Neurotransmitter Systems Overview
A survey of the major neurotransmitter systems, their signaling mechanisms, and their relevance to pharmacological intervention.
## Introduction
Neurotransmitters are endogenous chemical messengers that transmit signals across synapses. Understanding these systems is foundational to neuropharmacology because most CNS drugs act by modifying neurotransmitter synthesis, release, receptor binding, reuptake, or degradation.
## Classification of Neurotransmitters
Neurotransmitters fall into several chemical classes:
- **Amino acids** -- glutamate (excitatory) and GABA (inhibitory) account for the majority of fast synaptic transmission in the CNS
- **Monoamines** -- dopamine, norepinephrine, serotonin, and histamine modulate mood, arousal, reward, and autonomic function
- **Acetylcholine** -- mediates neuromuscular transmission and parasympathetic signaling; central cholinergic pathways regulate attention and memory
- **Neuropeptides** -- endorphins, substance P, neuropeptide Y, and oxytocin act as co-transmitters or neuromodulators
- **Endocannabinoids** -- anandamide and 2-AG signal retrogradely to suppress presynaptic neurotransmitter release
- **Purines** -- ATP and adenosine modulate pain, sleep, and neuroinflammation
## Synaptic Transmission Steps
1. **Synthesis** in the presynaptic terminal or cell body
2. **Vesicular storage** via vesicular transporters (e.g., VMAT2 for monoamines)
3. **Calcium-dependent exocytosis** upon action potential arrival
4. **Receptor binding** at postsynaptic ionotropic or metabotropic receptors
5. **Signal termination** by reuptake transporters (SERT, DAT, NET), enzymatic degradation (MAO, COMT, AChE), or diffusion
## Pharmacological Targets
Most CNS drugs intervene at one or more of these steps. SSRIs block serotonin reuptake. Benzodiazepines enhance GABA-A receptor function. Levodopa increases dopamine synthesis. Each subsequent guide in this series explores individual systems and their drug targets in depth.
## Key Takeaways
- Glutamate and GABA dominate fast excitatory and inhibitory transmission, respectively
- Monoamine systems are targets of antidepressants, antipsychotics, and psychostimulants
- Drugs can act at any step from synthesis to signal termination
- Most CNS disorders involve dysregulation of one or more neurotransmitter systems