Periodic Reporting for period 4 - CoordinatedDopamine (Coordination of regional dopamine release in the striatum during habit formation and compulsive behaviour)
Okres sprawozdawczy: 2019-11-01 do 2020-04-30
The basal ganglia consist of a set of brain structures responsible for the selection and execution of behavioral actions in a given context. Release of the neurotransmitter dopamine into the striatum, the main input nucleus of the basal ganglia, is a fundamental mechanism involved in learning and regulation of such actions. The striatum consists of multiple functional units: the limbic striatum is thought to mediate motivational aspects of actions, the associative striatum goal-directedness of actions, and the sensorimotor striatum their automation (e.g. habit formation). A long-standing question in the field of neuroscience is how these different striatal domains communicate with one another, and specifically if they do so during the automation of actions. An influential hypothesis suggests that such coordination is implemented by semi-reciprocal connections, so-called loops, between striatal projection neurons and the dopaminergic midbrain. Although very influential in theory, this cross-domain “bridging” principle has yet to be verified. And even more so, it has to be tested whether regionally coordinated dopamine signaling governed by such loop-based striatal hierarchy is involved in automation of behavior. We discovered both anatomical and functional evidence for the existence and importance of this neurobiological organizational principle of the basal ganglia, in accordance with what we proposed in the project proposal. However, the specifics of this dopaminergic principle and its role in behavioral function were unexpected. We communicated our findings not only to other scientists, but also to clinicians and researchers at the AMC Psychiatry Department (University of Amsterdam). At the AMC Psychiatry Department, many otherwise therapy-resistant psychiatric patients are treated with deep-brain stimulation of fiber tracts that contain basal ganglia and dopamine projections. Efforts to target subdivisions of these fiber tracts patient-specifically to optimize clinical outcomes are already underway. Our findings will hopefully impact this targeting in order to alter the occurrence of unwanted, extinction-resistant automatic behaviors.
To test our hypotheses, we conducted electrochemical measurements with real-time resolution simultaneously in limbic, associative, and sensorimotor aspects of the striatum to assess regional coordination of dopamine release in behaving animals, supported by rabies virus-mediated anatomical circuit mapping and in-vivo optogenetics in transgenic rats. Our results demonstrate that reward-seeking-related dopamine release into limbic, associative, and sensorimotor regions of the striatum are much more heterogeneous in its specific signal features within regions and less coordinated in its occurrence between regions than commonly assumed. Our work answers long-standing questions in the field about whether limbic, associative, and sensorimotor domains communicate with each other via dopamine-centric loop projections, and whether such inter-region coordination is of importance for the automation of action sequences (i.e. habit formation). In contrast to previous assumptions, dopamine signaling in the associated striatum (DMS), but not the sensorimotor striatum (DLS), is at the center of loop circuitry and crucial for the automation of behavior.
In this project, we set out to decipher a hierarchical organization of neural circuits potentially responsible for habit formation by investigating the functional connectivity of basal-ganglia networks involved in the coordination of dopamine signaling. In this context, we made several significant achievements:
1) We identified and demonstrated the existence of a complex neuroanatomical organizational principle in rats that was known to exist in primates only (inter-species confirmation), and, for the first time in any species, we provided functional evidence to verify this organizing principle.
2) We extended and updated this organizational principle by finding bi-directional connectivity instead of only uni-directional information flow through the anatomical structure.
3) We identified dopamine release in the so-called associative aspects of the striatum as at the center of habitual behavioral performance. This striatal region was previously only thought of as important for the control of flexible, goal-directed behavior (the opposite of habitual behavior). This new insight challenges the current beliefs in our field of research and may induce a paradigm shift.
In this project, we set out to decipher a hierarchical organization of neural circuits potentially responsible for habit formation by investigating the functional connectivity of basal-ganglia networks involved in the coordination of dopamine signaling. In this context, we made several significant achievements:
1) We identified and demonstrated the existence of a complex neuroanatomical organizational principle in rats that was known to exist in primates only (inter-species confirmation), and, for the first time in any species, we provided functional evidence to verify this organizing principle.
