GABA suppresses stimulation‐induced release of [³H]‐noradrenaline from sympathetic nerve fibres in bovine ovarian follicles

Strips from the bovine ovarian follicle wall were incubated in Krebs-Ringer solution containing [³H]-noradrenaline in order to saturate sympathetic nerve fibres with radiolabelled transmitter. This allowed the study of field stimulation-evoked transmitter release. 80.3 ± 3.9% of the tritium released upon stimulation (10 Hz, pulse duration 1 ms, 10 V between the electrodes) was noradrenaline. The stimulated release of tritium was totally blocked in calcium-free, EGTA(1mM) containing medium or by 1 μM tetrodotoxin. Chemical sympathectomy (6-hydroxydopamine treatment) in vitro reduced the tritium content of the strip by 85%. The neuronal amine uptake blocker desipramine (0.6 μM) was almost equally effective in inhibiting the incorporation of tritium. The extraneuronal amine uptake blocker normetanephrine (10 μM) reduced the tritium content by 30%. Together, the results suggest that the electrically evoked release of tritium reflects the release of [³H]-noradrenaline from sympathetic nerve fibres. γ-Aminobutyric acid (GABA) concentration-dependently reduced the electrically evoked tritium release. Also the GABA(B)-receptor agonist baclofen (30 μM) reduced the stimulated tritium release whereas muscimol (100 μM), a GABA(A)-receptor agonist, failed to affect the release. The selective GABA(A)-receptor antagonist bicuculline (3 and 100 μM) did not block the effect of GABA, while 3-amino-1-propanesulphonic acid (3-APA), a blocker of GABA(B)-receptors reversed the inhibitory effect of GABA. The results suggest that neuronal GABA(B)-receptors are involved in the GABA-evoked suppression of stimulated noradrenaline release.