Luteinizing Hormone-releasing Hormone Fibers Research Articles

Luteinizing hormone-releasing hormone distribution in the anterior hypothalamus of the female rats.

Luteinizing hormone-releasing hormone (LHRH) neurons and fibers are located in the anteroventral hypothalamus, specifically in the preoptic medial area and the organum vasculosum of the lamina terminalis. Most luteinizing hormone-releasing hormone neurons project to the median eminence where they are secreted in the pituitary portal system in order to control the release of gonadotropin. The aim of this study is to provide, using immunohistochemistry and female brain rats, a new description of the luteinizing hormone-releasing hormone fibers and neuron localization in the anterior hypothalamus. The greatest amount of the LHRH immunoreactive material was found in the organum vasculosum of the lamina terminalis that is located around the anterior region of the third ventricle. The intensity of the reaction of LHRH immunoreactive material decreases from cephalic to caudal localization; therefore, the greatest immunoreaction is in the organum vasculosum of the lamina terminalis, followed by the dorsomedial preoptic area, the ventromedial preoptic area, and finally the ventrolateral medial preoptic area, and in fibers surrounding the suprachiasmatic nucleus and subependymal layer on the floor of the third ventricle where the least amount immunoreactive material is found.

Immortalized hypothalamic luteinizing hormone-releasing hormone (LHRH) neurons induce a functional switch in the growth factor responsiveness of astroglia: involvement of basic fibroblast growth factor

Recent evidence indicates that astroglial-derived growth factors (GFs) participate in the development of luteinizing hormone-releasing hormone (LHRH) neurons, but it is still unknown whether LHRH neurons may exert a reciprocal modulation of glial cell function. Using immortalized hypothalamic LHRH (GT1-1) neurons in co-culture with glial cells, we have recently shown that basic fibroblast growth factor (bFGF) plays a prominent role in the glial-induced acquisition of the mature LHRH phenotype by GT1-1 cells. We have resorted to this model and combined biochemical and morphological approaches to study whether the response of glial cells to a number of GFs (including bFGF, insulin-like growth factor I, IGF-I, epidermal growth factor, EGF and insulin) expressed during LHRH neuron differentiation, is modulated by co-culture with pure LHRH neurons. Pre-treatment of hypothalamic astrocytes with an inactive (‘priming’) dose of bFGF for 12 h powerfully increased astroglia proliferative response to IGF-I (10 ng/ml), EGF (10 g/ml) and insulin (10 μg/ml), inducing a 65–100% increase in the [3H]thymidine incorporation compared to untreated cultures. When astroglial cells and developing GT1-1 neurons were co-cultured for 5 days in vitro (DIV), the [3H]thymidine incorporation was significantly higher than in astroglial cells cultured without neurons. Application of the different GFs to the co-culture for either 12 or 24 h further stimulated DNA synthesis to various extent according to the GF applied and the time of application. Localization of the proliferating cells by dual immunohistochemical staining, followed by cell counting and bromodeoxiuridine (BrdU) labeling index calculation, revealed that the incorporation of BrdU was restricted to the nuclei of LHRH-immunopositive neurons. Such changes were accompanied by extensive morphological alterations of astroglial and LHRH fiber networks, whereas neutralization of bFGF activity in GT1-1 neuron–glial co-cultures by a bFGF-antibody, dramatically counteracted the observed effects. The functional switch of astroglia proliferative response to GFs coupled to the potent morphological and functional modifications of developing glia and pure LHRH neurons observed in vitro, support a bidirectional interaction between immortalized LHRH neurons and astroglial cells and identify bFGF as a key player in this crosstalk.

Neuroendocrine Mechanisms Underlying the Control of Gonadotropin Secretion by Steroids

There is considerable evidence that although estradiol may trigger the preovulatory surge of gonadotropins, progesterone is required for its full magnitude and duration and that glucocorticoids bring about selective follicle-stimulating hormone release. The luteinizing hormone-releasing hormone (LHRH) neuron does not have steroid receptors and is regulated by excitatory amino acid neurotransmission. Steroids do not appear to modulate excitatory amino acid receptors directly but increase release of glutamate in the preoptic area. This may be due to the suppression by steroids of the enzyme glutamatic acid decarboxylase 67 that converts glutamate into GABA. NMDA receptors colocalize with nitric oxide synthase-containing neurons that surround the LHRH neurons in the preoptic area and intersect the LHRH fibers in the median eminence. Other potential novel pathways of LHRH release that are currently being explored include carbon monoxide generated by the action of heme oxygenase-2 on heme molecules and bradykinin acting via bradykinin B 2 receptors.

