Retrograde transport of horseradish peroxidase in the magnocellular neurosecretory system of the rat

Topography of neurophysin-immunoreactive neurons projecting to the neurohypophysis: direct evidence as revealed by a double staining method

The topographic distribution of neurophysin-immunoreactive (NP-IR) cells projecting to the posterior pituitary gland has been studied in the cat using a double staining method: immunohistochemistry of neurophysin in conjunction with horseradish peroxidase (HRP) retrograde tracer technique. We found that almost all the hypothalamic NP-IR cells project directly to the neurohypophysis except those localized in the suprachiasmatic nucleus and in the caudal part of the paraventricular nucleus.

Characteristics of vasculature and neurovascular relations in intraventricular anterior hypothalamic transplants

Fetal hypothalami obtained from normal Long-Evans rats were transplanted to the lateral, third or fourth ventricle of adult male Brattleboro rats, homozygous for diabetes insipidus. The density of the capillary plexuses within the grafts did not vary as a function of their intraventricular location; all transplants exhibited a capillary density equivalent to that of the in situ hypothalamus. Intravascular injections of HRP resulted in retrograde neuronal labeling only in grafts that were attached to circumventricular organs of the host brain, especially the median eminence. Typically, HRP-associated label was confined to vascular and perivascular elements and not diffusely distributed within the graft parenchyma, indicating that capillaries within the transplants developed barrier properties similar to those of the native hypothalamus. Neural integration of the transplant with the recipient brain was limited; at most points of apposition the neuropil of graft and host were separated by an intervening ependymal layer or glia limitans. All surviving grafts contained neurophysin and/or vasopressin-immunoreactive (VP-ir) neurons. Three anatomically distinct populations of VP-ir neurons were identified. Magnocellular VP-ir neurons were identified in less than half of the grafts, and when present they were few in number distinct, but sparse vasopressinergic innervation of median eminence capillaries was observed in all cases where grafts containing magnocellular neurons were apposed to this structure. Most grafts contained numerous, parvicellular VP-ir neurons arranged in aggregations which resembled the suprachiasmatic nucleus (SCN). SCN-like cell groups projected to neural targets within the graft and the host brain, but they did not project onto blood vessels. A second, distinct class of parvicellular VP-ir neuron also was identified in a majority of transplants. In contrast to SCN-like ('type' I) cells, these 'type II' parvicellular neurons were somewhat larger and found in less discretely organized groups, and they projected to vascular targets; usually locally elaborated capillary plexuses intrinsic to the transplants. In the present study, there was no amelioration of the symptoms of diabetes insipidus in the host animals despite the presence of numerous VP-ir neurons in virtually all grafts. This was probably related to the limited survival of magnocellular VP-ir neurons, which appear to be the principal source of vasopressinergic projections from the graft to fenestrated capillary plexuses of the host brain.

Functional topography of the rat hypothalamic dopamine neuron systems: retrograde tracing and immunohistochemical study

The origins of the dopamine (DA)-containing nerve terminals in the external layer of the median eminence and in the neurointermediate pituitary were determined in rats by a combination of retrograde labeling with wheat germ agglutinin (WGA) and immunohistochemistry for tyrosine hydroxylase (TH). Biotinylated WGA (b-WGA) was injected into the posterior pituitary (group 1) and into the median eminence (group 2). In group 1 animals, all the magnocellular neurons of the paraventricular, supraoptic, and accessory nuclei, and many parvicellular neurons in the rostral periventricular region (RPR) were labeled with WGA. In group 2 animals, many neurons were labeled in the arcuate nucleus and the RPR, and in a small population of the preoptic-septal region. In group 1 animals, about 39% of TH neurons in the RPR were labeled with WGA, whereas only a few TH neurons (1%) in the arcuate nucleus were labeled with WGA. In group 2 animals, on the contrary, almost all TH neurons (73%) in the arcuate nucleus carried WGA, whereas in the RPR, only some of the TH neurons (19%) were labeled with WGA. It is concluded that DA neurons involved in the hypothalamic-anterior pituitary axis are located in the arcuate nucleus; those involved in neuro-intermediate lobe function in the RPR.

Prolactin secretion in posterior pituitary lobectomized rats: differential effects of 5-hydroxytryptophan and ether

The objective of this study was to determine whether the posterior pituitary mediates the prolactin (PRL) releasing activities of serotonin and ether. Ovariectomized (OVEX) rats were subjected to posterior pituitary lobectomy (LOBEX) or sham surgery (SHAM). Either 1 or 12 days after surgery, rats were injected i.v. with 20 mg/kg b. wt. of 5-hydroxytryptophan (5-HTP), which is a precursor of serotonin. A second group of rats was exposed to ether vapors for 10 min. Blood was collected from a jugular cannula before and after the treatments and analyzed for PRL. On either 1 or 12 days after surgery, injection of 5-HTP increased plasma PRL levels 5-10 fold in both LOBEX and SHAM rats. This was followed by a decline to preinjection levels within 60-90 min. LOBEX and SHAM male rats which were injected i.p. with 50 mg/kg b. wt. of 5-HTP, also showed marked and similar elevations of plasma PRL levels 12 days after surgery. Exposure of OVEX rats to ether elicited a 2-3 fold rise in plasma PRL levels only on day 1, but not on day 12, after LOBEX. The ether-induced rise in PRL was similar in SHAM rats tested on both days. These results indicate that the 5-HTP-induced rise in plasma PRL levels is independent of the posterior pituitary, regardless of the sex, the route of drug administration or the length of time after surgery. In contrast, the PRL response to ether stress is diminished within 12 days after LOBEX. The evidence that the PRL responses to 5-HTP and to ether might be mediated via different neuronal mechanisms is discussed.

Determination of the source of 5-hydroxytryptaminergic neuronal projections to the neural and intermediate lobes of the rat pituitary gland through the use of electrical stimulation and lesioning experiments

5-Hydroxytryptamine (5-HT)-containing axons and terminals have been visualized in the neural and intermediate lobes of the rat pituitary gland, but the origin of these fibers remains in question. This study was designed to determine if 5-HT cell bodies in the brainstem or in the dorsomedial nucleus of the hypothalamus project to either of these pituitary lobes. Since lesions and electrical stimulation of 5-HT cell bodies decrease and increase, respectively, the rate of 5-HT synthesis in regions innervated by these cells, these techniques were employed. The in vivo rate of 5-HT synthesis was determined by quantifying the rate of accumulation of the 5-HT precursor, 5-hydroxytryptophan (5-HTP), in the neural and intermediate lobes of the pituitary gland 30 min after the administration of a decarboxylase inhibitor (NSD 1015, 100 mg/kg, i.p.). The application of 30 min of stimulating current (monophasic cathodal pulses of 1 ms duration and 0.3 mA current delivered at a frequency of 10 Hz) to electrodes implanted in the dorsal and median raphe nuclei increased the rate of 5-HT synthesis in both the neural and intermediate lobes of the pituitary gland. 5,7-Dihydroxytryptamine lesions of these nuclei altered neither 5-HTP accumulation nor 5-HT concentrations in the neural and intermediate lobes, but similar lesions of the nuclei raphe pontis and raphe magnus decreased both the concentration of 5-HT and the accumulation of 5-HTP in these pituitary regions.(ABSTRACT TRUNCATED AT 250 WORDS)

