Abstract

Single units in the region of the medial nucleus tractus solitarius (NTS), responding to electrical stimulation of gastric vagal fibers, were recorded in an in vitro neonatal rat brainstem-gastric preparation. γ-Aminobutyric acid (GABA) subreceptor agonists and antagonists were applied to the gastric and brainstem compartments of the bath chamber to evaluate the peripheral gastric and central brainstem GABAergic effects on NTS neuronal activity. The gastric effects of the GABA_A receptor agonist muscimol and GABA_B receptor agonist baclofen were evaluated on 55 tonic units that received the gastric vagal inputs. For ∼58% (32 of 55) and 38% (21 of 55) of the units observed, muscimol (30 μM; IC₅₀ = 2.0 μM) and baclofen (30 μM; IC₅₀ = 1.5 μM) in the gastric compartment induced a concentration-dependent inhibition of 36.2 ± 3.1% (mean ± S.E.) and 31.0 ± 2.9% of the control level of the NTS neuronal activity, respectively. The brainstem effects of muscimol and baclofen were tested on 51 units. For ∼90% (46 of 51) and 78% (40 of 51) of the units tested, muscimol (30 μM; IC₅₀ = 1.3 μM) and baclofen (30 μM; IC₅₀ = 1.1 μM) in the brainstem 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. Bicuculline (10 μM) and saclofen (10 μM), the GABA_A and GABA_Bsubreceptor antagonists, competitively antagonized the gastric and brainstem effects by muscimol and baclofen, respectively. Our results demonstrated that both GABA_A and GABA_Breceptors in the stomach and brainstem play an important role in activity modulation of the medial NTS neurons receiving gastric vagal inputs in neonatal rats.