Organization of tyrosine hydroxylase-immunoreactive neurons in the di-and mesencephalon of the American bullfrog (Rana catesbeiana) during metamorphosis

Summary We examined the immunocytochemical distribution of tyrosine hydroxylase, the rate-limiting enzyme in catecholamine synthesis, in the di-and mesencephalon of developing bullfrog tadpoles. Special attention was given to catecholaminergic innervation of the median eminence and pituitary. In premetamorphic tadpoles, tyrosine hydroxylase-immunoreactive neurons were visualized in the suprachiasmatic and infundibular hypothalamus, the ventral thalamus, and midbrain tegmentum by Taylor-Kollros stage V. The number of labeled neurons in all these areas increased as metamorphosis progressed. By mid-prometamorphosis, labeled neurons appeared in the preoptic recess organ as well as in the posterior thalamic nucleus. The majority of cells in the preoptic recess organ, as well as occasional neurons in the suprachiasmatic nucleus, exhibited labeled processes which projected through the ependymal lining of the preoptic recess to contact cerebrospinal fluid. The modified CSF-contacting neurons of the nucleus of the periventricular organ were devoid of specific staining. By late prometamorphosis, labeled fibers from the suprachiasmatic nucleus were observed projecting caudally to enter the hypothalamo-hypophysial-tract en route to innervating the median eminence and pituitary. Labeled fibers arising from the dorsal infundibular nucleus projected ventrolaterally to contribute to catecholaminergic innervation of the median eminence and pituitary. Immunoperoxidase staining of tyrosine hydroxylase-immunoreactive fibers and terminal arborizations in the median eminence were restricted to non-ependymal layers, while labeled fibers in the pituitary were observed in the pars intermedia and pars nervosa. Staining of tyrosine hydroxylase-immunoreactive fibers in the median eminence and pituitary was sparse or absent in premetamorphic tadpoles, but became increasingly more intense as metamorphosis progressed.     

Sauvagine-like and corticotropin-releasing factor-like immunoreactivity in the brain of the bullfrog (Rana catesbeiana)

Summary Immunocytochemical methods were used to investigate the occurrence and distribution of sauvagine, corticotropin-releasing factor-, or urotensin I-like immunoreactivities (SVG-ir, CRF-ir, UI-ir, respectively) in the bullfrog (Rana catesbeiana) brain, using specific antisera raised against non-conjugated SVG, ovine CRF, rat/human CRF, and UI. In the hypothalamus, SVG-ir was found in the magnocellular perikarya, in the dorsal and ventral regions of the preoptic nucleus, and in the hypothalamo-hypophyseal projections to the external zone as well as the internal zone of the median eminence, to pars nervosa, and in fibres running from the pars nervosa to the pars intermedia of the pituitary. In contrast, CRF-ir was found only in parvocellular perikarya, mainly localized in the rostro-ventral part of the preoptic nucleus, with fine processes protruding through the ependyma of the third ventricle, fibre projections terminating in the anterior preoptic area and in the neuropil of the periventricular gray, and a caudal projection to the external zone of the median eminence. No CRF-ir staining was seen in the pars nervosa and pars intermedia. The use of UI-specific antisera failed to give a positive response in the frog brain. It is concluded that, in the frog brain, two anatomically different CRF-like (or SVG-like) systems co-exist, comparable to the reported co-existence of UI-ir and CRF-ir neuronal systems in fish brain.     

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     

Mitotic Figures in the Median Eminence of the Hypothalamus

The median eminence of the hypothalamus is part of the avenue by which neurosecreted hormones from the hypothalamic nuclei reach the pars nervosa (neural lobe) of the pituitary and eventually the bloodstream. Lithium treatment and osmotic stress increases the transport of neurosecretory hormones to the pituitary in the adult rat. Specialized astrocytes termed pituicytes in the pars nervosa of the pituitary participate in the secretory process and also develop considerable mitotic activity. The present work reveals similar mitotic figures in cells within the median eminence following 3 days of lithium treatment. The location and appearance of these mitoses add to the evidence that pituicytes are present in the median eminence. Moreover, mitoses occur within the ependymal (tanycyte) layer of the median eminence. Thus, the present results suggest that the tanycyte layer may contain pituicytes, indicating that the hypothalamus possesses specialized cells for modulating neurosecretion in response to osmotic challenges.     

