Neurotrophin Modulation of NMDA Receptors in Cultured Murine and Isolated
Rat Neurons.
C.R. Jarvis, Z-G. Xiong, J.R. Plant, D. Churchill, W-Y. Lu, B. A. MacVicar and
J.F. MacDonald.
NeuroScience Research Group, Faculty of Medicine, University of Calgary,
Alberta, Canada, T2N 4N1, Departments of Physiology and Pharmacology,
University of Toronto, Medical Sciences Building, Toronto Ontario, Canada, M5S
1A8.
APStracts 4:148N, 1997.
ABSTRACT
Patch clamp and calcium imaging techniques were used to assess the acute
effects of the neurotrophins, brain derived neurotrophic factor (BDNF),
neurotrophin-3 (NT-3), and nerve growth factor (NGF), on the responses of
cultured and acutely isolated hippocampal and cultured striatal neurones to
the glutamate receptor agonist N-methyl-d-aspartic acid (NMDA). The effects of
BDNF on NMDA-activated currents were examined in greater detail. Currents
evoked by NMDA, and the accompanying changes in intracellular calcium, were
enhanced by low concentrations of the neurotrophins (1 to 20 ng/ml). The
potentiation by the neurotrophins was rapid in onset and offset (less than one
second). The neurotrophins also reduced desensitization of these currents in
most cells.
The enhancement of NMDA-activated currents by BDNF was observed using both
perforated and whole cell patch recording techniques and could be demonstrated
in outside-out patches. Furthermore, its effects were not attenuated by
pretreatment with the protein kinase inhibitors genistein or 1-(5-
isoquinolynesulfony)2-methylpiperazine (H7). Therefore, the actions of BDNF do
not appear to be mediated by phosphorylation. Similar enhancements were
observed with NT-3, NT-4 and with NGF in spite of the fact that hippocampal
neurons lack TrkA receptors. All together this evidence suggests that the
enhancement of NMDA-evoked currents is unlikely to be mediated through the
activation of growth factor receptors. Modulation of NMDA responses by BDNF
was dependent upon the concentration of extracellular glycine. The most
pronounced potentiation by BDNF was observed at low concentrations whilst no
potentiation was observed in saturating concentrations of glycine suggesting
that BDNF may have increased the affinity of the NMDA receptor for glycine.
However, the competitive glycine-site antagonist 7-chloro-kynurenic acid
blocked the enhancement by BDNF without shifting the dose-inhibition
relationship for this antagonist and Mg2- consistently depressed the
potentiation of NMDA-evoked currents by BDNF indicating that BDNF does not
alter glycine affinity. BDNF also reversibly increased the probability of
opening of NMDA channels recorded from outside-out patches taken from cultured
hippocampal neurones.
Other unrelated peptides including dynorphin and somatostatin also caused a
glycine-dependent enhancement of NMDA currents and depressed the currents in
saturating concentrations of glycine. In contrast, a shortened analogue
dynorphin (6-17) which lacks N-terminus glycine residues and another peptide
met-enkephalin were without effects on NMDA currents recorded in low
concentrations of glycine. Our results suggest that neurotrophins and other
peptides can serve as glycine-like ligands for the NMDA receptor.
Received 30 March 1997; accepted in final form 8 July 1997.
APS Manuscript Number J238-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 27 August 1997