Inhibitory actions of taurocyamine, hypotaurine, homotaurine, taurine and GABA on spike discharges of Purkinje cells, and localization of sensitive sites, in guinea pig cerebellar slices.

The effects of the possible metabolites of taurine--taurocyamine and hypotaurine--and of homotaurine on spike discharge frequency of guinea pig cerebellar Purkinje cells were investigated in comparison with taurine and GABA. Taurocyamine, hypotaurine and homotaurine all dose-dependently suppressed the spike frequency with potencies relative to taurine (ED50 = 1.0 mM) of 6, 0.5 and 500, respectively. Both picrotoxin and strychnine competitively blocked the inhibition by all the substances tested, except for the non-competitive antagonism by picrotoxin on taurocyamine. The sites sensitive to taurocyamine and hypotaurine were situated primarily in the dendritic region of the cerebellar molecular layer, while those to homotaurine were primarily near the Purkinje cell soma. The onset of the inhibitory action of taurine and related sulfur-containing substances was found to be slower than that of GABA.