Application of avidin-ferritin and peroxidase as contrasting electron-dense markers for simultaneous electron microscopic immunocytochemical labelling of glutamic acid decarboxylase and tyrosine hydroxylase in the rat arcuate nucleus

TIDAL WAVES: Network mechanisms in the neuroendocrine control of prolactin release

Neuroendocrine tuberoinfundibular dopamine (TIDA) neurons tonically inhibit pituitary release of the hormone, prolactin. Through the powerful actions of prolactin in promoting lactation and maternal behaviour while suppressing sexual drive and fertility, TIDA neurons play a key role in reproduction. We summarize insights from recent in vitro studies into the membrane properties and network behaviour of TIDA neurons including the observations that TIDA neurons exhibit a robust oscillation that is synchronized between cells and depends on intact gap junction communication. Comparisons are made with phasic firing patterns in other neuronal populations. Modulators involved in the control of lactation - including serotonin, thyrotropin-releasing hormone and prolactin itself - have been shown to change the electrical behaviour of TIDA cells. We propose that TIDA discharge mode may play a central role in tuning the amount of dopamine delivered to the pituitary and hence circulating prolactin concentrations in different reproductive states and pathological conditions.

Luteinizing hormone-releasing hormone and gamma-aminobutyric acid neurons in the medial preoptic area are synaptic targets of dopamine axons originating in anterior periventricular areas

The aim of this study was to characterize further the transmitter content and the location of the parent cells of tyrosine hydroxylase-immunoreactive boutons terminating on luteinizing hormone-releasing hormone- and glutamic acid decarboxylase-immunoreactive neurons in the rat medial preoptic area. Electron microscopic immunostaining for luteinizing hormone-releasing hormone, tyrosine hydroxylase or glutamic acid decarboxylase was performed on desipramine-pretreated (to protect norepinephrine and epinephrine axons) rats which received a stereotaxic injection of 6-hydroxydopamine into the medial preoptic area anteroventral periventricular nucleus 48 h prior to sacrifice. This treatment induced acute degeneration of dopamine axon terminals characterized by the development of autophagous cytolysosomes, an early morphological sign of catecholamine axon degeneration. To further define the cells of origin of these dopamine boutons, the anterograde marker Phaseolus vulgaris leucoagglutinin was iontophoretically applied to the zona incerta. Six days later, rats received a 6-hydroxydopamine injection into the zona incerta or the lateral ventricle, and 48 h later, double immunostaining was performed for Phaseolus vulgaris leucoagglutinin and tyrosine hydroxylase, luteinizing hormone-releasing hormone, or glutamic acid decarboxylase on preoptic area vibratome sections. Following the 6-hydroxydopamine injection into the anteroventral periventricular nucleus, autophagous cytolysosome-containing degenerated axons were found in synaptic contact with both luteinizing hormone-releasing hormone and GABA neurons in the medial preoptic area, confirming that these are dopaminergic connections. Following the double injection treatment, 6-hydroxydopamine-induced degenerated, Phaseolus vulgaris leucoagglutinin-labeled dopamine axons originating in the zona incerta were not found to contact luteinizing hormone-releasing hormone-containing or GABA cells.(ABSTRACT TRUNCATED AT 250 WORDS)

GABAergic and catecholaminergic innervation of mediobasal hypothalamic beta-endorphin cells projecting to the medial preoptic area

