A New Optogenetic Tool to Investigate the Role of Dopamine Signaling in the Basal Ganglia

For millions of years, dopamine has served as a neurotransmitter in the nervous system of both invertebrates and vertebrates. Throughout evolution, this molecule has maintained conserved roles in a myriad of functions such as regulation of movement, learning, motivation, memory formation, reward-seeking, and feeding behavior. Clusters of dopamine-synthesizing neurons are distributed all over the brain; however, the largest population is found in midbrain nuclei that project to the frontal cortex, dorsal striatum, and nucleus accumbens (NAc; Costa and Schoenbaum, 2022).

Like other catecholamines, dopamine acts on G-protein–coupled receptors (GPCRs) to modulate intracellular effector protein activity. Dopamine receptors are classified into two main families: D1-like receptors (DRD1), which include D1 and D5 subtypes, and D2-like receptors (DRD2), comprising D2, D3, and D4 subtypes. DRD1 activation triggers a signaling cascade that enhances adenylate cyclase activity, increasing intracellular cAMP and activating cAMP-dependent protein kinase (PKA), which phosphorylates downstream proteins, including the dopamine- and cAMP-regulated phosphoprotein (DARPP-32), ultimately increasing neuronal excitability. Conversely, DRD2 activation inhibits adenylate cyclase and lowers intracellular cAMP levels, reducing neuronal excitability (Tritsch and Sabatini, 2012).

The key site where dopamine regulates motor control is the basal ganglia, which comprise the striatum, substantia nigra pars compacta (SNpc), substantia nigra pars reticulata (SNpr), globus pallidus externus (GPe), globus pallidus internus (GPi), and subthalamic nucleus (STN; Calabresi et al., 2014). Phasic dopamine release from the SNpc modulates the activity of spiny projection neurons (SPNs) in the striatum, playing a crucial role in the initiation, transition, and termination of voluntary movements (Klaus et al., 2019). Most SPNs can be categorized into those that express DRD1 and project directly to the main output nuclei of the basal ganglia (GPi and SNpr) and those that express DRD2 and influence the output nuclei via intermediate nuclei (GPe), which in turn projects to the glutamatergic neurons of the STN (Tritsch and Sabatini, 2012; Calabresi et al., 2014). These pathways are coactivated during movement initiation, and it has been hypothesized that the direct pathway facilitates the execution of desired movements, while the indirect pathway suppresses competing motor programs (Klaus et al., 2019). Activation of DRD1 and DRD2 also influences striatal neuroplasticity (Calabresi et al., 2014; Costa and Schoenbaum, 2022), a fundamental process for learning new motor actions (Klaus et al., 2019). In addition, recent studies have identified a small subset of SPNs that coexpress DRD1 and DRD2 receptors, intensifying the prokinetic and antikinetic roles of DRD1- and DRD2-SPNs, respectively (Bonnavion et al., 2024).

Although the direct and indirect pathways described above are the most studied basal ganglia circuits, additional projections exist to, from, and among basal ganglia nuclei. For instance, the GPe contributes to motor control through its connections with the cortex, thalamus, and other basal ganglia structures (dorsal striatum, SNpc, SNpr, and STN; Courtney et al., 2023). The use of techniques like optogenetics to modulate neuronal activity has enhanced our understanding of how each of these pathways contributes to the movement. In particular, channelrhodopsins and halorhodopsins, which form ion channels that open in response to specific wavelengths of light, have been useful in exploring how activation of subsets of neurons affects behavior. However, this approach cannot be used to study how distinct metabotropic receptors influence the activity of specific neurons to modulate complex behaviors. Moreover, ion channels and metabotropic receptors contribute to changes in neuronal excitability with different kinetics. Because of this limitation, chimeric GPCRs, known as OptoXRs, were developed to examine catecholaminergic signaling across different brain regions (Kim et al., 2017).

Kim et al. (2025) recently added to the OptoXR toolbox by developing an OptoDRD2 in which bovine rhodopsin is fused with human DRD2. EYFP was inserted at the C-terminal end to allow assessment of receptor expression patterns. When expressed in HEK293A cells, the chimeric OptoDRD2 was properly translated, correctly folded, and localized to both the plasma membrane and to intracellular organelles. Activation of OptoDRD2 by blue light reduced cAMP levels in HEK cells treated with forskolin, an adenylate cyclase activator. Examination of downstream signaling pathways revealed that light-mediated OptoDRD2 and agonist-mediated activation of endogenous DRD2 had comparable effects on cAMP levels and ERK phosphorylation, classic markers of DRD2 activation. Furthermore, whereas photostimulation of OptoDRD1-expressing cells induced DARPP-32 phosphorylation by cAMP-PKA, the stimulation of OptoDRD2 inhibited DARPP-32 phosphorylation, even when the cells were treated with forskolin. This suggests that OptoDRD2 activation can offset the intracellular signaling effects of DRD1 activation.

Next, Kim et al. (2025) evaluated the behavioral effects of DRD2 signaling by expressing OptoDRD2 in cells that usually express DRD2 in the right dorsal striatum of mice. Upon light stimulation, the animals displayed a gradual increase in movement. Other movement-related parameters, including speed, vigor, and acceleration, were also significantly higher in OptoDRD2-expressing mice than in wild-type mice. These results are consistent with the role of DRD2-expressing SPNs in the indirect pathway: because their activity typically suppresses unwanted movement, decreasing their excitability by activating DRD2 signaling results in increased movement (Klaus et al., 2019).

