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

Abstract

Many neurons within the ventrolateral hypothalamus in guinea pigs contain estrogen-induced progestin receptors as well as substance P. Retrograde tracing combined with immunocytochemistry was used to determine the specific projections of this subset of steroid-sensitive cells. Unilateral Fluoro-Gold injections into the dorsal midbrain, including the central gray, labeled a large proportion of the ventrolateral hypothalamic neurons immunoreactive for both progestin receptors and substance P (approximately 30%); substantially fewer of these neurons were labeled by unilateral Fluoro-Gold injections into the preoptic area (approximately 6%), medial amygdala (approximately 10%), or the bed nucleus of the stria terminalis (approximately 11%). The projections of progestin receptor-immunoreactive neurons in the ventrolateral hypothalamus were similar to those of progestin receptor/substance P double-labeled neurons, while a slightly lower percentage of the ventrolateral hypothalamic, substance P-immunoreactive neurons tended to project to each of these areas. These pathways may prove to be components of the neural circuitry underlying a variety of functions influenced by gonadal steroid hormones and substance P, such as female sexual behavior, salt intake, nociception and aggression.

Projections of the Estrogen Receptor-Immunoreactive Ventrolateral Hypothalamus to Other EstrogenReceptor-Immunoreactive Sites in Female Guinea Pig Brain

“…Because the VLH was so heavily stained by the Pha-L, the quantification of ERIR cells containing Pha-Lir was not possible. However, the terminal fields observed were regions in which estrogen receptor-containing cells have been reported to project [26, 43, 48, 49, 50]. Thus, the location of projections from the VLH determined in this study using anterograde labeling is consistent with previous reports of projections of estrogen receptor-containing cells in the VLH.…”

“…Although differences between this experiment and previous experiments include the sex of the animals used, the particular steroid hormone receptor investigated (estrogen receptor or progestin receptor), species (rat or guinea pig), and precise hypothalamic locus studied, the studies are all in agreement with the idea that the estrogen receptor-containing neurons within the general area of the VLH project to each of these areas [26, 43, 48, 50, 51, 52, 53]. Thus, many of the estrogen receptor-containing neurons in the VLH project to those regions which also receive the heaviest projections from the VLH and also contain the largest number of boutons closely associated with ERIR cells.…”

“…This area consists of the ventrolateral nucleus (VLN) and rostrally, a band of cells lateral to the ventromedial hypothalamus, which extends from the VLN to the perifornical region. The VMHVL [20, 21, 22, 23]as well as the region of the VLH in guinea pigs [24, 25]has been reported to project to other brain regions where estrogen receptor-containing cells have been localized [2, 3, 4, 5, 6, 9, 26]. Many of those regions also project back to the VLH or VMHVL, forming reciprocally connected pathways [27, 28, 29].…”

“…Hypothalamic projections from the guinea pig VLH terminating in other estrogen receptor-containing regions [24]include projections from estrogen receptor-containing cells [26]. These projections suggest the possibility that steroid hormone receptor-containing cells may be directly linked.…”

Expanding the role of tachykinins in the neuroendocrine control of reproduction

“…A number of studies have suggested that SP may be an important participant in this circuitry, as SP injections in the periaqueductal central gray of OVX, estrogen-primed rats produced a long-lasting increase of lordosis behavior (Dornan et al 1987), whereas SP antiserum injections in the same region inhibit the behavior (Dornan et al 1987). 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).…”