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We investigated whether electrical stimulation of the cerebellar fastigial nucleus (FN) can protect pyramidal neurons of the CA1 zone of dorsal hippocampus from delayed neuronal death caused by global ischemia. Stimulation of the FN for 1 h prior to transient 4-vessel occlusion in anesthetized rats salvaged 57% (p < 0.01) of pyramidal neurons from degeneration. This effect could be preconditioned. Sham simulation of FN or stimulation of the rostral ventrolateral medulla (RVL) were without effect (p > 0.5). Excitation of intrinsic neuronal pathways represented in FN can protect central neurons from global as well as focal ischemic degeneration. The brain contains systems designed to protect it from ischemia by mechanisms of central neurogenic neuroprotection acting independently of actions on cerebral blood flow.

A group of clinically useful antihypertensive agents, including clonidine, moxonidine, and rilmenidine, are all ligands at alpha(2)-adrenergic and imidazoline (I-) receptors, the latter principally of the I1-subclass. These agents all lower blood pressure by reducing the activity of tonically active sympathoexcitatory reticulospinal neurons of the C1 area of the rostroventrolateral medulla (RVL). They tonically excite preganglionic sympathetic neurons in the spinal cord by release of L-glutamate and mediate most reflexes influencing blood pressure. The RVL contains alpha(2)-adrenergic and I1-receptors, and there is evidence to suggest that both receptors may participate in the hypotensive actions of the drugs. However, because only activation of the alpha(2)-adrenergic receptors appears responsible for somnolence, the imidazoline-receptor agonists moxonidine and rilmenidine, both relatively selective for I-receptors, may have superior clinical utility in antihypertensive therapy, since they are sympatholytic and also suppress the generation of angiotensin II.

We investigated whether astrocytes store and synthesize agmatine (decarboxylated arginine), an endogenous ligand for imidazoline and alpha 2-adrenergic receptors, in brain. Agmatine, detected chemically and immunocytochemically, is contained in cultured astrocytes and C6 glioma cells (8.5 +/- 1.4 and 1.8 +/- 0.6 nmol mg-1 protein, respectively). Glial membranes express activity for arginine decarboxylase (ADC), the biosynthetic enzyme for agmatine (astrocytes 85.4 +/- 9.2; C6 cells 18.2 +/- 3.12 nmol h-1 mg-1 protein). Lipopolysaccharide, and inducer of glial nitric oxide synthase (iNOS), significantly reduced (C6) or did not affect (astrocytes) ADC activity. Inferferon-gamma, not affecting iNOS, elevated ADC activity in both cell types. Astrocytes are a site of synthesis and storage of agmatine. ADC and iNOS enzymes synthesizing distinct bioactive products from L-arginine, may be reciprocally regulated.

MEDULLARY ROSTRAL VENTROLATERAL RETICULAR NUCLEUS (RVL): Reticulospinal neurons are critical to control of the circulation by the brain. Its actions are implemented by a few reticulospinal neurons, 200 in the rat. These directly innervate and excite preganglionic sympathetic neurons of the spinal cord by releasing L-glutamate. The RVL-spinal sympathetic premotor neurons are innervated by neurochemically diverse afferents from local and remote sources. They maintain arterial pressure tonically, mediate vasomotor reflexes elicited by stimulation of baro- or chemoreceptors or in response to pain or muscular exercise, and couple vasomotor responses to defense and conditioned fear behaviors. RVL-spinal neurons are central oxygen sensors, directly excited by hypoxia, and initiate sympathetic responses to cerebral ischemia or distortion (Cushing reflex). Stimulation of the RVL directly elevates cerebral flow independently of metabolism and initiates much of the cerebrovascular vasodilation in response to hypoxemia. RVL-SPINAL NEURONS IN RELATION TO HYPERTENSION AND SHOCK: RVL-spinal neurons are sites of action for many centrally acting antihypertensive drugs and some vasoactive hormones. Their integrity is required for expression of the elevated arterial pressure in neurogenic hypertension and for the compensatory sympathetic responses to hemorrhage. We propose that RVL-spinal neurons (1) maintain the activity of sympathetic neurons in mid-range amplifying, thereby, their signaling capacities; (2) initiate and integrate circulatory responses to a lack of oxygen so as to protect the brain from real or threatened hypoxia; (3) maintain, by tonic activity, normal expression of genes and gene products of central and peripheral sympathetic neurons and their peripheral targets that relate to their structure and neurotransmission-associated functions.