In the absence of cellular estrogen receptors or proven direct estrogen action in the rat, it is assumed that estrogen indirectly regulates the secretory activity of the preoptic area luteinizing hormone-releasing hormone-producing cells. We have previously shown that pro-opiomelanocortin neurons in the arcuate nucleus of the rat send axons rostrally to connect with luteinizing hormone-releasing hormone neurons of the preoptic area. An experiment combining retrograde tracing and double-immunostaining was used to test the hypothesis that rat GABAergic and/or catecholaminergic neurons can influence luteinizing hormone-releasing hormone-producing cells via mediobasal hypothalamic beta-endorphin neurons. The retrograde tracer horseradish peroxidase was injected into the medial preoptic area; two days later, arcuate nucleus Vibratome sections were double-immunostained for beta-endorphin and glutamate decarboxylase or tyrosine hydroxylase. Light and electron microscopic analysis of these triple-labeled sections demonstrated that a population of beta-endorphin-immunoreactive neurons concentrated in the ventromedial arcuate nucleus contain retrogradely transported horseradish peroxidase granules and form synaptic contacts with glutamate decarboxylase- and tyrosine hydroxylase-immunoreactive axon terminals. The present data suggest that arcuate nucleus GABA and catecholamine fibers may influence luteinizing hormone-releasing hormone-containing neurons via projective pro-opiomelanocortin cells.

Use of ultrastructural immunohistochemistry in human pituitary pathology

Recent advances in ultrastructural immunohistochemistry have provided insight into not only the subcellular localization of single antigens but also the colocalization of two distinct antigens in the same cellular constituent. In the field of pituitary pathology, precise identification of cell types, mechanism of processing, and dynamic intracellular transportation of hormones, as well as production of multiple hormones in the same cells of nontumorous and neoplastic adenohypophyses, have been documented by use of these techniques. The present review deals with the use of major methods for ultrastructural immunohistochemistry including pre-, post-, and non-embedding methods, particularly focusing on their application to human pituitary pathology. Problems of tissue processing and a protocol for double labeling technique using the protein A-gold complex are also described.

Dual immunocytochemistry using 125I-labeled protein A: a new electron microscopic technique applied to the investigation of chemical connectivity and axonal transmitter co-localization in the brain

We have developed a double labeling immunocytochemical method utilizing peroxidase conjugated Fab fragments and 125I-labeled protein A to localize two neuronal markers on the same light or electron microscopic section with primary antibodies raised in the same animal species. The technique is applicable to the study of chemical connectivity in the brain, as illustrated by data obtained in the hypothalamus using rabbit polyclonal antisera against tyrosine hydroxylase (TH), phenylethanolamine-N-methyltransferase (PNMT), neuropeptide Y (NPY), and vasoactive intestinal peptide (VIP). Moreover, due to a high level of sensitivity and resolution, the technique offers considerable advantages over many previously developed dual labeling immunocytochemical methods for the demonstration of transmitter axonal co-localizations. Utilizing the peroxidase Fab/[125I]protein A method, we present here the first direct evidence that PNMT is present in many endings also containing NPY in the thalamic and hypothalamic paraventricular nuclei and in the arcuate nucleus. The method also may be combined as required with other labeling methods for localizing more than two neurochemical markers on one and the same electron microscopic section.

Two different types of dynorphin-A-immunoreactive terminals in rat substantia nigra

The opioid peptide dynorphin A (1-17) is the third transmitter identified in the striatonigral projection, the other two being gamma-aminobutyric acid (GABA) and substance P. The ultrastructural features of the dynorphinergic terminals in substantia nigra/pars reticulata were studied using pre-embedding immunocytochemistry with the classical peroxidase-antiperoxidase-diaminobenzidine-method; these features were compared with GABAergic boutons visualized with an immunogold method. Two distinct types of dynorphin-A-immunoreactive boutons could be identified: (1) type A (81%) possessing characteristics similar to the GABAergic nerve endings in this region, i.e., large pleomorphic vesicles and symmetric synaptic contacts; (2) type B (19%) displaying asymmetric synaptic zones and small, mostly round vesicles. These results are in agreement with physiological studies suggesting a dual action of dynorphin A in substantia nigra.