Kim et al. (2025) then used OptoDRD2 to investigate a small, poorly understood population of neurons located in the GPe that coexpress DRD2 and CamKIIa, a marker of glutamatergic neurons (Saunders et al., 2018). Activation of OptoDRD2 expressed in this cell group increased total mobility time and acceleration of movements in the animals without affecting average speed (Fig. 1). This result suggests that this population contributes to the regulation of movement.

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Figure 1.

Overview of the sequence of events leading to changes in movement behavior following OptoDRD2 activation in excitatory neurons of the GPe. A, Light-induced stimulation of OptoDRD2 triggers an intracellular signaling cascade that inhibits adenylate cyclase activity, reduces cAMP levels, and ultimately attenuates DARPP-32 phosphorylation mediated by PKA. B, This chain of biochemical events results in altered neuronal excitability of GPe glutamatergic neurons, which is reflected in behavior changes such as increased motility time and enhanced movement acceleration. DRD2, dopamine receptor type 2; OptoDRD2, light-responsive chimeric dopamine receptor type 2; GPe, globus pallidus externus; cAMP, cyclic adenosine monophosphate; PKA, protein kinase A; DARPP-32, dopamine- and cyclic-AMP-regulated phosphoprotein. Figure created on Biorender.com.

In summary, the findings of Kim et al. (2025) shed light on the function of GPe glutamatergic neurons and demonstrate the novel role of dopamine in motor function. Although the functions of the excitatory neurons in the GPe have not yet been explored at all, it is believed that these cells may reduce SPN-mediated inhibition within the GPe itself. Future work should further clarify the synaptic targets of this particular group of neurons as well as detailing its function.

The work by Kim et al. (2025) suggests a potential link between GPe function and Parkinson's disease (PD). In PD, the degeneration of dopaminergic neurons in the SNpc leads to progressive dopamine depletion. Although motor symptoms typically emerge only after substantial neuronal loss, disruptions in dopaminergic signalizing are already evident in early stages of the disease (Gerfen and Surmeier, 2011). These early effects may stem from the fact that DRD2 receptors exhibit a higher affinity for dopamine than DRD1 receptors. Moreover, the dendritic architecture of DRD2-SPNs in the indirect pathway—which possess a smaller surface area compared with DRD1-SPNs of the direct pathway—would render them more excitable despite having similar spine densities.

As dopamine levels progressively decline in PD, DRD1-SPNs are likely affected earlier, resulting in reduced activation of the direct pathway and diminished inhibition of GPi/SNpr. In contrast, DRD2-SPNs—due to their higher dopamine affinity—are impacted later in the disease course (Gerfen and Surmeier, 2011). Once dopamine depletion becomes severe, the loss of DRD2-mediated suppression leads to hyperexcitability of the indirect pathway. This results in excessive inhibition of the GPe and subsequent overactivity of the STN, thereby enhancing excitatory drive to the GPi/SNpr and promoting pathological suppression of the thalamocortical pathway. The combined alterations in these pathways ultimately result in motor impairment (Calabresi et al., 2014).

OptoDRD2-mediated modulation of GPe glutamatergic neurons reduces immobility and enhances locomotor activity, highlighting their pivotal role in motor control circuits. In PD, progressive dopaminergic denervation likely disrupts their activity, leading to excessive motor suppression. Targeted manipulation of GPe glutamatergic signaling in parkinsonian models may clarify their distinct contribution to bradykinesia and rigidity, the core motor symptoms of PD.

Looking ahead, the application of OptoDRD2 could be extended to other basal ganglia structures, such as the GPi, SNpr, and STN, to investigate how dopaminergic signaling modulates movement across these nuclei, known to be hyperactive in PD. Understanding the specific role of neuronal activity within each structure may reveal novel therapeutic targets. As gene therapy advances, approaches like chemogenetics may offer a less invasive and more targeted means of modulating these circuits, potentially reducing pathological activity in PD patients.

When considering clinical implications of the work by Kim et al. (2025), it is worth noting that constant light exposure for OptoDRD2 activation does not accurately mimic the phasic nature of dopamine release, particularly in the midbrain (Costa and Schoenbaum, 2022), where firing patterns variations play a critical role in motor actions (Klaus et al., 2019). Furthermore, effective OptoDRD2 activation requires a high-power laser. One of the main technical challenges in optogenetics, limiting its application in treating neurological disorders, is achieving sufficient neuronal stimulation and behavioral control while avoiding heat-induced damage from laser pulses, particularly during prolonged stimulation.

In conclusion, the development of OptoDRD2 opens the door to explore the role of dopaminergic signaling in multiple areas. DRD2 is expressed in several brain regions beyond the basal ganglia (Costa and Schoenbaum, 2022), including the frontal cortex, hippocampus, amygdala, and NAc. The expression of OptoDRD2 in these areas may provide valuable insights into both the normal effects of dopamine and the pathophysiology of dopaminergic system disorders, including schizophrenia, addiction, depression, and attention-deficit/hyperactivity disorder.