Medianosomes as integrative units in the external layer of the median eminence. Studies on grf/catecholamine and somatostatin/catecholamine interactions in the hypothalamus of the male rat

Somatostatin/catecholamine as well as growth hormone releasing factor/catecholamine interactions have been characterized in the hypothalamus and the preoptic area using morphometrical and quantitative histofluorimetrical analyses.

• 1.(1) The morphometrical analysis of adjacent coronal sections of the rat median eminence demonstrated a marked overlap of somatostatin and tyrosine hydroxylase immunoreactive nerve terminals as well as of growth hormone releasing factor and tyrosine hydroxylase immunoreactive nerve terminals in the medial and lateral palisade zones of the rostral and central parts. Furthermore, the studies on codistribution of growth hormone releasing factor and tyrosine hydroxylase immunoreactivity indicate that only a limited proportion of the growth hormone releasing factor and the dopamine nerve terminals may costore dopamine and growth hormone releasing factor respectively in the medial and lateral palisade zones (see Meister et al., 1985).
• 2.(2) Intravenous injections of somatostatin 1–14 (100 μg/kg, 2 h) into the hypophysectomized male rat produced an increase in dopamine utilization in the medial and lateral palisade zones of the median eminence.
• 3.(3) Intravenous injections of rat hypothalamic growth hormone releasing factor (80 μg/kg, 2 h) in the hypophysectomized male rat did not change dopamine utilization in the median eminence but increased noradrenaline utilization in the ventral zone of the hypothalamus and produced a depletion of noradrenaline stores in the paraventricular hypothalamic nucleus.
• 4.(4) Intravenous injections of human pancreatic growth hormone releasing factor 1–44 (80 μg/kg, 2 h) in the hypophysectomized male rat did not change dopamine utilization in the median eminence, but reduced noradrenaline utilization in the subependymal layer and increased noradrenaline utilization in the suprachiasmatic preoptic nucleus.

The combined results of the present and previous studies have led us to put forward the medianosome concept. The medianosome is defined as an integrative unit, which consists of well defined aggregates of transmitter identified nerve terminals interacting with one another in the external layer of the median eminence. Our present data indicate the existence of putative medianosomes consisting predominantly of growth hormone releasing factor nerve terminals costoring dopamine as well as of somatostatin and dopamine nerve terminals, which interact locally to control growth hormone secretion. A complementary control of growth hormone secretion may be exerted by noradrenaline mechanisms in the subependymal layer, in the ventral zone and/or in the suprachiasmatic preoptic nucleus. However, further analyses in view of the differential effects seen with the present doses of rat hypothalamic and human pancreatic growth hormone releasing factor have to be done. The results also indicate the possible existence of growth hormone releasing factor receptors in the median eminence which may participate in the feedback control of the growth hormone releasing factor immunoreactive neurons in the ventral zone of the hypothalamus.     

Access of dopamine to the median eminence and brain throughout local vascular pathways in sheep

In female sheep, estradiol-dependent dopaminergic inhibition exerted by the A15 nucleus during long days (LD) results in a blockade of reproductive activity. This effect could involve the GnRH cell bodies or their terminals in the median eminence (ME). However, a vast majority of terminals of the A15 nucleus are located in neurohypophysis and only a few in the ME. Previously we demonstrated that tritiated dopamine (DA) was transferred from the venous blood of the cavernous sinus to the arterial blood supplying the brain. In the present paper, we tested the hypothesis that the transferred dopamine could reach further the brain and ME. Using isolated sheep heads harvested on short days vs. long days, we examined radioactivity in brain tissues following infusion of tritiated dopamine into the cavernous sinus. The experiment was performed in ovariectomized ewes treated with estradiol (E2) or vehicle. The mean level of radioactivity in brain was affected by season (p<0.001) and E2 (p<0.05) and was the highest during LD in E2-treated animals. In the next experiment on isolated sheep head we measured dopamine and its metabolites levels in blood and pituitary after infusion of non-radiolabeled dopamine. We observed an increase (p<0.01) in dopamine concentration in arterial blood but not in the brain. The pituitary was the only structure examined in which a tendency (p=0.06) towards increased dopamine concentration following dopamine infusion was observed. Thus, even if part of DA released from terminals within the posterior and intermediate lobes of the pituitary reaches the vessels of the ME through local vascular pathways, it is unlikely that it could affect the LHRH terminals located in ME. In addition, our results suggest that brain capillaries in the isolated head are able to maintain a functional blood brain barrier.     

