Study of projections from the entopeduncular nucleus to the thalamus of the rat.

The entopeduncular nucleus (EP) is a major outflow nucleus of the basal ganglia and innervates the lateral habenula, parafascicular, pedunculopontine, ventrolateral (VL), ventromedial (VM), and mediadorsal thalamic nuclei. This study investigated the morphology of single axons of entopeduncular neurons projecting to the motor thalamus by placing small injections of dextran biotin into the EP and reconstructing drawings of single axone from serial sections. There were two populations of entopeduncular-thalamic projection axone: those that projected only to the motor thalamus (VL and VM) and those that projected to both the motor thalamus and other nuclei (e.g., the habenula). The neurochemistry of EP neurons projecting to the thalamus was investigated by injecting the retrograde tracer FluoroCold into the VL and VM thalamic nuclei to retrogradely fill entopeduncular projection neurons. These were subsequently immunohistochemically labeled for choline acetyl transferase, gamma-aminobutyric acid (GABA), and glutamate. Consistent with previous studies, significant proportions of these neurons were GABA immunoreactive. In addition, approximately half of the entopeduncular-thalamic projecting neurons were found to be cholinergic. This excitatory input is most likely derived from axons that branch as they pass through the motor thalamus to the lateral habenula. (C) 2000 Wiley-Liss, Inc.