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

The periventricular anteroventral third ventricle (AV3V): its relationship with the subfornical organ and neural systems involved in maintaining body fluid homeostasis

The periventricular tissue surrounding the anteroventral third ventricle (AV3V) is critically involved in the maintenance of normal body fluid balance and distribution. The present review examines the anatomical, neurochemical, and functional relationship of the AV3V with neural systems subserving body fluid homeostasis. In particular, the nature of AV3V afferents from the subfornical organ (SFO) and from brainstem noradrenergic cell groups is discussed. A model is presented proposing that specific structures within the AV3V, particularly along the ventral lamina terminalis, function to integrate information derived from blood-borne angiotensin II (via the SFO) with input arising from vascular pressure/volume receptors. The resultant of this integration is important for the generation of a normal component of thirst (i.e., drinking) associated with extracellular dehydration.

Distribution of atrial natriuretic factor-like immunoreactive neurons in the rat brain

Using antisera generated in rabbits against rat atriopeptin III [alpha-rANP(5-28)] and human alpha-atrial natriuretic polypeptide we mapped the distribution of atrial natriuretic factor-like immunoreactivity throughout the rat central nervous system. Cell bodies were observed in the telencephalon (nucleus interstitialis striae terminalis and between the amygdala centralis and medialis), throughout the diencephalon in all nuclei of the "anteroventral third ventricle", the base of the hypothalamus, the subzona incerta area, the medial forebrain bundle and the medial habenula, in the mesencephalon (mamillary body, substantia nigra lateralis, dorsal and ventral parabrachial nuclei) and very sparse in the medulla oblongata along the fourth ventricle towards the vestibular nuclei, the nucleus tractus solitarii and nervi trigemini. Fibers were present wherever cell bodies were located. The highest relative densities were observed in the anteroventral third ventricle area and the medial habenula. Sparse fibers were also seen in the spinal cord (dorsal and ventral horn and around the central canal) and in the posterior pituitary. The predominance of the atrial natriuretic factor-like perikarya and fibers in the anteroventral third ventricle area suggests an involvement of this peptide in central blood pressure control.

Colocalization of neuropeptide Y immunoreactivity in brainstem catecholaminergic neurons that project to the paraventricular nucleus of the hypothalamus

Immunohistochemical methods were used in the rat to plot the distribution of neuropeptide Y (NPY) immunoreactivity in the paraventricular (PVH) and supraoptic (SO) nuclei of the hypothalamus, and a combined retrograde transport-double immunohistochemical labeling technique was used to determine the extent to which NPY immunoreactivity is coexpressed in brainstem cell groups that stain with antisera to phenylethanolamine-N-methyltransferase (PNMT; a marker for adrenergic neurons) or dopamine-beta-hydroxylase (DBH; a marker for adrenergic and noradrenergic neurons) and also project to the PVH. The results confirm the existence of a major NPY-immunoreactive pathway that is in a position to influence each major class of output neurons in the PVH. Thus, most parts of the parvicellular division receive a dense input that is similar to, though somewhat more extensive than, the one stained by DBH antisera. However, in the magnocellular division catecholaminergic inputs are preferentially associated with vasopressinergic neurons, while NPY-stained fibers tend to be more evenly distributed in regions containing both oxytocinergic and vasopressingergic neurons, and their density appear to be lower than that of DBH-stained fibers. In the SO, only a moderate NPY-stained input was apparent, while, as described previously, DBH-immunoreactive fibers are rather dense and are preferentially distributed in vasopressinergic regions of the nucleus. The results of combined retrograde transport-double immunohistochemical labeling experiments may be summarized as follows: the vast majority of cells in the medulla that were retrogradely labeled after discrete implants of the fluorescent tracer true blue into the PVH, and were PNMT-immunoreactive, also stained for NPY. However, less extensive co-localization was detected in noradrenergic cell groups of the caudal medulla. About 60% of the retrogradely labeled-DBH positive cells in the A1 cell group were also NPY-positive, while those in the caudal part of the nucleus of the solitary tract (the A2 cell group) usually failed to stain with anti-NPY. Similarly, in the locus coeruleus (the A6 cell group) where virtually all retrogradely labeled neurons were DBH-positive, only rarely were triply labeled cells detected. These results suggest that NPY immunoreactivity is extensively co-contained within adrenergic neurons of the C1, C2, and C3 groups that project to the PVH, while the correspondence in noradrenergic cell groups is less complete, and generally limited to a subset of neurons in the A1 cell group.(ABSTRACT TRUNCATED AT 400 WORDS)

Selective cardiovascular and neuroendocrine effects of a kappa-opioid agonist in the nucleus tractus solitarii of rats

The cardiovascular and neuroendocrine effects of a selective kappa-opiate receptor agonist (U50488H) microinjected into the nucleus tractus solitarii have been investigated in urethane-anaesthetized rats. Comparative experiments were conducted using 8-arginine vasopressin (AVP)-deficient Brattleboro rats and an opiate agonist selective for delta receptors. Unilateral injection of U50488H elicited a significant dose-dependent increase in mean arterial pressure and a small decrease in heart rate in Sprague-Dawley rats. The pressor effect was blocked preferentially by the relatively selective kappa-receptor antagonist MR2266BS compared to naloxone. Bilateral injections of U50488H elicited a relatively greater increase in mean arterial pressure than unilateral injections and a significant decrease in heart rate. U50488H did not elicit a pressor effect in Brattleboro rats, whereas a marked response (associated with a significant increase in AVP secretion) was found in parent strain Long-Evans rats. In contrast, no such differential effects in the response of Brattleboro and Long-Evans rats were observed in parallel experiments using equimolar doses of the selective delta-opiate agonist Tyr-D-Ser-Gly-Phe-Leu-Thr which elicited a transient pressor response. An antagonist [1-(beta-mercapto-beta, beta-cyclopentamethylene-propionic acid)2-(0-methyl) tyrosine] arginine vasopressin (1,d(CH2)5Tyr(ME)AVP) specific for the vasopressor action of AVP blocked the U50488H-induced pressor response in a dose-dependent manner when administered intravenously 10 min prior to the kappa agonist, but did not significantly attenuate the response to the delta agonist. Conversely, the U50488H-induced response was not modified by pre-treatment with phenoxybenzamine whereas the delta-agonist pressor response was completely blocked by it. The results provide evidence for specific kappa-opiate cardiovascular and neuroendocrine responses in the nucleus tractus solitarii and suggest that a kappa-receptor mechanism, possibly involving a peptide of the dynorphin group as the endogenous ligand, may operate in the central control of blood pressure and AVP secretion.

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.

Inhibiting the rabbit caudal ventrolateral medulla prevents baroreceptor-initiated secretion of vasopressin

The A 1 noradrenergic neurones are known to project from the caudal ventrolateral medulla to the vasopressin-secreting neuroendocrine cells in the hypothalamus. They therefore represent a possible central pathway from the medulla to the hypothalamus for baroreceptor-initiated secretion of vasopressin. We tested this hypothesis in the anaesthetized rabbit. Muscimol, a gamma-aminobutyric-acid-receptor agonist, was injected into the caudal ventrolateral medulla to inhibit the A 1 noradrenergic neurones. Secretion of vasopressin, measured by radioimmunoassay, was initiated either by arterial haemorrhage or by constriction of the inferior vena cava. After injection of vehicle into the caudal ventrolateral medulla, or after injection of muscimol into nearby control areas, both haemorrhage and constriction of the inferior vena cava produced the expected elevation in plasma vasopressin. After injection of muscimol into the caudal ventrolateral medulla, secretion of vasopressin in response to haemorrhage and to constriction of the inferior vena cava, was completely abolished. The A 1 noradrenergic neurones may be the sole pathway transmitting the reflex for baroreceptor-initiated secretion of vasopressin from the medulla to the hypothalamus.