We sought to determine whether high spatial resolution magnetic resonance imaging is useful for noninvasive quantitation of the ischemic infarct produced by occlusion of the middle cerebral artery and for detection of reduced infarct volume elicited by electric stimulation of the cerebellar fastigial nucleus. Male rats of the spontaneously hypertensive strain were anesthetized, the middle cerebral artery was occluded, and the fastigial nucleus was stimulated for 1 hour. Twenty-four hours later, rats were reanesthetized and T1- and T2-weighted images were obtained. Rats were killed and the volume and distribution of the lesion was established by histopathology. Magnetic resonance imaging estimates of the lesion volume were 271 +/- 41.0 mm3 (middle cerebral artery, n = 5) and 148 +/- 8.4 mm3 (middle cerebral artery + fastigial nucleus stimulation, n = 6; 45% reduction, p less than 0.05). Histopathological analysis revealed a lesion of 229.8 +/- 15.4 mm3 involving somatosensory cortex, lateral caudate putamen, and lateral hippocampus. Fastigial nucleus stimulation resulted in a 36% reduction in infarct volume to 146.0 +/- 10.3 mm3. The retrieved zone was largely in the cortex dorsal and ventral to the lesion and mostly posterior to the lesion. The estimates of lesion volume by magnetic resonance imaging and histopathology did not differ and were highly correlated (r = 0.90; p less than 0.001). This study confirms our previous finding that fastigial nucleus stimulation reduces the volume of a focal ischemic infarct and demonstrates that magnetic resonance imaging not only accurately estimates the volume of the lesion but also can detect changes as small as 50-100 mm3.

Bilateral electrolytic lesions of the anterior hypothalamus in unrestrained rats resulted in the development, within 2 hours, of arterial hypertension, tachycardia, hyperthermia, and increased locomotor activity, often leading to pulmonary edema and death. Similar lesions in paralyzed, artificially ventilated rats produced comparable changes in arterial blood pressure and body temperature with a similar time course. The arterial hypertension was a consequence of an increase in total peripheral resistance to 15% of control with a reduction in cardiac output to 49% of control. Arterial hypertension, elevated peripheral resistance, and diminished cardiac output were reversed toward normal by alpha-receptor blockade with phentolamine (1 mg/kg, iv). Bilateral adrenalectomy, adrenal demedullation, or adrenal denervation performed prior to lesion placement prevented the development of arterial hypertension and pulmonary edema as well as the changes in peripheral resistance, cardiac output, and body temperature. We conclude that arterial hypertension following lesions of the anterior hypothalamus is due to a neurally mediated increase in peripheral resistance initiated by the release of adrenal medullary catecholamines and that pulmonary edema is due to myocardial failure secondary to the ensuing ventricular overload. Structures originating in or passing through the anterior hypothalamus may exert selective control over the adrenal medulla independent of vasomotor neurons.

Polyclonal antisera raised against para-aminoclonidine coupled to haemocyanin exhibit high affinity for para-aminoclonidine, clonidine and chloroethylclonidine (IC50 less than 100 nmol/l). Anti-para-aminoclonidine antibodies also cross-react with naphazoline, oxymetazoline and tolazoline at moderate concentrations (IC50, 300-500 mumol/l); the phenyl-imidazoles detomidine, medetomidine and MPV830 are weakly cross-reactive (IC50 greater than 0.2 mmol/l). All of these compounds bind with high affinity to both imidazole and alpha 2-adrenergic receptors. Compounds which are imidazole- or alpha 2-specific do not cross-react with anti-para-aminoclonidine antibodies (IC50 greater than 1 mmol/l). Anti-para-aminoclonidine also recognizes an endogenous clonidine-displacing substance in the brain. Thus, (a) binding to anti-para-aminoclonidine antibodies defines a subset of phenyl-imidazol(in)e ligands which bind to both imidazole and alpha 2-adrenergic receptors, suggesting that anti-para-aminoclonidine recognition sites resemble a hybrid of the two receptor types; (b) antibodies to imidazole- or alpha 2-specific agents may be useful as models for differentiating between these types; (c) since clonidine-displacing substance is recognized by anti-para-aminoclonidine antibodies, it may have phenyl and imidazole rings as parts of its chemical structure.

