The Interaction Between A2A Receptor and CB1 Receptor in GABA and Glutamate Release in the Basal Ganglia

Background

The basal ganglia are key structures in the brain responsible for motor control and reward-based behaviors. They receive excitatory inputs from the cortex and thalamus, with 80% of the synapses being glutamatergic, while the second most common synapse type is GABAergic. The regulation of these synapses relies on various neuromodulators, such as adenosine, acetylcholine, dopamine, and endocannabinoids. Adenosine modulates synaptic transmission through A1 receptors (A1R) and A2A receptors (A2AR), while endocannabinoids inhibit neurotransmitter release via CB1 receptors (CB1R). However, the interaction between A2AR and CB1R in GABAergic synapses has not been fully elucidated.

This study aimed to explore the functional interaction between A2AR and CB1R in GABAergic nerve terminals of the basal ganglia and compare it with their interaction in glutamatergic synapses. By investigating the regulatory mechanisms of these two receptors in GABA and glutamate release, the researchers hoped to reveal their complex relationship in neuromodulation and provide new insights for the treatment of related neurological disorders.

Source of the Paper

This paper was co-authored by Samira G. Ferreira, Rafael M. Bitencourt, Pedro Garção, Rodrigo A. Cunha, and Attila Köfalvi. The authors are affiliated with the Center for Neuroscience and Cell Biology (CNC) and the Center for Innovative Biomedicine and Biotechnology (CIBB) at the University of Coimbra, Portugal. The paper was published in 2025 in the European Journal of Neuroscience, with the DOI 10.1111/ejn.16642.

Research Process and Results

1. Experimental Design and Methods

The study was divided into the following steps:

a) Synaptosome Preparation and Neurotransmitter Release Assay

The researchers isolated synaptosomes from the striatum of rats and mice and induced GABA and glutamate release through high potassium (KCl) stimulation. Radioactively labeled GABA ([3H]GABA) and glutamate ([14C]glutamate) were used to monitor neurotransmitter release. By adding the CB1R agonist WIN55212-2 and the A2AR selective agonist CGS21680 or antagonists SCH58261 and ZM241385, the researchers observed the effects of these drugs on neurotransmitter release.

b) Binding Experiments

To further investigate the interaction between A2AR and CB1R, the researchers conducted radioligand binding experiments. They used [3H]ZM241385 and [3H]SR141716A to label A2AR and CB1R, respectively, and measured the binding parameters (Bmax and Kd) of these receptors under different drug treatments.

c) Experiments with Gene Knockout Mice

The researchers also used A2AR and CB1R knockout (KO) mice to compare differences in neurotransmitter release and receptor binding between these mice and wild-type (WT) mice.

2. Main Results

a) CB1R-Mediated Inhibition of GABA Release is Modulated by A2AR Ligands

The study found that WIN55212-2 significantly inhibited high potassium-evoked GABA release by activating CB1R. However, when the A2AR selective agonist CGS21680 or antagonist SCH58261 was added simultaneously, the inhibitory effect of WIN55212-2 was significantly attenuated. Notably, the A2AR antagonist ZM241385 completely blocked the inhibition of GABA release by WIN55212-2.

b) Effects of A2AR Ligands on CB1R Binding Parameters

Binding experiments showed that A2AR ligands CGS21680 and SCH58261 reduced the affinity of CB1R (increased Kd), while ZM241385 decreased the maximum binding sites (Bmax) of CB1R. These results indicate that A2AR ligands modulate CB1R function by altering its binding properties.

c) Results from Gene Knockout Mice

In A2AR knockout mice, WIN55212-2 failed to inhibit GABA release but still exhibited inhibitory effects on glutamate release. This suggests that A2AR uniquely regulates CB1R function in GABAergic synapses.

3. Conclusions and Significance

This study provides the first functional evidence of A2AR and CB1R forming heterotetramers in GABAergic nerve terminals of the basal ganglia. The findings reveal that A2AR ligands modulate CB1R function by altering its binding properties, thereby influencing neurotransmitter release. This discovery not only deepens our understanding of the complex interaction between A2AR and CB1R in neuromodulation but also offers new potential therapeutic targets for related neurological disorders, such as Parkinson’s disease and schizophrenia.

Research Highlights

1. A2AR and CB1R Heterotetramers: This study is the first to identify A2AR and CB1R heterotetramers in GABAergic nerve terminals and reveal their regulatory role in neurotransmitter release.
2. Unique Effects of A2AR Ligands: The study found that different A2AR ligands (e.g., CGS21680, SCH58261, and ZM241385) have distinct modulatory effects on CB1R function, providing new insights for the development of selective drugs.
3. Validation with Gene Knockout Mice: By using A2AR and CB1R knockout mice, the researchers validated the unique regulatory role of A2AR in CB1R function in GABAergic synapses.

Additional Valuable Information

This study also revealed the interdependence of A2AR and CB1R in striatal neuromodulation. The researchers found that the absence of A2AR leads to a reduction in CB1R binding sites, while the absence of CB1R also affects A2AR binding properties. This finding provides important experimental evidence for further research into the roles of these two receptors in neurological disorders.

This study not only enhances our understanding of the interaction between A2AR and CB1R in the basal ganglia but also offers new potential therapeutic targets for related diseases.