In acute experiments on nembutal-urethan-anesthetized rats, structures
selectively sensitive to capsaicin were found near the ventral surfac
e of the medulla at the exit of hypoglossal nerve roots. Microinjectio
n of 5-50 nl 0.01% capsaicin to the rostral region of the capsaicin-se
nsitive area mostly activated; respiration, arterial pressure and hear
t rate (HR) while that to the caudal region inhibited arterial pressur
e and HR. In chronic experiments on rats, injection of 25 nl 1% capsai
cin to the caudal capsaicin-sensitive area led to a decrease in arteri
al pressure by 35-45% and in HR by 10-15% within a week after operatio
n. Arterial pressure and HR virtually reached the control level and th
e rostral and caudal ventral medulla showed asymmetric distribution of
nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d)-posi
tive cells by the end of the 2nd week. It is suggested that nitric oxi
de may be involved in the mechanisms of neurochemical rearrangements i
n the brainstem after application of capsaicin to the caudal ventrolat
eral medulla (CVLM).