Glutamate Receptors in Superior Colliculus

Glutamate Receptors and Visual Responses in the Adult Superior Colliculus

NMDA and AMPA Rs in SCThe excitatory amino acid L-glutamate is a major transmitter in the vertebrate central nervous system. We have shown that antagonists which block glutamate receptors of either the NMDA type or AMPA type reduce visual responses of neurones in the adult superficial superior colliculus when these antagonists are applied locally by iontophoresis (Binns & Salt, 1994). This indicates that both NMDA receptors and AMPA receptors mediate visual responses of these neurones. Furthermore, involvement of NMDA receptors in visual responses appears to be related to those neurones which are likely to receive an input from the visual cortex: this may be a mechanism for the cortex to gate activity in the superficial layers of the superior colliculus.


mGluRs in SCWe have also investigated the role of metabotropic glutamate (mGlu) receptors in visual processing in the superficial superior colliculus. We have found that these receptors have distinct roles in modulating responses under different conditions of visual stimulation (Cirone & Salt, 2001).   More recently we have discovered that this is due to specific  functional localisation of different mGlu receptor subtypes so as to allow specific modulation of either excitatory or inhibitory transmission (Lacey et.al., 2005Neale & Salt 2006).     

 

 

 

 

In the deeper layers of the superior colliculus, neurones are found which respond to combinations of visual, auditory and somatosensory stimuli. Such multimodal responses are particularly sensitive to NMDA receptor antagonists (Binns & Salt, 1996). NMDA receptors are unique amongst the ionotropic glutamate receptors in that the ion channel is blocked by magnesium ions in a voltage dependent manner. The Mg2+ block is strongest when the membrane is hyperpolarized, but if the neurone is partially depolarized then the block is relieved and the responsiveness of the cell to further depolarising inputs is enhanced. This confers the ability to summate inputs from several sources in a non-linear fashion on neurones which have synaptic NMDA receptors. Furthermore, given the kinetics of NMDA-gated channels, these receptors can also be utilized in temporal summation of inputs. Multi-modal integration by DSC sensory neurones contributes to the generation of goal directed orientation responses to novel sensory information. Thus, NMDA receptors underlie the control of appropriate behavioural responses to new elements in the local environment.

Glutamate Receptors and Visual Responses in the Developing Superior Colliculus

As NMDA receptors are important in the developmental plasticity of the nervous system, we have also investigated how NMDA receptor-mediated transmission of visual responses in the superfical superior colliculus may change during development. We found that after eye opening, there was an increase in the the degree of NMDA receptor involvement in visual responses which peaked a week later (Binns & Salt 1998). Furthermore, this increase is sustained and enhanced into adulthood in the absence of visual experience (Binns & Salt 1998), and may be a result of changes in the molecular properties/composition of the NMDA receptors involved in visual responses in the superior colliculus (Binns et. al. 1999).

Changes in transmitter receptor function in the Superior Colliculus in retinal degeneration and/or regeneration

Given our previous findings that the various glutamate receptors are major players in the mediation of visual input to the superior colliculus, we have begun to investigate how the role and function of these receptors may change during retinal degeneration and/or lesion . It is evident that changes do occur in the superior colliculus and it will be important to define these changes and how to modulate them.


This page was written by Tom Salt, and is part of the Neurotransmitters in Sensory Systems Home Page.


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