The central organization of the vagus nerve innervating the stomach of the rat

We employed the neural tracers cholera toxin-horseradish peroxidase and wheat germ agglutinin-horseradish peroxidase to examine the organization of the afferent and efferent connections of the stomach within the medulla oblongata of the rat. The major finding of this study is that gastric motoneurons of the dorsal motor nucleus (DMN) possess numerous dendrites penetrating discrete regions of the overlying nucleus of the solitary tract (NTS). In particular, dendritic labelling was present in areas of NTS which also received terminals of gastric vagal afferent fibers such as the subnucleus gelatinosus, nucleus commissuralis, and medial nucleus of NTS. This codistribution of afferent and efferent elements of the gastric vagus may provide loci for monosynaptic vagovagal interactions. A small number of dendrites of DMN neurons penetrated the ependyma of the fourth ventricle and a few others entered the ventral aspect of the area postrema, thus making possible the direct contact of preganglionic neurons with humoral input from the cerebrospinal fluid and/or the peripheral plasma. Nucleus ambiguus neurons projecting to the stomach predominantly innervate the forestomach. The dendrites of these cells, when labelled, were generally short, and extended beyond the compact cluster of ambiguus neurons in a ventrolateral direction, parallel to the fascicles of vagal efferent fibers traversing the medulla.

Excitation of neuronal function in rabbit caudal ventrolateral medulla elevates plasma vasopressin

Injection of L-glutamate into the caudal ventrolateral medulla of the rabbit caused a dose-dependent increase in plasma vasopressin. Activity of the A1 noradrenergic cells within the caudal ventrolateral medulla appears to excite the vasopressin-secreting neuroendocrine cells within the hypothalamus.

Newly identified GABAergic neurons in regions of the ventrolateral medulla which regulate blood pressure

Glutamic acid decarboxylase (GAD), the enzyme which synthesizes the inhibitory transmitter gamma-aminobutyric acid (GABA), was localized immunocytochemically within cells and processes distributed throughout the ventrolateral medulla. In caudal regions, GAD-stained cells were adjacent to the 'precerebellar' lateral reticular nucleus and partially overlapped the A1 area of norepinephrine synthesizing neurons. The largest number of labeled neurons filled the rostral ventrolateral medulla (RVL), coinciding with and extending beyond the C1 adrenergic area. GAD-positive cells also occupied the nucleus reticularis parvocellularis, raphe magnus (RM) and lateral wings of RM in the region of the pararaphe. Intrinsic GAD-containing cells in the ventrolateral medulla may tonically inhibit sympathoinhibitory neurons in the caudal ventrolateral medulla (CVL) and sympathoexcitatory neurons in the RVL.

Subarachnoid haemorrhage in the rat: effect on the development of vasospasm of selective lesions of the catecholamine systems in the lower brain stem

Intracisternal injection of blood in the rat produces an angiographically demonstrable biphasic vasospasm. Lesioning at the level of the mesencephalon of the ascending catecholamine pathways from locus coeruleus in the pons and the A1 and A2 nuclei in the medulla oblongata prior to cisternal blood injection prevents the development of both acute and late spasm. Selective lesioning in the medulla oblongata of ascending fibres from A1 and A2 also prevents development of spasm, indicating that these nuclei, which project to the hypothalamus-pituitary, are essential for the spasm syndrome. It is suggested that a substance vasospasm is produced by a substance liberated either by the hypothalamus or by the pituitary is involved in the occurrence of spasm.

Clonidine effects on catecholamine levels and turnover in discrete hypothalamic and extra-hypothalamic areas

In rats treated with alpha-methyl-p-tyrosine (alpha-MpT) or saline, the effects of clonidine on the levels and turnover of norepinephrine (NE), epinephrine (EPI) and dopamine (DA) were analyzed in microdissected regions of the hypothalamus and extra-hypothalamic structures. In 7 of the 9 brain sites examined (namely dorsomedial nucleus, ventromedial nucleus, medial preoptic area, midlateral perifornical hypothalamus, frontal cortex, dorsal hippocampus and cerebellum), clonidine (50 micrograms/kg) caused a significant decrease in NE turnover, with no change in steady-state levels. In the two remaining areas, namely the hypothalamic paraventricular nucleus and the locus coeruleus, clonidine produced different patterns of effects. In the paraventricular nucleus (PVN), clonidine significantly reduced NE content in saline-treated rats, and in rats injected with alpha-MpT + clonidine, no further change in NE concentration was observed. In the locus coeruleus, both NE levels and turnover were unaltered. Epinephrine and DA turnover, in contrast to NE turnover, was unaffected by clonidine in all brain areas, with the exception of the midlateral hypothalamus, where the alpha-MpT-induced depletion of EPI and DA was totally reversed by clonidine, and in the frontal cortex, where DA turnover was also significantly reduced. These data are discussed relative to the proposed physiological actions of clonidine in the hypothalamus.

Inputs from the A1 noradrenergic region to hypothalamic paraventricular neurons in the rat

Electrical stimulation of the rat A1 noradrenergic region produced excitation (77%) of the activity of putative vasopressin (VP)-secreting neurons in the paraventricular nucleus (PVN) and produced excitation (4%), inhibition (26%) and excitation-inhibition (11%) of the activity of PVN neurons that were not antidromically identified by neurohypophysial stimulation. The excitatory response of putative VP-secreting neurons was blocked by microiontophoretically applied phentolamine, an alpha-adrenoceptor antagonist, but not by timolol, a beta-adrenoceptor antagonist. The inhibitory response of unidentified PVN neurons, on the other hand, was blocked by timolol, but not by phentolamine.

2-Deoxyglucose uptake in the central nervous system during systemic hypercapnia in the peripherally chemodenervated rat

Changes in 2-deoxyglucose (2-DG) uptake in the central nervous system during systemic hypercapnia were determined by the [3H]2-DG autoradiographic method in peripherally chemodenervated rats. Autoradiographs were made from serial transverse sections of the brain and analyzed by a computer-based interactive image processing system for areas having increases or decreases in metabolic activity compared with control animals. The most pronounced change shown by autoradiographs of the hypercapnic animals was a generalized decrease in the metabolism of the gray matter throughout the central nervous system with respect to the normocapnic controls. However, several central structures showed evidence of either no change or an increased metabolism in the hypercapnic animals. In the brain stem these areas were localized to the ventrolateral region of the nucleus of the solitary tract rostral to the obex, around the region of the nucleus retroambiguus, in a region of the ventrolateral medullary reticular formation extending rostrally from the obex to the level of the intramedullary rootlets of the facial nerve, in the region of the ventral nucleus raphe pallidus, and in the region of the lateral parabrachial nucleus. In the diencephalon these regions included the supraoptic nucleus and the dorsal hypothalamic area, extending into the caudal portion of the paraventricular nucleus. The thoracolumbar cord showed activation of the lateral aspects of the dorsal horns, the region of lamina X and the region of the intermediolateral nucleus. These data may be interpreted as a functional map of the central structures activated in hypercapnia in the peripheral chemodenervated rat. It appears likely that these structures are involved in mediating the cardiorespiratory responses associated with the activation of central chemoreceptors by the increased carbon dioxide concentrations.

Response of phasically firing paraventricular neurons to A1 noradrenergic region stimulation and its attenuation by adrenoceptor antagonists

In urethane-anesthetized rats electrical stimulation of the A1 noradrenergic region in the ventrolateral medulla produced excitatory (27%), excitatory-inhibitory (24%), or inhibitory (34%) responses of spontaneous activity of phasically firing units in the paraventricular nucleus (PVN) of the hypothalamus. The latency of excitatory responses was significantly shorter in those followed by inhibition, suggesting the existence of two distinct excitatory pathways. The latency of inhibitory responses, however, was not different between primary and postexcitatory inhibitory responses. Effects of alpha- and beta-adrenoceptor antagonists on responses of phasically firing PVN units to stimulation of the A1 region were tested. The excitatory responses that were not followed by inhibition were significantly attenuated by intraperitoneal phenoxybenzamine, whereas the other responses were not affected. Both primary and postexcitatory inhibitory responses were significantly attenuated by intravenous propranolol, whereas neither type of excitatory response was affected. These results suggest that the medullary (A1 region) inputs to phasically firing PVN units are mediated by alpha-adrenergic (excitatory), beta-adrenergic (inhibitory), and nonaminergic (excitatory) pathways.

