Hypotensive action of 5-HT receptor agonists in the vitamin B6-deficient hypertensive rat

Dietary excess of vitamin B-6 affects the concentrations of amino acids in the caudate nucleus and serum and the binding properties of serotonin receptors in the brain cortex of rats

Vitamin B-6 is a cofactor in many reactions of nitrogen metabolism. Deficiency alters tissue amino acid concentrations but effects of excess vitamin B-6 have not been well described. We fed female rats (218 g, 7 per group) 1 (control), 10, 100, 175 or 250x) the National Research Council recommended level of pyridoxine HCl (7 mg/kg) for 10 wk and measured serum amino acids, amino acids and neurotransmitters in brain regions and the binding properties of serotonin receptors in the cerebral cortex using a ketanserin binding assay. Rats were decapitated, and unheparinized blood was obtained. In the caudate nucleus, concentrations of glutamate, threonine, taurine, methionine, gamma-amino-butyric acid and the sum of the essential amino acids in groups 10X and 100X were approximately 130 to 180% of control levels (P < 0.05); groups 1X, 175X and 250X were not different. A similar pattern was seen in the serum for serine, glycine, aspartate and ornithine; the latter two amino acids increased to over 200% of control in group 100X. In the ketanserin binding assay, both the antagonist binding affinity and the maximal number of binding sites were higher for group 100X than for 1X, 175X and 250X, and were higher for 10X than for 1X. Norepinephrine in the raphe nucleus followed a similar biphasic pattern. Excess dietary pyridoxine affected brain and serum concentrations of some amino acids and binding properties of cortical serotonin receptors in a biphasic pattern over the range of concentrations fed in this study.

Effects of serotonergic agents on plasma prolactin levels in pyridoxine-deficient adult male rats

Plasma concentration of prolactin was significantly reduced in pyridoxine-deficient as compared to control (pyridoxine-supplemented) adult male rats. Administration of pyridoxine to deficient rats resulted in a significant increase in plasma prolactin. The reduction in plasma prolactin in pyridoxine-deficient rats corresponded with the significantly reduced hypothalamic contents of pyridoxal phosphate and serotonin in pyridoxine-deficient rats. Plasma prolactin concentrations were also measured in response to serotonergic agents in both groups of rats. The administration of the 5HT1A agonist (8-hydroxy 2-n-dipropylamino tetralin) resulted in a significant increase in plasma prolactin and that of the specific 5HT1A antagonist spiroxatrine had the opposite effect. The results suggest that the hypothalamic serotonergic regulation of prolactin release is impaired in pyridoxine deficiency.