Dopamine D1 and D2 Receptor Immunoreactivities in the Arcuate-Median Eminence Complex and their Link to the Tubero-Infundibular Dopamine Neurons

Dopamine D1 and D2 receptor immunohistochemistry and Golgi techniques were used to study the structure of the adult rat arcuate-median eminence complex, and determine the distribution of the dopamine D1 and D2 receptor immunoreactivities therein, particularly in relation to the tubero-infundibular dopamine neurons. Punctate dopamine D1 and D2 receptor immunoreactivities, likely located on nerve terminals, were enriched in the lateral palisade zone built up of nerve terminals, while the densities were low to modest in the medial palisade zone. A codistribution of dopamine D1 receptor or dopamine D2 receptor immunoreactive puncta with tyrosine hydroxylase immunoreactive nerve terminals was demonstrated in the external layer. Dopamine D1 receptor but not dopamine D2 receptor immnunoreactivites nerve cell bodies were found in the ventromedial part of the arcuate nucleus and in the lateral part of the internal layer of the median eminence forming a continuous cell mass presumably representing neuropeptide Y immunoreactive nerve cell bodies. The major arcuate dopamine/ tyrosine hydroxylase nerve cell group was found in the dorsomedial part. A large number of tyrosine hydroxylase immunoreactive nerve cell bodies in this region demonstrated punctate dopamine D1 receptor immunoreactivity but only a few presented dopamine D2 receptor immunoreactivity which were mainly found in a substantial number of tyrosine hydroxylase cell bodies of the ventral periventricular hypothalamic nucleus, also belonging to the tuberoinfundibular dopamine neurons. Structural evidence for projections of the arcuate nerve cells into the median eminence was also obtained. Distal axons formed horizontal axons in the internal layer issuing a variable number of collaterals classified into single or multiple strands located in the external layer increasing our understanding of the dopamine nerve terminal networks in this region. Dopamine D1 and D2 receptors may therefore directly and differentially modulate the activity and/or Dopamine synthesis of substantial numbers of tubero-infundibular dopamine neurons at the somatic and terminal level. The immunohistochemical work also gives support to the view that dopamine D1 receptors and/or dopamine D2 receptors in the lateral palisade zone by mediating dopamine volume transmission may contribute to the inhibition of luteinizing hormone releasing hormone release from nerve terminals in this region.Key words: Dopamine D1 and D2 receptors, tubero-infundibular dopamine neurons, dopamine receptor colocalization, arcuate-median eminence complex, volume transmission, luteinizing hormone releasing hormone     

Distribution of immunoreactive somatostatin in the primate hypothalamus

Immunocytochemical methods were used to compare the localization of somatostatin (SRIF) in the human and rhesus monkey hypothalamus. The distribution of SRIF-containing cell bodies and fibers is similar in the two species. Perikarya are located predominantly in the periventricular region and to a lesser extent in the ventromedial nucleus. Fibers occur in dense clusters within the periventricular region, ventromedial nucleus, arcuate nucleus, median eminence, and pericommissural area of both species. Analysis of serial sections suggests that fibers originate from cells in the periventricular region, extend ventrally through the ventromedial and arcuate nuclei to terminate around the portal vessels of the infundibular stalk, and thereby participate in the regulation of anterior pituitary function. Somatostatinergic fibers are also found surrounding non-immunoreactive perikarya in the ventromedial nucleus and periventricular region of both primates. This arrangement may support somatostatin's postulated role as a neurotransmitter or neuromodulator. The strong similarity between the localization of hypothalamic SRIF in the human and rhesus monkey supports the use of the rhesus monkey as a model for the study of somatostatin as a neuroendocrine regulatory in the human.     

Myelinated Herring bodies in the median eminence of the cat.

An electron-microscopic study was carried out on the median eminence of cats during post-natal development. From the moment of birth (observations performed 12 hours later) Herring bodies were seen in the fibrillary layer of the median eminence. At 45 days after birth, myelinated nerve fibres could be observed, some of them containing neurosecretory granules. The number of myelinated fibres in the median eminence increased with age and at 90 days some Herring bodies appeared surrounded by myelin sheaths; these mainly contained neurosecretory granules and a few mitochondria.     

Immunocytochemical localization of somatostatin in cell bodies of the rat hypothalamus.

