Naloxone-induced depolarization and synaptic activation of myenteric neurons in morphine-dependent guinea pig ileum

To investigate the cellular basis of opiate dependence, intracellular microelectrodes were used to record from both electrophysiologically defined classes of neurons (S and AH) in myenteric plexus longitudinal muscle preparations from morphine pretreated guinea pigs. These preparations responded to naloxone with the characteristic contraction of the longitudinal smooth muscle, indicative of morphine dependence. Depolarization in response to naloxone was observed in 42% of S neurons, but there were no consistent changes in input resistance. In some cells the depolarization was reduced or abolished after blockade of synaptic transmission, suggesting that it was due in part to the release of an excitatory transmitter producing a slow depolarization in the impaled neuron. Synaptic activation of S neurons during withdrawal was further indicated by the observation that fast postsynaptic potentials appeared after abrupt displacement of morphine from its receptors by naloxone. Morphine withdrawal, therefore, involves both the final motor neurons and interneurons. During naloxone-induced withdrawal, 25% of S neurons discharged action potentials. In contrast, no action potentials were discharged in AH neurons. Furthermore, naloxone did not alter the resting membrane potential, input resistance, soma action potential configuration, or the slow hyperpolarization following a soma spike in AH neurons. The specificity of the withdrawal response for S neurons and the relatively small proportion of neurons involved suggests that morphine withdrawal occurs in quite specific neuronal circuits in the myenteric plexus.