Impaired diet-induced thermogenesis in brown adipose tissue from rats made obese with parasagittal hypothalamic knife-cuts

Two experiments were performed to determine if bilateral parasagittal hypothalamic knife-cuts (KCs), which produce long-term overeating and obesity, after biochemical indices of brown adipose tissue (BAT) reactivity to thermogenic stimuli. In the first study, responses to environmental cold were tested. Four weeks after surgery, KC rats had gained 4-5 times more weight than controls and were obese (increased Lee Obesity Index and weight of gonadal white fat). Before being sacrificed, groups of KC and control rats were exposed to 4 degrees C for 21 hr or remained at 28 degrees C. Interscapular BAT weighed 300% more in KC rats, due largely to increased white fat content. Functional indices of BAT thermogenic capacity (protein content, DNA content, cytochrome oxidase activity and mitochondrial guanosine diphosphate (GDP) binding) were normal at 28 degrees C. Exposure to 4 degrees C produced greatly enhanced responses but these were equivalent for both groups. This suggested an intact capacity for non-shivering thermogenesis in obese KC rats. In the second study, the same BAT responses were examined in other rats fed a palatable "cafeteria" diet (CAFE). One week after surgery, KC and control rats were subdivided into groups that received chow alone or chow plus four different palatable foods daily. Before sacrificing 4-5 weeks later, KC rats had gained 3-4 times more weight than controls and were obese. Interscapular BAT weighed 200-300% more in KC rats. CAFE feeding produced larger increments in all variables for KC vs. control rats. Most importantly, GDP binding was reduced in both KC groups, and significantly more so after CAFE feeding.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuropeptide Y injected in the paraventricular hypothalamus: a powerful stimulant of feeding behavior

Neuropeptide Y (NPY) was injected directly into the paraventricular nucleus of the hypothalamus (PVN) of satiated, brain-cannulated rats, and food and water intake were measured 0.5, 1, 2, 4, and 22 hr postinjection. NPY (24, 78, 235, 783, and 2351 pmol/0.3 mul) produced a large, dose-dependent increase in food intake as well as small increase in water intake. The latency to eat was about 10 min, with substantial feeding occurring in the first 30 min. At dose below 78 pmol, the eating generally occurred only within the first hour. At doses above 235 pmol, however, the subjects' food intake continued to increase such that by 4 hr postinjection they had consumed the equivalent of normal 22-hr intake, and 22 hr postinjection they had also eaten significantly more than control subjects. Previous studies have shown that norepinephrine injected into the PVN stimulates feeding through alpha-adrenergic receptors. To investigate a possible interaction, subjects were given PVN injections of phentolamine (60 nmol) prior to injections of either NPY (78 pmol) or norepinephrine (20 nmol). Phentolamine pretreatment significantly decreased feeding elicited by norepinephrine without affecting feeding elicited by NPY. This suggests that NPY does not stimulate feeding through the release of endogenous norepinephrine. The powerful stimulation of feeding elicited by this neuropeptide suggests an important role for hypothalamic NPY, or a structurally related peptide, in the regulation of feeding behavior.

Importance of endogenous vasopressin in the hypertensive and bradycardic response to central alpha 1-adrenergic stimulation

To determine the interaction between alpha 1-adrenergic and vasopressinergic mechanisms in the central regulation of cardiovascular functions, the effects of intracerebroventricular (i.c.v.) administration of the alpha 1-agonists, methoxamine and phenylephrine, were examined in conscious Long-Evans (LE) rats and Brattleboro rats with hereditary hypothalamic diabetes insipidus (DI). In LE rats, i.c.v. methoxamine and phenylephrine (3-30 micrograms/kg) increased blood pressure and decreased heart rate in a dose-related manner, while they had no detectable cardiovascular effects in DI rats. Neither i.c.v. (0.5 ng/kg per min, 1 h) nor intravenous (i.v., 2 ng/kg per min, 2 h) infusion of vasopressin (AVP) restored the cardiovascular response to i.c.v. phenylephrine in DI rats. In LE rats, however, i.v. pretreatment with the specific antagonist to the pressor effect of AVP, d(CH2)5Tyr(Me)AVP (10 micrograms/kg), attenuated the hypertensive and bradycardic effects of i.c.v. phenylephrine, while i.c.v. pretreatment with AVP antagonist (300 ng/kg) did not alter the cardiovascular response to i.c.v. alpha 1-agonist. The cardiovascular response to i.c.v. phenylephrine was blocked by i.c.v. pretreatment with the alpha 1-antagonist, prazosin (2 micrograms/kg). Intracerebroventricular phenylephrine increased plasma AVP levels 14-fold without affecting plasma angiotensin II levels. The present study clearly demonstrated that endogenous AVP plays a significant role in the cardiovascular response to i.c.v. alpha 1-agonist.

Extra-hypothalamic afferent inputs to the supraoptic nucleus area of the rat as determined by retrograde and anterograde tracing techniques

To detect neuronal cell bodies whose axon projects to the hypothalamic supraoptic nucleus, small volumes (10-50 nl) of 30% horseradish peroxidase or 2% fast blue solutions were pressure-injected into the area of one supraoptic nucleus of rats. Both dorsal and ventral approaches to the nucleus were used. In animals where the injection site extended beyond the limits of the supraoptic nucleus, retrogradely labelled cell bodies were found in many areas of the brain, mainly in the septum, the nucleus of the diagonal band of Broca and ventral subiculum in the limbic system; the dorsal raphe nucleus, the locus coeruleus, the nucleus of the dorsal tegmentum, the dorsal parabrachial nucleus, the nucleus of the solitary tract and the catecholaminergic A1 region in the brain stem; in the subfornical organ and the organum vasculosum of the lamina terminalis, as well as in the median preoptic nucleus. In contrast, when the site of injection was apparently restricted to the supraoptic nucleus, labelling was only clearcut in the two circumventricular organs, the median preoptic nucleus, the nucleus of the solitary tract and the A1 region. Injections of wheat germ agglutinin coupled with horseradish peroxidase (60-80 nl of a 2.5% solution) made in the septum and in the ventral subiculum anterogradely labelled fibers coursing in an area immediately adjacent to the supraoptic nucleus but not within it. In contrast, labelling within the nucleus was found following anterograde transport of tracer deposited in the A1 region and in an area that includes the nucleus of the solitary tract. Neurones located in the perinuclear area were densely labelled by small injections into the supraoptic nucleus; they may represent a relay station for some afferent inputs to the supraoptic nucleus. These results suggest that the supraoptic nucleus is influenced by the same brain areas which project to its companion within the magnocellular system, the paraventricular nucleus.

