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

The objective was to determine the effects of the opioid peptide receptor antagonist, naloxone 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). After naloxone treatment, 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<or=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.