CHARACTERIZATION OF THE SHORT DAY-INDUCED DECREASE IN MEDIAN-EMINENCETYROSINE-HYDROXYLASE ACTIVITY IN THE EWE - TEMPORAL RELATIONSHIP TO THE CHANGES IN LUTEINIZING-HORMONE AND PROLACTIN SECRETION AND SHORT DAY-LIKE EFFECT OF MELATONIN

In the ewe, photoperiod modulates LH and PRL secretion as well as medi an eminence (ME) dopaminergic activity. The studies reported here were designed to characterize the functional significance of this photoper iodic modulation of ME dopaminergic neuron activity in relation to the regulation of LH and PRL secretion. The aim of the first experiment w as to assess whether photoperiodic changes in hypothalamic dopaminergi c activity were temporally linked to changes in either PRL or LH secre tion. The purpose of the second experiment was to determine whether me latonin mimicked the effects of photoperiod on ME dopaminergic activit y. In the first experiment, LH and PRL secretion, hypothalamic tyrosin e hydroxylase (TH) activity, and catecholamine contents were determine d in ovariectomized estradiol-treated ewes either during long days (LD ; control group) or after 5, 25, and 76 short days (SD). SD were assoc iated with a stimulation of LH secretion and a decrease in ME TH activ ity, which were both expressed only in the 76 SD group. In contrast, t he SD-induced inhibition of PRL secretion was already maximal in the 2 5 SD group. In the second experiment, LH secretion and hypothalamic do paminergic activity were studied in ovariectomized estradiol-treated e wes kept in LD and then treated for 0 (control), 25, or 77 days with m elatonin implants producing a SD-like effect on LH secretion. Melatoni n induced a decrease in PRL secretion (observed after 25 days of treat ment), as well as a stimulation of LH secretion and a decrease in ME T H activity and dopamine content (observed only after 77 days of treatm ent). In conclusion, the decrease in ME dopaminergic activity associat ed with SD exposure or the SD-like effect of melatonin appears unrelat ed to the regulation of PRL secretion. The SD-like effect of melatonin on ME dopaminergic activity suggests that melatonin mediates the effe ct of SD on this activity. The regulation of ME dopaminergic activity can thus be considered a probable step in the photoperiodic regulation of LH secretion.