Single neurons in the region of the medial nucleus tractus solitarius (NTS), responding or not responding to gastric vagal branch stimulation, were recorded in an in vitro neonatal rat brainstem-gastric preparation. The spontaneous activity of the majority of these two types of NTS units was inhibited by GABA(A) receptor agonist, muscimol (30 microM), and this inhibition (approximately 52% compared to 100% of the control level) could be antagonized by selective GABA(A) receptor antagonist, bicuculline (10 microM). Application of Panax quinquefolium L. extracts (3.0 microg/ml) into the brainstem compartment of the preparation also significantly reduced the discharge rate of these NTS neurons (approximately 27% compared to the control level), but this reduction could not be reversed by bicuculline (10 microM). Pretreatment with Panax quinquefolium L. (3.0 microg/ml) significantly decreased the NTS inhibitory effects induced by muscimol (30 microM), approximately from 51 to 33%. Our results demonstrated the interactions of Panax quinquefolium L. with ligand-bindings of GABA(A) receptors, and the modulation of the brainstem GABAergic mechanism by Panax quinquefolium L. Our data suggest that the regulation of GABAergic neurotransmission may be an important action of Panax quinquefolium L.