GABAergic synapses in the brain identified with antisera to GABA and its synthesizing enzyme, glutamate decarboxylase

GABA is a known inhibitory neurotransmitter in the mammalian brain. The site of GABAergic synapses can be determined with immunocytochemical methods that localize either GABA or its synthesizing enzyme, glutamate decarboxylase (GAD). In general, GABAergic axon terminals contain pleomorphic synaptic vesicles and form symmetric synapses. However, a small number of GABAergic axon terminals in selected brain regions (spinal cord, cerebellum, superior colliculus, striatum, globus pallidus, inferior olive, and substantia nigra) form asymmetric synapses. GAD- and GABA-immunoreactive processes that contain synaptic vesicles participate in every known morphological type of chemical synapse. These include axosomatic, axodendritic, axospinous, initial segment, axoaxonic, dendrodendritic, serial, reciprocal, and ribbon synapses. Although GABAergic synapses form a heterogeneous group, they most commonly form axosomatic, axodendritic, and initial segment synapses in the brain and spinal cord. These findings provide helpful guidelines for the identification of GABAergic synapses in future studies.

Organization of the septal region in the rat brain: cholinergic-GABAergic interconnections and the termination of hippocampo-septal fibers

This study deals with two characteristic cell types in the rat septal complex i.e., cholinergic and GABAergic neurons, and their synaptic connections. Cholinergic elements were labeled with a monoclonal antibody against choline acetyltransferase (ChAT), the acetylcholine synthesizing enzyme. Antiserum against glutamate decarboxylase (GAD), the GABA synthesizing enzyme, was employed to identify GABAergic perikarya and terminals, by using either the peroxidase-antiperoxidase (PAP) technique or a biotinylated second antiserum and avidinated gold or ferritin. With these contrasting immunolabels we have studied the cholinergic-GABAergic interconnections in double-labeled sections of intact septal regions and the GABAergic innervation of medial septal area cholinergic neurons in sections taken from animals 1 week following lateral septal area lesion. In other electron microscopic experiments we have studied cholinergic and GABAergic neurons in the septal complex for synaptic contacts with hippocamposeptal fibers, which were identified by anterograde degeneration following fimbria-fornix transection. Our results are summarized as follows: (1) GAD-positive terminals form synaptic contacts on ChAT-immunoreactive dendrites in the medial septum/diagonal band complex (MSDB), (2) surgical lesion of the lateral septal area resulted in a dramatic decrease of the number of GABAergic boutons on MSDB cholinergic neurons, (3) cholinergic terminals establish synaptic contacts with GAD immunoreactive cell bodies and proximal dendrites in the MSDB as well as in the lateral septum (LS), (4) degenerated terminals of hippocampo-septal fibers were mainly observed in the LS, where they formed asymmetric synaptic contacts on dendrites of GABAergic neurons and on nonimmunoreactive spines. We did not observe degenerated boutons in contact with ChAT-positive dendrites or cell bodies in the MSDB. From these results and from data in the literature we conclude that excitatory hippocampo-septal fibers activate GABAergic cells, and as yet unidentified spiny neurons in the LS, which may control the discharge of medial septal cholinergic neurons known to project back to the hippocampal formation.

Immunohistochemical evidence for synaptic connections between pro-opiomelanocortin-immunoreactive axons and LH-RH neurons in the preoptic area of the rat

Connections between adrenocorticotropic hormone (ACTH)-immunoreactive neurons in the arcuate nucleus and the preoptic area were studied in the female rat. ACTH-immunopositive terminals were observed in the medial preoptic area in contact with dendritic shafts, while in the ventrolateral preoptic area the majority of ACTH-immunoreactive synapses were found on dendritic spines. Double-label electron microscopic immunocytochemistry using peroxidase and avidin-ferritin as contrasting electron-dense markers revealed numerous synaptic contacts between ACTH-immunopositive boutons and luteinizing hormone-releasing hormone (LH-RH)-immunoreactive dendritic shafts in the medial preoptic area. Following injection of horseradish peroxidase (HRP) into the medial preoptic area, retrogradely HRP-labeled perikarya were observed throughout the arcuate nucleus. Double-staining experiments revealed that a proportion of these retrogradely labeled cells, in the ventromedial arcuate nucleus, are also immunoreactive for ACTH. These results suggest that pro-opiomelanocortin peptide-producing neurons in the ventromedial arcuate nucleus project to the medial preoptic area. Some of these neurons establish direct synaptic contacts with LH-RH-immunoreactive cells.

