Estradiol increases tyrosine hydroxylase activity of the A15 nucleus dopaminergic neurons during long days in the ewe

In the ewe, the inhibition of LH secretion during long days results from increased negative feedback by estradiol. This effect depends on aminergic systems, mainly in the lateral retrochiasmatic area, a region that includes the dopaminergic A15 nucleus. Recently we demonstrated that the inhibition of LH secretion by estradiol under long days is accompanied by an increase in extracellular levels of amine metabolites (3,4-dihydroxyphenylacetic acid [DOPAC], homovanillic acid [HVA], and 5-hydroxyindolacetic acid [5-HIAA]) in the lateral retrochiasmatic area of the hypothalamus. The increase in catecholamine metabolite levels could be due to stimulation of tyrosine hydroxylase (TH) activity in the catecholaminergic systems of this region. In the present study, we tested this hypothesis by incorporating in vivo measurement of TH activity in the brain into our microdialysis model. We used this approach to first examine the effect of estradiol on TH activity in both the lateral retrochiasmatic area and the caudate nucleus during long days (experiment 1). Two dialysis sessions were carried out on each of six ewes: one after a 10-day estradiol treatment (s.c. implant) and the other after 10 days without estradiol treatment. Estradiol treatment reduced pulsatile LH secretion without affecting the secretion of prolactin. The steroid also significantly increased TH activity in the lateral retrochiasmatic area as assessed by the formation of L-3-4 dihydroxyphenylalanine (L-DOPA). There was no variation in the TH activity of the catecholaminergic systems of the caudate nucleus, suggesting that the effect of estradiol is specifically hypothalamic. In experiment 2, we estimated the relative contribution to the accumulation of L-DOPA in the lateral retrochiasmatic area by dopamine (DA) synthesis in the dopaminergic neurons and noradrenaline (NA) synthesis in the noradrenergic afferents of the A15 nucleus. This study also allowed us to test whether DOPAC and HVA molecules released in this medium originate from DA metabolism in dopaminergic cells of the A15 nucleus or in its noradrenergic afferents. We monitored TH activity as well as amine metabolite concentrations in the lateral retrochiasmatic area after a partial destruction of noradrenergic nerves in the brain using intracerebroventricular (i.c.v.) injections of nomifensine followed by 6-hydroxydopamine (6-OHDA) injections. We compared the responses in four experimental ewes with those in four control ewes receiving a single i.c.v. injection of nomifensine only. Decreases in extracellular concentrations of 4-hydroxy-3-methoxyphenylethyleneglycol (MHPG) in the left lateral retrochiasmatic area and the tissue content of NA in the contralateral structure confirmed the effectiveness of the lesion of the noradrenergic endings. By contrast, the lack of any modification of the tissue content of DA was evidence of the integrity of dopaminergic cells of the A15 nucleus. These studies show that extracellular L-DOPA, DOPAC, and HVA originate from dopaminergic cells of the A15 nucleus and not from its noradrenergic afferents. The long-day inhibition of LH secretion by estradiol results from a stimulation of TH activity of the A15 nucleus dopaminergic neurons.