Developmental alterations in thermal nociceptive threshold and the distribution of immunoreactive calcitonin gene-related peptide and substance P after neonatal administration of capsaicin in the rat.

Age-dependent changes in thermal nociceptive threshold, as determined by the hot plate test, occurred in rats treated neonatally with capsaicin. From 4 weeks of age, hot plate latency increased to a maximum at 8 weeks of age and then declined to control values by 16 weeks of age. Although substantially diminished by 10 days of age, calcitonin gene-related peptide (CGRP)-like immunoreactivity increased in laminae I and II between 8 and 16 weeks of age. In addition, a dense plexus of CGRP-like immunoreactive fibers appeared in laminae III and IV. Similar changes in substance P-like immunoreactivity were not observed. The parallel timecourse of these alterations suggests that CGRP is involved in nociceptive processing in the spinal cord.