Synaptic Inputs to Morphologically Identified Myenteric Neurons in Guinea Pig Rectum from Pelvic Nerves

Neurobiotin-filled microelectrodes were used to investigate electrical and synaptic behavior and morphological characteristics of rectal myenteric neurons that received synaptic inputs from the pelvic nerves. Stimulation of the pelvic nerve at low frequencies (< 3.3 Hz) evoked nicotinic fast excitatory postsynaptic potentials (fast EPSPs) in 45.3% of rectal neurons. Pelvic fast EPSPs were found in S/type 1, AH/type 2, type 3, or single-spike neurons that had a single long process preferentially projecting in the orad direction. Stimulation of the pelvic nerve at higher frequencies (5-20 Hz) elicited slow membrane excitation in 13.9% of the neurons. They were either AH/type 2 neurons with Dogiel II morphology or S/type 1 neurons with a single long process. Hexamethonium (100 microM) blocked pelvic fast EPSPs more quickly than those evoked by fiber tract stimulation but did not affect slow excitatory response. The results suggested the presence of more than one nicotinic-cholinergic synapse in the pelvic nerve pathway and the possible release of a noncholinergic excitatory substance from the afferent nerve terminals. It is possible that a subpopulation of rectal neurons, which receive a fast EPSP and have a single long process that projects in the orad direction, might be interneurons that mediate the defecation reflex.