Picrotoxinin Receptor Research Articles

Alkyl-substituted γ-butyrolactones act at a distinct site allosterically linked to the TBPS/picrotoxinin site on the GABA A receptor complex

Effects of alkyl-substituted gamma-butyrolactones and gamma-thiobutyrolactones on [35S]t-butylbicyclophosphorothionate (35S-TBPS) dissociation from the picrotoxinin receptor were studied. Unlike picrotoxinin, these lactones accelerated the dissociation rate of 35S-TBPS. Thus, previous reports that these lactones change the Kd but not the Bmax of 35S-TBPS in equilibrium binding experiments is explained not by competitive inhibition, but by an allosteric interaction with the 35S-TBPS binding site. These results indicate that modulatory effects of alkyl-substituted gamma-butyrolactones may result from their action at a distinct site on the gamma-aminobutyric acid (GABA)A receptor.

Two insecticidal monoterpenes, telfairine and aplysiaterpenoid A, from the red alga Plocamium telfairiae: Structure elucidation, biological activity, and molecular topographical consideration by a semiempirical molecular orbital study

Two insecticidal polyhalogenated monoterpenes, aplysiaterpenoid A and telfairine were isolated from the red alga Plocamium telfairiae. Both compounds showed strong insecticidal activities against the German cockroach ( Blatella germanica) and mosquito larvae ( Anopheles gambiae). The dieldrin-resistant strain of mosquito larvae ( A. gambiae) which exhibited the significant cross-resistance to ( 1246 35 )-1,2,3,4,5,6- hexachlorohexane (γ-BHC) also revealed weak but definite cross-resistance to aplysiaterpenoid A and telfairine. The electrophysiological symptom that telfairine increases the frequency of spontaneous discharge and prolongs the synaptic after-discharge of the central nervous system of the German cockroach ( B. germanica) in the same manner as γ-BHC indicated that telfairine also has some excitatory effect on the central nervous system. The molecular topographical studies by semiempirical molecular orbital calculation about these insecticidal monoterpenes were carried out in terms of their electrostatic similarity to picrotoxinin and extending volumes from the surface of picrotoxinin molecule. The result suggested that the orientations of the steric bulky regions and electronegative centers in aplysiaterpenoid A and telfairine could satisfy the geometrical requirement for interacting with the picrotoxinin receptor as well as γ-BHC, while the large extending volumes of both compounds led to the loose fitting into the receptor. Accordingly, we propose that two insecticidal monoterpenes from marine alga seem to interact with the picrotoxinin receptor, indicating that their mode of action is cyclodiene-type.

“Central” and “peripheral” benzodiazepines and kinetics of lindane-induced toxicity

Because hypothermia and anorexia were previously found to be more sensitive indices of the effects of lindane than were convulsions, these endpoints were used to quantify the ability of benzodiazepines (BDs) and phenytoin either to ameliorate or exacerbate the toxicity of lindane in the rat. After administration of lindane (40 or 50 mg/kg) in oil per os, toxicity was counteracted by phenytoin and the “central” BD agonists diazepam and clonazepam, but was worsened by Ro 5-4864 a “peripheral” BD agonist. Clonazepam and diazepam were each more effective in counteracting lindane-induced anorexia than in stimulating food intake, presumably because the animals had been fasted and probably even controls ate maximally when food was presented. Diazepam alone (3 injections in 1 day) produced withdrawal-induced decreased food intake the following day. Clonazepam and diazepam alone each transiently decreased colonic temperature, yet effectively blocked the more severe hypothermia produced by lindane. Ro 5-4864 by itself did not produce any measurable effects, yet exacerbated all of the effects, including lethal effects, of lindane. The present findings are compatible with other evidence that lindane and Ro 5-4864 act at the picrotoxinin receptor of the GABA A-activated chloride channel and that systemic administration of agents acting at this site may produce a constellation of effects, including seizures, hypothermia and anorexia.

Picrotoxinin receptor ligand blocks anti-punishment effects of alcohol

Ethanol at low doses produces a release of punished responding in an operant rat conflict test similar to that observed for benzodiazepines and phenobarbital. It has been hypothesized that these anti-punishment effects are mediated via the GABA-benzodiazepine receptor-ionophore complex but not at the benzodiazepine binding site. In the present study isopropylbicyclophosphate (IPPO), which binds at the picrotoxinin site, reversed the release of punished responding produced by ethanol, pentobarbital and chlordiazepoxide; at low doses IPPO (<10 micro g/kg) appeared to be most effective against ethanol but at higher doses (>15 micro g/kg) was also effective against pentobarbital and chlordiazepoxide. At still higher doses IPPO produced a decrease in punished and unpunished responding. These results suggest that the “anxiolytic” actions of ethanol may involve a direct action on the GABA-benzodiazepine receptor-ionophore complex and this action may underlie some of the intoxicating effects of ethanol.

