Central action as a key to past and future therapy of hypertension

Electrophysiological responses of angiotensin peptides on the rat isolated nodose ganglion

Previous autoradiographic studies have identified angiotensin II (AII) binding sites over the nodose ganglion and along the vagal afferent neurons. In the present study, we examined whether these binding sites are functional receptors by measuring d.c. potential changes by extracellular recording techniques in the rat isolated nodose ganglion preparation in response to superfusion of angiotensin peptides. It was found that AII, as well as AI and AIII elicited concentration-dependent depolarisation of the nodose ganglion. However, the amino terminal angiotensin heptapeptide, A(1-7), failed to evoke any significant response. The AII receptor antagonist, saralasin had no intrinsic activity, but caused a concentration-dependent blockade of AII-induced depolarisation. This study provides evidence for direct neuronal effects of angiotensin peptides on rat vagal afferent neurons. Moreover, this preparation is a relatively convenient one in which to study functional neuronal AII receptor mechanisms on central or peripheral terminals of vagal sensory neurons.

Reduced V1 vasopressin binding in the rat nucleus solitarii after nodose ganglionectomy

1. Anaesthetized male Sprague-Dawley rats underwent unilateral nodose ganglionectomy or sham operation. 2. One week later hindbrain V1 vasopressin binding site density was assessed by in vitro autoradiography with the selective V1 antagonist radioligand [125I] [d(CH2)5, Sar7]AVP. 3. Specific binding was observed in the nodose ganglion, nucleus tractus solitarius (NTS), area postrema, and inferior olive. Only binding in the medial subnucleus of the NTS was reduced by ipsilateral nodose ganglionectomy. 4. These findings are consistent with V1 vasopressin receptors occurring presynaptically on vagal primary viscerosensory afferent fibres. 5. Such receptors may be involved in the effect of AVP on blood pressure and/or the baroreflex.

Localization of angiotensin II receptor binding in rabbit brain by in vitro autoradiography

Binding of 125I-[Sar1,Ile8] angiotensin II (AII) to sections of brains from both wild and laboratory rabbits was determined by in vitro autoradiography. In the forebrain, specific high density binding was observed in the olfactory bulb, organum vasculosum of the lamina terminalis (OVLT), subfornical organ, median eminence, lateral septum, median preoptic nucleus and hypothalamic paraventricular, supraoptic and arcuate nuclei. In the midbrain, binding of the radioligand was observed in the interpeduncular and parabrachial nuclei, in the locus coeruleus, and ventrolateral pons. In the hind brain, there was dense binding of 125I-[Sar1,Ile8] AII to the nucleus of the solitary tract (NTS) and to both rostral and caudal parts of the reticular formation of the ventrolateral medulla oblongata. Weaker specific binding of the radioligand to the molecular layer of the cerebellum, to the nucleus of the spinal trigeminal tract, dorsal motor nucleus of the vagus, area postema, and to a band of tissue connecting the NTS to the ventrolateral medulla was also observed. Binding of the ligand to circumventricular organs such as the OVLT, subfornical organ, and median eminence suggests that these are sites in the brain of the rabbit at which blood-borne AII may exert influences on the central regulation of fluid balance and pituitary hormone secretion, although AII of neuronal origin could also act at these sites. Binding of the radioligand in several other brain regions suggests that angiotensin II of cerebral origin may be involved in a number of different aspects of brain function in the rabbit. The finding of dense binding in the NTS and ventrolateral medulla, which are involved in autonomic activity and are also sites of catecholamine-containing neurons, raises the possibility of angiotensin interaction with these neurons and involvement in autonomic function.

Angiotensin II receptor binding and the baroreflex pathway

Angiotensin II (Ang II) acts centrally to modulate autonomic activity and cardiovascular function. Using in vitro autoradiography we have determined the distribution of putative receptors for ANG II in brain regions associated with the baroreflex pathway. Ang II receptor binding sites were observed in the nucleus of the solitary tract (NTS), the dorsal motor nucleus of vagus, the rostral and caudal ventrolateral medulla and intermediolateral cell column of the spinal cord. Receptor binding sites were also observed in the nodose ganglion and in association with nerve fibres in the heart. Nodose ganglionectomy and vagal ligation studies revealed that Ang II receptor binding sites are produced in the nodose ganglion and transported in the vagus nerve to terminals of vagal afferent neurones. In the NTS these presynaptic receptors could mediate the known baroreflex inhibitory action of Ang II by inhibition of neurotransmitter release. In the cat, Ang II receptors were sharply localized in the rostral ventrolateral medulla to the subretrofacial nucleus. Microinjection of Ang II (10-50 pmoles) into this region induced a sympathetically-mediated pressor response. Together these results demonstrate several regions within the baroreflex arc which contain Ang II receptors, at which Ang II may modulate cardiovascular control and autonomic function.

Angiotensin receptors and the vagal system

Angiotensin II (Ang II) is known to attenuate the vagal component of the baroreflex at both central and peripheral cardiac sites. Ang II receptor binding sites occur in both the nucleus of the solitary tract (NTS), where they are associated with vagal afferent terminals, and in the dorsal motor nucleus of vagus. In this study we have examined the distribution of Ang II binding sites in the cell bodies of vagal afferents in the nodose ganglion, and investigated whether these receptors are transported in the vagus nerve. Dense Ang II receptor binding was observed over neuronal cell bodies in the nodose ganglion and, in streaks, in the vagus nerve. Vagal ligation distal to the nodose ganglion resulted in a marked accumulation of receptor binding sites, proximal to the ligature, with a moderate increase on the distal side. These results demonstrate that Ang II receptor binding sites occur in the nodose ganglion and are transported centrally in the vagus to be located on presynaptic terminals in the NTS and also peripherally where they may occur on terminals of the vagus.

Pharmacology of centrally acting hypotensive drugs

1. A survey is given of the various aspects of centrally acting hypotensive drugs. The majority of centrally acting hypotensive agents act by way of central alpha-adrenoceptors, probably located in the pontomedullary region of the brain. These central receptors are stimulated by clonidine, guanfacine and various related compounds, and also by alpha-methylnoradrenaline, generated in vivo upon biotransformation of alpha-methyldopa within the brain. The stimulation of the alpha-adrenoceptors induces a decrease in peripheral sympathetic tone and thus a fall in arterial blood pressure and bradycardia. 2. The possibility that presynaptic alpha-adrenoceptors in the brain are involved in the central hypotensive action of clonidine, guanfacine and related compounds is discussed. Also, the possible involvement of central histaminergic and cholinergic receptors in central hypotensive effects is reviewed. 3. Various experimental compounds with a central hypotensive effect different from that of clonidine and related drugs are mentioned. 4. Finally, it is pointed out that the hypotensive effects of various beta-sympatholytic drugs and of prazosin are probably not of central origin.