Comparison of norepinephrine release in hypertensive rats: I. Hypothalamic and brainstem tissues

Enhanced assymetrical noradrenergic transmission in the olfactory bulb of deoxycorticosterone acetate-salt hypertensive rats

The ablation of olfactory bulb induces critical changes in dopamine, and monoamine oxidase activity in the brain stem. Growing evidence supports the participation of this telencephalic region in the regulation blood pressure and cardiovascular activity but little is known about its contribution to hypertension. We have previously reported that in the olfactory bulb of normotensive rats endothelins enhance noradrenergic activity by increasing tyrosine hydroxylase activity and norepinephrine release. In the present study we sought to establish the status of noradrenergic activity in the olfactory bulb of deoxycorticosterone acetate (DOCA)-salt hypertensive rats. Different steps in norepinephrine transmission including tyrosine hydroxylase activity, neuronal norepinephrine release and uptake were assessed in the left and right olfactory bulb of DOCA-salt hypertensive rats. Increased tyrosine hydroxylase activity, and decreased neuronal norepinephrine uptake were observed in the olfactory bulb of DOCA-salt hypertensive rats. Furthermore the expression of tyrosine hydroxylase and its phosphorylated forms were also augmented. Intriguingly, asymmetrical responses between the right and left olfactory bulb of normotensive and hypertensive rats were observed. Neuronal norepinephrine release was increased in the right but not in the left olfactory bulb of DOCA-salt hypertensive rats, whereas non asymmetrical differences were observed in normotensive animals. Present findings indicate that the olfactory bulb of hypertensive rats show an asymmetrical increase in norepinephrine activity. The observed changes in noradrenergic transmission may likely contribute to the onset and/or progression of hypertension in this animal model.

Nerve stimulation induced overflow of neuropeptide Y and modulation by angiotensin II in spontaneously hypertensive rats

The sympathetic nervous system and renin-angiotensin system are both thought to contribute to the development and maintenance of hypertension in experimental models such as the spontaneously hypertensive rat (SHR). We demonstrated that periarterial nerve stimulation (NS) increased the perfusion pressure (PP) and neuropeptide Y (NPY) overflow from perfused mesenteric arterial beds of SHRs at 4-6, 10-12, and 18-20 wk of age, which correspond to prehypertensive, developing hypertensive, and maintained hypertensive stages, respectively, in the SHR. NS also increased PP and NPY overflow from mesenteric beds of Wistar-Kyoto (WKY) normotensive rats. NS-induced increases in PP and NPY were greater in vessels obtained from SHRs of all three ages compared with WKY rats. ANG II produced a greater increase in PP in preparations taken from SHRs than WKY rats. ANG II also resulted in a greater increase in basal NPY overflow from 10- to 12-wk-old and 18- to 20-wk-old SHRs than age-matched WKY rats. ANG II enhanced the NS-induced overflow of NPY from SHR preparations more than WKY controls at all ages studied. The enhancement of NS-induced NPY overflow by ANG II was blocked by the AT1 receptor antagonist EMD-66684 and the angiotensin type 2 receptor antagonist PD-123319. In contrast, ANG II greatly enhanced norepinephrine overflow in the presence of PD-123319. Both captopril and EMD-66684 decreased neurotransmitter overflow from SHR mesenteric beds; therefore, we conclude that an endogenous renin-angiotensin system is active in this preparation. It is concluded that the ANG II-induced enhancement of sympathetic nerve stimulation may contribute to the development and maintenance of hypertension in the SHR.

Modulation of neurotransmitter release by NO is altered in mesenteric arterial bed of spontaneously hypertensive rats

Nitric oxide (NO) reacts with catecholamines resulting in their deactivation. In the present study with the use of the perfused mesenteric arterial bed as a model of the sympathetic neuroeffector junction, the NO synthase (NOS) inhibitor Nω-nitro-l-arginine methyl ester (l-NAME) resulted in the enhancement of the periarterial nerve stimulation-induced increase in perfusion pressure and norepinephrine overflow while decreasing neuropeptide Y (NPY) overflow. These changes were prevented by l-arginine, demonstrating that the effects of l-NAME were specific to the inhibition of NOS. From the fact that norepinephrine acts on prejunctional α2-adrenoceptors to inhibit the evoked release of sympathetic cotransmitters, we carried out experiments in the presence of the α2-adrenergic receptor antagonist yohimbine to investigate the possibility that the decrease in NPY observed in the presence of l-NAME was due to the increase in bioactive norepinephrine acting on its autoreceptor. Periarterial nerve stimulation in the presence of both l-NAME and yohimbine prevented the previously observed decrease in NPY, indicating that the cause of this decrease was, as predicted, due to α2-adrenoceptor activation. The periarterial nerve stimulation-induced increase of norepinephrine overflow was greater in the spontaneously hypertensive rat compared with normotensive rats. In contrast to what was observed in the isolated perfused mesenteric arterial bed obtained from normotensive animals, inhibition of NOS did not result in a further increase in the overflow of norepinephine or in a subsequent decrease in NPY. These results demonstrate that, in addition to being a direct vasodilator, NO, by deactivating norepinephrine, can modulate sympathetic neurotransmission and that this modulation is altered in the spontaneously hypertensive rat.

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.

Central interactions between noradrenaline and neuropeptide Y in the rat: implications for blood pressure control

Neuropeptide Y (NPY) and noradrenaline are co-localised in central neurones and both transmitters exert cardiovascular effects. Using microdialysis and push-pull techniques to measure transmitter release in vivo, and microinjection studies, we examined the role(s) of central noradrenaline and NPY in blood pressure regulation in the hypothalamus and nucleus tractus solitarius (NTS) of the rat. Hypothalamic noradrenaline release was increased following haemorrhage and reduced after phenylephrine infusion. Ageing is associated with markedly reduced NPY concentrations in the hypothalamus. 18-month old animals showed a reduced ability to release both NPY and noradrenaline to a potassium depolarisation stimulus. NTS administration of NPY induced dose-dependent decreases in blood pressure and heart rate. The depressor but not the bradycardic response was attenuated by prior administration of yohimbine. NTS microinjection of 23 pmol NPY induced similar cardiovascular effects in spontaneously hypertensive and Wistar Kyoto rats. NPY and noradrenaline appear to interact at several sites in the brain known to be important for blood pressure control.

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.

3H-norepinephrine release in hypothalamus and brainstem of Dahl-salt sensitive and resistant rats in vitro

Electrically evoked 3H-norepinephrine release was measured in slices of hypothalamus and brainstem of Dahl-salt sensitive (Dahl-S) and Dahl-salt resistant (Dahl-R) rats on both high and low sodium chloride diets. Only those Dahl-S rats fed a high sodium chloride diet became hypertensive, systolic blood pressure above 150 mmHg, although the Dahl-S rats on a low sodium diet, and Dahl-R rats fed a high sodium diet, both had blood pressures that were elevated compared to the Dahl-R rats maintained on a low sodium diet, when measured at the time of sacrifice. The Dahl-S rats on a high sodium diet also showed an enhancement in the field-stimulation induced release of 3H-norepinephrine in the posterior hypothalamus. Evoked 3H-norepinephrine release was not altered in low sodium diet Dahl-S rats or in low or high sodium salt Dahl-R rats. The stimulation induced 3H-norepinephrine release was also not different in the anterior hypothalamus or the A2 region of the nucleus tractus solitarius in either Dahl-S or Dahl-R animals on either sodium chloride diet. These results suggest that the alteration of evoked 3H-norepinephrine release, specifically in the posterior hypothalamus may play a role in the development and/or maintenance of hypertension.