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Title: Effect of naloxone treatment on luteinizing hormone and testosterone concentrations in boars with high and low libido

Author
item ESTIENNE, MARK - Virginia Polytechnic Institution & State University
item HARPER, ALLEN - Virginia Polytechnic Institution & State University
item SPEIGHT, SUSAN - Virginia Polytechnic Institution & State University
item CRAWFORD, RUSSEL - Virginia Polytechnic Institution & State University
item Barb, Claude

Submitted to: Reproductive Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/21/2009
Publication Date: 11/1/2009
Citation: Estienne, M.J., Harper, A.F., Speight, S.M., Crawford, R.J., Barb, C.R. 2009. Effect of naloxone treatment on luteinizing hormone and testosterone concentrations in boars with high and low libido. Reproductive Biology. 9:241-252.

Interpretive Summary: Luteinizing hormone (LH) secreted from the pituitary gland stimulates testosterone secretion from the testis which is necessary for sexual maturation and maintenance of semen production and libido in the boar. Brain opioid peptides, hormones with morphine like activity, inhibit gonadotropin releasing hormone (GnRH) release, necessary for the release of LH from the pituitary, and subsequent libido in the boar. It was determined that boars with heighten sexual activity were more responsive to brain opioid receptor antagonism than boars with low libido. Thus, these results may serve as the basis for development of a test for predicting libido.

Technical Abstract: The objective was to determine the effects of naloxone, an opioid peptide receptor antagonist on circulating concentrations of luteinizing hormone (LH) and testosterone in boars characterized as having high (n = 8) or low libido (n = 8) based on the willingness to mount an artificial sow and allow semen collection. On the day of the experiment, blood was sampled every 15 min for 4 h before and 4 h after i.v. injection of naloxone (1 mg/kg body weight). For the 4-h period prior to naloxone administration, concentrations of LH and testosterone in serum, and LH pulse frequency and amplitude were similar (p > 0.12) for the High- and Low-libido boars. After naloxone, a libido status by time interaction was detected and concentrations of LH within 15 min after treatment were greater (p < 0.05) for High Libido boars than for Low Libido boars. Concentrations of testosterone were highly variable amongst boars and there were no effects of libido status (p = 0.66) or libido status by time (p = 0.66). There was, however, an effect of time (p = 0.01), and concentrations of testosterone in samples collected between 0.5 and 1.25 h after naloxone were greater than concentrations in samples collected prior to injection. In summary, the responsiveness of the hypothalamic-gonadotropic-gonadal axis to opioid receptor antagonism was heightened in boars displaying a high level of sexual motivation.