Possible involvement of ovarian mechanisms other than estrogen-progesterone secretion in the regulation of glutamic acid decarboxylase activity of the rat fallopian tubes

This study was performed to clarify the physiological role of the ovary in regulating the glutamic acid decarboxylase (GAD) activity in rat Fallopian tubes. To this purpose, GAD activity of the oviduct was evaluated in the following experimental conditions: immature or adult castrated (CX) rats; immature or adult CX rats treated with graded doses of estradiol benzoate (EB) or a fixed dose of EB and progesterone; adult CX rats bearing Silastic implants able to produce steady state estradiol plasma levels in the range of diestrous values; and prepubertal rats treated with ovulatory or anovulatory doses of exogenous gonadotropins (PMS and hCG). Moreover, the possible fluctuations of both gamma-aminobutyric acid (GABA) concentrations and GAD activity in the Fallopian tubes were studied during the estrous cycle. The results show that the prepubertal rat oviduct possesses a GABA content and a GAD activity analogous to those of normal diestrous rats. The GAD activity measured with the CO2 formation method was well correlated with the formation of labeled GABA, indicating that tubes of prepubertal rats are able to form the neurotransmitter by means of specific decarboxylation of glutamate. GAD activity, but not GABA levels, was increased over control values by the administration of exogenous gonadotropins. The role of the ovary in both adult and prepubertal rats to regulate this enzymatic activity is further stressed by the results of the experiments performed in CX animals which showed that ovariectomy produced a 4- to 5-fold decrease in GAD activity independent of the age of the animals. However, implantation of Silastic estradiol-containing capsules in adult CX animals or the administration of EB for 5 days in a dose range from 0.001-6.4 micrograms/day to adult ovariectomized animals and from 0.001-0.2 microgram/day to prepubertal animals did not modify GAD activity in spite of marked peripheral estrogenization of the animals evidenced by increases in uterine weight. Moreover, no variation of the enzymatic activity was observed at puberty (assessed by the age at vaginal opening). The administration of progesterone (0.2 mg) plus EB (0.01 microgram) did not produce any significant variation in GAD activity. GABA content and GAD activity of the tubes did not change during the estrous cycle. We, therefore, believe that other ovarian, still unidentified, secretions might be involved in the regulation of GAD activity in rat Fallopian tubes.