Separate studies with ewes have shown that catecholamines play an inhibitory role in the control of LH secretion during anestrus, and that there are structures in the lateral retrochiasmatic area (L-RCh), which could be involved in the regulation of gonadotrophin secretion. These observations have led to the hypothesis that the catecholaminergic structures in the L-RCh mediate the inhibition of pulsatile LH secretion by estradiol in the anestrous ewe. We tested this hypothesis by injecting 6-hydroxydopamine (60H-DA) into the L-RCh of ovariectomized ewes during the anestrous season, and comparing the secretion of LH and prolactin in these animals with that in sham (injected with vehicle) and control (no injection) animals, in the presence and absence of exogenous estradiol. Finally, the effectiveness of the toxin was assessed by immunocytochemical techniques. When the ewes were treated with estradiol, LH pulse frequency was significantly lower in the controls (mean 1.1 pulses/4 h) and shams (0 pulses/4 h) than in the ewes treated with 60H-DA (3.1 pulses/4 h). When the estradiol implants were removed, the frequencies increased to 5.1 pulses/4 h for the controls and 5.7 pulses/4 h in the ewes treated with 60H-DA. These were not significantly different. Plasma prolactin levels were significantly reduced by 60H-DA treatment. The 60H-DA ewes recovered their response to estradiol by 14 weeks after the injection. The anatomical study at the end of the experiment revealed a difference between treated and control ewes of only 15% in the numbers of dopaminergic cells in the L-RCh. It was concluded that the catecholaminergic structures of the L-RCh mediate the inhibitory effect of estrogen on the secretion of LH pulses during seasonal anestrus. It also appears that catecholaminergic systems in the same region of the hypothalamus stimulate the secretion of prolactin.
Peripheral prolactin levels were measured by radioimmunoassay in nine Ile de France rams, born in autumn and reared under natural conditions of light and temperature until three years old. During December, when the rams were 80–100 days old, a brief and transitory peak of prolactin occurred (70 ng PS6/ml plasma ± 10 ng). During each following year, the prolactin level fell in winter to approximately 20 ng/ml. However, in summer the prolactin level was 4 times higher than the winter level during the first year and 11 times higher during the 2nd and 3rd years. A possible role of prolactin variation throughout the breeding season is discussed.
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