Group II and III metabotropic glutamate receptors contribute to different aspects of visual response processing in the rat superior colliculus.

1. Neurones in the superior colliculus (SC) respond to novel sensory stimuli and response habituation is a key feature of this. It is known that both ionotropic and metabotropic glutamate (mGlu) receptors participate in visual responses of superficial SC neurones. A feature of Group II and Group III mGlu receptors is that they may modulate specific neural pathways, possibly via presynaptic mechanisms. However, less is known about how this may relate to functions of systems in whole animals. We have therefore investigated whether these receptors affect specific attributes of visual responses in the superficial SC. 2. Recordings were made from visually responsive neurones in anaesthetised rats, and agonists and antagonists of Group II and III mGlu receptors were applied iontophoretically at the recording site. 3. We found that application of the Group III metabotropic glutamate receptor agonist L-2-amino-4-phosphonobutyric acid (L-AP4) produced an increase in visual response habituation, whilst Group III antagonists decreased habituation. These effects were independent of the response habituation mediated via GABA(B) receptors. In contrast, modulation of Group II mGlu receptors with the specific agonist LY354740 or the antagonist LY341495 did not affect response habituation, although these compounds did modulate visual responses. This suggests a specific role for Group III mGlu receptors in visual response habituation. 4. The magnitude of Group II effects was smaller during presentation of low contrast stimuli compared with high contrast stimuli. This suggests that activation of Group II receptors may be activity dependent and that these receptors can translate this into a functional effect in adapting to high contrast stimuli.