The regulation of cardiovascular functions by monoamine neurotransmitters in the brain

Organization of peptidergic and catecholaminergic efferents from the nucleus of the solitary tract to the rat amygdala

Previous studies have focused on the role of the central nucleus of the amygdala (CeA) in cardiovascular and other amygdaloid functions. The combined retrograde tracing/immunohistochemical method was used to test for the presence of enkephalin, neurotensin, neuropeptide Y, and catecholamine neurons within the nucleus of the solitary tract that send efferents to the CeA. After injections of retrograde tracer into the CeA, retrogradely labeled neurons were observed within the caudal, medial nucleus of the solitary tract. Most CeA-projecting neurons were located ipsilaterally within the medial nucleus of the solitary tract at the level of the area postrema. Retrogradely labeled enkephalin- and neurotensin-immunoreactive neurons were found within the medial nucleus of the solitary tract at this level, while retrogradely labeled neuropeptide Y-immunoreactive neurons were found within the medial nucleus of the solitary tract rostral to the area postrema. About 60-74% of CeA-projecting cells were also immunoreactive for tyrosine hydroxylase. Approximately 9% of retrogradely neurons were phenylethanolamine-N-methyltransferase immunoreactive. The results provide evidence that within the nucleus of the solitary tract, peptidergic CeA-projecting neurons have a topographic distribution. In addition, noradrenergic neurons within the A2 group, rather than adrenergic neurons of the C2 group, provide the bulk of catecholaminergic input to the CeA from the nucleus of the solitary tract. Cell counts indicate that each of these peptides may be colocalized (to varying extents) within catecholamine-producing neurons. Also the catecholaminergic and enkephalinergic contribution to the ascending pathway from the nucleus of the solitary tract to the CeA distinguishes it neurochemically from the descending pathway. Thus, although there are afferent and efferent connections between the nucleus of the solitary tract and CeA, their peptidergic/neurotransmitter connections are not necessarily reciprocal. Input from nucleus of the solitary tract peptidergic and catecholaminergic neurons to the CeA may be important in the etiology of a number of pathophysiological conditions including hypertension, gastric ulcers, and schizophrenia.

Effect of prazosin on spontaneous sympathetic-cholinergic activity

Tonic sympathetic-cholinergic electrodermal (sudomotor) activity was measured in intact anesthetized and unanesthetized decerebrate and decerebrate-spinalized cats. Prazosin (3-100 micrograms/kg i.v.) depressed spontaneous electrodermal activity in intact anesthetized cats in a dose-dependent fashion (ED50 4.8 micrograms/kg). Prazosin's action was almost totally abolished by monoamine depletion with reserpine and a synthesis inhibitor. The alpha 2-adrenoceptor blocker, yohimbine (0.5 mg/kg i.v.) also antagonized prazosin-induced sympatho-inhibition. Prazosin reduced tonic sudomotor activity in unanesthetized decerebrate cats in a dose-dependent fashion (ED50 5.5 micrograms/kg i.v.) but was without effect in spinalized preparations. These results support the hypothesis that, in this system, prazosin produces sympatho-inhibition indirectly by means of an alpha 2-adrenoceptor-mediated mechanism, an effect which appears to be prejunctional. It is proposed that prazosin acts at the level of the spinal cord to facilitate ongoing alpha 2-adrenergic inhibition arising from supra-spinal loci.

On the use of fibroblasts in tissue culture to study the neurochemistry of brain mechanisms controlling blood pressure

Fibroblasts grown in tissue culture from skin biopsies taken from the genetic model of spontaneously hypertensive rats (SHR), show a reduction in the accumulation of choline when compared to fibroblasts grown from skin biopsies taken from normotensive Wistar Kyoto rats. Choline uptake by fibroblasts from both strains of rats is sodium dependent but there is no difference between the strains in the effects of sodium depletion. These results suggest that fibroblasts in tissue culture may be a useful source of tissue for analyzing differences between hypertensive and normotensive individuals that are genetically determined rather than due to prolonged exposure to elevated blood pressure. In addition, the fibroblast shares several biochemical characteristics with the neuron and, at least as far as these common characteristics are concerned, appears to be a good model for neurochemical study.

Studies on the mechanism of prazosin induced sympatho-inhibition

Intravenous administration of the alpha 1-adrenoceptor antagonist, prazosin (3-300 micrograms/kg), produced a depression of sympathetic-cholinergic electrodermal responses evoked by electrical stimulation of the posterior hypothalamus in pentobarbital anesthetized cats. Pretreatment with the alpha 2-adrenoceptor antagonists yohimbine (0.5 mg/kg) or idazoxan (0.1 mg/kg) significantly blocked the depressant effects of prazosin but had no effect on hypothalamic evoked electrodermal responses when given alone. Electrodermal responses were readily elicited in animals depleted of CNS monoamines. Monoamine depletion, however, totally abolished prazosin's depression of centrally evoked electrodermal responses. Prazosin also depressed the amplitude of electrodermal responses evoked by electrical stimulation of the cervical spinal cord in spinalized cats. In contrast to hypothalamic stimulation, yohimbine when given alone potentiated spinally evoked electrodermal responses which suggests that both excitatory and inhibitory mechanisms were being activated. Taken together these results suggest that prazosin produces its CNS sympatholytic effect by enhancing inhibition mediated by alpha 2-adrenoceptor mechanisms and not directly by blockade of excitatory alpha 1-adrenergic receptors in the central nervous system. A spinal cord site of action for prazosin is also implicated.

