A Disynaptic Circuit in the Globus Pallidus Controls Locomotion Inhibition

The basal ganglia (BG) inhibit movements through two independent circuits: the striatal neuron-indirect and the subthalamic nucleus-hyperdirect pathways. These pathways exert opposite effects onto external globus pallidus (GPe) neurons, whose functional importance as a relay has changed drastically with the discovery of two distinct cell types, namely the prototypic and the arkypallidal neurons. However, little is known about the synaptic connectivity scheme of different GPe neurons toward both motor-suppressing pathways, as well as how opposite changes in GPe neuronal activity relate to locomotion inhibition. Here, we optogenetically dissect the input organizations of prototypic and arkypallidal neurons and further define the circuit mechanism and behavioral outcome associated with activation of the indirect or hyperdirect pathways. This work reveals that arkypallidal neurons are part of a novel disynaptic feedback loop differentially recruited by the indirect or hyperdirect pathways and that broadcasts inhibitory control onto locomotion only when arkypallidal neurons increase their activity.

Translocator protein (18 kDa) mapping with [125I]-CLINDE in the quinolinic acid rat model of excitotoxicity: a longitudinal comparison with microglial activation, astrogliosis, and neuronal death

Excitotoxicity leads to an inflammatory reaction involving an overexpression of: translocator protein 18 kDa (TSPO) in cerebral microglia and astrocytes. Therefore, we performed ex vivo explorations with [125]-CLINDE, a TSPO-specific radioligand, to follow the time course of TSPO expression, in parallel with lesion progression, over 90 days after induction of cerebral excitotoxicity in rats intrastriatally injected with quinolinic acid. Biodistribution data showed a significant increase in CLINDE uptake on the injured side from 1 days postlesion (dpl); the maximal striatal binding values evidenced a plateau between 7 and 30 dpl. [125I]-CLINDE binding was displaced from the lesion by PK11195, suggesting TSPO specificity. These results were confirmed by ex vivo autoradiography. Combined immunohistochemical studies showed a marked increase in microglial expression in the lesion, peaking at 14 dpl, and astrocytic reactivity enhanced at 7 and 14 dpl, whereas a prominent neuronal cell loss was observed. At 90 dpl, CLINDE binding and immunoreactivity targeting activated microglia, astrogliosis, and neuronal cell density returned to a basal level. These results show that both neuroinflammation and neuronal loss profiles occurred concomitantly and appeared to be transitory processes. These findings provide the possibility of a therapeutic temporal window to compare the differential effects of antiinflammatory treatments in slowing down neurodegeneration in this rodent model, with potential applications to humans.