SUMMARY According to current models, the involvement of the basal ganglia in motor and non-motor functions is explained in the context of information processing in segregated basal ganglia-thalamocortical loops. These models predict that striatal dopamine loss in Parkinson’s disease (PD) eventually leads to abnormal processing in the “motor” thalamocortical network, and the antiparkinsonian effects of deep brain stimulation (DBS) of the sensorimotor internal globus pallidus (GPi) is explained as a release of movement-related thalamic neurons from overactive inhibitory GPi inputs. However, recent evidence suggests that descending basal ganglia output, specifically the massive projection of GPi to the pedunculopontine nucleus (PPN), may also be relevant for normal behavior and parkinsonism. Thus, manipulations of the PPN influence limb movements and postural adjustments, PPN activation has antiparkinsonian effects in monkeys, and DBS of the PPN ameliorates gait disturbances in some PD patients. The PPN is a highly heterogeneous brain region that gives rise to widespread ascending and descending projections. Our lack of knowledge of the anatomical targets of GPi projections to the PPN, and the effects of activation of the GPi-PPN pathway on PPN activity limits our understanding of the normal role of the GPi-PPN interaction and its role in the pathophysiology of PD, particularly in primates. The proposed studies aim therefore to examine the functional connectivity between the GPi and the PPN (aims 1 and 2), determine whether the anatomy and physiology of these networks are altered in the parkinsonian state (aims 2 and 3), and how GPi-DBS alters firing rates and patterns of GPi-receiving PPN neurons, as well as local field potential activity in the PPN (aim 3). These studies will be done in normal and MPTP-treated parkinsonian monkeys, using a combination of state-of-the-art optogenetic, anatomical and electron microscopy procedures. The knowledge gained from these studies is needed to develop or refine antiparkinsonian therapies that target the PPN or its projections for treatment of PD or other basal ganglia disorders.