A Golgi study of the monkey paraventricular nucleus: neuronal types, afferent and efferent fibers

The neuronal organization of the paraventricular nucleus (PVN) was examined in Golgi impregnations of adult monkey. Results showed that at least six types of neurons could be identified in the nucleus on the basis of morphological features of the somata, dendrites, and axons. Four types of neurons with sparse to densely spined cell bodies and dendrites exhibited long axons and included large neurons (types I and II), medium-sized to large neurons (type III), and small to medium-sized cells (type IV). Axons of type I, III, and IV neurons had different diameters and were followed out of the PVN. Axon collaterals that arborized within the PVN were seen on the axons of types III and IV cells. Two types of interneurons with small somata were also found. One (type V) exhibited varicose dendrites and a profusely arborizing local axon. The other cell (type VI) had recurved dendrites with long appendages and no impregnated axon. Afferent fibers were also identified. Type 1 was a fine-caliber axon that coursed long distances in the PVN and exhibited numerous short branches. Additional observations suggested that type 1 afferents originated from the stria terminalis. The other afferent axon (type 2) was thicker and gave rise to terminal arborizations containing clusters of small swellings. The efferent fibers of the PVN were also examined in impregnations of the paraventriculosupraopticohypophysial tract. Fibers formed an extensive plexus as they coursed ventrally and passed through the lateral hypothalamus. Axons coursing more laterally in the tract were much larger than those more medially located. Our findings show a diverse organization of neuronal types within the monkey PVN with evidence for intrinsic connections through axon collaterals of efferent neurons and the locally arborizing axons of interneurons. Correlations are proposed between morphological subtypes of neurons seen in this Golgi study and the known functional output pathways of the PVN.

Ultrastructural demonstration of oxytocin and vasopressin release sites in the neural lobe and median eminence of the rat by tannic acid and immunogold methods

We have investigated the localization of oxytocin (OXT) and vasopressin (AVP) release sites in the neural lobe and median eminence (ME) of the rat. Ultrastructural and post-embedding immunocytochemical studies with protein A-gold as a marker were used for the identification of OXT and AVP neurosecretory granules (NSG). Release of the contents of these OXT and AVP NSG by exocytosis has been clearly demonstrated with the tannic acid-Ringer incubation method in axon terminals of the neural lobe and in varicose fibres with OXT and AVP NSG in the internal zone of the ME. As these fibers lack synaptic specializations, and are not located in the direct vicinity of capillaries of the primary portal plexus, these observations suggest that OXT and AVP are released in a paracrine way in the ME of the rat.

A gamma-aminobutyric-acid-mediated baroreceptor input to supraoptic vasopressin neurones in the rat

1. Extracellular recordings in pentobarbitone anaesthetized male Long-Evans rats examined the influence of electrical stimulation in the diagonal band of Broca on the excitability of 113 putative vasopressin-secreting and 22 putative oxytocin-secreting neurosecretory neurones in the hypothalamic supraoptic nucleus. 2. Single pulse or repetitive (5-20 Hz) stimulation in the ventral part of the diagonal band evoked a prominent reduction in the excitability of 83% of vasopressin-secreting neurones with no effect on the remainder. Amongst oxytocin-secreting neurones, 59% were unresponsive, 27% responded with an increase in activity while only 14% revealed an inhibitory pattern similar to vasopressin-secreting neurones. 3. Diagonal band stimulation-evoked inhibitions were reversibly abolished by local pressure applications of bicuculline methiodide (100 microM) to twenty out of twenty vasopressin secreting cells tested, whereas strychnine sulphate (100 microM) was without effect on four out of four cells tested. 4. In five out of five vasopressin-secreting cells tested, bicuculline applications reversibly abolished the reduction in their activity that follows peripheral baro-receptor activation. Failure to alter baroreflex-evoked depressions in firing during similar trials with prazosin hydrochloride (10 microM, six cells tested), timolol maleate (20 microM, six cells tested) or strychnine sulphate (100 microM, three cells tested) indicated the specificity of bicuculline's action. 5. These findings suggest that a GABAergic pathway from the diagonal band of Broca preferentially innervates vasopressin-secreting neurosecretory supraoptic nucleus (s.o.n.) neurones, and support the view that the baroreflex-induced depression in firing of s.o.n. vasopressin-secreting neurones is mediated in large part through this input.

Projections from the hypothalamus and its adjacent areas to the posterior pituitary in the rat

Cholera toxin conjugated horseradish peroxidase was injected into the posterior pituitary and its afferents traced in 21 albino rats. The neuronal processes as well as the perikarya were elaborately displayed. The principal and retrochiasmatic supraoptic nuclei and the magnocellular paraventricular subnuclei were densely labelled. The accessory cell groups or nuclei labelled included: the medial preoptic and anterior hypothalamic areas, the anterior and posterior fornical nuclei, the lateral hypothalamic area, the nucleus circularis and nucleus of the forebrain bundle and hitherto unknown or not fully appreciated retrochiasmatic area, the dorsal accessory groups in an area between the stria medullaris and fornix, on the one hand, and the stria terminalis and internal capsule, on the other, and a well developed subependymalperiventricular zone. The medial preoptic nucleus, subfornical organ and organ vasculosum laminae terminalis were also weakly stained. Dendrites of the magnocellular paraventricular nucleus have been said by some to be largely confined to the subnuclei in which they lie. Immunohistochemical studies have proved that they extended beyond their nuclear confinement. The present study has found much wider extension of their dendritic fields. In fact, dendrites of the magnocellular neurosecretory cells in general were long and had a certain degree of directional bias. Several sites projecting to the posterior pituitary were closely related to the cerebrospinal fluid. Namely, the subependymal neuronal plexuses along the third ventricle and beneath the interventricular foramen, and the subpial dendritic plexuses of the supraoptic and retrochiasmatic supraoptic nuclei. Neurons were seen to squeeze in-between the ependymal cells, bringing themselves very close to the cerebrospinal fluid. No direct cerebrospinal fluid-contacting elements, either cell bodies or processes, however, could be ascertained. It is proposed that these plexuses may monitor changes in the cerebrospinal fluid. Besides the principal neurohypophysial tract the posterior pituitary was found in the present study to receive its afferents via two accessory fasciculi, one coursing in the medial forebrain bundle and the other running along the lateral wall of the infundibular recess subependymally.

