Locus coeruleus effects on baroreceptor responsiveness and activity of neurosecretory vasopressin cells

Induction of hypertension blunts baroreflex inhibition of vasopressin neurons in the rat

Vasopressin secretion from the posterior pituitary gland is determined by action potential discharge of hypothalamic magnocellular neurosecretory cells. Vasopressin is a potent vasoconstrictor, but vasopressin levels are paradoxically elevated in some patients with established hypertension. To determine whether vasopressin neurons are excited in hypertension, extracellular single-unit recordings of vasopressin neurons from urethane-anaesthetized Cyp1a1-Ren2 rats with inducible angiotensin-dependent hypertension were made. The basal firing rate of vasopressin neurons was higher in hypertensive Cyp1a1-Ren2 rats than in non-hypertensive Cyp1a1-Ren2 rats. The increase in firing rate was specific to vasopressin neurons because oxytocin neuron firing rate was unaffected by the induction of hypertension. Intravenous injection of the α1-adrenoreceptor agonist, phenylephrine (2.5 μg/kg), transiently increased mean arterial blood pressure to cause a baroreflex-induced inhibition of heart rate and vasopressin neuron firing rate (by 52 ± 9%) in non-hypertensive rats. By contrast, intravenous phenylephrine did not inhibit vasopressin neurons in hypertensive rats, despite a similar increase in mean arterial blood pressure and inhibition of heart rate. Circulating angiotensin II can excite vasopressin neurons via activation of afferent inputs from the subfornical organ. However, the increase in vasopressin neuron firing rate and the loss of inhibition by intravenous phenylephrine were not blocked by intra-subfornical organ infusion of the angiotensin AT1 receptor antagonist, losartan. It can be concluded that increased vasopressin neuron activity at the onset of hypertension is driven, at least in part, by reduced baroreflex inhibition of vasopressin neurons and that this might exacerbate the increase in blood pressure at the onset of hypertension.

Role of the locus ceruleus in baroreceptor regulation of supraoptic vasopressin neurons in the rat

The goal of this study was to identify the source of baroreceptor-related noradrenergic innervation of the diagonal band of Broca (DBB). Male Sprague-Dawley rats underwent sinoaortic denervation (SAD, n = 13) or sham SAD surgery (n = 13). We examined Fos expression produced by baroreceptor activation and dopamine-beta-hydroxylase immunofluorescence in hindbrain regions that contain noradrenergic neurons. Baroreceptors were stimulated by increasing blood pressure >40 mmHg with phenylephrine (10 microgram. kg(-1). min(-1) iv) in sham SAD and SAD rats. Controls were infused with 0.9% saline. Only the locus ceruleus (LC) demonstrated a baroreceptor-dependent increase in Fos immunoreactivity in dopamine-beta-hydroxylase-positive neurons. In a second experiment, normal rats received rhodamine-labeled microsphere injections in the DBB (n = 12) before phenylephrine or vehicle infusion. In these experiments, only the LC consistently contained Fos-positive cells after phenylephrine infusion that were retrogradely labeled from the DBB. Finally, we lesioned the LC with ibotenic acid and obtained extracellular recordings from identified vasopressin neurons in the supraoptic nucleus. LC lesions significantly reduced the number of vasopressin neurons that were inhibited by acute baroreceptor stimulation. Together, these results suggest that noradrenergic neurons in the LC participate in the baroreflex activation of the DBB and may thus be important in the baroreflex inhibition of vasopressin-releasing neurons in the supraoptic nucleus.

Projections of medullary and pontine noradrenergic neurons to the horizontal limb of the nucleus of diagonal band in the rat

Recent investigations in the rat have implicated a noradrenergic innervation to the horizontal nucleus of the diagonal band of Broca as a critical link in a neural circuit that conveys baroreceptor information centrally to inhibit the firing of vasopressin-secreting neurons in the hypothalamic supraoptic nucleus. In this study we used small intra-diagonal band injections of a retrograde tracer, rhodamine latex microspheres, in combination with tyrosine hydroxylase histochemistry to identify brainstem noradrenergic cells contributing to this innervation. In three cases where tracer injections were limited to the horizontal limb of the diagonal band, we observed 20-50 double-labelled neurons ipsilaterally in the dorsal part of the locus coeruleus (A6) and the caudal nucleus tractus solitarius (A2), and bilaterally in the caudal ventrolateral medulla (A1). Double-labelled neurons were also noted in the ventral tegmental area (dopaminergic A10 cell group). Although all major brainstem noradrenergic cell groups contribute fibers to the horizontal limb of the nucleus of diagonal band, data from physiological studies suggest that the noradrenergic A2 neurons in the nucleus tractus solitarius are the most likely pathway through which it receives this baroreceptor information.

Norepinephrine uptake modifications in circumventricular organs, pons and myelencephalic areas and nuclei in prehepatic portal hypertensive rats

Peripheral noradrenergic activity is enhanced in portal hypertension and correlates with the progression of the disease. However, little is known about the status of central norepinephrine (NE) in portal hypertension. The aim of the present work was to study the uptake of NE in several areas rich in NE in experimental prehepatic portal hypertension. The experiments were performed in vitro in several encephalic areas and nuclei, obtained according to the 'punch-out technique'. Results showed that in portal hypertensive rats NE uptake enhanced in all areas and nuclei studies (subfornical organ, organum vasculosum lamina terminalis, area postrema, locus coeruleus and nucleus tractus solitarius). The present results suggest that these encephalic areas and brainstem nuclei may be related to the development and/or maintenance of portal hypertension in this animal model.

CNS pathways mediating cardiovascular regulation of vasopressin

1. The release of vasopressin from the neurohypophysial terminals of hypothalamic magnocellular neurosecretory neurons is subject to regulation by peripheral baroreceptors, cardiopulmonary volume receptors and circulating angiotensin II. Information from these sources is transmitted through different pathways to achieve different influences on the excitability of the vasopressin-secreting cells. 2. A brief increase in arterial pressure, sufficient to activate baroreceptors, is associated with a transient and selective GABAergic inhibition of these neurosecretory neurons, achieved through a multisynaptic pathway that involves ascending catecholaminergic fibres and neurons in the diagonal band of Broca. A decrease in arterial pressure activates peripheral low volume receptors and initiating neural inputs that result in an increase in the excitability of vasopressin-secreting neurons, achieved via pathways that include direct projections from caudal ventrolateral medulla A1 neurons. 3. Hypotension also releases renal renin and leads to the formation of angiotensin II; binding of this hormone to AT1 receptors on subfornical organ neurons promotes activation of a central angiotensinergic input that evokes a predominantly excitatory effect on vasopressin neurons.