The 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.
We 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., 2005; Neale & 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.
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