Toxicity of chlorinated bornane (toxaphene) residues isolated from Great Lakes lake trout (Salvelinus namaycush).

Lake trout samples from the Great Lakes contain residues of a complex pattern of chlorinated bornanes similar to the insecticide toxaphene. These residues, although structurally similar to toxaphene, are composed of a different profile of compounds than the analytical standard. Chlorinated bornane residues were isolated from tissues of Lake Michigan and Siskiwit Lake (Isle Royale) lake trout using a variety of purification techniques. Toxicity studies were conducted to determine the toxicological properties of the isolated residues in comparison to both technical material and the procedural standard. Static 24-hr acute bioassays with mosquito larvae(Aedes egypti) demonstrated that the residues were as toxic as the toxaphene standard. Additionally, experiments performed on the picrotoxinin receptor of the GABA (γ-aminobutyric acid)-chloride ionophore complex of the central nervous system revealed that the residues were very potent at the [35S]-t-butylbicyclophosphorothionate binding site. Therefore, both non-specific and specific measures of toxicologic potency show that environmentally derived residues of toxaphene retain significant biologic activity. There is no trend for decreasing toxicity of toxaphene residues for the period of 1982–85. Also, the residues found in samples from Siskiwit Lake were as toxic as those found in Lake Michigan.

Decreased GAB A, Benzodiazepine, and Picrotoxinin Receptor Binding in Brains of Rats After Cobalt-Induced Epilepsy

In previous studies of experimental and human epilepsy, defects have been shown in the gamma-aminobutyric acid (GABA) receptors. We further investigated the role of the GABA/benzodiazepine/picrotoxinin receptor complex in the epileptic focus and also in other regions of the rat brain. The focus was induced by cobalt implantation to the right motor cortex, and the brains were dissected 16-19 days after the operation. Benzodiazepine (using [3H]flunitrazepam as a ligand; FLU), GABA [3H]muscimol; MUS), and picrotoxinin [( 35S]t-butylbicyclophosphorothionate; TBPS) receptor bindings were measured in different brain areas and the values were compared with glass-implanted controls. In the focal area, the specific receptor binding decreased in the order TBPS greater than FLU greater than MUS. In the perifocal area only TBPS binding decreased, and Scatchard analysis showed a decrease in the number of binding sites (p less than 0.05) without any effect on binding affinity. No change was seen in the binding characteristics of the other areas studied. According to our results, in cobalt-induced epilepsy the GABA/benzodiazepine/picrotoxinin receptor complex is modulated in the focal area; this may lead to a defect in chloride conductance, which in turn induces disturbed control of neuronal activity in the epileptic focus.

Mode of Action of Insecticidal Compounds Acting at Inhibitory Synapse

All cyclodiene-resistant strains of the German cockroach, the house fly and the mosquito are also resistant to picrotoxinin, a well-known GABA antagonist. By using electrophysiological techniques, picrotoxinin was confirmed to act at a presynaptic region, resulting in a stimulation of the release of an excitatory transmitter. The electrophysiological symptoms caused by picrotoxinin were quite similar to those of γ-BHC and dieldrin. It was determined by using a 3H-dihydropicrotoxinin binding method that the nerve membrane in CNS of the American cockroach contained a component having a high affinity toward picrotoxinin and heptachlor epoxide, which can be regarded as a picrotoxinin receptor. By using a sucrose density centrifugation technique, it was determined that the fraction sedimented at the interphase of 1.0 and 1.2M sucrose at 100, 000×g contained the highest level of specific binding site. The 3H-dihydropicrotoxinin binding on the receptor was inhibited by all cyclodienes tested and γ-BHC. Toxaphene, bicyclic phosphate and TETS also bound with the receptor. Dieldrin resistant German cockroach showed cross-resistance to the chemicals having binding potencies to picrotoxinin receptor. Milbemycin and avermectin were found to stimulate Cl- uptake by the leg muscles of the American cockroach within 4min at 10-7M. This stimulatory action could be antagonized by picrotoxinin (10-4M). It was concluded that the action of milbemycin and avermectin is to open the chloride channel on the plasma membrane. Their actions do not seem to be mediated through GABA receptor, picrotoxinin receptor and benzodiazepam receptor, suggesting their direct attack on the chloride channel proper both in the central nervous and the neuromuscular systems.