Cell bodies of small to moderate-sized neurons in the female rat hypothalamus were stained specifically for somatostatin (SRIF) by means of the unlabeled antibody-peroxidase-antiperoxidase immunocytochemical method. SRIF-positive perikarya were scattered throughout the periventricular nucleus in a limited region extending from the middle of the optic chiasm to the rostral margin of the median eminence. The same neurons were revealed with either rabbit (R) or guinea pig (GP) anti-SRIF antisera. Positive cell bodies were more readily assessed with GP antibodies because nonspecific background staining was much less with these than with R anti-SRIF. Positive perikarya were not observed in other hypothalamic nuclei and ependymal elements were also immunocytochemically negative.     

Intragranular colocalization of arginine vasopressin and methionine-enkephalin-octapeptide in CRF-axons in the rat median eminence

Summary Ultrastructural appearances of axonal terminals containing corticoliberin (CRF) were examined in the rat median eminence prepared by a freeze-drying procedure. Immunolabeling was performed by using 5-, 8-, or 15-nm gold-antibody complexes for CRF, arginine vasopressin (VP) and methionine-enkephalin-octapeptide (Enk-8), singly or in combination. In intact animals, the CRF-containing secretory granules were only slightly labeled with goldanti-VP or -Enk-8. In adrenalectomized rats, granules within single axons appeared to be labeled with all the immunogold complexes. This intragranular colocalization of the three antigens was confirmed by using three neighboring sections of the same axon terminals which were stained separately with each one of the antibodies and visualized with the avidin-biotin-peroxidase complex method. The granules labeled for CRF had decreased 9 days after adrenalectomy but had increased again by day 21, while those labeled for VP steadily increased after adrenalectomy. However, this did not correspond with the appearances of cell bodies in the paraventricular nucleus; the cell bodies labeled for both CRF and VP steadily increased in number and in stainability. By contrast, Enk-8 immunoreactivity in the axonal terminals and cell bodies was not affected by adrenalectomy. These findings suggest that although the three peptides could be released simultaneously from the axonal terminals, VP may play some special role in the expression of CRF activity.     

Immunohistochemical Distribution and Subcellular Localization of the Somatostatin Receptor Subtype 1 (sst1) in the Rat Hypothalamus

The aim of the present study was to examine the cellular and sub-cellular distribution of the somatostatin (SRIF) receptor subtype sst1 in the rat hypothalamus. Receptors were immunolabeled using an antibody directed against an antigenic sequence in the N-terminus of the receptor. Immunopositive neuronal cell bodies and dendrites were observed throughout the mediobasal hypothalamus, including the medial preoptic area, paraventricular, periventricular, and arcuate nuclei. Immunoreactive axons and axon terminals were also observed in the median eminence, suggesting that sst1 is also located pre-synaptically. Electron microscopic examination of the arcuate nucleus revealed a predominant association of immunoreactive sst1 with perikarya and dendrites. Most immunoreactive receptors were intracellular and localized to tubulovesicular compartments and organelles such as the Golgi apparatus, but 14% were associated with the plasma membrane. Of the latter, 47% were apposed to abbuting afferent axon terminals and 20% localized immediately adjacent to an active synaptic zone. These results demonstrate a widespread distribution of sst1 receptors in rat hypothalamus. They also show that somatodendritic sst1 receptors in the arcuate nucleus are ideally poised to mediate SRIF’s modulation of afferent synaptic inputs, including central SRIF effects on growth hormone-releasing hormone neurons documented in this area.Special Issue Dedicated to Miklós Palkovits.     

Morphological effects of somatostatin on rat somatotrophs previously activated by growth hormone-releasing factor

Summary Correlative morphological and physiological analysis was carried out in order to clarify the role of somatostatin in the inhibition of the secretion of growth hormone (GH) from somatotrophs of the rat anterior pituitary gland in vivo. Transmission electron microscopy combined with immunogold labelling showed an increased number of exocytotic GH-containing secretory granules in somatotrophs fixed between 2 and 10 min after injection of GH-releasing factor (GRF). Injection of GRF also induced the appearance of immunopositive material in cisternae of the Golgi apparatus, many coated vesicles and multivesicular bodies. Microtubules were observed more frequently throughout the cytoplasm, particularly in and near the Golgi region. At 2 and 10 min after injection of somatostatin (SRIF), both the number of exocytotic figures in the somatotrophs previously stimulated by GRF and the amount of radioimmunoassayable GH in the plasma were clearly decreased. Undulation of the plasma membrane (PM) induced by GRF rapidly disappeared, and the number of granules just beneath the plasma membrane was significantly reduced. After injection of SRIF, parallel bundles of microfilaments were often observed in the space between the granules and the plasma membrane. SRIF did not cause a noticeable decrease in the amount of immunopositive material, coated vesicles and multivesicular bodies in the Golgi areas or any significant changes in the distribution of microtubules. SRIF therefore appears to inhibit hormone release mainly at the level of the plasma membrane, probably through changes in the distribution of microfilaments.     

