Cholinergic receptor site binding, choline acetyltransferase, and acetylcholinesterase activity in the forebrain and brainstem of the Dahl rat model of essential hypertension

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.

Cardiovascular responses to cholinergic agonists in sinoaortic denervated rats

1. The cardiovascular effect of systemic nicotinic and muscarinic cholinergic stimulation were studied in conscious sham operated and sinoaortic denervated (SAD) rats, 7 days after the corresponding operation. 2. The administration of the nicotinic ganglionic agent, 1,1-dimethyl-4-phenylpiperazinium (DMPP, 50-100 micrograms.kg-1, i.v.) induced a fall of heart rate that was significantly higher in SAD rats than in sham rats. DMPP induced in sham rats an increase of arterial pressure but in SAD animals a biphasic response: an initial hypotension followed by an increase of arterial pressure. 3. Under muscarinic blockade, DMPP only showed a pressor and tachycardic action in both groups of rats without differences between them. 4. The muscarinic agonist, carbachol (0.1-10 micrograms.kg-1, i.v.) showed the same hypotensive and bradycardic action in both groups of rats. 5. Our results suggest that after 7 days of SAD, differences in the response to DMPP between sham and denervated animals could be due to the loss of baroreflex mechanisms. The increased bradycardic effect of DMPP in SAD rats could be mediated by a supersensitivity of parasympathetic ganglionic nicotinic receptors, whilst the sympathetic ganglionic nicotinic receptors remained unaltered. On the other side, the cardiovascular muscarinic responses to carbachol remain unaffected in SAD rats.

Cardiac acetylcholinesterase molecular forms and choline acetyltransferase in the Dahl (salt-sensitive) hypertensive strain of rats: a regional study

A regional study of acetylcholinesterase (AChE) molecular forms and choline acetyltransferase (ChAT) in the hearts of Dahl-salt sensitive (DS) and salt resistant (DR) rats was performed in animals administered either 8% or 0.35% dietary NaCl. Atria isolated from DS rats, regardless of dietary NaCl intake, had lower activities of all of the AChE molecular forms and ChAT when compared to their dietary-matched DR controls. In the ventricles, the activities of AChE molecular forms and ChAT were lower in DS rats compared to dietary-matched DR rats only when 8% NaCl diets were administered. The percent contribution of each of the molecular forms to the total AChE pool was not affected by animal strain or diet.

Changes in regional brain synaptosomal high affinity choline uptake during the development of hypertension in spontaneously hypertensive rats

The high-affinity uptake of choline (HAChU) by freshly prepared crude synaptosomal fractions was employed as relative measure of regional brain cholinergic activity. The Vmax for uptake as determined by the accumulation of a tracer amount of 3H-choline in the presence of unlabeled choline (0.2-2 microM) varied 6 fold depending upon the region examined (stratum greater than hypothalamus greater than medulla-pons). HAChU was hemicholinium-3-sensitive and linear at 37 degrees C from 1 to 8 min in all brain regions. Respective brain synaptosomal fractions derived from adult (12 week old) spontaneously hypertensive (SH) rats and normotensive Wistar Kyoto (WK) rats revealed no difference in the Vmax for HAChU from synaptosomes derived from the striatum of either strain. However, there was a significant increase in the Vmax for HAChU measured from the medulla-pons of SH rats compared with WK rats. In older (22 weeks) rats, the Vmax for HAChU was 78% greater than age-matched WK control rats. In addition, a highly significant correlation was found between resting systolic blood pressure and the Vmax for HAChU both in the medulla-pons (r = 0.76) and hypothalamus (r = 0.48). That the increase in HAChU in SH rats was not a consequence of elevated pressure, was indicated by the lack of effect of prolonged i.v. infusion of pressor agents in normotensive rats on HAChU. These findings are consistent with a role for brain cholinergic neurons in the maintenance of hypertension in SH rats.

The effect of atropine or atropine methylnitrate on salt-induced hypertension in the inbred S/JR and R/JR Dahl rat

The cholinergic antagonists atropine (ATR) and atropine methylnitrate (ATRMN) were chronically administered to inbred Dahl hypertension-sensitive (S/JR) and -resistant (R/JR) rats maintained on an 8.0% NaCl-containing diet. The effects on blood pressure (BP), heart rate (HR), and mortality were then examined during and after a four week period of treatment. Administration of ATR (7.2 mg/day) or ATRMN (2.4 mg/day) attenuated the development of salt-induced hypertension (HT) in the S/JR strain but had relatively little effect on BP in the R/JR strain. HR during the treatment period was significantly greater in S/JR and R/JR rats that received ATR or ATRMN than vehicle-treated controls. Each drug also reduced HT-related mortality in S/JR rats. In general, the effects of ATR on BP and mortality were greater than those of ATRMN. However, the results suggest that the central and peripheral cholinergic systems participate in the development of salt-induced HT in the S/JR rat.

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.

Genetic predisposition to hypertension, elevated blood pressure and pain sensitivity: a functional analysis

Pain sensitivity was examined in Dahl hypertension-sensitive (DS) and Dahl hypertension-resistant (DR) rats. The results indicated that hypertensive DS rats had a reduced sensitivity to painful stimulation compared to normotensive DS rats or compared to DR rats. Furthermore, the lower pain sensitivity of even normotensive DS vis-a-vis normotensive DR rats was not due to genetic factors but due to slight variations in blood pressure between lines. These results provide additional experimental support for the contention that elevations in blood pressure result in decreased pain sensitivity.