Control of goal-directed and inflexible actions by dorsal striatal melanocortin systems, in coordination with the central nucleus of the amygdala

Neurology Medicine Basic Medical Sciences Cell Biology Life Sciences
dorsal striatum melanocortin system goal-directed behavior inflexible actions α-msh caudate

This academic paper, authored by researchers Elizabeth C. Heaton, Esther H. Seo, Laura M. Butkovich, Sophie T. Yount, and Shannon L. Gourley, primarily explores the role of the melanocortin system in the dorsal striatum in controlling goal-directed behavior and inflexible behavior, particularly in inhibiting habitual behavior in the dorsolateral striatum. This study was published in the journal Progress in Neurobiology, Issue 238, 2024 (Article No. 102629), and was published online on May 17, 2024.

Research Background

In daily life, people often need to modify familiar behaviors based on new information. For example, when encountering road construction, a driver may need to abandon a familiar route and choose a new one. The dorsomedial striatum (DMS) plays a significant role in this behavioral flexibility. The activity of the DMS is enhanced when familiar behaviors are flexibly updated. Recent studies have shown that the DMS is indispensable in behavioral learning and response strategy adjustment when training laboratory mice to press a lever to obtain food.

In this study, the researchers propose that melanocortin-4 receptors (MC4R) in the dorsal striatum may play an important role in this type of behavioral regulation. Alpha-melanocyte-stimulating hormone (α-MSH) is its high-affinity ligand, typically released by hypothalamic arcuate nucleus neurons in a satiated state and widely distributed throughout the brain. However, despite in-depth studies on the role of MC4R in inhibiting food intake and increasing metabolism, its specific function in the dorsal striatum, particularly in the DMS, remains poorly understood.

Research Objective

The objective of this study is to reveal the role of MC4R in the dorsal striatum, particularly how it affects behavioral flexibility and habitual behavior. The researchers hypothesize that MC4R can functionally regulate the DMS and DLS, respectively inhibiting flexible and habitual behaviors. Additionally, they explore how the interaction between MC4R and the central nucleus of the amygdala (CEA) regulates flexible behavior.

Research Institutions and Publication Information

This study was completed by a research team at Emory University, involving the Neuroscience Graduate Program, Emory National Primate Research Center, Departments of Pediatrics and Psychiatry and Behavioral Sciences, Molecular and Systems Pharmacology Graduate Program, and the Children’s Healthcare of Atlanta. The study is published in the journal Progress in Neurobiology.

Research Process

1. Experimental Subjects and RNA In Situ Hybridization Analysis

The experimental subjects were adult male and female mice (≥56 days postnatal). The researchers employed multiple mouse strains, including some transgenic mice from the Jackson Laboratory, such as MC4R-2A-Cre knock-in mice and homozygous MC4R-flox mice. The mice were kept in a 14-hour light cycle and conducted training under voluntary feeding and drinking conditions, with behavioral training starting when their body weight was reduced to approximately 90% of the baseline.

RNA in situ hybridization (RNA scope) was used to observe the expression of MC4R. MC4R and Drd1 (dopamine D1 receptor) mRNA significantly colocalized in the DMS, indicating that MC4R is primarily expressed on D1 receptor-containing medium spiny neurons (MSNs).

2. Surgery and Viral Vectors

To achieve Chemogenetic manipulation, the researchers injected viral vectors, such as the Cre-dependent Chemogenetic receptor constructs AAV5-HSyn-DIO-hM3Dq-mCherry or AAV5-HSyn-DIO-hM4Di-mCherry, into the mice’s brains, including injection sites such as the DMS, DLS, and Ventral Striatum (VS). Different viral vectors were used for MC4R manipulation, as mentioned above, such as AAV8-CamKII-hi-GFP-Cre-WPRE-SV40 or AAV8-CamKII-EGFP for MC4R gene knockout.

3. Behavioral Testing and Data Analysis

The researchers trained the mice to perform operant tasks at specific nose pokes to obtain food rewards, such as chocolate or purified grain pellets. The design followed a fixed ratio 1 (FR1) schedule to ensure each response received a food reward. Subsequently, an independent response task tested the mice’s behavioral flexibility when one of the response apertures closed while others still provided rewards, observing how the mice responded to changes in action-outcome relationships.

4. Postoperative Observation

The researchers manipulated MC4R+ cells using chemogenetic techniques. They found that inhibiting MC4R+ cells in the DMS led to an inability in mice to flexibly adjust to habitual behavior, while excitation of these cells enhanced behavioral flexibility.

Research Results

  1. Role of MC4R in the DLS: Activation of MC4R+ cells in the DLS led to habitual behavior, while reducing the availability of MC4R inhibited this behavior, making the behavior more flexible.
  2. Behavioral Flexibility Test: In initial and subsequent behavioral tests, mice showed a tendency to revert to previously trained habitual behaviors when faced with changing reward mechanisms. This pattern was particularly evident after long-term training, but silencing MC4R restored the mice’s flexible response to changes in rewards.
  3. Interaction between CEA and DMS: Direct projections from the CEA to the DMS played a crucial role in behavioral flexibility. Particularly, inhibiting local cell activity in the CEA led to stronger behavioral flexibility in mice.

Research Conclusion

MC4R in the dorsal striatum serves as an endogenous brake on behavioral flexibility and habitual behavior. This finding highlights the dual role of MC4R in different brain regions: inhibiting flexible behavior in the DMS while inhibiting habitual behavior in the DLS. This phenomenon helps understand interactions between different brain regions during behavioral strategy transitions.

Research Significance

This study reveals the profound impact of MC4R in the dorsal striatum on behavioral regulation, specifically its balanced regulation mechanism between habit formation and behavioral flexibility. The study enhances the basic understanding of MC4R’s role in the brain and provides new research directions for potential pharmacological interventions. For example, regulating MC4R activity may have potential applications in treating drug addiction and habitual behavior-related disorders (such as obsessive-compulsive disorder).

Other Important Findings

Finally, the researchers noted that the role of MC4R might extend beyond food-seeking behaviors, despite the study focusing mainly on this aspect. Future research can further explore the role of MC4R in different behavioral patterns, such as motivation and reward mechanisms, which will aid in comprehensively understanding the receptor’s multifaceted functions.

In this way, the research team not only deepened the understanding of behavioral regulation but also provided an important foundation for future studies on MC4R.