The alpha-adrenergic receptor: radiohistochemical analysis of functional characteristics and biochemical differences

The partial agonist [3H]para-aminoclonidine was used to label alpha 2-adrenergic binding sites in intact sections of the rat central nervous system using in vitro labeling receptor autoradiographic techniques. The distribution of alpha 2-agonist binding sites closely parallels the reported distribution of noradrenergic and adrenergic cell groups and their terminal fields, particularly the projections of the medullary catecholamine neurons. This distribution of alpha 2 binding sites confirms physiological studies which indicate that the anti-hypertensive actions of alpha 2-agonist compounds are mediated centrally in medullary and spinal centers involved in the control of parasympathetic and sympathetic outflow. Further, the high concentrations of alpha 2 binding sites in pontine and limbic areas such as the locus coeruleus, parabrachial nucleus, dorsal raphe, hypothalamus, amygdala, bed nucleus of the stria terminalis, septum and entorhinal cortex offer an anatomical basis for understanding the anxiolytic and antidepressant actions of drugs like clonidine. The antagonists [3H]prazosin and [3H]WB4101 were used to study the distribution of alpha 1-adrenergic binding sites in the rat forebrain and biochemical studies were performed to analyze the marked differences that were initially seen in the distribution of [3H]prazosin and [3H]WB4101 binding sites. Several pieces of evidence derived from both biochemical and autoradiographic studies suggest that [3H]prazosin and [3H]WB4101 act at distinctly different binding sites. However, both sites may represent components of an alpha 1-adrenergic receptor-effector complex since a high degree of overlap was seen in the binding site distribution of these two ligands and since kinetic interactions could be demonstrated in at least one region of the brain, the hippocampus. Differences noted in the relative displacements of [3H]prazosin and [3H]WB4101 binding in various forebrain regions could reflect differences in the coupling efficiency of the [3H]prazosin and [3H]WB4101 component of the hypothesized complex. Further, in some regions, [3H]WB4101 labeled a binding site that is different from the alpha 1-receptor. Thus, [3H]prazosin and [3H]WB4101 binding sites seen in forebrain regions such as lamina V of the cortex, thalamic nuclei and dorsal raphe probably represent alpha 1-adrenergic receptors and confirm electrophysiological and biochemical studies which demonstrate that adrenergic transmission in these regions can be mediated through an alpha 1-receptor.

The central neural connections of the area postrema of the rat

We applied the neuroanatomical tracers cholera toxin-horseradish peroxidase and wheat germ agglutinin-horseradish peroxidase to investigate the neural connections of the area postrema (AP) in the rat. We find that the AP projects to the nucleus of the solitary tract (NTS) and dorsal motor nucleus of the vagus bilaterally both rostral and caudal to obex; the nucleus ambiguus; the dorsal aspect of the spinal trigeminal tract and nucelus and the paratrigeminal nucleus; the region of the ventrolateral medullary catecholaminergic column; the cerebellar vermis; and a cluster of structures in the dorsolateral pons which prominently include a discrete set of subnuclei in the lateral parabrachial nucleus. The major central afferent input to the area postrema is provided by a group of neurons in the paraventricular and dorsomedial hypothalamic nuclei whose collective dendrites describe a horizontally oriented plexus which encircles the parvocellular nucleus of the hypothalamus bilaterally. In addition, the caudal NTS may project lightly to the AP. The lateral parabrachial nucleus provides a very light input as well. These connections, when considered in the context of the known vagal afferent input and reduced blood-brain barrier of AP, place this structure in a unique position to receive and modulate ascending interoceptive information and to influence autonomic outflow as well.

Caudal ventrolateral medulla can alter vasopressin and arterial pressure

The effects of specific treatments of the caudal ventrolateral medulla (CVLM) on vasopressin (AVP) release and arterial pressure (AP) were examined in urethane-anesthetized rabbits. Electrical stimulation of the CVLM for 5 minutes did not increase plasma AVP; it did however elicit an acute fall in arterial pressure. Similarly, stimulation of cells in the CVLM by microinjection of L-glutamate caused an acute decrease in AP but no change in plasma AVP. Muscimol injected into the CVLM increased AP but not AVP. On the other hand, bicuculline decreased AP and markedly increased plasma AVP, and the increase in AVP following bicuculline injection was not secondary to the fall in AP. These results demonstrate that the CVLM is involved in the regulation of AVP release. Furthermore, taken together with previous findings, these studies indicate that AVP and AP respond differently to treatments of the CVLM, suggesting that different cells in the CVLM are involved in the regulation of AVP release and AP.

Vasopressin secretion in progressive autonomic failure: evidence for defective afferent cardiovascular pathways

Patients with progressive autonomic failure with multiple system atrophy show a severely blunted response of plasma arginine vasopressin to the stimulus of head-up tilt. Whether this could be due to lesions either at one or more sites within ascending neural pathways from cardiovascular stretch receptors in the thorax or, alternatively, to lesions affecting vasopressin secreting cells within the hypothalamus was investigated. The arginine vasopressin response to an intravenous infusion of hypertonic saline was determined in six patients with progressive autonomic failure. The mean plasma concentration of arginine vasopressin rose from 1.0 to 3.7 pmol/l, a change comparable to that observed in normal controls. This demonstrates normal functioning of the efferent connections from the osmoreceptors within the hypothalamus and suggests that the loss of vasopressin response to head-up tilt is due to lesions in ascending pathways from cardiovascular receptors. There was a significant rise in mean blood pressure during the infusions on patients with progressive autonomic failure, a change which was not observed with the controls. This may have been at least partly caused by the rise in circulating arginine vasopressin concentrations, since these patients have been reported to be extremely sensitive to the pressor effects of arginine vasopressin.

Cardiovascular effects of intracisternal 6-hydroxydopamine and of subsequent lesions of the ventrolateral medulla coinciding with the Al group of noradrenaline cells in the rabbit

The acute cardiovascular effects of intracisternal injections of 6-hydroxydopamine (6-OHDA), 5,6-dihydroxytryptamine and 5,7-dihydroxytryptamine and the degree of neurotransmitter depletion achieved by such injections were studied. The two different vehicles used--0.2% ascorbic acid in 0.9% NaCl, or 0.9% NaCl--had little effect on the cardiovascular response to 6-OHDA injections but had a striking effect on levels of noradrenaline (NA) subsequently measured in the thoracic spinal cord. 6-OHDA (600 micrograms kg-1 free base) dissolved in normal saline depleted spinal cord NA to less than 1% of control levels whereas the same dose of 6-OHDA dissolved in ascorbate saline only depleted spinal cord NA to 24% of control levels. The degree of depletion of NA in medulla, pons and hypothalamus was similar in the two groups. Ascorbic acid also appeared to contribute to the non-specific toxicity of intracisternal injections of 6-OHDA. The hypertension and bradycardia that followed lesions of the ventrolateral medulla coinciding with the A1 group of noradrenergic cells (Al lesions) were attenuated in animals in which spinal cord NA had been depleted to 2% of control using 6-OHDA in normal saline. However, pretreatment with 6-OHDA in ascorbate saline, which only reduced spinal cord NA to 23% of control, had no effect on the cardiovascular response to Al lesions. It seems likely that the effects of Al lesions are mediated, at least in part, by NA projections descending within the spinal cord.

Neuropeptide Y: stimulation of feeding and drinking by injection into the paraventricular nucleus

Neuropeptide Y (NPY), a peptide contained within numerous presynaptic terminals in the hypothalamic paraventricular nucleus (PVN), was injected directly into the PVN of satiated, brain-cannulated rats, and food and water intake were measured 0.5, 1, 2 and 4 hrs postinjection. Neuropeptide Y (24 and 78 pmoles/0.3 microliter isotonic saline) caused a dose-dependent increase in food intake, as well as a small, dose-dependent increase in water intake. This effect on feeding occurred even when food was not presented until 4 hrs postinjection. To determine the behavioral specificity of this effect, the impact of PVN injection of NPY (78 pmoles) on various behaviors was observed. With food available, only feeding and drinking behavior were affected. No change in other behaviors, including grooming, rearing, sleeping, resting or different levels of activity, was observed. With food absent, NPY still elicited drinking, suggesting that this is a primary effect, rather than secondary to the feeding. In addition to drinking, NPY reliably increased activity while decreasing sleep and grooming. These results suggest an important role for hypothalamic NPY, or a structurally-related peptide, in the regulation of feeding and drinking behavior.