Peripherin fibers in the main olfactory bulb are different from olfactory fibers and from LHRH fibers: an immunocytochemical and DiI study

Fibers coming from the olfactory epithelium and entering the olfactory bulb expressed peripherin. These were not sensory olfactory fibers because: (1) they did not form glomeruli, (2) they did not express olfactory marker protein (OMP), (3) they entered deeply into the olfactory bulb, reaching its white matter. Cells and fibers in the olfactory bulb expressing luteinizing hormone-releasing hormone (LHRH) differed from those expressing peripherin because LHRH and peripherin did not coexist and the distributions of LHRH-positive and peripherin-positive fibers were found to be different.

Presence of LHRH (luteinizing hormone-releasing hormone) fibers in the optic nerve, optic chiasm and optic tract of the adult rat

In mammals LHRH (luteinizing hormone-releasing hormone) is synthesized and released by a set of neurons that have their embryonic origin in the olfactory placode. We have observed that, besides their classical location, LHRH fibers can also be seen in the optic nerve and optic chiasm. Some LHRH fibers could also be traced in the optic tract. The possible course of these projections, and their functional significance are discussed.

2418 Effect of abdominal vagotomy on interleukin-1β induced noradrenaline release in the paraventricular nucleus in conscious rats

Seasonal breeding mammals exhibit a reversible annual cycle of fertility, and thus represent valuable models for investigating the organization of luteinizing hormone-releasing hormone (LHRH) neurons which mediate the neuroendocrine control of reproduction. Electron microscopic immunocytochemistry was used to examine the ultrastructure of LHRH neurons and their projections in a seasonal breeder, the golden hamster, housed under photoperiodic conditions which are reproductively stimulatory. LHRH perikarya in the diagonal band, medial septum, and preoptic area were bipolar or unipolar cells which possessed nuclei with prominent indentations and multiple nucleoli. LHRH reaction product within these cells was associated with neurosecretory granules and the rough endoplasmic reticulum (RER), particularly those portions of RER adjacent to the outer nuclear envelope. LHRH cell bodies and dendrites in the hamster received an extremely limited amount of synaptic input; of a total of twenty-five cells analyzed, we found only five instances of morphologically identified synapses onto immunoreactive dendrites. Occasionally close somatic appositions were seen between LHRH cells and non-immunoreactive neurons, and in one instance between two LHRH somas. In the organum vasculosum of the lamina terminalis (OVLT), LHRH fibers appeared as isolated strings of varicosities. In contrast, within the median eminence immunoreactive axonal profiles were frequently bundled together in fascicles although synaptic specializations were not observed between individual axons. The existence of contacts between LHRH axons in the median eminence, coupled with the extreme paucity of synaptic inputs onto LHRH neurons in this species, suggests that the median eminence may be a site where neural or hormonal signals could influence the coordinated release of LHRH from cells of dispersed origin.

Central peptidergic mechanisms controlling reproductive hormone secretion: novel methodology reveals a role for the natriuretic peptides.

A variety of neural factors can influence reproductive hormone secretion by neuromodulatory actions within the hypothalamus or neuroendocrine actions within the anterior pituitary gland. Passive immunoneutralization and antagonist administration protocols have suggested physiological roles for a number of these factors; however, both experimental approaches have severe technical limitations. We have developed novel methodology utilizing cytotoxin cell targeting with neuropeptides linked to the toxic A chain of the plant cytotoxin ricin. With this methodology we can target and destroy in vivo or in vitro cells bearing receptors for that peptide. Ricin A chain conjugated to atrial natriuretic peptide (ANP), a neuropeptide known to pharmacologically inhibit luteinizing hormone-releasing hormone (LHRH) release, was injected into the cerebroventricular system of intact, cycling rats and ovariectomized rats. Cytotoxin conjugate treatment significantly lengthened the estrous cycle. In ovariectomized rats the luteinizing hormone surge induced by steroid priming was completely inhibited. LHRH content of the median eminences of these rats was not significantly altered. These data suggest that ANP binding to clearance receptors in the hypothalamus displaces the C-type natriuretic peptide (CNP) from the shared clearance receptor, making more CNP available to inhibit LHRH release. In the absence of cells bearing the clearance receptor all available CNP binds to the ANPR-B receptor and exerts its effect via an inhibitory interneuron, since LHRH fibers are spared by this treatment.

