Long-term effects of synaptic activation at low frequency on excitability of myenteric AH neurons

Intracellular microelectrodes were used to record the effects of extended p eriods (1-30 min) of synaptic activation on AH neurons in the myenteric gan glia of the guinea-pig ileum. Low-frequency (I Hz) stimulation gave rise to a slowly developing, sustained increase in excitability of the neurons ass ociated with depolarization and increased input resistance. The increased e xcitability lasted for up to 3.5 h following the stimulus period. Successiv e stimulus trains (1-4 min) elicited successively greater increases in exci tability. The neurons went through stages of excitation. Before stimulation , 500-ms depolarizing pulses evoked up to three action potentials (phasic r esponse) and anode break action potentials were not observed. As excitabili ty increased, more action potentials were evoked by depolarization (the res ponses became tonic), anode break action potentials were observed, prolonge d afterhyperpolarizing potentials that follow multiple action potentials we re diminished and, with substantial depolarization of the neurons, invasion by antidromic action potentials was suppressed. It is concluded that a state of elevated excitability is induced in myenter ic AH neurons by synaptic activation at law frequency and that changes in e xcitability can outlast stimulation by several hours. (C) 1999 IBRO. Publis hed by Elsevier Science Ltd.