Mediodorsal and reticular thalamic nuclei receive collateral axons from prefrontal cortex and laterodorsal tegmental nucleus in the rat.

Quantitative analysis of the branching pattern of output projections from the prefrontal cortex and the laterodorsal tegmental nucleus of the brainstem, has been carried out using the newly developed method for double retrograde labelling of neurons with fluorescent labelled latex microspheres. At least 34% of cortical mediodorsal projection neurones in lamina VI send collateral axons to the reticular thalamic nucleus, and at least 40% of cortical reticular projection neurones and collateral axons to mediodorsal nucleus. In the brainstem, laterodorsal neurones are also highly collateralized; at least 44% of mediodorsal projection neurones also innervating nucleus reticularis, and at least 47% of reticularis projection neurones innervating the mediodorsal nucleus.