Impact of age-related changes in serotonin and norepinephrine metabolism on reproductive function in female rats: an analytical review.

This review attempts to reconcile changes in brain NE and 5HT metabolism with the effects of aging, drug treatment and altered photoperiods on phasic and tonic release of LH in the female. A hypothesis is presented in which 5HT has "indirect" and "direct" effects on LH surges. The "indirect" ones facilitate LH surges via a generalized effect of 5HT on temporal organization within multiple neuroendocrine axes. Thus, 5HT promotes circadian rhythmicity in many systems due to its participation in the biological "clock" mechanism of the hypothalamus. The "direct" ones that are specific to the LH releasing mechanism also occur for 5HT (inhibitory) as well as for NE (facilitory) due to the interaction of these transmitters with LH-RH secreting neurons. It is proposed that metabolic changes during aging that favor 5HT at the expense of NE obliterate the facilitatory effects of both amines while enhancing the inhibitory component of 5HT's influence on LH secretion.