Wybren de Jong

Regional concentrations of noradrenaline and dopamine in rat brain

The concentrations of noradrenaline and lopamine of 92 brain regions have been measured by a radiometric method which enabled discrimination between noradrenaline and adrenaline. Almost all brain regions investigated contained both noradrenaline and lopamine in measurable amount. However, both catecholamines appeared to be unevenly distributed. Very high dopamine concentrations were measured in the olfactory tubercle, the nucleus accumbens, the caudate nucleus and the rostral part of the medial forebrain bundle; the globus pallidus, the nucleus tractus diagonalis and the nucleus septalis lateralis were also very rich in dopamine. Outside the telencephalon the dopamine concentrations were rather low, except in the median eminence and the area tegmentalis ventralis (Tsai), an area corresponding to the A10 region. High noradrenaline concentrations were measured in most hypothalamic nuclei. Relatively high concentrations of this catecholamine were also measured in several mesencephalic (the ventral part of the central gray, the nucleus raphe dorsalis and the nucleus cuneiformis) and pontine (the locus coeruleus and the nuclei parabrachiales) regions. The highest noradrenaline concentrations in the medulla oblongata were observed in the A2 region and the nucleus commissuralis, which contained at least twice as much noradrenaline as did the more rostral part of the nucleus tractus solitarii.

Effect of catecholamine-receptor stimulating agents on blood pressure after local application in the nucleus tractus solitarii of the medulla oblongata

The effect of various catecholamines and alpha-mimetics, given by microinjection in the A2-region of the nucleus tractus solitarii (NTS), on blood pressure was investigated in anesthetized male rats. A dose-dependent decrease of blood pressure and heart rate was induced by adrenaline as the most effective drug, followed by noradrenaline, dopamine, alpha-methylnoradrenaline and octopamine. Ablation of the rostral or caudal part of the NTS, or removal of the area postrema did not diminish the effect of alpha-methylnoradrenaline. Higher doses of noradrenaline and alpha-methylnoradrenaline caused an initial rise of blood pressure, while the blood pressure lowering effect of noradrenaline was diminished, and that of alpha-methylnoradrenaline and dopamine delayed. Isoprenaline and the (+)-stereoisomers of noradrenaline and alpha-methylnoradrenaline were ineffective. The hypotensive effect of dopamine was not prevented by systemic injection of the dopamine beta-hydroxylase inhibitor FLA 63. Prior application of haloperidol, yohimbine and phentolamine antagonized the hypotensive response to dopamine and alpha-methylnoradrenaline. Application of peripherally effective alpha-mimetics into the A2-region had no or little effect, while high doses increased blood pressure. Tyramine and clonidine caused some decrease of blood pressure. Clonidine also decreased blood pressure when it was applied in the area of the locus coeruleus. Application of isoprenaline in the locus coeruleus also decreased blood pressure while in contrast adrenaline, noradrenaline, dopamine and alpha-methylnoradrenaline increased blood pressure. The present data suggest that the catecholaminergic receptors in the A2-region of the NTS differ from the classic vascular alpha-receptor and that the NTS also may contain structures which can antagonize the decrease in blood pressure.

Catecholamine content of individual brain regions of spontaneously hypertensive rats (SH-rats)

Many reports have contributed to the notion that catecholamine neurons in the brain stem participate in the central regulation of arterial blood pressure (for reviews see refs. 2, 6). In recent years considerable effort has been made to relate the increase in blood pressure of spontaneously hypertensive rats (SH-rats 5) to catecholamine metabolism in the brain. Initially, a lower noradrenaline level and aromatic amino acid decarboxylase activity 2a, and also a decreased noradrenaline synthesis 9, were observed in the brain stem of SH-rats compared to that of normotensive controls. However, in later studies, in which SH-rats were compared with rats of the genetically related normotensive Wistar-Kyoto strain (W/K-rats), these findings were not corroborated2L Moreover, considerable differences in the activity of catecholamine synthesizing enzymes occurred in the brain stem of various strains of rats with substantial differences in bloodpressure level 10, whereas no correlation was evident between the activity of any of these enzymes and blood pressure 10. Although the latter observations seem to argue against the existence of obvious correlates, it might be that the occurrence of changes in catecholamine content and metabolism in relatively small brain regions escaped detection in these studies, a possibility which was also suggested by Yamabe et al.2L In fact, it was observed in our laboratory that noradrenaline levels were slightly elevated in the pons-medulla of recent generation SH-rats compared to those of W/K-rats 7 and 10 weeks after birthL The present study was undertaken to investigate this phenomenon in more detail. Using a sensitive radiometric method for the simultaneous assay of noradrenaline, dopamine and adrenaline 20, we measured the catecholamine content of individual nuclei and brain regions of SH-rats and W/K-rats. Male Wistar-Kyoto-NIH Cpb (F6) and SHR-NIH Cpb (F32) were obtained from TNO Zeist (The Netherlands) at an age of 14 weeks (for genealogy see ref. 5). Systolic blood pressure measurements were carried out with a tail-plethysmographic method on trained conscious rats s. The rats were killed by decapitation at an age of