Monoclonal Antibodies to Luteinizing Hormone-Releasing Hormone: Production, Characterization, and Immunocytochemical Application1

Luteinizing hormone-releasing hormone (LHRH) was conjugated to bovine thyroglobulin and used to immunize a BALB/c mouse. Spleen lymphocytes were subsequently fused to SP2/0 myeloma cells and two of the resulting hybridoma clones were found to produce high titer antibodies to LHRH (HU4H and HU11B); both belonged to the IgG1 subclass. Characterization of the monoclonal antibodies revealed that HU4H and HU11B have conformational and sequential specificity to LHRH, respectively, and that neither one shows significant immunoactivity with pro-LHRH. The value of these antibodies in immunocytochemical applications is demonstrated by their ability to cause intense specific staining of LHRH neuronal cell bodies and fibers in brain sections from several mammalian species.

Synaptology of luteinizing hormone‐releasing hormone (LHRH)‐immunoreactive cells in the nervus terminalis of the gray short‐tailed opossum (monodelphis domestica)

Light and electron microscopic immunocytochemistry were used to examine the structure of LHRH neurons and fibers in the nervus terminalis of the gray short-tailed opossum (Monodelphis domestica). LHRH-immunoreactive neurons and fibers form a loose plexus within the fascicular network of the ganglion terminale on the median surface of the olfactory bulb. There are at least two populations of LHRH-immunoreactive neurons within the network of the ganglion terminale: fusiform and round neurons similar to those described in the forebrain. At the ultrastructural level, axosomatic and axodendritic contacts were seen between LHRH-immunoreactive and nonimmunoreactive elements in the ganglion terminale. These contacts were classified as 1) synaptic input, with asymmetric synapses seen between a nonimmunoreactive axon terminal and a LHRH-immunoreactive cell body or a nonimmunoreactive axon terminal and a LHRH-immunoreactive dendritic process. 2) synaptic output, with symmetric synapses seen between LHRH-immunoreactive and nonimmunoreactive processes. This study is the first systematic examination of the ultrastructure of the LHRH-immunoreactive neurons and their synaptic contacts in the nervus terminalis. The possible integrative roles for this LHRH-immunoreactive system are discussed.

Luteinizing hormone-releasing hormone (LHRH)-expressing cells do not migrate normally in an inherited hypogonadal (Kallmann) syndrome

Kallmann syndrome inherited hypogonadotropic hypogonadism with anosmia, is associated with an X-chromosome deletion at Xp 22.3. In a Kallmann fetus, we have found an absence of luteinizing hormone-releasing hormone (LHRH)-expressing cells in the brain despite dense clusters of LHRH cells and fibers in the nose. LHRH-containing cells and neurites end in a tangle beneath the forebrain, within the dural layers of the meninges, on the dorsal surface of the cribriform plate of the ethmoid bone. Normal fetal brains, matched for age and sex, had LHRH cells and fibers, as expected, in the hypothalamus and preoptic area. Since LHRH-expressing cells recently were discovered to migrate from the olfactory placode into the brain, it appears that the hypogonadotropism of the Kallmann syndrome can be accounted for by a failure of LHRH cells to migrate into the brain.

Topographic relations between tyrosine hydroxylase- and luteinizing hormone-releasing hormone-immunoreactive fibers in the median eminence of adult rats

Employing electron microscopic double immunolabeling, we determined a close apposition of tyrosine hydroxylase (TH) and luteinizing hormone-releasing hormone (LHRH) nerve fibers in the rat median eminence (ME). These axo-axonic contacts occurred frequently in the internal and palisade zones, i.e. at the level of the fiber preterminals. In the superficial area of the ME, major TH fibers abutted on the basal lamina and some were projected into the pericapillary space of the portal vessels. Conversely, LHRH fibers were arrested by the endfeet of tanycytes in reaching the basal lamina.

Immunocytochemical localization of luteinizing hormone-releasing hormone and proopiomelanocortin neurons within the preoptic area and hypothalamus of the bovine brain.

