Ultrastructural characteristics of vasopressin-containing neurons in the paraventricular nucleus of the hypothalamus

The AMPA glutamate receptor GluR3 is enriched in oxytocinergic magnocellular neurons and is localized at synapses

The cellular localization of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate glutamate receptor, GluR3, was identified using antibodies that recognize the N-terminus of the predicted polypeptide sequence of GluR3. Regional immunoblot analysis of monkey brain homogenates identified a protein of approximately 102,000 mol. wt that was enriched in hypothalamus. Immunocytochemistry demonstrated that GluR3 was enriched within the hypothalamic magnocellular neurosecretory nuclei and axons of the hypothalamo-neurohypophysial tract in rat and monkey. GluR3 immunoreactivity co-localized to oxytocin-containing, but not vasopressin-containing, neurons of the hypothalamic paraventricular nucleus, supraoptic nucleus and accessory magnocellular nuclei. Ultrastructurally, GluR3 immunoreactivity was enriched throughout cytoplasm of the somatodendritic compartment and was associated with postsynaptic and presynaptic structures. GluR3 immunoreactivity was frequently observed to be clustered at the plasma membrane of the somatodendritic compartment, consistent with the predicted localization of a membrane-bound ion channel. Additionally, GluR3-immunoreactive axon terminals in synaptic contact with unlabeled dendrites within the retrochiasmatic area and bed nucleus of the stria terminalis were observed, providing morphological evidence for a presynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor. By immunoblot analysis and immunocytochemistry using antibodies directed against a specific alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor in rat and monkey brain, our findings suggest a highly selective hypothalamic distribution of the GluR3 subunit that may have functional significance in the glutamatergic regulation of oxytocinergic neurons.

The hypothalamo-neurohypophysial system in streptozotocin-diabetic rats: ultrastructural and immunocytochemical evidence for alterations of oxytocin- and vasopressin-containing neuronal profiles

The hypothalamic paraventricular and supraoptic nuclei and the neurohypophysis of rats were investigated 8 weeks after streptozotocin (STZ)-induction of type 1 diabetes. Vasopressin (VP)- and oxytocin (OT)-containing neuronal profiles were examined using the pre-embedding peroxidase-antiperoxidase technique for electron microscopy. Ultrastructural alterations were observed in the somata, dendrites and axons of VP- and OT-labelled profiles. There was no evidence, however, for alterations in the synapses associated with VP- or OT-labelled somata, dendrites and axons. The results indicate that both VP- and OT-containing neuronal profiles are involved in the ultrastructural reorganisation of the hypothalamo-neurohypophysial complex during diabetic conditions. Depletion of VP- and OT-containing axon profiles in the neurohypophysis may suggest increased release of both neurohypophysial hormones in STZ-induced diabetes.

Fine structural studies of growth-hormone-releasing-factor (GRF)-immunoreactive neurons and their synaptic connections in the guinea pig arcuate nucleus

The fine structure of neurons containing human growth-hormone-releasing factor (hGRF) immunoreactivity located in the arcuate nucleus of the guinea pig was studied by means of the preembedding immunohistochemical technique. The perikaryon of labeled neurons was fusiform or ovoid; the nucleus was regular in shape and contained a prominent nucleolus. The main ultrastructural features of the hGRF-immunoreactive neurons were the presence of numerous labeled secretory granules (100-120 nm in diameter) and the abundance and the enlargement of the organelles involved in the synthesis of the peptides: a well-developed rough endoplasmic reticulum and a conspicuous Golgi apparatus. Synaptic inputs were observed on immunoreactive perikarya but, above all, on the labeled dendrites. The unstained presynaptic nerve endings most often contained only small clear vesicles and formed symmetrical contacts. In rare cases, the presynaptic terminals exhibited both small clear and large dense vesicles and constituted asymmetrical contacts. Immunoreactive nerve endings were also observed in this area: the synaptic boutons contained large, stained vesicles and small, unlabeled, clear vesicles. These axon terminals made synaptic contacts with unstained dendritic processes; the contacts were symmetrical. The results indicate that hGRF-immunoreactive neurons of the guinea pig arcuate nucleus present morphological features of neuroendocrine cells. Moreover, the presence of hGRF-labeled nerve endings in the arcuate nucleus itself suggests that a substance related to hGRF might be a neuromodulator, at least in this area.

Ultrastructural characteristics of peptidergic neurons using pre-embedding immunocytochemical methods

The immunocytochemical staining technique represents a valuable method for electron microscopic analysis with which to elucidate the ultrastructural characteristics, synaptology, and potential interactions of selectively labeled neuronal systems. The unlabeled antibody peroxidase-antiperoxidase (PAP) technique is one of the most widely used immunocytochemical methods and has been applied to both pre-embedding and postembedding EM procedures. This review provides a detailed protocol for the former method and discusses advantages and disadvantages associated with the use of both approaches. A descriptive fine structural analysis of peptidergic neurons localized in the hypothalamus of brain illustrates the applicability and specificity of utilizing this method for the localization of cellular antigens of other systems.