Galanin: presence and distribution in the brain and pituitary of Rhinella arenarum (Amphibia: Anura) during development

The immunohistochemical distribution of galanin (Gal) in the brain and pituitary of Rhinella arenarum was studied during development. Gal-immunoreactivity was first observed in the brain just after hatching in anterior preoptic area, infundibular area, median eminence and pars distalis of the pituitary as well as in the olfactory epithelium. At the beginning of prometamorphosis new Gal-immunoreactive (ir) cells were observed in the olfactory nerve and bulb. Later in prometamorphosis new Gal-ir cells were observed in the telencephalon, suprachiasmatic nucleus, rostral rhombencephalon and in the pars nervosa of the pituitary. The most numerous accumulations of Gal-ir neurons throughout the larval development were observed in the ventral hyphothalamus where numerous Gal-ir cells of cerebrospinal fluid-contacting type were found. During metamorphic climax and soon after we did not detect Gal-ir neurons in the pallium, medial or pretectal dorsal thalamus. In the median eminence and pars distalis of the pituitary many Gal-ir fibers were found during development indicating that Gal may play a role in the modulation of hypophyseal secretion. Furthermore, the distribution of Gal-ir elements observed throughout larvae development indicates that galaninergic system maturation continues until sexual maturity.     

Layer-specific innervation of the dopamine-deficient frontal cortex in weaver mutant mice by grafted mesencephalic dopaminergic neurones

Summary The dopamine innervation of the frontal cortex originates in the A9 and A10 mesencephalic dopamine cell groups. In weaver mutant mice, there is a 77% frontocortical dopamine deficiency associated with losses of dopamine neurones in areas A9 and A10. The dopamine-depleted cortical areas of weaver mutant mice are receptive to reinnervation by afferent fibres originating in dopamine-containing mesencephalic grafts from normal donor embryos. In the anteromedial frontal lobe, reinnervation by tyrosine hydroxylase immunoreactive fibres is largely confined to the basal cortical layers whereas in the anterior cingulate cortex, tyrosine hydroxylase immunoreactive fibres also occupy superficial layers, including the molecular layer. Normally, the dopaminergic innervation of the anteromedial frontal lobe is distributed among the basal cortical layers (IV–VI), and the dopaminergic innervation of the cingulate cortex occupies both basal and superficial cortical layers. The pattern of innervation following transplantation indicates that, in repopulating dopamine-deficient cortical areas of recipient weaver mutants, graft-derived dopamine fibres show a preference for those layers which are normally invested by dopamine afferents.     

Ultrastructural study of the uptake of peroxidase by the rat median eminence

An active role of the ependymal cells (tanycytes) of the median eminence in the transport of hypothalamic hormones has been recently suggested. In order to investigate the fate of material present in the cerebrospinal fluid, a protein tracer, horse-radish peroxidase (HRP) was injected into the left lateral ventricle of rats. Two minutes after the injection, HRP had largely diffused between tanycytes and hypendymal cells. As soon as 5 min after the injection, HRP had completely penetrated all the layers of the median eminence. A few labelled vesicles and lysosomes were occasionally seen in ependymal and glial cells. At longer time intervals (20 min, 1 and 4 hrs), a reaction was observed in the lumen of fenestrated capillaries of the pituitary portal plexus. In many nerve endings of the external zone, vesicles and lysosomes were seen to contain HRP. An interesting observation was the localization of HRP between nerve endings and cells in both the pars nervosa and the pars intermedia of the pituitary gland. No reaction was recorded in the anterior pituitary and the kidney. Seventeen hours after the injection, the extracellular space was free of reaction but a few positive intracellular structure were still found. These results clearly indicate that some material from the third ventricle can rapidly diffuse between cells and axons of the median eminence to reach the fenestrated capillaries of the pituitary portal plexus and the posterior pituitary without involving an active transport by tanycytes.     

