Morphology and synaptic connections of crossed corticostriatal neurons in the rat.

The neurons of origin of the bilateral corticostriatal projection arising from the medial agranular cortical field in rats were identified by antidromic activation from contralateral neostriatal stimulation. The same cells were tested for antidromic activation from the contralateral neocortex and for orthodromic responses to stimulation of neocortex of the contralateral hemisphere or ipsilateral rostral thalamus. The neurons were then stained by intracellular injection of horseradish peroxidase. The laminar distribution of these neurons was compared to that of cortical cells stained retrogradely after injection of wheat germ agglutinin/HRP in the ipsilateral or contralateral neostriatum. The morphological features of physiologically identified corticostriatal neurons, their laminar organization, and their responses to stimulation were examined and compared with crossed corticocortical and brainstem-projecting cells. Crossed corticostriatal cells of the medial agranular cortical field were medium-sized pyramidal neurons found in the superficial part of layer V and in the deep part of layer III. Their basilar dendritic fields and initial intracortical axon collateral arborizations were coextensive with the layer defined by the distribution of corticostriatal neurons. The apical dendrites were thin and sparsely branched but consistently reached layer I, where they made a small arborization. These morphological features were shared by cortical neurons projecting to contralateral neocortex but not responding antidromically to stimulation of contralateral neostriatum, but they were not shared by brainstem-projecting cortical cells. Orthodromic responses to contralateral cortical stimulation consisted of brief excitatory postsynaptic potentials that were followed by powerful and longer-lasting inhibitory postsynaptic potentials. Corticostriatal cells also exhibited small excitatory postsynaptic potentials in response to thalamic stimulation. Many crossed corticostriatal neurons were also commissural corticocortical neurons. The results of reciprocal collision tests showed that this was due to the existence of two separate axonal branches, one projecting to contralateral neocortex and one to contralateral neostriatum. Intracellular staining of these neurons revealed ipsilateral axonal projections to the neostriatum and cortex.