Interactions between GABAergic and serotoninergic systems with excitatory amino acid neurotransmission in the hypothalamic control of gonadotropin secretion in prepubertal female rats

Abstract

The present studies were designed to study the interrelationships between GABAergic, serotoninergic and excitatory amino acids systems (EAAs) in the control of gonadotropin secretion in prepubertal female rats. For this purpose we determined the effects of N-methyl- d-aspartate (NMDA), an exogenous agonist of EAAs receptors, on LH and FSH secretion in 16-day-old female rats in which the GABA-A and GABA-B receptors were blocked by bicuculline and baclofen or serotonin (5-HT) depleted by p-choloroamphetamine (PCA). In addition the effects of the GABAergic and serotoninergic systems on LH and FSH secretion were evaluated in animals treated with dibenzocycloalkenimine (diocilpine MK-801), an antagonist of NMDA neurotransmission. While muscimol, a GABA- A agonist, induced a significant increase in LH and FSH levels ( P<0.01), baclofen, a GABA-B agonist, had an inhibitory effect on these hormones ( P<0.01). MK 801, a NMDA receptor antagonist, not only suppressed the stimulatory effect of NMDA on LH and FSH but also blocked the stimulatory effect of muscimol without modifying the inhibitory action of baclofen on both gonadotropins. Bicuculline, a GABA-A receptor antagonist, did not modify the release effect of NMDA on LH and FSH. 5-HTP, a precursor of 5-HT that increases the levels of this neurotransmitter in the central nervous system significantly increased ( P<0.01) the plasma levels of LH and FSH, and this effect was blocked by the NMDA receptor antagonist MK-801. We conclude that the stimulatory effects of GABAergic and serotoninergic systems in prepubertal female rats are connected with the activation of EAA neurotransmission, while the stimulatory effects of NMDA appear to be independent of serotoninergic and GABAergic actions on LH and FSH secretion. Since both GABA and serotonin systems change their effects on LH and FSH during sexual maturation from a stimulatory action in prepubertal to an inhibitory action in adult rats and since NMDA neurotransmission has a stimulatory effect on gonadotropin secretion both in prepubertal and adult rats, it is clear that the interrelationships between GABAergic and serotoninergic systems with EAAs in the gonadotropin control are different in prepubertal and in adult rats.