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.