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

Abstract

The retrograde tracer, FluoroGold, was used to trace the neuronal inputs from the septum, hypothalamus, and brain stem to the region of the GnRH neurons in the rostral preoptic area of the ram and to compare these imputs with those in the ewe. Sex differences were found in the number of retrogradely labeled cells in the dorsomedial and ventromedial nuclei. Retrogradely labeled cells were also observed in the lateral septum, preoptic area, organum vasculosum of the lamina terminalis, bed nucleus of the stria terminalis, stria terminalis, subfornical organ, periventricular nucleus, anterior hypothalamic area, lateral hypothalamus, arcuate nucleus, and posterior hypothalamus. These sex differences may partially explain sex differences in how GnRH secretion is regulated. Fluorescence immunohistochemistry was used to determine the neurochemical identity of some of these cells in the ram. Very few tyrosine hydroxylase-containing neurons in the A14 group (<1%), ACTH-containing neurons (<1%), and neuropeptide Y-containing neurons (1-5%) in the arcuate nucleus contained FluoroGold. The ventrolateral medulla and parabrachial nucleus contained the main populations of FluoroGold-containing neurons in the brain stem. Retrogradely labeled neurons were also observed in the nucleus of the solitary tract, dorsal raphe nucleus, and periaqueductal gray matter. Virtually all FluoroGold-containing cells in the ventrolateral medulla and about half of these cells in the nucleus of the solitary tract also stained for dopamine beta-hydroxylase. No other retrogradely labeled cells in the brain stem were noradrenergic. Although dopamine, beta-endorphin, and neuropeptide Y have been implicated in the regulation of GnRH secretion in males, it is unlikely that these neurotransmitters regulate GnRH secretion via direct inputs to GnRH neurons.

Progesterone-Receptive β-Endorphin and Dynorphin B Neurons in the Arcuate Nucleus Project to Regions of High Gonadotropin-Releasing Hormone Neuron Density in the Ovine Preoptic Area

“…The location of FG-stained neurons (fig. 2A–C) is similar to that previously obtained in the sheep using the same technique [27, 39, 40]. In brief, large concentrations of FG cells were observed throughout the rostrocaudal extension of the arcuate nucleus (fig.…”

“…Our distribution of FG retrogradely labeled neurons in the ovine hypothalamus is consistent with previous data in the ewe brain using FG [27, 39, 40]. However, little is known about the neuronal projections of steroid-receptive cells in the brain of non-rodent species.…”

Progesterone-Receptive Dopaminergic and Neuropeptide Y Neurons Project from the Arcuate Nucleus to Gonadotropin-Releasing Hormone-Rich Regions of the Ovine Preoptic Area

“…In this study, only 5% of the total TH-immunoreactive cell population was found to contain FG. This is similar to a study on the ram [25], where a mere 3% of TH neurons located in A12 and A14 contained FG when the tracer was injected into the rostral preoptic area. Indeed, Tillet et al [26] did not detect any FG/TH in the A12 dopaminergic nucleus.…”

“…However, high temperature antigen retrieval causes the complete bleaching of FG fluorescence from our sections and to perform quantitative analysis we had to take photographs before and after the antigen retrieval procedure. One advantage of this dual-photography approach is that it avoids FG fading following immunocytochemistry – a problem noted by others [25]. …”

“…However, it is not known if the arcuate nucleus dopaminergic neurons are involved in the regulation of GnRH neuron activity. Moreover, previous reports in sheep [25, 26] have suggested a limited projection of TH neurons from the arcuate nucleus to the rostral medial preoptic area.…”