GABAA receptors involved in sleep and anaesthesia: β1-versus β3-containing assemblies

The histaminergic neurons of the posterior hypothalamus (tuberomamillary nucleus-TMN) control wakefulness, and their silencing through activation of GABA(A) receptors (GABA(A)R) induces sleep and is thought to mediate sedation under propofol anaesthesia. We have previously shown that the beta 1 subunit preferring fragrant dioxane derivatives (FDD) are highly potent modulators of GABA(A)R in TMN neurons. In recombinant receptors containing the beta 3N265M subunit, FDD action is abolished and GABA potency is reduced. Using rat, wild-type and beta 3N265M mice, FDD and propofol, we explored the relative contributions of beta 1- and beta 3-containing GABA(A)R to synaptic transmission from the GABAergic sleep-on ventrolateral preoptic area neurons to TMN. In beta 3N265M mice, GABA potency remained unchanged in TMN neurons, but it was decreased in cultured posterior hypothalamic neurons with impaired modulation of GABA(A)R by propofol. Spontaneous and evoked GABAergic synaptic currents (IPSC) showed beta 1-type pharmacology, with the same effects achieved by 3 mu M propofol and 10 mu M PI24513. Propofol and the FDD PI24513 suppressed neuronal firing in the majority of neurons at 5 and 100 mu M, and in all cells at 10 and 250 mu M, respectively. FDD given systemically in mice induced sedation but not anaesthesia. Propofol-induced currents were abolished (1-6 mu M) or significantly reduced (12 mu M) in beta 3N265M mice, whereas gating and modulation of GABA(A)R by PI24513 as well as modulation by propofol were unchanged. In conclusion, beta 1-containing (FDD-sensitive) GABA(A)R represent the major receptor pool in TMN neurons responding to GABA, while beta 3-containing (FDD-insensitive) receptors are gated by low micromolar doses of propofol. Thus, sleep and anaesthesia depend on different GABA(A)R types.