Muscarinic cholinoceptors and choline acetyltransferase activity in the hypothalamus of spontaneously hypertensive rats

Intranasal Losartan Decreases Perivascular Beta Amyloid, Inflammation, and the Decline of Neurogenesis in Hypertensive Rats

The contribution of the local angiotensin receptor system to neuroinflammation, impaired neurogenesis, and amyloid beta (Aβ) accumulation in Alzheimer's disease (AD) and in hypertension is consistent with the remarkable neuroprotection provided by angiotensin receptor blockers (ARBs) independent of their blood pressure-lowering effect. Considering the causal relationship between hypertension and AD and that targeting cerebrovascular pathology with ARBs does not necessarily require their systemic effects, we tested intranasal losartan in the rat model of chronic hypertension (spontaneously hypertensive stroke-prone rats, SHRSP). Intranasal losartan at a subdepressor dose decreased mortality, neuroinflammation, and perivascular content of Aβ by enhancing key players in its metabolism and clearance, including insulin-degrading enzyme, neprilysin, and transthyretin. Furthermore, this treatment improved neurologic deficits and increased brain IL-10 concentration, hippocampal cell survival, neurogenesis, and choroid plexus cell proliferation in SHRSP. Losartan (1 μM) also reduced LDH release from cultured astroglial cells in response to toxic glutamate concentrations. This effect was completely blunted by IL-10 antibodies. These findings suggest that intranasal ARB treatment is a neuroprotective, neurogenesis-inducing, and Aβ-decreasing strategy for the treatment of hypertensive stroke and cerebral amyloid angiopathy acting at least partly through the IL-10 pathway.

Altered urinary sodium excretion response after central cholinergic and adrenergic stimulation of adult spontaneously hypertensive rats

In this study, we hypothesized that blunting of the natriuresis response to intracerebroventricularly (i.c.v.) microinjected cholinergic and adrenergic agonists is involved in the development of hypertension in spontaneously hypertensive rats (SHR). We evaluated the effect of i.c.v. injection of cholinergic and noradrenergic agonists, at increasing concentrations, and of muscarinic cholinergic and α1 and α2-adrenoceptor antagonists on blood pressure and urinary sodium handling in SHR, compared with age-matched Wistar Kyoto rats (WR). We confirmed that CCh and NE microinjected into the lateral ventricle (LV) of conscious rats leads to enhanced natriuresis. This response was associated with increased proximal and post-proximal sodium excretion accompanied by an unchanged rate of glomerular filtration. We showed that cholinergic-induced natriuresis in WR and SHR was attenuated by previous i.c.v. administration of atropine and was significantly lower in the hypertensive strain than in WR. In both groups the natriuretic effect of injection of noradrenaline into the LV was abolished by previous local injection of an α1-adrenoceptor antagonist (prazosin). Conversely, LV α2-adrenoceptor antagonist (yohimbine) administration potentiated the action of noradrenaline. The LV yohimbine pretreatment normalized urinary sodium excretion in SHR compared with age-matched WR. In conclusion, these are, as far as we are aware, the first results showing the importance of interaction of central cholinergic and/or noradrenergic receptors in the pathogenesis of spontaneous hypertension. These experiments also provide good evidence of the existence of a central adrenergic mechanism consisting of α1 and α2-adrenoceptors which works antagonistically on regulation of renal sodium excretion.

Is the spontaneously hypertensive stroke prone rat a pertinent model of sub cortical ischemic stroke? A systematic review

The spontaneously hypertensive stroke prone rat is best known as an inducible model of large artery stroke. Spontaneous strokes and stroke propensity in the spontaneously hypertensive stroke prone rat are less well characterized; however, could be relevant to human lacunar stroke. We systematically reviewed the literature to assess the brain tissue and small vessel pathology underlying the spontaneous strokes of the spontaneously hypertensive stroke prone rat. We searched systematically three online databases from 1970 to May 2010; excluded duplicates, reviews, and articles describing the consequences of induced middle cerebral artery occlusion or noncerebral pathology; and recorded data describing brain region and the vessels examined, number of animals, age, dietary salt intake, vascular and tissue abnormalities. Among 102 relevant studies, animals sacrificed after developing stroke-like symptoms displayed arteriolar wall thickening, subcortical lesions, enlarged perivascular spaces and cortical infarcts and hemorrhages. Histopathology, proteomics and imaging studies suggested that the changes not due simply to hypertension. There may be susceptibility to endothelial permeability increase that precedes arteriolar wall thickening, degeneration and perivascular tissue changes; systemic inflammation may also precede cerebrovascular changes. There were very few data on venules or tissue changes before hypertension. The spontaneously hypertensive stroke prone rat shows similar features to human lacunar stroke and may be a good spontaneous model of this complex human disorder. Further studies should focus on structural changes at early ages and genetics to identify factors that predispose to vascular and brain damage.

