Differential regulation of γ‐aminobutyric acid receptor channels by diazepam and phenobarbital

Abstract

The anticonvulsant activity of diazepam and phenobarbital may be mediated in part by enhancement of inhibition involving gamma-aminobutyric acid (GABA). While both diazepam and phenobarbital increase GABA receptor chloride current, they may have different mechanisms of action, since they bind to different sites on the GABA receptor-chloride channel complex. We used the patch clamp technique to compare the effects of diazepam and phenobarbital on single GABA receptor currents. Outside-out patches were obtained from mouse spinal cord neurons grown in cell culture for 2 to 4 weeks. GABA (2 microM) evoked single channel currents that occurred as single brief openings or in bursts of multiple openings. Diazepam (20 nM) and phenobarbital (500 microM) both increased the GABA receptor current by increasing mean open time without altering channel opening frequency. However, the temporal grouping of openings into bursts suggested that the enhancement occurred via different mechanisms. Diazepam increased the frequency of bursting GABA receptor currents with minimal effect on the duration of bursts. Phenobarbital increased the duration of bursting GABA receptor currents without altering the frequency of bursts. These results suggest that diazepam binds to a site that may enhance single channel burst frequency by increasing the affinity of GABA binding, while phenobarbital may stabilize the bursting open state of the channel by binding to a different modulatory site at or near the chloride channel.

Enteric GABA‐containing nerves projecting to the guinea‐pig inferior mesenteric ganglion modulate acetylcholine release.

“…Baclofen (10 /M), a GABAB receptor agonist, had no effect, suggesting that the presynaptic GABA receptors located on peripheral lumbar colonic nerves were of the GABAA receptor subtype. Diazepam, which is reported to have no intrinsic activity of its own, enhances GABAA synaptic transmission during release of endogenous GABA (Study & Barker, 1981;Twyman, Rogers & Macdonald, 1989). In the present study, diazepam (5/M) facilitated release of [3H]acetylcholine by 55 % (P < 0'05) during LCN stimulation (Fig.…”

Seizures triggered by pentylenetetrazol in marmosets made chronically epileptic with pilocarpine show greater refractoriness to treatment

“…This might be associated with both PTZ and PB acting on the same GABA-A channel. PB binds to the GABA-A receptor in the site for benzodiazepine, which extends the time the chloride channel remains opened (Twyman et al, 1989), whereas PTZ blocks the chloride influx by means of the binding to the picrotoxin site of the same GABA-A channel (Huang et al, 2001). Even though the antagonism between PTZ and PB does not rely on a competition for the same binding site, the previous PB administration may prompt conformational changes, which might reduce the responsiveness of GABA-A channel to PTZ.…”

Effects of Halothane on GABAAReceptor Kinetics: Evidence for Slowed Agonist Unbinding

“…These similarities again suggest a common mechanism of action. Although there is evidence that benzodiazepines increase the microscopic agonist binding rate (Twyman et al, 1989;Lavoie and Twyman, 1996) and alter channel conductance (Eghbali et al, 1997), it has also been proposed that benzodiazepines slow IPSC decay, at least in part by reducing the agonist dissociation rate (Mellor and Randall, 1997).…”