METABOTROPIC GLUTAMATE RECEPTORS DEPRESS AFFERENT EXCITATORY TRANSMISSION IN THE RAT NUCLEUS-TRACTUS-SOLITARII

1. We have previously demonstrated that the metabotropic glutamate rec eptor (mGluR) agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate [( 1S,3R)-ACPD] presynaptically inhibits evoked glutamatergic excitatory postsynaptic currents (EPSCs) in patch-clamped rat nucleus tractus sol itarius (NTS) neurons recorded in thin slices. The present study inves tigated the ability of endogenously released glutamate to modulate EPS Cs in the NTS. 2. A low-frequency tetanus of the tractus solitarius (T S) resulted in either posttetanic potentiation (PTP) (8 of 21 cells) o r depression (13 of 21 cells) of monosynaptic EPSCs recorded in the pr esence of D(-)2-amino-5-phosphonopentanoic acid (AP5) and bicuculline. 3. The amplitude of the EPSC was not significantly affected by the ba th application of the mGluR antagonist (+) alpha-methyl-4-carboxypheny lglycine (MCPG). 4. In the presence of MCPG, a low-frequency tetanus r esulted in PTP of the EPSC in all neurons. PTP was significantly enhan ced in those cells previously exhibiting PTP. 5. The results suggest t hat presynaptic mGluRs on TS projections to the NTS may be activated b y endogenously released glutamate at physiologically relevant stimulus frequencies and therefore play a role in the modulation of autonomic afferent transmission.