Electron microscopic examination of the catecholaminergic innervation of neurophysin- or vasopressin-containing neurons in the rat hypothalamus

The ultrastructural relationships between neurophysin (NP)- or vasopressin (VP)-containing neurons and catecholamine terminals in the paraventricular nucleus and supraoptic nucleus of the rat were observed by means of a technique combining immunocytochemistry using NP I/II or VP antiserum with autoradiography after [3H]noradrenaline (NA) injection or 5-hydroxydopamine (5-OHDA) uptake. NP- or VP-like immunoreactive nerve cell bodies and fibers received synaptic contacts from a large number of immunonegative axon terminals. The presynaptic elements that innervate the neurosecretory neurons were studied. Axon terminals labeled with [3H]NA or 5-OHDA made synaptic contacts with NP- or VP-like immunoreactive nerve cell bodies and fibers. Furthermore, axodendritic and/or axo/axonic and axosomatic synapses occurred between the same NP- or VP-like immunoreactive neurons. These findings suggest that at least NA- and 5-OHDA-containing neurons play some important role in the control of neurosecretion in the NP- or VP-producing neurons of the rat hypothalamus and that the axon collaterals of NP- and VP-containing neurons make synaptic contacts with the same kind of neurons to form a recurrent collateral circuit.

Relationship of CRF-immunostained cells and magnocellular neurons in the paraventricular nucleus of rat hypothalamus

The distribution of corticotropin-releasing factor (CRF), vasopressin (VP) and oxytocin (OXY) containing neurons within the magnocellular and parvocellular divisions in the paraventricular nucleus (PVN) of rat hypothalamus is described in brains from normal untreated, colchicine treated and adrenalectomized animals. Double immunostained preparations using glucose oxidase-antiglucose oxidase (GAG) complex combined with PAP complex to visualize two antigens with contrasting colors in the same tissue sections were employed. Separate and distinct populations of cells containing the immunoreactive (ir) elements were seen. Immunostained CRF neurons present in the ventral medial portion of the posterior magnocellular division were juxtaposed to oxytocin-ir perikarya in colchicine treated and adrenalectomized animals. CRF-ir cells were for the most part concentrated in the medial parvocellular component of PVN. An intimate anatomical proximity between CRF-ir and VP-ir perikarya was evident in this medial parvocellular division in brains of adrenalectomized animals; this area is normally VP-ir poor except in the adrenalectomized rats. This extension of VP-ir cells into this CRF rich region and the very close approximation between the two cell bodies suggests potential cell to cell communication following perturbation of the brain-pituitary-adrenal axis. No evidence for the co-existence of two peptidergic systems in the same neuron was apparent in the present study.

Relationship of ACTH1-39-immunostained fibers and magnocellular neurons in the paraventricular nucleus of rat hypothalamus

Dual antigen immunocytochemical staining procedures were used in the same tissue section to determine the distribution of ACTH immunostained fibers and varicosities within the magnocellular and parvocellular divisions in the paraventricular nucleus (PVN) of rat hypothalamus and elucidate its anatomical relationship to vasopressin (VP) and oxytocin (OXY)-containing neurons. Double immunostained preparations using glucose oxidase-antiglucose oxidase complex combined with PAP complex to visualize two antigens with contrasting colors in the same tissue section were employed. ACTH-immunoreactive (ir) fibers were distributed throughout the periventricular stratum and the parvocellular component of the PVN; in the latter area fibers were particularly dense in the ventral medial portion of the medial parvocellular division. Dual immunostained sections revealed a close anatomical association between opiocortin fibers and oxytocin and vasopressin parvocellular neurons. ACTH immunostained fibers were present in the anterior and medial magnocellular component of PVN and in the ventral medial portion of the posterior magnocellular division; these immunoreactive fibers were in intimate proximity to oxytocin-ir perikarya. The very close approximation between the ACTH-ir fibers and oxytocin-containing cell bodies suggests potential cell to cell communication between the two peptidergic systems in PVN. Few ACTH immunostained fibers were seen in the dorsal lateral portion of the posterior magnocellular division in which vasopressinergic neurons predominate. The present anatomical study supports pharmacological and physiological studies which indicate that opioids can influence the activity of magnocellular PV neurons. This study also elucidates an anatomical relationship between opiocortins (ACTH1-39) and parvocellular PV neurons which suggests that the opiocortin system may play a role in the regulation of both the neuroendocrine and autonomic activities of specific PV neurons.

Ultrastructural studies on the afferent synaptic input to oxytocin-containing neurons in the paraventricular nucleus of the hypothalamus

A diverse afferent synaptic input to immunostained oxytocin magnocellular neurons of the paraventricular nucleus of the rat hypothalamus is described. By electron microscopy, immunoreactive material is present within cell bodies and neuronal processes and it is associated primarily with neurosecretory granules and granular endoplasmic reticulum. Afferent axon terminals synapse on perikarya, dendritic processes, and possibly axonal processes of oxytocin-containing neurons. The presynaptic elements of the synaptic complexes contain clear spherical vesicles, a mixture of clear spherical and ellipsoidal vesicles, or a mixture of clear and dense-centered vesicles. The postsynaptic membranes of oxytocinergic cells frequently show a prominent coating of dense material on the cytoplasmic face which gives the synaptic complex a marked asymmetry.