EFFECTS OF THE SOMATOSTATIN ANALOG OCTREOTIDE ON RECTAL AFFERENT NERVES IN HUMANS

Somatostatin (Som) administered intrathecally to humans has been shown to exert potent analgesic effects on somatic pain, and anecdotal evid ence suggests that Som may also relieve visceral pain. In the current study, we used rectal balloon distension in seven healthy volunteers t o evaluate the effect of the Som analogue octreotide (Oct; 1.25 mug/kg sc) on four pathways mediated by different visceral afferents that or iginate in the rectum: conscious perception, receptive relaxation, ref lex inhibition of internal anal sphincter, and a viscerosomatic reflex . Rectal mechanoreceptors were stimulated either by distending the rec tum tonically (volume ramp at 20-40 and 400 ml/min) or phasically (int ermittent pressure steps of 60 s duration). Pressure thresholds for no nnoxious and noxious sensations in response to slow tonic distension w ere increased in the presence of rectal lidocaine (20 ml of 2% solutio n), whereas those to phasic distension were unaffected. Oct significan tly increased pressure and volume thresholds for nonnoxious and noxiou s sensations in response to slow tonic distension but did not further increase thresholds in the presence of intrarectal lidocaine. In contr ast, no effect of Oct on rectal sensations was observed during rapid t onic or phasic distension. Oct had no effect on any of the monitored r eflex responses. The effect of Oct on rectal sensation in the concentr ation used in this study was not associated with changes in the rectal wall pressure-volume relationship during any distension protocol. The se findings indicate that the inhibitory effect of Oct on rectal sensa tion is likely to represent a direct effect on a subset of extrinsic p rimary afferent neurons, with receptive fields in the mucosa.