title: Noradrenaline upregulates T-type calcium channels in rat pinealocytes.
abstract: CONCLUSIONS
  The mammalian pineal gland is a neuroendocrine organ that responds to circadian and seasonal rhythms. Its major function is to secrete melatonin as a hormonal night signal in response to nocturnal delivery of noradrenaline from sympathetic neurons. Culturing rat pinealocytes in noradrenaline for 24 h induced a low-voltage activated transient Ca(2+) current whose pharmacology and kinetics corresponded to a CaV3.1 T-type channel. The upregulation of the T-type Ca(2+) current is initiated by β-adrenergic receptors, cyclic AMP and cyclic AMP-dependent protein kinase. Messenger RNA for CaV3.1 T-type channels is significantly elevated by noradrenaline at 8 h and 24 h. The noradrenaline-induced T-type channel mediated an increased Ca(2+) entry and supported modest transient electrical responses to depolarizing stimuli, revealing the potential for circadian regulation of pinealocyte electrical excitability and Ca(2+) signalling.
  Our basic hypothesis is that mammalian pinealocytes have cycling electrical excitability and Ca(2+) signalling that may contribute to the circadian rhythm of pineal melatonin secretion. This study asked whether the functional expression of voltage-gated Ca(2+) channels (CaV channels) in rat pinealocytes is changed by culturing them in noradrenaline (NA) as a surrogate for the night signal. Channel activity was assayed as ionic currents under patch clamp and as optical signals from a Ca(2+)-sensitive dye. Channel mRNAs were assayed by quantitative polymerase chain reaction. Cultured without NA, pinealocytes showed only non-inactivating L-type dihydropyridine-sensitive Ca(2+) current. After 24 h in NA, additional low-voltage activated transient Ca(2+) current developed whose pharmacology and kinetics corresponded to a T-type CaV3.1 channel. This change was initiated by β-adrenergic receptors, cyclic AMP and protein kinase A as revealed by pharmacological experiments. mRNA for CaV3.1 T-type channels became significantly elevated, but mRNA for another T-type channel and for the major L-type channel did not change. After only 8 h of NA treatment, the CaV3.1 mRNA was already elevated, but the transient Ca(2+) current was not. Even a 16 h wait without NA following the 8 h NA treatment induced little additional transient current. However, these cells were somehow primed to make transient current as a second NA exposure for only 60 min sufficed to induce large T-type currents. The NA-induced T-type channel mediated an increased Ca(2+) entry during short depolarizations and supported modest transient electrical responses to depolarizing stimuli. Such experiments reveal the potential for circadian regulation of excitability.
authors.affiliation: Department of Physiology and Biophysics, University of Washington, Seattle, WA, USA.
authors.forename: Haijie
authors.initials: H
authors.lastname: Yu
authors.forename: Jong Bae
authors.initials: JB
authors.lastname: Seo
authors.forename: Seung-Ryoung
authors.initials: SR
authors.lastname: Jung
authors.forename: Duk-Su
authors.initials: DS
authors.lastname: Koh
authors.forename: Bertil
authors.initials: B
authors.lastname: Hille
mesh_terms: D000818:Animals; D002118:Calcium / Q000502:physiology; D020747:Calcium Channels, T-Type / Q000502:physiology*; D000242:Cyclic AMP / Q000378:metabolism; D008297:Male; D009638:Norepinephrine / Q000502:physiology*; D010870:Pineal Gland / Q000166:cytology* / Q000502:physiology*; D012333:RNA, Messenger / Q000378:metabolism; D017207:Rats, Sprague-Dawley
chemical_list: D020747:Calcium Channels, T-Type; D012333:RNA, Messenger; D000242:Cyclic AMP; D002118:Calcium; D009638:Norepinephrine