Gonadotropin-releasing hormone immunoreactive neurons with access to fenestrated capillaries in mouse brain

Gonadotropin-releasing hormone (GnRH) producing neurons which have access to fenestrated capillaries were identified through a combination of indirect immunofluorescence for GnRH with a fluorescein-taged second antibody and histochemical demonstration of the localization of blood borne and retrogradely transported horseradish peroxidase. In the mouse, GnRH positive neurons were present in the septum, which includes neurons originating from the nervus terminalis, the nucleus medialis and triangularis septi, and the nucleus of the diagonal band. Also, GnRH immunoreactive neurons could be seen in the lateral anterior hypothalamus, the nucleus preopticus medianus, the rostral nucleus periventricularis hypothalami and, to a lesser extent, the nucleus preopticus medialis. Single GnRH positive neurons were found in the nucleus supraopticus, the bed nucleus of the stria terminalis and the cingulate cortex. GnRH neurons which showed uptake of horseradish peroxidase were located in all of these regions and intermingled with unlabeled GnRH neurons. No preferential topographical concentration of GnRH neurons with access to the fenestrated vasculature was apparent. In animals in which GnRH secretion was stimulated by castration for 2 weeks, 65% of all GnRH neuronal perikarya contained horseradish peroxidase. This was reduced to 35% after a 2-week treatment of ovariectomized animals with 10 micrograms/day estradiol while the total number of immunoreactive GnRH cells remained unchanged. No differences in the number of GnRH-horseradish peroxidase positive cells was seen when the dose of horseradish peroxidase of the survival time were increased. While the presence of certain GnRH neurons with dual actions via collaterals cannot be excluded, the results suggest that there are two populations of GnRH neurons, one with access to fenestrated capillaries which is probably related to neurosecretory endocrine regulation of anterior pituitary gonadotropin secretion, and one without access to fenestrated capillaries which is probably related to intracerebral neurotransmission only.

Periventricular dopaminergic neurons terminating in the neuro-intermediate lobe of the cat hypophysis

The aim of the present study was to determine the exact origins of the dopaminergic hypothalamohypophyseal projections in the cat brain. For this purpose, we used a retrograde tracer technique with horseradish peroxidase (HRP) in conjunction with tyrosine hydroxylase (TH) immunohistochemistry as a marker for the dopaminergic neurons. After injections of the tracer into the neuro-intermediate lobe, a substantial number of HRP-labeled neurons was observed in the supraoptic and paraventricular neurosecretory nuclei. Furthermore, a cluster of HRP-positive neurons was found in the tuberal component of the periventricular nucleus where few, if any, neurosecretory magnocellular cells are identified. TH immunohistochemistry on the same sections further revealed that virtually all these HRP-containing neurons showed TH immunoreactivity. These double-labeled neurons were medium in size and fusiform or ovoid and appeared to belong to the A14 dopamine cell group. In addition to these medium-sized double-labeled neurons, a magnocellular type of double-labeled cell body was identified just adjacent to the organum vasculosum of the lamina terminalis and in and around the supraoptic and paraventricular nuclei. These double-labeled cells appeared to be members of the A14 and A15 dopamine cell groups. In conclusion, the present study indicated that the dopaminergic projections to the cat neurointermediate lobe might originate mainly in the medium-sized cells located in the tuberal periventricular nucleus and partly in the large-sized cells located in and around the supraoptic and paraventricular neurosecretory nuclei.

Elimination of vasopressin analgesia following lesions placed in the rat hypothalamic paraventricular nucleus

Pain thresholds are increased following central administration of arginine vasopressin (AVP), an effect which appears not to be mediated through opioid analgesic processes. In addition to magnocellular projections to the posterior lobe of the pituitary gland and parvocellular projections to the zona externa of the median eminence, the paraventricular nucleus (PVN) of the hypothalamus contains VP parvocellular neurons which also project to extrahypothalamic structures involved in pain inhibition. The present study examined whether AVP analgesia as measured by the tail-flick test was altered in animals with lesions placed in the PVN at either 7 or 35 days after surgery. VP levels in the pons-medulla and the lumbo-sacral spinal cord were measured by radioimmunoassay, as well as VP-like immunoreactivity in the PVN and spinal cord with immunocytochemistry. Lesions placed in the PVN eliminated AVP analgesia on the tail-flick test at both 7 and 35 days after surgery, and decreased radioimmunoassayable VP by 59% in the lumbo-sacral spinal cord and 36% in the pons-medulla. The extent of the lesions ranged from complete destruction of the PVN to partial sparing of ventro-medial PVN cells with VP-like immunoreactivity. These data indicate that the PVN is a critical structure for the integrity of AVP analgesia.

Neurotensin-like immunoreactivity and neurotensin receptors in the rat hypothalamus and in the neurointermediate lobe of the pituitary gland

In the rat hypothalamus, cell bodies containing neurotensin-like immunoreactivity were mainly found in the medial preoptic area, the periventricular nucleus, the paraventricular nucleus, the supraoptic nucleus and the arcuate nucleus. [3H]neurotensin binding sites were observed throughout the hypothalamus with a dense accumulation of silver grains over the paraventricular nucleus, the arcuate nucleus and the median eminence region. By radioimmunoassay neurotensin-like immunoreactivity was also found in the neurointermediate lobe of the pituitary gland of various mammalian species and in human postmortem posterior pituitary glands. In the rat studies involving pituitary stalk transections and the neurotoxin monosodium glutamate indicated the presence of a neurotensinergic pathway from the arcuate nucleus to the neurointermediate lobe of the pituitary gland. [3H]neurotensin binding sites were found to be concentrated over the intermediate lobe of the pituitary gland and their presence was not affected by pituitary stalk transection, indicating their localization on endocrine cells of the intermediate lobe of the pituitary gland.

Dye-coupled magnocellular peptidergic neurons of the rat paraventricular nucleus show homotypic immunoreactivity

Magnocellular neurons in rat hypothalamic slices are known to exhibit dye coupling: the transfer of the fluorescent dye, Lucifer Yellow, from an intracellularly-injected neuron to one or more nearby neurons. The question of the hormonal identity of coupled cells and the possibility of dye coupling as an artefact led us to determine the immunoreactivity of dye-coupled magnocellular neurons in the paraventricular nucleus of the rat hypothalamus using antisera to oxytocin- and vasopressin-associated neurophysins. In 23 pairs, one triplet, and one quadruplet, immunoreactivity to one or the other antiserum was always exclusive, and dye coupling was always homotypic, that is, coupled neurons in each instance were reactive to the same antiserum. The quadruplet, triplet and 17 pairs were immunoreactive to vasopressin-associated neurophysin, and oxytoxin-associated neurophysin immunoreactivity was observed in the remaining pairs. Immunoreactivity to each antiserum was found for somasomatic and non somasomatic modes of coupling and for coupled neurons in the three magnocellular areas of the nucleus. A relationship between mode of coupling and hormone content was not detected. The data support the hypothesis that coupling is a real, functionally significant mechanism for coordinating neuronal activity in this nucleus, particularly under conditions of high hormone demand. They do not support the idea that coupling is artefact. The possibility of a relationship between hormone content and mode of coupling, and the projection pathway(s) of the coupled neurons of each type require further study.

