BIDN (3,3-bis(trifluoromethyl)bicyclo[2,2,1]heptane-2,2-dicarbonitrile) at 10(-5) M blocked GABA-induced inhibitory postsynaptic potentials (IPSPs) recorded from an identified, giant interneuron (G12) of the cockroach (Periplaneta americana). The same concentration of this bicyclic dinitrile also blocked Cl- -mediated responses of G12 to GABA applied by pressure microinjection into the terminal abdominal ganglion neuropile containing dendrites of G12. BIDN (10(-5) M) was without effect on a response of G12 to GABA known to be mediated by a GABAB type receptor. In studies of the cell body of an identified motor neurone, the fast coxal depressor (Df) in the cockroach metathoracic ganglion, BIDN (10(-5) M) blocked reversibly an extrasynaptic GABA-gated Cl- channel, but not an extrasynaptic L-glutamate-gated Cl- channel. Glycine-gated Cl- channels observed when rat brain messenger RNA was expressed in Xenopus laevis oocytes were unaffected by BIDN at concentrations up to 10(-4) M, whereas this same concentration of BIDN completely blocked GABA-gated Cl- responses recorded from the same preparations. Unlike picrotoxin, which antagonises a variety of ligand-gated Cl- channels, to date BIDN has been found to block only Cl- channels gated by GABA, both in insect and vertebrate preparations.

Local application of GABA to rat cerebral cortical neurons in brain slices elicited biphasic responses mediated via GABAA receptors. The fast component of the response, which was most apparent with somatic application of GABA, was hyperpolarizing at the normal resting membrane potential (GABAh response). The slower component could be elicited by GABA application to nearly all regions of the cell, and was depolarizing at the resting membrane potential (GABAd response). The reversal potential of evoked IPSCs recorded with whole-cell patch electrodes (-68 mV) was comparable to the reversal potential of the GABAh response (-69 mV), and was significantly different from the reversal potential of the GABAd response (-56 mV). The GABAd response was more sensitive to enhancement by pentobarbital and more readily antagonized by both bicuculline and picrotoxin than the GABAh response. Recording in bicarbonate-free buffer changed the reversal potential of the GABAd response significantly, but had no effect on the GABAh response. In contrast, superfusion with ethanol significantly enhanced the GABAh response, while having no effect on the GABAd component. Although a localized collapse of the Cl- gradient, which has been proposed to underlie the GABAd response, could explain the greater sensitivity of the GABAd response to pentobarbital and the GABAA antagonists, this could not account for the greater sensitivity of the GABAh response to ethanol. Differences in GABAA receptor subunit composition may result in the expression of dendritic and somatic GABAA receptors that have different kinetics, reversal potentials, and sensitivity to pharmacological agents, including ethanol.

General anesthetics variably enhance inhibitory synaptic transmission that relies on (-aminobutyric acid (GABA) and GABAA receptor function with distinct differences across brain regions. Activation of "extra-synaptic" GABAA receptors produces a tonic current considered the most sensitive target for general anesthetics, particularly in forebrain neurons. To evaluate the contribution of poor drug access to neurons in slices, we tested the intravenous anesthetic propofol in mechanically isolated neurons from the solitary tract nucleus (NTS). Setting chloride concentrations to ECl=-29 mV made GABA currents inward at holding potentials of -60 mV. Propofol triggered pronounced but slowly-developing tonic currents that reversed with 5 min washing. Effective concentrations in isolated cells were lower than in slices and propofol enhanced phasic IPSCs more potently than tonic currents (1 microM increased phasic decay-time constant vs. >3 microM tonic currents). Propofol increased IPSC frequency (>3 microM), a presynaptic action. Bicuculline blocked all propofol actions. Gabazine blocked only phasic IPSCs. IPSCs persisted in TTX and/or cadmium but these agents prevented propofol-induced increases in IPSC frequency. Furosemide (>1 mM) reversibly blocked propofol-evoked IPSC frequency changes without altering waveforms. We conclude that presynaptic actions of propofol depend on a depolarizing chloride gradient across presynaptic inhibitory terminals. Our results in isolated neurons indicate that propofol pharmacokinetics intrinsically trigger the tonic currents slowly and the time course is not related to slow permeation or delivery. Unlike forebrain, phasic NTS GABAA receptors are more sensitive to propofol than tonic receptors but that presynaptic GABAA receptor mechanisms regulate GABA release.