The anatomical locations of proopiomelanocortin (POMC) and luteinizing hormone-releasing hormone (LHRH) neurons were examined in brain tissue from peripubertal female calves and from mature, luteal-phase cows. Biotin-avidin-peroxidase procedures were used for single- and double-labeled immunostaining. LHRH perikarya formed a loosely arranged continuum, extending posteriorly from the diagonal band of Broca, passing through the medial and lateral preoptic areas, and ending within the anterior hypothalamic area. LHRH fibers, apparently directed toward the median eminence, passed (1) posteroventrally in the periventricular area and through the arcuate nucleus, and (2) ventromedially lateral to the arcuate nucleus and medial to the supraoptic nucleus. POMC perikarya were located within and about the arcuate nucleus, some penetrating into the median eminence. Fibers from these POMC perikarya passed ventrally to terminate in the median eminence and to regions ventral to the mammillary nuclei. POMC fibers also projected dorsally and laterally from the arcuate nucleus to other hypothalamic regions as well as anteriorly along the third ventricular wall to the preoptic area, bed nucleus of stria terminalis, and stria terminalis. Less than 6% of the POMC or LHRH processes were in close anatomical apposition to LHRH perikarya and dendrites. Extensive intermingling of LHRH and POMC fibers occurred within zona externa of the median eminence.

Immunocytochemical localization of luteinizing hormone-releasing hormone (LHRH) in the brain and nervus terminalis of the adult and early neonatal gray short-tailed opossum (Monodelphis domestica).

Luteinizing hormone-releasing hormone (LHRH) was detected by immunocytochemical procedures in cells and fibers in the brain and in the nervus terminalis of the adult and neonatal gray, short-tailed opossum (Monodelphis domestica). As in several species of eutherian mammals, LHRH-immunoreactive cells and fibers were seen in the medial septal nuclei, nucleus, and tract of the diagonal band and olfactory tubercle. Surprisingly, few LHRH-immunoreactive cells were found in the hypothalamus or in the preoptic area in either the adult or neonate. LHRH-immunoreactive fibers were seen in these regions and were numerous in the median eminence. The nervus terminalis in the gray opossum showed several distinctive characteristics. Immunoreactive and nonreactive cells, in ganglia along the peripheral and intracranial course of this nerve, were accompanied by thick fascicles of LHRH fibers and frequently lay adjacent to blood vessels. No LHRH-immunoreactive branches of the nervus terminalis were found in contact with the vomeronasal nerves as they traversed the medial surfaces of the main olfactory bulbs en route to the accessory olfactory bulbs. The LHRH-immunoreactive fibers in the central roots of the nervus terminalis formed a compact bundle on either side of midline, which coursed obliquely from clusters of ganglion cells on the ventromedial surface of the olfactory bulbs into the septum. Traced through serial sagittal sections, LHRH-immunoreactive fibers, in continuity with those in the triangular nucleus of the septum, ran down and around the rostral face of the anterior commissure and fanned out into the medial preoptic area. As previously observed in the fetal guinea pig and rat, LHRH was detected in ganglion cells of the nervus terminalis of the newborn gray opossum preceding its detection in any other area of the brain.

Slice cultures of LHRH neurons in the presence and absence of brainstem and pituitary.

Luteinizing hormone releasing hormone (LHRH) neurons from the preoptic area (POA)/hypothalamus of the postnatal rat were cultured for up to 7 weeks using a slice explant roller culture technique. The slices thinned to quasi-monolayers, but maintained organotypic distributions of large numbers of immunocytochemically identifiable LHRH, neurotensin, tyrosine hydroxylase, neurophysin and corticotropin releasing hormone-containing neurons. The distribution, survival and morphology of LHRH cells in co-cultures with brainstem and anterior pituitary was quantitated, and found to be similar to that observed in single cultures. LHRH fibers grew into either pituitary or brainstem tissue, however when all three tissues were co-cultured, LHRH fibers preferentially invaded the pituitary. LH immunoreactive anterior pituitary gonadotropes were maintained only in co-cultures containing POA/hypothalamic slices, and addition of an LHRH antagonist in such cultures, inhibited LH immunoreactivity in the gonadotropes. This slice explant roller culture method effectively maintains the cyto- and chemoarchitecture and functional properties of the LHRH system for long periods in vitro and should provide excellent models for studying the interactive and molecular characteristics of postnatal LHRH neurons.