Localization and pharmacology of some dopamine receptor complexes in the striatum and the pituitary gland: synaptic and son-synaptic communication

The striatum receives massive dopaminergic projections from neurons in the ventral tegmental area, the substantia nigra and the retro-rubral cell group. Dopaminergic neurons in the arcuate nucleus and periventricular hypothalamic nuclei project to the median eminence and the neuro-intermediate lobe of the pituitary gland. The anterior lobe of the pituitary gland is not innervated by dopaminergic neurons, but receives dopamine via a vascular route from the median eminence. Two categories of dopamine receptors (D-1 and D-2) can be identified on the basis of the ability of various drugs to discriminate between these two entities. Dopamine stimulates both D-1 and D-2 receptors. The affinity of dopamine for the D-2 receptor is approximately 1000 times higher than for the D-1 receptor. Dopamine is involved in synaptic as well as non-synaptic communication. Examples of non-synaptic communication via D-2 receptors are the dopamine induced inhibition of prolactin release from the anterior pituitary gland and most likely the D-2 receptor mediated inhibition of the release of acetylcholine in the striatum. Examples of synaptic communication have been found in the striatum where (with ultrastructural techniques) synaptic contacts between dopaminergic nerve terminals and elements from cells containing GABA, substance P or enkephalin have been demonstrated. It is tempting to speculate that synaptic and non-synaptic communication occurs via D-1 and D-2 receptors respectively.     

Existence of noradrenalin cells and serotonin cells in the pituitary gland of Rana catesbeiana

The localization of catecholamine-synthesizing enzymes [tyrosine hydroxylase (TH), dopamine-beta-hydroxylase (DBH), and phenylethanolamine-N-methyltransferase (PNMT)], of serotonin (5-HT), and adrenocorticotropin (ACTH) in the pituitary of bullfrog (Rana catesbeiana), rat, hamster, and dog was examined by the immunofluorescence method. Many TH- and DBH-positive but PNMT-negative noradrenalin cells and 5-HT-positive serotonin cells were first observed in the pars distalis of the frog pituitary together with small numbers of ACTH-positive cells, in marked contrast to our previous findings that TH-positive but DBH-negative dopamine cells are rare in the anterior lobe of rat and dog pituitary. The entire population of cells of the pars intermedia showed a weak ACTH-like immunoreaction. Although most of these cells were TH- or 5-HT-negative, TH-positive but DBH-negative dopaminergic varicose fibers surrounded these cells. Among cells of the pars intermedia of the frog, 5-HT-positive cells with processes were also scattered. In the neural lobe, TH- and 5-HT-immunoreactive fibers were rarely seen.     

The hypophyseal pars tuberalis is enriched with distinct phosphotyrosine-containing proteins not detected in other areas of the brain and pituitary

The regulation of cell activity, growth and metabolism by a number of growth factor receptors and proto-oncogene products involves tyrosine kinase activity resulting in autophosphorylation of the receptors and production of phosphorylated tyrosine-containing protein substrates. The identification and precise localization of phosphotyrosine (PY)-containing proteins are first steps in elucidating the functional role of tyrosine kinases in the modulation of the central nervous system and related areas. In the present report, we describe PY-containing proteins in the median eminence and adjacent pars tuberalis of the rat adenohypophysis by immunocytochemistry using light and electron microscopy, and by Western blotting analysis. PY-immunoreactivity was found to be most intense throughout the cytoplasm of a population of epithelial pars tuberalis cells. Polyacrylamide gel electrophoresis and Western blotting of tissue extracts from various brain and pituitary regions demonstrated a general pattern of 4 major bands of PY-proteins, with an additional dense band representing a 44 kDa protein that was highly phosphorylated on tyrosines and that was exclusively found in the pars tuberalis. Additional investigation for the presence of insulin receptors, a tyrosine kinase previously correlated with the distribution of PY-proteins, demonstrated a receptor localization in axons and nerve terminals in the external and internal zone of the median eminence. However, the large amount of different PY-proteins present in the secretory cell population of the pars tuberalis could not be attributed to the insulin receptor. Our findings demonstrate that there is a large amount of cell-specific tyrosine kinase activity in the median eminence and contacting the pars tuberalis; these may play a significant role for transduction of biological signals or metabolic regulation in the neuroendocrine region.This paper is dedicated to Professor Dr. Leonhardt on the occasion of his 75th birthday     

Neurotensin-induced activation of hypothalamic dopaminergic neurons is accompanied by a decrease in pituitary secretion of prolactin and alpha-melanocyte-stimulating hormone.