Evidence for excitatory actions of histamine on supraoptic neurons in vitro: mediation by an H1-type receptor

The effects of histamine on the firing of supraoptic neurosecretory neurons in the rat were examined in vitro using acutely prepared, hypothalamo-neurohypophysial explants perifused with an artificial cerebrospinal fluid. Extracellular action potentials meeting the criteria of antidromic invasion from neurohypophysial stalk stimulation were recorded from 135 neurons in the tuberal portion of the supraoptic nucleus, which lies superficially along the tuber cinereum and consists of mostly vasopressin-containing neurons. Units could be classified as slow/silent (76.3%), phasic (21.5%) or continuous (2.2%) on the basis of their spontaneous activity. Histamine applied briefly to the perifusate excited approximately one-third of the slow silent neurons and approximately two-thirds of the phasic neurons, with a wide range (10(-3)-10(-9)) in the effective concentration across neurons. The H1-receptor agonists 2-pyridylethylamine and 2-thiazolylethylamine mimicked these excitations in 10 of 12 and 3 of 6 neurons tested, respectively. The H2-receptor agonists dimaprit (4 neurons) and impromidine (5 neurons) failed to excite any of the tested neurons previously excited by histamine. The H1-receptor antagonist promethazine antagonized histamine's excitatory effect in 8 of 9 cells, while the H2-receptor antagonist cimetidine had little effect on the 9 cells tested. Histamine also modified bursts of activity induced in some slow/silent neurons by antidromic stimulation without having an observable effect in the absence of an antidromic burst. In 10 of 18 neurons histamine produced an elongation of burst duration and a modest increase in intraburst firing rate when applied during an antidromically evoked burst. In an additional 5 of 17 neurons, which had neither previously responded to histamine nor shown an antidromically-evoked burst, the pairing of histamine application and antidromic shocks resulted in an antidromically evoked burst. The effects of histamine on evoked bursts also appeared to be mediated by an H1-receptor. Histamine's excitation of supraoptic neurons is thus dependent on the electrical activity expressed by the neuron at the time of testing. Conductances activated by depolarization of the neuron may be modified by histamine or this compound may alter the threshold for burst generation. Considered with data showing H1-receptor localization and histamine-immunoreactive fibers within the supraoptic nucleus, the present results, as well as those showing the potency of centrally applied histamine in releasing vasopressin, suggest histamine may act physiologically by altering the electrical activity of vasopressin-secreting neurons.

The effects of paraventricular hypothalamic lesions on maternal behavior in rats

The present study was undertaken to determine whether the disruptive effects of knife cuts which sever the lateral connections of the medial preoptic area (MPOA) on maternal behavior are mediated by interfering with the output of the paraventricular hypothalamic nucleus (PVN). Postpartum rats received one of the following: Knife cuts severing the lateral connections of the MPOA; knife cuts severing the lateral connections of the PVN; radiofrequency lesions of the PVN; sham lesions or knife cuts. Only females that received knife cuts severing the lateral connections of the MPOA showed severe deficits in maternal behavior. These results indicate that the influence of the MPOA on maternal behavior is not mediated by the output of the PVN. Since the PVN is the major source of oxytocin input to other brain regions, these results also suggest that oxytocinergic neural pathways are not critical for postpartum maternal behavior. Another important finding was that females with MPOA knife cuts that did not retrieve their young were capable of hoarding candy, suggesting that the retrieval deficit was not the result of a general oral motor deficit.

The cytoarchitecture, histochemistry and projections of the tuberomammillary nucleus in the rat

The normal morphology, efferent projections and possible neurotransmitter content of neurons in the tuberomammillary nucleus (caudal magnocellular nuclei of Bleier et al.) [Bleier, Cohn and Siggelkow (1979) In Anatomy of the Hypothalamus, Vol. 1, pp. 137-220] have been examined in the adult male rat. In Nissl-stained sections, the nucleus can be divided into a dorsomedial, ventral and diffuse part, each of which consists of large, darkly stained neurons cradling the mammillary body. The ventral part is by far the largest and consists of some 2500 neurons on each side of the brain. Immunohistochemical studies indicate that a majority of the large neurons in all three parts of the nucleus stain with antisera against glutamate decarboxylase and [Met]enkephalyl-Arg6-Phe7 heptapeptide and that a smaller subset of these neurons (about 10%) also stain with an antiserum against substance P. Single injections of retrogradely transported fluorescent tracers were made into 18 different sites in 86 animals and the results indicate that all three parts of the tuberomammillary nucleus on one side of the brain send fibers to or through various parts of the neocortex, hippocampal formation, amygdala, basal ganglia, thalamus, superior colliculus and cerebellum on both sides of the brain and that the projection neurons are not organized in a highly topographic way. Injections of two different fluorescent tracers in the same animal indicate that individual neurons in the nucleus may give rise to both ascending and descending projections, as well as projections to widely divergent parts of the forebrain. Together with previous results, this evidence suggests that the tuberomammillary nucleus has widespread projections to the numerous brain structure located in the forebrain and in the caudal medulla (it may not project to the spinal cord), and that its axons may release a mixture of neuroactive substances including gamma-amino butyrate and several peptides. Although its functional significance remains to be investigated, morphological evidence suggests that the tuberomammillary nucleus may constitute one of a series of neurotransmitter-specific cell groups in the brainstem and basal forebrain with diffuse efferent projections that may be involved in the modulation of attention or behavioral state, rather than the processing of specific sensory or motor information.

Granule populations in oxytocin and abnormal perikarya of the supraoptic nucleus of homozygous Brattleboro rats: effects of colchicine administration

Magnocellular neurones in the supraoptic nucleus of the homozygous Brattleboro rat, which are unable to produce vasopressin, were investigated by immunocytochemistry to identify both the oxytocin cells and the abnormal neurones, which in normal animals would produce vasopressin. The abnormal cell profiles were significantly more rounded than those of the oxytocin cells. Both cell types showed evidence of hyperactivity, but the Golgi apparatus was more extensive in the oxytocin cells, probably as a result of the failure of the abnormal cells to produce vasopressin and its neurophysin and the resultant reduction in hormone packaging. Neurosecretory granules (NSG) 160 nm in diameter were found in the oxytocin perikarya but were absent from the abnormal cell bodies. In addition, a population of small dense granules (SDG) 100 nm in diameter was observed in both types of neurone, in numbers equal to the NSG in oxytocin cells. Injection of a low, non-lethal dose of the axonal transport inhibitor colchicine resulted in a rapid and equal accumulation of both NSG and SDG in oxytocin perikarya and of SDG in the abnormal perikarya after one day. The effects of colchicine were reversed 2-3 days after administration. The SDG, which may contain a co-transmitter or co-hormone substance, are thus produced at a similar rate to NSG, and appear to be transported from the perikarya for subsequent release at the nerve endings.

