Pregnanolone, a metabolite of progesterone, stimulates LH-RH release: in vitro and in vivo studies

The effect of 5β- and 5α-reduced progestins on luteinizing hormone-releasing hormone (LH-RH) release was examined using either an in vitro superfusion or an in vivo push-pull perfusion (PPP) technique. Ovariectomized rats (at least 6 days post operation: OVX) were implanted subcutaneously in the neck region with a silatic capsule containing 17β-estradiol (235 μg/ml: E₂). Two days afterwards, these OVX+E₂ rats were subjected to experimental conditions. Pregnanolone(5β-pregnane-3β-ol-20-one) at a low concentration of 0.01 ng/ml and an apparent latency of 1 h stimulated in vitro LH-RH release from superfused hypothalamic fragments containing the preoptic-anterior hypothalamic-mediobasal hypothalamic area (POA-AHA-MBH). The stimulatory effect of pregnanolone required estrogen-priming and a pulsatile mode of administration. In addition, the effect appeared quite specific since other closely related steroids such as epipregnanole, R-5020 and 5α-DHP were ineffective. In OVX+E₂ rats bearing a push-pull cannula (PPC) in the hypothalamic area, in vivo infusion of repetitive pulses of pregnanolone through the PPC was also effective, confirming in vivo the stimulatory effect of pregnanolone on LH-RH release obtained from in vitro preparations. In these experiments, the same OVX+E₂ rats served both as an experimental animal receiving pulses of pregnanolone and as a control subject when perfused only with medium after a week interval. The basal release rate of LH-RH and the response to pulsatile pregnanolone were variable among animals apparently due to different locations of PPC in the hypothalamus. However, in all animals treated with pregnanolone, increases in in vivo LH-RH release were observed. Compared to controls, the augmented output seemed to be attributable to a significant increase in the amplitude of LH-RH pulses with no change in the frequency. This study demonstrates a high biological activity of this 5β-metabolite of progestone on in vitro as well as on in vivo hypothalamic preparations. The possible physiological significance of this robust effect of a low dose of pregnanolone in the control of LH-RH release remains to be elucidated.