The noradrenergic innervation of vasopressin neurons in the paraventricular nucleus of the hypothalamus: an ultrastructural study using radioautography and immunocytochemistry

Immunocytochemical and radioautographic procedures were combined at the ultrastructural level to study the noradrenergic synaptic input to vasopressin neurons in selected portions of the paraventricular nucleus of the hypothalamus (PVN) of the rat. Radioactive norepinephrine (NE) was infused into the lateral ventricle or applied topically to the region of the PVN. After appropriate survival times, brain tissues were processed for ultrastructural immunocytochemical demonstration of vasopressin using a monoclonal antibody. [3H]NE varicosities were detected by electron microscopic radioautography. In the periventricular zone of the PVN, radioactive varicosities were numerous accounting for 20-30% of all nerve terminals in this zones. These NE terminals primarily innervated dendritic processes of non-vasopressinergic neurons. Although an occasional axosomatic synapse was observed, input to vasopressin positive neurons was exclusively to their dendrites. In the lateral magnocellular sub-nucleus of the PVN (designed pvl2), noradrenergic terminals were fewer in number accounting for only 1-2% of the total. These terminals were found predominately but not exclusively making axodendritic synapses onto non-vasopressin processes. In both regions, many of the radiolabeled terminals had well-defined membrane appositions with their post-synaptic partners which included a synaptic cleft and post-synaptic density of varying thickness. In both the periventricular zone and the lateral magnocellular regions, noradrenergic varicosities were seen in close proximity to numerous blood vessels.

Anatomy of brain alpha 1-adrenergic receptors: in vitro autoradiography with [125I]-heat

Much useful information on the localization of alpha 1-adrenergic binding sites has been gained by using tritiated radioligands for in vitro autoradiography. However, the iodinated alpha 1-adrenergic antagonist HEAT [( 2-beta (4-hydroxyphenyl)-ethylaminomethyl)-tetralone], BE 2254), a radioligand with high affinity and specificity, provides autoradiographs with a higher signal to noise ratio. This has allowed us to describe the anatomy of these binding sites in much greater detail than previously possible. Regions showing the highest levels of binding include external plexiform layer of the olfactory bulb, layers Va and Vc of frontoparietal cortex, lateral and central amygdaloid nuclei, thalamus, and inferior olive. Other regions were generally less intensely labeled, with the least evidence of labeling in white matter, such as corpus callosum. Some regions (e.g., hippocampus) had only moderate labeling, but the binding appeared in a discrete pattern that reflected the functional organization of the structure. Although the [125I]-HEAT binding sites were distributed in a pattern similar to that previously reported for [3H]-WB 4101 and [3H]-prazosin, the anatomical detail seen with the iodinated ligand is greater. As a result, an association of alpha 1-adrenergic antagonist binding sites with specific layers in the cortex and with some catecholamine-containing nuclei in the brainstem, such as the locus coeruleus, have been seen for the first time.

The mechanism of hypertension and bradycardia following lesions of the caudal ventrolateral medulla in the rabbit: the role of sympathetic nerves, circulating adrenaline, vasopressin and renin

Lesions of the ventrolateral medulla of the rabbit, coinciding with the A1 noradrenaline cell bodies (A1 lesions) produced fortyfold increases in the plasma levels of vasopressin and adrenaline, a twofold increase in plasma noradrenaline and a substantial increase in plasma renin activity. These increases accompanied the hypertension and bradycardia that follow A1 lesions. The vasoconstriction and hypertension were completely abolished by phentolamine, an alpha-adrenoceptor antagonist, when it was administered before lesions and were markedly reduced when it was given after lesions. On the other hand, administration of an antagonist to the vasoconstrictor action of vasopressin (d(CH2)5Tyr(Me)AVP) or an angiotensin converting enzyme inhibitor had little effect. Prior removal of the adrenal glands prevented any rise in plasma adrenaline levels but had no effect on the pressure response to subsequent A1 lesions. These results indicate that the vasoconstriction and hypertension were predominantly mediated by alpha-adrenoceptor stimulation, acting mainly through sympathetic vasoconstrictor nerves. The fall in heart rate following A1 lesions was approximately halved by pretreatment either with d(CH2)5Tyr(Me)AVP alone, or by blockade of the vagus and sympathetic with scopolamine and propranolol; it was completely abolished by combined pretreatment with all three agents. The experiments show that vasopressin release makes a major contribution to the bradycardia acting at least in part through mechanisms that are independent of cardiac vagal or sympathetic nerves.

Evidence for local corticotropin releasing factor (CRF)-immunoreactive neuronal circuits in the paraventricular nucleus of the rat hypothalamus. An electron microscopic immunohistochemical analysis

The interrelationships of corticotropin-releasing factor (CRF) immunoreactive neuronal cell bodies and processes have been examined in the paraventricular nucleus (PVN) of adrenalectomized-dexamethasone treated rats. Antisera generated against ovine CRF (oCRF) were used in the peroxidase-anti-peroxidase-complex (PAP)-immunocytochemical method at both the light and electron microscopic levels. In this experimental model, a great number of CRF-immunoreactive neurons were detected in the parvocellular subdivisions of the PVN and a few scattered labelled parvocellular neurons were also observed within the magnocellular subunits. Characteristic features of immunolabeled perikarya included hypertrophied rough endoplasmic reticulum with dilated endoplasmic cisternae, well developed Golgi complexes and increased numbers of neurosecretory granules. These features are interpreted to indicate accelerated hormone synthesis as a result of adrenalectomy. Afferent fibers communicated with dendrites and somata of CRF-immunoreactive neurons via both symmetrical and asymmetrical synapses. Some neurons exhibited somatic appendages and these structures were also observed to receive synaptic terminals. Within both the PVN and its adjacent neuropil, CRF-immunoreactive axons demonstrated varicosites which contained accumulations of densecore vesicles. CRF-containing axons were observed to branch into axon collaterals. These axons or axon collaterals established axo-somatic synapses on CRF-producing neurons in the parvocellular regions of the PVN, while in the magnocellular area of the nucleus they were found in juxtaposition with unlabeled magnocellular neuronal cell bodies or in synaptic contact with their dendrites. The presence of CRF-immunoreactive material in presynaptic structures suggests that the neurohormone may participate in mechanisms of synaptic transfer. These ultrastructural data indicate that the function of the paraventricular CRF-synthesizing neurons is adrenal steroid hormone dependent. They also provide morphological evidence for the existence of a neuronal ultrashort feed-back mechanism within the PVN for the regulation of CRF production and possibly that of other peptide hormones contained within this complex.

Embryonic hypothalamic tissue transplanted to the IVth ventricle of newborn Brattleboro rats

The hypothalamic region, dissected from normal rat embryos, was transplanted into the IVth ventricle of newborn mutant Brattleboro rats. Water intake and urine osmolality were measured in both the recipient animals and unoperated littermate controls during a 7-week period following weaning. No differences were found between operated and unoperated animals. Ten weeks after transplantation, host animals were fixation perfused and the brains prepared for either catecholamine fluorescence or vasopressin immunohistochemistry. Well-developed grafts were found in the IVth ventricle of the hosts. They received innervation from the host locus coeruleus and contained many neurons with vasopressin-like immunoreactivity. Vasopressin-containing fibers were found running from the grafts into the host medulla.

Plasticity of catecholaminergic neurons in aged rat brain: reinnervation and functional recovery after axotomy

Regenerative growth at the lesion site, reinnervation of a target nucleus and functional manifestations of recovery were studied in aged (20 and 30 months old) rats subjected to long-term transection of catecholaminergic (CA) fibers which contact and influence neurons of the supraoptic nucleus (SON). Small bilateral knife cuts were placed stereotaxically just caudal and medial to the SON. CA histofluorescence, induced by formaldehyde-glutaraldehyde (FAGLU) or aluminum-formaldehyde (ALFA) methods, was examined in hypothalamus at 2, 14, 21 and 60 days postsurgically. Water consumption, and urine volume and osmolality, were monitored presurgically, and through survival times. Subtotal CA denervation in the SON, and typical axonal transmitter "pile-up" at the lesion site, were evident two days after surgery. Among these degenerative profiles, which persisted for up to three weeks, fine-sized new fibers were apparent at the lesion, beginning between 2 and 14 days, and persisting throughout the period studied. At 21 days, and progressively thereafter, SON neurons were rimmed with fluorescent varicosites. Water consumption initially was depressed, but returned to presurgical mean levels by nine days. Urine volume returned to normal by 32 days. Urine osmolality showed a recovery by approximately three weeks. These functional parameters rebounded to levels higher than presurgical means among 20 month old, but not 30 month old, rats beyond 6 weeks survival, concurrent with a morphological hyperinnervation. The results reaffirm morphological regeneration, and support reinnervation and functional recovery, which extend considerably into the aging process.

