Intracellular study of effects of morphine on electrical activity of myenteric neurons in cat small intestine.

Intracellular recording methods were used to study the effects of morphine on electrical behavior of myenteric neurons in cat small intestine. Two general categories of myenteric ganglion cells were found. AH/Type 2 neurons and relatively high resting membrane potentials, low input resistances, spikes only at the onset of a depolarizing current pulse and long-duration hyperpolarizing after-potentials. S/Type 1 neurons discharged spikes throughout depolarizing current pulses, received nicotinic-cholinergic synaptic input (EPSPs), and tended to have lower membrane potentials and higher input resistances than AH/type 2 cells. Morphine suppressed the excitability of S/type 1, but not AH/type 2 neurons. Morphine suppressed current-evoked spike discharge, and this action was associated with membrane hyperpolarization and decreased input resistance. This action of morphine was reversed by naloxone. Morphine did not reduce the amplitudes of nicotinic-cholinergic EPSPs in the neurons. The results suggest that the spasmogenic and constipating effects of morphine may be related to suppression of excitability of inhibitory enteric neurons and consequent reduction of tonic inhibitory influences which normally suppress pacesetter-controlled myogenic activity within the circular muscle coat of the bowel.