MAJOR GLUTAMATERGIC PROJECTION FROM SUBPLATE INTO VISUAL-CORTEX DURING DEVELOPMENT

Subplate neurons, the first neurons of the cerebral cortex to differen tiate and mature, are thought to be essential for the formation of con nections between thalamus and cortex, such as the system of ocular dom inance columns within layer 4 of visual cortex. To learn more about th e requirement for subplate neurons in the formation of thalamocortical connections, we have sought to identify the neurotransmitters and pep tides expressed by the specific class of subplate neurons that sends a xonal projections into the overlying visual cortex. To label retrograd ely subplate neurons, fluorescent latex microspheres were injected int o primary visual cortex of postnatal day 28 ferrets, just prior to the onset of ocular dominance column formation. Subsequently, neurons wer e immunostained with antibodies against glutamate, glutamic acid decar boxylase (GAD-67), parvalbumin, neuropeptide Y (NPY), somatostatin (SR IF), or nitric oxide synthase (NOS). Retrograde labeling results indic ate that the majority of subplate neurons projecting into the cortical plate reside in the upper half of the subplate. Combined immunostaini ng and microsphere labeling reveal that about half of cortically proje cting subplate neurons are glutamatergic; most microsphere-labeled sub plate neurons do not stain for GAD-67, parvalbumin, NPY, SRIF, or NOS. These observations suggest that subplate neurons can provide a signif icant glutamatergic synaptic input to the cortical plate, including th e neurons of layer 4. If so, excitation from the axons of subplate neu rons may be required in addition to that from lateral geniculate nucle us neurons for the activity-dependent synaptic interactions that lead to the formation of ocular dominance columns during development. J. Co mp. Neurol. 398:105-118, 1998. (C) 1998 Wiley-Liss, Inc.