The effects of intracerebral implants of progesterone on multiple measures of female rat reproductive behavior

Abstract

The purpose of the present investigation was to determine if progesterone-facilitated reproductive behaviors in the rat share common neural structures as sites of progesterone action. The study included repeated measures of lordosis, pacing, and soliciting behaviors following the intracranial administration of progesterone or cholesterol, or the subcutaneous administration of oil or progesterone in oil. Progesterone or cholesterol were implanted via indwelling cannulae into either the anterior hypothalamic area (AHA), caudate-putamen (CPU), interpeduncular- ventral tegmental area (IP-VTA), or medial reticular formation (MRF) in estrogen-primed ovariectomized rats. Each subject was randomly assigned a balanced schedule of hormone administration such that no subject received the same manipulation more than once. Of the three reproductive behaviors investigated, only the lordosis reflex demonstrated a clear increase with progesterone delivered to the IP-VTA and CPU. No facilitation of pacing or solicitation behaviors was found with any of the intracranial progesterone implants. Progesterone was clearly shown to have a physiological role, however, since it produced changes in all behaviors across all cannula groups when administered subcutaneously. The present investigation also compared the effects of progesterone on pacing behavior with those produced by vaginal anesthetization. Since stimulation of the vagina received during vaginal penetration appears to be an important variable for pacing, it was thought the effects of progesterone might be mimicked by reducing vaginal sensitivity. Subjects were given a single test under one of four treatment conditions: oil, progesterone, lidocaine hydrochloride administered topologically to the vaginal walls, or lidocaine administered intraperitoneally. Vaginal anesthetization was effective in reducing pacing behavior to levels comparable to those produced by progesterone administered subcutaneously. Vaginal anesthetization was without effect on lordosis or soliciting behaviors. In summary, two conclusions are suggested from the results of the present study: (1) the facilitatory effects of progesterone on reproductive behaviors in the female rat appears to operate by modulating diverse neural systems; (2) progesterone's regulation of pacing may be functionally related to a reduction of vaginal sensitivity to penetration.