Modulation of visual processing in the rat superior colliculus by metabotropic glutamate receptors
Neurones in the superficial layers of the superior colliculus (SSC) respond to novel visual events. Cells in the SSC project via neurones in the deep layer of the superior colliculus to motor nuclei which generate appropriate behavioural and avoidance responses to novel sensory stimuli. Glutamate is a neurotransmitter at the retino-collicular and cortico-collicular synapse. Glutamate receptors can be classified as either ionotropic or metabotropic (mGluRs). At present, 8 mGluRs have been cloned (mGluRI - mGluR8), and these can be divided into 3 groups based on sequence homology, pharmacology and coupling to 2nd messenger pathways. There is evidence that metabotropic glutamate receptors may be present on SSC neurones and SSC afferents. This study examines how mGluRs may modulate the response properties of visually responsive cells in the SSC. Iontophoretic application of pharmacological agents including selective mGluR agonists and antagonists are used to probe the functional effects of mGluR manipulation in an in- Wvo preparation. All three groups of receptor appear to be activated by endogenous glutamate during visual synaptic transmission. Activation of Group I mGluRs (mGluRl and mGluR5) cause a depression of the visual response. Activation of both Group 11 (mGluR2 and mGluR3) and Group HI mGluRs (mGluR4, mGluR6, mGluR7 and mGIuR8) causes a facilitation or inhibition of the visual response in individual neurones. Neurones in the SSC detect novel visual stimuli by producing a decline in the response to repeated stimuli, this is called habituation. Group HI (but not Group I or Group H) mGluRs contribute to response habituation in the SSC and therefore have a functional role in detecting stimulus novelty. Activation of Group R receptors is dependent upon the intensity of the stimulus, probably due to their location away from the central region of the synapse. Immunohisto chemical data presented here details the distribution of selected mGluRs in the sub-cortical retinofugal pathway of the rat, ferret and cat. Analysis shows that the distribution in these three species is dissimilar. This suggests that mGluRs may have different functional roles in visual processing in different species.