The mechanisms by which general anaesthetics produce a loss of consciousness are unknown. However, the EEG suggests similarities between this state and that of non-REM sleep. We are investigating the hypothesis that these drugs interact with endogenous sleep pathways. In particular, a GABAergic nucleus in the hypothalamus called the ventrolateral preoptic area (VLPO) is activated specifically during sleep [1], and increases its firing in proportion to sleep depth. Moreover, lesions of this nucleus produce insomnia. These neurons are believed to cause sleep through the inhibition of wake-active nuclei [2], including the histaminergic tuberomamillary nucleus (TMN) and the orexinergic neurons of the perifornical area (PeF). The GABAergic nature of the VLPO is critical to this hypothesis of anaesthetic action because these drugs potentiate the GABA_A receptor in vitro. Furthermore, mice with a point-mutation (N265M) in the β₃ subunit of this receptor are markedly less sensitive to the hypnotic action of propofol and etomidate, but retain sensitivity to the steroid anaesthetic alphaxalone [3]. Previous work from this laboratory suggests that the TMN is an important neuronal target for general anaesthetics [4]. The TMN receives additional excitation from the orexinergic neurons of the PeF during the waking state [5] and c-fos data (unpublished) indicates that these neurons are inhibited by GABAergic anaesthetics. We are using hypothalamic coronal slices to compare the ability of propofol (1.5 µM) and alphaxalone (2 µM) to prolong the decay of whole-cell GABAergic IPSCs in both the TMN and PeF of adult male wild-type and N265M knock-in 129/SvJ mice. The decay time constant was calculated as the IPSC integral divided by its peak. All recordings were made at room temperature. Under control conditions, the IPSC decay time was not significantly different in wild-type compared to knock-in animals. In the wild-type TMN, IPSCs were prolonged by both propofol (141 ± 24%, mean ± s.e.m. shown for all data; n=7) and alphaxalone (237 ± 25%; n=5). Strikingly, IPSCs in the TMN of the N265M knock-in mouse were found to be insensitive (P<0.001, 2-tailed, unpaired Student’s t test) to prolongation by propofol (23 ± 8%; n=9). However, again mirroring in vivo findings, the ability of alphaxalone to prolong the IPSC decay time (218 ± 48%; n=4) was retained in the knock-in mouse. Similar data were obtained for the orexinergic neurons of the PeF: these neurons were insensitive (P<0.01) to propofol (10 ± 10%; n=4) in the knock-in mouse compared to wild-type (116 ± 25%; n=5) whereas responses to alphaxalone did not differ. These findings are consistent with the hypothesis that enhancement of GABAergic input into hypothalamic wake-active nuclei is an integral part of propofol's mechanism of action in vivo.