Catecholaminergic innervation of pyramidal and GABAergic nonpyramidal neurons in the rat hippocampus. Double label immunostaining with antibodies against tyrosine hydroxylase and glutamate decarboxylase

This study describes the catecholaminergic innervation of rat hippocampal neurons at the electron microscopic level by using an antibody against tyrosine hydroxylase (TH) and immunocytochemical techniques. In a first series of experiments, the course and distribution as well as the synaptic contacts of TH-immunoreactive fibers were analyzed with the peroxidase-antiperoxidase (PAP) method. Next, peroxidase immunostaining of TH fibers was combined with glutamate decarboxylase (GAD) immunostaining, using avidinated ferritin as a second electrondense marker. Our results demonstrate that TH-immunostained terminals establish asymmetric synaptic contacts with spines of pyramidal neurons, and symmetric synaptic contacts with cell bodies and dendritic shafts of ferritin-labeled GAD-immunoreactive nonpyramidal cells.

Light and electron microscopic study of tyrosine hydroxylase-immunoreactive neurons within the developing rat arcuate nucleus

The topography, fine structure, and patterns of connections of tyrosine hydroxylase (TH)-immunoreactive tubero-infundibular dopaminergic (TIDA) neurons were examined by light and electron microscopic immunocytochemistry in the arcuate nucleus of 2-, 15- and 30-day-old female Wistar rats. In 2-day-old animals, TH-immunoreactive perikarya were mainly located in the ventrolateral portion of the arcuate nucleus. In 15-day-old rats numerous TH-positive cell bodies were still present ventrolaterally, but a cluster of labeled cells was also apparent in the mediodorsal segment of the nucleus. In the 30-day-old rats, most TH-immunoreactive neurons were concentrated mediodorsally, as seen in the adult. At the ultrastructural level, TH-immunoreactive somata exhibited, in all age groups, a large nucleus surrounded by a thin rim of cytoplasm containing mitochondria, Golgi apparatus, endoplasmic reticulum, multivesicular bodies and lysosomes. These labeled somata were synaptically contacted by unlabeled axon terminals and often laid adjacent to either labeled or unlabeled dendrites. Similarly, in all age groups, labeled dendrites were synaptically contacted by unlabeled axon terminals and were often directly apposed to either labeled or unlabeled perikarya and dendrites, or to tanycytic processes. These results indicate that TIDA neurons establish extensive connections early in development, and that their pattern of intercellular relationships remains qualitatively unchanged from 2 days to adulthood. It is suggested that TIDA neurons may be already functional at birth, and could therefore, influence the maturation of other arcuate neuronal populations.

Immunocytochemical evidence for direct synaptic connections between corticotrophin-releasing factor (CRF) and gonadotrophin-releasing hormone (GnRH)-containing neurons in the preoptic area of the rat

Electron microscopic double-label immunostaining with peroxidase and avidin-ferritin was used to study connections between corticotrophin-releasing factor (CRF) and gonadotrophin-releasing hormone (GnRH) immunoreactive elements in the medial preoptic area of the rat. Synaptic contacts were observed between CRF-immunoreactive axon terminals and the dendrites of GnRH-immunopositive neurons. These results suggest that the inhibitory effects of stress-induced CRF release on reproductive function may involve a direct CRF input to the GnRH-producing cells.

The cellular effects of estrogens on neuroendocrine tissues

Estrogen action on sensitive neurons in the rat diencephalon has been studied by morphologic techniques; evidence of estrogen action at every level is presented, including tracts, cells, circuitry and subcellular organelles. The demonstration in the arcuate nucleus of estrogen-induced synaptic remodelling, estrogen-induced postsynaptic membrane phenotypes, changes in intracellular membranes and rapid estrogen actions on neuronal endo-exocytosis indicates that cellular estrogen actions may underlie the neuronal control of reproduction.

