ELECTROPHYSIOLOGICAL CHARACTERISTICS OF SUBMUCOSAL NEURONS IN THE PROXIMAL COLON OF GUINEA-PIGS - COMPARISONS WITH CECUM AND DESCENDING COLON

A systematic examination has been made of the active and passive elect rophysiological properties and synaptic inputs of forty-four randomly impaled submucosal neurones in the proximal colon of the guinea-pig to compare these characteristics directly with those of submucosal neuro nes in the caecum (n = 70) and descending colon (n = 45). Within each of the three electrophysiological classes of submucosal neurones ident ified (S, S/AH and AH), no statistically significant regional differen ces were found with respect to the resting membrane potential, membran e time constant or input resistance between neurones of the proximal c olon, descending colon and caecum. Of submucosal neurones from the pro ximal colon, forty-three of forty-four (98%) received fast excitatory synaptic potentials (fast EPSPs); thirty-nine (91%) were S neurones an d the others were S/AH neurones; only one of the forty-four cells (2%) was an AH neurone. An idazoxan-sensitive slow inhibitory postsynaptic potential (slow IPSP) was induced in thirty of forty-three S and S/AH neurones (70%) of the proximal colon, compared with sixty-one of sixt y-six caecal neurones (92%) and twelve of forty-one neurones (29%) in the descending colon. The mean (+/- S.E.M.) amplitude of the slow IPSP in proximal colonic neurones was 17 +/- 1 mV (range, 6-30 mV; n = 30) , compared with the significantly larger synaptic response (25 +/- 1 m V; range, 7-38 mV; n = 66; P < 0.05) recorded in the caecum; the mean slow IPSP amplitude in the descending colon was significantly smaller (12 +/- 2 mV; range, 5-27 mV; n = 12; P < 0.05) than that in the caecu m. In the proximal colon and caecum, only those neurones with a slow I PSP had a hyperpolarizing response to noradrenaline, whereas about 50% of those neurones of the descending colon that lacked a slow IPSP wer e hyperpolarized by noradrenaline, acting via alpha(2)-adrenoceptors. Thus, the electrophysiological characteristics of the submucosal neuro nes of the proximal colon more closely resemble those of the caecum th an those of the descending colon, of which many do not have a function al noradrenergic synaptic input. Furthermore, the results confirm that there are fundamental regional differences in the guinea-pig large in testine with respect to the synaptic organization of submucosal neuron es of particular electrophysiological classes.