Effects of steroid hormones on synaptosomal ectonucleotidase activities from hippocampus and cortex of adult female rats.

Over the last few years, the effects of steroid hormones on the brain have been intensively discussed. It has been demonstrated that ATP (acting as a neurotransmitter) is hydrolyzed to adenosine in the synaptic cleft by the conjugated action of ectonucleotidases, which include an enzyme of the E-NTPDase family (NTPDase3, apyrase, EC 3.6.1.5) and a 5'-nucleotidase (EC 3.1.3.5). The 5'-nucleotidase enzyme is able to hydrolyze AMP as well as other monophosphate nucleotides. The importance of this enzyme in the central nervous system is to participate in the adenosine formation, a nucleoside with neuroprotective properties and modulatory effects. However, several questions have been raised about the mechanisms of steroid hormones and the possible neuroprotective effects of estrogen. Thus, we examined the effects of gonadal steroid hormone deprivation, induced by ovary removal (OVX) and estradiol replacement therapy, on the ectonucleotidase activities in synaptosomes from hippocampus and cerebral cortex of adult rats. ATP and ADP hydrolysis in synaptosomes from cerebral cortex and hippocampus did not change as a function of OVX and results demonstrated an increase in AMP hydrolysis (82%) in the animals submitted to OVX in cerebral cortex, but not in hippocampus, when compared to control and sham-operated groups. Estradiol replacement therapy reversed this effect. RT-PCR analysis showed that the enhancement of enzyme activity in cerebral cortex could be explained by the higher expression of 5'-nucleotidase, following OVX. The hormones 17beta-estradiol (cyclodextrin-encapsulated 17beta-estradiol), DHEAS, and pregnenolone (1.0, 2.5, and 5.0 microM) did not alter the nucleotide hydrolysis, in vitro, in synaptosomes from cortex and hippocampus of female adult rats. Results presented, herein, should be considered relevant for hormone replacement therapy, since much controversy exists surrounding this area and the relationship between adenosine and sex steroids is still poorly understood.