Facilitation by GABA of the potassium-evoked release of 3H-noradrenaline from the rat occipital cortex

Summary The effects of GABA were studied on the release of ³H-noradrenaline (³H-NA) evoked by potassium or by tyramine from slices of the rat occipital cortex and on the release of ³H-NA elicited by nerve stimulation from the cat nictitating membrane.GABA (30–1000M) facilitated the potassium-evoked release of ³H-NA in a concentration dependent manner. This facilitatory effect was not antagonized by bicuculline (1–100 M) or by picrotoxin (1–100 M). Exposure to the GABA agonist muscimol (1–100 M) did not affect either the spontaneous or the potassium-evoked release of ³H-NA.The facilitatory effect of GABA on the release of ³H-NA elicited by potassium was observed when the occipital cortex slices were exposed to 20 mM K⁺ during 1 min. When depolarization was induced by 35 mM K⁺ exposure to GABA failed to enhance the release of the neurotransmitter. GABA 300 M did not affect the release of the labelled neurotransmitter evoked by exposure to tyramine 0.6 M.In the cat nictitating membrane prelabelled with ³H-NA, the stimulation evoked release of the labelled neurotransmitter was not affected by GABA (10–300 M).In conclusion, GABA has a facilitatory effect on the calcium-dependent, potassium evoked release of ³H-NA when the depolarization is of moderate degree. This effect of GABA appears to be selective for the central nervous system.Some of the findings described in this paper have been presented at the Meeting of the British Pharmacological Society (March, 1978, Guildford, U.K.)