Depletion of central catecholamines reduces pressor responses to arginine vasopressin

Previous reports that central administration of arginine vasopressin (AVP) increases turnover of brain catecholamines raise the possibility that the pressor responses which follow central administration of AVP may be mediated, in part, by central catecholamines. To test this hypothesis, rats were given intraventricular injections of vehicle, or of the neurotoxin, 6-hydroxydopamine, which resulted in significant depletions of hypothalamic and medulla oblongata noradrenalin and hypothalamic dopamine, but not of medullary dopamine or of hypothalamic and medullary 5-hydroxytryptamine. Following a one week recovery, these conscious rats, fitted with indwelling arterial catheters, were given intraventricular injections of AVP; the increases in arterial pressure and heart rate were significantly reduced in the catecholamine-depleted animals. These data support the hypothesis that the pressor and tachycardia responses to intraventricular AVP are mediated, in part, by central catecholamine-containing neurons.

Comparison of neuropeptide Y immunoreactivity in hypothalamic and brainstem nuclei of young and mature spontaneously hypertensive and normotensive Wistar-Kyoto rats

The concentration of neuropeptide Y immunoreactivity (NPY-ir) was measured in 8 hypothalamic and 5 brainstem nuclei of 6- and 14-week-old spontaneously hypertensive (SH) and Wistar-Kyoto (WKY) rats. Strain differences were observed in 3 hypothalamic nuclei and age-dependent changes occurred in 3 hypothalamic and 2 brainstem nuclei. In both the ventromedial hypothalamic nucleus and locus coeruleus the observed change in NPY-ir with age in SH rats was significantly different to the change observed in the WKY. These strain- and age-related differences in NPY-ir may be of relevance in the development of hypertension in the SH rat.

Pupillary dilation as an index of central nervous system alpha 2-adrenoceptor activation

In recent years there has been increasing evidence that some antihypertensive drugs like clonidine and alpha-methyldopa (after conversion in the brain to alpha-methylnorepinephrine) may decrease sympathetic tone by stimulating central nervous system (CNS) alpha 2-adrenoceptors. These same drugs also produce pupillary dilation in cats and rats. In this review, evidence is presented supporting the hypothesis that clonidinelike drugs act either directly or indirectly on CNS postsynaptic alpha 2-adrenoceptors to cause pupillary dilation by reduction of parasympathetic neural tone to the iris. It is further suggested that the underlying physiologic mechanism for this mydriatic action is activation of an ascending pathway that provides tonic inhibitory input by releasing norepinephrine on neurons in the Edinger-Westphal complex. Yohimbine-sensitive pupillary dilation in these species may provide a simple and effective model for quantitatively accessing CNS alpha 2-adrenoceptor activity.

Animal models of depression: parallels and correlates to severe depression in humans

Drugs with antidepressant properties in patients with severe depression also have various behavioral and neurochemical effects in animals. This has given rise to numerous animal models that have been suggested to be valid for research into the neurobiology of depression and the neurochemical mechanisms of the antidepressant drugs. However, considerable evidence from many avenues of research indicates that severe depression is a biochemical disorder that develops in those individuals with some predisposing neurochemical vulnerability. Although the predisposing biochemical abnormality has not been identified, it may be related to the neurochemical mechanisms that regulate impulse traffic in various neural systems and maintain the homeostatic balance of neural activity within the brain. Therefore, the appropriate animal model for severe depression should have some disruption of neural functioning that is returned to normal by the chronic administration of antidepressant drugs. Of the numerous animal models of depression that have been presented in the literature, only the rat with olfactory bulb lesions meets this requirement. The behavioral and endocrine abnormalities induced by the olfactory bulb lesions are reversed by chronic (but not acute) treatment with antidepressants of various classes. Of the existing animal models of severe depression, the olfactory bulbectomy model holds the most promise for elucidating the neurobiology of depression and the neurochemistry of antidepressant drugs.

Alpha 1- and alpha 2-adrenoceptor binding in the Dahl rat model of hypertension

Dahl sensitive rats on a high salt diet (DSH group) developed significant elevations in blood pressure (BP). Sensitive rats maintained on a low salt diet (DSL group) and Dahl resistant rats on a high or low salt diet (DRH and DRL groups, respectively) remained normotensive. The DSH and DRH groups displayed a lower density of alpha 2-adrenoceptors (as measured with [3H]-clonidine) in the cerebral cortex than normotensive DSL and DRL groups. In contrast, the density of alpha 2-adrenoceptors in the medulla was significantly lower in the DSH group than the DSL group, but significantly higher in the DRH group compared to the DRL group. The density of alpha 1-adrenoceptors (as measured with [3H]-WB4101) in the hypothalamus was lower in the DSH group than the DSL group but greater in the DRH group than the DRL group. The results suggest that the sensitive and resistant lines can be distinguished by the density of alpha 1- adrenoceptors in the hypothalamus and medulla, respectively. The interactive effects of dietary NaCl and susceptibility to hypertension on adrenoceptors lend further support to the hypothesis that the genetic predisposition to hypertension is associated with a disruption in central adrenergic activity.