Cholinergic innervation of hippocampal GAD- and somatostatin-immunoreactive commissural neurons

This study describes the cholinergic innervation of chemically defined nonpyramidal neurons in the hilar region of the rat hippocampus. Cholinergic terminals were identified by immunocytochemistry employing a monoclonal antibody against choline acetyltransferase (ChAT), the acetylcholine-synthesizing enzyme, and the avidin-biotin-peroxidase (ABC) technique. Nonpyramidal neurons in the hilar region were characterized by immunostaining with antibodies against glutamate decarboxylase (GAD), the gamma aminobutyric acid (GABA)-synthesizing enzyme, and somatostatin (SS). The immunoreactivity to these antibodies was detected by using biotinylated secondary antibodies and avidinated ferritin as an electron-dense marker. This electron microscopic double immunostaining procedure enabled us to demonstrate that immunoperoxidase-labeled ChAT-immunoreactive terminals established symmetric synaptic contacts on the ferritin-labeled GAD- and SS-immunoreactive hilar cells. In additional experiments at least some of the GAD- and SS-immunoreactive hilar neurons were further characterized as commissural neurons by retrograde filling with horseradish peroxidase (HRP) following an injection of the tracer into the contralateral hilus. From these triple labeling experiments, we concluded that at least some GABAergic and somatostatin-containing neurons in the hilar region, which are postsynaptic to cholinergic terminals, project to the contralateral hippocampus. Together with previous studies on the cholinergic innervation of the hippocampus and fascia dentata, our present results thus demonstrate that different types of hippocampal cells, including GABAergic and peptidergic commissural neurons in the hilar region, receive a cholinergic input.

GABAergic input of cholecystokinin-immunoreactive neurons in the hilar region of the rat hippocampus. An electron microscopic double immunostaining study

Double immunostaining was performed for electron microscopy to analyze the synaptic connections between glutamate decarboxylase (GAD)-immunoreactive axons and cholecystokinin (CCK)-immunoreactive neurons in the hilar region of the rat hippocampal formation. Following immunostaining for CCK, the diaminobenzidine (DAB) reaction product was silver-intensified and gold-substituted. In a subsequent second immunostaining for GAD, the immunoreactive elements were labeled using a single DAB reaction. Electron microscopic analysis of the double-stained Vibratome sections demonstrated that the single DAB-labeled GAD-immunoreactive boutons form symmetrical synaptic connections on the soma and primary dendrites of the DAB-gold-labeled CCK-immunoreactive neurons.

Simultaneous detection of indoleamines and dopamine in rat dorsal raphe nuclei using specific antibodies

Using a monoclonal antibody against dopamine and a rabbit antiserum against serotonin, 5-methoxytryptamine or tryptamine, we were able to achieve the simultaneous localization of two amines in glutaraldehyde-fixed sections of rat dorsal raphe nuclei. In this staining procedure, the first antigen was localized using 3,3'-diaminobenzidine (DAB), while the second antigen was stained using the 1-naphthol basic dye (2-NBD) method. The two antigens were localized in different cells or structures. No overlap of the staining was observed, thus indicating that dopamine is not localized with serotonin, 5-methoxytryptamine or tryptamine.

GABAergic input to cholinergic forebrain neurons: an ultrastructural study using retrograde tracing of HRP and double immunolabeling

