Abstract The present study aimed to investigate whe- ther the pedunculopontine projection to the thalamus overlaps with identified thalamostriatal neurons. These projections were studied using a dual tract-tracing proce- dure combining anterogradely transported biotinylated dextran amine (pedunculopontine projections) and retro- gradely transported Fluoro-Gold (thalamostriatal projec- tions). Overlapping thalamic territories between thala- mostriatal neurons and the axon terminals arising from the pedunculopontine tegmental nucleus were observed in the midline (paraventricular) and in the intralaminar (centrolateral, central medial, paracentral and parafasci- cular) thalamic nuclei. Other thalamic nuclei, such as the ethmoid, intermediodorsal, mediodorsal, paratenial, pos- teromedian, ventromedian, ventrolateral and rhomboid thalamic nuclei, displayed a lesser degree of overlap. These observations suggest the existence of presumptive contacts between thalamostriatal neurons and axons emerging from the pedunculopontine tegmental nucleus, therefore supporting the possible existence of feedback circuits in the rat basal ganglia in which the tegmento- thalamic projection would play a major role. Key words Basal ganglia · Pedunculopontine tegmental nucleus · Fluoro-Gold · BDA Abbreviations aca Anterior commissure, anterior part · Acb accumbens nucleus · acp anterior commissure, posterior part · AD anterodorsal thalamic nucleus · APTD anterior pretectal nucleus, dorsal part · Aq aqueduct (Sylvius) · B basal nucleus (Meynert) · cc corpus callosum · CL centrolateral thalamic nucleus · CM central medial thalamic nucleus · CnF cuneiform nucleus · CPu caudate putamen · ECIC external cortex of the inferior colliculus · EP entopeduncular nucleus · Eth ethmoid thalamic nucleus · F nucleus of the fields of Forel · fr fasciculus retroflexus · GP globus pallidus · IAM interanteromedial thalamic nucleus · IMD intermediodorsal thalamic nucleus · LD laterodorsal thalamic nucleus · LHb lateral habenular nucleus · LP lateral posterior thalamic nucleus · LV lateral ventricle · MD mediodorsal thalamic nucleus · MHb medial habenular nucleus · MiTg microcellular tegmental nucleus · ml medial lemniscus · mt mammillothalamic tract · PC paracentral thalamic nucleus · PF parafascicular thalamic nucleus · Po posterior thalamic nuclear group · PPTg pedunculopontine tegmental nucleus · PV paraventricular thalamic nucleus · Re reuniens thalamic nucleus · Rh rhomboid thalamic nucleus · Rt reticular thalamic nucleus · scp superior cerebellar peduncle · sm stria medullaris of the thalamus · SN substantia nigra · SPF subparafascicular thalamic nucleus · SPTg subpeduncular tegmental nucleus · STh subthalamic nucleus · Sub submedius thalamic nucleus · xscp decussation of the superior cerebellar peduncle · VL ventrolateral thalamic nucleus · VM ventromedial thalamic nucleus · VPM ventral posteromedial thalamic nucleus · VPL ventral posterolateral thalamic nucleus · VPPC ventral posterior thalamic nucleus, parvicellular part · ZI zona incerta Introduction The basal ganglia are a group of subcortical structures related to the organization of movement as well as to cognitive and motivational processes (Albin et al. 1995; Graybiel 1995; Parent and Hazrati 1995; Gerfen and Wilson 1996). The main output of the basal ganglia is di- rected to the cortex with a relay in the thalamus. The most caudal brain nucleus receiving projections from the basal ganglia is the pedunculopontine tegmental nucleus (PPTg). Anterograde and retrograde tract-tracing proce- dures and neurophysiological studies have reported the existence of a close and reciprocal relationship between E. Erro · J.L. Lanciego · J.M. Giménez-Amaya ( ✉ ) Departamento de Anatomía, Facultad de Medicina, Universidad de Navarra, C/Irunlarrea, S/N, E-31080 Pamplona, Spain e-mail: jmga@unav.es Tel.: +34-48-42-56-85, Fax: +34-48-42-56-49 Exp Brain Res (1999) 127:162–170 © Springer-Verlag 1999 RESEARCH ARTICLE E. Erro · J.L. Lanciego · J.M. Giménez-Amaya Relationships between thalamostriatal neurons and pedunculopontine projections to the thalamus: a neuroanatomical tract-tracing study in the rat Received: 7 December 1998 / Accepted: 8 March 1999