Glucocorticoid-induced hypertension in rats: role of the central muscarinic cholinergic system

Interaction of basal forebrain cholinergic neurons with the glucocorticoid system in stress regulation and cognitive impairment

A substantial number of studies on basal forebrain (BF) cholinergic neurons (BFCN) have provided compelling evidence for their role in the etiology of stress, cognitive aging, Alzheimer’s disease (AD), and other neurodegenerative diseases. BFCN project to a broad range of cortical sites and limbic structures, including the hippocampus, and are involved in stress and cognition. In particular, the hippocampus, the primary target tissue of the glucocorticoid stress hormones, is associated with cognitive function in tandem with hypothalamic-pituitary-adrenal (HPA) axis modulation. The present review summarizes glucocorticoid and HPA axis research to date in an effort to establish the manner in which stress affects the release of acetylcholine (ACh), glucocorticoids, and their receptor in the context of cognitive processes. We attempt to provide the molecular interactive link between the glucocorticoids and cholinergic system that contributes to BFCN degeneration in stress-induced acceleration of cognitive decline in aging and AD. We also discuss the importance of animal models in facilitating such studies for pharmacological use, to which could help decipher disease states and propose leads for pharmacological intervention.

Glucocorticoid treatment increases inhibitory m(2) muscarinic receptor expression and function in the airways

M(2) muscarinic receptors on parasympathetic nerve endings inhibit acetylcholine release in the airways. In this study, the effects of dexamethasone on M(2) receptors in vivo and in primary cultures of airway parasympathetic neurons were tested. Treating guinea pigs with dexamethasone (0.1 mg/kg, daily for 2 d) substantially increased inhibitory M(2) muscarinic receptor function, decreasing airway responsiveness to electrical stimulation of the vagi. At the same time, dexamethasone decreased the response to acetylcholine but not to methacholine, suggesting that cholinesterase activity was increased. When both cholinesterase and M(2) receptors were blocked (using physostigmine and gallamine, respectively) vagally induced bronchoconstriction was increased to control values. In primary cultures of airway parasympathetic neurons, dexamethasone significantly decreased the release of acetylcholine in response to electrical stimulation. Blocking inhibitory M(2) receptors using atropine (10(-5) M) increased acetylcholine release. After the M(2) receptors were blocked there was no difference in acetylcholine release between control and dexamethasone-treated cultures. M(2) receptor gene expression was increased by more than fivefold in dexamethasone-treated cultures. Immunostaining of dexamethasone-treated neurons demonstrated more intense staining. Thus, decreased vagally mediated reflex bronchoconstriction after glucocorticoid treatment may be the result on increased M(2) receptor expression and function as well as increased degradation of acetylcholine by cholinesterase.

Cholinergic mechanism and blood pressure regulation in the central nervous system

Cholinergic neurons in numerous brain regions have been implicated in blood pressure regulation. One of the most important brain regions where cholinergic neurons play a role in the pathogenesis of hypertension is the rostral ventrolateral medulla (RVL), an essential source of efferent sympathetic activity. Pharmacological and biochemical studies have revealed that acetylcholine release in the RVL is increased in experimental hypertension regardless of its etiology and that this enhanced release of acetylcholine leads to hypertension. The lateral parabrachial nucleus, another important hindbrain area involved in blood pressure regulation, is responsible for the enhanced release of acetylcholine in the RVL of hypertensive animals. Moreover, recent studies have demonstrated the involvement of the hypothalamic defence area, an area believed to be involved in the hypertension induced by chronic stress, in the release of acetylcholine in the RVL and also have demonstrated the existence of direct projections from the hypothalamic structures to the lateral parabrachial nucleus. More studies about mechanisms of the enhanced release of acetylcholine in the RVL of experimentally hypertensive animals will provide important information for central mechanisms of hypertension.

Possible involvement of glucocorticoids in the modulation of interleukin-1-induced cardiovascular responses in rats

1. In freely moving rats, we investigated whether glucocorticoids modulate the cardiovascular responses to intraperitoneal (I.P.) injection of interleukin-1 beta (IL-1 beta). 2. A lower dose of IL-1 beta (1 microgram kg-1, I.P.) induced monophasic increases, and a higher dose (10 micrograms kg-1, I.P.) induced biphasic increases in both blood pressure and heart rate. Plasma concentration of corticosterone increased significantly after injection of IL-1 beta (10 micrograms kg-1). 3. Systemic pretreatment with an exogenous glucocorticoid, dexamethasone (DEX; 0.5 mg kg-1) reduced the monophasic pressor response, the first phase of the biphasic pressor response and also the initial tachycardia. By contrast, the second phase of the biphasic pressor response was enhanced. 4. After bilateral adrenalectomy, the IL-1 beta (10 micrograms kg-1)-induced pressor effect was reduced; it was restored by treatment with DEX (0.5 mg kg-1). The heart rate response was enhanced in adrenalectomized (ADX) rats; this enhancement was attenuated by DEX. 5. IL-1 beta (10 micrograms kg-1)-induced increases in plasma noradrenaline (NA) were suppressed in intact rats pretreated with DEX (0.5 mg kg-1). The IL-1 beta-induced NA response was greater in ADX rats than in sham-ADX rats. 6. We suggest that glucocorticoids are an important modulator of cardiovascular responses induced in rats by systemically administered IL-1 beta.

Analysis of circadian variation of blood pressure and heart rate in dexamethasone-induced hypertensive rats

We studied the effect of the chronic oral administration of dexamethasone (dexa) on arterial blood pressure (BP) in conscious rats. Special attention was paid to the effects of dexa on circadian rhythm of BP. As determined by the tail cuff-method, BP in the dexa-treated group was significantly higher than in the control group 24 h after treatment, then increased gradually, reaching a plateau on the 7th day of treatment. At that time, the difference in BP between the two groups was approximately 30 mmHg. When monitored directly and continuously on day 10, mean arterial pressure (MAP) in the dexa-treated group exceeded that of the control group by approximately 15 mmHg throughout the monitoring period. Thus, the circadian rhythm of MAP was sustained in the dexa-treated group, which was in contrast to the previously reported elimination of circadian rhythm in humans. In addition, the increase in BP may have been overestimated by tail-cuff plethysmography, possibly owing to a hightended cardiovascular reactivity to environmental stimuli in dexa-treated animals.

Brain amines in glucocorticoid-induced hypertension in the rat

A two week administration of the glucocorticoid betametasone to male Wistar rats produced a mild hypertensive state. The brain of these rats showed some significant changes in amine and metabolite content with respect to normotensive controls. Epinephrine and metanephrine were increased in the rostral ventrolateral medulla and in the preoptic area. Epinephrine also increased in the septal area. Normetanephrine decreased in the rostral ventrolateral medulla. Dopamine and homovanillic acid increased in septal and preoptic areas. Dopamine alone increased in rostral ventrolateral medulla. Serotonin and 5-hydroxyindole-3-acetic acid increased in the septal area and dorsal medulla. These changes suggest significant alterations in the aminergic activity of the brain circuitry known to regulate cardiovascular functions; the changes may play a basic role in the development and maintenance of glucocorticoid-induced hypertension.