Summary: Recent research by Tan et al "Drugs of abuse hijack a mesolimbic pathway that processes homeostatic need", published at Science (AAAS) has shed light on how commonly abused drugs like cocaine and morphine impact the brain's reward system, which normally responds to natural incentives. This study shows that these drugs exploit the same neurons in the nucleus accumbens (NAc), a key area in the brain for processing reward and pleasure, leading to addictive and compulsive drug-seeking behaviors.
Key Facts:
Activation of Neurons: Both natural rewards (like food and water) and drugs activate neurons in the NAc, but drugs do so more robustly and can overshadow natural rewards
Changes with Repeated Exposure: Neurons in the NAc become increasingly responsive to drugs with repeated use, a pattern that doesn't occur with natural rewards. This suggests that drugs can hijack and modify the brain's reward system over time.
Drug-Specific Effects: Different drugs affect different types of neurons in the NAc. Cocaine primarily activates and amplifies the activity of D1 type neurons, while morphine affects both D1 and D2 types. This distinction could explain the varied addictive and withdrawal effects of different drugs.
Withdrawal Symptoms: Stopping drug use leads to a rebound effect in neuron activity, which can lead to negative feelings or a lack of pleasure from activities that a person normally enjoys. For cocaine, this might mean less response to positive things, while for morphine, it might mean an increased response to negative feelings.
Molecular Changes: The study also looks at how repeated drug use changes certain molecules in these neurons, specifically focusing on a molecule called RHEB, which helps control whether the neuron is more or less active. By influencing RHEB, drugs can make the neurons more sensitive to their effects, further pushing the brain's reward system to favor drug use over natural rewards.
Brain Wiring and Decision Making: The drugs might alter the connections between different parts of the brain, particularly how the orbitofrontal cortex (part of the frontal lobe responsible for decision-making) communicates with the NAc. This could make the brain value drugs more than natural rewards, changing how decisions are made.
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