Electrochemical and electrical stimulation of the medial preoptic area (MPA) was shown to induce release of LH in rats. Owing to differences in cytoarchitecture and neural afferents between the medial (mMPA) and lateral (IMPA) parts of the MPA, we decided to explore whether this difference in organization would distinctly influence the secretion of gonadotropin. Both parts of the MPA were electrochemically stimulated on the day of proestrus in freely behaving rats bearing chronic implanted electrodes. An anodic direct current of 100 µA was delivered for 40 s at 11.00 h and blood samples were obtained every hour until 17.00 h. Serum LH concentrations in rats stimulated in the medial part of the MPA showed a robust rise 1 h after the stimulus was applied and the values remained high up to the end of the bleeding period. All these animals ovulated. An initial rise in serum LH was also seen in rats stimulated in the IMPA but serum values thereafter returned to basal levels, significantly lower than those in the mMPA-stimulated or in control, nonstimulated rats. Only 2 rats showed full ovulation and the others failed to ovulate or had partial ovulation. Injection of the serotonin antagonist, methysergide, did not affect the response of rats stimulated in either the mMPA or IMPA. The GABA antagonist, bicuculline, had no effect on LH release evoked by IMPA stimulation but blocked the release induced by mMPA stimulation. This later blockade was partially reversed by the administration of the opioid receptor antagonist, naloxone, suggesting the existence of GABA facilitatory influences on LH release via inhibition of opioidergic inputs to the GnRH neurons. On the other hand, naloxone administration had no effect on LH release evoked by stimulation of the mMPA but partially reversed the inhibition resulting from stimulating the IMPA. These data indicate that the mMPA and IMPA have opposite effects on LH secretion and provide evidence for the possible neurotransmitters involved.

Keywords:
Medial preoptic area, Preoptic nucleus, Gonadotropins, Electrochemical stimulation, Serotonin, γ-Aminobutyric acid, Opioid peptides
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© 1997 S. Karger AG, Basel
1997
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