Subtype 2 of the metabotropic glutamate receptor (mGluR2) is expressed
in the presynaptic elements of hippocampal messy fiber-GAS synapses.
Knockout mice deficient in mGluR2 showed no histological changes and n
o alterations in basal synaptic transmission, paired-pulse facilitatio
n, or tetanus-induced long-term potentiation (LTP) al the messy fiber-
CA3 synapses. Long-term depression (LTD) induced by low-frequency stim
ulation, however, was almost fully abolished. The mutant mice performe
d normally in water maze learning tasks. Thus, the presynaptic mGluR2
is essential for inducing LTD at the mossy fiber-CA3 synapses, but thi
s hippocampal LTD does not seem to be required for spatial learning.