, according to a new study using a mouse model of disease.
Parkinson’s disease is characterized by a range of cognitive and motor symptoms, which appear at different disease stages. While recent research has pointed to specific neuronal subpopulations, or microcircuits, operating in the basal ganglia, researchers lacked a clear understanding of how they might correspond with specific symptom domains.
In a study published online March 15 in Nature Neuroscience, lead author Varoth Lilascharoen, PhD, of the University of California, San Diego, and colleagues reported that two different neuronal subpopulations within the external globus pallidus, an important nucleus within the basal ganglia, are associated, respectively, with movement and with reversal learning (having to adapt to a reward pattern that is the reverse of a previous pattern). This is the first time, the investigators said, that the contributions of specific microcircuits in the basal ganglia have been linked to different behaviors.
Using electrophysiology, viral tracing, and other approaches, Dr. Lilascharoen and colleagues demonstrated that two microcircuits or populations of parvalbumin-expressing neurons could be manipulated to exacerbate or alleviate the motor or cognitive deficits in the dopamine-depleted mice.
One of these microcircuits, made up of substantia nigra pars reticulata-projecting GPe-PV neurons, could be manipulated in ways that promoted or inhibited the mice’s movement. The other, which comprises parafascicular thalamus-projecting GPe-PV neurons, could be manipulated to affect reversal learning, the researchers found. Activation or inhibition of either circuit was not seen affecting function in the other.
The results shed light on the functional organization of the different basal ganglia nuclei at the circuit level, and suggest, the authors argued, that differences in how different neuronal subpopulations adapt to dopamine loss could explain some of the patterns of progression seen in Parkinson’s disease.
The findings “establish the differential contributions from two distinct GPe-PV microcircuits in specific Parkinsonian-like behaviors linked to early and late stages of the disease,” Dr. Lilascharoen and colleagues wrote in their analysis. “[F]urther elucidation of the detailed connectivity of GPe subpopulations to their downstream targets … is needed to fully define the function of each microcircuit and design better therapeutic strategies for the various behavioral impairments of Parkinson’s disease.”
Commenting on the research, Stefan Lang, MD, PhD, of the University of Calgary in Alberta said, “While Parkinson’s disease is often referred to as a movement disorder, it is well known that nonmotor symptoms, including cognitive and behavioral impairment, are common and debilitating. Impairment of basal ganglia function is known to contribute to these different symptom domains, though the specific circuits have never been elucidated. [Dr.] Lilascharoen et al. tease apart specific basal ganglia circuits associated with motor and behavioral symptoms, thereby providing evidence that distinct microcircuits might contribute to unique behaviours. As technological advances in neuromodulatory therapies continue to improve the spatial and temporal resolution of stimulation, future treatments may allow for specific targeting of behavioral impairment symptoms in Parkinson’s disease.”
Dr. Lilascharoen and Dr. Lang did not report outside funding or conflicts of interest.