Abstract
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 ([Ca(2+) ](i) ); however, the role of NE with respect to modulating [Ca(2+) ](i) in PVN neurones and the mechanisms by which this may occur remain unclear. We evaluated the effects of NE on [Ca(2+) ](i) of acutely isolated PVN neurones using Fura-2 imaging. NE induced a slow increase in [Ca(2+) ](i) compared to artificial cerebrospinal fluid vehicle. NE-induced Ca(2+) elevations were mimicked by the α(1) -AR agonist phenylephrine (PE) but not by α(2) -AR agonist clonidine (CLON). NE and PE but not CLON also increased the overall number of neurones that increase [Ca(2+) ](i) (ie, responders). Elimination of extracellular Ca(2+) or intracellular endoplasmic reticulum Ca(2+) stores abolished the increase in [Ca(2+) ](i) and reduced responders. Blockade of voltage-dependent Ca(2+) channels abolished the α(1) -AR induced increase in [Ca(2+) ](i) and number of responders, as did inhibition of phospholipase C inhibitor, protein kinase C and inositol triphosphate receptors. Spontaneous phasic Ca(2+) events, however, were not altered by NE, PE or CLON. Repeated K(+) -induced membrane depolarisation produced repetitive [Ca(2+) ](i) elevations. NE and PE increased baseline Ca(2+) , whereas NE decreased the peak amplitude. CLON also decreased peak amplitude but did not affect baseline [Ca(2+) ](i) . Taken together, these data suggest receptor-specific influence of α(1) and α(2) receptors on the various modes of calcium entry in PVN neurones. They further suggest Ca(2+) increase via α(1) -ARs is co-dependent on extracellular Ca(2+) influx and intracellular Ca(2+) release, possibly via a phospholipase C inhibitor-mediated signalling cascade.