Sexual dimorphism in the synaptogenic effect of estradiol in prepuberal female rats

A study has been made of the effects of estradiol benzoate (EB) on synaptogenic induction in the hypothalamic arcuate nucleus (ARC), medial preoptic area (MPOA), and medial septal area (MSA) of prepuberal male and female rats. Subcutaneous administration of EB to 25-day-old female rats induces accelerated synaptogenesis in the ARC concomitant with a precocious surge of plasma luteinizing hormone (LH) at 1600 h on day 27 of age. The synaptic area densities (No. synapses/unit area of tissue section) of the MPOA and MSA are not increased at 27 days of age in rats treated with EB on day 25 of age. The synaptic area densities of the ARC, MPOA, and MSA are significantly higher on day 31 compared to day 27. However, only the ARC exhibits a further increase of synaptic numbers in EB-treated female rats. Similar estrogen treatment to prepuberal male rats induces neither an LH surge in plasma nor increased synaptogenesis in the ARC, MPOA, or MSA. Male 27-day-old rats of both estrogen-treated and control groups exhibit a synaptic area density in the ARC similar to 27-day-old control female rats. These data indicate that the sexually dimorphic positive response to estrogen with LH secretion has an equally sexually dimorphic neuromorphological response in rats. In addition, the synaptogenic effect of EB in the prepuberal female rat ARC remains manifest for at least 6 days after estrogen treatment.