Variation of CS salience reveals group II mGluR-dependent and -independent forms of conditioning in the rat.

There is good evidence that metabotropic glutamate receptors (mGluRs) are involved in some types of learning, and we have previously suggested that this involvement may reflect the modulation by mGluRs of the signal-to-noise ratio in neural networks. This hypothesis supposes that unspecific activation of mGluRs increases background noise level, so reducing the effectiveness of behaviourally relevant stimuli as signals in the network. We report here that intraperitoneal (i.p.) injection of 4-aminopyrrolidine-2,4-dicarboxylic acid (APDC), a specific agonist of group II mGluRs, disrupts conditioning to context (but not to cue) using conventional procedures. The hypothesis predicts, however, that the effect of the drug should be counteracted by the use of more salient stimuli, which would provide stronger signals to the network. In accordance with this prediction, we find that increases in the salience of either the CS (context) or the UCS (shock) abolish the drug-induced disruption of conditioning. These results suggest that group II mGluRs modulate neural networks involved in association formation.