Electrical stimulation of the fastigial nucleus in anesthetized, paralyzed, and artificially ventilated rats for 10 seconds (50 Hz) induced a stimulus-locked elevation of arterial pressure (the fastigial pressor response) and increased plasma vasopressin. Cervical spinal cord transection abolished the stimulus-locked fastigial pressor response and augmented the vasopressin response to a 10-fold increase (19 +/- 1 to 188 +/- 58 pg/ml, P less than 0.05; n = 8). Grading the pressor elevations occurring during the fastigial nucleus stimulus changed the amounts of vasopressin released in the same animal: acute adrenalectomy and chemosympathectomy by guanethidine reduced the magnitude of the fastigial pressor response and facilitated the vasopressin release to fastigial nucleus stimulation (intact: 52 +/- 11 pg/ml; after adrenalectomy and chemosympathectomy, 254 +/- 73 pg/ml, P less than 0.05, n = 6). Subsequent intravenous administration of a bolus of phenylephrine to increase mean arterial pressure during fastigial nucleus stimulus, as in intact situation, reduced the vasopressin release (47 +/- 9 pg/ml). After sinoaortic denervation plus vagotomy, the fastigial pressor response was preserved; however, vasopressin still increased 11-fold (from 11 +/- 1 to 126 +/- 23 pg/ml, P less than 0.01, n = 8). Vagotomy alone did not affect the vasopressin resting level nor the 4-fold increase in response to fastigial nucleus stimulation. Therefore, stimulus-locked elevations of arterial pressure oppose, by reflex mechanisms mediated through baroreceptors, but do not prevent the release of vasopressin elicited by stimulation of the fastigial nucleus.

The sympathetic drive to the cardiovascular system depends on the integrity of neurons in the medulla oblongata. The adrenergic neurons of the C1-area of the cortical ventrolateral medulla are tonically active and provide the background excitation of the spinal preganglionic neurons, which relay to the postganglionic sympathetic neurons. The noradrenergic neurons of the A1-area of the caudal ventrolateral medulla act as tonic inhibitors of the sympathetic discharge; they mediate the depressor response to centrally acting alpha-adrenergic agonists such as clonidine. Imbalance between those two types of neuronal control systems leads to increased sympathetic discharge and hence elevated arterial blood pressure.

The role of the sympathetic nervous system in generating behaviourally linked blood pressure (BP) responses was investigated in awake rats instrumented for chronic intra-arterial BP recording. All data from the recording sessions were digitalized and processed by a computer. Frequency interval histograms of behaviourally associated changes of BP and heart rate (HR) were generated. 6-Hydroxydopamine (100 and 200 mg/kg intravenously) was used for chemosympathectomy. In the intact rats (n = 8) three different frequency histograms of mean BP values for three behavioural groupings were generated (from lowest to highest average values): (1) resting, (2) exploring-grooming and (3) eating-drinking. Chemosympathectomy (n = 5) abolished this order of BP changes; mean BP did not rise from rest to other behaviours, nor did it vary during different behaviours. It is concluded that the sympathetic nervous system is responsible for producing the changes in BP that are appropriate and characteristic of each behaviour in awake, unrestrained rats.

An area of the rostral ventrolateral medulla (RVL), particularly that surrounding a cluster of cells of the adrenergic C1 group (the C1 area), contains neurons that function as the classical sympathoexcitatory region of the medulla oblongata. Pharmacological evidence demonstrates that these neurons are tonically regulated by inhibitory GABAergic and possibly noradrenergic inputs and by excitatory cholinergic mechanisms. Immunocytochemical studies indicate an abundance of intrinsic cholinergic and GABAergic neurons in the C1 area. Local interneurons in RVL play a critical role in regulating resting and reflex control of arterial pressure, conceivably by an action on adrenergic neurons of the C1 group.

