EFFECT OF GAMMA-AMINOBUTYRIC-ACID ON SYNAPTIC TRANSMISSION AND LONG-TERM POTENTIATION IN RAT SUPERIOR CERVICAL-GANGLION

The effect of gamma-aminobutyric acid (GABA) on synaptic transmission in rat superior cervical ganglion (SCG) was assessed in vitro by extra cellular recording. Postganglionic compound action potentials (CAPs) t riggered by preganglionic stimulation were blocked in a reversible and concentration-dependent fashion by short, 60 s long, superfusion with GABA (IC50 = 39.3 mu M), with the GABA(A) agonist muscimol (IC50 = 8. 7 mu M) or with the GABA(B) agonist baclofen (IC50 = 145 mu M). Respon ses to GABA and muscimol, but not to baclofen, exhibited desensitizati on after 5 min long superfusions with the drugs. In a long-term potent iation (LTP) paradigm, the degree of potentiation found 30 min after a tetanic train of stimuli (20 Hz for 20 s) was strongly inhibited by G ABA (100-250 mu M), when superfused at the time of tetanic stimulus or shortly thereafter. The effect of GABA on SCG LTP was mimicked by mus cimol but not by baclofen. The results are compatible with the view th at GABA exerts overall inhibitory effects in rat SCG, including transm ission blockade of single impulses (through activation of GABA(A) and GABA(B) receptors) and impairment of activity-dependent potentiation o f nicotinic transmission (through activation of GABA(A) receptors).