gamma‐Aminobutyric acid efflux from sympathetic glial cells: effect of ‘depolarizing’ agents.

Abstract

Isolated desheathed rat superior cervical ganglia were incubated in [3H]GABA solution (1-10 .mu.M for 2-3 h) in the presence of 10 .mu.M-amino-oxyacetic acid (AOAA). The subsequent efflux of 3H into a stream of superfused non-radioactive GABA-free Krebs solution at 25.degree. C was measured. In the presence of 10 .mu.m-AOAA the mean basal efflux rate coefficient (Ko) for exit of tritium into the superfusion fluid was 0.7 .times. 10-3 min-1. Of effluent 3H > 98% comprised unchanged [3H]GABA. The rate coefficient showed no correlation with the amount of [3H]GABA previously accumulated by the ganglion. Elevation of [K+]o to > 50 nM increased the rate coefficient for [3H]GABA release by up to 4 times. Changes in efflux rate were not correlated with osmotic changes, and persisted after re-accumulation of effluent [3H]GABA by the inward carrier was inhibited. The effect of alkali metal cations diminished in the order Rb+ > K+ > Cs+Li+. Effects of K+ solutions were not reduced by omitting Ca2+ ions with or without the addition of Mg2+. Application of electrical pulses (0.1-1 ms duration, 1-10 Hz, 4 min trains) to the ganglion soma or to the preganglionic nerve trunk also raised ko. This effect declined with repeated stimulus trains, without an accompanying diminution in the response to K+. Responses to electrical stimulation were not reduced by amethocaine (300 .mu.M), tetrodotoxin (3 .mu.M) or raised [M2+] (0 mM-[Ca2+]/30 mM-[Mg3+]). Separate local superfusion of the pre- and post-ganglionic nerve trunks and of the ganglion soma showed that the response to electrical stimulation was localized to the vicinity of the stimulus and was not propagated along the nerve trunks or across the synapses. Electrical recording from impaled inexcitable cells (presumed to be neuroglial cells) indicated that the quantities of K+ accumulating during repetitive nerve stimulation are insufficient to stimulate the release of GABA from the glial cells. No physiological role for the release process in modulating neuronal excitability could be adduced.