Syed M. Ghiasuddin

Evidence for similarities between cyclodiene type insecticides and picrotoxinin in their action mechanisms

Evidence has been obtained to indicate that cyclodiene-type insecticides, e.g., heptachlor epoxide and gamma-BHC, mimic the action of picrotoxinin. These insecticides inhibit the GABA (gamma-aminobutyric acid)-stimulated chloride uptake in the coxal muscle of the American cockroach, and directly compete against [3H]a-dihydropicrotoxinin for binding in the rat brain synaptosomes. Moreover, several cyclodiene-resistant insect strains are also resistant to picrotoxinin. This cross-resistance is specific to picrotoxinin and does not extend to other neuroexcitants. These insecticides, like picrotoxinin, cause central nerve excitation by stimulating transmitter release. Similarity in molecular structures also has been pointed out. These results indicate that some of the nerve excitation symptoms that insecticides cause are likely due to their interaction with picrotoxinin receptor.

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

Abstract 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 [ 3 H]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.

Inhibition of gamma-aminobutyric acid (GABA)-induced chloride uptake by gamma-BHC and heptachlor epoxide

Abstract 1. 1. A bioassay system was established using the coaxal muscle of the American cockroach to measure the stimulatory effect of γ-aminobutyric acid (GABA) on 36 Cl − permeability and the antagonistic effect of picrotoxinin. In this system half maximum stimulation by GABA is achieved at 80 μM. 2. 2. This stimulatory action by GABA was found to be antagonized by two cyclodiene-type insecticides, heptachlor epoxide and γ-BHC. Other insecticidal analogs, which do not posses cyclodiene-type neuro-excitant property, failed to show this phenomenon. 3. 3. These results clearly demonstrate that heptachlor and γ-BHC have picrotoxinin-type action.

Pyrethroid insecticides: potent, stereospecific enhancers of mouse brain sodium channel activation

Abstract Deltamethrin and NRDC 157, pyrethroid insecticides that produce different poisoning syndromes in mammals, enhanced veratridine-dependent, sodium channel-mediated 22Na+ uptake in mouse brain synaptosomes. Concentrations producing half-maximal enhancement were 2.5 × 10−8 M (deltamethrin) and 2.2 × 10−7 M (NRDC 157). This effect was stereospecific: The nontoxic 1S enantiomers had no significant effect on veratridine-dependent activation. At high deltamethrin concentrations, enhancement was maximal at 5 × 10−5−1 × 10−4 M veratridine. Pyrethroid enhancement was completely blocked by 5 × 10−6 M tetrodotoxin, and neither pyrethroid affected 22Na+ uptake in the absence of veratridine at concentrations up to 1 × 10−5 M. The relative potencies of deltamethrin and NRDC 157 in the synaptosomal sodium channel assay agree well with their relative acute toxicities to mice when administered by intracerebral injection. These findings demonstrate that pyrethroids exemplifying both characteristic poisoning syndromes are potent, stereospecific modifiers of sodium channel function in mammalian brain.

Metabolism of 2,5,2′-trichloro-, 2,5,2′,5′-tetrachloro-, and 2,4,5,2′,5′-pentachlorobiphenyl in rat hepatic microsomal systems☆☆☆

The polychlorinated biphenyls (PCBs) are important environmental contaminants. The relationship between degree of chlorination of these compounds and metabolism in rat liver microsomes was studied. [14C]2,5,2′-trichloro-, [14C]2,5,2′,5′-tetrachloro-, and [14C]2,4,5,2′,5′-pentachlorobiphenyl were incubated with a NADPH-generating system in microsomal systems prepared from phenobarbital-pretreated rats. Metabolites were isolated, purified by thin-layer chromatography and identified by mass spectroscopy. It was found that the number and quantitative significance of metabolites decreased with increasing degree of chlorination of PCBs. The approximate percentages of the applied doses recovered as metabolites after 4-hr incubation were 62% for trichlorobiphenyl, 36% for tetrachlorobiphenyl, and 7% for pentachlorobiphenyl. The major metabolite of each PCB was identified as a monohydroxylated product.

