METABOTROPIC GLUTAMATE RECEPTORS SWITCH VISUAL RESPONSE MODE OF LATERAL
GENICULATE NUCLEUS CELLS FROM BURST TO TONIC.
Godwin, Dwayne W., J. William Vaughan, and S. Murray Sherman.
Department of Neurobiology, State University of New York, Stony Brook, New
APStracts 3:0093N, 1996.
SUMMARY AND CONCLUSIONS
1. Metabotropic glutamate receptors (mGluRs) on relay cells of the lateral
geniculate nucleus appear to be activated exclusively by cortical inputs. We
thus sought to manipulate these receptors in an effort to gain insight into
the possible role of the corticogeniculate pathway. We used in vivo recording
and pharmacological techniques in cats to activate or inactivate these
receptors on geniculate neurons while analyzing their response properties. 2.
Iontophoretic application of the mGluR agonist 1-amino-cyclopentane-1,3-
dicarboxylic acid (ACPD) to X and Y cells in the geniculate A-laminae
diminished or abolished burst activity characteristic of low threshold Ca 2+
spikes. This was accompanied by pronounced changes in the visual response,
including a decrease in signal detectability as measured with receiver
operating characteristic (ROC) curves. 3. ACPD effects appear specific to
mGluRs, because a specific antagonist of ionotropic glutamate receptors
(iGluRs) failed to affect the ACPD-evoked responses, and antagonists of ACPD
failed to affect iGluR mediated responses. We found that 3,5-
dihydroxyphenylglycine (DHPG), an agonist reported to be specific for PI-
linked mGluRs, had effects similar to those of ACPD, implying that these
effects are mediated by phosphatidylinositol (PI)-coupled mGluRs. Furthermore,
antagonists reported to be effective against PI-linked mGluRs were effective
in antagonizing the ACPD mediated effects, and substances reported to be
agonists to mGluRs coupled to the cyclic AMP cascade did not affect neuronal
responses on their own. These data, when added to our preliminary anatomical
data, indicate that the receptor responsible for the observed effects may be
mGluR1, or a functionally-equivalent mGluR. 4. Activation of mGluRs produces
changes in geniculate relay cell activity consistent with depolarization of
these cells seen during in vitro studies. Such membrane depolarization has
been shown to control the activation state of a voltage-dependent Ca 2+
conductance, and this, in turn, determines whether the relay cell fires in
tonic or burst mode. Our data show that application of ACPD produces a shift
in response mode from burst to tonic. Since response mode is an important
characteristic of the geniculate relay and since the activation state of
certain mGluRs, which helps determine response mode, may be controlled by
corticogeniculate input, we conclude that an important function of this input
is to provide a visuotopically discrete transition from burst to tonic
Received 27 October 1995; accepted in final form 30 April 1996.
APS Manuscript Number J725-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 5 June 96