COLONIC MECHANOSENSORY AFFERENT INPUT TO NEURONS IN THE MOUSE SUPERIOR MESENTERIC GANGLION

Electrical activity and synaptic responses were recorded intracellular ly in 415 neurons of the mouse superior mesenteric ganglion (SMG) atta ched to a segment of distal colon in vitro. Eighty-seven percent of ne urons tested received ongoing nicotinic cholinergic fast excitatory po stsynaptic potentials (fEPSPs). Colonic distension caused an initial t ransient followed by a sustained, slowly adapting increase in fEPSP ac tivity. Application of hexamethonium only to the colon reduced, but di d not completely abolish, distension-evoked responses, suggesting dire ct projection of some distension-sensitive fibers. Ongoing fEPSPs were abolished when nerve trunks connecting the SMG to the colon were tran sected or blocked with tetrodotoxin applied to the colon. Intracellula r labeling with horseradish peroxidase or lucifer yellow revealed that about 90% of neurons receiving colonic synaptic input had a caudally projecting axon; about 60% that did not receive colonic input had a ro strally projecting axon. The latter neurons were found only in the cep halad ganglion region. These results show that mouse SMG neurons recei ve colonic mechanosensory afferent synaptic input and thus may partici pate in sympathetic intestinal reflexes.