Structural requirements for bridged bicyclic compounds acting on picrotoxinin receptor

Structural requirements for bridged bicyclic compounds acting on picrotoxinin receptor

Convulsant component of a depressant benzodiazepine

The convulsant influence of high doses of diazepam, in the presence of the benzodiazepine receptor antagonist Ro 15-1788, was studied in rats. Animals were implanted with permanent cortical screw electrodes for EEG recording. EEG spiking and accompanying clonic activity was observed in rats receiving ≥ 20 mg/kg diazepam, followed 10 minutes later by Ro 15-1788 (20 mg/kg). Pentylenetetrazole and picrotoxin seizure thresholds, measured during constant rate iv infusion, were significantly lowered by pretreatment with diazepam (250 mg/kg) and Ro 15-1788 (20 mg/kg) administered 30 and 20 minutes, respectively, before seizure threshold measurement. It is proposed that this convulsive activity of diazepam is mediated through the picrotoxinin receptor.

Action of avermectin B 1a on the leg muscles and the nervous system of the American cockroach

The action of avermectin was studied in the leg muscle and the central nervous system of the American cockroach, Periplanata americana L. Avermectin at a low concentration (10 −7 M) causes a failure of the leg muscles to respond to external stimuli within 30 min without affecting the magnitude of contraction. Avermectin was found to stimulate Cl − uptake by the leg muscles within 4 min at 10 −7 M. The threshold concentration to cause such stimulation was on the order of 10 −8 M. This stimulatory action could be antagonized by picrotoxinin (10 −4 M) and to a lesser extent by bicuculline methiodide (10 −4 M). The phenomenon is observable under both Na +-free and K +-free conditions. It was concluded that the action of avermectin is to open the chloride channel on the plasma membrane. This action of avermectin does not seem to be mediated through GABA, GABA receptors, diazepine receptors, or picrotoxinin receptor in this insect species, and therefore suggests that avermectin directly attacks the chloride channel proper both in the central nervous and the neuromuscular systems.

Picrotoxinin receptor in the central nervous system of the American cockroach: Its role in the action of cyclodiene-type insecticides

The nature of the picrotoxinin receptor was studied using the central nervous system (CNS) of the American cockroach. It first was confirmed by using an electrophysiological technique that the abdominal nerve cord of the American cockroach was sensitive to picrotoxinin. By using a [ 3H]α-dihydropicrotoxinin binding test it was determined that the picrotoxinin receptor in CNS of this insect had a higher affinity toward picrotoxinin and heptachlor epoxide than the corresponding receptor in the rat brain. Also, the cockroach brain preparation had a higher percentage of specific binding in the total binding, making this material suitable for receptor studies. By using a sucrose density centrifugation technique, it was determined that the fraction sedimented at the interphase of 1.0 to 1.2 M sucrose at 100,000 g contained the highest level of specific binding site. The receptor showed a sensitivity to all insecticidal cyclodienes tested, namely photodieldrin, oxychlordane, endrin, heptachlor epoxide, γ-chlordane, dieldrin, aldrin, heptachlor, and isodrin (expressed in the order of potency). Among four BHC isomers, the γ-isomer showed the highest potency to bind with this receptor.

Difference in the picrotoxinin receptor between the cyclodiene-resistant and susceptible strains of the german cockroach

As a result of toxicity tests, it was established that all cyclodiene-resistant strains of the German cockroach are also resistant to picrotoxinin, a plant-origin neurotoxicant. Two of the cockroach strains which exhibit a distinct cross-resistance pattern to picrotoxinin (i.e., LPP and FRP) are the ones that have been purified genetically by backcrossing against the susceptible (CSMA) strain. This cross-resistance pattern appears to be specific to picrotoxinin and does not extend to other neuroexcitants such as bicuculline, beta-bungarotoxin, and DDT. The nervous system of the resistant cockroach was found to be less sensitive to picrotoxinin. Furthermore, it was determined that nerve components from the resistant cockroaches have significantly lower binding capacity to [ 3H]α-dihydropicrotoxinin. The most likely explanation for the above phenomenon is that these cockroaches have developed the cyclodiene resistance by altering the nerve receptor for picrotoxinin.