The effects of neurotensin on the activity of hypothalamic tuberoinfundibular and periventricular-hypophysial dopaminergic (DA) neurons, and on the secretion of pituitary hormones that are tonically regulated by these neurons (i.e. prolactin and alpha-melanocyte-stimulating hormone [alpha MSH], respectively) were examined in estrogen-primed ovariectomized rats. The activity of tuberoinfundibular and periventricular-hypophysial DA neurons was estimated by measuring concentrations of the dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the terminals of these neurons in the median eminence and intermediate lobe of the posterior pituitary, respectively. Intracerebroventricular administration of neurotensin caused a dose- and time-related increase in DOPAC concentrations in both the median eminence and intermediate lobe, and a concurrent decrease in plasma levels of prolactin and alpha MSH. These results suggest that neurotensin-induced inhibition of secretion of prolactin and alpha MSH from the pituitary may be due to the stimulatory action of this neuropeptide on the release of dopamine from tuberoinfundibular and periventricular-hypophysial neurons.     

Differential effects of morphine on the rates of dopamine turnover in the neural and intermediate lobes of the rat pituitary gland

The acute administration of morphine to male rats decreased the rate of dopamine turnover in the median eminence and in the neural lobe of the pituitary, but was without effect in the intermediate lobe of the pituitary. Pretreatment with the opiate antagonist, naltrexone, reduced the effects of morphine. These results indicate that morphine, by acting on opiate receptors, inhibits the activity of tuberoinfundibular dopaminergic neurons that terminate in the median eminence and those tuberohypophysial dopaminergic neurons that terminate in the neural lobe of the pituitary.     

Effects of the serotonin agonist, quipazine, on luteinizing hormone and prolactin release: evidence for serotonin-catecholamine interactions

The present study tested whether administration of the serotonin agonist, quipazine maleate, affects the secretion of luteinizing hormone (LH) and prolactin (PRL) and concomitantly, the activity of central noradrenergic and dopaminergic systems. Quipazine (15 mg/kg, ip) significantly reduced LH and increased PRL when administered to ovariectomized rats. Associated with these changes, the depletion of dopamine seen after synthesis inhibition with alpha-methyl tyrosine was reduced by quipazine in the caudate nucleus and median eminence, suggesting a depression of dopaminergic activity. The depletion of norepinephrine in the median eminence was unaffected. In a second experiment, quipazine (1 microM) diminished the potassium-induced release of both norepinephrine and dopamine from fragments of medial basal hypothalamus, in vitro. Release from preoptic area was unaffected. These results suggest that central serotonergic systems may interact with noradrenergic and dopaminergic systems that regulate LH and PRL secretion, respectively.     

Histochemically demonstrable monoamines in the pituitary gland and median eminence of the female rat during the postnatal development

Summary The postnatal development of formaldehyde induced fluorescence (FIF) was studied in the pituitary glands of female rats. The effects of 3,4-dihydroxyphenylalanine (L-dopa), D,L-5-hydroxytryptophan (DL-5-HTP) and dopamine (DA) treatments on the FIF were followed during the postnatal period.The appearance of specifically fluorescing monoamines into the cells of the pars intermedia occurred postnatally and the level of the adult fluorescence was reached at 4–5 weeks' age. The intensity of the fluorescence was independent on the density of the fluorescing nerves. Among the fluorescing nerves droplet fibres were regularly observed from the age of 3 weeks, which confirms the theory that these fibres are caused by toxic factors when the blood-brain barrier is not functioning.There was no change postnatally in the number of fluorescing cells in the pars distalis.The fluorescing innervation of the median eminence, developed most rapidly at the age of 1–3 weeks and the level of the adult fluorescence was reached at the age of 4–5 weeks.The first specifically fluorescing cells after L-dopa treatment were observed at 6 days age. A remarkable increase in the number of fluorescing cells was seen between 12 and 18 days. After DL-5-HTP treatment fluorescent cells were seen but at later stages. These observations suggest that the cells in the pituitary gland, which store amine-precursors and monoamines developmentally differ from the APUD-cells. The rapid increase of the fluorescing cells between 12 and 18 days and the simultaneous development of the fluorescing innervation of the median eminence suggest the following correlations: the development of dopaminergic innervation of the median eminence — the secretion of releasing hormones — the activity of PAS-positive cells (FSH, LH and TSH secretion) — the uptake of L-dopa and DL-5-HTP into the PAS-positive cells.Dopamine was not uptaken into the cells of pars distalis. The walls of the blood vessels began to show fluorescence suggesting a barrier mechanism, which prevents the DA-uptake into the PAS-positive cells.This work was supported by the Grant for Young Research Workers, University of Helsinki.     

