Norepinephrine reversibly blocked both the fast and slow stimulus-evoked EPSPs in myenteric ganglion cells. This action of norepinephrine was offset by the alpha-adrenergic blocking agent, phentolamine. 2. The putative neurotransmitter for the fast EPSP is acetylcholine, and serotonin (5-HT) is the suspected transmitter for the slow EPSP. 3. Norepinephrine did not block the excitatory effect of microiontophoresis of 5-HT onto the ganglion cells. 4. Norepinephrine decreased the input resistance and suppressed neuronal excitability. This effect was reversed by addition of 5-HT to the perfusion solution and was attributed to presynaptic blockade by norepinephrine of ongoing release of 5-HT. 5. Addition of norepinephrine after the slow EPSP had been evoked did not shorten the duration of the slow EPSP. 6. The results suggested that the mechanism of the blocking action of norepinephrine was inhibition of release of the excitatory neurotransmitters form presynaptic nerve terminals. This is probably one of the mechanisms that is involved in suppression of gastrointestinal motor function during activation of the sympathetic innervation of the bowel.
