GABAERGIC EFFECTS ON NUCLEUS-TRACTUS-SOLITARIUS NEURONS RECEIVING GASTRIC VAGAL INPUTS

Single units in the region of the medial nucleus tractus solitarius (N TS), responding to electrical stimulation of gastric vagal fibers, wer e recorded in an in vitro neonatal rat brainstem-gastric preparation. gamma-Aminobutyric acid (GABA) subreceptor agonists and antagonists we re applied to the gastric and brainstem compartments of the bath chamb er to evaluate the peripheral gastric and central brainstem GABAergic effects on NTS neuronal activity. The gastric effects of the GABA(A) r eceptor agonist muscimol and GABA(B) receptor agonist baclofen were ev aluated on 55 tonic units that received the gastric vagal inputs. For similar to 58% (32 of 55) and 38% (21 of 55) of the units observed, mu scimol (30 mu M; IC50 = 2.0 mu M) and baclofen (30 mu M; IC50 = 1.5 mu M) in the gastric compartment induced a concentration-dependent inhib ition of 36.2 +/- 3.1% (mean +/- S.E.) and 31.0 +/- 2.9% of the contro l level of the NTS neuronal activity, respectively. The brainstem effe cts of muscimol and baclofen were tested on 51 units. For similar to 9 0% (46 of 51) and 78% (40 of 51) of the units tested, muscimol (30 mu M; IC50 = 1.3 mu M) and baclofen (30 mu M; IC50 = 1.1 mu M) in the bra instem compartment produced a concentration-dependent inhibition of 54 .1 +/- 3.4% and 48.9 +/- 3.5% of the control level, respectively. The remaining NTS units were not affected by these two GABA agonists. Bicu culline (10 mu M) and saclofen (10 mu M), the GABA(A) and GABA(B) subr eceptor antagonists, competitively antagonized the gastric and brainst em effects by muscimol and baclofen, respectively. Our results demonst rated that both GABA(A) and GABA(B) receptors in the stomach and brain stem play an important role in activity modulation of the medial NTS n eurons receiving gastric vagal inputs in neonatal rats.