Infusion of progestins into the hypothalamus of female New Zealand white rabbits: effect on in vivo luteinizing hormone-releasing hormone release as determined with push-pull perfusion.

Previously, we have reported that intermittent infusion of progesterone (P4; 10 ng/ml) into the hypothalamus of conscious unrestrained female New Zealand White rabbits stimulates LHRH release in vivo. To further investigate this phenomenon, in the present studies, the effect of pulsatile (six pulses; 10 min on, 30 min off) and continuous infusion of P4 (Exp I) and 4-pregnen-20 alpha-ol-3-one or 20 alpha-hydroxyprogesterone (20 alpha-OH-P; Exp II) on LHRH release were studied in vivo using push-pull cannulae. Furthermore, the effect of pulsatile infusion (six pulses; 10 min on, 30 min off) of low doses of the following three progestins [5 beta-pregnan-3 beta-ol-20-one (pregnanolone), 5 alpha-pregnan-3 alpha-ol-20-one (3 alpha-5 alpha-P), and 20 alpha-OH-P] into the hypothalamus of does using push-pull cannulae were examined in Exp III. In Exp I, continuous infusion of P4 at 10 ng/ml was unable to stimulate mean LHRH release. However, pulses of P4 at 0.01 ng/ml (n = 4) were found to significantly increase the amplitude of the largest LHRH pulse (control period, 1.18 +/- 0.41; versus treatment period, 3.15 +/- 0.75 pg; P less than 0.035) as well as the frequency of LHRH pulses (control period, 0.72 +/- 0.26; treatment period, 1.37 +/- 0.12 pulses/h; P less than 0.035). On the other hand, there was no effect of pulses of P4 at 0.001 ng/ml (n = 4) on the activity of the LHRH neural apparatus. In Exp II, pulses of 20 alpha-OH-P at 10 ng/ml (n = 4) were found to significantly increase the mean LHRH release (control period, 1.24 +/- 0.10; treatment period, 2.07 +/- 0.52 pg/10 min) as well as the amplitude of the largest LHRH pulse (control period, 0.99 +/- 0.36; treatment period, 8.15 +/- 3.75 pg). Interestingly, continuous infusion of 20 alpha-OH-P (10 ng/ml) also significantly increased the mean amplitude (control period, 0.58 +/- 0.34; treatment period, 1.90 +/- 0.29 pg) as well as the amplitude of the largest LHRH pulse (control period, 0.64 +/- 0.37; treatment period, 4.30 +/- 1.97 pg).(ABSTRACT TRUNCATED AT 400 WORDS)