Glutamic acid decarboxylase immunoreactivity is present in perikarya of neurons in nucleus tractus solitarius of rat

Immunohistochemical procedures reveal perikarya containing glutamic acid decarboxylase immunoreactivity in the nucleus tractus solitarius of the rat. After colchicine pretreatment, neurons were observed in all subdivisions of the nucleus with a particular concentration in the ventrolateral and intermediate subdivisions.

Interdependence of rostral and caudal ventrolateral medullary areas in the control of blood pressure

Pharmacologic experiments were carried out to test the degree of interdependence of rostral vasopressor and caudal vasodepressor neuron pools in the ventrolateral medulla (VLPA and VLDA, respectively). In two groups of urethane-anesthetized rats, the gamma-aminobutyric acid (GABA) agonist muscimol (10 ng/site) was bilaterally microinjected into both the VLPA and VLDA to inhibit neuronal activity at these sites. In one group of experiments, muscimol was microinjected first into the VLPA and then into the VLDA. Following muscimol microinjection in the VLPA the mean arterial pressure (MAP) and heart rate (HR) decreased to 40 +/- 6 mm Hg and 310 +/- 21 beats/min (bpm) from a control level of 90 +/- 3 mm Hg and 403 +/- 23 bpm. Subsequent microinjection of muscimol in the VLDA had no significant effect on BP or HR. This lack of response was not due to severe fall in BP caused by microinjection of muscimol into the VLPA. In the second group of experiments muscimol was first injected into the VLDA followed by muscimol microinjection into the VLPA. In the VLDA muscimol significantly increased MAP and HR to 139 +/- 4 mm Hg and 427 +/- 4 bpm from a control level of 87 +/- 2 mm Hg and 356 +/- 23 bmp. The aortic depressor nerve response (-37 +/- 1 mm Hg and -47 +/- 4 bpm) was converted to an aortic 'pressor' response (+20 +/- 1 mm Hg and -13 +/- 6 bpm). Subsequent microinjection of muscimol into the VLPA caused MAP and HR to fall to 43 +/- 5 mm Hg and 338 +/- 17 bpm. The aortic 'pressor' response was also abolished (2 +/- 2 mm Hg). These results indicate that neuronal activity in the rostral VLPA is an important determinant for changes in BP and its reflex regulation mediated by the VLDA. However, BP changes mediated by the rostral VLPA are independent of the level of neuronal activity in the VLDA. Sites of VLPA and VLDA interaction are discussed.

Determination of the course of brainstem catecholamine fibers mediating amphetamine anorexia

Previous studies suggest that brainstem catecholamine (CA) fibers which mediate amphetamine (AMPH)-induced anorexia ascend through the midlateral medical forebrain bundle and perifornical region and terminate in the perifornical hypothalamic region (PFH) at the level of the hypothalamic ventromedial nucleus. Through studies of wire-knife cuts (KCs) placed in the lower brainstem, the present paper further delineates the course of fibers mediating AMPH feeding suppression, as they ascend through the medullary, pontine and midbrain tegmentum. The results indicate that the crucial CA fibers ascend through the ventrolateral medulla just dorsal to the nucleus of the seventh cranial nerve, 1.1-1.9 mm lateral to midline. In their rostral course, these fibers apparently maintain a relatively straight position in the ventral pons and then enter the ventrolateral midbrain just dorsal to the medial lemniscus, between 0.7 and 1.1 mm lateral to midline. These medullary fibers, possibly originating from the norepinephrine/epinephrine-containing ventrolateral cell group (A1/C1), then appear to join additional fibers from the scattered dopamine-containing neurons positioned in the caudal midbrain (A8 CA cell group). Together, these dopamine, epinephrine and norepinephrine systems are believed to ascend into the medial aspect of the medial forebrain bundle on their way to the PFH at the level of the ventromedial nucleus.

Lesions of the locus coeruleus abolish baroreceptor-induced depression of supraoptic neurones in the rat

Urethane-anaesthetized rats were used to investigate the influence of lesions within the locus coeruleus on the inhibition of phasically discharging supraoptic neurones that normally follows the activation of arterial baroreceptors. Carotid sinus baroreceptors were stimulated by the inflation of a blind sac of the carotid bifurcation. A general activation of arterial baroreceptors was evoked by increasing arterial blood pressure following the intravenous injection of the pure alpha-adrenoreceptor agonist phenylephrine. The locus coeruleus of one side only was destroyed either by thermal (radio-frequency) lesions, or by the injection of 6-hydroxydopamine (1 microliter, 0.5 mg/ml). The extent of each lesion was assessed histologically in stained tissue and with fluorescence histochemistry. Lesions in locus coeruleus abolished all baroreceptor input to supraoptic neurones on the side ipsilateral to the lesion. The lesions had no effect on the cardiovascular responses to the stimulus, and did not abolish the excitation of supraoptic neurones after ipsilateral carotid body chemoreceptor activation. 6-Hydroxydopamine lesions (1 microliter, mg/ml) in the rostral part of the ventrolateral A1 catecholamine neurones were less consistent in their abolition of baroreceptor input to the supraoptic nucleus. When the input from ipsilateral carotid sinus baroreceptors was abolished, there was an equivalent effect on the influence of the carotid body chemoreceptors. Input from other arterial baroreceptors, activated by phenylephrine injection, was not affected. From these results, it is proposed that the baroreceptor-induced depression of-phasically discharging supraoptic neurones is mediated via a direct noradrenergic input from the locus coeruleus.

Facilitatory influence of noradrenergic afferents on the excitability of rat paraventricular nucleus neurosecretory cells

The role of the A1 and A2 noradrenergic cell groups of the caudal medulla in regulating the activity of paraventricular nucleus neurosecretory cells was examined with electrophysiological methods in anaesthetized male Sprague-Dawley rats. Antidromically identified neurosecretory cells were classified as vasopressin or oxytocin secreting on the basis of spontaneous firing patterns and responsivity to baroreceptor activation. The effect on cell firing of single pulses (25-200 microA) delivered to either the A1 or A2 cell group areas was then examined using peri-stimulus histograms. Stimulation of the A1 region enhanced the activity of 78% of putative vasopressin-secreting neurones tested (n = 18), but failed to affect the activity of the majority (73%) of putative oxytocin-secreting units (n = 15). A2 stimulation enhanced the firing rate of both putative vasopressin- (60%, total n = 14) and putative oxytocin-secreting (70%, total n = 27) neurones. Destruction of the paraventricular nucleus catecholamine terminal plexus by pre-treatment with the neurotoxin 6-hydroxydopamine abolished the facilitatory effects of both A1 and A2 stimulation. These findings suggest that noradrenergic afferents of medullary origin facilitate the activity of paraventricular nucleus neurosecretory cells. The role of the projection from the A1 cell group appears to differ from that of the A2 group, however, in that its effects are specific to putative vasopressin-secreting units.

The action of the A1 noradrenergic region on phasically firing neurons in the rat paraventricular nucleus

Electrical stimulation of the rat A1 noradrenergic region produced orthodromic inhibition (34%), excitation (23%) or excitation-inhibition sequence (25%) of the spontaneous activity of phasically firing units in the paraventricular nucleus of the hypothalamus (PVH). Excitatory responses could be distinguished into two types on the basis of their latency and duration. Both the primary and post-excitatory inhibitory responses were significantly attenuated by intravenous injection of propranolol, a beta-adrenoceptor antagonist, whereas neither type of excitatory response was affected.

