GABAergic actions on cholinergic laterodorsal tegmental neurons: implications for control of behavioral state

Abstract: Cholinergic neurons of the pontine laterodorsal tegmentum (LDT) play a critical role in regulation of behavioral state. Therefore, elucidation of mechanisms that control their activity is vital for understanding of how switching between wakefulness, sleep and anesthetic states is effectuated. In vivo studies suggest that GABAergic mechanisms within the pons play a critical role in behavioral state switching. However, the postsynaptic, electrophysiological actions of GABA on LDT neurons, as well as the identity of GABA receptors present in the LDT mediating these actions is virtually unexplored. Therefore, we studied the actions of GABA agonists and antagonists on cholinergic LDT cells by performing patch clamp recordings in mouse brain slices. Under conditions where detection of Cl− -mediated events was optimized, GABA induced gabazine (GZ)-sensitive inward currents in the majority of LDT neurons. Post-synaptic location of GABAA receptors was demonstrated by persistence of muscimol-induced inward currents in TTX and low Ca2+ solutions. THIP, a selective GABAA receptor agonist with a preference for δ-subunit containing GABAA receptors, induced inward currents, suggesting the existence of extrasynaptic GABAA receptors. LDT cells also possess GABAB receptors as baclofen-activated a TTX- and low Ca2+-resistant outward current that was attenuated by the GABAB antagonists CGP 55845 and saclofen. The tertiapin sensitivity of baclofen-induced outward currents suggests that a GIRK mediated this effect. Further, outward currents were never additive with those induced by application of carbachol, suggesting that they were mediated by activation of GABAB receptors linked to the same GIRK activated in these cells by muscarinic receptor stimulation. Activation of GABAB receptors inhibited Ca2+ increases induced by a depolarizing voltage step shown previously to activate VOCCs in cholinergic LDT neurons. Baclofen-mediated reductions in depolarization-induced Ca2+ were unaltered by prior emptying of intracellular Ca2+ stores, but were abolished by low extracellular Ca2+ and pre-application of nifedipine, indicating that activation of GABAB receptors inhibits influx of Ca2+ involving L-type Ca2+ channels. Presence of GABAC receptors is suggested by the induction of inward current by (E)-4- amino-2-butenoic acid (TACA) and its inhibition by 1,2,5,6-tetrahydropyridine-4-ylmethylphosphinic (TPMPA), a relatively selective agonist and antagonist, respectively, of GABAC receptors. All of these GABA-mediated actions were found to occur in histochemically-identified cholinergic neurons. Taken together, these data indicate for the first time that cholinergic neurons of the LDT exhibit functional GABAA, B and C receptors, including extrasynaptically located GABAA receptors, which may be tonically activated by synaptic overflow of GABA. Accordingly, the activity of cholinergic LDT neurons is likely to be significantly affected by GABAergic tone within the nucleus, and so, demonstrated effects of GABA on behavioral state may be mediated, in part, via direct actions on cholinergic neurons in the LDT. [Copyright &y& Elsevier]