Neurotransmitters: Criteria and Categories

Criteria for a Molecule to be a Neurotransmitter

• Synthesis: Must be synthesized within the presynaptic neuron.
• Release: Released by the presynaptic neuron in response to stimulation.
• Effect: Must produce the same effect in the postsynaptic neuron when applied by a researcher as when released naturally.

Categories of Neurotransmitters

1. Small Molecule Transmitters

   • Include amino acid neurotransmitters and biogenic amines (monoamines).
   • Synthesized in the cytoplasm and packaged in the presynaptic terminal.
   • Neurons typically synthesize and release one type of small molecule neurotransmitter, but multiple neuropeptides.

2. Peptide Transmitters (Neuropeptides)

   • Synthesized in the cell body and transported to the terminal, leading to slower release.
   • Encoded by genes; mRNA translated to amino acids forming pre-propeptides, processed in the Golgi apparatus.

Small Molecule Neurotransmitters

Amino Acid Neurotransmitters

• Glutamate: Primary excitatory transmitter in the brain; synthesized from glutamine via glutaminase.
• GABA: Primary inhibitory transmitter in the brain; synthesized from glutamate via glutamic acid decarboxylase.
• Glycine: Inhibitory, more common in spinal cord; synthesized from serine via serine hydroxymethyl transferase.

Biogenic Amines (Monoamines)

• Catecholamines
  • Dopamine: Involved in reward and movement; synthesized from tyrosine via tyrosine hydroxylase and dopa-decarboxylase.
  • Norepinephrine: Synthesized from dopamine by dopamine beta-hydroxylase within vesicles.
  • Epinephrine: Considered a hormone; synthesized from norepinephrine via phenylethanolamine N-methyltransferase.
• Serotonin: Role in mood; synthesized from tryptophan via tryptophan hydroxylase and aromatic L-amino acid decarboxylase.
• Histamine: Synthesized from histidine via histidine decarboxylase.

Acetylcholine

• Synthesized from acetyl CoA and choline via choline acetyltransferase.
• Best known for its role at the neuromuscular junction.

Neuropeptide Synthesis and Transport

• Synthesized in the cell body, larger vesicles than small molecule transmitters.
• Pre-propeptide processing in the rough ER, Golgi apparatus processes and packages into vesicles.
• Transported to terminals via fast anterograde axonal transport.

Transport Mechanisms

• Anterograde Transport: Moves materials from the cell body to the terminal.
• Retrograde Transport: Moves materials from the terminal to the cell body.
• Fast Anterograde Transport: Used for transporting packaged neuropeptides to synaptic terminals.

This summary captures the essential information regarding neurotransmitter criteria, categorization, synthesis, and transport mechanisms for small molecule transmitters and neuropeptides.