The present study has used a rat neuronal nitric oxide synthase (nNOS) antibody to examine the relationship of nNOS immunoreactivity to GnRH neurons in the ovariectomized and intact diestrous and proestrous rat. A striking band of nNOS-immunoreactive cells was identified in the rostral preoptic area which began in the median preoptic nucleus and organum vasculosum of the lamina terminalis and formed an inverted Y-type distribution above the rostral third ventricle at the level of the anteroventral periventricular nucleus. Another band of nNOS-immunoreactivity was found extending through the internal zone of the median eminence into the arcuate nucleus. Although nNOS immunoreactivity was not detected within GnRH neuronal cell bodies in any of the experimental groups, GnRH perikarya located in the rostral preoptic area, but not elsewhere, were found to be surrounded by nNOS-containing cells. In the median eminence, nNOS and GnRH immunoreactivities were distributed separately in the internal and external zones, respectively. These results provide evidence that, regardless of their pattern of activity, GnRH neurons in the female rat do not express nNOS. Instead, a close anatomical relationship between nNOS-immunoreactive cells and GnRH perikarya and fibers has been identified within specific sub-regions of the rostral preoptic area and in the median eminence. Such findings are compatible with a role for NO at both sites in regulating the release of GnRH throughout the estrous cycle.

Gonadal steroids exert a powerful regulatory influence upon the functioning of gonadotropin‐releasing hormone (GnRH) neurons despite the apparent absence of gonadal steroid receptors in these cells. By using retrograde‐tracing techniques combined with dual‐labeling immunocytochemistry, we show here that distinct populations of estrogen receptor alpha (ERα)‐containing neurons located in the hypothalamus and caudal brainstem project to the vicinity of the GnRH perikarya located in the rostral preoptic area (rPOA). The strongest estrogen‐receptive afferent projection to this area originated from neurons located in the anteroventral periventricular and medial preoptic nuclei of the preoptic area. Approximately 50% of arcuate nucleus neurons projecting to the rPOA were demonstrated to synthesize either neuropeptide Y or β‐endorphin, but little evidence was found for ERα immunoreactivity in either of these specific subpopulations. Over 80% of all tyrosine hydroxylase‐expressing neurons in the arcuate nucleus expressed ERα, but none projected to the rPOA. In the caudal brainstem, the A1 and A2 norepinephrine neurons comprised nearly all of the retrogradely labeled neurons. However, only the A2 afferents expressed ERα immunoreactivity, whereas the A1 afferents coexpressed neuropeptide Y. These observations, combined with the anterograde labeling data of others, provide neuroanatomical evidence for the existence of specific estrogen‐receptive neuronal cell populations that project to the rPOA and may be involved in the estrogen‐dependent transsynaptic regulation of GnRH neurons in the rat. J. Comp. Neurol. 411:346–358, 1999. © 1999 Wiley‐Liss, Inc.

An understanding of the functional significance of the newly identified estrogen receptor (ER beta) in the brain will require definition of its expression pattern and relationship to ER alpha. Using an antibody generated against the C-terminus of rat ER beta, we report the presence of ER beta immunoreactivity in the lateral septum, medial amygdala, hippocampus and paraventricular nucleus (PVN) of ovariectomized rats. Double labelling studies in the PVN revealed that approximately 35% of oxytocin neurons located principally in the medial and lateral parvocellular divisions of the caudal PVN were immunoreactive for ER beta while vasopressin, somatostatin and magnocellular oxytocin neurons exhibited no ER beta staining with this antibody. No ER alpha immunoreactive cells were identified in the caudal PVN. These observations provide direct evidence for the differential expression of ER sub-types within neurons and indicate that ER beta may be of physiological significance in the regulation of hypothalamic parvocellular oxytocin neurons by estrogen.

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We have examined here the role of glutamate in regulating the process of tangential neuronal migration during embryogenesis by investigating the roles of AMPA and NMDA receptors in the migration of the gonadotropin-releasing hormone (GnRH) neurons from the nose to the hypothalamus. We first determined that GluR1-4 subunit mRNAs were present from embryonic day (E) 12.5 along the complete nose-brain migratory pathway of the GnRH neurons, whereas that of the obligatory NMDAR1 transcript was present only in brain regions of GnRH migration. In vivo studies revealed that AMPA receptor antagonism between E12.5 and E16.5 resulted in a significant (p < 0.05) accumulation of GnRH neurons in the nose adjacent to the cribiform plate. In contrast, NMDA receptor antagonism over E12.5-E16.5 or E13.5-E16.5 caused a selective increase (p < 0.05) in the number of GnRH neurons located in their final resting place within the diagonal band of Broca and preoptic area. Dual-labeling studies using GnRH promoter-LacZ transgenic mice, which facilitate the identification of receptors in GnRH neurons, identified the presence of NMDAR1 receptors in approximately 6% of embryonic GnRH neurons located throughout the migratory pathway. Postnatally, the percentage of GnRH neurons expressing NMDAR1 increased to 50%. These results indicate that tonic AMPA receptor activation enhances the migration of GnRH neurons from the nose into the brain, whereas that of NMDA receptor activation slows the final phase of GnRH migration within the forebrain. These in vivo observations demonstrate differing, spatially restricted roles for AMPA and NMDA receptor activation in the process of tangential neuronal migration.

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Noradrenergic neurons are implicated in the estrogen-dependent neural regulation of luteinizing hormone secretion in a variety of mammalian species. The current study has used immunocytochemical methods to determine whether estrogen receptors (ER) are expressed within the brainstem of the ewe and to establish their relationship to noradrenergic neurons. Using a monoclonal mouse antiserum directed against the N-terminal of ERα, four distinct populations of ERα-immunoreactive cells were identified in ovine medulla and pons. The largest population was found in the superficial laminae of the spinal nucleus of the trigeminal nerve, followed by the nucleus tractus solitarius, lateral area postrema, and ventrolateral medulla. Double-labelling immunocytochemistry using antisera directed against the ERα and dopamine-β-hydroxylase revealed that noradrenergic neurons expressing ER immunoreactivity were only found in ventrolateral medulla (A1 cell group) and nucleus tractus solitarius (A2 cell group). No double-labelled cells were identified in the A5, A6, or A7 noradrenergic cell groups. ERs were expressed with a clear rostrocaudal topography within the A1 and A2 populations, with 80–90% of noradrenergic neurons expressing ERα in the caudalmost medulla as compared with less than 5% rostral to the obex. Our findings demonstrate that, as in the rat, the ovine A1 and A2 neurons express ERs in a defined topographical manner, while, dissimilar to the rat, ERα is not synthesized by noradrenergic neurons in the other cell groups. These observations indicate that A1 and A2 noradrenergic neurons in the ovine brainstem are likely to be influenced by circulating estrogens and lay the neuroanatomical foundations for investigating the functional role of these cell populations within the gonadotropin-releasing hormone neuron network of the sheep.