Ultrastructure of luteinizing hormone-releasing hormone (LHRH) neurons and their projections in the golden hamster

Seasonal breeding mammals exhibit a reversible annual cycle of fertility, and thus represent valuable models for investigating the organization of luteinizing hormone-releasing hormone (LHRH) neurons which mediate the neuroendocrine control of reproduction. Electron microscopic immunocytochemistry was used to examine the ultrastructure of LHRH neurons and their projections in a seasonal breeder, the golden hamster, housed under photoperiodic conditions which are reproductively stimulatory. LHRH perikarya in the diagonal band, medial septum, and preoptic area were bipolar or unipolar cells which possessed nuclei with prominent indentations and multiple nucleoli. LHRH reaction product within these cells was associated with neurosecretory granules and the rough endoplasmic reticulum (RER), particularly those portions of RER adjacent to the outer nuclear envelope. LHRH cell bodies and dendrites in the hamster received an extremely limited amount of synaptic input; of a total of twenty-five cells analyzed, we found only five instances of morphologically identified synapses onto immunoreactive dendrites. Occasionally close somatic appositions were seen between LHRH cells and non-immunoreactive neurons, and in one instance between two LHRH somas. In the organum vasculosum of the lamina terminalis (OVLT), LHRH fibers appeared as isolated strings of varicosities. In contrast, within the median eminence immunoreactive axonal profiles were frequently bundled together in fascicles although synaptic specializations were not observed between individual axons. The existence of contacts between LHRH axons in the median eminence, coupled with the extreme paucity of synaptic inputs onto LHRH neurons in this species, suggests that the median eminence may be a site where neural or hormonal signals could influence the coordinated release of LHRH from cells of dispersed origin.

Luteinizing hormone-releasing hormone in the vomeronasal system and terminal nerve of the hamster.

The vomeronasal system of mammals is a specialized chemosensory pathway that parallels the main olfactory system but is anatomically distinct from its peripheral aspect through its projections into the central nervous system.'.' Recent evidence suggests a predominant role for the vomeronasal system in conveying the influence of odor stimuli upon neuroendocrine and on a variety of motivated behaviors, including sex,6 aggression,' and maternal behavior.' This functional role probably reflects the relatively direct projections of this chemosensory system to areas of the ventral forebrain important in the control of neuroendocrine and behavioral events. The sole central target of the vomeronasal organ is the accessory olfactory bulb (AOB), neurons which in turn project to several nuclei of the medial amygdala.'.' In particular, the medial nucleus of the amygdala (M), has projections to the preoptic area and hypothalamus, and has been strongly implicated in mediating the influence of odor cues upon the neuroendocrine system and behavior.'.'' Axons containing immunoreactive luteinizing hormone-releasing hormone (LHRH) are found throughout the vomeronasal system in the rat and Sparse LHRH fibers are seen within the neuroepithelium of the vomeronasal organ itself.'*. LHRH fibers and cells, defined as a subset of the terminal are present in close proximity to the vomeronasal nerve in the nasal cavity. Centrally, a dense plexus of LHRH fibers innervates the glomerular layer of the AOB, the site of the first synapse in this sensory system. Occasionally, isolated LHRH perikarya are found within the AOB as well. Further caudally, LHRH fibers are seen adjacent to the optic tract within the superficial plexiform layer of M, that layer which receives

Nervus terminalis, olfactory nerve, and optic nerve representation of luteinizing hormone-releasing hormone in primates.

The luteinizing hormone-releasing hormone (LHRH) system was examined immunocytochemically in olfactory bulbs of adult monkeys, including two New World species (squirrel monkey, Saimiri sciureus and owl monkey, Aotus trivirgatus) and one Old World species (cynomolgus macaque, Macaca fasciculata), and in the brain and nasal region of a fetal rhesus macaque Macaca mulatta. LHRH neurons and fibers were found sparsely distributed in the olfactory bulbs in all adult monkeys. There was more LHRH in the accessory olfactory bulb (which is absent in Old World monkeys). In the fetal macaque there was a rich distribution of LHRH neurons and fibers along the pathway of the nervus terminalis, anterior and ventral to the olfactory bulb, and in the nasal septum, with fibers branching into the olfactory epithelium. In addition, there were LHRH neurons and fibers in the optic nerve.

