Different pharmacological anatomy in the paraventricular hypothalamic nucleus, supraoptic nucleus, and suprachiasmatic nucleus of rats: quantitative autoradiography on angiotensin II receptor binding sites

Changes of hypothalamic and plasma vasopressin in rats with deoxycorticosterone-acetate induced salt appetite

Mineralocorticoids play a predominant role in development of salt appetite and hypertension. Since vasoactive peptides could mediate the central effects of mineralocorticoids, we evaluated changes of immunoreactive (IR) arginine vasopressin (AVP) in the paraventricular (PVN) and supraoptic (SON) hypothalamic nucleus during DOCA-induced salt appetite. In one model, rats having free access to water and 3% NaCl during 9 (prehypertensive stage) or 21 days (hypertensive stage) received DOCA (s.c., 10 mg/rat/in alternate days). A decrease in the IR cell area, number of IR cells and staining intensity was obtained in magnocellular PVN of rats treated during 9 days. After 21 days IR cell area and number of cells in the PVN also decreased, but staining intensity of remaining cells was normal. The same parameters were unchanged in the SON. In another model, animals treated with DOCA during 9 days had only access to 3% NaCl or water. The IR cell area in PVN and SON significantly increased in mineralocorticoid-treated and control animals, both drinking 3% NaCl. Staining intensity (PVN and SON) and number of IR cells (PVN) also augmented in DOCA-treated animals drinking salt respect of a group drinking water. Plasma AVP in rats treated with DOCA and offered salt and water, exhibited a 2-2.5 fold increase at the time of salt appetite induction. Plasma AVP was substantially higher in rats drinking salt only, while the highest levels were present in salt-drinking DOCA-treated rats. Thus, peptide depletion in the PVN may be due to increased release, because reduced levels of hypothalamic and posterior pituitary AVP were measured in this model. In rats drinking salt only the substantial increase of IR AVP in the PVN and SON, may be due to dehydration and hyperosmosis. Because DOCA-salt treated rats showed higher AVP levels in the PVN compared to untreated rats drinking salt only, it is possible that DOCA sensitized PVN cells to increase AVP production. The results suggest the vasopressinergic system could mediate some central functions of mineralocorticoids.

Analysis of angiotensin type 2 receptors in vasopressinergic neurons and pituitary in the rat

Previous functional studies indicated that an angiotensin type 2 (AT2) receptor subtype may participate in the regulation of vasopressin release by angiotensin II (AngII). In the present study, AT2 receptor-directed antiserum immunohistochemically detected AT2 receptors within the hypothalamic paraventricular (PVN) and the supraoptic nuclei (SON) of the rat brain, more specifically, in identified vasopressinergic neurons. Considering the lack of AT2 binding in the PVN and the SON using receptor autoradiography, we tested the hypothesis that these AT2 receptors are transported to the posterior pituitary. Western blot analysis detected AT2 immunoreactivity in the posterior pituitary. However, no AT2 binding was detected in posterior pituitary membranes, and no AT2 binding was detected with quantitative receptor autoradiography in the neurohypophysis. Thus, if AT2 receptors are transported from the magnocellular vasopressin neurons to the posterior pituitary, their role in AngII regulation of vasopressin release at the neurohypophyseal terminals remains to be clarified.

Differential effects of angiotensin II on the activities of suprachiasmatic neurons in rat brain slices

The effects of angiotensin II (AII) on the firing rates of suprachiasmatic neurons were determined in rat brain slices. AII in pmol ranges stimulated 25% and inhibited another 25% of 52 irregular firing neurons, while it stimulated 23% and inhibited 4% of 30 regular firing neurons. Three "oscillating" neurons whose firing rates oscillated with rather constant amplitudes and periods were recorded. AII induced the occurrence of oscillation in one unit and modulated the oscillation amplitude of the other two. Pretreatment with saralasin, an AII antagonist, effectively blocked (100%) the actions of AII (n = 5). The present findings suggest that AII may act as an important mediator in the suprachiasmatic nucleus and its mode of action may be variable in different neurons.

