Correlation of electrophysiology, morphology, and functions in corticotectal and corticopretectal projection
neurons in rat visual cortex
A. Rumberger, M. Schmidt, H. Lohmann & K.-P. Hoffmann
Experimental Brain Research, 119:375-390, 1998
- 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.
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