Noradrenaline: Central inhibitory control of blood pressure and heart rate

Noradrenaline injected bilaterally into the brainstem in the area of the nucleus tractus solitarii decreased systemic arterial blood pressure and heart rate of anesthetized rats. The effect of noradrenaline was prevented by a preceding injection of the α-adrenergic blocking agent phentolamine, at the same site. The results suggest an inhibitory role of an α-adrenoceptor in the area of the nucleus tractus solitarii in the central control of blood pressure.

Cardiovascular effects of β-endorphin after microinjection into the nucleus tractus solitarii of the anaesthetised rat

The cardiovascular effects of beta-endorphin after administration directly into the nucleus tractus solitari (NTS) of urethane-anaesthetised rats were investigated. Unilateral injection resulted in a U-shaped dose-response relationship with a fall in mean arterial pressure and heart rate occurring at low doses (less than 10 ng). No change in respiratory frequency was observed at any of the doses examined. The hypotensive effects of beta-endorphin were anatomically specific and restricted to the NTS. The depressor response was prevented and bradycardia reduced by naloxone (1 mg/kg s.c. or 10 ng injected into the NTS) and also by beta-endorphin antiserum (1:50 dilution) but not by antiserum to [Met5]enkephalin (1:50 dilution) applied locally into the NTS. The beta-endorphin antiserum caused a rise in blood pressure when administered alone. Conversely microinjection of antiserum to [Met 5]enkephalin resulted in a brief depressor response. Doses of beta-endorphin larger than 10 ng induced a rise in blood pressure accompanied by variable effects on heart rate. Similarly unilateral administration of Des-tyr-endorphin (100 pg) resulted in a blood pressure increase and [D-Ala2,Met5]enkephalin produced a dose-related pressor response and tachycardia. The results indicate that at least two separate endorphin systems are involved in cardiovascular control at the level of NTS, one being depressor in nature (beta-endorphin-like) and the other pressor ([met5]enkephalin-like).

Central Inhibitory Noradrenergic Cardiovascular Control

Publisher Summary This chapter presents several experiments that indicate that the central hypotensive effect of L-DOPA (L- 3,4-dihydroxyphenylalanin) may be related to an increase of brain stem noradrenaline level. Depletion of brain noradrenaline in neonate or adult rats by intraventricular administration of 6-hydroxydopamine did not affect blood pressure. The hypotensive effect of L-DOPA has also been shown in rats, dogs and cats. Inhibition of extracerebral decarboxylase activity to prevent the peripheral effects of dopamine and noradrenaline, which generate from L-DOPA, is required for the expression of the hypotensive effect of L-DOPA in the rat. A number of experiments were performed to investigate whether low levels of brain noradrenaline would lead to opposite effects. In addition, the chapter also studies the cardiovascular effect of bilateral electrolytic brain stem lesions at the level of the obex in rats anesthetized with ether. Blood pressure in the conscious rats was recorded from a permanent indwelling iliac cannula. Lesioning this area was followed by an acute and severe hypertension.

