Preoptic rather than Mediobasal Hypothalamic Amino Acid Neurotransmitter Release Regulates GnRH Secretion during the Estrogen-Induced LH Surge in the Ovariectomized Rat

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Abstract
Inhibitory and excitatory amino acid neurotransmitters have been suggested to participate in the feedback actions of estradiol (E2) on LH secretion. In the rat estrogen-receptive neurons have been demonstrated in the preoptic/anterior hypothalamic area (POA) and mediobasal hypothalamus/median eminence (MBH) and many of these neurons utilize γ-aminobutyric acid (GABA) as neurotransmitter. The actions of excitatory amino acids (EAA) differ in ovariectomized (ovx) and ovx E2-substituted rats indicating that EAAs also participate in the positive feedback action of E2 on LH release. However, little information is available as to whether in vivo these transmitters exert their effects in the POA, where most of the GnRH perikarya are located, or in the MBH, i.e. at the nerve terminals. Therefore we conducted push pull cannula perfusions to compare the release rates of GABA, aspartate (ASP) and glutamate (GLU) in the MBH and POA. A subcutaneous implant of a silastic tube containing E2 resulted in LH surges in the afternoon of all treated animals. Prior to and during this LH surge the MBH release rates of neither GABA nor ASP nor GLU were significantly altered. In contrast, a conspicuous drop in preoptic GABA release occurred prior to and during the time of estrogen-induced LH surges and this was accompanied by enhanced preoptic secretion of ASP and GLU. In conclusion, we present the first data about amino acid release in the MBH during the E2-induced LH surge. Since only in the POA the LH surge is associated with changes in amino acid release, it appears that both inhibitory and excitatory amino acids act at the level of the GnRH cell bodies and/or dendrites and not on GnRH nerve terminals to mediate the feedback mechanism of E2 on LH release.

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