Region-specific changes of tyrosine hydroxylase-immunoreactivity by estrogen treatment in female rat hypothalamus

The effect of 17β-estradiol (E₂)_treatment for 28 days on tyrosine hydroxylase-immunoreactive (TH-IR) neurons in the periventricular preoptic nucleus (PPN) and medial preoptic area (MPA) of ovariectomized (OVX) rats was examined by morphometric analysis. The number of TH-IR nuerons in the PPN of the E₂-treated group was smalle than that of the OVX group, whereas the opposite result was found in the MPA; the number of TH-IR neurons in the MPA of the E₂-treated group was larger than that of the OVX group. Numerous TH-IR neurons were found in the ventromedial portion of the MPA of the E₂-treated group. In both the OVX and E₂-treated groups, TH-IR neurons contained many short processes up to 40 μm in length. E₂ treatment caused a significant decrem,ent of the number of neurons containing the processes in the range of 10–40 μm length in the PPN, however it caused a significant increment of the number of neurons containing the processes in the range of 5–10 μm length in the MPA. These results suggested that immunoreactivity of TH in the PPN and MPA neuron are affected by E₂ treatment and that E₂ might modulate the production of TH in a region-specific pattern within the hypothalamus of the female rat.