A cholinergic link in the reflex release of vasopressin by hypotension in the rat

Inhalation of amyl nitrite in the water-loaded rat under ethanol anaesthesia produced a brief fall of blood pressure followed by a prolonged antidiuretic response. The antidiuretic response to amyl nitrite was accompanied by increased urinary excretion of vasopressin, it was blocked by a specific vasopressin antagonist and by a barbiturate and it was absent in the Brattleboro rat with congenital diabetes insipidus. These results show that the antidiuretic response to the hypotension induced by amyl nitrite is due to the release of vasopressin and that this release is mediated by a neuroendocrine reflex acting through the brain stem. Carbachol and nicotine produced an antidiuretic response on injection into a lateral cerebral ventricle (i. vent.). Carbachol was almost ineffective, but nicotine much more effective, when injected into the cisterna magna (i.cist.) from which in the rat there is no access to the ventricles. Carbachol therefore acts at a site reached from the ventricles, possibly the paraventricular nucleus. Nicotine acts at a more distal site reached from the subarachnoid space. This site may correspond with the nicotine-sensitive area on the ventral surface of the brain stem which has been described in the cat. Atropine blocked the antidiuretic response to carbachol but not that to amyl nitrite. Hexamethonium blocked the antidiuretic response to amyl nitrite as well as that to nicotine and was more effective on i.cist. than i.vent. injection. These results reveal a cholinergic link with a nicotinic but not a muscarinic receptor in the neural pathways controlling the release of vasopressin in response to hypotension. A hypothetical model is presented in which the release of vasopressin is stimulated by a pathway arising from chemoreceptors and inhibited by a second pathway arising from stretch- and baroreceptors. Hypotension acts by suppressing the normally predominant inhibitory pathway and stimulating the excitatory pathway. Hexamethonium is presumed to block transmission at a synapse in the excitatory pathway at the ventral surface or, less probably, at the paraventricular and supraoptic nuclei.

Vasopressin effects on food-rewarded learning tasks might be due to its action on carbohydrate/lipid metabolism, not memory

Vasopressin (VP) has been implicated in memory processes on the basis of effects observed in aversively motivated learning situations. Therefore researchers have tried to confirm this role by using food-motivated learning tasks. However, the well-established physiological influences of VP on carbohydrate and lipid metabolism were not taken into consideration. At various times following administration, VP might act as a feeding stimulant or as a satiating agent. Experimental designs should allow for these effects when food-rewarded learning paradigms are used to determine whether VP acts on memory.

Brain stem catecholamine mechanisms in tonic and reflex control of blood pressure

Neurons of the lower brain stem maintain resting levels of arterial pressure (AP), mediate reflex responses from cardiopulmonary receptors, and are an important site of the hypotensive actions of alpha 2-adrenergic agonists. Details of the pathways and transmitters that mediate tonic and reflex control of AP are emerging. Afferent fibers of cardiopulmonary receptors in the ninth and tenth nerves terminate bilaterally in the nucleus of the tractus solitarius (NTS). Although some neurons contain substance P, the primary neurotransmitter appears to be the excitatory amino acid L-glutamate (L-glu). Neurons in rostral ventrolateral medulla, which most probably comprise the C1 group of epinephrine neurons, are also critical in AP control. C1 neurons project to innervate cholinergic preganglionic sympathetic neurons in the spinal cord. Stimulation of the C1 area electrically or with L-glu increases AP, while lesions or local injection of the inhibitory amino acid gamma-aminobutyric acid (GABA) lowers AP to levels comparable to spinal cord transection. Lesions of C1 neurons or their pathways abolish vasodepressor reflexes from baroreceptors and vagal afferents. In contrast, noradrenergic neurons of the caudal ventrolateral medulla, the A1 group, project rostrally to innervate, in part, vasopressin neurons of the hypothalamus. Stimulation of A1 neurons lowers AP, while lesions or GABA elevates it. We propose that C1 neurons comprise the so-called tonic vasomotor center of the brain stem and also mediate, via a projection from the NTS, the vasodepressor limb of baroreflexes. The NTS-C1 projection may be GABAergic.

Ultrastructural analysis of the noradrenergic innervation of rat supraoptic nucleus

The noradrenergic innervation of the rat supraoptic nucleus (SON) was examined by high resolution radioautography after animals had been intracerebrally injected with [3H]noradrenaline (NA). In the medial portion of the SON, noradrenergic varicosities were scattered throughout this nucleus, but were preferentially concentrated in its ventral subdivision. 25.5% of these NA varicosities displayed typical synaptic contacts, with either the somatic or proximal dendritic portions of neurosecretory neurons (9.2%) or with small dendritic processes of such neurons (16.3%). The possibility is discussed that at least some of the dendritic NA synapses detected in the ventral subdivision of the SON, which preferentially contains vasopressinergic perikarya, may affect the neurosecretory neurons located in the dorsal part of this nucleus, which preferentially contains oxytocinergic perikarya.

Distribution of tyrosine-hydroxylase-immunoreactive neurons in the hypothalamus of rats

The distribution and morphology of cells containing tyrosine hydroxylase (TH) immunoreactivity in the hypothalamus of rats were studied by using a modified immunoperoxidase technique. The TH cell system is more complexly organized than was previously thought. On the basis of their clustering patterns, hypothalamic TH neurons could be subdivided into two groups: dorsal and ventral. The ventral group consists of a prominent aggregate of cells located in the caudal part of the arcuate nucleus. From here, cells extend around the caudal part of the ventromedial and dorsomedial nuclei and the base of the diencephalon. Tyrosine hydroxylase-positive cells are present throughout the arcuate nucleus, except in its ventromedial part. Anteriorly, immunoreactive cells appear in the suprachiasmatic and supraoptic nuclei, in the retrochiasmatic area, and in the ventral part of the anterior hypothalamic nucleus. The dorsal group has its main concentration of cells in the medial part of the zona incerta, from which two clusters of cells, one medial and one lateral, extend rostralward. The medial group comprises cells in the medial part of the dorsomedial, paraventricular, and anterior hypothalamic nuclei. These cells adjoin the periventricular cells. The lateral group of cells emanating from the zona incerta occupies the lateral part of the dorsomedial and anterior hypothalamic nuclei and the dorsal hypothalamic area. The dorsal and ventral TH cell groups are in continuity medially in the periventricular layer, and laterally through the cells that surround the ventromedial nucleus. Although the cells vary widely in size, shape, and dendritic arborization pattern, there are two main cell types. Small (21 X 11 microns), round to fusiform cells, with two or three dendrites arborizing simply, were frequently seen in the arcuate, suprachiasmatic, periventricular, supramammillary nuclei and at the borders of the ventromedial nucleus. The other cell type is larger (40 X 15 microns) and multipolar, with three to five frequently branching dendrites. The dendritic field is large and the cells are intensely TH-immunoreactive. Although the larger cells occur occasionally in every hypothalamic nucleus, their principal locations are in the dorsal parts of the dorsomedial, posterior hypothalamic nuclei and the dorsal and lateral parts of the zona incerta, and in the areas dorsal and medial to the mammillothalamic tract at caudal hypothalamic levels. In this paper we give a detailed description of TH-immunoreactive fibers and terminals in the hypothalamus and a comparison with previous studies of catecholamine cells in the hypothalamus.

