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Neurotransmitters

Overview of Neurotransmitters and Neurotransmitter Systems

  • Neurotransmitters: Chemical messengers that facilitate communication between neurons.

  • Types of Neurotransmitters:

    • Small-molecule neurotransmitters

    • Neuropeptides

Categories of Small-Molecule Neurotransmitters

  • Amino Acids:

    • Glutamate: Major excitatory neurotransmitter.

    • GABA (Gamma-aminobutyric acid): Primary inhibitory neurotransmitter.

    • Glycine: Inhibitory neurotransmitter in the spinal cord and brainstem.

  • Monoamines:

    • Dopamine: Involved in reward and motor control.

    • Serotonin: Regulates mood, sleep, and anxiety.

    • Norepinephrine: Affects arousal and alertness.

  • Purines:

    • ATP (Adenosine triphosphate): Cellular energy molecule also acts as a neurotransmitter.

  • Gasotransmitters:

    • Nitric Oxide: Function in neuronal signaling.

  • Others:

    • Acetylcholine: Critical for muscle activation and autonomic functions.

    • Anandamide: Involved in pain regulation and appetite.

Neurotransmitter Systems

  • Components:

    • Synthesis: The production of neurotransmitters in neurons.

    • Packaging: Storage of neurotransmitters in synaptic vesicles.

    • Breakdown: Degradation of neurotransmitters by enzymes.

    • Receptors: Proteins on the postsynaptic membrane that interact with neurotransmitters.

Methods of Neurotransmitter Detection

Immunohistochemistry (IHC)

  • Purpose: Detection of neurotransmitters in tissue samples.

  • Techniques:
    a. Inject neurotransmitter candidate.
    b. Withdraw specific antibodies from ear vein.
    c. Use radioactively labeled probe with a complementary nucleic acid sequence.
    d. Apply visible markers to label neurons containing neurotransmitter candidates.
    e. Use microiontophoresis with drug-containing micropipette to localize neurotransmitter effects.

Acetylcholine (ACh)

Synthesis and Breakdown

  • Components:

    • Synthesized from: Acetyl CoA + Choline via Choline acetyltransferase (ChAT).

    • Broken down by: Acetylcholinesterase (AChE) into Choline and Acetic acid.

Cholinergic Synapses

  • Characteristics: Critical for learning and memory.

  • Projections to brain areas:

    • Reticular formation: Regulates circadian cycles.

    • Amygdala, Hippocampus, Cerebral Cortex.

  • Association with Alzheimer's Disease: Widespread loss of ACh is a characteristic of Alzheimer's disease (senile dementia).

Peripheral ACh Distribution

  • Functions in:

    • Neuromuscular Junction: Role in muscle contraction.

    • Autonomic Nervous System:

    • Parasympathetic Division: Constricts pupils, stimulates salivation, slows heartbeat.

    • Sympathetic Division: Dilates pupils, inhibits salivation, accelerates heartbeat.

  • Summary of Responses:

    • ACh effects in various organs, e.g., stimulates digestion in the stomach and peristalsis in intestines.

Pharmacology of Acetylcholine Receptors

  • Types of Receptors:

    • Nicotinic Receptors: Ionotropic receptors that mediate fast synaptic transmission.

    • Mechanism: Two ACh molecules bind to the receptor to open the channel allowing ions to pass (Na+, K+, Ca2+).

    • Muscarinic Receptors: Metabotropic, causing either EPSPs or IPSPs through G-proteins.

Effects of Curare

  • Mechanism: Competitively inhibits ACh from binding to nicotinic receptors.

  • Muscle Function Impact: Results in paralysis and respiratory failure due to inability to initiate muscle contraction.

Glutamate and Its Role

Overview

  • Glutamate: Major excitatory neurotransmitter in central nervous system (CNS).

  • Production: Synthesized from glutamine via glutaminase.

  • Key for learning and memory.

Synaptic Mechanisms

  • Excitatory Amino Acid Transporters (EAAT) clear excess glutamate from the synaptic cleft, recycling it through neurons and glial cells.

  • Key Receptors:

    • Ionotropic: NMDA, AMPA, Kainate—cause excitatory postsynaptic potentials (EPSPs).

    • Metabotropic: Slow acting, higher magnitude EPSPs than ionotropic.

Dopamine Synthesis and Function

Pathway

  • Synthesis: Tyrosine → L-DOPA → Dopamine, facilitated by tyrosine hydroxylase and dopa decarboxylase.

  • Significance: Associated with motor control, reward, reinforcement, and motivation.

Pharmacological Effects

  • MAO inhibitors: Increase dopamine levels—used in treating depression and Parkinson's disease. Potential side effects related to diet must be managed to avoid hypertensive crises.

Reuptake and Breakdown

  • Mechanisms: Dopamine transporters (DAT) regulate reuptake; metabolites like DOPAC and HVA indicate dopamine activity.

Norepinephrine System

Function and Synthesis

  • Locus Coeruleus: Main source of norepinephrine (
    NE) influencing mood, arousal, and memory.

  • Synthesis: Tyrosine → L-DOPA → Dopamine → Norepinephrine, also impacted by stress responses.

Serotonin System

Importance and Synthesis

  • Serotonin (5-HT): Regulates mood, anxiety, and sleep. Mostly found in the gut.

  • Synthesis: Tryptophan → 5-Hydroxytryptophan → Serotonin, affected by enzymes such as tryptophan hydroxylase.

Therapeutic Uses

  • SSRIs (Selective Serotonin Reuptake Inhibitors): Block SERT to elevate serotonin levels and treat mood disorders.

Neuropeptides

Overview and Function

  • Characteristics: Gene-encoded, released with high-frequency action potentials, have modulatory roles in pain, stress, and appetite regulation.

  • Types:

    • Opioids: Natural depressants involved in pain relief, reward systems, and addictive behaviors.

Pharmacological Notes

  • Naloxone: Used to block mu receptors and reverse opioid overdose.

Conclusion

  • Neurotransmitters play essential roles in modulating various physiological and psychological functions, serving as vital targets for therapeutic intervention in numerous neurological and psychiatric conditions.