Stimulation of extrinsic sympathetic nerves differentially affects neurogenic motor activity in guinea pig distal colon

The speed of pellet propulsion through the isolated guinea pig distal colon in vitro significantly exceeds in vivo measurements, suggesting a role for inhibitory mechanisms from sources outside the gut. The aim of this study was to investigate the effects of sympathetic nerve stimulation on three different neurogenic motor behaviors of the distal colon: transient neural events (TNEs), colonic motor complexes (CMCs), and pellet propulsion. To do this, segments of guinea pig distal colon with intact connections to the inferior mesenteric ganglion (IMG) were set up in organ baths allowing for simultaneous extracellular suction electrode recordings from smooth muscle, video recordings for diameter mapping, and intraluminal manometry. Electrical stimulation (1-20 Hz) of colonic nerves surrounding the inferior mesenteric artery caused a statistically significant, frequency-dependent inhibition of TNEs, as well as single pellet propulsion, from frequencies of 5 Hz and greater. Significant inhibition of CMCs required stimulation frequencies of 10 Hz and greater. Phentolamine (3.6 μM) abolished effects of colonic nerve stimulation, consistent with a sympathetic noradrenergic mechanism. Sympathetic inhibition was constrained to regions with intact extrinsic nerve pathways, allowing normal motor behaviors to continue without modulation in adjacent extrinsically denervated regions of the same colonic segments. The results demonstrate differential sensitivities to sympathetic input among distinct neurogenic motor behaviors of the colon. Together with findings indicating CMCs activate colo-colonic sympathetic reflexes through the IMG, these results raise the possibility that CMCs may paradoxically facilitate suppression of pellet movement in vivo, through peripheral sympathetic reflex circuits.