Activation of alpha‐1 adrenergic receptors increases cytosolic calcium in neurones of the paraventricular nucleus of the hypothalamus

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.