Effects of castration on volumes of the preoptic nucleus and the amygdala and on immunoreactivity of LH-RH fibers in the brain of the toad, Bufo japonicus

We examined the influence of castration on the volumes of sexually dimorphic nuclei, the amygdala pars medialis (Am) and lateralis (Al), and the anterior part of the preoptic nucleus, and on the immunoreactivity of luteinizing hormone-releasing hormone (LH-RH) in brains of Japanese toads captured in spring and autumn. Animals were castrated (GnX) and half were implanted with testosterone (T) (GnX + T) and then killed and dissected after 1 month. Compared with sham-operated (Sham) toads, plasma androgen levels in autumn toads were significantly decreased by the castration, and those in both spring and autumn GnX toads were significantly elevated by the T implantation. The volume of Am in autumn toads was significantly reduced by GnX. Although not significant, changes in the volumes of the other nuclei, except for Al in spring toads, showed the following tendency 30 days after the operation: GnX + T > Sham > GnX. GnX did not alter LH-RH immunoreactivity in the median eminence. However, dense immunoreactive LH-RH fibers were found in the mesencephalic tegmental region in spring GnX toads but not in the other operation groups in both spring and autumn. LH-RH immunoreactivity was not altered in autumn toads. In spring GnX toads, thumb pads degenerated and evoked release calling was infrequent. These results suggest that (i) the volumes of sexually dimorphic nuclei, especially Am in the autumn toad, are androgen-dependent, and (ii) castration can modulate activity of the extrahypothalamic LH-RH-ergic projection in toad brain.

Extrahypothalamic projection of luteinizing hormone-releasing hormone fibers in the brain of the toad, Bufo japonicus

Extrahypothalamic projection of luteinizing hormone-releasing hormone (LHRH) fibers in the brain of the toad ( Bufo japonicus) was examined immunohistochemically by the avidin-biotin-peroxidase complex (ABC) method. Immunoreactive LHRH perikarya are localized in the nuclei medialis septi and of the diagonal band of Broca. A part of the LHRH fibers are sent anteriad to the medial and dorsal pallia. Some fibers reach the olfactory bulb. Dorsocaudally, LHRH neurons in the medial septum project their fibers to the deep layers of the optic tectum and the posterior mesencephalon including the nucleus pretrigeminalis, which is considered to be a generator of mate calling behavior, via the habenular and posterior thalamic regions. In addition, LHRH fibers which run caudad through the dorsal infundibular region and then the mesencephalic reticular formation were widely distributed in both the gray and the white matter of the medulla oblongata. These findings suggest that LHRH acts as a neurotransmitter or a neuromodulator in the various neuronal circuitries for reproductive behavior in the central nervous system, because LHRH has been considered to be related to amphibian seasonal breeding, and many regions where the immunoreactive LHRH fibers were observed are the loci concerned with mating behavior.

Immunocytochemical localization of luteinizing hormone-releasing hormone (LHRH) pathways in the sheep brain during anestrus and the mid-luteal phase of the estrous cycle.

The luteinizing hormone-releasing hormone (LHRH) system of the sheep brain was examined by light microscopic immunocytochemistry with thick, unembedded sections. We compared the distribution and morphology of LHRH cells and their fibers in intact and ovariectomized anestrous ewes, and in breeding season ewes during the mid-luteal phase of their estrous cycle. In all animals, a majority of LHRH neurons were found in the medial preoptic area adjacent to the organum vasculosum of the lamina terminalis. These cells formed a continuum rostrally with immunoreactive neurons in the diagonal band of Broca and medial septum and caudally with cells in the ventrolateral anterior hypothalamus and lateral hypothalamus. Relatively few cells (1-2%) were seen in the arcuate nucleus or its vicinity. Preoptic LHRH neurons project to the tubero-infundibular sulcus of the median eminence by at least two routes: a major ventrolateral projection above the optic tract in the anterior and lateral hypothalamus, and a less prominent periventricular pathway along the third ventricle. LHRH fibers were also observed in a number of extrahypothalamic regions, including the medial amygdala and the accessory olfactory bulb. Immunoreactive LHRH neurons in the sheep exhibited a complex light microscopic morphology unlike that seen in LHRH cells of any other species to date. LHRH cells with extensive, branching processes were frequently found in clusters with close somatic appositions between neighboring cells. Multiple thin protuberances emanated from the soma of many immunoreactive neurons. Immunoreactive fibers with beaded varicosities often were intimately associated with both cell bodies and their dendritic processes. Morphometric analyses revealed that preoptic LHRH neurons in three of four mid-luteal phase ewes had a shorter total dendritic length than those neurons in either intact or ovariectomized anestrous ewes, but this difference between breeding season and anestrous ewes was not statistically significant. Evidence for possible seasonal and/or steroid-induced alterations in the morphology of LHRH neurons must be documented by further studies, including immunocytochemical observations at an ultrastructural level.