Vascular determinants of cholinergic deficits in Alzheimer disease and vascular dementia

Alzheimer's disease (AD) and vascular dementia (VaD) are widely accepted as the most common forms of dementia. Cerebrovascular lesions frequently coexist with AD, creating an overlap in the clinical and pathological features of VaD and AD. This review assembles evidence for a role for cholinergic mechanisms in the pathogenesis of VaD, as has been established for AD. We first consider the anatomy and vascularization of the basal forebrain cholinergic neuronal system, emphasizing its susceptibility to the effects of arterial hypertension, sustained hypoperfusion, and ischemic cerebrovascular disease. The impact of aging and consequences of disruption of the cholinergic system in cognition and in control of cerebral blood flow are further discussed. We also summarize preclinical and clinical evidence supporting cholinergic deficits and the use of cholinesterase inhibitors in patients with VaD. We postulate that vascular pathology likely plays a common role in initiating cholinergic neuronal abnormalities in VaD and AD.

The hypothalamus and hypertension

Most forms of hypertension are associated with a wide variety of functional changes in the hypothalamus. Alterations in the following substances are discussed: catecholamines, acetylcholine, angiotensin II, natriuretic peptides, vasopressin, nitric oxide, serotonin, GABA, ouabain, neuropeptide Y, opioids, bradykinin, thyrotropin-releasing factor, vasoactive intestinal polypeptide, tachykinins, histamine, and corticotropin-releasing factor. Functional changes in these substances occur throughout the hypothalamus but are particularly prominent rostrally; most lead to an increase in sympathetic nervous activity which is responsible for the rise in arterial pressure. A few appear to be depressor compensatory changes. The majority of the hypothalamic changes begin as the pressure rises and are particularly prominent in the young rat; subsequently they tend to fluctuate and overall to diminish with age. It is proposed that, with the possible exception of the Dahl salt-sensitive rat, the hypothalamic changes associated with hypertension are caused by renal and intrathoracic cardiopulmonary afferent stimulation. Renal afferent stimulation occurs as a result of renal ischemia and trauma as in the reduced renal mass rat. It is suggested that afferents from the chest arise, at least in part, from the observed increase in left auricular pressure which, it is submitted, is due to the associated documented impaired ability to excrete sodium. It is proposed, therefore, that the hypothalamic changes in hypertension are a link in an integrated compensatory natriuretic response to the kidney's impaired ability to excrete sodium.

Activation of the reticulothalamic cholinergic pathway by the major metabolites of aniracetam

The aim of the study was to further investigate the effects of aniracetam, a cognition enhancer, and its metabolites on the brain cholinergic system. We measured choline acetyltransferase activity and acetylcholine release using in vivo brain microdialysis in stroke-prone spontaneously hypertensive rats (SHRSP). The enzyme activity in the pons-midbrain and hippocampus, and basal acetylcholine release in the nucleus reticularis thalami were lower in SHRSP than in age-matched Wistar Kyoto rats, indicating central cholinergic deficits in SHRSP. Repeated treatment of aniracetam (50 mg/kg p.o. x 11 for 6 days) preferentially increased the enzyme activity in the thalamus, whereas decreased it in the striatum. Among the metabolites of aniracetam, local perfusion of N-anisoyl-gamma-aminobutyric acid (GABA, 0.1 and/or 1 microM) and p-anisic acid (1 microM) into the nucleus reticularis thalami, dorsal hippocampus and prefrontal cortex of SHRSP produced a significant but delayed increase of acetylcholine release. We failed, however, to find any effect of aniracetam itself. A direct injection of N-anisoyl-GABA (1 nmol) into the pedunculopontine tegmental nucleus of SHRSP enhanced the release in the nucleus reticularis thalami. Thus, these data prove that aniracetam can facilitate central cholinergic neurotransmission via both metabolites. Based on its pharmacokinetic profile, N-anisoyl-GABA may contribute to the clinical effects of aniracetam, mainly by acting on the reticulothalamic cholinergic pathway.