Systems in the brain control blood pressure by maintaining life-sustaining, resting (tonic) levels and adjusting blood pressure in association with changes in regional blood flow appropriately coupled to behavior or environmental stimulation. Tonic levels of blood pressure are mediated by neurons in the medulla oblongata. The critical neurons appear to correspond to the epinephrine-containing cells of the so-called C1 group located in the rostral ventral lateral medulla. These also mediate baroreceptor reflex responses. Behaviorally coupled changes in blood pressure are often highly stereotyped and vary depending on the behavior being performed in both animals and humans. These reactive circulatory adjustments are largely mediated by forebrain regions working in concert with the medullary centers. Some of the largest increases in blood pressure occur in response to aversive emotional arousal. In hypertensive animals and humans, such changes, which are normally buffered by reflex mechanisms in the brainstem, are exaggerated. In the rat, the neural pathway mediating the coupling, through aversive emotional conditioning, of blood pressure responses to acoustic stimuli involves the transmission of sensory signals through the primary auditory projection system to the medial geniculate body, where the input is then relayed subcortically to the amygdala. The amygdala, presumably by way of connections with the hypothalamus, and from there to the brainstem or spinal cord, controls the learned emotional response. These findings, which implicate a largely unrecognized sensory relay to the amygdala in emotional learning, represent the first demonstration of a direct link between primary sensory system and autonomic control regions in the mammalian brain.

Changes in the fractional distribution of cardiac output (FF), organ blood flow, and regional vascular resistance were measured by the isotope dilution technique of Sapirstein using 86Rb as indicator in unanesthetized rats during acute arterial hypertension produced by bilateral lesions of the nucleus tractus solitarii (NTS). After NTS lesions, the FF was significantly reduced in skin, muscle, and colon, increased in ventricular myocardium, spleen, and adrenal glands, and was unchanged elsewhere. Because of a marked reduction in cardiac output (CO) during hypertension, the absolute organ blood flow (FF X CO) was reduced in lesioned rats to 30-40% of control in skin, muscle, and colon and between 60% and 75% of control in most of the remainder of the gastrointestinal tract and renal cortex; it was unchanged in myocardium and endocrine glands. Resistance was substantially increased (4- to 6-fold) in skin, muscle and colon but was only moderately increased (1.5- to 2.5-fold) in the remaining organs. The results indicate that, while NTS lesions will increase resistance in most vascular beds, the response is unequally distributed, influencing skin, muscle, and colon disproportionately to other tissues. Because of an interaction between a reduction in CO and little autoregulation, blood flow is reduced primarily in skin, muscle, and colon. The pattern of redistribution of CO was consistent with the interpretation that NTS hypertension results from interrupting baroreceptor reflexes centrally. The pattern of redistribution of blood flow in rats with NTS lesions differs from that produced by deoxycorticosterone acetate-salt and renal ischemia.

Bilateral electrolytic lesions of the nucleus tractus solitarii (NTS) were made at the level of the obex in seven cats. Within 1 hour the mean arterial pressure (MAP) rose to a maximum of 144 mm Hg (141% of control), and by 7 hours heart rate reached a peak of 236 beats/min (148% of control). The baroreceptor reflexes were abolished. After 24 hours the arterial pressure became extremely labile, with variations of 80-100 mm Hg observed. The lability occurred spontaneously and during behaviors that were self-initiated or elicited by environmental stimuli. The MAP in the lesion group was 144 mm Hg (180% of control) during the day, and 96 mm Hg (120% of control) at night. The lability, measured by the standard deviation, during the day in the lesion group was 4 times greater than in the control group and at night there were no differences. The heart rate of the lesion group was always higher than that of the control group but the lability of both groups was the same. We conclude that lesions of the NTS produced labile hypertension, probably by disinhibition of sympathetic activity through central interruption of the baroreceptor reflexes. The higher, more labile arterial pressures during the day may be caused by uninhibited increases in sympathetic activity elicited by environmental stimuli that are present during the day and absent at night. The daily variation of pressure may also be caused by somatomotor activity or by a daily rhythm of sympathetic activity which is unmasked by the lesions.

Clonidine acts within the rostral ventrolateral medulla to lower arterial pressure. The receptor mechanism for this action is unknown. In the rostral ventrolateral medulla clonidine binds not only to the alpha 2-adrenergic receptor but also to a novel class of sites which are specific for imidazolines and imidazoles and are distinct from adrenergic or histaminergic receptors. In order to distinguish whether a putative imidazole receptor or the alpha 2-receptor mediates the clonidine hypotensive response, a series of clonidine analogues were tested (1) in radioligand binding assays of their affinity at imidazole and alpha 2-adrenergic receptors, and (2) by microinjection into the rostral ventrolateral medulla of anaesthetized rats to measure their capacity to lower arterial pressure. The hypotensive response elicited by the test agents was strongly correlated with affinity at imidazole sites (r = 0.92) but not with alpha 2-adrenergic affinity (r = 0.36). An imidazole receptor in the rostral ventrolateral medulla may participate in the hypotensive action of clonidine.

