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

Abstract

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 μM were tested. At 100 μM, propofol completely inhibited the firing of all neurons tested ( 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 μM) or BaCl 2 (300 μM), but were completely blocked by bicuculline methiodide (100 μM) or picrotoxin (100 μM), 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 (γ-aminobutyric acid). Potentiation of the propofol effect by other general anesthetics or other drugs was also investigated. When pentobarbital (100 μM) 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 μM propofol, it caused a 6.1-fold greater increase in membrane hyperpolarization and a 9.7-fold greater reduction in input resistance than 30 μM propofol alone. A relatively low concentration of alphaxalone (10 μM), when tested 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 μM propofol, it caused a 9.3-fold greater hyperpolarization and an 8.6-fold greater reduction in input resistance compared with 30 μM 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 γ subunit.