Spontaneously hypertensive rats cholinergic hyper-responsiveness: central and peripheral pharmacological mechanisms

1. The mechanisms and the subtypes of muscarinic receptors implicated in the cardiovascular effects of physostigmine were investigated in conscious normotensive and spontaneously hypertensive rats. 2. Intravenous (i.v.) physostigmine (50 microg kg-1) induced in both strains a long pressor response, accompanied by a bradycardia. This pressor response was larger in spontaneously hypertensive (+41+/-6 mmHg) than in Wistar-Kyoto (+25+/-2 mmHg) rats (P<0.05). 3. Pretreatment with atropine sulphate (0.4 mg kg-1 i.v.), completely abolished the physostigmine-induced pressor response in both normotensive and hypertensive rats. In both strains, the physostigmine pressor response was significantly reduced by the systemic administration of either an alpha1-adrenoceptor antagonist (prazosin, 1 mg kg-1) or a V1A-vasopressin receptor antagonist (AVPX, 20 microg kg-1). This physostigmine pressor effect was completely abolished in both strains when both antagonists were administered concomitantly. 4. In WKY rats, the pressor response to physostigmine (50 microg kg-1 i.v.) was inhibited in a dose-dependent manner by i. c.v. administration of atropine (ID50=3.70 nmoles), the M1 receptor antagonist pirenzepine (ID50=10.71 nmoles), the M2 receptor antagonist methoctramine (ID50=4.31 nmoles), the M3 receptor antagonist p-F-HHSiD (ID50=60.52 nmoles) and the M4 receptor antagonist tropicamide (ID50=214.20 nmoles). In the hypertensive strain, the ID50 were found to be significantly higher for atropine (7.34 nmoles), pirenzepine (21.60 nmoles) and p-F-HHSiD (139.50 nmoles) (P<0.05). 5. The present results indicate that physostigmine acts in normotensive and spontaneously hypertensive rats, through stimulation of both central M2 and M1 cholinoceptors to induce a rise in blood pressure mediated by an increase in plasma vasopressin and sympathetic outflow. Moreover, our results suggest that some modifications of the M1 receptor subtypes in terms of expression or affinity could be responsible for the hyper-responsiveness of the hypertensive strain to cholinomimetic agents.

Enhanced release of acetylcholine in the rostral ventrolateral medulla of spontaneously hypertensive rats

We examined whether the altered rostral ventrolateral medulla (RVLM) cholinergic function in spontaneously hypertensive rats (SHR) results from enhanced presynaptic cholinergic tone. Male 12- to 16-week-old SHR and age-matched Wistar Kyoto rats (WKY) were anesthetized, paralyzed and artificially ventilated. Unilateral microinjection of cholinergic agents into the RVLM produced a pressor response. The pressor response to physostigmine was greater in SHR than that of WKY whereas the response to ACh and carbachol was the same in WKY and SHR. Bilateral microinjection of scopolamine produced a decrease in blood pressure. The depressor response was greater in SHR than that of WKY. When a microdialysis probe was placed in the RVLM, ACh release in the RVLM was greater in SHR than that of WKY. Choline acetyltransferase (CAT) activity was increased only in the rostro-ventral part of the medulla, which contained the RVLM, but not in other parts of the medulla oblongata. Physostigmine (0.5 mg/kg, i.p.)-induced increases in ACh content were also enhanced only in the rostro-ventral part of the medulla. These results provide direct evidence that ACh release in the RVLM is enhanced in SHR. It appears that the enhanced cholinergic activity in the RVLM of SHR results from an increase in cholinergic impulse flow in the RVLM of SHR. This abnormality may play a role in the maintenance of hypertension in SHR.

Distribution of [3H]quinuclidinyl benzilate, [3H]nicotine, and [125I]alpha-bungarotoxin binding sites in the nucleus tractus solitarii of the cat

The distribution of muscarinic and nicotinic cholinergic binding sites in the cat nucleus tractus solitarii was studied by the technique of in vitro autoradiography. Using the antagonist [3H]quinuclidinyl benzilate, muscarinic binding sites were differentially located in subdivisions of the nucleus tractus solitarii. The majority of muscarinic binding sites were located predominantly in the caudal half of the nucleus, reaching their greatest amounts at the mid levels of the nucleus tractus solitarii. The medial, dorsolateral, intermediate, and interstitial subdivisions contained the highest densities of quinuclidinyl benzilate binding sites. Nicotinic cholinergic binding sites, using [3H]nicotine and [125I]alpha-bungarotoxin, had unique patterns of distribution. With [3H]nicotine the majority of binding sites were located in rostral levels of the nucleus with very few binding sites present in the caudal half. In contrast, [125I]alpha-bungarotoxin binding sites were present mainly in subdivisions located in the caudal half of the nucleus, i.e., commissural, ventrolateral, dorsolateral, medial, and intermediate subdivisions, and dropped off precipitously at more rostral levels. The differential distribution of [3H]nicotine and [125I]alpha-bungarotoxin suggests the two ligands may be labeling different types of nicotinic binding sites in the nucleus tractus solitarii. The unique distribution of muscarinic and nicotinic cholinergic binding sites in the various subdivisions of the nucleus solitarii suggests that muscarinic and nicotine mechanisms may play an active role in the regulation of the diverse autonomic functions at the level of the nucleus tractus solitarii.