Clonidine-induced feeding: analysis of central sites of action and fiber projections mediating this response

Clonidine (CLON), an alpha-adrenergic agonist, was used in conjunction with norepinephrine (NE) to elicit feeding in satiated rats that had sustained hypothalamic electrolytic lesions, or coronal knife cuts at the hypothalamic, midbrain or pontine level of the brainstem. Electrolytic lesions of the paraventricular nucleus (PVN) of the hypothalamus significantly attenuated feeding normally stimulated by intraperitoneal injection of CLON. This contrasts with lesions in the dorsomedial or perifornical hypothalamic regions which had no effect on CLON-elicited eating. Knife cuts placed in the posterior hypothalamus and throughout the midbrain tegmentum also left intact the CLON eating response, in contrast to specific cuts in the dorsal pontine tegmentum which disrupted feeding elicited by PVN injections of NE and CLON, as well as by peripheral administration of CLON. Analyzed together, these results with effective and ineffective cuts relative to CLON and NE feeding provide evidence for an alpha-adrenergic feeding circuit which originates in the PVN and descends from this nucleus, via a dorsal periventricular course, through the diencephalon and midbrain. Further caudally, these fibers mediating NE and CLON feeding then appear to traverse ventrolaterally into the dorsolateral pontine tegmentum on their way to the dorsal medulla.

Electrophysiological identification of neurons in ventrolateral medulla sending collateral axons to paraventricular and supraoptic nuclei in the cat

Experiments were done in chloralosed, paralyzed and artificially ventilated cats to identify single units in the ventrolateral medulla (VLM) that send collateral axons directly to the region of the paraventricular (PVH) and supraoptic (SON) nuclei, and responding to peripheral inputs carrying cardiovascular afferent information. Twenty-six single units were antidromically activated in the VLM to stimulation of both the PVH and SON, and in each case the antidromic potential evoked by stimulation of one site was cancelled by stimulation of the other site. These units responded with latencies corresponding to conduction velocities of 5.1 +/- 0.4 m/s. Of these 26 units, 10 responded orthodromically to stimulation of either the carotid sinus or aortic depressor nerves. These data have demonstrated the existence of VLM neurons which send collateral axons to the PVH and SON and have provided evidence for their role in mediating cardiovascular afferent information directly to hypothalamic regions involved in autonomic and neuroendocrine regulation.

Substance P, neurotensin, enkephalin, and catecholamine-synthesizing enzymes: light microscopic localizations compared with autoradiographic label in solitary efferents to the rat parabrachial region

The immunocytochemical localizations of substance P, neurotensin, enkephalin and the catecholamine-synthesizing enzymes tyrosine hydroxylase and dopamine-beta-hydroxylase were examined in the rat parabrachial region. The immunoreactivity for each marker was compared with the distribution of superimposed autoradiographic labeling of parabrachial afferents after unilateral injection of 3H-amino acids into the caudal portion of the medial nucleus of the solitary tract (m-NTS). Substance-P- and neurotensinlike immunoreactivity (SPLI and NTLI, respectively) were localized primarily in varicose processes in the ventrolateral quadrant of the parabrachial region. The SPLI and NTLI were differentially localized with respect to each other; however, both peptides were detected in regions of the parabrachial containing dense autoradiographic label. In contrast, enkephalinlike immunoreactivity (ELI), tyrosine hydroxylase, and dopamine-beta-hydroxylase were detected in processes and a few perikarya located outside the ventrolateral parabrachial region. The ELI was primarily in the dorsolateral, and the catecholamine-synthesizing enzymes were primarily in the medial parabrachial regions which contained sparse autoradiographic labeling of transported amino acids. We conclude that in the rat parabrachial region, SPLI and NTLI are contained within two distinct populations of afferents which may originate from perikarya in the caudal m-NTS, whereas ELI and the catecholamines are more likely to be found in other afferents or possibly in intrinsic neurons.

Lesions of the ventrolateral medulla in normal and vasopressin-deficient (Brattleboro) rats

The cardiovascular response to electrolytic lesions of the A1 noradrenaline cell group (A1 lesion) were studied in the rat. A1 lesions in normal rats cause acute hypertension and bradycardia and elevate plasma arginine-vasopressin (AVP), as previously described in the rabbit. After A1 lesions in vasopressin deficient rats the hypertension is not reduced compared to that seen in normal rats, but the bradycardia is attenuated. AVP makes a greater contribution to the bradycardia than to the hypertension after A1 lesions.

Electrophysiological evidence that noradrenergic afferents selectively facilitate the activity of supraoptic vasopressin neurons

The functional role of the ascending projection from A1 noradrenergic neurons of the caudal ventrolateral medulla to the supraoptic nucleus of the hypothalamus was investigated by examining the effects of electrical stimulation of the A1 region on the activity of supraoptic neurons deemed to be vasopressinergic or oxytocinergic on the basis of basal firing patterns and responsivity to baroreceptor activation. A1 stimulation enhanced the activity of all putative vasopressin-secreting supraoptic neurons tested. This effect appeared to be selective in that no putative oxytocin-secreting neurons were excited by A1 stimulation. Destruction of the supraoptic noradrenergic terminal plexus by local application of the neurotoxin 6-hydroxydopamine abolished the facilitatory effects of A1 stimulation but did not noticeably alter basal activity patterns, nor the influence of baroreceptor inhibitory pathways. These findings suggest a facilitatory role for noradrenergic afferents in regulating the activity of neurohypophysially-projecting vasopressin neurons of the supraoptic nucleus.

The organization of projections from the central nucleus of the amygdala to brainstem sites involved in central autonomic regulation: a combined retrograde transport-immunohistochemical study

The central nucleus of the amygdala (ACe) in the rat sends a considerable projection to, and receives projections from, the parabrachial nucleus (PB) and the dorsal vagal complex (DVC; the nucleus of the solitary tract and the dorsal motor nucleus of the vagus nerve). In each part of this 'triangle', immunohistochemical staining for the following peptides has been observed in perikarya and fibers: neurotensin, somatostatin, substance-P, Leu-enkephalin and corticotropin-releasing factor. The aim of the present study was to investigate whether any of these peptides are involved in projections to the brainstem from the ACe, and to characterize the distribution of each cell type in the ACe. The results of double retrograde tracing studies indicate that most of the ACe neurons projecting to the PB and DVC are present in the medial part of ACe (ACem), and that many of them project to both the 1 B and the DVC. The combined use of immunohistochemistry with a retrograde fluorescent tracer, True Blue, indicated that the peptide-containing perikarya are found predominantly in the lateral part of ACe (ACe1), and that only a small proportion of neurotensin, somatostatin and corticotropin-releasing factor-stained neurons contained True Blue after injections into the PB or the DVC. The results suggest that most of the fibers in the descending projection from the ACe to the brainstem do not contain the peptides examined here.

Electrophysiological properties of spinally-projecting A5 noradrenergic neurons

Spinally-projecting A5 neurons were studied with anatomical and electrophysiological techniques in the rat. A detailed study of the number and distribution of spinally-projecting catecholaminergic (CA) and non-catecholaminergic neurons present in a defined area of ventrolateral pontine reticular formation was performed using a sequential technique for the detection of CA fluorescence and retrogradely transported HRP. Using control animals and rats with 6-hydroxydopamine-induced lesions of spinal CA axons, it was concluded that up to 93% of all noradrenergic (NE) neurons present in the area investigated send an axonal process to the thoracic spinal cord and that NE neurons constitute at least 90% of all spinally-projecting neurons present in the same area. Single unit recordings of spinally-projecting neurons were obtained in the same area of the reticular formation in urethane-anesthetized, paralyzed and respirated rats. Based on the above-mentioned anatomical data, antidromic activation from thoracic spinal cord provided a necessary and sufficient criterion for the identification of A5 NE cells. These neurons had a conduction velocity of 2.5 m/s, a discharge rate of up to 4 spikes/s and all were inhibited by i.v. clonidine or desmethylimipramine (DMI). The inhibition produced by the latter drugs was always reversed by the alpha-2 adrenergic antagonists piperoxan or yohimbine. Antidromic (AD)-activation was followed by a period of inhibition whose duration was increased by raising the intensity of the stimulus or by administration of the NE-uptake inhibitor DMI. The effect of the latter was reversed by administration of the alpha-2 antagonist piperoxan.