Regional brain variations of tryptophan, monoamines, monoamine oxidase activity, plasma free and total tryptophan during the estrous cycle of the rat.

Normal 4-day cyclic rats were sacrificed at 10 a.m. and 3 p.m. on proestrus or estrus and at 10 a.m. on metestrus. Noradrenaline (NA), dopamine (DA) and tryptophan (T) concentrations varied only slightly. Serotonin (5-HT) concentrations showed characteristic changing patterns in many hypothalamic, limbic and midbrain structures with a decrease during proestrus and an increase during estrus being observed. Monoamine oxidase (MAO) activity changes were often parallel to the 5-HT changes, but were not as great. The marked changes in 5-HT early in proestrus are a further indication of its inhibitory effect on gonadotropin release mechanisms before the 'critical period'.