Cardiovascular regulation by cholinergic mechanisms in rostral ventrolateral medulla of spontaneously hypertensive rats

This study aimed to demonstrate the role of acetylcholine receptors in the rostral ventrolateral medulla (RVL) in the central regulation of the cardiovascular system in normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). The effects of cholinergic drugs, microinjected into the rostral ventrolateral medullary vasopressor area, on blood pressure and heart rate in anesthetized and artificially ventilated rats were investigated. Unilateral microinjection of carbachol (1 nmol/site), physostigmine (300 pmol/site) or 3,4-diaminopyridine (500 pmol/site) into the RVL elicited a pressor and tachycardiac response, of which only the pressor response was significantly greater in SHR than in WKY. Bilateral microinjection of atropine (1 nmol/site) caused a depressor and bradycardiac response. The depressor response produced by atropine injected in the RVL was also significantly greater in SHR than in WKY. These results suggest that there are tonic cholinergic mechanisms in the RVL of the rats, which exert an excitatory cardiovascular action, and that the enhanced responsiveness to acetylcholine receptor stimulation in the RVL may contribute to the sustained elevation of blood pressure in the SHR.

Comparative studies of muscarinic and dopamine receptors in three strains of rat

Cholinesterase activities and characteristics of muscarinic and dopamine receptors from 9 week old male Sprague-Dawley (SD), Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) were studied. Plasma cholinesterase activity in WKY was significantly lower (50%) than activity in the other strains. In studies of muscarinic receptors, the number of [3H]QNB binding sites in striata from SD rats was lower (18%) than those from WKY and SHR. However, muscarinic receptor properties (Kd and Bmax) were the same in hypothalami. Studies of dopamine receptors revealed that the densities of both D-1 and D-2 receptors in both striata and hypothalami were significantly higher in SHR than in other strains. However, there were no differences in the affinity constant (Kd). The higher densities in hypothalami from SHR were mainly due to the high population of D-1 and D-2 receptors in the posterior hypothalamus. In the anterior hypothalamus, there was no difference in the population of D-2 receptors. These results provide a substantive basis, i.e. demonstration of alterations in drug metabolizing enzymes and receptor populations, on which to build an understanding of the genetic predisposition to the actions of xenobiotic agents.

Enhanced parasympathetic nerve activities in experimentally-induced nasal hypersensitivity

In order to verify the role of cholinergic nerves in nasal allergy and nonspecific nasal hypersensitivity we have evaluated the muscarinic acetylcholine receptors and the activity of choline acetyltransferase in nasal mucosa of TDI-sensitized guinea-pigs at specific points in time during a pre-determined sensitization period. The density of muscarinic receptors was significantly elevated in animals sensitized for 8 and/or 12 weeks as well as in the group which received 12 weeks of sensitization followed by a 4 week rest period. Choline acetyltransferase activity was also enhanced at 4-, 8- and 12 weeks. These observations provide biochemical evidence of the hyperactivity of cholinergic nerves in nasal allergy which has been suggested by clinical and pharmacological studies, and lead to the speculation that enhanced parasympathetic nerve function is implicated in causing and maintaining hypersensitivity and hypersecretion in nasal allergy.

Brain nicotine cholinoceptor binding in spontaneous hypertension

To study the role of central cholinergic mechanisms in hypertension, we have determined nicotinic and muscarinic agonist binding sites in the brain regions of stroke-prone spontaneously hypertensive rats (SHRSP), using [3H]nicotine and [3H]cismethyldioxolane (CD). There was a significant decrease in specific [3H]nicotine binding in the cerebral cortex, thalamus, midbrain, cerebellum and medulla oblongata of SHRSP at 16-24 weeks of age compared to that of age-matched Wistar Kyoto rats (WKY). Scatchard analysis revealed 35% decrease in the Bmax value for [3H]nicotine binding in the SHRSP medulla oblongata without a change in the Kd value, suggesting a change in the receptor density. Similar reduction of nicotinic cholinoceptor binding sites was also observed in the discrete brain regions of young (5-week-old) SHRSP. In contrast, there was no alteration in specific [3H]CD binding in the SHRSP brains regions, except the hypothalamus which showed a significant increase. The SHRSP medulla oblongata showed no change in the ChAT activity. Thus, the present study suggests an important role for medullary nicotinic cholinoceptors in the pathogenesis of spontaneous hypertension.