Receptor-like stereospecific binding of a pyrethroid insecticide to mouse brain membranes

A heterogeneous particulate fraction of mouse brain homogenates binds NRDC 157 (3-phenoxybenzyl [1R, cis]-3-(2,2-dibromovinyl)-2,2-dimethylcyclopropanecarboxylate), a potent pyrethroid insecticide, stereospecifically and with high affinity. Stereospecific binding is a minor component of total binding (2.8%); the remainder of observed binding is predominantly nonspecific and unsaturable. Stereospecific binding is half-saturated at 4 X 10(-8)M and fully saturated at concentrations in excess of 1 X 10(-7)M. The stereospecific binding capacity of this preparation was 200-250 pmoles of NRDC 157 per gram equivalent of brain tissue (2.3-2.8 pmol/mg protein). This binding site may represent the neural receptor involved in the stereospecific toxic action of pyrethroids.

Hydrolysis of pyrethroid insecticides by soluble mouse brain esterases

trans-Permethrin, a pyrethroid insecticide, was hydrolyzed by one or more carboxylesterases located in the soluble fraction of mouse brain homogenates. The apparent affinity of this activity for trans-permethrin was greater than that reported for mouse hepatic carboxylesterase activity, but the apparent maximum velocity was considerably lower than that of the hepatic activity. Soluble brain esterases also hydrolyzed several other pyrethroid esters with a substrate specificity different from that of the hepatic esterases. In particular, alpha-cyano-3-phenoxybenzyl esters of noncyclopropane acids (e.g., fenvalerate and fluvalinate) were hydrolyzed by brain esterases at rates equal to or greater than that measured for trans-permethrin. These results suggest that hydrolysis in the brain may contribute to the detoxication of some pyrethroids in mammals.

Mouse brain synaptosomal sodium channels: Activation by aconitine, batrachotoxin, and veratridine, and inhibition by tetrodotoxin

Batrachotoxin, veratridine and aconitine, activators of the voltage-dependent sodium channel in excitable cell membranes, increase the rate of 22Na+ uptake by mouse brain synaptosomes. Batrachotoxin was both the most potent (K0.5, 0.49 microM) and most effective activator of specific 22Na+ uptake. Veratridine (K0.5, 34.5 microM) and aconitine (K0.5, 19.6 microM) produced maximal stimulations of 22Na+ uptake that were 73% and 46%, respectively, of that produced by batrachotoxin. Activation of 22Na+ uptake by veratridine was completely inhibited by tetrodotoxin (I50, 6 nM ), a specific blocker of nerve membrane sodium channels. These results identify appropriate conditions for measuring sodium channel-dependent 22Na+ flux in mouse brain synaptosomes. The pharmacological properties of mouse brain synaptosomal sodium channels described here are distinct from those previously described for sodium channels in rat brain synaptosomes and mouse neuroblastoma cells.

Characteristics of DDT-Sensitive Ca-ATPase in the Axonic Membrane

A highly DDT-sensitive Ca-ATPase was found in an axonic nerve preparation from the lobster legs. As a result of several attempts to isolate the enzyme apart from other ATPases, an assay condition was devised. This method gives rise to the ATPases activity which is totally sensitive to DDT. By using such an approach the characteristics of the Ca-ATPase was studied. The 150 of the enzyme toward DDT was found to be in the neighborhood of 10−9 M. It has a rather high Ca++ optimum (0.3 mM) suggesting that it is likely an ecto Ca-ATPase located at the surface of the axons. This was confirmed by using an axonic suspension preparation which gave essentially identical results on Ca-ATPase activities and its characteristics. The inhibition of the Ca-ATPase appears to result in the suppression of Ca++ uptake process at the surface of the membrane as judged by a 45Ca++ uptake experiment by using both the membrane and the axonic suspension preparations in the presence of ATP. These results help to explain why DDT poisoned nerve membranes are prone to destabilization and subsequent excitation.

Reduced sensitivity of a Ca-ATPase in the DDT-resistant strains of the German cockroach

Abstract 1. DDT-resistant cockroaches were found to have a relatively DDT-insensitive Ca-ATPase as compared to the susceptible one. 2. This Ca-ATPase present in the 90,000 g membrane fraction from the brains of German cockroaches showed identical enzyme characteristics as the ecto Ca-ATPase of the lobster nerve which has been previously shown to be highly sensitive to DDT. 3. The resistant strain also shows decreased sensitivities in vivo toward various inhibitors of this ATPase.