Electrophysiology of the subfornical organ and its hypothalamic connections--an in-vivo study in the rat

The connections of the subfornical organ to a defined population of hypothalamic neurons have been explored with extracellular recording methods in the rat. Electrical stimulation in the subfornical organ has a predominantly excitatory action on a majority of oxytocin and vasopressin-secreting neurosecretory neurons in the supraoptic and paraventricular nuclei. Subfornical organ stimulation also enhances the excitability of paraventricular nucleus neurons that project to the median eminence, and to the dorsomedial medulla. These observations provide initial evidence of functional connectivity of subfornical organ neurons with other hypothalamic cells that are engaged in central regulation of pituitary secretions and autonomic activities.

An enkephalinergic projection from the hypothalamic paraventricular nucleus to the hypothalamic ventromedial nucleus of the rat: an experimental immunohistochemical study

The distribution and origins of enkephalin-like immunoreactive (ENK-IR) fibers in the nucleus ventromedialis hypothalami (vm) of the rat were examined using immunohistochemistry. A dense plexus was evenly distributed in the vm with no regional differences. A group of ENK-IR neurons was concentrated in the ventrolateral portion of the vm. The destruction of the hypothalamic paraventricular nucleus (pv) which contained numerous ENK-IR neurons, resulted in a marked decrease in ENK-IR fibers in the vm on the operated side. In addition, the destruction of the magnocellular portion of the pv, while leaving most of parvocellular portion intact, failed to substantially decrease the ENK-IR fibers in the vm, suggesting that these fibers originate from ENK-IR neurons located in the parvocellular portion of the pv. The present study further showed that the axons of these neurons first proceeded laterally to the perifornical area, next ran ventrolaterally to the ventrolateral portion of the anterior hypothalamic nucleus and finally turned medially to the vm.

Extranuclear axon collaterals of paraventricular neurons in the rat hypothalamus: intracellular staining, immunocytochemistry and electrophysiology

Recent studies have suggested that some paraventricular nucleus (PVN) neurons projected to more than one target and, thereby, perhaps coordinate some aspects of seemingly diverse functions. We have systematically investigated the existence, location, hormonal contents and functional integrity of some axon collaterals arising from PVN neurons. This was done using intracellular injections of the fluorescent dye, Lucifer Yellow, extracellular ejections of horseradish peroxidase (HRP), immunocytochemistry with antisera directed against vasopressin (VP) and oxytocin (OX) and electrophysiological analysis of synaptic activation of perifornical neurons in response to electrical stimulation of the PVN in hypothalamic slices. Each of the three morphological techniques revealed clear axon collaterals, arising in the lateral hypothalamus and generally ventrolateral to the PVN. Most branching axons appeared to have a small number of branch points, and many collaterals appeared to terminate near their parent axon. Electrical stimulation of the PVN was found to activate synaptically perifornical neurons located in the areas where the other methods revealed collaterals. Stimulation outside of the nucleus was ineffective unless current intensities were increased 10-30-fold over those applied to the PVN. We conclude that many PVN neurons, at least some of these containing OX and other VP, give rise to axons that branch in the perifornical and more ventral lateral hypothalamus, and that some of their collaterals probably terminate on neurons close to the PVN.

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.

Alpha-adrenergic activation of rat hypothalamic supraoptic neurons maintained in vitro

Extracellular recordings from 141 rat supraoptic nucleus neurons maintained in vitro in a perfused hypothalamic explant indicated that the excitability of 85% of cells was enhanced by the addition of 10-200 microM norepinephrine (NE) and the alpha-agonist methoxamine (MOXY), but not by isoproterenol, to the perfusion medium or by pressure application. Similar responses were observed from 7 cells maintained in synaptic isolation in medium containing 15 mM Mg2+. At the lowest effective concentration (10 microM), NE and MOXY induced bursting activity without an overall increase in firing frequency; at higher concentrations an initial increase in firing frequency was followed by the appearance of phasic activity patterns. The actions of NE and MOXY were reversibly blocked by administration of the alpha-adrenergic antagonists phenoxybenzamine (1-10 microM), yohimbine (greater than 10 microM) and irreversibly blocked by prazosin (10 nM-1 microM). These observations suggest that NE has a predominantly facilitatory role to enhance the excitability and promote bursting activity in supraoptic neurosecretory neurons through an alpha-1 adrenoreceptor mechanism.

Endogenous peroxidase-like activity in the feline dorsal column nuclei and spinal cord

Endogenous peroxidase-like activity was investigated with a combined light and electron microscopical technique in 15 cats. The lateral cervical nucleus, the dorsal column nuclei, and segments C6 and L5 of the spinal cord were incubated with diaminobenzidine-tetrahydrochloride (DAB) or tetramethylbenzidine (TMB). After histochemical reaction with DAB a considerable amount of activity was found in nerve cells, astrocytes and pericytes. The neuronal labelling was mainly located in mitochondria of axon terminals and in dendrites whereas the astrocytic and pericytic activity was found in cytoplasmic dense bodies. The quantity of stained structures differed considerably between the animals. In TMB reacted tissue endogenous peroxidase-like activity was only sparsely seen. It was found mainly in frozen sections, in which the neuropil and perivascular structures sometimes contained granules and irregular filaments. The significance of the findings is discussed in relation to observations in tracer studies using horseradish peroxidase.

Effects of dehydration and renin on vasopressin concentration in the subfornical organ area

The subfornical organ (SFO), one of the brain circumventricular organs, contains immuno-reactive arginine-vasopressin (AVP). AVP in the SFO area may originate in neurons of the hypothalamo-neurohypophysial system. If AVP in the SFO area is part of the magnocellular neuroendocrine system and is important in the regulation of hydration, then its concentration [(]) should change during prolonged dehydration. The SFO is also a target for angiotensin II when the peptide stimulates drinking and releases AVP from the hypothalamo-neurohypophysial system. For these reasons, it was reasoned that hormones of the renin-angiotensin system may also influence [AVP] in the SFO area. To test these hypotheses [AVP] was measured in the SFO area, hippocampal commissure-fornix (HC-F), neural lobe and plasma of rats after 24, 48 and 72 h of water deprivation and at various times after intracerebroventricular (i.v.t.) administration of 5 milli -Goldblatt Units of renin. Before examining the responsiveness of [AVP] in these brain regions to stimulation, we characterized the extraction and recovery of AVP from brain tissue and determined the variance of [AVP] in the SFO area and HC-F among different groups of animals. AVP was extracted from pooled brain tissue into 0.1 N HCl. The homogenate was centrifuged and AVP in the supernatant was quantified by radioimmunoassay either directly or after bentonite extraction. AVP extracted from the SFO area and HC-F displaced labelled antigen bound to antisera in a manner similar to that displaced by standard AVP. The recovery of AVP, added to the 0.1 N HCl extract and assayed directly, averaged 78-108%, whereas 51% was recovered after bentonite extraction. [AVP] in the SFO area from 47 or more groups of 2-6 organs each, averaged 16 +/- 5 pg/organ, 16 +/- 4 pg/mg wet wt, 153 +/- 31 pg/mg protein, and was not significantly different from that contained in the HC-F, 10 +/- 1 pg/mg wet wt and 111 +/- 16 pg/mg protein. A frequency histogram of these data revealed a normal (HC-F) and skewed distribution (SFO). Water deprivation for 24, 48 and 72 h stimulated drinking and increased plasma [AVP]. The elevation in plasma [AVP] plateaued after 48 and 72 h of water deprivation, whereas [AVP] in the neural lobe was reduced (P less than 0.05). Water deprivation increased [AVP] in the HC-F (control vs 72 h water deprivation), but did not alter hormone in the SFO area when expressed as pg/mg protein or pg/mg wet wt.(ABSTRACT TRUNCATED AT 400 WORDS)

