The present studies were carried out to determine whether pyridoxine (B6) inhibits PRL release in the rat and to define its site of action. Adult female rats were injected with 50 mg B6 of diluent (C) ip at 1200 h on proestrus. B6 prevented the increase in PRL secretion normally observed at 1300 h and this inhibition was prolonged until at least 1500 h by a second injection of B6 at 1400 h. All B6-treated and control animals had normal proestrous LH surges and ovulated the next day. To determine whether the inhibitory effect of B6 on PRL release was due to enhanced dopamine synthesis or to a direct effect on the pituitary, ovariectomized rats were treated with a potent inhibitor of dopamine synthesis, α-methyl-p-tyrosine (α-MPT). The administration of B6 either with or 4 h after α-MPT significantly decreased the PRL rise observed in animals treated with α-MPT alone. B6 also significantly suppressed the TRH-induced PRL rise in both α-MPTtreated and normal rats, but had no effect on the TRHinduced rise in plasma TSH concentration. These studies demonstrate that pharmacological doses of Be reduce the proestrous PRL surge and that this reduction may be at least partially due to a direct effect on the pituitary, inasmuch as B6 decreased the PRL rise after α-MPT and/or TRH administration.
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