TRPM4 conductances in thalamic reticular nucleus neurons generate persistent firing during slow oscillations
During sleep, neurons in the thalamic reticular nucleus (TRN) participate in distinct types of oscillatory activity. While the reciprocal synaptic circuits between TRN and sensory relay nuclei are known to underlie the generation of sleep spindles, the mechanisms regulating slow (<1 Hz) forms of thalamic oscillations are not well understood. Under in vitro conditions, TRN neurons can generate slow oscillations in a cell-intrinsic manner, with postsynaptic Group 1 metabotropic glutamate receptor (mGluR) activation leading to the generation of plateau potentials mediated by both T-type Ca2+ currents and Ca2+ -activated nonselective cation currents (ICAN). However, the identity of ICAN and the possible contribution of thalamic circuits to slow rhythmic activity remain unclear. Using thalamic slices derived from adult mice of either sex, we recorded slow forms of rhythmic activity in TRN neurons, which were mediated by fast glutamatergic thalamoreticular inputs but did not require postsynaptic mGluR activation. For a significant fraction of TRN neurons, synaptic inputs or brief depolarizing current steps led to long-lasting plateau potentials and persistent firing (PF), and in turn, resulted in sustained synaptic inhibition in postsynaptic relay neurons of the ventrobasal thalamus (VB). Pharmacological approaches indicated that plateau potentials were triggered by Ca2+ influx through T-type Ca2+ channels and mediated by Ca2+ and voltage-dependent transient receptor potential melastatin 4 (TRPM4) channels. Taken together, our results suggest that thalamic circuits can generate slow oscillatory activity, mediated by an interplay of TRN-VB synaptic circuits that generate rhythmicity and TRN cell-intrinsic mechanisms that control PF and oscillation frequency. Significance StatementSlow forms of thalamocortical rhythmic activity are thought to be essential for memory consolidation during sleep and the efficient removal of potentially toxic metabolites. In vivo, thalamic slow oscillations are regulated by strong bidirectional synaptic pathways linking neocortex and thalamus. Therefore, in vitro studies in the isolated thalamus can offer important insights about the ability of individual neurons and local circuits to generate different forms of rhythmic activity. We found that circuits formed by GABAergic neurons in the thalamic reticular nucleus (TRN) and glutamatergic relay neurons in the ventrobasal thalamus generated slow oscillatory activity, which was accompanied by persistent firing in TRN neurons. Our results identify both cell-intrinsic and synaptic mechanisms that mediate slow forms of rhythmic activity in thalamic circuits.