Major glutamatergic projection from subplate into visual cortex during development.

Subplate neurons, the first neurons of the cerebral cortex to differentiate and mature, are thought to be essential for the formation of connections between thalamus and cortex, such as the system of ocular dominance columns within layer 4 of visual cortex. To learn more about the requirement for subplate neurons in the formation of thalamocortical connections, we have sought to identify the neurotransmitters and peptides expressed by the specific class of subplate neurons that sends axonal projections into the overlying visual cortex. To label retrogradely subplate neurons, fluorescent latex microspheres were injected into primary visual cortex of postnatal day 28 ferrets, just prior to the onset of ocular dominance column formation. Subsequently, neurons were immunostained with antibodies against glutamate, glutamic acid decarboxylase (GAD-67), parvalbumin, neuropeptide Y (NPY), somatostatin (SRIF), or nitric oxide synthase (NOS). Retrograde labeling results indicate that the majority of subplate neurons projecting into the cortical plate reside in the upper half of the subplate. Combined immunostaining and microsphere labeling reveal that about half of cortically projecting subplate neurons are glutamatergic; most microsphere-labeled subplate neurons do not stain for GAD-67, parvalbumin, NPY, SRIF, or NOS. These observations suggest that subplate neurons can provide a significant glutamatergic synaptic input to the cortical plate, including the neurons of layer 4. If so, excitation from the axons of subplate neurons may be required in addition to that from lateral geniculate nucleus neurons for the activity-dependent synaptic interactions that lead to the formation of ocular dominance columns during development.