RESUMO
Norepinephrine (NE) activates adrenergic receptors (ARs) in the hypothalamic paraventricular nucleus (PVN) to increase excitatory currents, depolarise neurones and, ultimately, augment neuro-sympathetic and endocrine output. Such cellular events are known to potentiate intracellular calcium ([Ca2+ ]i ); however, the role of NE with respect to modulating [Ca2+ ]i in PVN neurones and the mechanisms by which this may occur remain unclear. We evaluated the effects of NE on [Ca2+ ]i of acutely isolated PVN neurones using Fura-2 imaging. NE induced a slow increase in [Ca2+ ]i compared to artificial cerebrospinal fluid vehicle. NE-induced Ca2+ elevations were mimicked by the α1 -AR agonist phenylephrine (PE) but not by α2 -AR agonist clonidine (CLON). NE and PE but not CLON also increased the overall number of neurones that increase [Ca2+ ]i (ie, responders). Elimination of extracellular Ca2+ or intracellular endoplasmic reticulum Ca2+ stores abolished the increase in [Ca2+ ]i and reduced responders. Blockade of voltage-dependent Ca2+ channels abolished the α1 -AR induced increase in [Ca2+ ]i and number of responders, as did inhibition of phospholipase C inhibitor, protein kinase C and inositol triphosphate receptors. Spontaneous phasic Ca2+ events, however, were not altered by NE, PE or CLON. Repeated K+ -induced membrane depolarisation produced repetitive [Ca2+ ]i elevations. NE and PE increased baseline Ca2+ , whereas NE decreased the peak amplitude. CLON also decreased peak amplitude but did not affect baseline [Ca2+ ]i . Taken together, these data suggest receptor-specific influence of α1 and α2 receptors on the various modes of calcium entry in PVN neurones. They further suggest Ca2+ increase via α1 -ARs is co-dependent on extracellular Ca2+ influx and intracellular Ca2+ release, possibly via a phospholipase C inhibitor-mediated signalling cascade.