Collateral input to the paraventricular and supraoptic nuclei in rat. I. Afferents from the subfornical organ and the anteroventral third ventricle region

Injections of two fluorescent retrograde tracers were used to investigate the existence of collateral branching of input to the hypothalamic magnocellular neuroendocrine neurons. Injection of one tracer (either Fluoro-Gold or rhodamine-labeled microspheres) into the supraoptic nucleus and the other tracer into the ipsilateral paraventricular nucleus produced labeled neurons within the subfornical organ and the anteroventral third ventricle area. Some labeled cells were found to contain both fluorescent tracers (double-labeled cells), suggesting that they project to both the paraventricular and supraoptic nuclei via branching axons. Most double-labeled cells were found within the subfornical organ. Fewer of these cells were located within the nucleus medianus preopticus, and still fewer were distributed in the organum vasculosum lamina terminalis, the bed nucleus of the stria terminalis, and the medial and the lateral preoptic areas. These data present the first direct evidence that single cells may provide input to more than one magnocellular neuroendocrine nucleus. Hypothetically, hormonal release would require coordinated firing of many magnocellular cells. Thus, the branched input to these neurons may assist in the organization and the timely activation of this system in response to physiological stimuli.     

Synchronization of oxytocin neurons in suckled rats: possible role of bilateral innervation of hypothalamic supraoptic nuclei by single medullary neurons

We have previously shown that oxytocin neurons located in the four hypothalamic magnocellular nuclei display synchronous bursts of action potentials before each milk ejection. The mechanisms involved in such a synchronization have, however, not yet been elucidated. In this study, we test the hypothesis of an extranuclear synchronization arising from a common extrahypothalamic input innervating bilateral magnocellular nuclei. First, two different retrograde tracers were injected into the right and left supraoptic nuclei of rats that were fixed 5-7 days later. Each tracer labelled numerous neurons in various brain regions ipsilateral or contralateral to the injection site, but colocalization of the two tracers within the same cell body could only be detected bilaterally in neurons in the ventromedial regions of the medulla oblongata. The axonal projections of these medullary neurons were then visualized by the unilateral microinjection of an anterograde tracer (BDA) within the ventromedial medulla oblongata. BDA-labelled axons afferent to the hypothalamus were found to branch towards both supraoptic nuclei through medial portions of the optic chiasma. Finally, in anaesthetized lactating rats, surgical lesions were placed medially through the optic chiasma and the electrical activity of oxytocin neurons in bilateral supraoptic nuclei was pair-recorded during suckling. The incidence of synchronous bursts in oxytocin neurons located within bilateral supraoptic nuclei were dramatically altered only when the medial portions of the optic chiasma were totally lesioned. Taken together, these data suggest that medullary neurons afferent to bilateral supraoptic nuclei are involved in the recruitment and synchronization of bursting in oxytocin neurons during suckling.     

Projections of the paratrigeminal nucleus to the ambiguus, rostroventrolateral and lateral reticular nuclei, and the solitary tract

The paratrigerminal nucleus (Pa5), a constituent of the spinal interstitial system, was linked to the pressor effect caused by bradykinin injected in the dorsal lateral medulla of the rat. The nucleus receives primary afferent sensory fibers contained in branches of the trigeminal, glossopharyngeal and vagus nerves. In this investigation connections of the paratrigeminal nucleus to other medullary structures were studied with the use of retrograde and anterograde neuronal tracers. Fluorescent light microscopy analyses of medullary sections of rats injected with the retrograde transport tracer Fluoro-gold in the nucleus of the solitary tract (NTS) or in the pressor area of the rostral ventrolateral medulla (RVLM) revealed labeled neuronal cell bodies in the ipsi- and contralateral Pa5. FluoroGold microinjections in the caudal ventrolateral medulla (CVLM) did not produce fluorescent labeling of Pa5 neurons. Microinjection of the anterograde transport neuronal tracer biocytin in the Pa5 produced bilateral labeling of the solitary tract (sol). rostroventrolateral reticular nucleus (RVL), ambiguus nucleus (Amb), lateral reticular nucleus (LRt) and ipsilateral parabrachial nuclei, but not the contralateral Pa5. Confocal laser microscopy showed fluorescence labeling of fibers and presumptive terminal varicosities in the NTS, RVL, Amb and LRt. The present findings showing the paratrigeminal nucleus interposed between sensory afferent and stuctures associated to cardiovascular and respiratory functions, suggest that the structure may act as a medullary relay nucleus for sensory stimuli directly connecting primary afferents to structures mediating cardiovascular and respiratory reflexes.     

