THE INFLUENCE OF GABA ON CELLS IN THE GUSTATORY REGION OF THE HAMSTER SOLITARY NUCLEUS

A brainstem slice preparation was used to investigate GABA-induced responses in the gustatory region of the nucleus of the solitary tract (NST) of the hamster. The baseline activities of 91 cells in the rostral NST were examined extracellularly; 59 cells were located in the rostral central (RC), 21 in the rostral lateral (RL), six in the ventral (V) and five in the medial (M) subdivision of the NST. Of the 80 cells in the gustatory region of the NST (RC and RL subdivisions), application of GABA produced dose-dependent inhibition in 55 (69 %), excitation in 9 (11 %) and no effect in 16 cells (22 %). In contrast, only nine cells were responsive to baclofen, a GABA(B) agonist. In all subdivisions of the rostral NST, 57 cells were inhibited by GABA and the responses of 48 of these were blocked by the specific GABA(A) antagonist, bicuculline methiodide (BICM). Application of BICM alone often yielded an excitatory burst of impulses; this effect was eliminated when synaptic release was blocked by perfusion with a high magnesium physiological saline solution (PSS/Mg++). The GABA(A)-responsive cells were distributed predominantly within the RC subdivision, whereas the GABA(B)-responsive neurons were mostly in the RL subdivision of the NST. The influences of GABA on the membrane properties of cells within the gustatory region (RC and RL subdivisions) of the NST were recorded using conventional intracellular (16 cells) or whole-cell patch (17 cells) recording methods. Intracellular recording revealed that GABA produced hyperpolarisation of the membrane, decreased the firing frequency, and increased the membrane conductance. In the patch-clamp experiments, the application of GABA evoked both inward and outward currents, and an increase in membrane conductance. The reversal potential produced by GABA was close to the Cl- equilibrium potential. The effects of GABA were blocked by BICM. These results suggest that (i) GABA has a strong inhibitory influence on rostral NST neurons. which in the majority of cells is mediated through GABA(A) receptors: and (ii) the gustatory region of the NST may contain a tonically active GABAergic network.