endrin and tert-butylbicyclophosphorothionate

The intrinsic lethality of a series of chloride-channel-directed convulsants and insecticides was determined by intracerebral injection into mice. The toxicities of the cyclodiene insecticides and picrotoxinin were potentiated by intracerebral injection, when compared to their reported intraperitoneal LD50s. In contrast, the toxicities of lindane, abamectin, and the bicyclic convulsants TBPS and a TBOB analog were approximately the same by either route of administration. These results suggest that the brain is the primary target site for the cyclodienes and picrotoxinin, while the peripheral nervous system may be relatively more important in the toxic action of lindane, abamectin, and bicyclic convulsants such as TBPS. With the exception of picrotoxinin these compounds showed, overall, a good correlation between acute intracerebral toxicity and potency for inhibiting GABA-dependent chloride uptake. The evidence that multiple chloride-channel subtypes serve as targets for these compounds and the potential impact this had on the results of these studies are discussed.

Topics: Animals; Bridged Bicyclo Compounds; Bridged Bicyclo Compounds, Heterocyclic; Chloride Channels; Convulsants; Dose-Response Relationship, Drug; Endrin; Hexachlorocyclohexane; Injections, Intraperitoneal; Injections, Intravenous; Insecticides; Ivermectin; Lethal Dose 50; Membrane Proteins; Mice; Mice, Inbred Strains

1. In assays of 36Cl- uptake into mouse brain vesicles, 100 microM GABA markedly increased both the initial rate of 36Cl- uptake and the total amount of chloride taken up over a 120-sec incubation period. Specific GABA-dependent 36Cl- uptake (the difference between total and background uptake) was essentially complete within 15 sec of incubation. 2. Incubation with GABA following preincubation with 10 microM endrin, a polychlorocycloalkane insecticide and established blocker of GABA-gated chloride channels, showed a stimulation of uptake over background levels that was much slower in onset than that observed with GABA alone but nevertheless achieved virtually the same level of stimulation above background levels after 90 sec of incubation with GABA. 3. In electrophysiological assays of GABA receptors expressed in Xenopus oocytes following injection with rat brain mRNA, endrin (20 microM) effectively blocked the transient currents elicited by brief exposure of oocytes to GABA (200 microM). However, prolonged exposure to GABA in the absence of perfusion produced a large, slowly-developing inward current. 4. The actions of several known GABA antagonists were also compared as inhibitors of GABA-dependent 36Cl- uptake into mouse brain vesicles at short (4 sec) and long (120 sec) incubation times using concentrations of inhibitors known to produce approximately 70-90% inhibition of GABA-dependent chloride uptake in 4-sec incubations. Picrotoxinin and TBPS, like endrin, were completely ineffective as inhibitors in 120-sec incubations. In contrast, bicuculline was almost as effective at 120 sec as at 4 sec, and avermectin Bla produced approximately 50% inhibition of the GABA response after 120 sec.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Animals; Brain; Bridged Bicyclo Compounds; Bridged Bicyclo Compounds, Heterocyclic; Chloride Channels; Chlorides; Convulsants; Endrin; GABA Antagonists; gamma-Aminobutyric Acid; Ion Channel Gating; Kinetics; Male; Membrane Proteins; Mice; Mice, Inbred ICR; Picrotoxin; Rats; Rats, Inbred Strains; Receptors, GABA-A; Sesterterpenes; Xenopus

Six polychlorinated convulsant insecticides that potently inhibit t-[35S]butylbicyclophosphorothionate ([35S]TBPS) binding to rat brain membranes also potentiate the protective effect of NaCl (200 mM) against heat inactivation of [3H]flunitrazepam binding sites on the same membranes. Similar effects were obtained with all "cage" convulsants tested. The rank order of potencies as heat protection potentiators was similar to the rank order of potencies as inhibitors of [35S]TBPS binding (alpha-endosulfan greater than endrin greater than dieldrin greater than toxaphene greater than lindane). alpha-Endosulfan and endrin are more potent in both respects than any previously reported picrotoxin-like (cage) convulsant, but are much less toxic to mammals. The greatly reduced toxicities of alpha-endosulfan and endrin in mammals may reflect partial gamma-aminobutyric acid (GABA)-neutral properties of these compounds. Time courses of heat inactivation of [3H]flunitrazepam binding sites in the presence of 200 mM NaCl plus saturating concentrations of endrin or picrotoxin revealed monophasic components constituting about 88% of the binding sites, suggesting that virtually all [3H]flunitrazepam binding sites are coupled to picrotoxin binding sites in the GABA/benzodiazepine/picrotoxin receptor complex. Protection against heat inactivation constitutes a useful tool for characterizing the various allosterically linked binding sites in neurotransmitter receptor complexes.