Thyrotropin-releasing hormone (TRH)-immunoreactive structures in the brain of the domestic mallard

Summary The distribution of immunoreactive thyrotropin-releasing hormone (TRH) in the central nervous system of the domestic mallard was studied by means of the peroxidase-antiperoxidase technique. After colchicine pretreatment, the highest number of TRH-immunoreactive perikarya was found in the parvocellular subdivision of the paraventricular nucleus and in the preoptic region; a smaller number of immunostained perikarya was observed in the lateral hypothalamic area and in the posterior medial hypothalamic nucleus. TRH-immunoreactive nerve fibers were detected throughout the hypothalamus, forming a dense network in the periventricular area, paraventricular nucleus, preoptic-suprachiasmatic region, and baso-lateral hypothalamic area. TRH-containing nerve fibers and terminals occurred in the organon vasculosum of the lamina terminalis and in the external zone of the median eminence in juxtaposition with hypophyseal portal vessels. Scattered fibers were also seen in the internal zone of the median eminence and in the rostral portion of the neural lobe. Numerous TRH-immunoreactive fibers were detected in extra-hypothalamic brain regions: the highest number of immunoreactive nerve fibers was found in the lateral septum, nucleus accumbens, olfactory tubercle, and parolfactory lobe. Moderate numbers of fibers were located in the basal forebrain, dorsomedial thalamic nuclei, hippocampus, interpeduncular nucleus, and the central gray of the mesencephalon. The present findings suggest that TRH may be involved in hypophysiotropic regulatory mechanisms and, in addition, may also act as neuromodulator or neurotransmitter in other regions of the avian brain.     

Light and electron microscopic observation on the appearance of immunoreactive LHRH in perinatal rat hypothalamus

Summary The appearance and localization of LHRH were studied in the developing hypothalamus of perinatal rats using the unlabelled antibody method. By light microscopy, immunoreactive LHRH was first detected as brown dots on day 18.5 of gestation in the OVLT and on day 19.5 in the median eminence, respectively. When the median eminence was examined by the preembedding immunohistochemistry technique for electron microscopy, the occurrence of immunoreactive LHRH fibers could be demonstrated on day 18.5. These fibers were thin and very occasionally encountered near the surface of the lateral regions of the median eminence. The axoplasm contained a few immunopositive secretory granules and also extragranular immunoreactive products. With development, a gradual increase was noted both in number and size of nerve fibers with a concomitant accumulation of secretory granules within the axoplasm.A possible physiological significance of LHRH is discussed in relation to the onset of hypothalamo-hypophysial system in fetal life.     

Ontogeny of luteinizing hormone releasing hormone (LHRH) and somatostatin (SRIF) in the hypothalamus of the sheep

Using the immunoperoxidase method, luteinizing hormone releasing hormone (LHRH) and somatostatin (SRIF) were demonstrated in the hypothalamus of fetal sheep. Both hormones were found in the perikarya at about day 60 of fetal life, i.e., at the end of the first half of pregnancy. Immunoreactive LHRH (irLHRH) perikarya were situated in the vicinity of the organum vasculosum of the lamina terminalis (OVLT), i.e., in the medial preoptic nucleus and in the nucleus of the diagonal band of Broca. They were scattered and generally sparse in these areas. In the earliest stages of fetal life (60, 75, 90 days of gestation) irSRIF perikarya grouped in the ventromedial nucleus and in the lateral preoptic nucleus, were very numerous. In the oldest fetuses (120 and 135 days of gestation) they had disappeared from these nuclei but could be found in some extrahypothalamic regions--the amygdala, septo-olfactory area and sometimes in the anterior periventricular zone of the hypothalamus. Neither irLHRH nor irSRIF material were stored in the nerve terminals of the external layer of the median eminence (ME) before day 75 of gestation. In all developmental stages examined, irLHRH material in the ME was very scarce whereas irSRIF material very aboundant.     

