Abstract
To examine the role of gamma-aminobutyric acid (GABA)B receptor mediating systems in the ventromedial hypothalamus-infundibular nucleus region (VMH/NI) of anoestrous ewes in controlling gonadotropin-releasing hormone (GnRH) release, the extracellular concentrations of GnRH, beta-endorphin, norepinephrine, dopamine, 4-hydroxy-3-methoxy-glycol and 3,4-dihydroxy-phenylacetic acid were quantified during infusion of baclofen or phaclofen (agonist and antagonist of GABAB receptors, respectively) in this structure. The stimulation of GABAB receptors activates GnRH/luteinising hormone (LH) release, attenuates noradrenergic and beta-endorphinergic tone but has no evident effect on the dopaminergic system. Blockade of GABAB receptors in this structure increases the extracellular beta-endorphin concentration but has no significant influence on GnRH release or catecholaminergic activity. It is suggested that activation of GnRH/LH release in the VMH/NI of anoestrous ewes may result from a decrease of norepinephrine output and hence its inhibitory effect on GnRH secretion. Activation of GABAB receptors, as well as their blockade, did not change dopaminergic system activity, indicating that GABAB does not affect GnRH release indirectly by a GABAB receptor mechanism acting on dopaminergic neurones in the VMH/NI. Increased activity of the beta-endorphinergic system during blockade of GABAB receptors does not change GnRH release, suggesting that beta-endorphin does not play a significant role in the control of GnRH secretion in anoestrous ewes.
Published Version
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