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The Journal of Neuroscience, September 15, 2001, 21(18):7001-7012
Activation of Group I Metabotropic Glutamate Receptors Produces a
Direct Excitation and Disinhibition of GABAergic Projection Neurons in
the Substantia Nigra Pars Reticulata
Michael J.
Marino1,
Marion
Wittmann4,
Stefania
Risso
Bradley1,
George W.
Hubert2,
Yoland
Smith2, and
P. Jeffrey
Conn3
Departments of 1 Pharmacology and
2 Neurology and Yerkes Regional Primate Research Center,
Emory University, Atlanta, Georgia 30322, 3 Merck Research
Laboratories, West Point, Pennsylvania 19486, and
4 Tierphysiologie, University of Tuebingen, D-72076
Tuebingen, Germany
A pathological increase in excitatory glutamatergic input to
substantia nigra pars reticulata (SNr) from the subthalamic nucleus (STN) is believed to play a key role in the pathophysiology of Parkinson's disease. We present an analysis of the
physiological roles that group I metabotropic glutamate receptors
(mGluRs) play in regulating SNr functions. Immunocytochemical analysis
at the light and electron microscopic levels reveal that both mGuR1a and mGluR5 are localized postsynaptically in the SNr. Consistent with
this, activation of group I mGluRs depolarizes SNr GABAergic neurons.
Interestingly, although both group I mGluRs (mGluR1 and mGluR5) are
expressed in these neurons, the effect is mediated solely by mGluR1.
Light presynaptic staining for mGluR1a and mGluR5 was also observed in
some terminals forming symmetric synapses and in small unmyelinated
axons. Consistent with this, activation of presynaptic mGluR1a and
mGluR5 decreases inhibitory transmission in the SNr. The combination of
direct excitatory effects and disinhibition induced by activation of
group I mGluRs could lead to a large excitation of SNr projection
neurons. This suggests that group I mGluRs are likely to play an
important role in the powerful excitatory control that the STN exerts
on basal ganglia output neurons.
Key words:
substantia nigra pars reticulata; group I metabotropic
glutamate receptors; movement disorders; slow excitatory postsynaptic
potential; disinhibition; basal ganglia output nucleus
Copyright © 2001 Society for Neuroscience 0270-6474/01/21187001-12$05.00/0
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