GABAA-Receptor-Mediated Rebound Burst Firing and Burst Shunting in Thalamus

The role of gamma-aminobutyric acid-A (GABA(A))-receptor-mediated inhibitory postsynaptic potentials (IPSPs) in 1) generating rebound burst firing and 2) burst inhibition in thalamocortical (TC) relay cells and inhibitory neurons of nucleus reticularis thalami (nRt) was investigated. Experimental data from previous studies were used to generate artificial synaptic responses in neurons via a computer-driven dynamic clamp. On average, in nRt neurons trains of six or more 10-nS GABA(A) IPSPs generated rebound bursts of action potentials with a mean delay of 605 +/- 32 (SE) ms. In contrast, 10 IPSPs were required for rebound bursts in relay cells, and these occurred with a significantly shorter delay of 327 +/- 35 ms. Ca2+-dependent burst responses could be shunted by single IPSPs. Half-maximal burst inhibition was obtained in nRt cells when IPSP conductance was 1.5 times the whole cell input conductance. Burst shunting in TC cells was less effective and required a synaptic- to input-conductance ratio of 3. The relative time window of IPSP burst shunting was broader in nRt (approximately 20 ms) than TC cells (approximately 10 ms). We conclude that in nRt cells GABA(A)-dependent rebound burst responses would occur with a latency that is incompatible with pacemaking of fast (>3-Hz) thalamic rhythm generation such as spindles, yet burst inhibition is powerful. Therefore a likely role for reciprocal intra-nRt connectivity is to mediate lateral inhibition between nRt cells.