Presynaptic depolarization of trigeminal cutaneous afferent fibers induced by ethanol

Abstract

The effect of ethyl alcohol (ethanol) on the level of polarization of trigeminal cutaneous afferents was studied in cats. A modification of Wall's technique was utilized to determine excitability changes of trigeminal cutaneous neurons at two different structural levels: at the central terminals located in the spinal trigeminal nucleus; and at the level of the cell bodies contained within the trigeminal ganglion. The excitability of these neuronal structures was expressed in the form of stimulus-response curves. In addition, the effect of ethanol on cerebral cortically conditioned stimulus-response curves and on time courses of cortically induced excitability changes was determined. Intravenous ethanol injections (0.08 g/kg/min) consistently produced primary afferent depolarization (PAD) of central terminals of trigeminal fibers, whereas ganglion cells and fiber processes, contained in the trigeminal ganglion, were not significantly altered in their level of polarization. Ethanol-induced changes in PAD were greater in cortically conditioned than in nonconditioned states. In preparations with complete precollicular transection, ethanol failed to induce consistent and significant PAD. The observed changes in PAD of presynaptic (central) terminals were discussed with respect to the concept of presynaptic inhibition. It was concluded that at least part of the depressant action of ethanol on the trigeminal somesthetic system is due to an enhancement of presynaptic depolarization. It appears that descending cerebral-cortical influences play an important role in this inhibitory mechanism.