Calretinin is differentially localized in magnocellular oxytocin neurons of the rat hypothalamus. A double-labeling immunofluorescence study

By use of a double-labeling immunofluorescence method with a confocal laser scanning microscope, we have examined whether a calcium-binding protein, calretinin, is localized in magnocellular oxytocin and vasopressin neurons of the rat hypothalamus. In the supraoptic nucleus, all oxytocin-labeled cells were stained for calretinin. However, in the magnocellular part of the paraventricular nucleus, almost all oxytocin-stained cells were devoid of calretinin immunoreactivity. All vasopressin-positive cells of both the supraoptic nucleus and the magnocellular part of the paraventricular nucleus lacked calretinin immunoreactivity. No calretinin immunoreactivity was found in oxytocin-labeled cells of the the anterior commissural nucleus or in vasopressin-labeled cells of the suprachiasmatic nucleus. We previously showed that another calcium-binding protein, calbindin-D28k, was localized in magnocellular oxytocin neurons of the supraoptic nucleus but not in those of the paraventricular nucleus. These findings suggest that, in general, magnocellular oxytocin neurons of the supraoptic nucleus and those of the paraventricular nucleus can be chemically distinguished, that is, the former contain both calretinin and calbindin-D28k but the latter lack the two calcium-binding proteins.     

Localization of agmatine in vasopressin and oxytocin neurons of the rat hypothalamic paraventricular and supraoptic nuclei

Agmatine (decarboxylated l-arginine), an endogenous ligand of imidazoline and alpha(2) adrenoreceptors, is particularly enriched in the rat hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei. The present study utilized light and electron microscopic immunocytochemical methods to determine the distribution and extent of colocalization of agmatine relative to subpopulations of vasopressin- (VP) and oxytocin- (OT) producing neurons in PVN and SON nuclei. By light microscopy, agmatine-immunoreactive perikarya were found in both the magnocellular and the parvocellular neuronal subdivisions of PVN and SON. Confocal and electron microscopy revealed that agmatine-immunoreactivity (I) within neuronal perikarya was associated with the nuclear membrane as well as mitochondria, Golgi complexes, endoplasmic reticula, and plasmalemma. Additionally, agmatine-I was identified in both axons and axonal terminals, which were enriched in large dense-core vesicles. Dual and triple immunocytochemical labeling experiments also demonstrated that agmatine coexists with VP or OT in most PVN and SON magnocellular neurons. Combinations of iontophoretic injections of Fluorogold into the dorsomedullary complex with immunocytochemical labeling revealed that many retrogradely labeled neurons in the parvocellular region of the PVN contained agmatine-I and either VP or OT. These findings provide evidence that agmatine may function as a modulator of both hypothalamically mediated neuroendocrine and autonomic responses.     

Comparison of the Number of Vasopressin-Producing Hypothalamic Neurons in Rats and Humans

The aim of this study was to assess the number and proportion of vasopressin-producing neurons in the hypothalamic magnocellular nuclei in rats and humans. Accurate and unbiased neuronal counts were estimated using the optical disector method. Arginine vasopressin-containing neurons were immunohistochemically visualized in formalin-fixed tissue sections. The magnocellular neurons were similar in size and morphology in both species. While the human hypothalamus contained significantly more vasopressin-containing neurons compared with the rat (36-fold increase), the proportion of vasopressin-containing neurons between species was similar. In both species, the majority of supraoptic neurons contained vasopressin, however the proportion of vasopressin-containing neurons in the human paraventricular nucleus was double that of the rat (nearly a 100-fold increase in number). These results suggest that the paraventricular nucleus contributes significantly to the release of vasopressin from the posterior pituitary in humans, whereas in rats vasopressin is mainly released by supraoptic neurons.     

