Evidence for suppression of Na-dependent Ca2+ efflux from rat brain synaptosomes by ovarian steroids in vivo.

The possibility that intracellular Ca2+, which mediates neurotransmitter release, regulation of membrane permeability, microtubule polymerization and axonal transport, is influenced by gonadal steroids via a Na-Ca exchange mechanism was examined. The resting Ca2+ uptake into synaptosomes was measured using crude synaptosomal pellets (P2 fraction), isolated from the brain stem, mesencephalic reticular formation (MRF), nucleus caudatus (NC) and the hippocampus of intact, long-term ovariectomized (OVX) and OVX plus progesterone (P) or estradiol-17 beta benzoate (EB) treated adult female rats. Irrespective of the brain structure investigated, the uptake was 1) markedly increased in synaptosomes from OVX animals in comparison to intact controls, and 2) reduced to near control values in synaptosomes from OVX rats treated s.c. with a single dose of 2 mg P or 5 micrograms EB. Since Ca2+ influx into synaptosomes was shown earlier to depend on external sodium concentration, which was the same in all experiments described in this work, the results obtained indicate that ovarian steroids modulate basal synaptic activity in the rat brain by suppressing Na-dependent Ca2+ efflux from the nerve cell.