Opiate-induced inhibition of the visceral distension reflex by peripheral and central mechanisms.

Distension of the proximal ileum by elevation of the intraluminal pressure, caused a transient, reflex fall in blood pressure of urethane-anaesthetised rats. The threshold intraluminal pressure required to evoke a reflex response was increased by codeine, but not by the quaternary opiate agonist N-methylmorphine. The magnitude of the reflex hypotension was decreased by codeine, morphine and N-methylmorphine. The quaternary opiate antagonist N-methylnalorphine reduced the effects of N-methylmorphine, but not those of codeine or morphine. Codeine-induced inhibition of reflex depressor responses was reduced by pretreatment with naloxone. Bilateral vagotomy caused a significant decrease in effects of codeine on the threshold, but not the magnitude, of reflex depressor responses. Furthermore N-methylnalorphine antagonised both codeine- and morphine-induced inhibition of depressor responses following transsection of the vagus nerves. Effects of opiates on the threshold for the reflex appear to be centrally mediated and to require an intact vagal innervation. The magnitude of depressor responses is largely independent of vagal innervation and may be influenced by opiates acting via peripheral mechanisms.