Localization of hypophysiotropic neurohormones by assay of sections from various brain areas.

The results of studies of the localization of the hypothalamic hypophysiotropic factors based on their direct determination in sections or nuclear punches are described. Luteinizing hormone-releasing hormone was found in high concentrations in the median eminence-arcuate nucleus complex, in lower concentrations in the mediobasal zone of the preoptic area. In addition to these hypothalamic sites, it is present in all four periventricular organs, especially in the organum vasculosum laminae terminalis. Thyrotropin releasing hormone has a widespread distribution. High concentrations are in the median eminence, arcuate nucleus, dorsomedial nucleus, and anterior part of the ventromedial nucleus. Lower concentrations are in several other structures of the hypothalamus, preoptic area and septum, and low but measurable quantities are found in most of the structures of the brain. Somatostatin is also present in most structures of the central nervous system, with highest concentrations in the median eminence, arcuate nucleus, ventromedial nucleus and periventricular nucleus. There are indications that the ventromedial nucleus or its immediate vicinity contains growth hormone releasing factor. Prolactin releasing activity was present in the median eminence and mediobasal parts of the anterior hypothalamus, whereas prolactin inhibitory activity was in the dorsolateral parts of the anterior hypothalamus and/or preoptic area.     

Decreased expression of the mRNA for somatostatin in the periventricular nucleus of depression-model rats.

Expression of the mRNA for somatostatin (SRIF) in the periventricular nucleus (PeN), the level of SRIF in the stalk-median eminence (SME) and the concentration of growth hormone (GH) in the plasma were examined in depression-model rats in an attempt to confirm the hypothesis that SRIF neurons in the hypothalamus are hypofunctional in this model. We exposed male Wistar rats to intermittent walking stress for two weeks and then we measured their spontaneous running activity for 12 days. We divided the rats into a depression-model group and a partial-recovery group according to the spontaneous running activity of each rat after the termination of exposure to stress. Expression of SRIF mRNA in the PeN of the hypothalamus was monitored by in situ hybridization and relative levels were determined with an image analysis system. The relative level of expression of SRIF mRNA in the PeN was lower in rats in the depression-model group than in the control group and the partial-recovery group. The level of SRIF in the SME was lower and the plasma concentration of GH was higher in the depression-model group than in the other groups. Our findings suggest that reduced expression of mRNA for SRIF in the PeN might be associated with the pathophysiology of rats with this particular model of depression.     

Ontogenetic development of thyrotropin-releasing hormone (TRH)-immunoreactive structures in the brain of the mallard embryo

Summary Developmental changes of thyrotropin-releasing hormone (TRH)-immunoreactive structures in the brain of mallard embryos were studied by means of immunocytochemistry (PAP technique). The primary antibody was generated against synthetic TRH. Immunoreactive neurons were first detected in the hypothalamus of 14-day-old embryos. By day 20, increasing numbers of immunoreactive perikarya were observed in the paraventricular nucleus, anterior preoptic region and supraoptic region. Immunoreactive fiber projections were seen in the median eminence as early as embryonic day 20; they occurred also in some extrahypothalamic regions (lateral septum, accumbens nucleus). The number and staining intensity of the cell bodies increased up to hatching, and continued to increase during the first week after hatching.     

Changes of beta-endorphin and somatostatin concentrations in different hypothalamic areas of female rats after chronic starvation

To study the effect of starvation on hypothalamic beta-endorphin and somatostatin (SRIF) concentrations in relation to starvation induced anestrus, groups of 8 rats were fed 50% of their normal daily chow consumption. Rats were sacrificed after 4, 8, 12, and 16 days during diestrus or anestrus. beta-endorphin concentrations decreased in the preoptic suprachiasmatic area (0.52 +/- 0.13 vs 0.21 +/- 0.05 ng/mg tissue wet weight) and increased in the posterior hypothalamus (0.31 +/- 0.06 vs 0.57 +/- 0.11 ng/mg) after 4 days of starvation. No significant change occurred in the arcuate nucleus or in the median eminence. On day 8 and 12 of starvation, beta-endorphin was unaltered in all areas compared to controls. Vaginal smears showed constant diestrus in a significant number of rats (5 out of 8) after 12 days. beta-endorphin concentrations in the arcuate nuclei of these rats were significantly reduced on day 16 (1.00 +/- 0.33 vs 0.30 +/- 0.11 ng/mg). The SRIF levels changed only in the median eminence with increased concentrations on day 12 (45.2 +/- 8.4 vs 79.5 +/- 14.8 ng/mg). At this time serum levels of luteinizing hormone (LH), prolactin (PRL), and growth hormone (GH) were significantly reduced. The results indicate that changes in hypothalamic beta-endorphin accompany the events leading to starvation induced anestrus.     

