Noradrenergic Projections from the A1 Field to the Preoptic Area in the Brain of the Ewe and Fos Responses to Oestrogen in the A1 Cells

Abstract

Previous studies have shown that there is a population of noradrenergic cells in the caudal A1 field of the brainstem of the ewe that contain oestrogen receptors and project to the preoptic area, where gonadotrophin releasing hormone (GnRH) neurones are located. There are some discrepancies in the literature regarding the extent of this projection and the location of the cells in the A1 region. The issue has been a focus of attention because the positive feedback response to oestrogen that causes the ovulatory GnRH/luteinizing hormone surge may originate from this brainstem region. The aim of the present study was to determine the extent of the projections to the preoptic area and to determine whether the caudal A1 cells are activated by oestrogen. Eleven ovariectomized ewes received an injection of the retrograde tracer FluoroGold into the preoptic hypothalamus and four of these also received an i.m. injection of oestrogen 2 h before tissue collection. A further three sheep received i.m. oil injections to act as controls for those receiving oestrogen. Dopamine-beta-hydroxylase (DBH)-positive, retrogradely labelled cells were found within the A1 field in sheep that received preoptic FluoroGold injections. Cells in the vicinity of the A2 and A6 fields, that were retrogradely labelled with FluoroGold, were not DBH-positive. Thus, cells in the A1 field provide a direct noradrenergic projection to the preoptic area and may be involved in the control of the secretion of GnRH in this species. Cells that project to the preoptic hypothalamus from more rostrally located areas of the brainstem are not noradrenergic. In the animals that received oestrogen, double-labelling immunohistochemistry was performed throughout the A1 field for FluoroGold, DBH and Fos. DBH cells of the A1 field expressed Fos only in the oestrogen-treated animals and not in the oil-treated animals. There was a decline in the number of DBH cells that were retrogradely labelled from the caudal region of A1 towards obex. There was a similar gradient in the number of cells that were double-labelled for Fos and FluoroGold. We conclude that there is a population of noradrenergic cells in the caudal A1 field that project to the preoptic area; this is a larger group of cells than previously reported. Oestrogen elicits an acute Fos response in these cells, which may be involved in the time-delayed positive feedback response on GnRH cells. The caudal-to-rostral gradient in the labelling with FluoroGold and Fos in DBH-positive cells is similar to that seen previously for oestrogen receptor in DBH-positive cells in the A1 field.

Neuronal Inputs from the Hypothalamus and Brain Stem to the Medial Preoptic Area of the Ram: Neurochemical Correlates and Comparison to the Ewe1

“…TH-ir fibers have been observed in close association with GnRH neurons in the ewe [17], but this input most likely is noradrenergic. Retrograde tracing studies [11] have shown that numerous DBH-ir (presumably noradrenergic) neurons in the ventrolateral medulla of the brain stem (A1 cell group) project to the rostral preoptic area in the ewe. The present study indicates that a similar pathway exists in the ram, although the physiological significance of this pathway remains unknown.…”

“…In the ewe, some orexin-containing neurons in the lateral hypothalamus provide direct input to GnRH neurons [10]. Noradrenergic neurons of the ventrolateral medulla in the caudal brain stem also project to the medial preoptic area in the ewe [11]. These neurons contain estrogen receptors [12] and thus may be involved in the feedback regulation of GnRH secretion by this steroid.…”

Seasonal Plasticity within the Gonadotropin-Releasing Hormone (GnRH) System of the Ewe: Changes in Identified GnRH Inputs and Glial Association

“…Anatomical and physiological studies in this species have implicated catecholaminergic, neuropeptide Y (NPY)ergic, ␥-aminobutyric acid (GABA)ergic and opioidergic systems, among others, as potential transducers of steroid feedback onto the GnRH system. Specifically, dopamine (DA) is suggested to mediate estrogen-negative feedback via an inhibitory influence on the GnRH system during AN (29 -32), whereas the role of norepinephrine may vary depending on the stage of the annual reproductive cycle (30,33,34). An inhibitory influence of GABA on the GnRH system is suggested and may be expressed during negative feedback in AN and during negative feedback throughout the estrous cycle (35,36).…”

“…Yet despite their possible presence, our findings would suggest that noradrenergic systems are not able to influence GnRH in a fashion that involves altering their numbers of inputs onto GnRH neurons. However, as for DA, a possible change in activation of these afferents by estradiol cannot be discounted and indeed may be an important mechanism for regulating GnRH, especially during the estrous cycle (34). A final possibility is that NE could modulate GnRH via corelease at terminals other than those expressing TH-ir.…”

Neuroendocrine Control of Gonadotropin Secretion

“…In spontaneous ovulators, NE release (estimated by microdialysis or turnover rate) increases in the POA and median eminence before and during the preovulatory LH surge in rats, 258,259 sheep, [260][261][262] and monkeys. 263 These species differ in the source of this NE input; both the A2 264-266 and A6 267-269 NE groups appear to be important in rats, whereas the A1 270,271 and A6 263 NE groups have been implicated in sheep and monkeys, respectively. The relative importance of NE to the normal surge also varies among these species ( Figure 33.8), with α-adrenergic antagonists completely blocking the surge in rats, 27,259 but having no effect in primates.…”