Effect of Oestradiol and Photoperiod on TH mRNA Concentrations in A15 and A12 Dopamine Cell Groups in the Ewe

In the sheep, photoperiod, through melatonin, and oestradiol negative‐feedback are two major regulators of seasonal changes in luteinizing hormone (LH) and prolactin secretion. Melatonin and oestradiol act on dopamine neurons of the hypothalamus to modify the enzymatic activity of tyrosine hydroxylase (TH). To further understand how melatonin and oestradiol regulate TH activity, we have studied the level of TH mRNA by in situ hybridization with an homologous cDNA probe, in A12 and A15 dopamine neurons of four groups of ovariectomized ewes: long‐day exposed ewes with or without subcutaneous oestradiol implants and short‐day exposed ewes with or without oestradiol. Results were analysed in relation to the concentration of LH and prolactin in the peripheral circulation. In the A15 cell group, TH mRNA levels were elevated in the short‐day, oestradiol‐treated ewes compared to all other groups. In this group, the level of TH mRNA was elevated simultaneously with LH concentration. The low level of TH mRNA found in the long‐day, oestradiol‐treated ewes may indicate that the increase of TH enzymatic activity previously reported by this treatment is not caused by an increase of the level of enzyme. In the A12 cell group, the level of TH mRNA in both long‐day and short‐day oestradiol‐treated ewes was significantly higher than in ewes without oestradiol replacement. Prolactin concentrations were not correlated with TH mRNA variations in the A12 cell group.