2) We extended and updated this organizational principle by finding bi-directional connectivity instead of only uni-directional information flow through the anatomical structure.
3) We identified dopamine release in the so-called associative aspects of the striatum as at the center of habitual behavioral performance. This striatal region was previously only thought of as important for the control of flexible, goal-directed behavior (the opposite of habitual behavior). This new insight challenges the current beliefs in our field of research and may induce a paradigm shift.
Our findings advance beyond the state of the art, because we deciphered an organizational neuroanatomical principle of great behavioral relevance. Our experiments revealed, against existing theories, dopamine signaling in the associative striatum as the centerpiece of striatum-midbrain loops and habit formation. Our work introduced several innovative methodologies:
1) A novel behavioral paradigm to test the automation of behavior
We developed a behavioral test that elucidates automation of behavior in a sophisticated and versatile way, thus, extending beyond traditionally employed tests for habitual behavior. Importantly, this test can be administered reliably many times across a time span of weeks (in contrast to traditional habit measures) and, thus, embodies a powerful tool for tracking the developmental state of a slowly forming habit (via repeated testing).
2) Multi-region in-vivo fast-scan cyclic voltammetry (FSCV)
We advanced the development of FSCV for dopamine detection by using it in combination with optogenetic stimulation in transgenic rats to implement both control and detection of dopamine in the same animal. We further innovated this technology by increasing the number of chronically implanted FSCV recording electrodes per rat (three electrodes simultaneously in freely-moving rats), sampling dopamine in three different functional units of the striatum.
3) FSCV for functional brain-circuit mapping
We combined optogenetic stimulation of dopamine release with detection of dopamine release using FSCV to conduct brain-circuit mapping experiments in vivo. Specifically, we used optogenetic stimulation of different inputs to dopamine neurons projecting to associative and sensorimotor striatum, respectively, to test the functional existence of presumed striato-nigral pathways for hierarchical region-specific dopaminergic control in the basal ganglia and also to identify the most effective inputs.
4) Rabies virus-based anatomical brain-circuit in rats
To use rabies virus for brain-circuit mapping in rats, we successfully implemented a combination of rabies virus plus helper AAVs (necessary for rabies virus to replicate and transfect). The use of rabies virus is well-established in mice, but was not for rats as of yet. Rats offer several advantages including a bigger brain size enabling bigger and a greater number of brain implants, as well as a more sophisticated cognitive and behavioral repertoire.
1) A novel behavioral paradigm to test the automation of behavior
We developed a behavioral test that elucidates automation of behavior in a sophisticated and versatile way, thus, extending beyond traditionally employed tests for habitual behavior. Importantly, this test can be administered reliably many times across a time span of weeks (in contrast to traditional habit measures) and, thus, embodies a powerful tool for tracking the developmental state of a slowly forming habit (via repeated testing).
2) Multi-region in-vivo fast-scan cyclic voltammetry (FSCV)
We advanced the development of FSCV for dopamine detection by using it in combination with optogenetic stimulation in transgenic rats to implement both control and detection of dopamine in the same animal. We further innovated this technology by increasing the number of chronically implanted FSCV recording electrodes per rat (three electrodes simultaneously in freely-moving rats), sampling dopamine in three different functional units of the striatum.
3) FSCV for functional brain-circuit mapping
We combined optogenetic stimulation of dopamine release with detection of dopamine release using FSCV to conduct brain-circuit mapping experiments in vivo. Specifically, we used optogenetic stimulation of different inputs to dopamine neurons projecting to associative and sensorimotor striatum, respectively, to test the functional existence of presumed striato-nigral pathways for hierarchical region-specific dopaminergic control in the basal ganglia and also to identify the most effective inputs.
4) Rabies virus-based anatomical brain-circuit in rats
To use rabies virus for brain-circuit mapping in rats, we successfully implemented a combination of rabies virus plus helper AAVs (necessary for rabies virus to replicate and transfect). The use of rabies virus is well-established in mice, but was not for rats as of yet. Rats offer several advantages including a bigger brain size enabling bigger and a greater number of brain implants, as well as a more sophisticated cognitive and behavioral repertoire.