Microfluorimetric quantitation of catecholamine fluorescence in rat median eminence. II. Turnover changes in hormonal states.

Catecholamine nerve terminals in the rat median eminence have been studied using the fluorescence histochemical technique of Falck and Hillarp in combination with quantitative microfluorimetry. The catecholamine fluorescence intensities recorded from various parts of the median eminence were all found to be within the linear part of the dopamine or noradrenaline concentration-fluorescence relationship as studied in an agar-albumin model system. The catecholamine fluorescence was also found to disappear with time in an exponential manner following tyrosine hydroxylase inhibition produced by alpha-methyl-p-tyrosine methylester (H44/68). Similar results were obtained when measuring the dopamine decline by mass fragmentography in the median eminence after H44/68 treatment. These results and analysis of fluorescence frequency histograms strongly indicate that the catecholamine fluorescence values recorded are proportional to the catecholamine concentration. It is concluded that the microfluorimetric technique used is a reliable method for catecholamine quantitation in discrete nerve terminal areas of the median eminence. The main advantages of the technique are that a high sensitivity and quantitative data on the transmitter content can be obtained in strict relation to the neuroanatomy. Measurement of the catecholamine fluorescence disappearance after H44/68 was used to evaluate catecholamine turnover during various endocrine states. The results showed that two dopamine systems with different transmitter turnover may be distinguished. Tuberinfundibular dopamine neurons projecting to the lateral palisade zone were thus shown to have a slower turnover than those projecting medially to the capillary loops. No definite changes in catecholamine turnover were observed after adrenalectomy and castration in the male, although there was a tendency toward increased noradrenaline turnover in both states. During pregnancy an increase in noradrenaline as well as dopamine turnover was noted. The present results therefore give further evidence for the view that catecholamine nerve terminals in the median eminence may participate in the regulation of gonadotrophin secretion.     

Possible involvement of brain-derived neurotrophic factor (BDNF) in the innervation of dopaminergic neurons from the rat periventricular nucleus to the pars intermedia

Developing neurons are guided to their appropriate targets by specific guidance substances that have neurotrophic actions. The aim of the present study was to elucidate the mechanism by which hypothalamic neurons reach the pars intermedia (PI) by correlating the development of dopaminergic (DA) neurons arising in the periventricular nucleus (PeV) of fetal rats with the expression of brain-derived neurotrophic factor (BDNF) in the rat pituitary. The differentiation of DA neurons was observed by immunohistochemistry using an antibody against tyrosine hydroxylase (TH), whereas the ontogenesis of BDNF mRNA in the PI was examined by in situ hybridization and RT-PCR. Immunoreactive TH-neurons were first observed in the PeV at embryonic day (E) 16.5, following which time their axons elongated toward the pituitary. TH-positive reactions were observed in the connective tissue between the PI and the pars nervosa at E20.5. Innervation of the PI by TH-positive neurons was determined at postnatal day (P) 1.5; however, BDNF mRNA was first detected in the PI cells at E17.5, with an increase in its expression clearly visible at E21.5 and continuing high expression levels in the PI thereafter. These results suggest that BDNF is a specific guidance cue for DA neurons elongating from the PeV to the PI.     

Studies on hypothalamo-pituitary corticoliberin system. II. Corticotropin-releasing factor (CRF) and neurophysin (NP) immunoreactive neurocytes in the hamster

The aim of the study was to investigate CRF- and neurophysin-immunoreactive neurocytes in hypothalamo-pituitary system of the hamster. CRF-immunoreactive nerve fibers were observed mainly in the outer layer of the median eminence and pituitary stalk and also in the neurohypophysis. On the contrary, neither intermediate lobe nor anterior pituitary contained CRF-immunoassayable substance. The pattern of distribution of neurophysin-immunoreactive fibres was different from CRF-immunoreactive fibres as far as a median eminence, pituitary stalk and neurohypophysis are concerned. Between the tannocytes of the III ventricle and nervous fibres forming the internal layer of the median eminence a CRF- and neurophysin-immunoreactive perikaryons of neurocytes were found. Results of the study suggest regulatory function of CRF-immunoreactive neurons of the hamster hypothalamo-pituitary system in controlling of ACTH secretion. Moreover, the distribution of CRF-immunoreactive substances in hamster hypothalamo-pituitary system shows some peculiarities if compared with other rodents.