Actions of avermectin B1a on the gamma-aminobutyric acidA receptor and chloride channels in rat brain.

The interaction of avermectin B1a (AVM) with the gamma-aminobutyric acid (GABA) receptor of rat brain was studied using radioactive ligand binding and tracer ion flux assays. Avermectin potentiated the binding of [3H]flunitrazepam and inhibited the binding of both [3H]muscimol and [35S]t-butylbicyclophosphorothionate to the GABAA receptor. Inhibition of muscimol binding by AVM suggested competitive displacement. Two kinds of 36chloride (Cl) flux were studied. The 36Cl efflux from preloaded microsacs was potentiated by AVM and was highly inhibited by the Cl-channel blocker 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS). However, it was not potentiated by GABA nor was it sensitive to the convulsants picrotoxin or bicuculline. On the other hand, 36Cl-influx measurement in a different microsac preparation of rat brain was very sensitive to GABA and other GABA-ergic drugs. Avermectin induced 36Cl influx into these microsacs in a dose-dependent manner, but to only 35% of the maximal influx induced by GABA. The AVM-induced 36Cl influx was totally blocked by bicuculline. It is suggested that AVM opens the GABAA-receptor Cl channel by binding to the GABA recognition site and acting as a partial receptor agonist, and also opens a voltage-dependent Cl channel which is totally insensitive to GABA but is very sensitive to DIDS.

Effects of insecticides on GABA-induced chloride influx into rat brain microsacs.

The actions of different insecticides known to affect binding of ligands to gamma-aminobutyric acid (GABA) receptors were studied on the function of GABAA receptors in rat brain as assayed by GABA-induced 36Cl- influx into membrane microsacs. This flux was inhibited by the competitive antagonist bicuculline and the noncompetitive antagonist t-butylbicyclophosphorothionate, and the GABA effect was potentiated by the tranquilizer flunitrazepam and the depressant pentobarbital, as expected for effects on a GABAA receptor. The GABA-induced 36Cl- flux was inhibited by several cyclodienes and gamma-hexachlorocyclohexane (gamma-BHC) with the following order of decreasing potency: endosulfan I greater than endrin greater than endosulfan II greater than dieldrin greater than heptachlor epoxide greater than gamma-BHC greater than heptachlor. The noninsecticidal beta-BHC had no effect, while the IC 50 values for gamma-BHC and endrin were 1 and 0.2 microM, respectively. The four pyrethroids tested also inhibited the GABA-induced 36Cl- flux with the following decreasing potencies: 1R,cis,alpha S-cypermethrin greater than 1R,trans, alpha S-cypermethrin greater than fluvalinate greater than allethrin. Avermectin B1a was the only insecticide tested that, in the absence of GABA, stimulated 36Cl- flux in a dose-dependent manner, and this flux was inhibited by bicuculline. The stereospecific inhibition of the GABA-induced 36Cl- influx by the cyclodienes and gamma-BHC supports previously published data on their binding to mammalian brain GABAA receptor and suggests that these insecticides inhibit this receptor's function. It is also suggested that type II pyrethroids are potent inhibitors of the same receptor. However, avermectin B1a appears to act as a partial agonist of GABAA receptors.

gamma-Aminobutyric acid-activated 36Cl- influx: a functional in vitro assay for CNS gamma-aminobutyric acid receptors of insects.

The effects of gamma-aminobutyric acid (GABA) on the uptake of 36Cl- into a membrane microsac preparation from isolated nerve cords of the cockroach Periplaneta americana was studied. On addition of 1 microM GABA (after 4-s incubation, then rapid quenching) the influx of 36Cl- was stimulated to a level 75% above that of the control value. This stimulation was reduced by picrotoxin (100 microM), but was not significantly affected by bicuculline (100 microM). Results of 36Cl- influx experiments are in agreement with data obtained from radiolabelled ligand binding assays and electrophysiological investigations on the same tissue. The method described represents a functional in vitro assay for CNS GABA receptors of insects.

Muscarinic receptor subtype specificity of 5'-(isobutylthio)-adenosine (SIBA) and its analogs.

1. 5'(Isobutylthio)-adenosine (SIBA) and its analogs, at 100 microM, inhibited [3H]N-methyl-scopolamine binding to homogenates of whole brain and cortex (mainly M1 subtype receptors) by 11-30% and to cerebellum (mainly M2 subtype receptors) by 20-39%. 2. At 0.01-1.0 microM, stimulation of [3H]QNB and NMS-inaccessible [3H]QNB binding was observed, with the most induced by 1 microM 3-deaza-SIBA. 3. In contrast, [3H]pirenzepine ([3H]PZ) binding to whole brain and cortex was inhibited in a dose-dependent manner with Ki values in the microM range. 4. As antagonists of acetylcholine-induced contraction of guinea pig ileum (mainly M2 subtype receptors), the analogs were slightly more potent than pirenzepine, but several orders of magnitude less than atropine; the order of potency was opposite that determined for the binding of [3H]PZ to cortex. 5. Thus, SIBA and its analogs may have differential effects on muscarinic receptor subtypes and show some specificity for the M1 receptor subtype.