Correlation of electrophysiology, morphology, and functions in corticotectal and corticopretectal projection neurons in rat visual cortex.

In most mammals the superior colliculus (SC) and the pretectal nucleus of the optic tract (NOT) receive direct input from the ipsilateral visual cortex via projection neurons from infragranular layer V. We examined whether these projection neurons belong to different populations and, if so, whether it is possible to correlate the electrophysiological features with the suggested function of these neurons. Projection cells were retrogradely labeled in vivo by rhodamine-coupled latex beads or fast blue injections into the SC or the NOT 2-5 days prior to the electrophysiological experiment. Intracellular recordings of prelabeled neurons were made from standard slice preparations and cells were filled with biocytin in order to reveal their morphology. Both cell populations consist of layer V pyramids with long apical dendrites that form terminal tufts in layer I. In electrophysiological terms, 12 of the corticotectal cells could be classified as intrinsically bursting (IB), while two neurons showed a doublet firing characteristic and one neuron was classified as regular-spiking (RS). Intracortical microstimulation of cortical layer II/III revealed that SC-projecting neurons responded optimally to stimulation sites up to a distance of 1000 microm from the recorded cell. The morphological features of the SC-projecting cells reveal an apical dendritic tuft in layer I with a lateral extension of 300 microm, a mean spine density of 65 spines per 40 microm on the apical dendrites located in layer II/III, and a bouton density of 13 boutons per 100 microm on the intracortical axons. Sixteen NOT-projecting neurons exhibited an IB and five cells an RS characteristic. Intracortical microstimulation of cortical layer II/III showed that NOT-projecting neurons responded optimally to stimulation sites up to a distance of 1500 microm. Their morphological features consist of an apical dendritic tuft with a lateral extension of 500 microm, a mean spine density of 25 spines per 40 microm on the apical dendrites located in layer II/III, and a bouton density of 6 boutons per 100 microm on the intracortical axons. When the passive membrane parameters, responses to intracortical microstimulation in layer V, the extension of the basal dendritic field, and spine densities in layers I or V were compared between SC- and NOT-projecting cells, no differences were revealed. Differences were only consistently found in the supragranular layers, either for morphological parameters or for intracortical microstimulation. The results suggest that NOT-projecting and SC-projecting neurons, although biophysically similar, could integrate and transmit different spatial aspects of cortical visual information to their target structures.