Correlations of catecholamine release in the medial preoptic area with proestrous surges of luteinizing hormone and prolactin: effects of aging.

The roles of hypothalamic norepinephrine (NE) and dopamine (DA) in the regulation of LH and PRL are controversial. In the present studies, we used HPLC and push-pull perfusion to measure NE and DA releases in the medial preoptic area (MPA) of conscious, freely moving rats. Serum LH and PRL were determined in separate groups of rats with indwelling venous cannulas. In young (4- to 5-month-old) rats, concomitant with proestrous surges of serum LH and PRL, NE release in the MPA increased gradually to reach a peak at 1800 h, whereas DA release decreased gradually to its lowest level at 2000 h. Compared to the young animals, in middle-aged (8- to 10-month-old) animals, the proestrous surge of PRL was unaltered, but the LH surge was delayed and attenuated. The pattern of DA release in the middle-aged animals was also unaltered, but the peak in NE release was markedly attenuated, although the average NE release was increased compared to that in the young proestrous animals. In young diestrous rats and old (22- to 24-month-old), persistently diestrous rats, in which serum LH and PRL are known to be stable, both NE and DA releases were devoid of fluctuations. However, in the young diestrous animals, the average NE and DA releases were significantly increased compared to those in the young proestrous animals, whereas in the old, persistently diestrous animals, NE and DA releases were markedly reduced compared to those in the young diestrous animals. These data lead us to conclude that NE, through its stimulatory action, may be the primary regulator of LH release, and that it is the pattern, rather than the level, of NE release that appears to be critical in determining the pattern of LH release. DA, through its inhibitory action, appears to control PRL only and probably has no association with LH release. By tracing NE and DA activities from adulthood through middle-age to senescence, these studies revealed that the marked reductions in catecholamines in old age are preceded by a transitory increase in NE activity in middle-age, and that the cyclic increase in NE activity associated with the LH surge begins to diminish in middle age and disappears completely in old age.