Changes in synaptosomal pH and rates of oxygen radical formation induced by chlordecone.

The resting pH of 7.14 +/- 0.02 within rat cortical synaptosomes is elevated in vitro by the insecticide chlordecone, in a dose-dependent manner. Chlordecone also reduces the rate of oxygen radical formation within synaptosomes. Both of these changes can also be demonstrated following in vivo treatment of rats with chlordecone (75 mg/kg body wt). Although chlordecone increases the permeability of the plasma membrane, the increase in pH observed is unlikely to be caused by this, since in vivo administration of chlordecone does not appreciably alter membrane order as evaluated by both a lipophilic probe, and a probe with an ionic segment. Another xenobiotic agent, methyl mercuric chloride, and a free radical generating system, an ascorbic acid-ferrous sulfate mixture, did not modulate synaptosomal pH, although membrane permeability was increased. Other evidence of the ability of synaptosomes to maintain homeostasis was the failure of mitochondrial inhibitors to significantly reduce pH. The drop in synaptosomal pH effected by amiloride, an inhibitor of Na+/H+ exchange, and the transient rise in pH caused by ammonium chloride further suggested that synaptosomes may be a good model in the study of the regulation of intracellular pH. The elevation of cytosolic pH, and depression of oxygen radical formation by chlordecone, may result from both the attenuation of respiratory metabolism and an impaired capacity of the plasma membrane to maintain ionic gradients.

Toluene-induced alterations in rat synaptosomal membrane composition and function.

Toluene is a widely used organic solvent that can produce acute central nervous system (CNS) effects. Since toluene reaches relatively high concentrations in the CNS and is extremely lipophilic, we investigated its effects on rat brain membrane composition and function. Toluene (1 g/kg, lh) did not alter total brain microsomal phospholipid (PL) or cholesterol (CL) content. However, synaptosomal PL was decreased (24%), while synaptosomal CL was unaltered. The PL/CL ratio, an indirect index of membrane fluidity, did not change, suggesting that toluene did not affect membrane fluidity. Fluorescence polarization studies employing 1,6-diphenyl-1,3,5-hexatriene (DPH) showed that toluene did not alter synaptosomal membrane fluidity after administration in vivo (1 g/kg) or in vitro (0.5 to 5.0 mM). Dose-response and time-course studies showed that toluene maximally decreased synaptosomal PL after 1 g/kg, 1 h. The dose-response and time-course studies also showed that the toluene-induced decreases in PL were a result of specific decreases in phosphatidylethanolamine (PE). Since PE was decreased, we assessed whether toluene altered synaptosomal membrane function by investigating phospholipid methylation, a reaction which uses PE as its initial substrate. Toluene decreased the incorporation of methyl groups into lipid when [3H]-methionine was used as the methyl donor, but did not affect methylation when [3H]-adenosylmethionine was the methyl donor. These data suggest that toluene-induced specific decreases in synaptosomal PE and inhibition of phospholipid methylation may alter normal synaptic function and play a critical role in the mechanism(s) of action of toluene's CNS effects.