Activity patterns and osmosensitivity of rat supraoptic neurones in perfused hypothalamic explants

Extracellular recordings were obtained in vitro from supraoptic neurones in an explant of rat hypothalamus maintained viable through intravascular perfusion with artificial media. The spontaneous activity observed from 64% of cells included continuously active neurones (mean frequency 3.2 +/- 0.8 Hz) located throughout the nucleus, and phasically active neurones (8.6 +/- 0.6 Hz) located predominantly in the caudal half of the nucleus. Supraoptic neurones displayed antidromic activation (latency range 6.5-18.2 ms) following stimulation of the pituitary stalk and orthodromic excitatory or inhibitory responses following stimulation in the anteroventral third ventricular (AV3V) region. Synaptic responses were reversibly abolished during perfusion with media containing 12 mM-Mg2+. A 15-40 mosmol/l increase in the osmotic pressure of the perfusion media, through addition of NaCl, sucrose or mannitol, prompted nineteen of twenty-three cells to increase their discharge frequency. The patterns of response included the induction or simple increase in continuous firing frequency, the induction or enhancement of phasic or bursting activity and a change from continuous to phasic activity. Similar responses to an osmotic stimulus were obtained from thirteen of twenty-one supraoptic neurones, including two phasic neurones, where synaptic activity had been abolished during perfusion with 12 mM-Mg2+. Osmosensitivity appeared to be selective for supraoptic cells; no significant change in firing frequency was observed from any of six cells recorded in the lateral hypothalamus or thirteen cells recorded in the medial hypothalamus during exposure to a change in osmotic pressure of +20 to +40 mosmol/l, using either control media (seven cells) or media containing 12 mM-Mg2+ (twelve cells). These observations indicate that supraoptic neurones maintained in vitro can display spontaneous, antidromie , orthodromic and osmotically induced activity patterns identical to those observed with in vivo recordings. The persistence of a reduced osmosensitivity among supraoptic neurones in the absence of synaptic transmission indicates that although these cells can function as osmoreceptors, their osmosensitivity may be enhanced through synaptic input from adjacent neurones, possibly located in the AV3V area. The presence of phasic activity among supraoptic neurones maintained in media where synaptic transmission has been abolished suggests that the mechanisms responsible for such activity patterns are endogenous membrane properties of a subpopulation of supraoptic neurones.

Study of the efferent connections of the enkephalinergic magnocellular dorsal nucleus in the guinea-pig hypothalamus using lesions, retrograde tracing and immunohistochemistry: evidence for a projection to the lateral septum

The efferents of enkephalin-immunoreactive neurons in the magnocellular dorsal nucleus of the guinea-pig were studied using different neuroanatomical methods and indirect immunocytochemical technique. Following unilateral implantation of the fluorescent dye 4',6-diamidino-2-phenylindole in the lateral septal nucleus, retrogradely-labeled perikarya were found in the magnocellular dorsal nucleus. These labeled perikarya reacted with antiserum against enkephalin, demonstrating that enkephalin-immunoreactive neurons in the magnocellular dorsal nucleus project to the lateral septal nucleus. In other experiments, complete bilateral lesions were produced in the magnocellular dorsal nucleus by electrocoagulation. Enkephalin-immunoreactive nerve fibers and terminals were totally depleted in the lateral septal nucleus. This confirms that septal enkephalin-immunoreactive terminals originate in the magnocellular dorsal nucleus and further suggests that this nucleus is the source of all the enkephalin-immunoreactive material found in the septum. Experiments utilizing two different fluorescent dyes, 4',6-diamidino-2-phenylindole and propidium iodide, injected in each side of the lateral septal nucleus, respectively, demonstrated that the magnocellular dorsal nucleus gives off axon collaterals to both sides of the septum, since double-labeling of individual cell bodies was detected in the nucleus. By relating this finding to the results obtained after unilateral destruction of the nucleus, which caused an incomplete loss of enkephalin- immunoreactive material in the lateral septal nucleus ipsilaterally, it is suggested that the enkephalinergic hypothalamo-septal pathway contains unbranching neurons projecting ipsilaterally and branching neurons distributing fibers ipsilaterally and contralaterally. Lesion experiments, and experiments based on the retrograde axonal transport of horseradish peroxidase after intravenous injections, demonstrated that the magnocellular dorsal nucleus contributes neither to the tubero-infundibular nor to the hypothalamo-neurohypophyseal tracts. The lateral septal nucleus receives numerous aminergic and peptidergic projections, indicating the potential importance of this region in physiological and behavioral events. In the guinea-pig, the well-demarcated enkephalinergic pathway demonstrated in this study provides a convenient model for the experimental study of the enkephalinergic innervation of the lateral septal nucleus.

Evidence that spinal cord thyrotropin-releasing hormone is independent of the paraventricular nucleus

To determine whether neuronal perikarya containing thyrotropin-releasing hormone (TRH) in the paraventricular nucleus project to the spinal cord, this region of the hypothalamus was ablated by a midline electrolytic lesion and the spinal cord assayed for its content of TRH. In contrast to the marked diminution of TRH in the median eminence, there was no significant change in the TRH content in the spinal cord. These results indicate the absence of significant afferent input of TRH-containing perikarya in the paraventricular nucleus to the spinal cord.

Horseradish peroxidase: a tool for study of the neuroendocrine cell and other peptide-secreting cells

The versatility of horseradish peroxidase is its usefulness both as an antigenic marker and as a probe molecule. We have demonstrated in the neuroendocrine cell that an HRP-bound antibody offers a high order of resolution for determining in which cellular compartment an antigen is located and where it is not. When native peroxidase is applied as an intracellular probe, it labels organelles associated with endocytosis in retrograde axonal transport and with the lysosomal system in both retrograde and orthograde axonal transport. The investigation that remains is the application of lectin-bound HRP to determine the pathways of membrane flow at the time when the neuroendocrine cell is stimulated to synthesize, transport, and secrete its peptide. For example, we are interested to know (1) whether internalized axon terminal membrane tagged with wheat germ agglutinin-HRP is channeled to all Golgi saccules engaged in the production of secretory granules in salt stimulated supraoptic neurons; and (2) if internalized cell membrane of the supraoptic cell body is tagged with wheat germ agglutinin-HRP and channeled to GERL, will this membrane be transferred from GERL to secretory granules, lysosomes in the cell body and axon, the axonal endoplasmic reticulum, and to autophagic/crinophagic vacuoles in axon terminals of salt-stressed supraoptic neurons? These additional studies should provide a more comprehensive, morphological picture of membrane flow in a neuroendocrine cell that is responding to the metabolic demands placed upon it.

