THE ROLE OF STEROID MODULATION OF AMINO-ACID TRANSMITTERS IN THE REGULATION OF FEMALE REPRODUCTION

The amino acid transmitters can be placed in two general categories, e xcitatory and inhibitory. This discussion focuses on the role of the i nhibitory transmitter GABA and the excitatory amino acids aspartate an d glutamate in the control of gonadotropin secretion and reproductive behavior. GABAergic neurotransmission in the preoptic area inhibits go nadotropin 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 G ABA(A) receptors increases gonadotropin release by inhibiting a curren tly unidentified inhibitory interneuron. In regard to reproductive beh avior, GABA acting in the preoptic area inhibits female sexual recepti vity whereas GABA in the mediobasal hypothalamus and the midbrain cent ral gray facilitates this behavior. The effects of GABA on reproductiv e behavior do not appear to be secondary to actions on defensive or lo comotor behavior. Gonadal steroids modulate activity at the GABA(A) re ceptor 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 secreti on, exerting a stimulatory effect both in the preoptic area and at the level of the median eminence. When a specific antagonist for one of t he excitatory amino acid receptors is infused into the preoptic area o r when an excitatory amino acid receptor agonist is infused into the m ediobasal hypothalamus, female sexual behavior is inhibited. There hav e only been limited reports of steroid modulation of excitatory amino acid neurotransmission.