THE INFLUENCE OF GABA ON CELLS IN THE GUSTATORY REGION OF THE HAMSTERSOLITARY NUCLEUS

A brainstem slice preparation was used to investigate GABA-induced res ponses in the gustatory region of the nucleus of the solitary tract (N ST) of the hamster. The baseline activities of 91 cells in the rostral NST were examined extracellularly; 59 cells were located in the rostr al 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 i n the gustatory region of the NST (RC and RL subdivisions), applicatio n of GABA produced dose-dependent inhibition in 55 (69 %), excitation in 9 (11 %) and no effect in 16 cells (22 %). In contrast, only nine c ells were responsive to baclofen, a GABA(B) agonist. In all subdivisio ns of the rostral NST, 57 cells were inhibited by GABA and the respons es of 48 of these were blocked by the specific GABA(A) antagonist, bic uculline methiodide (BICM). Application of BICM alone often yielded an excitatory burst of impulses; this effect was eliminated when synapti c release was blocked by perfusion with a high magnesium physiological saline solution (PSS/Mg++). The GABA(A)-responsive cells were distrib uted predominantly within the RC subdivision, whereas the GABA(B)-resp onsive neurons were mostly in the RL subdivision of the NST. The influ ences of GABA on the membrane properties of cells within the gustatory region (RC and RL subdivisions) of the NST were recorded using conven tional intracellular (16 cells) or whole-cell patch (17 cells) recordi ng methods. Intracellular recording revealed that GABA produced hyperp olarisation of the membrane, decreased the firing frequency, and incre ased the membrane conductance. In the patch-clamp experiments, the app lication of GABA evoked both inward and outward currents, and an incre ase in membrane conductance. The reversal potential produced by GABA w as close to the Cl- equilibrium potential. The effects of GABA were bl ocked by BICM. These results suggest that (i) GABA has a strong inhibi tory influence on rostral NST neurons. which in the majority of cells is mediated through GABA(A) receptors: and (ii) the gustatory region o f the NST may contain a tonically active GABAergic network.