Disturbances of amino acids from temporal lobe synaptosomes in human complex partial epilepsy.

We have studied the levels of neuroactive amino acids in synaptosomes (P2 fraction) isolated from brain tissue of ten patients with medically intractable epilepsy who were undergoing temporal lobectomy. First, lateral temporal tissue (nonfocal) was removed followed by medial temporal tissue (focal). A synaptosomal fraction (P2) was immediately prepared from each tissue and analyzed for free amino acid concentrations. Statistically significant reductions were seen in glutamine and GABA concentrations in focal tissue compared to nonfocal tissue. The ratio of excitatory amino acids (aspartate and glutamate) to inhibitory amino acids (taurine and GABA) was significantly higher in focal tissue compared to nonfocal. The glutamine/glutamate ratio was significantly reduced. These data support the hypothesis that alterations in the balance between excitatory and inhibitory amino acids may be involved in the expression of epilepsy.

The relationship between reactivity of metabolites of pyrrolizidine alkaloids and extrahepatic toxicity.

Pyrrolizidine alkaloids (PAs) are a large group of structurally similar toxins. In animals, including man, they are hepatotoxic and in some cases pneumo- and neurotoxic. PAs are metabolized by the liver P450 system to reactive dehydroalkaloid (DHA) intermediates. PA toxicity is a result of alkylation of macromolecules by DHAs. We have measured the relative reactivity of a series of semi-synthetic DHAs by recording the rate at which they alkylate a model nucleophile, 4-(p-nitrobenzyl)pyridine. Rate data fit mono- or biexponential equations. Rank order of reactivity for the macrocyclic and open ester DHAs was the same as those measured for DHA hydrolysis. The reaction with 4-(p-nitrobenzyl)pyridine was easier to follow, however, as rates of reaction can easily be controlled by temperature or level of acid catalysis, and the final product can be measured colorimetrically. DHAs of the primarily hepatotoxic alkaloids, retrorsine and seneciphylline, were more reactive than DHAs of monocrotaline and trichodesmine, which additionally produce pneumo- and neurotoxicity, respectively. This suggests that DHAs with greater stability (longer half-lives) are able to survive long enough to reach target organs downstream form the liver. We believe that differences in PA metabolism and the nature of toxicity ultimately produced are in part related to differences in reactivity of the primary toxic intermediate, the DHA.

Chronic administration of taurine to aged rats improves the electrical and contractile properties of skeletal muscle fibers.

A reduction of resting chloride conductance (GCl) and a decrease of the voltage threshold for contraction are observed during aging in rat skeletal muscle. The above alterations are also observed in muscle of adult rat after taurine depletion. As lower levels of taurine were found by others in aged rats compared to young rats, we tested the hypothesis that a depletion of taurine may contribute to the alteration of the electrical and contractile properties we found in skeletal muscle during aging. This was accomplished by evaluating the potential benefit of a pharmacological treatment with the amino acid. To this aim 25-mo-old Wistar rats were chronically treated (2-3 mo) with taurine (1 g/kg p.o. daily) and the effects of such a treatment were evaluated in vitro on the passive and active membrane electrical properties of extensor digitorum longus muscle fibers by means of current-clamp intracellular microelectrode technique. Excitation-contraction coupling was also evaluated by measuring the voltage threshold for contraction with the intracellular microelectrode "point" voltage clamp method. In parallel muscle and blood taurine contents were determined by high-performance liquid chromatography. Taurine supplementation significantly raised taurine content in muscle toward that found in adult rats. Supplementation also significantly increased GCl vs. the adult value, in parallel the excitability characteristics (threshold current and latency) related to this parameter were ameliorated. The increase of GCl induced by taurine was accompanied by a restoration of the pharmacological sensitivity to the R(+) enantiomer of 2-(p-chlorophenoxy) propionic acid, a specific chloride channel ligand. In parallel also the protein kinase C-mediated modulation of the channel was restored; in fact the potency of 4-beta-phorbol 12, 13-dibutyrate in reducing GCl was lower in taurine-treated muscles vs. untreated aged, being rather similar to that observed in adult. The treatment also improved the mechanical threshold for contraction of striated fibers which in aged rats is shifted toward more negative potentials, moving it toward the adult values. Our results suggest that the reduction of taurine content could play a role in the alteration of electrical and contractile properties observed during aging. These findings may indicate a potential application of taurine in ensuring normal muscle function in the elderly.