Thalamic projections to the paleostriatum and neostriatum in the pigeon (Columba livia)

Thalamic projections to the paleostriatum and neostriatum in the pigeon have been studied using wheat germ agglutinin-horseradish peroxidase in anterograde and retrograde tracing experiments. Injections centred on different mediolateral regions of the dorsal thalamus produced terminal labelling in correspondingly different mediolateral regions of the striatal complex, comprising paleostriatum augmentation and lobus parolfactorius, within the ventral paleostriatum, and within the neostriatum. Injections into various loci within these regions retrogradely labelled numerous neurons within the dorsal thalamus, the location of which varied systematically with the injection placement; and within various regions of the midbrain and pons. These experiments demonstrate a major thalamic projection to the striatum analogous to that from the midline and intralaminar nuclei to the caudatoputamen in mammals, although the patchy characteristic of mammalian striatal afferent terminations was observed only within ventral regions of the pigeon paleostriatum. In addition the experiments demonstrate that striatal afferents also originate from certain nuclei of the mesencephalic midline which are possibly equivalent to the raphe nuclei.