Topics: Animals; Brain; Bridged Bicyclo Compounds; Bridged Bicyclo Compounds, Heterocyclic; Cell Membrane; Child, Preschool; Dieldrin; Drug Synergism; Endosulfan; Endrin; Flunitrazepam; Hexachlorocyclohexane; Hot Temperature; Humans; Insecticides; Rats; Receptors, GABA-A; Sodium Chloride; Toxaphene

Certain populations of mosquitofish (Gambusia affinis) are highly resistant to endrin (42-fold), but are not cross-resistant to cypermethrin. These populations show relatively low levels of resistance (2-fold) to 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) and t-butylbicyclophosphorothionate (TBPS). Studies comparing specific [35S]TBPS binding to brain membrane preparations from resistant and susceptible fish indicate a reduced binding capacity for TBPS in membranes from resistant fish. Endrin was about twice as potent in competitively inhibiting [35S] TBPS binding in vitro to membranes from susceptible fish as compared to membranes from resistant fish, suggesting that TBPS binding sites in resistant fish brain are less sensitive to endrin. Endrin was 37 times as potent in competitively inhibiting [35S]TBPS binding in vivo to membranes from susceptible fish compared to membranes from resistant fish, indicating that a contributory factor is involved in endrin resistance. Data for TBPS inhibition of [35S]TBPS binding revealed an approximate 2-fold difference between resistant and susceptible for both in vitro and in vivo IC50, indicating that an altered TBPS binding site is the only factor involved in TBPS resistance. DDT did not inhibit [35S] TBPS binding, suggesting that DDT resistance is not due to changes at the TBPS receptor. These data support the hypothesis that two factors operate in vertebrate resistance to endrin, whereas only one of these factors confers TBPS resistance. These resistance mechanisms are not functional against cypermethrin.

Topics: Animals; Brain; Bridged Bicyclo Compounds; Bridged Bicyclo Compounds, Heterocyclic; Bridged-Ring Compounds; Cyprinodontiformes; DDT; Drug Resistance; Endrin; In Vitro Techniques; Pyrethrins; Receptors, GABA-A

Ontogenesis of the convulsant (picrotoxinin or t-butylbicyclophosphorothionate (TBPS] site(s) was defined in chicken embryonic brain with the radioligands [35S]TBPS and [3H]t-butylbicycloorthobenzoate [( 3H]TBOB). Binding of the radioligands is detectable from day 6 of incubation. The increase in binding after day 14 of incubation is due to an increase in the number of binding sites. The pharmacological properties of the embryonic recognition site(s) do not undergo significant changes during hatching based on the affinity of embryonic and chick brain membranes for [3H]TBOB; the rate of association and dissociation for TBOB; non-competitive inhibition of [3H]TBOB binding and modulation of 1R, alpha S-cis-cypermethrin interaction with the recognition site(s) by GABA; and inhibition of radioligand binding by endrin, picrotoxinin and TBPS. Early development and in vitro susceptibility of the recognition site to convulsive toxicants makes the embryo a possible target for a variety of drugs and environmental toxicants acting at this site.

Topics: Aging; Animals; Brain Chemistry; Bridged Bicyclo Compounds; Bridged Bicyclo Compounds, Heterocyclic; Chick Embryo; Convulsants; Endrin; Kinetics; Membranes; Picrotoxin; Receptors, GABA-A; Sesterterpenes

Three major classes of chlorinated hydrocarbon insecticides, i.e., the lindane/hexachlorocyclohexane, toxaphene and aldrin/dieldrin types, are potent, competitive, and stereospecific inhibitors of t-butylbicyclophosphorothionate (TBPS) binding to brain-specific sites, thereby indicating an action at the gamma-aminobutyric acid (GABA)-regulated chloride channel. The most inhibitory and toxic of four isomers of hexachlorocyclohexane is lindane and of greater than 188 components of toxaphene is 2,2,5-endo, 6-exo,8,9,9,10-octachlorobornane. 12-Ketoendrin (IC50 = 36 nM) is twice as active as the most potent previously known inhibitor of TBPS binding and it is also the most inhibitory and toxic of 22 cyclodienes examined. Within each of these three series of polychlorocycloalkanes the mammalian toxicity is closely related to the potency for inhibition of TBPS binding. A modified receptor assay incorporating liver microsomes and reduced nicotinamide-adenine dinucleotide phosphate compensates in part for oxidative detoxification and bioactivation. Specific TBPS binding is reduced in a dose-dependent manner in dieldrin-poisoned rats. DDT, mirex and kepone are not inhibitors of TBPS binding, even at 10 microM.

Topics: Animals; Binding, Competitive; Brain; Bridged Bicyclo Compounds; Bridged Bicyclo Compounds, Heterocyclic; Cycloparaffins; Endrin; Hexachlorocyclohexane; Insecticides; Isomerism; Kinetics; Microsomes; NADP; Picrotoxin; Rats; Receptors, Cell Surface; Receptors, GABA-A; Sesterterpenes; Synaptic Membranes; Toxaphene