Neuroanatomical connections between corticotropin-releasing factor (CRF) and somatostatin (SRIF) nerve endings and thyrotropin-releasing hormone (TRH) neurons in the paraventricular nucleus of rat hypothalamus.

In order to investigate the possible involvement of corticotropin-releasing factor (CRF) and somatostatin (SRIF) on thyrotropin-releasing hormone (TRH) neuronal cell activity in the rat hypothalamic paraventricular nucleus, we have proceeded to the simultaneous localization of CRF or SRIF and TRH. For this purpose, we used a dual immunostaining procedure that employed antibodies to CRF and SRIF and peroxidase-labeled goat anti-rabbit IgG as a first sequence, and antibodies to a cryptic fragment (Phe178-Glu199) of pro-TRH (to label TRH neurons) and alkaline phosphatase-labeled goat anti-rabbit IgG as the second sequence. A rich innervation of the paraventricular nucleus by immunoreactive CRF and SRIF fibers was observed. A large number of CRF and SRIF nerve endings were seen intimate anatomic proximity and often appeared to surround TRH-containing cell bodies. These results strongly suggest that TRH neurons might be regulated by both CRF and SRIF. These interactions might be the neuroanatomical basis for the already observed inhibitory effects of CRF and SRIF on TRH release.     

Distribution of catecholamines in the median eminence of rats following adrenalectomy]

Distribution of adrenergic and peptidergic nerve fibers in rat median eminence was studied three weeks after bilateral adrenalectomy. Fluorescence intensity in the external zone and in some of the nerve cell-bodies proved to be increased in the nucleus arcuatus. There were many nerve fibers with a bright fluorescence in the internal zone. A great number of the peptidergic nerve fibers appeared in the external zone. Reactions in the rostral, medial and caudal regions of the median eminence differed and were described.     

A histochemical study of the primary catecholamines in the hypothalamic neurons of the rat in relation to the ontogenetic and sexual differentiation

Summary The alterations in the content of the primary catecholamines in the hypothalamus have been studied with the histochemical technique of para-formaldehyde induced fluorescence.In the adult normal rats, independent of the sex, the fluorescence is located in the cell bodies of a few arcuate neurons, around the perikarya of the arcuate, para-ventricular and supra-optic neurons, and in the nerve endings of the arcuate neurons in the median eminence.The appearance of the primary catecholamines takes place at the 20th day of gestation in the para-ventricular and arcuate-peri-ventricular regions. In the supra-optic nucleus the fluorescent nerve terminals are not seen before birth. In the outer layer of the median eminence the fluorescence develops around the 5th post-natal day. No sexual differences were observed in the maturation of the primary catecholamines during the ontogenic development of the rat.More fluorescent cell bodies and nerve endings are seen in the arcuate neurons during the late diestrus than during estrus. The number and intensity of the catecholamine fluorescent neurons in the arcuate nucleus increases during the pregnancy. Castration increases slightly the number and intensity of the fluorescent cell bodies in the arcuate nucleus, but it diminishes the fluorescence in the median eminence. The changes were compensated by a treatment with testosterone propionate. Hypophysectomy alone has no effect on the fluorescence of the hypothalamic neurons.Supported by a grant from The Finnish Medical Society Duodecim.     

