Neurotransmitters in Psychology

Neurotransmitters

• Neurotransmitters are endogenous substances that exist in presynaptic axon terminals.
• These substances are synthesized in presynaptic terminals.
• Neurotransmitters are released when action potentials reach the terminal.
• They're recognized by specific receptors on the postsynaptic membrane.
• Application of neurotransmitters alters the postsynaptic cell.
• Blocking their release prevents presynaptic activity.

Types of Neurotransmitters

• Small molecule neurotransmitters: Amines (e.g., dopamine, norepinephrine, serotonin), amino acids (e.g., glutamate, GABA, glycine), acetylcholine.
• Neuropeptides (e.g., endorphins, enkephalins, substance P).
• Gases (e.g., nitric oxide).

Neurotransmitter Synthesis

• Enzymes within the presynaptic terminal synthesize neurotransmitters.
• Rate limiting steps (often an enzyme) control the rate of synthesis.

Neurotransmitter Storage

• Small molecules are stored in synaptic vesicles.
• Large molecules (peptides) are created in the soma and transported to terminals via microtubules (axonal transport).

Neurotransmitter Release

• Action potentials trigger the opening of voltage-gated calcium channels.
• Calcium influx causes synaptic vesicles to fuse with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft.

Neurotransmitter Receptors

• Multiple types of receptors exist for each neurotransmitter.
• Different receptors can determine if the neurotransmitter is excitatory or inhibitory.
• Receptors can be ionotropic (e.g., AMPA, NMDA, GABA-A) or metabotropic (e.g., muscarinic, dopamine, serotonin).

Neurotransmitter Removal

• Neurotransmitters are cleared from the synapse through:
  • Reuptake by transporters
  • Degradation by enzymes
  • Diffusion away from the synapse.

Specific Neurotransmitters

Dopamine (DA)

• Involved in motor control, reward, reinforcement, and learning.
• Loss of dopamine neurons is linked to Parkinson's disease.
• Dopamine pathways include mesostriatal and mesolimbocortical pathways.
• Related to reward-seeking behaviors (e.g., drug use, food, sex).
• Dopamine receptors are metabotropic, with different subtypes (e.g., D1, D2) having varying effects.
• Dopamine removal involves dopamine transporters and monoamine oxidase.

Norepinephrine (NE)

• Released from the locus coeruleus (pons and lateral tegmental system).
• Plays a role in arousal, vigilance, attention, and mood.
• Used as a hormone in the blood to increase heart rate and blood vessel constriction.
• Released predominantly from axonal varicosities allowing for widespread effects.
• Norepinephrine receptors are metabotropic (e.g., α1, α2, β1, β2).

Serotonin (5-HT)

• Located in the raphe nuclei (brainstem)
• Plays a role in sleep-wake cycles, mood, appetite, and pain.
• Involved in sexual behavior and regulation of nausea and vomiting
• Has 14 different receptors
• Removal by reuptake.

Acetylcholine(ACh)

• First recognized neurotransmitter
• Crucial for skeletal muscle contraction (neuromuscular junction).
• Important in the autonomic nervous system (parasympathetic and sympathetic).
• Involved in memory and learning.
• Nicotinic and muscarinic are two important types of acetylcholine receptor
• Removed in the cleft rapidly (50 % of choline recycling in synaptic cleft).

Gamma-aminobutyric acid (GABA)

• Primary inhibitory neurotransmitter in the central nervous system.
• Influencing neuronal excitability.
• GABA receptors: GABA_A, GABA_B, GABA_C, vary in their signaling mechanisms and impact
• Removal from synapse by transporters (or through degradation by GABA-aminotransferase).

Glycine

• Inhibitory neurotransmitter that controls chloride channels.
• Involved in rapid and long-lasting inhibitory effects often with GABA.

Nitric oxide (NO)

• Diffuses out of the cell and into other neurons.
• Involved in retrograde signaling (in which the postsynaptic neuron signals back to the presynaptic).
• Important in synaptic plasticity.
• Vasodilator.

Neuropeptides

• Short chains of amino acids.
• They have diverse roles, from pain relief to social bonding.
• Often involved in modulatory roles rather than direct excitation or inhibition.
• Includes endorphins and other related peptides involved in stress response and pleasure/pain

Other Relevant Information

• Multiple types of neurotransmitters can interact with several types of receptors.
• Co-localization, coexistence and co-release of neurotransmitters in the same neuron is possible.
• Some neurotransmitters act as hormones as well.
• Many drugs target neurotransmitters and their receptors.