EFFECTS OF TAURINE, HOMOTAURINE AND GABA ON HYPOTHALAMIC AND STRIATAL DOPAMINE METABOLISM

To elucidate the effects of taurine on hypothalamic and striatal dopaminergic neurotransmission we compared its effects to those of gamma-aminobutyric acid (GABA) and homotaurine (a GABA(A)-receptor agonist) on hypothalamic and striatal concentrations of dopamine (DA) and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and, in the case of striatum, 3-methoxytyramine (3-MT) in rats. In addition, hypothalamic and striatal 5-hydroxytryptamine (5-HT) und 5-hydroxyindoleacetic acid, hypothalamic noradrenaline (NA) and 3-methoxy-4-hydroxyphenylglycol sulfate, and pituitary DA concentrations were also measured. The amino acids were injected into the lateral brain ventricles of conscious male rats in doses of 10 and 36-mu-mol/rat, and rat were sacrificed 15 and 60 min later, respectively. Homotaurine (by 11 %) but not the other two amino acids elevated striatal DA, whereas hypothalamic DA was increased by both taurine (36 %) and homotaurine (31 %). All three amino acids at 36-mu-mol elevated striatal DOPAC, homotaurine (51 %) more than taurine (31 %) or GABA (30 %), and hypothalamic DOPAC, both taurine (102 %) and homotaurine (82 %) clearly more than GABA (34 %). Neither striatal nor hypothalamic HVA was altered by any of the amino acids. At 10-mu-mol the amino acids decreased striatal 3-MT by about 40 %. At 36-mu-mol taurine and homotaurine reduced 3-MT by about 70 %, whereas increasing the dose of GABA did not further reduce 3-MT. Both taurine and homotaurine at 36-mu-mol decreased hypothalamic NA content. Neither hypothalamic nor striatal 5-HT metabolism was altered. In the neurointermediate lobe of the pituitary gland taurine at 10-mu-mol but not at 36-mu-mol slightly (20 %) increased DA. These results show that taurine and homotaurine clearly alter the hypothalamic DA metabolism, and give further support to the suggestion that taurine and homotaurine similarly to GABA reduce the release of DA in the striatum. Homotaurine seems to alter the striatal DA metabolism more effectively than taurine or GABA. On the other hand, hypothalamic DA seems to be equally sensitive to taurine and homotaurine, and is clearly less sensitive to GABA. The pronounced elevation of hypothalamic DA and DOPAC after taurine and homotaurine could be related to changes in hypothalamic NA.