Ultrastructural localization of inhibin β- and somatostatin-28-immunoreactivities in the paraventricular and supraoptic nuclei

Recent studies have supported the existence of projections to the paraventricular and supraoptic nuclei of the hypothalamus that arise from non-catecholaminergic neurons in the nucleus of the solitary tract, whose terminal distribution is suggestive of interactions with both parvocellular and magnocellular neurosecretory neurons. Pre-embedding immunolabeling methods were used to compare and characterize the termination patterns of axons immunoreactive for two putative markers for this projection system, inhibin β and somatostatin-28, at the ultrastructural level. Axon terminal profiles stained fro either peptide were found to form symmetric or asymmetric junctions predominantly with the shafts of unlabeled dendrites of varying caliber. A small percentage of peptidergic terminals was found in both hypothalamic nuclei to engage in so-called ‘shared synapses’, where a single terminal profile contacted two postsynaptic elements. Axo-somatic terminations were relatively rarely seen in the supraoptic nucleus, but were somewhat more abundant in the paraventricular nucleus. These comprised principally symmetric junctions onto the somatic membranes of an ostensibly mixed population of cells, some of which bore apparent neurosecretory specializations. Combined immunoperoxidase and immuno-autoradiographic staining methods were used to estimate the extent to which either terminal type interacts with oxytocin neurons. Oxytocin stained elements comprised a minority of the postsynaptic targets of both peptidergic terminal types in the paraventricular nucleus, and a scant majority of those in the supraoptic nucleus. These results support the view that peptidergic neurons in the caudal nucleus of the solitary tract interact synaptically with multiple cell types in the parvocellular division of the paraventricular nucleus, and preferentially with oxytocinergic elements in the magnocellular neurosecretory system.     

Upregulation of nitric oxide synthase and galanin message-associated peptide in hypothalamic magnocellular neurons after hypophysectomy. Immunohistochemical and in situ hybridization studies

The expression of several bioactive molecules in magnocellular hypothalamic neurons is modified when the axons of these cells are transected. In this stuyd we have evaluated by means of immunocytochemistry and in situ by hybridization the effect of hypophysectomy on the expression of nitric oxide synthase (NOS)- and of galanin message-associated peptide (GMAP)-like immunoreactivities (-LIs) as well as on their respective mRNAs in hypothalamic magnocellular neurosecretory neurons. The results show a transient increase in NOS- and GMAP-LIs in magnocellular neurons of both the paraventricular and supraoptic nuclei when compared to normal animals. The maximal increase in staining was observed between 5 and 7 days, and by 14 days NOS-LI was back to normal levels, whereas strong GMP-LI could still be detected in a few cella. A similar picture was observed for the NOS and GMAP mRNAs. The functional significance of the present findings is unclear, but they indicate a possible role of nitric oxide and GMAP in neurosecretory neurons after injury.     

Chronic stress elevates enkephalin expression in the rat paraventricular and supraoptic nuclei.

Numerous studies have implicated opioids in the regulation of hypothalamic functions. Dynorphin, which is co-expressed with vasopressin in the magnocellular neurons of the paraventricular and supraoptic nuclei, is co-regulated with vasopressin in response to hyperosmolality and appears to inhibit vasopressin and oxytocin release from the posterior pituitary. Enkephalin is present in paraventricular parvocellular neurons and its expression is elevated in response to various stresses. However, enkephalin's presence and roles in paraventricular and supraoptic magnocellular neurons are uncertain. By giving rats daily intraperitoneal injections of hypertonic saline for up to 12 days, we induced a marked increase in enkephalin expression in magnocellular neurons of the paraventricular and supraoptic nuclei, beyond what develops from drinking hypertonic saline. Our results suggest that enkephalin expression in both vasopressin and oxytocin neurons may increase in response to chronic stresses and provide another source of enkephalin in addition to the parvocellular neurons.     

