivities and binding of the respective radioligands. The data demonstrated a different action of capsaicin on SP and CGRP and their specific binding sites in the spinal cord of rats and chicken and were discussed with regard to functional differences between these two animal species.