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DAVID LEÓN NAVARRO Y COLS. AN. R. ACAD. NAC. FARM.

of calcium responses to ATP (4, 8). Although we have not investigate
the inhibitory effect of L-Glu on individual nucleotide receptors,
results obtained have clearly shown that all ATP responses, including
both ionotropic and metabotropic nucleotide receptors, undergo a
significant decrease after L-Glu addition, suggesting, therefore, that
inhibition of ATP-mediated responses evoked by L-Glu was a general
phenomenous and not associated to a specific group of nucleotide
receptor. The inhibitory effect of L-Glu on ATP-mediated responses
was dose-dependent. Thus, 100 µM and 10 µM L-Glu caused a strong/
medium effect on ATP-evoked Ca2+ rises whereas 1 µM L-Glu did
not affect responses induced by ATP. These data suggest, therefore,
that L-Glu concentrations play an important role in the interaction
between both groups of receptors. The possibility that L-Glu
inhibited ATP-mediated Ca2+ rises by a direct action on nucleotide
receptors was excluded when different agonists of glutamate
receptors were tested and data showed that they were also able to
depress ATP currents.

In the present work glutamate receptor agonist not only acutely
depresses ATP-elicited calcium rises, but also induces a depression
in ATP responsiveness that remains even after 5 minutes the agonist
gone. By using selective agonists of ionotropic glutamate receptor,
NMDA and AMPA, and Group I-mGluR, DHPG, we could investigate
the individual contribution of each receptor to the cross-talk process.
Thus, both ionotropic agonists, NMDA and AMPA, caused a similar
decrease on ATP-mediated responses whereas the inhibitory effect of
DHPG was slightly higher. Another interesting point treated was if
the loss of ATP responsiveness observed after L-Glu exposure could
be due to desensitization of nucleotide receptor and not by a cross-
talk process between both groups of receptors. In that sense, previous
works have shown that different P2X and P2Y receptors can undergo
a decline in current amplitude in response to repeated applications
of ATP (24, 25). Results obtained have allowed excluding this
possibility since the amplitude of the ATP-mediated response remains
unchanged in granule neurons challenged three consecutives times
with ATP. Therefore, these results demonstrate that L-Glu through
binding ionotropic (both NMDA and non-NMDA receptors) and
metabotropic glutamate receptor (Group I) caused a prolonged
heterologous desensitization in nucleotide receptors.

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