SPONTANEOUS AND EVOKED INHIBITORY JUNCTION POTENTIALS IN THE CIRCULARMUSCLE LAYER OF MOUSE COLON

Intracellular microelectrodes were used to record spontaneous and evok ed inhibitory junction potentials (IJPs) from the circular muscle laye r of an in vitro preparation of whole mouse colon. Membrane potential recordings were made from cells of the mid to distal region of colon a t 36 +/- 1 degrees C in a modified Krebs' solution that contained atro pine (1-2 mu M) and nifedipine (1-2 mu M). Spontaneously occurring hyp erpolarisations of irregular amplitude and frequency (range: up to 20 mV and 2 Hz) were recorded that were resistant to N-G-nitro-L-arginine (NOLA, 100 mu M), but were abolished by tetrodotoxin(TTX, 1.6 mu M) o r apamin (250 nM). These were considered to be spontaneous IJPs as a c onsequence of activity in inhibitory motor neurons. Single electrical stimuli (0.6 ms, 15 V), elicited a fast IJP, whose time course could b e superimposed on spontaneous IJPs of similar amplitude. The amplitude of evoked IJPs was not depressed by NOLA (100 mu M). However, in NOLA (100 mu M), further addition of apamin (250 nM) significantly depress ed the amplitudes of the evoked IJPs by 44%. NOLA- and apamin-resistan t evoked IJPs were abolished by TTX (1.6 mu M). It is suggested, that in the circular muscle layer of mouse colon, NO does not mediate the f ast hyperpolarisations associated with spontaneous or evoked IJPs. Apa min abolished spontaneous IJPs, but electrical stimuli evoked an IJP w ith apamin-sensitive and resistant components both of which were non-n itrergic in origin. (C) 1998 Elsevier Science B.V. All rights reserved .