Lesions of the tissue surrounding the preoptic recess (AV3V region) affect neurosecretory cells in the paraventricular nuclei in the rat

Lesions of the tissue surrounding the preoptic recess (AV3V region) have severe effects on body fluid homeostasis; these include acute adipsia and failure of the antidiuretic response. Because neurosecretory cells in supraoptic nuclei comprise the major source of antidiuretic hormone (ADH) in this species, we have previously observed the fine structure of supraoptic nuclei in rats with AV3V lesions. Paraventricular nuclei are the other major source of ADH in rats. Therefore, in this investigation we compared the fine structure of paraventricular nuclei in rats which had received AV3V lesions 3 days earlier with that of control rats which had received sham lesions and either had drinking water available or had water withheld for 3 days. Degenerating axons and axon terminals were present in paraventricular nuclei of lesioned rats. The degenerating terminals were in axodendritic and less often in axosomatic synapses. Morphometric evaluation revealed that neurosecretory cells did respond to the dehydrated state of the adipsic-lesioned animals, but the response was significantly attenuated compared to that which occurred in sham-lesioned rats deprived of water for 3 days. It appears that AV3V lesions damage afferent connections and impair the response of neurosecretory cells to dehydration in paraventricular as well as supraoptic nuclei. However, in paraventricular nuclei the response is not completely prevented by AV3V lesions during the adipsic period as was observed in supraoptic nuclei. The presence of a response in paraventricular nuclei may be at least partially stimulated by reduced body fluid volume. Information from volume receptors would be carried from the medulla to paraventricular nuclei by ascending pathways which are not affected by AV3V lesions.     

Collateral axonal projections from the A1 noradrenergic cell group to the paraventricular nucleus and bed nucleus of the stria terminalis in the rat

The A1 noradrenergic cell group in the caudal ventrolateral medullary reticular formation of the rat sends efferent projections to a number of regions in the basal forebrain and hypothalamus, but the extent to which these projections represent collateral branches of individual axons is not known. Immunohistochemical labeling of medullary neurons containing the catecholamine biosynthetic enzymes tyrosine hydroxylase, dopamine beta-hydroxylase, and phenylethanolamine N-methyltransferase was used to reveal the anatomical location of A1 noradrenergic neurons within the ventrolateral medulla. Subsequently, the retrograde fluorescence double-labeling technique was employed to investigate the collateralization of ascending A1 efferent axons. The subcommissural bed nucleus of the stria terminalis (BST) was injected with rhodamine-fluorescent latex microspheres and the ipsilateral left paraventricular nucleus of the hypothalamus (PVN) was injected with Fast blue. Within the ventrolateral medulla, single- and double-labeled neurons were identified in a distribution corresponding to that demonstrated for A1 noradrenergic perikarya. The results indicate that some ascending axons from cells within the A1 region collateralize to effect a simultaneous innervation of the BST and PVN. The innervation of multiple efferent targets by single neurons within the A1 region may have important implications with respect to A1's postulated role in central cardiovascular regulation.     

