Effects of gamma-aminobutyric acid on the compound action potential of the rat superior cervical ganglion.

The effects on the ganglionic transmission of gamma-aminobutyric acid (GABA) and related drugs were studied in the rat isolated superior cervical ganglion. The extracellularly recorded postganglionic compound action potential was used as an index for ganglionic transmission. GABA reversibly inhibited the ganglionic transmission by blocking the action potential. The effect of GABA is markedly antagonized by bicuculline (20 microM) or picrotoxin (200 microM). Furosemide (1 mM) was also effective in antagonizing the ganglionic effect of GABA. The antagonism by bicuculline was not affected in the presence of pentobarbital (50 microM). Baclofen inhibited the ganglionic transmission through an activation of GABAB receptors since it was markedly antagonized by phaclofen. Muscimol and isoguvacine were less effective than GABA in blocking the ganglionic transmission. The GABA uptake inhibitor isonipecotic acid blocked the ganglionic transmission, probably by activating GABAA receptors, because the effect was antagonized by bicuculline, but the GABA uptake inhibitor guvacine had no significant effect on the ganglionic transmission. These results suggest that GABA causes a ganglionic blockade by activation of both GABAA and GABAB receptor subtypes.