Biphasic responses of thalamic neurons to GABA in isolated rat brain slices--II.

In isolated thalamic slices, responses of relay neurons to electrophoretically applied GABA were recorded intracellularly and compared with inhibitory postsynaptic potentials evoked by electrical stimulation of the reticularis nucleus of the thalamus. Both reduced the excitability of thalamic neurons and were biphasic in the majority of neurons studied, consisting of an early, negative-going and a later, positive-going component, when recorded close to reversal potential (mean reversal potentials -66.6 and -57.7 mV). Bicuculline and picrotoxin applied electrophoretically reduced conductance increases evoked by GABA in all neurons. The later, positive-going component was more sensitive to these antagonists (applied with submaximal doses) than the early component. Current-voltage relations for responses to GABA, like those for inhibitory postsynaptic potentials, were non-linear in the majority of neurons. In particular, there was a region of reduced slope resistance close to the reversal potential. Holding the membrane at a conditioning potential was found to change the subsequent response and its reversal potential. Positive holding potentials shifted reversal potentials in the positive direction only when GABA was applied during the conditioning period. Negative holding potentials were effective whether GABA was applied during the conditioning period or not. Recovery from these effects followed a similar time course at all membrane potentials tested. Injection of Cl- produced a positive shift in the reversal potential for both components of the response to GABA and of the evoked inhibitory postsynaptic potential. Inhibitory postsynaptic potentials evoked in thalamic relay neurons by stimulation of the nucleus reticularis resembled responses to GABA in their biphasic nature, reversal potentials and sensitivity to antagonists and to changes in intracellular chloride.