Activation of rat locus coeruleus neuron GABAA receptors by propofol and its potentiation by pentobarbital or alphaxalone

The action of propofol on the rat locus coeruleus was examined using intracellular recording from in vitro brain slice preparations. Concentrations of propofol between 3 and 300 mu M were tested. At 100 mu M, propofol completely inhibited the firing of all neurons rested (n = 34); this was associated with a 5.7-mV hyperpolarization (range 0-16 mV, n = 33) and a 35.6% reduction in input resistance (range 7.3-66.1%, n = 33). The propofol-induced responses were not affected by 2-hydroxysaclofen (50 mu M) or BaCl2 (300 mu M), but were completely blocked by bicuculline methiodide (100 mu M) or picrotoxin (100 mu M), indicating that propofol acts on GABA(A) receptors. As assessed by inhibition of the spontaneous firing rate, propofol was 5.6-fold more potent than GABA (gamma-aminobutyric acid). Potentiation of the propofol effect by other general anesthetics or other drugs was also investigated. When pentobarbital (100 mu M) was tested alone on locus coeruleus cells. no change in membrane potential or input resistance was seen and there was only a 20.3 +/- 7.2% (n = 8) inhibition of firing rate: however, in combination with 30 mu M propofol, it caused a 6.1-fold greater increase in membrane hyperpolarization and a 9.7-fold greater reduction in input resistance than 30 mu M propofol alone. A relatively low concentration of alphaxalone (10 mu M), when tasted alone, had little effect on the membrane potential or input resistance and only produced a 46.0 +/- 8.9% (n = 8) inhibition of firing rate, however, in combination with 30 mu M propofol, it caused a 9.3-fold greater hyperpolarization and an 8.6-fold greater reduction in input resistance compared with 30 mu M propofol alone. In contrast, diazepam caused no potentiation of either propofol- or GABA-induced responses. Our data also indicate that locus coeruleus neuron GABA(A) receptors possess distinctive pharmacologic characteristics. such as blocking of the propofol effects by zinc and insensitivity to diazepam and the direct action of pentobarbital. On the basis of these pharmacologic properties, we suggest that locus coeruleus neuron GABA(A) receptors do not contain the gamma subunit. (C) 1999 Elsevier Science B.V. All rights reserved.