Kainic acid (KA), an analogue of L-glutamate, was microinjected in 0.1 microliter of saline into the nucleus tractus solitarii (NTS) of adult rats. In rats anesthetized with halothane or alpha-chloralose, KA injected unilaterally elicited hypotension, bradycardia, and apnea. The threshold dose was 0.1-0.2 ng (10(-13) mol). Doses greater than 0.2 ng blocked responses to subsequent injections for at least 30 minutes. Doses of KA greater than 15 ng reduced the reflex bradycardia elicited by raising the arterial pressure with phenylephrine and produced arterial hypertension in rats anesthetized with alpha-chloralose or in other rats within 15 minutes of terminating halothane anesthesia. Bilateral injection of KA in doses greater than 15 ng completely blocked baroreflexes and resulted in a dose-dependent elevation of arterial pressure (167 +/- 9.4; P less than 0.001) both in alpha-chloralose-anesthetized rats and in awake rats after the termination of halothane anesthesia. The hypertension rapidly led to pulmonary edema and death. Procaine microinjected also elicited fulminating hypertension; vehicle did not. Doses of KA producing hypertension caused no histological or biochemical evidence of neuronal death. The cardiovascular responses to KA were restricted to sites in the intermediate one-third of NTS and could not be elicited by injection into adjacent sites in brainstem. The results indicate that, in low doses, KA injected into NTS stimulates neurons which mediate the baroreflex, whereas, in higher doses, it produces baroreflex blockade and neurogenic hypertension. The results suggest that fulminating hypertension can be produced by nondestructive perturbations of neurochemical transmission in brain. Since the cardiovascular responses of KA are similar to those produced by microinjection into NTS of the amino acid neurotransmitter glutamic acid, the study adds further support to the hypothesis that L-glutamate is the neurotransmitter released by baroreceptor afferent nerves.

In 1948 knowledge of the role of the central nervous system in controlling blood pressure was rudimentary and the possibility that the brain could contribute to hypertension was barely acknowledged. Over the past 35 years the development of new technologies, particularly those of the neurosciences, and their application to cardiovascular medicine have markedly increased our understanding of how the brain governs the circulation. Notable has been the increase in knowledge of the neuroanatomy, neurochemistry, and physiology of the representation, within the brain, of the pathways critical in governing blood pressure, particularly those involved in tonic and baroreflex control of the circulation. The observation that perturbations in these networks can lead to the development of hypertension or the reversal of established hypertension and the demonstration that some of these networks are targets of drugs used in the clinical treatment of hypertension provide obvious clinical relevance to the discoveries. In the future, it is expected that more will be discovered about the biochemical neuroanatomy, and hopefully the molecular biology, of these neuronal networks. Such advances will permit more rigorous testing of the proposition that defects in these systems may underlie the disorder of hypertension in human beings.

The integrated effects of stimulation of fastigial nucleus (FN) and of concurrent activation of baroreceptors on arterial pressure (AP), heart rate (HR) and vasopressin (VP) release in anaesthetized, paralysed, artificially ventilated rats were investigated. Stimulation of FN blocked the fall in AP and HR elicited by afferent vagal stimulation. It also abolished the atropine-sensitive bradycardia occurring at the peak of the stimulus-elicited pressor elevation in 6-hydroxydopamine treated rats. Sinoaortic denervation plus vagotomy left the cardiovascular responses to FN stimulus unmodified but facilitated FN elicited VP secretion. Vagotomy alone was ineffective. After high spinal cord transection more VP than in the intact condition was secreted after FN stimulation: simultaneous afferent vagal stimulation did not affect the facilitated VP release. Therefore FN stimulation can inhibit the reflex effects on AP and HR from baroreceptors, while the high-(but not the low-) pressure receptors can partly inhibit the VP release elicited by FN stimulation.