Evidence for a functional and anatomical relationship between the lateral septum and the hypothalamus in the control of flank marking behavior in Golden hamsters

Golden hamsters with established dominant/subordinate relationships communicate their social status by rubbing pheromone-producing flank glands against objects in the environment. This behavior, called flank marking, is controlled by vasopressin-sensitive neurons localized to the anterior hypothalamus. Vasopressinergic magnocellular neurons in the nucleus circularis and medial aspect of the supraoptic nucleus are thought to be a source of neurotransmitter for the initiation of flank marking. The present study was undertaken to examine the extrahypothalamic control of flank marking. The anatomical and functional connections between the lateral septum and the vasopressin-containing nuclear groups in and around the anterior hypothalamus were examined by: (1) tracing afferent and efferent connections following microinjection of horseradish peroxidase and Phaseolus vulgaris-leucoagglutinin into the lateral septum, and (2) recording odor-induced flank marking prior to and following ibotenate lesions in the septum. The greatest number of perikarya retrogradely labeled with horseradish peroxidase were found lateral to the anterior hypothalamus and ventral to the fornix in the area of the lateral hypothalamus. The vasopressin-containing nuclear groups, e.g., paraventricular, supraoptic, suprachiasmatic nuclei, and the nucleus circularis, were devoid of labeled perikarya. Nerve terminals anterogradely labeled with Phaseolus vulgaris-leucoagglutinin were primarily localized to the anterior hypothalamus, in and around the nucleus circularis, and the medial aspect of the supraoptic nucleus. The lateral aspect of the supraoptic nucleus was devoid of nerve terminals as were the paraventricular and suprachiasmatic nuclei. The anatomical connections between the lateral septum and the hypothalamus appear to be necessary for the control of flank marking, since the microinjection of ibotenate into this limbic site significantly reduced odor-induced flank marking as compared to control microinjections of 0.9% NaCl.     

Hypothalamic and brainstem sources of pituitary adenylate cyclase-activating polypeptide nerve fibers innervating the hypothalamic paraventricular nucleus in the rat

The hypothalamic paraventricular nucleus (PVN) coordinates major neuroendocrine and behavioral mechanisms, particularly responses to homeostatic challenges. Parvocellular and magnocellular PVN neurons are richly innervated by pituitary adenylate cyclase-activating polypeptide (PACAP) axons. Our recent functional observations have also suggested that PACAP may be an excitatory neuropeptide at the level of the PVN. Nevertheless, the exact localization of PACAP-producing neurons that project to the PVN is not understood. The present study examined the specific contribution of various brain areas sending PACAP innervation to the rat PVN by using iontophoretic microinjections of the retrograde neuroanatomical tracer cholera toxin B subunit (CTb). Retrograde transport was evaluated from hypothalamic and brainstem sections by using multiple labeling immunofluorescence for CTb and PACAP. PACAP-containing cell groups were found to be retrogradely labeled from the PVN in the median preoptic nucleus; preoptic and lateral hypothalamic areas; arcuate, dorsomedial, ventromedial, and supramammillary nuclei; ventrolateral midbrain periaqueductal gray; rostral and midlevel ventrolateral medulla, including the C1 catecholamine cell group; nucleus of the solitary tract; and dorsal motor nucleus of vagus. Minor PACAP projections with scattered double-labeled neurons originated from the parabrachial nucleus, pericoeruleus area, and caudal regions of the nucleus of the solitary tract and ventrolateral medulla. These observations indicate a multisite origin of PACAP innervation to the PVN and provide a strong chemical neuroanatomical foundation for interaction between PACAP and its potential target neurons in the PVN, such as parvocellular CRH neurons, controlling physiologic responses to stressful challenges and other neuroendocrine or preautonomic PVN neurons.     

