Identification of the anterior nucleus of the ansa lenticularis in birds as the homolog of the mammalian subthalamic nucleus

In mammals, the subthalamic nucleus (STN) is a glutamatergic diencephalic c ell group that develops in the caudal hypothalamus and migrates to a positi on above the cerebral peduncle. By its input from the external pallidal seg ment and projection to the internal pallidal segment, STN plays a critical role in basal ganglia functions. Although the basal ganglia in birds is wel l developed, possesses the same major neuron types as in mammals, and plays a role in movement control similar to that in mammals, it has been uncerta in whether birds possess an STN. We report here evidence indicating that th e so-called anterior nucleus of the ansa lenticularis (ALa) is the avian ho molog of mammalian STN. First, the avian ALa too develops within the mammil lary hypothalamic area and migrates to a position adjacent to the cerebral peduncle. Second, ALa specifically receives input from dorsal pallidal neur ons that receive input from enkephalinergic striatal neurons, as is true of STN. Third, ALa projects back to avian dorsal pallidum, as also the case f or STN in mammals. Fourth, the neurons of ALa contain glutamate, and the ta rget neurons of ALa in dorsal pallidum possess AMPA-type glutamate receptor profiles resembling those of mammalian pallidal neurons. Fifth, unilateral lesions of ALa yield behavioral disturbances and movement asymmetries rese mbling those observed in mammals after STN lesions. These various findings indicate that ALa is the avian STN, and they suggest that the output circui try of the basal ganglia for motor control is similar in birds and mammals.