Estradiol pulses induce progestin receptors selectively in substance P‐immunoreactive neurons in the ventrolateral hypothalamus of female guinea pigs

Abstract

Low doses of estradiol, administered as pulses, are as effective as higher doses for priming ovariectomized (OVX) guinea pigs to display progesterone-facilitated lordosis. High doses of estradiol, administered by constant-release implants, induce progestin receptors in many substance P-immunoreactive (SP-IR) neurons in the ventrolateral hypothalamus (VLH), a site at which estradiol primes OVX guinea pigs to respond behaviorally to progesterone. To test the hypothesis that behaviorally effective estradiol pulses induce progestin receptors selectively in substance P-containing neurons in the VLH, OVX females received estradiol implants 1 week prior to perfusion, or two pulses of estradiol-17 beta, injected 39 and 11 h before perfusion. Colchicine was administered intracerebroventricularly prior to perfusion. No significant differences were observed in the total number of progestin receptor-immunoreactive (PR-IR) or substance P-immunoreactive cells in the VLH and VLH/ventromedial hypothalamus (VMH), respectively, of females receiving the two estradiol treatments. However, the percentage of PR-IR cells in the VLH also immunoreactive for SP was significantly higher in the estradiol pulse-treated (53%), than in the estradiol capsule-implanted animals (36%). These data suggest that behaviorally effective estradiol pulses induce progestin receptors selectively in substance P-containing neurons in the VLH and are consistent with the hypothesis that substance P is involved in progesterone-facilitated lordosis in guinea pigs.

Projections from Ventrolateral Hypothalamic Neurons Containing Progestin Receptor‐ and Substance P‐Immunoreactivity to Specific Forebrain and Midbrain Areas in Female Guinea Pigs

“…The proportion of PR-IR neurons that also had SP-IR (39%) observed in this study is within the range of those previously reported (2,6). By adding together the mean percentages of PR & SP-IR neurons retrogradely labeled by each of the four injections, we can account for the projections of about 58% of this group of VLH neurons.…”

“…The pattern of immunostaining for both progestin receptors and substance P matched previous reports (2,6). VLH sections (Figure 1 ) were analyzed from three guinea pigs for each type of injection site (Table 1 ).…”

“…However, many of these steroid sensitive cells in guinea pigs clearly lie outside of the cytoarchitecturally defined ventromedial nucleus. In both rats and guinea pigs, many of the estrogen and progestin receptor-containing neurons in this region, also contain substance P (2,5,6,7). This finding suggests that substance P may be used as a neurotransmitter by these neurons and that its release may be influenced by estradiol and progesterone.…”

Expanding the role of tachykinins in the neuroendocrine control of reproduction

“…Interestingly, FluoroGold injections into the dorsal midbrain labeled a large proportion (approximately 30%) of the vl VMN neurons immunoreactive for SP in the guinea pig (Ricciardi & Blaustein 1994). Furthermore, pulsatile administration of estradiol selectively induces the expression of progesterone receptors in SP neurons located in this area (Olster & Blaustein 1992), and this process is necessary for the induction of lordosis (Rubin & Barfield 1983). Collectively, these results suggest that SP originating in the vl VMN may participate in the onset of lordosis behavior (Dornan et al 1990), however, further detailed components of the anatomy and physiology of this neurocircuitry is missing.…”

Sex differences in the physiology of eating

“…Because reproductive hormone levels cycle in female rats, however, constant-release pellets cannot be considered physiological. Indeed, daily or continuous peripheral administration of low estradiol doses or even single high doses of long-lasting estrogens such as estradiol valerate can disrupt ovarian cycling, induce pseudopregnancy, elicit progressive, aphysiological changes in several brain neurochemical receptor systems and in behavior, and greatly accelerate the degeneration of the arcuate nucleus (83,179,347,467,468,528,529,647). In contrast, a weekly cyclic estradiol injection regimen in which 10 g estradiol benzoate was injected on Tuesdays and Wednesdays and progesterone priming and sexual-receptivity tests were done on Fridays led to stable, normal levels of progestin and oxytocin receptors and in sexual receptivity (647).…”