The effect of [met]enkephalin on the periaqueductal gray neurons of the rat: An in vitro study

Abstract

Intracellular and extracellular recording techniques and in vitro preparation were used to examine the effect of [Met]enkephalin on the rat periaqueductal neurons. In the 20 cells that were recorded intracellularly, [Met]enkephalin caused an increase in the resting membrane conductance, hyperpolarization of the cell membrane, an increase in the firing threshold and a decrease in the spontaneous firing rate. This effect of [Met]enkephalin could be blocked by naloxone. The effect of [Met]enkephalin on 99 neurons was also examined using extracellular recording. In 59% of cells, pressure application of [Met]enkephalin caused a dose-dependent inhibition that could be blocked by naloxone; 15% of the cells were excited and the remaining neurons (26%) did not respond. Nineteen per cent of responsive cells were located in the dorsolateral subdivision; 41% in the ventrolateral and 13% in the dorsal regions. In 10 cells, perfusion with physiological saline solution/Mg did not alter the inhibitory effect of [Metjenkephalin. However, perfusion with physiological saline solution/Mg abolished the excitatory response to [Met]enkephalin in four cells. It is concluded that: (1) the major effect of [Met]enkephalin on periaqueductal gray cells is inhibition that occurs through a direct postsynaptic process. This inhibition is probably due to an increase in permeability to potassium; (2) a small population of periaqueductal gray cells are excited by [Met]enkephalin, probably through a presynaptic process.