The distribution of corticotropin releasing factor-like immunoreactive neurons in rat brain

Using the indirect immunofluorescent technique, corticotropin releasing factor (CRF)-like immunoreactive nerve fibers and cell bodies were observed to be widely distributed in rat brain. A detailed stereotaxic atlas of CRF-like immunoreactive neurons was prepared. Large numbers of CRF-containing perikarya were observed in the nucleus paraventricularis, with scattered cells in the following nuclei: accumbens, interstitialis stria terminalis, preopticus medialis, supraopticus, periventricularis hypothalami, amygdaloideus centralis, dorsomedialis, substantia grisea centralis, parabrachialis dorsalis and ventralis, tegmenti dorsalis lateralis, vestibularis medialis, tractus solitarius and reticularis lateralis. The most intense staining of CRF-containing fibers was observed in the external lamina of the median eminence. Moderate numbers of CRF-like fibers were observed in the following nuclei: lateralis and medialis septi, tractus diagonalis, interstitialis stria terminalis, preopticus medialis, supraopticus, periventricularis thalami and hypothalami, paraventricularis, anterior ventralis and medialis thalami, rhomboideus, amygdaloideus centralis, habenulae lateralis, dorsomedialis, ventromedialis, substantia grisea centralis, cuneiformis, parabrachialis dorsalis and ventralis, tegmenti dorsalis lateralis, cerebellum, vestibularis medialis, reticularis lateralis, substantia gelatinosa trigemini and lamina I and II of the dorsal horn of the spinal cord. The present findings suggest that a CRF-like peptide may be involved in a neurotransmitter or neuromodulator role, as well as a hypophysiotropic role.

The organization of noradrenergic pathways from the brainstem to the paraventricular and supraoptic nuclei in the rat

Axonal transport and immunohistochemical methods have been used to clarify the organization of pathways from noradrenergic and adrenergic cell groups in the brainstem to the paraventricular (PVH) and supraoptic (SO) nuclei of the hypothalamus. First, the location of such cells was determined with a combined retrograde tracer-immunofluorescence method. The fluorescent tracer, True Blue, was injected into the PVH or the SO, and sections through the brainstem were stained with anti-(rat) DBH, a specific marker for noradrenergic and adrenergic neurons. It was found that, after injections in the PVH, doubly labeled neurons were confined almost exclusively to 3 cell groups, the A1 region of the ventral medulla, which contained a majority of such cells, the A2 region in the dorsal vagal complex, and the locus coeruleus (A6 region). After injections centered in the SO an even greater proportion of doubly labeled cells were found in the A1 region, although some were also found in the A2 and A6 regions. The topography of doubly labeled cells indicates that these projections arise primarily from noradrenergic neurons, although adrenergic cells in both the C1 and the C2 groups probably contribute as well. Because well over 80% of the retrogradely labeled cells in these three regions were also DBH-positive, we next placed injections of [3H]amino acids into each of them in different groups of animals, and traced the course and distribution of the ascending (presumably DBH-positive) projections to the PVH and SO in the resulting autoradiograms. Injections centered in the A1 region labeled a substantial projection to most parts of the parvocellular division of the PVH, and was most dense in the dorsal and medial parts. In addition, terminal fields were labeled on those parts of the magnocellular division of the PVH, and of the SO, in which vasopressinergic cell bodies are concentrated. Injections centered in the A2 region also labeled a projection to the parvocellular division of the PVH that was topographically similar, but less dense, than that from the A1 region. In contrast, [3H]amino acid injections centered in the locus coeruleus labeled a moderately dense projection to the PVH that was limited to the medialmost part of the parvocellular division. Neither the A2 nor the A6 cell groups project to the magnocellular parts of PVH, or to the SO. The autoradiographic material, and additional double-labeling experiments, were used to identify and to characterize projections that interconnect the A1, A2 and A6 regions, as well as possible projections from these cell groups to the spinal cord. These results may be summarized as follows: a substantial projection from the nucleus of the solitary tract to the A1 region was identified, but this pathway does not arise from catecholaminergic neurons in the A2 cell group. DBH-stained cells in the A1 region project back to the dorsal vagal complex, as well as quite massively to the locus coeruleus (A6 region)...

Alterations in local cerebral glucose utilization during hemorrhagic hypotension in the rat

The alterations in local cerebral glucose utilization in 58 anatomically discrete regions which occur during a period of hemorrhagic hypotension have been investigated in conscious rats, using the quantitative autoradiographic 14C-deoxyglucose technique. Hemorrhagic hypotension (mean arterial pressure reduced by approximately 50 mm Hg) effected significant increases in glucose utilization in eight areas of the central nervous system, namely, the nucleus of the tractus solitarius (glucose utilization increased by 38%), the dorsal motor nucleus of the vagus (by 36%), locus coeruleus (by 38%), lateral habenular nucleus (by 40%), periventricular nucleus of the hypothalamus (by 41%), paraventricular nucleus of the hypothalamus (by 97%), supraoptic nucleus (by 86%), and the interstitial nucleus of the stria terminalis (by 84%). In five of these eight areas (nucleus of the tractus solitarius, dorsal motor nucleus of the vagus, paraventricular and supraoptic nuclei, and the interstitial nucleus of the stria terminalis), a significant relationship could be demonstrated between the level of glucose utilization and mean arterial blood pressure. In the majority of the CNS regions examined (neocortex, hippocampus, thalamus, extrapyramidal and motor areas), hemorrhagic hypotension was without significant effect upon local cerebral glucose utilization. The results provide direct evidence of the functional involvement of specific brain areas of conscious rats (thus obviating complicating anesthetic influences) in the response of the CNS to hemorrhagic hypotension.

Central neurohypophyseal peptide pathways: interactions with endocrine and other autonomic functions

Electrophysiological and pharmacological studies have been carried out in rats and rabbits to attempt to identify possible functional roles for neurohypophyseal peptides in brain. In anesthetized rats, single unit recordings and antidromic activation criteria were utilized to identify projections of the paraventricular nucleus (PVN) to neurohypophysis and to extrahypothalamic areas (amygdala or nucleus tractus solitarius). None of the cells tested innervated more than one of these areas and, when tested for their responses to haemorrhage, increased body osmolarity, or suckling of pups, only the osmotic stimulus caused increased activity in some cells projecting to amygdala or nucleus tractus solitarius. Indirect evidence as well as direct measurement by radioimmunoassay of arginine vasopressin (AVP) in brain perfusates revealed probable central release of AVP in response to stimuli known to activate pituitary secretion of this peptide. These observations raise the possibility that certain brain and pituitary peptidergic systems may function in a co-ordinated manner.