Axonal projections of medullary swallowing neurons in guinea pigs

A central pattern generator (CPG) for swallowing in the medulla oblongata generates spatially and temporally coordinated movements of the upper airway and alimentary tract. To reveal the medullary neuronal network of the swallowing CPG, we examined the cytoarchitecture of the swallowing CPG and axonal projections of its individual neurons by extracellular recording and juxtacellular labeling of swallowing-related neurons (SRNs) in the medulla in urethane-anesthetized and paralyzed guinea pigs. Three major types of neuronal discharge patterns were identified during fictive swallowing induced by stimulation of the superior laryngeal nerve: early (burst-like activation during the pharyngeal stage), late (activation after the pharyngeal stage), and inhibited (inhibition during the pharyngeal stage) types. Sixteen neurons were successfully labeled in the nucleus tractus solitarii (NTS) and in the medullary reticular formation (RF). No motoneuron was labeled. The SRNs in the NTS had axons projecting to the NTS, RF, nucleus ambiguus, nucleus hypoglossus, and dorsal motor nucleus of the vagus on the ipsilateral side. Some NTS SRNs projected only within the NTS. The axons of SRNs in the RF projected also to the NTS, RF, motor nuclei on the ipsilateral side, and to the other side RF. These findings show anatomic substrates for the neuronal network of the CPG for swallowing, which consists of complex neuronal connections among SRNs in the NTS, RF, and motor nuclei.     

CaBP D-28k and NADPH-diaphorase coexistence in the magnocellular neurosecretory nuclei.

Coexistence of the calcium binding protein calbindin D-28k and NADPH-diaphorase activity was studied in the magnocellular secretory nuclei of the rat hypothalamus using both immunocytochemical and histochemical techniques. Coexistence was found in all the nuclei considered (supraoptic, paraventricular, circularis and fornicals nuclei) with the exception of the hypothalamic area situated between the supraoptic and the paraventricular nuclei. Since both stainings are reliable markers, not based upon the physiological characteristics at a given moment, our study provides a further characterization of the neurons in the magnocellular neurosecretory nuclei.     

Calbindin D-28K- and parvalbumin-reacting neurons in the hypothalamic magnocellular neurosecretory nuclei of the rat.

The distribution of calbindin D-28K- and parvalbumin-reacting neurons in the hypothalamic magnocellular neurosecretory nuclei of the rat was studied using the avidin-biotin-immunoperoxidase method and highly specific monoclonal antibodies. Incubation with anticalbindin D-28K-antiserum revealed immunoreactive neurons in the following nuclei: supraoptic, paraventricular (both in the magnocellular and parvicellular regions), circularis, fornicals and medial forebrain bundle. Incubation with parvalbumin antiserum displayed immunoreactive neurons only in the circularis nucleus. Additionally, it was possible to observe scattered calbindin and parvalbumin immunoreactive neurons (which do not form part of the nuclei considered) located in the hypothalamic area between the supraoptic and the paraventricular nuclei, especially for the calbindin D-28K antiserum.     

The organization of chemically characterized afferents to the perivascular neuronal groups of the hypothalamic magnocellular neurosecretory system in the rat

The hypothalamic magnocellular neurosecretory system consists of the paraventricular nucleus and supraoptic nucleus and a number of accessory nuclei. There is evidence that each of the accessory nuclei has a preferential source of afferents. Two of the accessory nuclei, namely the nucleus circularis (NC) and the lateral hypothalamic perivascular nucleus (LHPN), are particularly interesting due to their very close relationship with the blood vessels. The NC is composed of small dense clusters of neurons in the medial anterior hypothalamus. The groups of lateral hypothalamic neurons gathering around vascular branches are collectively called the LHPN. Their close topographical relationship with the blood vessels may indicate that the latter may serve as a source of input to these nuclei. As a part of the effort to investigate this issue, the present study examined in these two nuclei the distribution pattern of terminal-like elements containing 11 transmitters/modulators. Only a few, if any, terminal-like elements of the transmitters/modulators studied could be found distributed in the NC proper, although its immediate vicinity could be densely innervated. On the contrary, the LHPN proper was often densely innervated by fibers expressing the examined markers. These terminal patterns were found to be quite different from those of the paraventricular and supraoptic nuclei. The present findings further substantiate the notion of a functional differentiation among the subnuclei of the magnocellular neurosecretory system. The significance of the relationship of these two perivascular nuclei with the blood vessels is discussed.     