Electrically stimulated [3H]dopamine and [14C]acetylcholine release from nucleus caudatus slices: differences between spontaneously hypertensive rats and Wistar-Kyoto rats

Using an in vitro superfusion method it was found that nucleus caudatus slices of 8- and 12-week-old spontaneously hypertensive rats (SHR) release significantly less [3H]dopamine and [14C]acetylcholine upon electrical stimulation than do slices of normotensive Wistar-Kyoto rats (WKY) at all frequencies tested. At 4 weeks similar trends were seen, but the difference in [14C]acetylcholine release was not significant. That the difference in release of dopamine was already present prior to the onset of the development of hypertension, i.e. at the age of 4 weeks, indicates that it is probably not a consequence of, but rather associated with the development of hypertension. Addition of the dopamine uptake inhibitor nomifensine to the superfusion medium caused an increase in the net release of [3H]dopamine by inhibiting re-uptake, but did not influence the difference in release between SHR and WKY. The release of labelled dopamine and acetylcholine was inhibited in the presence of the dopamine D2 receptor agonist quinpirole. The concentration-response curve for the inhibition of the release of [3H]dopamine, but not that of [14C]acetylcholine, by quinpirole was shifted to the left and the maximum inhibition was higher for SHR than for WKY. These results suggest that the difference in stimulus-evoked release of labelled dopamine in the nucleus caudatus is not the consequence of changes in the uptake mechanism of dopamine, but is associated with differences between SHR and WKY in dopamine D2 autoreceptor regulation.

Elevated adrenaline content in nuclei of the medulla oblongata and the hypothalamus during the development of spontaneous hypertension

We have previously investigated catecholamine (CA) levels in brain nuclei involved in blood pressure regulation in various models of hypertension 15,17. The adrenaline (A) content in several nuclei of older spontaneously hypertensive rats (SH rats) was found to be higher than that of normotensive Wistar/Kyoto rats (WK rats). A markedly elevated content of this catecholamine was found in the nucleus tractus solitarius (NTS), in the medulla oblongata and in the nucleus periventricularis (NPE) and the nucleus paraventricularis (NPV) in the hypothalamus. There were no such changes however in rats with renalor DOCA/salt-hypertension at a similar phase of hypertension development. On the basis of these findings it was postulated that the Acontaining neurons in the medulla oblongata and in the hypothalamus might play a role in the development of genetic hypertension 15. The present study was undertaken to investigate how the differences in the A content of the individual nuclei are related to the early development of spontaneous hypertension. Male rats, SHR-NIH-Cpb, F32 and WK rats (Wistar/Kyoto Cpb, F6) were used, with the latter as controls. Rats were obtained from the Central Proefdieren Bedrijf, TNO, Zeist, The Netherlands. They were caged in groups of 6 and kept in the laboratory for l week. Blood pressure measurements were carried out with a tailsphygmographic method 6. All rats were given rat chow and water ad libitum. They were kept at a 14 h light, 10 h dark schedule, with lights on from 5.00 a.m. to 7.00 p.m. On the day of the experiment the rats were decapitated between 9.00 a.m. and 10.30 a.m. and the brains rapidly taken out and frozen on dry ice. Serial sections (300 #m) were cut in a cryostat at -10 °C. From the frozen sections, the following 6 nuclei were punched out according to the method of Palkovits s : the paraventricular nucleus and the periventricular nucleus from the hypothalamus; the Al-catecholaminergic cell groups (lateral reticular formation); 3 parts of the nucleus tractus solitarii complex: the nucleus tractus solitarius proper (rostral to the obex) (NTS), the Az-catecholamin-

Enkephalins induce a centrally mediated rise in blood pressure in rats

The cardiovascular effects of enkephalins after administration directly into the nucleus tractus solitarii (NTS) of urethane anesthetized rats were investigated. Unilateral microinjection of Met-enkephalin and its stable analogue D-Ala2-Met5-enkephalin resulted in a dose-related rise in mean arterial pressure which in the case of the analogue was accompanied by tachycardia. The elevation in blood pressure was anatomically specific and restricted to the intermediate third of the NTS, as verified histologically. These cardiovascular changes were prevented by pretreatment with locally applied naloxone (10 ng). A similar pressor effect was obtained with Leu-enkephalin. Antiserum to Met-enkephalin (1:50 dilution) caused a fall in blood pressure on injection into the NTS, and completely blocked the pressor response and tachycardia induced by D-Ala2-Met5-enkephalin. These results suggest that enkephalins have a pressor role in the central nervous system.

Hypertension after localized transection of brainstem fibres.

Abstract Transections in the brainstem of the rat just lateral to the nucleus tractus solitarii (NTS) caused severe hypertension. The results of this study suggest that the fibres involved are entering the NTS laterally and that deafferentation of the cells of this nucleus results in hypertension by removing an inhibitory central control.