Effect of Pulsatile Infusion of Progesterone on thein VivoActivity of the Luteinizing Hormone-Releasing Hormone Neural Apparatus of Awake Unrestrained Female and Male Rabbits*

Abstract

Eight female and eight male New Zealand White rabbits were outfitted with push-pull cannulae aimed at the tuberal region of the hypothalamus and were used in 19 perfusion experiments. Animals were treated under 2 conditions: a control condition in which female (n = 4) and male (n = 4) rabbits were perfused only with artificial medium for 5-9 h, and an experimental condition in which female (n = 4) and male (n = 5) rabbits were subjected to perfusion with artificial medium, followed by 6 pulses of progesterone (P4) (10 min on, 30 min off; 10 ng/ml) during a 240-min period. Two female rabbits were also subjected to perfusion with artificial medium, followed by 6 pulses of cholesterol (10 min on, 30 min off; 10 ng/ml) during a 240-min period. The LHRH concentration in perfusates collected every 10 min was measured by RIA. In the 4 females undergoing 6- to 7-h control push-pull perfusions, spontaneous pulses were observed, with about 1 pulse every 60 min, and a variable amplitude of the LHRH signal, with a mean release of about 0.91 pg/10 min. In the 4 does treated with pulsatile P4 at 10 ng/ml, the mean LHRH release rate increased significantly from 0.91 +/- 0.13 to 1.66 +/- 0.20 pg/10 min (P less than 0.035), primarily due to an increase in the amplitude of the LHRH pulses, which were significantly greater than controls. This response occurred with an apparent mean latency of about 50 min. Cholesterol pulses did not affect the spontaneous activity of the LHRH neural apparatus. In the 4 males undergoing 5- to 9-h control push-pull perfusions, spontaneous LHRH pulses were observed with about 1 pulse every 60 min, and the mean release increased from 1.25 +/- 0.56 to 1.54 +/- 0.55 (P less than 0.035, by Wilcoxon's matched pairs signed rank test) in the late afternoon primarily due to an increase in the amplitude of the pulses. P4 pulses did not affect the spontaneous activity of the LHRH neural apparatus compared to that in the control animals. Overall, these results clearly demonstrate that although spontaneous pulsatile LHRH release from the hypothalamus of awake unrestrained female and male rabbits is similar, pulses of P4 can activate only the female LHRH neural apparatus, with an apparent latency to peak LHRH release of approximately 50 min. In addition, there appears to be an increase in LHRH mean release levels in the late afternoon to early evening in male rabbits.