Neurotransmitters and Receptors

Neurotransmitter Basics - Tiny Messenger Traffic

• Definition: Endogenous chemicals transmitting signals across a synapse from one neuron (presynaptic) to another neuron, muscle, or gland cell (postsynaptic).
• Classification:
  • Small Molecules:
    • Amino Acids: Glutamate (major excitatory), GABA (major inhibitory), Glycine.
    • Biogenic Amines (Monoamines): Dopamine, Norepinephrine, Epinephrine, Serotonin, Histamine.
    • Acetylcholine (ACh).
  • Neuropeptides: Endorphins, Enkephalins, Substance P.
  • Others: Nitric Oxide (NO - gasotransmitter), Anandamide (endocannabinoid).
• Lifecycle (General Steps):
  • Synthesis: From precursors, often enzyme-dependent.
  • Storage: In synaptic vesicles.
  • Release: Action potential triggers $Ca^{2+}$ influx, leading to vesicle fusion and exocytosis into synaptic cleft.
  • Receptor Binding: To specific postsynaptic receptors (ionotropic or metabotropic) or presynaptic autoreceptors.
  • Termination: Signal ends via reuptake (transporters), enzymatic degradation, or diffusion.

⭐ Most psychotropic medications exert their effects by modulating one or more stages of neurotransmitter lifecycle (synthesis, storage, release, receptor interaction, or termination).

Glutamate & GABA - Brain's Gas & Brakes

• Glutamate (Excitatory - "Gas")
  • Primary excitatory neurotransmitter in CNS.
  • Receptors:
    • Ionotropic: NMDA ($Ca^{2+}$ influx; learning, memory; $Mg^{2+}$ block), AMPA (fast excitation), Kainate.
    • Metabotropic: mGluRs.
  • Functions: Synaptic plasticity, excitotoxicity (↑).
  • Clinical: Ketamine (NMDA antagonist), Memantine.
• GABA (Inhibitory - "Brakes")
  • Primary inhibitory neurotransmitter in CNS. (Synthesized from glutamate; needs Vit B6).
  • Receptors:
    • GABA-A (Ionotropic): $Cl^{-}$ channel. Site for benzodiazepines, barbiturates, alcohol.
    • GABA-B (Metabotropic): ↑ $K^{+}$ efflux / ↓ $Ca^{2+}$ influx. Baclofen agonist.
  • Functions: Reduces anxiety, neuronal excitability.
  • Clinical: ↓ GABA linked to anxiety, seizures.

  ⭐ Benzodiazepines increase the frequency of GABA-A chloride channel opening; Barbiturates increase the duration.

Monoamine Mania - Mood Modulators

• Key: Dopamine (DA), Serotonin (5-HT), Norepinephrine (NE). Imbalances link to psychiatric disorders.

• Dopamine (DA)

  • Pathways & Roles:
    • Mesolimbic: Reward, psychosis (↑DA).
    • Mesocortical: Cognition, negative symptoms (↓DA).
    • Nigrostriatal: Motor (↓DA in Parkinson's).
    • Tuberoinfundibular: Prolactin inhibition.
  • Receptors: D1-like (Gs), D2-like (Gi).
  • 📌 DAWN: Drugs/Reward, Attention, Wellbeing, Normal movement.

• Serotonin (5-HT)

  • Origin: Raphe Nuclei. Regulates mood, sleep, anxiety.
  • Receptors:
    • 5-HT1A (Gi): Antidepressant/anxiolytic.
    • 5-HT2A (Gq): Atypical antipsychotics target.
    • 5-HT3 (Ion channel): Nausea.
  • Clinical: ↓5-HT in depression/anxiety. SSRIs ↑5-HT.

• Norepinephrine (NE)

  • Origin: Locus Coeruleus. Alertness, energy, mood.
  • Receptors: α (α1-Gq, α2-Gi), β (Gs).
  • Clinical: Implicated in mood/anxiety. SNRIs/TCAs act on NE.

⭐ Most antipsychotics achieve their effect primarily through D2 receptor blockade.

Receptors & Others - Lock, Key, Action!

• Receptor Superfamilies:
  • Ionotropic (Ligand-gated ion channels): Rapid, direct ion flow. E.g., Nicotinic AChR, GABA-A, 5-HT3.
  • Metabotropic (G-protein coupled receptors - GPCRs): Slower, indirect, via 2nd messengers (cAMP, IP3/DAG). E.g., Muscarinic AChR, most 5-HT, DA, Adrenergic Rs.
• Acetylcholine (ACh):
  • Key in CNS: Memory, learning, arousal. PNS: Muscle contraction.
  • Receptors: Nicotinic (ionotropic), Muscarinic (M1-M5, metabotropic).
  • ↓ ACh linked to Alzheimer's disease.
• Histamine:
  • From Histidine. Receptors: H1-H4 (all GPCRs).
  • CNS: Promotes wakefulness, regulates appetite.
  • 📌 H1 receptor antagonists cause sedation.
• Other Signaling:
  • Nitric Oxide (NO): Gaseous, retrograde messenger.
  • Endocannabinoids: Retrograde, modulate neurotransmission.

⭐ Metabotropic receptors (GPCRs) constitute the largest family of cell surface receptors and are the target for ~30-50% of all modern medicinal drugs.

High‑Yield Points - ⚡ Biggest Takeaways

• Dopamine pathways are key: mesolimbic (psychosis), mesocortical (negative symptoms), nigrostriatal (EPS), tuberoinfundibular (prolactin).
• Serotonin (5-HT) dysregulation links to depression, anxiety, OCD. SSRIs are primary treatment.
• Norepinephrine (NE) impacts mood, alertness, anxiety. SNRIs target NE and 5-HT.
• GABA is the main inhibitory neurotransmitter; its enhancement (e.g., benzodiazepines) reduces anxiety.
• Glutamate is the main excitatory neurotransmitter; NMDA receptors are novel antidepressant targets.
• Drug action depends on receptor profiles: D2 blockade (antipsychotics), 5-HT reuptake inhibition (antidepressants).