Amygdalopetal cholinergic neurons in the ventral pallidum were identified by combining choline acetyltransferase (ChAT) immunohistochemistry with retrograde tracing of horseradish peroxidase (HRP) following injections of the tracer in the basolateral amygdaloid nucleus. Although ChAT-positive terminals were identified in the ventral pallidum, they were never seen in contact with either immunonegative or ChAT-positive amygdalopetal neurons. In material, in which immunostaining against glutamic acid decarboxylase (GAD), the synthesizing enzyme for GABA was combined with retrograde tracing of HRP from the basolateral amygdaloid nucleus, GAD-positive terminals were seen to contact immunonegative amygdalopetal neurons. In addition, when sections of the rostral forebrain were processed, first to preserve and identify the transported HRP, and then were sequentially tested for both ChAT and GAD immunohistochemistry with the immunoperoxidase reaction for both tissue antigens, GAD-immunopositive terminals were seen to make synaptic contacts with cholinergic amygdalopetal neurons. These results suggest that amygdalopetal, presumably cholinergic, neurons receive GAD-positive terminals. In separate experiments using immunoperoxidase for ChAT and ferritin-avidin for GAD labeling, we confirmed the presence of GAD-containing terminals on cholinergic neurons. In addition, cholinergic terminals were seen in synaptic contact with GAD-positive cell bodies. These morphological studies suggest that direct GABAergic-cholinergic and cholinergic-GABAergic interactions take place in the rostral forebrain.

Ontogenesis of immunoreactive tyrosine hydroxylase-containing neurons in rat hypothalamus

By employing anti-tyrosine hydroxylase (TH) serum, the ontogenesis of hypothalamic dopamine (DA) neurons was immunohistochemically examined with special attention to the medial basal hypothalamic area. DA neurons first appeared in the lateral hypothalamic walls on day 13.5 of gestation and in the anterior periventricular region and arcuate nucleus on day 15.5-16.5. In the arcuate nucleus, the appearance of the neurons was confined to the ventrolateral (VL) region, but extended to the periventricular region thereafter. About day 10 postnatally, the population of the arcuate DA neurons conjoins anterodorsally with the cell population in the anterior periventricular region. Concomitant with this, DA neurons in the VL region of the nucleus diminished in number and in stainability, becoming barely visible. Interestingly enough, the latter neurons reappeared after an anterolateral deafferentation of the medial basal hypothalamus. This did not occur in pregnant and lactating rats. Although most of the arcuate DA neurons were retarded by neonatal administration of monosodium glutamate, the immunoreactive fibers remained almost intact in the medial portion of the median eminence. It is concluded that in the periventricular-arcuate complex, DA neurons seem to play different roles relating with their ontogenetic heterogeneity.

Activation of GABA receptors in the hypothalamus stimulates secretion of growth hormone and prolactin

Localized intracerebral microinjections of GABA, muscimol, picrotoxin and bicuculline were made in the anterior and basal hypothalamus to determine possible sites of action of GABA in the regulation of prolactin and growth hormone (GH) secretion. Studies were carried out in unanesthetized male rats with chronic indwelling atrial cannulae and intracerebral guide cannulae which permitted stress free blood sampling and intrahypothalamic injections, respectively. Preoptic/anterior hypothalamic area. (PO/AHA) injection of muscimol (0.16 nmol) stimulated both prolactin and GH secretion. GABA (1600 nmol) stimulated prolactin. Bicuculline (0.016 and 0.16 nmol) inhibited GH secretion. Medial basal hypothalamic (MBH) injection of muscimol (0.1 and 1.0 nmol) and GABA (1000 nmol) stimulated prolactin but had no effect on GH secretion. Picrotoxin and bicuculline did not stimulate GH. These findings indicate that activation of PO/AHA GABAergic receptors facilitates secretion of GH and prolactin and activation of MBH GABAergic receptors stimulates secretion of prolactin. It is proposed that GABA inhibits somatostatin neurons in the PO/AHA to facilitate GH and inhibits tuberoinfundibular dopamine or GABA neurons in the MBH to stimulate prolactin.

