Subsynaptic segregation of metabotropic and ionotropic glutamate receptors as revealed by immunogold localization

Glutamate is a major neurotransmitter in the brain that acts both through fast ionotropic receptors and through slower metabotropic receptors coupled to G proteins. Both receptors are present throughout the somatodendritic domain of neurons as shown by immunohistochemical and patch clamp recording studies. Immunogold labelling revealed a concentration of metabotropic receptors at the edge, but not within the main body of anatomically defined synapses, raising the possibility that ionotropic and metabotropic receptors are segregated. We applied double immunogold labelling to study glutamatergic parallel and climbing fibre synapses in the cerebellar cortex. The ionotropic AMPA type receptors occupy the membrane opposite the release site in the main body of the synaptic junction, whereas the metabotropic receptors are located at the periphery of the same synapses. Furthermore, immunoreactivity for AMPA receptors is at least twice as high in the parallel fibre synapses as in glutamatergic mossy fibre synapses. We suggest that the spatial segregation of ionotropic and metabotropic glutamate receptors permits the differential activation of these receptors according to the amount of glutamate released presynaptically, whereas the different densities of the ionotropic receptor at distinct synapses could allow the same amount of glutamate to evoke fast responses of different magnitude.