Neurotransmitters Pt 4

Overview: G-protein coupled receptors (GPCRs) mediate various intracellular signaling pathways through the activation of primary and secondary messengers which lead to diverse functional consequences in the brain.

Groups:
- Group I (mGluR1, mGluR5)
- G-protein Coupling: Gq
- Primary Messengers: Increased IP3, DAG
- Major Intracellular Effects:
  - Increased calcium (Ca²+) release leading to increased excitability and facilitating long-term potentiation (LTP), which is crucial for learning and memory.
- Group II (mGluR2, mGluR3)
- G-protein Coupling: Gi/o
- Primary Messenger: Decreased cAMP
- Major Intracellular Effects:
  - Inhibits Ca²+ influx and opens K+ channels, leading to presynaptic inhibition and reduced glutamate release.
- Group III (mGluR4, mGluR6-8)
- G-protein Coupling: Gi/o
- Primary Messenger: Decreased cAMP
- Major Intracellular Effects: Similar to Group II, it further reduces transmitter release.

Subtypes: GABAB1, GABAB2
G-protein Coupling: Gi/o
Primary Messengers: Decreased cAMP, GIRK activation
Major Intracellular Effects:
- Inhibits Ca²+ influx and opens K+ channels resulting in slow inhibitory postsynaptic potentials (IPSPs) and strong neuronal inhibition.

D1-like Receptors (D1, D5)
- G-protein Coupling: Gs
- Primary Messenger: Increased cAMP
- Major Effects: Increases protein kinase A (PKA) activity and channel phosphorylation, facilitating movement and reward.
D2-like Receptors (D2, D3, D4)
- G-protein Coupling: Gi/o
- Primary Messengers: Decreased cAMP and GIRK activation
- Major Effects: Causes inhibition and is a target for antipsychotic drugs.
D1-D2 Heterodimer
- G-protein Coupling: Gq/11
- Primary Messengers: Increased IP3, DAG
- Major Effects: Increased Ca²+ release from the endoplasmic reticulum (ER).

Subtypes:
- 5-HT₁ (1A, 1B, 1D)
- G-protein Coupling: Gi/o
- Major Effects: Decreased cAMP, opens K+ channels leading to increased Ca²+ influx.
- 5-HT2 (2A, 2C)
- G-protein Coupling: Gq
- Major Effects: Increased IP3, DAG, leading to increased calcium and excitability.
- 5-HT4, 6, 7
- G-protein Coupling: Gs
- Major Effects: Increased cAMP, enhancing excitability linked to mood and addiction circuits, with anxiety and mood-regulating functions.

Subtypes:
- α1 (Gq): Increased IP3, DAG, Ca²+ release leading to arousal and vigilance.
- α2 (Gi/o): Decreased cAMP, presynaptic inhibition.
- β (β1-3, Gs): Increased cAMP, PKA activation affecting various brain functions.

Subtypes:
- M1, M3, M5 (Gs): Increase cAMP, enhancing Ca²+ channel activity, involved in stress and arousal responses.
- M2, M4 (Gi/o): Decrease cAMP, causing inhibitory effects.

Subtypes:
- H1 (Gq): Increased IP3, DAG, enhances brain excitability.
- H2 (Gs): Increased cAMP, enhances excitability.
- H3, H4 (Gi/o): Decrease cAMP, involved in regulation of transmitter release with immune-related functions.

Subtypes:
- CB1, CB2 (Gi/o): Decrease cAMP, inhibit Ca²+ influx, and open K+ channels, affecting neurotransmitter release.

Subtypes: NK1, NK2, NK3 (Gq)**: Increased IP3, DAG leading to Ca²+ increase and involved in various brain functions.

Subtypes: μ, δ, κ (Gi/o)**: Decrease cAMP, inhibit Ca²+ influx opening K+ channels, leads to analgesia and reward mechanisms.

Learning and Memory: Mediation by LTP and LTD through mGluRs, serotonin, and dopamine signaling pathways.
Arousal and Vigilance: Modulated through adrenergic and histamine receptor activity.
Motor Control and Movement Facilitation: Predominantly via dopaminergic pathways and GABAergic inhibitory circuits.
Regulation of Emotional Responses: Primarily through serotonin and norepinephrine pathways which influence mood regulation and anxiety.

Overview:
- Glutamate is the most abundant excitatory neurotransmitter, constituting about 90% of the excitatory synapses in the brain.

Learning and Memory
- Activates NMDA and AMPA receptors crucial for synaptic plasticity, enabling LTP and LTD.
Developmental Significance
- Guides neuronal migration and synapse formation during brain development and regulates the pruning of unused connections.
Energy Metabolism Regulation
- Closely linked to glucose metabolism; glutamate cycling between neurons and astrocytes aids in regulating energy use in the brain.
Neurovascular Coupling
- Stimulates astrocytes to release signaling molecules that increase local blood flow—important for matching blood flow to neuronal activity.
Involvement in Sensory Processing
- Essential for vision, hearing, and somatosensation by relaying sensory information through cortical and subcortical pathways.
Risk of Excitotoxicity
- Excessive glutamate can lead to neuronal death via calcium overload, which is implicated in various neurological conditions, such as stroke and neurodegenerative diseases.

Corticospinal & Corticobulbar Pathways:
- Originates from motor cortex and innervates spinal cord and brainstem for voluntary motor control.
Corticostriatal Pathways:
- Originates from layer V pyramidal neurons in the cerebral cortex targeting the striatum, integrates sensory-motor control, cognition, and reward-related behavior.

Excitotoxicity in Neurological Disorders
- Overactivation of glutamate signaling pathways can lead to neurodegeneration.
Role in Alzheimer’s Disease (AD):
- Involves excess glutamate leading to NMDA receptor activation, neuroinflammation, and synaptic loss. Memantine, an NMDA antagonist, can mitigate these effects.

Riluzole:
- Used for ALS, it stimulates glutamate uptake and prevents release.
Memantine:
- A low-affinity uncompetitive NMDA receptor antagonist, effective in treating AD.

Question about the involvement of glutamate in ischemic brain injury and the mechanisms that lead to neuronal death.
Question discussing the differences in signaling pathways activated by synaptic versus extrasynaptic NMDA receptors.