Microinjections of angiotensin II into the supraoptic and paraventricular nuclei produce potent antidiureses by vasopressin release mediated through adrenergic and angiotensin receptors

We investigated the effects of angiotensin II (Ang II), microinjected into the supraoptic (SON) and paraventricular (PVN) nuclei of rats, on the urine outflow rate and underlying mechanisms. Ang II produced antidiuretic effects in a dose-dependent manner with ED50 values of 0.1 and 0.05 nmol in the SON and PVN, respectively. [Sar1, Ile8]Ang II at 0.1 nmol diminished the Ang II (0.5 nmol)-induced antidiureses in the SON more markedly than in the PVN. A high dose of [Sar1,Ile8]Ang II, 1 nmol, completely inhibited the effects in both the nuclei. In addition, the Ang II (1 nmol)-induced antidiuretic effects were partially inhibited by phenoxybenzamine (80 nmol) in the SON and by phenoxybenzamine, timolol (100 nmol) and propranolol (100 nmol) in the PVN. The microinjection of Ang II (1 nmol) into both the nuclei, after pretreatment with a vasopressin V1V2-antagonist, d(CH2)5-D-Tyr(Et)VAVP (i.v.) significantly increased the urine outflow rate. These findings suggest that 1) Two mechanisms account for the Ang II receptor-mediated antidiureses resulting from an increase in vasopressin release: direct stimulation on vasopressin-containing neurons and indirect stimulation on them through alpha-adrenoceptors in the SON and alpha- and beta-adrenoceptors in the PVN; 2) The Ang II-induced antidiuretic effect in the SON is slightly less potent than that in the PVN; and 3) Ang II receptors in the nuclei may possibly produce the diureses through mechanisms that are not presently understood.

Afferent connections of the rat's supraoptic nucleus

Neurons projecting to the supraoptic nucleus (SON) have been identified following stereotaxic injections of either horseradish peroxidase or fast blue into the SON region of adult rats. The subfornical organ, median preoptic nucleus, organum vasculosum of the lamina terminalis and medial septal nucleus were the source of the largest numbers of supraoptic-projecting neurons. Several smaller projections also originate from the ipsilateral locus coeruleus, preoptic area, lateral parolfactorial area, dorsomedial nucleus of the hypothalamus, lateral parabrachial nucleus and ventrolateral medulla. Several other areas appeared to project only to the region immediately dorsal to the SON: lateral septal nucleus, diagonal band of Broca, ventral tegmental nucleus, and the supramamillary nucleus. These areas may influence SON neurosecretory function by way of interneurons found immediately dorsal to SON. Additional areas were identified with retrograde fluorescent label only, and these projected to the area immediately dorsal to SON and/or to SON itself.

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.

Pre- and postsynaptic features of the central angiotensin systems. Indications for a role of angiotensin peptides in volume transmission and for interactions with central monoamine neurons

The transmitter receptor matches and mismatches in the angiotensin (ANG) immunoreactive (IR) neuronal systems of the rat CNS have been characterized in various regions by means of ANG II immunocytochemistry and 125I-angiotensin II receptor autoradiography. By means of in situ hybridization the distribution of angiotensinogen mRNA has been mapped out and related to the distribution of ANG IR. In some areas, high densities of ANG IR nerve terminals and ANG II receptors (e.g. paraventricular hypothalamic nucleus, locus coeruleus and nucleus tractus solitarius) or high densities of ANG II receptors alone (e.g. medial geniculate body, subthalamic nucleus and superficial layer of the superior collicle) were often associated with high levels of angiotensinogen mRNA, suggesting the existence of an extracellular formation of ANG II, mediating biological responses. These results underline a role of ANG peptides in volume transmission in addition to transmitter function. Other areas, such as nuc. n. hypoglossi, practically lacking ANG IR terminals, pericarya and receptors, also contained high levels of angiotensinogen mRNA, suggesting a different role of angiotensinogen in these areas. Evidence for presynaptic (turnover changes) and post-synaptic (receptor-receptor crosstalk) interactions with CA neuronal systems has been obtained especially in cardiovascular centers. Thus, ANG II reduces in a concentration related way the affinity of 3H-paraminoclonidine binding sites in the dorsomedial medulla without influencing the Bmax value. These results indicate the existence of intramembrane interactions between ANG II and alpha 2 adrenergic receptors. Finally paraventricular but not perifornical ANG immunoreactive nerve cells costore nuclear glucocorticoid receptor IR, suggesting that some ANG neurons may be directly regulated by glucocorticoids.