An immunohistochemical study of the organization of catecholaminergic cells and terminal fields in the paraventricular and supraoptic nuclei of the hypothalamus

The distribution of catecholaminergic fibers and cell bodies in the paraventricular and supraoptic nuclei of the hypothalamus was investigated with immunohistochemical methods in the adult albino rat. Sections through the nuclei were stained with antisera to the catecholamine synthesizing enzymes tyrosine hydroxylase (TH), dopamine-beta-hydroxylase (DBH), and phenylethanolamine-N-methyltransferase (PNMT). The results suggest that adrenergic (PNMT-stained) fibers innervate the entire parvocellular division of the paraventricular nucleus, although the highest density of fibers was found in the medial part of the division. Only widely scattered adrenergic fibers are found in the magnocellular division of the nucleus and in the supraoptic nucleus. Noradrenergic fibers appear to innervate the periventricular zone of the paraventricular nucleus and those parts of the paraventricular and supraoptic nuclei that contain predominantly vasopressinergic neurons in both the normal and in the homozygous Brattleboro rat. Significant numbers--somewhat more than 500--of dopaminergic (TH-stained) neurons are found in the paraventricular nucleus; the cells are distributed throughout the nucleus but are concentrated in the medial and periventricular parts of the parvocellular division. Double-labeling experiments with the retrogradely transported tracer true blue indicate that between 4% and 8% of the dopaminergic neurons in the paraventricular nucleus project to the region of the dorsal vagal complex and/or thoracic levels of the spinal cord. It is concluded that adrenergic inputs to the paraventricular nucleus may influence cells that project to the median eminence and to preganglionic autonomic cell groups in the medulla and spinal cord. Noradrenergic inputs to the supraoptic and paraventricular nuclei may influence primarily vasopressinergic cells that project to the posterior lobe of the pituitary, as well as cells in the periventricular part of the paraventricular nucleus that project to the median eminence.     

Entry of peroxidase into neurons of the central and peripheral nervous systems from extracerebral and cerebral blood.

Autonomic preganglionic, sensory, and lower motoneuron perikarya within the central nervous system, as well as cell bodies with axons projecting to the circumventricular organs, are retrogradely labeled with horseradish peroxidase (HRP) delivered to their axon terminals by cerebral and extracerebral blood. Subsequent to vascular injection of HRP into mice, blood-borne peroxidase passes across permeable vessels in muscle, ganglia, and in all circumventricular organs except for the subcommissural organ in which no leak could be discerned. Brain parenchyma adjacent to each of the permeable circumventricular organs quickly becomes inundated with the protein. By four to six hours post-injection, this extracellular HRP reaction product has disappeared, and by eight hours perikarya of specific hypothalamic nuclei contain HRP-positive granules indicative of the intra-axonal retrograde transport of the protein. Hypothalamic neurons so labeled are presumed to send axons to such circumventricular organs as the median eminence or neurohypophysis and include neurons of the magnocellular neurosecretory supraoptic and paraventricular nuclei, the accessory magnocellular nuclei, the parvicellular arcuate nucleus, and a band of periventricular cells extending rostrally into the medial preoptic area. Labeled somata are also adjacent to the organum vasculosum of the lamina terminalis and in the vertical limb of the nucleus of the diagonal band of Broca. No similarly labeled cell bodies were identified near the subfornical organ.     

Hypothalamic projections of medullary catecholamine neurons in the rabbit: A combined catecholamine fluorescence and HRP transport study

Injections of horseradish peroxidase into the rabbit hypothalamus, centered on the paraventricular nucleus, result in retrograde transport to A1 and A2 neurons in the medulla oblongata, in a bilateral distribution with ipsilateral predominance. Some HRP labeled cells in both A1 and A2 areas do not exhibit a positive histochemical reaction for catecholamines, but, in the Al area, nearly all HRP-positive neurons are catecholamine-containing A1 cells. Injection of radioisotope into the A1 area of the rabbit results in terminal labeling within the paraventricular and supraoptic nuclei. This study complements previous work showing that rabbit A1 and A2 cells have minimal projections to the spinal cord.