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In PMS- and HCG-treated immature rats, interruption of catecholamine synthesis by a-methyl-p-tyrosine (α-MT) is only able to block ovulation when treatment is applied between 3:00 and 4:00 PM of the day of proestrus, i.e., during a fraction of the “critical period.” Earlier treatment has only a slight effect upon ovulation intensity. Restoration of brain dopamine (DA) and norepinephrine (NE) levels by administration of their common precursor L-dihydroxyphenylalanine (DOPA) restores ovulation to subnormal levels in α-MT-blocked animals; however, L-threodihydroxyphenylserine(DOPS), which restores NE synthesis only, is ineffective. These results suggest that a dopaminergic mechanism is involved in LH-release regulation, and that its intervention is only critical during a very short time. The effect of αmethyl- DOPA administration, which induces synthesis of the false neurotransmitters α- methyl-DA and a-methyl-NE in catecholaminergic neurons, is in agreement with this hypothesis. Biochemical implications of these observations are discussed. (Endocrinology85: 924, 1969)
Kordon et al. (Sat,) studied this question.