The Brain - Structures & Functions
Neural circuits and neurotransmitters play a crucial role in substance use disorders (SUDs) and related behaviors. Understanding the underlying mechanisms can help in developing effective treatments. This page provides a brief overview of these concepts to aid in understanding the project's discussion of SUDs and related behaviors.
Original Image Source: https://www.bcheungbiomedicalillustration.com/brain
Original Image Source: https://www.bcheungbiomedicalillustration.com/brain
Image Source: Koob & Volkow, 2009.
Original Image Source: https://www.bcheungbiomedicalillustration.com/brain
NTs are extracellular chemical messengers that facilitate communication between neurons.
Neurotransmitters (NTs)
Neurotransmitters - A (Very) Quick Overview
Dopamine (DA):
Dopamine is known to be involved in the the brain's reward system. It plays a crucial role in motivation, reinforcement, and the experience of pleasure.
Serotonin (5-HT):
Serotonin affects mood regulation, sleep-wake cycles, and emotional processing. It is known to play a role in mood disorders and impulse control.
GABA (Gamma-Aminobutyric Acid):
GABA is the brain's primary inhibitory neurotransmitter, helps regulate neuronal excitability, and generally has calming and relaxing effects.
Glutamate:
Glutamate is the brain's primary excitatory neurotransmitter. It is known to be involved in learning and memory processes, and plays a strong role in modulating neuroplasticity.
Acetylcholine (ACh):
Acetylcholine is involved in various cognitive functions, including attention, memory, and learning.
Norepinephrine (Noradrenaline; NE):
Norepinephrine is involved in the body's stress response and arousal regulation. It affects attention, alertness, and vigilance.
Endogenous Cannabinoids (EC):
A family of molecules including anandamide (AEA) and 2-arachidonoylglycerol (2-AG) responsible for modulating other NT systems. Research points to its involvement in the reward circuitry (Volkow et al., 2017).
Corticotropin-Releasing Factor (CRF):
Involved in the body's stress responses and plays a role in anxiety and mood disorders.
Endorphins:
Endorphins are a family of endogenous opioids that modulate pain perception and generally contribute to feelings of pleasure and well-being. This family consists of three endogenous opioid peptides:
1. β-endorphins: Peptides primarily produced by the pituitary gland and hypothalamus. They are involved in pain modulation, mood regulation, and the body's stress response. Bind to μ-opioid receptors (MOR) in the brain and spinal cord, producing analgesic (pain-relieving) effects and feelings of euphoria.
2. Enkephalins: Small peptides produced in various parts of the brain and spinal cord. Involved in pain modulation and stress response. Enkephalins primarily bind to Δ-opioid receptors but can also bind to μ-opioid receptors. Their actions include pain relief and regulation of mood and emotions.
3. Dynorphins: Peptides produced in several brain and spinal cord regions, including the hypothalamus and striatum. Involved in pain modulation, stress response, and mood regulation. Dynorphins primarily bind to κ-opioid receptors (KOR), although they can also bind to other opioid receptors. Their actions include pain relief, dysphoria (feelings of dissatisfaction, anxiety, or unease), and regulation of mood and emotions.
The Reward Circuit
DA Pathways:
The exact reward mechanisms are complex, and research is still needed to decipher the roles of each opioid receptor subtype in the reward effect (Volkow et al., 2019). The following DA pathways are best understood to play a primary role in reward.
However, a study showed that even DA-deficient mice showed conditioned-place preference for cocaine, showing other neurotransmitters (NTs) also appear to influence reward, although this mechanism is not fully understood (Hnasko et al., 2005; Volkow et al., 2019).
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It is well established that all addictive drugs lead to the activation of DA neurons in the VTA - either directly or indirectly - resulting in the release of DA in the NAc. This pathway of activation is known as the mesolimbic pathway (Volkow et al., 2019).
The Mesolimbic Pathway:
This pathway is known to play a critical role in the rewarding, pleasurable effects associated with food, sexual behavior, and addictive substances (Meyer, et al., 2023).
The Mesocortical Pathway:
This pathway is also important in reinforcing rewarding behaviors. The mesocortical pathway projects from the VTA to the PFC, and is understood to facilitate the assignment of value and regulate self-control (Volkow et al., 2019).