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Journal of Neuroscience, Vol 15, 4602-4611, Copyright © 1995 by Society for Neuroscience
The mechanism of action for the block of NMDA receptor channels by the opioid peptide dynorphin
L Chen, Y Gu and LY Huang
Marine Biomedical Institute, University of Texas Medical Branch, Galveston 77555-1069, USA.
Dynorphin is one of the endogenous opioids that modulates the excitability
of nociceptive (pain-sensing) neurons. We have shown recently that
dynorphin blocks NMDA-activated currents directly without the participation
of kappa-opioid receptors. In order to understand the mechanism underlying
this novel action of dynorphin, we examined, in detail, the interactions
between dynorphin and NMDA receptors in isolated trigeminal neurons.
Dynorphin reversibly blocks NMDA-activated current (INMDA). The onset and
recovery of the block were determined with concentration jump experiments.
The association rate (k+) of dynorphin(1-17) is 4.9 x 10(6) sec-1 M-1 and
the dissociation rate (k-) is 7.5 sec-1. The apparent dissociation constant
(KD) of dynorphin, calculated from these rate constants, is 1.6 microM.
Dynorphin does not change the EC50 of NMDA, nor the potentiating action of
glycine. The binding site for dynorphin is distinct from that of Zn2+ or
H+. Upon treatment with the disulfide reducing agent dithiothreitol (DTT),
NMDA receptors become less susceptible to dynorphin block. The affinity of
dynorphin for the modified NMDA receptors is reduced by 2.7-fold. In
analyses of single NMDA channels in cell-free patches, we found that
dynorphin shortens the mean open time, decreases the probability of opening
of NMDA channels, but has no effect on the single channel conductance.
These results suggest that dynorphin interacts with a site conformationally
linked with the redox site(s) on the NMDA receptor, thus altering the
gating properties of the channel.
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