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A -Aminobutyric Acid_B Agonist Reverses the Negative Feedback Effect of Testosterone on Gonadotropin-Releasing Hormone and Luteinizing Hormone Secretion in the Male Sheep¹

Department of Veterinary Biosciences, University of Illinois, Urbana, Illinois 61802

Address all correspondence and requests for reprints to: Dr. Gary L. Jackson, Department of Veterinary Biosciences, 2001 South Lincoln Avenue, Urbana, Illinois 61802. E-mail: g-jackson{at}uiuc.edu

Infusion of baclofen, a GABA_B agonist, into the medial basal hypothalamus (MBH) of castrated rams rapidly increases LH pulse amplitude without altering pulse frequency. The objectives of this study were to determine whether baclofen infusion increased LH in testosterone (T)-treated and intact rams, the increased LH was due to increased GnRH release, and FSH secretion also was increased. In the first experiment we tested the main effects and interaction of baclofen and T on FSH and LH pulse patterns in castrated rams (n = 7). In the second experiment we determined whether baclofen affected GnRH and LH pulses in intact males. Microdialysis guide cannulae were implanted bilaterally into the MBH. After recovery of the animal from surgery, the MBH was perfused using concentric microdialysis probes (2-mm tip) with artificial cerebrospinal fluid (aCSF) for a 3-h control period followed by either aCSF or 1 mM baclofen for 4 h. Blood samples were taken at 10-min intervals. T suppressed mean LH concentrations (10.4 ± 1.3 vs. 3.3 ± 1.3 ng/ml) such that LH pulses were undetectable in some T-treated animals during the control period. The change (control period vs. drug infusion period) in mean LH was greater in response to baclofen than in response to aCSF and was not altered by T. The baclofen x T interaction was nonsignificant. Mean FSH was decreased by T, but was not altered by baclofen. In the second experiment hypophyseal portal blood was collected coincident with microdialysis. Infusion of baclofen into the MBH of intact males (n = 7) resulted within 1 h in the onset of frequent and robust GnRH pulses (0.10/h before baclofen vs. 1.57/h after baclofen) that were followed either immediately or gradually by coincident LH pulses. One interpretation is that baclofen acts downstream of the site of action of T. GABA_B receptors may regulate pulse amplitude in both the presence and absence of T and regulate pulse frequency by modulating the inhibitory effect of T.

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