Pigeon basal ganglia: insights into the neuroanatomy underlying telencephalic sensorimotor processes in birds

This paper reviews the organization of the avian and mammalian striatum. The striatum receives input from virtually the entire rostrocaudal and mediolateral expanse of the cerebral cortex. The corticostriatal projections appear to be glutamatergic, forming excitatory synapses in the striatum. Another major projection to the avian striatum that also appears to be glutamatergic stems from a set of nuclei in the dorsal zone of the avian thalamus that are comparable to the mammalian intralaminar, mediodorsal, and midline nuclei. Furthermore, the striatum receives a massive projection from dopaminergic neurons of the ventral tegmental area and substantia nigra in the midbrain tegmentum. In return, the midbrain tegmentum receives a direct GABAergic/substance P-ergic/ dynorphinergic projection from the striatum, as well as an indirect one formed by GABAergic/substance P-ergic/ dynorphinergic and GABA-ergic/enkephalinergic striatal neurons projecting to the pallidum in the first step, and pallidal GABAergic/LANT6/parvalbumin neurons projecting to the midbrain tegmentum in the second step. In addition to its projection neurons, the striatum possesses GABAergic and cholinergic interneurons. One motor output pathway of the striatum runs via the pallidum and dorsal thalamic ventral tier nulei to the motor cortex. In addition to this pathway, birds possess a major descending pathway from the basal ganglia to the tectum via the GABAergic nucleus spiriformis lateralis in the pretectum. On hodological and topological grounds, similar nuclei, although not GABAergic, can be found in mammals. Finally, an other striatal motor output is formed by a sequential GABAergic pathway from the basal ganglia via the substantia nigra to the tectum. In conclusion, it appears that the organization of the avian and mammalian basal ganglia is similar rather than different.