The Role of Steroid Modulation of Amino Acid Transmitters in the Regulation of Female Reproduction

Abstract

SYNOPSIS. The amino acid transmitters can be placed in two general categories, excitatory and inhibitory. This discussion focuses on the role of the inhibitory transmitter GAB A and the excitatory amino acids aspartate and glutamate in the control of gonadotropin secretion and reproductive behavior. GABAergic neurotransmission in the preoptic area inhibits gonadotropin secretion via direct synaptic contact with LHRH neurons and possibly through presynaptic inhibition of noradrenergic fibers that stimulate LH release. In the arcuate-median eminence, GABA acting at GABAA receptors increases gonadotropin release by inhibiting a currently unidentified inhibitory interneuron. In regard to reproductive behavior, GABA acting in the preoptic area inhibits female sexual receptivity whereas GABA in the mediobasal hypothalamus and the midbrain central gray facilitates this behavior. The effects of GABA on reproductive behavior do not appear to be secondary to actions on defensive or locomotor behavior. Gonadal steroids modulate activity at the GABAA receptor in a highly complex manner and these effects may be involved in the role GABA plays in controlling gonadotropin secretions as well as behavior. The excitatory amino acids also affect gonadotropin secretion, exerting a stimulatory effect both in the preoptic area and at the level of the median eminence. When a specific antagonist for one of the excitatory amino acid receptors is infused into the preoptic area or when an excitatory amino acid receptor agonist is infused into the mediobasal hypothalamus, female sexual behavior is inhibited. There have only been limited reports of steroid modulation of excitatory amino acid neurotransmission.