The external globus pallidus (GPe) contains two major neuron populations: the prototypical fast-spiking neurons and the arkypallidal neurons. Arkypallidal neurons are a distinct population identified relatively recently, characterized by their extensive projections back to the striatum—the input stage of the basal ganglia[1].
These neurons provide a crucial feedback loop within the basal ganglia circuit, modulating striatal activity and influencing motor control, action selection, and learning.
¶ Cell Types and Markers
GPe contains heterogeneous neuronal populations:
- Prototypical GPe neurons: Fast-spiking, mainly parvalbumin-positive (PV+)
- Arkypallidal neurons: Lower firing rate, Npas1+ and Pv- markers
- Protodendritic neurons: Another distinct population with intermediate properties
Key molecular markers:
- Npas1 (Neuronal PAS domain protein 1) - specific arkypallidal marker
- Pv (parvalbumin) - labels prototypical GPe neurons
- Foxp2 - enriched in arkypallidal neurons
- Ki67 - markers of arkypallidal identity
Morphology: Arkypallidal neurons have extensive dendritic fields and long-range axons targeting the striatum
GPe dysfunction contributes to PD pathophysiology:
- Activity changes: GPe neurons become overactive in PD due to reduced dopaminergic inhibition
- Arkypallidal role: These neurons provide excessive inhibition to striatum, contributing to bradykinesia
- Levodopa-induced dyskinesias: GPe hyperactivity correlates with dyskinesia development
- Therapeutic target: GPe is a target for deep brain stimulation (DBS) in PD[2]
- Early changes: GPe neuron loss is an early feature in HD
- Circuit dysfunction: Arkypallidal-striatal circuit disruption contributes to motor symptoms
- Cognitive effects: GPe dysfunction affects action selection and learning
- GPe dysfunction: Reduced GPe output leads to excessive movement
- Arkypallidal involvement: Aberrant arkypallidal signaling contributes to dystonic postures
- Treatment: GPe DBS is effective for generalized dystonia
The basal ganglia operate through direct and indirect pathways:
- Direct pathway (Go): Striatum → GPe (indirect) → Substantia nigra pars reticulata (SNr) → Thalamus → Cortex
- Indirect pathway (Stop): Striatum → GPe → Subthalamic nucleus → SNr
- Hyperdirect pathway: Cortex → Subthalamic nucleus → SNr
Arkypallidal function: These neurons provide feedback inhibition to striatum, regulating the gain of basal ganglia processing and preventing excessive motor output[3].
- Feedback projections: Arkypallidal neurons send dense projections to striatal medium spiny neurons
- Modulation: Provide inhibition that regulates striatal output and plasticity
- Learning: Involved in reinforcement learning and action value signals
- Prototypical interactions: Cross-talk between neuron types within GPe
- Local inhibition: GABAergic connections between GPe neurons
- Indirect pathway: Part of the classical indirect pathway
- Information integration: Receives and processes movement-related signals
- Deep brain stimulation: GPe is an effective target for PD and dystonia
- Biomarkers: GPe activity patterns may predict treatment response
- Neural interfaces: GPe recordings can inform closed-loop stimulation systems