Amino Acid Neurotransmitter Release in the Preoptic Area of Rats during the Positive Feedback Actions of Estradiol on LH Release

Abstract

To investigate the role of amino acid neurotransmitters in the regulation of LH secretion in ovariectomized (ovx) rats with or without estrogen substitution, we measured the release rates of γ-aminobutyric acid (GABA), taurine, glycine, aspartate, glutamate, homocysteic acid, and also of the neurally inactive amino acids serine and glutamine in push-pull perfusate samples of the preoptic/anterior hypothalamic area (PO/AH) collected at 30-min intervals. To achieve this we had to develop a highly sensitive assay utilizing phenylisothiocyanate prederivatization which was followed by HPLC chromatography. In confirmation of our earlier results we observed again a conspicuous drop of preoptic GABA release prior to and during the time of estrogen-induced LH surge. In addition, the release rates of the excitatory amino acid neurotransmitters aspartate and glutamate in the PO/AH increased during this time. Interestingly, also secretion of taurine and glycine was increased during the LH surge, whereas preoptic release rates of serine and glutamine and of homocysteic acid, the putative endogenous ligand of the so-called N-methyl-D-aspartate receptor, remained unchanged. No such changes of amino acid neurotransmitters release rates were observed in ovx rats. This finding underlines that the changes of amino acid secretion in ovx estrogen-primed rats are likely due to the influence of the steroid rather than due to a diurnal rhythm. We conclude that GnRH neurons are under a tonic inhibitory tone exerted by GABA which is relieved during the time of the estrogen-induced LH surge. During this time, aspartate and glutamate may have additional stimulatory effects on GnRH neurons. The unchanged preoptic release rates of the neurally inactive neurotransmitters serine and glutamine give evidence for the selectivity of the observed effects.

Dual-Phenotype GABA/Glutamate Neurons in Adult Preoptic Area: Sexual Dimorphism and Function

“…Considering that GABA and glutamate provide most of the input to GnRH neurons (Herbison, 2003), it seems likely that they are released from the VGAT-and VGLUT2-containing vesicles we observed in contact with GnRH neurons. Second, the decline in IRVGAT before the onset of LH surge parallels the E 2 -dependent suppression of GABA release into the POA (Jarry et al, 1992(Jarry et al, , 1995, as well as the decline in GAD67 gene expression in AVPV neurons (Curran-Rauhut and Petersen, 2002). Notably, work examining other brain regions shows that VGAT and GAD67 mRNA levels are correlated with GABA release (Litwak et al, 1990;Drengler and Oltmans, 1993;Falkenberg et al, 1997;Lamas et al, 2001;Ramirez and Gutierrez, 2001;Gutierrez, 2002;Kang et al, 2003).…”

The role of GABA_A receptors in the neural systems of the medial preoptic area in the control of GnRH release during the luteal phase of the oestrous cycle in ewes

“…Among these transsynaptic regulatory systems, γ-aminobutyric acid (GABA), a major inhibitory neurotransmitter in the hypothalamus (Decavel and van den Pol, 1990) and preoptic area (Robinson, 1995(Robinson, , 1997, affects gonadoliberin (GnRH) release by two classes of membrane receptors: GABA A (Sieghart, 1995) and GABA B (Mott and Lewis, 1994). Numerous experiments indicate that GABA mediates both inhibition (Scott and Clarke, 1993;Leonhardt et al, 1995) as well as stimulation (Jarry et al, 1992;Leonhardt et al, 2000) of GnRH release by GABA A receptor mechanism(s). The dual inhibitory-stimulatory action of GABA on GnRH release is not clearly understood.…”

The Arcuate Nucleus Mediates Facilitating Effect of Estrogen on Glutamate-Induced In Vitro GnRH Release from Nerve Terminals of Female Rats.

“…EAAs are considered to mediate the positive feedback effect of estrogen on LH secretion, because steroid-induced LH surges in female rats are blocked by EAA antagonists [3]. In addition, endogenous EAA release in the preoptic area (POA) [4] and mediobasal hypothalamus (MBH) [5] increases during steroid-induced LH surges. Estrogen may exert the feedback effect on LH secretion by modulating GnRH response to glutamate, because glutamate agonists stimulate in vivo LH release in the presence of estrogen but they do not stimulate or even inhibit LH secretion in the absence of estrogen [6][7][8][9][10].…”

“…Glutamate stimulates GnRH secretion by acting at the POA [1,4], where most GnRH cell bodies are located, and at the arcuate nucleus-median eminence (ARC-ME) region [11][12][13], where the majority of GnRH neurons terminate. In the latter region, both ME and ARC seem to be the action sites of glutamate in the enhancement of GnRH release.…”