title: Enhanced BDNF signalling following chronic hypoxia potentiates catecholamine release from cultured rat adrenal chromaffin cells.
abstract: We investigated the role of the neurotrophin BDNF signalling via the TrkB receptor in rat adrenomedullary chromaffin cells (AMCs) exposed to normoxia (Nox; 21% O2) and chronic hypoxia (CHox; 2% O2) in vitro for ∼ 48 h. TrkB receptor expression was upregulated in primary AMCs and in immortalized chromaffin (MAH) cells exposed to CHox; this effect was absent in MAH cells deficient in the transcription factor, hypoxia inducible factor (HIF)-2α. Relative to normoxic controls, activation of the TrkB receptor in chronically hypoxic AMCs led to a marked increase in membrane excitability, intracellular [Ca(2+)], and catecholamine secretion. The BDNF-induced rise of intracellular [Ca(2+)] in CHox cells was sensitive to the selective T-type Ca(2+) channel blocker TTA-P2 and tetrodotoxin (TTX), suggesting key roles of low threshold T-type Ca(2+) and voltage-gated Na(+) channels in the signalling pathway. Environmental stressors, including chronic hypoxia, enhance the ability of adrenomedullary chromaffin cells (AMCs) to secrete catecholamines; however, the underlying molecular mechanisms remain unclear. Here, we investigated the role of brain-derived neurotrophic factor (BDNF) signalling in rat AMCs exposed to chronic hypoxia. In rat adrenal glands, BDNF and its tropomyosin-related kinase B (TrkB) receptor are highly expressed in the cortex and medulla, respectively. Exposure of AMCs to chronic hypoxia (2% O2; 48 h) in vitro caused a significant increase to TrkB mRNA expression. A similar increase was observed in an immortalized chromaffin cell line (MAH cells); however, it was absent in MAH cells deficient in the transcription factor HIF-2α. A specific TrkB agonist, 7,8-dihydroxyflavone (7,8-DHF), stimulated quantal catecholamine secretion from chronically hypoxic (CHox; 2% O2) AMCs to a greater extent than normoxic (Nox; 21% O2) controls. Activation of TrkB by BDNF or 7,8-DHF increased intracellular Ca(2+) ([Ca(2+)]i), an effect that was significantly larger in CHox cells. The 7,8-DHF-induced [Ca(2+)]i rise was sensitive to the tyrosine kinase inhibitor K252a and nickel (2 mm), but not the Ca(2+) store-depleting agent cyclopiazonic acid. Blockade of T-type calcium channels with TTA-P2 (1 μm) or voltage-gated Na(+) channels with TTX inhibited BDNF-induced [Ca(2+)]i increases. BDNF also induced a dose-dependent enhancement of action potential firing in CHox cells. These data demonstrate that during chronic hypoxia, enhancement of BDNF-TrkB signalling increases voltage-dependent Ca(2+) influx and catecholamine secretion in chromaffin cells, and that T-type Ca(2+) channels play a key role in the signalling pathway.
authors.affiliation: Department of Biology, McMaster University, 1280 Main Street West, Hamilton, Ontario, L8S 4K1, Canada.
authors.forename: Angela L
authors.initials: AL
authors.lastname: Scott
authors.affiliation: Department of Biology, McMaster University, 1280 Main Street West, Hamilton, Ontario, L8S 4K1, Canada.
authors.forename: Min
authors.initials: M
authors.lastname: Zhang
authors.affiliation: Department of Biology, McMaster University, 1280 Main Street West, Hamilton, Ontario, L8S 4K1, Canada.
authors.forename: Colin A
authors.initials: CA
authors.lastname: Nurse
mesh_terms: D000818:Animals; D019208:Brain-Derived Neurotrophic Factor / Q000378:metabolism*; D020747:Calcium Channels, T-Type / Q000378:metabolism; D002395:Catecholamines / Q000378:metabolism*; D015687:Cell Hypoxia; D002478:Cells, Cultured; D019439:Chromaffin Cells / Q000378:metabolism*; D005089:Exocytosis; D010100:Oxygen / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D020813:Receptor, trkB / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction
chemical_list: D019208:Brain-Derived Neurotrophic Factor; D020747:Calcium Channels, T-Type; D002395:Catecholamines; D020813:Receptor, trkB; D010100:Oxygen