On the mechanism of action of picrotoxin on GABA receptor channels in dissociated sympathetic neurones of the rat.

Abstract

SUMMARY1. The mechanism of action of picrotoxin on GABA receptor channels in rat sympathetic neurones has been investigated with whole-cell clamp. In addition, the action of picrotoxin on single GABA channels has been examined in outside-out membrane patches from these cells.2. Picrotoxin, at concentrations which dramatically reduced the amplitude of whole-cell GABA currents, did not alter the spectral time constants or single-channel conductance estimated by analysis of GABA-activated current noise. This was observed at potentials both negative and positive to the GABA reversal potential (i.e. for both inward and outward GABA currents). In control conditions, the slow and fast time constants from GABA noise were 40 + 14 ms and 2 + 0 4 ms, while the estimated single-channel conductance was 14+2 pS. In the presence of picrotoxin, the time constants and estimated single-channel conductance were 41+5 ms, 2-7 +0-6 ms and 15+2-3 pS.3. Picrotoxin did not alter the shape of the whole-cell GABA current-voltage relationship, indicating that the steady-state block was not voltage dependent. The lack of effect of picrotoxin on the GABA noise spectra and the lack of outward rectification makes it unlikely that picrotoxin acts by a simple voltage-dependent (or voltage-independent) channel blocking mechanism. In the presence of picrotoxin the reversal potential for GABA remained at approximately 0 mV in symmetrical chloride.4. Distributions of total burst durations, obtained from single-channel records with low concentrations of GABA, were fitted with three or four exponential components. Picrotoxin had no consistent effect on the time constants of the total burst length distributions.--It also did not alter the amplitude of the main conductance state. However, picrotoxin did reduce the frequency of channel openings.5. The application of brief ionophoretic pulses of GABA, to cells under whole-cell voltage clamp, revealed that the rate of onset of block by picrotoxin was accelerated in the presence of GABA. In the absence of agonist, picrotoxin produced a more slowly equilibrating block.6. Our data are consistent with a mechanism whereby picrotoxin binds NMS 8382

Identification of a Novel Residue within the Second Transmembrane Domain That Confers Use-facilitated Block by Picrotoxin in Glycine α1 Receptors

“…However, single channel analysis has shown that picrotoxin lacks the flickery effect that is typical of a classical channel blocker (12). In addition, picrotoxin inhibition is voltage-independent in these channels (3,12,13). Furthermore, other reports have demonstrated little or no usefacilitated block by picrotoxin in glycine and GABA C receptors, respectively (13,21).…”

“…Based on the fact that the onset of picrotoxin block is facilitated in the presence of the agonist in GABA A and glutamate-gated Cl Ϫ channels, it has been suggested that picrotoxin acts as an open channel blocker (3,5). However, single channel analysis has shown that picrotoxin lacks the flickery effect that is typical of a classical channel blocker (12). In addition, picrotoxin inhibition is voltage-independent in these channels (3,12,13).…”

“…This trait is generally referred to as a use-dependent or use-facilitated block and suggests that the site of action of the antagonist may be in the channel lumen. Whereas picrotoxin block of GABA C and glycine ␣1 receptors displays weak or no use-facilitation (13, 21), picrotoxin inhibition of GABA A receptors and glutamate-gated Cl Ϫ channels is strongly use-facilitated (3,5,11,12). The molecular basis underlying this trait is unknown.…”

Propofol inhibits pressure-stimulated macrophage phagocytosis via the GABAA receptor and dysregulation of p130cas phosphorylation

“…Propofol and picrotoxin bind to GABA A receptors. Picrotoxin reduces GABA A receptor currents (38), whereas propofol increases GABA A receptor currents (16). Activation of p130cas is also regulated in a system without ion channels (46).…”

Eight genes are required for functional reconstitution of the Caenorhabditis elegans levamisole-sensitive acetylcholine receptor

“…Hence, we concluded that nicotine is mainly a noncompetitive L-AChR antagonist, possibly acting through allosteric inhibition. Such a mechanism has been proposed for picrotoxin at GABA A receptors (36), and for some AChR noncompetitive antagonists such as the steroid promegestone that efficiently inhibits Torpedo AChRs most probably by increasing the fraction of desensitized receptors (37).…”