Absence of histological changes
Absence of alterations in basal synaptic transmission
Absence of pulse-paired facilitation
Absence of tetanus-induced long-term potentiation (LTP) at the mossy fiber-CA3 synapses
Almost full abolition of long-term depression induced by low-frequency stimulation

Yokoi, K. Kobayashi, T. Manabe, T. Takahashi, I. Sakaguchi, G. Katsuura, R. Shigemoto, H. Ohishi, S. Nomura, K. Nakamura, K. Nakao, M. Katsuki & S. Nakanishi:
Impairment of Hippocampal Mossy Fiber LTD in Mice Lacking mGluR2.
Science 273, (1996)