Propofol enhances both tonic and phasic inhibitory currents in second-order neurons of the solitary tract nucleus (NTS)

Abstract

The anesthetic propofol is thought to induce rapid hypnotic sedation by facilitating a GABAergic tonic current in forebrain neurons. The depression of cardiovascular and respiratory regulation often observed during propofol suggests potential additional actions within the brainstem. Here we determined the impacts of propofol on both GABAergic and glutamatergic synaptic mechanisms in a class of solitary tract nucleus (NTS) neurons common to brainstem reflex pathways. In horizontal brainstem slices, we recorded from NTS neurons directly activated by solitary tract (ST) axons. We identified these second-order NTS neurons by time-invariant (“jitter” < 200 μs), “all-or-none” glutamatergic excitatory postsynaptic currents (EPSCs) in response to shocks to the ST. In order to assess propofol actions, we measured ST-evoked, spontaneous and miniature EPSCs and inhibitory postsynaptic currents (IPSCs) during propofol exposure. Propofol prolonged miniature IPSC decay time constants by 50% above control at 1.8 μM. Low concentrations of gabazine (SR-95531) blocked phasic GABA currents. At higher concentrations, propofol (30 μM) induced a gabazine-insensitive tonic current that was blocked by picrotoxin or bicuculline. In contrast, total propofol concentrations up to 30 μM had no effect on EPSCs. Thus, propofol enhanced phasic GABA events in NTS at lower concentrations than tonic current induction, opposite to the relative sensitivity observed in forebrain regions. These data suggest that therapeutic levels of propofol facilitate phasic (synaptic) inhibitory transmission in second-order NTS neurons which likely inhibits autonomic reflex pathways during anesthesia.

Introduction

Propofol (2,6-di-isopropylphenol) is widely favored as an intravenous general anesthetic (Rudolph and Antkowiak, 2004). Strong mechanistic links have been established, mostly in forebrain regions, between several of its clinical features and underlying cellular-molecular actions (Rudolph and Antkowiak, 2004, Franks, 2006). The hypnotic effects of propofol are primarily attributed to the enhancement of A-type γ-aminobutyric acid (GABA_A) receptor function by inducing extended GABA_A channel open times (Kitamura et al., 2004) and slowing desensitization (Bai et al., 1999, Bai et al., 2001), both attributes of phasic GABAergic transmission. Increasing interest has focused on propofol actions to induce an inhibitory tonic current that is pharmacologically and biophysically separable from phasic GABAergic currents (Hales and Lambert, 1991, Bai et al., 2001, Yeung et al., 2003). In hippocampal neurons, this tonic, “extrasynaptic” GABA_A component is more sensitive to anesthetics than actions on phasic currents in the same neurons (Hemmings et al., 2005) and seemingly plays a substantial role in suppressing neuronal excitability (Bieda and MacIver, 2004). In addition, ion channels and in particular voltage-gated sodium channels are reported to be inhibited by low micromolar concentrations of propofol (Frenkel and Urban, 1991, Ouyang et al., 2003, Jones et al., 2007). Given the diversity in composition and expression of native GABA_A receptors as well as potential ion channel targets, accurate predictions of general anesthetic actions within any specific brain region is problematic.

Hypotension, bradycardia, desaturation and apnea are noted side effects of propofol administration and are consistent with depression of cardiorespiratory control mechanisms (Trapani et al., 2000, Nieuwenhuijs et al., 2000). Neurons responsible for these homeostatic regulatory mechanisms are concentrated in the brainstem (Loewy, 1990, Andresen and Kunze, 1994, Guyenet, 2006) and actions in the brainstem are consistent with the substantial autonomic impact of propofol in humans (Ebert and Muzi, 1994, Ebert, 2005). Common to most of these reflex pathways is the nucleus of the solitary tract (NTS), the site of central terminations of visceral cranial afferent nerves (including the vagus) from the cardiovascular, respiratory and gastrointestinal systems (Saper, 2002, Andresen et al., 2004, Guyenet, 2006, Travagli et al., 2006). Arterial baroreceptors and respiratory afferents terminate within medial portions of the caudal NTS and are a source of excitatory glutamatergic input onto second-order NTS neurons (Mendelowitz et al., 1992, Doyle and Andresen, 2001, Kubin et al., 2006). GABAergic transmission in NTS is also critical to normal cardiorespiratory reflex performance (Andresen and Kunze, 1994, Kubin et al., 2006) as well as pathophysiological conditions (Urbanski and Sapru, 1988, Callera et al., 2000, Mei et al., 2003, Tolstykh et al., 2004).

To assess the cellular mechanisms of propofol in the NTS, we recorded GABAergic and glutamatergic synaptic currents from synaptically identified, second-order neurons in horizontal brainstem slices. Propofol facilitated spontaneous and miniature inhibitory postsynaptic currents (IPSCs) and evoked a tonic, GABA-mediated current without altering glutamatergic, excitatory postsynaptic currents (EPSCs). Interestingly and unlike that observed in some forebrain neurons, inhibitory phasic currents were more sensitive to propofol than the evoked GABA_A mediated tonic conductance. Taken together, these results suggest that the facilitation of inhibitory phasic neurotransmission dominates propofol actions within NTS.