The epileptogenic action of 6-aminomethyl-3-methyl,1-4H-1,2,6-benzothiadiazine-1,1-diazide hydrochloride (TAG): non-specific versus specific antitaurine pathogenesis

Further evidence in support of taurine as a mediator of synaptic transmission in the frog spinal cord

It has been reported that 6-aminomethyl-3-methyl-4H,1,2,4-benzothiadiazine-1, 1-dioxide (AMBD, TAG) is a specific blocker of taurine and beta-alanine responses in the central nervous system. We have re-examined the effect of AMBD on amino acid and synaptically evoked responses recorded from isolated hemisected frog spinal cords by means of the sucrose gap technique. When indirect responses were blocked by adding tetrodotoxin (0.2 microM) or manganese chloride (2 mM) to the normal Ringer solution, AMBD (0.01-0.5 mM) selectively antagonized taurine, beta-alanine, hypotaurine and kojic amine evoked depolarizations of primary afferents at their intramedullary part (dorsal root terminals, DRT) and on dorsal root ganglia (DRG), without significantly affecting responses to glutamate (on DRT), glycine (on DRT) or GABA (on DRT and DRG). Depolarizing responses to taurine and beta-alanine (1 mM) were depressed by up to 50% with 0.1 mM AMBD and often completely antagonized with 0.25 mM AMBD. In normal Ringer solution, AMBD selectively antagonized the dorsal root potential evoked by ventral root stimulation (VR-DRP, threshold at 0.02 mM AMBD, 90% block with 0.25 mM); other synaptic potentials increased in duration and/or amplitude, demonstrating a strong convulsant effect of AMBD. Thus, the depolarizing responses of taurine, beta-alanine and hypotaurine on primary afferents are pharmacologically indistinguishable from the VR-DRP. These results are in agreement with the proposal that taurine or a taurine-like substance (possibly beta-alanine or hypotaurine) is the mediator of VR-DRP in amphibian spinal cord.

Pentylenetetrazol seizure threshold and extracellular levels of cortical amino acids in taurine-deficient kittens

Directly after weaning, kittens were raised on a semisynthetic diet supplemented with 0.4% taurine or devoid of this amino acid. Eight to twelve weeks later the blood plasma concentration of taurine was decreased by 98-99% in kittens fed a taurine-free regimen. Parietal cortex dialysis, performed in anaesthetized kittens, revealed a selective, but less marked, reduction of extracellular taurine. When kainic acid was included in the dialysis buffer, taurine was doubled in taurine-supplemented kittens, but it was only slightly affected in deficient animals. The animals were also used for determination of the threshold of pentylenetetrazol-induced epilepsy 3 days after the dialysis experiment. This was not significantly different between the groups. The present work shows that taurine deficiency in its own right does not elevate interstitial glutamate, an effect previously observed in the 2-guanidino-ethane sulphonic acid model for taurine deficiency. The results further suggest that taurine is better retained in neural cells in the taurine-deficient state. Moreover, the findings argue against a role for endogenous taurine in the control of epileptiform discharge initiation and/or spread.