Localization of melanin-concentrating hormone-like immunoreactivity in the brain and pituitary of the frog Rana ridibunda

The distribution of melanin-concentrating hormone (MCH) in the central nervous system of the frog Rana ridibunda was determined by the indirect immunofluorescence technique using antibodies against synthetic salmon MCH, generated in rabbits. The most prominent group of MCH-like containing perikarya was detected in the preoptic nucleus. Comparatively, a moderate number of cell bodies was observed in the dorsal infundibular nucleus and in the ventral thalamic area. Brightly immunofluorescent nerve bundles were found in the preoptic nucleus and in the ventral infundibular nucleus, coursing towards the internal zone of the median eminence and the pituitary stalk. An intense network of immunofluorescent fibers was localized in the neural lobe of the pituitary. The subcellular localization of MCH-like material was studied in the neurohypophysis using the immunogold technique. It was demonstrated that MCH-like material was contained in dense core vesicles (80–90 mm in diameter) within specific nerve terminals. The present findings indicate that, in amphibians, MCH-like peptide is located in specific hypothalamic neurons. Our data suggest that MCH may be released by neurohypophyseal nerve endings as a typical neurohormone.     

Neuronal systems immunologically related to the somatostatin system in the garden dormouse

Cells and fibers containing somatostain (SRIF) or SRIF-like peptides were detected immunocytochemically in the brain of the garden dormouse (Eliomys quercinus L.). The periventricular preoptic nucleus and the paraventricular nucleus encompass a component of the SRIF-immunoreactive hypothalamo-infundibular and hypothalamo-neurohypophyseal systems. The suprachiastmatic, ventromedial and arcuate nuclei contain a number of SRIF-positive cells and receive a rich SRIF innervation. The extrahypothalamic systems containing SRIF can be subdivided into the following groups: (1) Afferents, the cellular origin of which is not always clearly evident, i.e., fibers of the septum, the lateral preoptic area, the thalamus, the superior olivary nucleus, the mesencephalic gray matter, and the subfornical organ; (2) dispersed cells with short projections (neocortex, caudate nucleus, putamen); (3) scattered cells with short projections (nucleus accumbens) or innervating remote territories (nucleaus interpeduncularis); (4) vascular organ of the lamina terminalis, a neurohemal area comparable to the median eminence. These observations lead to a theory of a functional bipotentiality of the somatostatin molecule. Immunocytological results depend on the antisera employed, the type of fixation and the experimental conditions. Adrenalectomy is followed by an accumulation of immunoreactive material in all SRIF-containing systems of the brain. These results clearly indicate the SRIF participates in the function of the CRF-ACTH-adrenal axis. The endocrine disturbance induced by adrenalectomy appears to modify the activity of both the neurohormonal and neuromodulator components of the SRIF system.     

Localization and development of chromogranin A and luteinizing hormone immunoreactivities in the secretory-specific cells of the hypophyseal pars tuberalis of the chicken

 The pars tuberalis mainly consists of the secretory cells specific to this portion of the pituitary. We examined the localization and development of luteinizing hormone (LH) and chromogranin A in the chicken pars tuberalis by immunohistochemistry. The vast majority of the chicken pars tuberalis was occupied by cells immunoreactive for both LH and chromogranin A. Furthermore, immunoblot analysis of chicken pars tuberalis extracts with LH antiserum demonstrated that two bands, the large α-subunit and small β-subunit of the LH molecule, were expressed in this tissue as well as in the pars distalis. A band for chromogranin A was also detected in pars tuberalis extracts with chromogranin A antiserum. In contrast to the cells of mammalian species that contain only a few small secretory granules, the specific cells of the chicken pars tuberalis were characterized by the presence of many secretory granules ranging from 90 to 400 nm in diameter. Postembedding immunogold labeling showed that gold particles representing immunoreactivity for LH were densely located on all secretory granules of the secretory-specific cells. Many secretory granules, especially the large ones, of the cells were also loaded with immunogold particles for chromogranin A. Double immunogold labeling confirmed that LH and chromogranin A were colocalized on the same secretory granules. During embryonic development, the primordium of the pars tuberalis was first detected at 8 days of incubation as a small group of cells containing LH- and chromogranin-immunoreactive cells. In the pars distalis, the onset of LH and chromogranin expression occurred earlier, at 6 days of incubation. At 10 days of incubation, the pars tuberalis primordium became large cell masses consisting of LH- and chromogranin-immunoreactive cells, which were located close to the median eminence. Subsequently, the primordium extended along the median eminence progressively with age. At 14 days of incubation, it reached to the rostral end and surrounded the median eminence as slender cell cords. These results indicate that specific cells of the chicken pars tuberalis synthesize a glycoprotein hormone related to the LH molecule, which is stored in the secretory granules together with chromogranin A. The pars tuberalis may be involved in the regulation of gonadal function in a different way from that of the pars distalis. Accepted: 26 August 1997