Immunocytochemical, Golgi and electron microscopic characterization of putative dendrites in the ventral glial lamina of the rat supraoptic nucleus

Processes of magnocellular neurosecretory cells in the rat supraoptic nucleus which project along the pial surface in the ventral glial lamina were investigated using immunocytochemistry, Golgi stains and electron microscopy. Immunocytochemical studies revealed that although both oxytocin- and vasopressin-containing processes were evident in the ventral glial lamina, vasopressin-containing processes predominated. Ventral processes were thicker and of a different morphology than dorsal axon-like processes which joined the hypothalamo-neurohypophysial tract and exhibited large varicosities along their length or at their apparent termination. Golgi stains revealed that classically defined dendrites of supraoptic neurons projected primarily ventrally and often invaded the ventral glial lamina. No axons were traced to the lamina. Ultrastructurally, processes within the ventral glial lamina characterized as dendrites could be stained immunocytochemically for neurophysin and were post-synaptic to a variety of presynaptic elements. The results suggest that many dendrites from magnocellular neurosecretory cells in the supraoptic nucleus project to the ventral glial lamina and form a restricted, receptive plexus. The previously demonstrated coexistence of catecholamine-containing varicosities and other axon types with these processes in the lamina indicates an important role for supraoptic dendrites in integrating a wide variety of information relevant to neurohypophysial hormone release.

Connections of the hypothalamic paraventricular nucleus with the neurohypophysis, median eminence, amygdala, lateral septum and midbrain periaqueductal gray: an electrophysiological study in the rat

Extracellular recordings were obtained from 555 paraventricular (PVN) nucleus neurons in pentobarbital-anesthetized male rats. Cells were examined for their spontaneous activity patterns and response to single 1-Hz electrical stimulation of the neurohypophysis, median eminence, amygdala, lateral septum (LS) and midbrain periaqueductal gray (PAG). Neurohypophyseal stimulation evoked antidromic activation from 109 neurons. Among spontaneously active neurohypophyseal neurons, evidence of a recurrent inhibitory pathway usually required pituitary stimulus intensities twice threshold for antidromic activation. Orthodromic excitatory or inhibitory responses followed amygdala and LS stimulation, but not PAG stimulation. The amygdala influence was predominantly inhibitory to 'phasic' (putative vasopressin-secreting) PVN neurohypophyseal neurons. Neurohypophyseal stimulation evoked orthodromic responses from 124 PVN cells; some of these neurons were also responsive to stimulation in other sites. Median eminence stimulation evoked antidromic responses from 37 PVN neurons; some of these cells also displayed phasic activity but no evidence for recurrent inhibition. Twelve cells in this group were also activated antidromically from both the median eminence and the neurohypophysis; collision tests suggest that the median eminence innervation may be an axon collateral of a neurohypophyseal pathway. Amygdala stimulation was inhibitory to some cells in this category. Amygdala, LS and PAG stimulation evoked antidromic activation from a small number of PVN cells, but none of these cells appeared to innervate more than one area, including the neurohypophysis, and none displayed phasic activity. Orthodromic responses were recorded among other PVN neurons after stimulation in these sites; however, PAG stimulation was the least effective stimulation area. These observations provide additional electrophysiological data that confirm efferent PVN connections to all areas tested, afferent connections from amygdala and LS but not PAG, and the possibility for coordinated activity among PVN neurons through local recurrent or common afferent connections.

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.

The paraventricular nucleus of the hypothalamus: cytoarchitectonic subdivisions and organization of projections to the pituitary, dorsal vagal complex, and spinal cord as demonstrated by retrograde fluorescence double-labeling methods

Experiments using two retrogradely transported fluorescent dyes (bisbenzimide-true blue, and Evans blue-granular blue) were performed in order to determine whether the same or different populations of neurons of the paraventricular nucleus of the hypothalamus (PVH) project to the pituitary gland, dorsal vagal complex, and spinal cord in the rat. The results suggest that cells projecting to the pituitary gland are concentrated in the magnocellular core of the nucleus, while the descending connections arise primarily from the surrounding parvocellular division. The occurrence of neurons double-labeled with both dyes further indicate that at lease 10-15% of the labeled cells in the parvocellular division send divergent axon collaterals to the dorsal vagal complex and to the spinal cord. Cell counts suggest that at least 1,500 cells in the PVH project to the medulla and/or spinal cord. These results, combined with a cytoarchitectonic analysis, show that the PVH consists of eight distinct subdivisions, three magnocellular and five parvocellular. The lateral hypothalamic area and zona incerta also contain a large number of cells projecting to the dorsomedial medulla and spinal cord; approximately 15% of such cells are the double-labeled following injections of separate tracers into these two regions of the same animal.

Efferents from the medial anterior hypothalamic area in the guinea pig

Medial anterior hypothalamic connections were studied with H3-proline and autoradiography. Most of the axons projected to other hypothalamic nuclei. The major pathways were found ventral medial to the fornix and in the periventricular tract. Substantial projections were apparent in the ventromedial and dorsomedial nuclei with less label in the arcuate nucleus. The dorsal premammillary nuclei were labeled bilaterally, particularly with more caudal injections of anterior hypothalamus. Efferents were evident in the posterior hypothalamus and continued into the central gray of the midbrain. Labeled fibers reached the ventral tegmental area and in the reticular formation were traced only through pons. Rostral projections were to the medial and lateral preoptic areas and ventral lateral septum. The bed nucleus of stria terminals was labeled a very few fibers reached the medial amygdaloid nucleus. The periventricular nucleus of thalamus was labeled.

Separate neurons in the paraventricular nucleus project to the median eminence and to the medulla or spinal cord

Double labeling experiments with the retrogradely transported fluorescent tracers bisbenzimide and True blue indicate that separate populations of cells in the paraventricular nucleus of the hypothalamus project to the median eminence and to the dorsomedial medulla or spinal cord in the rat. The cell populations giving rise to each projection are, however, at least partially intermixed in different parts of the nucleus.

Additional forebrain regions projecting to the posterior pituitary: preoptic region, bed nucleus of the stria terminalis, and zona incerta

Following the injections of horseradish peroxidase (HRP) into the posterior lobe of the pituitary gland of adult albino rats, scattered retrogradely labeled neurons were observed in the caudal part of the bed nucleus of the stria terminalis (ventrally and laterally), in the median and periventricular preoptic nuclei, in the medial and lateral preoptic areas, in the zona incerta, and in the substantia innominata. Immunohistochemical studies indicate that oxytocin-containing, or a mixture of vasopressin- and oxytocin-containing neurons are present in each of these areas. These findings suggest that the cells of the magnocellular neurosecretory system are more widely distributed than has previously been reported.