Enkephalin and neuropeptide Y: two colocalized neuropeptides are independently regulated in primary cultures of bovine chromaffin cells

We have found that Neuropeptide Y is colocalized with enkephalin in bovine adrenal chromaffin cells. The two peptides can be found in the same granules in those cells where they coexist. These cells correspond to the adrenergic subpopulation of chromaffin cells since they contain the epinephrine synthetic enzyme, phenylethanolamine N-methyltransferase. Despite their coexistence, production of the two peptides is independently regulated. Enkephalin levels are doubled after nicotinic depolarization (which increases enkephalin synthesis) or after treatment with reserpine (which increases enkephalin precursor processing). Neither of these treatments, acting by different mechanisms, has any effect on the levels of Neuropeptide Y.

Immunocytochemical evidence for synaptic regulation of paraventricular vasopressin-containing neurons by substance P

Employing a combination of pre-embedding peroxidase-antiperoxidase-labeling for substance P (SP) and postembedding immunogold labeling with protein A-colloidal gold-anti-arginine vasopressin (AVP) complex, we demonstrated immunoreactive SP containing nerve fibers, which terminate synaptically on the perikarya, contained gold-labeled secretory granules in the magnocellular paraventricular nucleus of rats. The perikarya were also synapsed with unlabeled nerve fibers. It is concluded that SP plays a role as an axosomatic neurotransmitter in diverse synaptic controls of vasopressinergic neurons.

A combined light and electron microscopic immunocytochemical method for the simultaneous localization of multiple tissue antigens. Tyrosine hydroxylase immunoreactive innervation of corticotropin releasing factor synthesizing neurons in the paraventricular nucleus of the rat

In order to study the morphological interrelationships between immunocytochemically identified neuronal systems, a double labelling procedure - suitable for correlative light and electron microscopic observations - is introduced. The technique is based on the consecutive use of the silver-gold (SG) intensified and non-intensified forms of the oxidized 3,3'-diaminobenzidine (DAB) chromogen in the framework of the peroxidase-antiperoxidase complex (PAP) indirect immunocytochemical procedure. The first tissue antigen is detected by the SG intensified DAB chromogen, which has a black color and high electron density. The structures containing the second antigen are visualized by the non-intensified DAB-endproduct, which is less electron-dense than the silver-gold amplified form and is brown. The metallic shield that forms around the labeled antibody sequences associated with the first antigen prevents non-specific binding of immunoglobulins used for the detection of the second tissue antigen. The application of this method for the simultaneous detection of tyrosine hydroxylase (TH)- and corticotropin releasing factor (CRF)-immunoreactive structures revealed that black colored TH-immunopositive fibers contacted brown colored CRF-synthesizing neurons in the hypothalamic paraventricular nucleus. The juxtaposition of TH- and CRF-containing elements was apparent in both thick vibratome (40 micron) and semithin (1 micron) sections. At the ultrastructural level, TH-positive terminals - labeled by silver-gold grains - were observed to establish asymmetric synapses with both CRF- and TH-immunoreactive neurons. The former finding indicates a direct, TH-immunopositive, catecholaminergic influence upon the hypothalamic CRF system, while the latter demonstrates the existence of intrinsic connections between TH-positive elements.

Adrenergic innervation of corticotropin releasing factor (CRF)-synthesizing neurons in the hypothalamic paraventricular nucleus of the rat. A combined light and electron microscopic immunocytochemical study

Corticotropin releasing factor (CRF), a neuropeptide synthesized in the parvocellular subnuclei of the hypothalamic paraventricular nucleus (PVN), takes part in the regulation of different stress evoked responses of the organism. In order to elucidate the role of the central adrenergic system in the regulation of these CRF-synthesizing neurons, a novel ultrastructural immunocytochemical dual localization technique was utilized. Phenylethanolamine-N-methyltransferase (PNMT), a specific enzyme marker for the central adrenaline system, and CRF-immunoreactive elements were simultaneously visualized in hypothalamic sections. PNMT-immunoreactive axon terminals established synaptic connections with somata, dendrites and spinous structures of CRF-producing neurons. This morphological finding indicates that the central adrenergic system directly influences CRF-synthesizing neurons in the PVN and provides basis for a more definitive pharmacological manipulation of this system.