4. CNS Pathways/Pharmacology Overview

Course and Foundations Overview

• Course Identification: Pharmacy 441, Integrated Chemical Structure (ICS): Neuropsychiatric Disorders I.

• Module: Foundations.

• Faculty: David Thomas, PhD.

• Academic Term: Fall 2026.

• Lecture Focus: Overview of CNS Pathways and Synaptic Pharmacology.

Epidemiological and Prescribing Overview of CNS Drugs

• Approved CNS Agents: Hundreds of pharmacological agents are currently approved for various CNS disease states.

• Prescribing Prevalence: CNS drugs represent the most commonly prescribed class of pharmaceutical agents.

  • Roughly $20\%$ of all drugs prescribed across healthcare are CNS agents.

  • Approximately $50\%$ of all adults are prescribed at least one CNS drug per year.

• Drugs of Abuse: Pharmacologically classified under CNS agents due to their direct action on central nervous system targets.

Fundamental Neurotransmitter Classes and Synaptic Physiology

• Chemical Synapse Focus: Signaling relying on chemical transmitters crossing the synaptic cleft to interact with specific receptors.

• Chemical Classes of Neurotransmitters:

  • Acetylcholine (ACh): Primary neurotransmitter family regulating central cognitive functions and peripheral motor/autonomic functions.

  • Biogenic Amines:

  • Catecholamines: Dopamine, Norepinephrine, Epinephrine.

  • Indolamines: Serotonin ($\text{5-hydroxytryptamine}$ or $\text{5-HT}$).

  • Amino Acids:

  • $\gamma\text{-aminobutyric acid}$ (GABA): Primary inhibitory neurotransmitter in the CNS.

  • Glutamate: Primary excitatory neurotransmitter in the CNS.

• Excitatory Synaptic Function (EPSP):

  • Excitatory Postsynaptic Potential (EPSP): A localized depolarization of the postsynaptic membrane that brings the neuron closer to the threshold required to fire an action potential.

  • Ionic Mechanism: Neurotransmitter binding to excitatory receptors opens ligand-gated ion channels, allowing an influx of positive ions such as $\text{Na}^+$ and/or $\text{Ca}^{2+}$, which depolarizes or excites the membrane.

  • Key Variables: Membrane potential ($\text{mV}$) measured over time ($\text{ms}$) relative to threshold stimulus.

• Inhibitory Synaptic Function (IPSP):

  • Inhibitory Postsynaptic Potential (IPSP): A localized hyperpolarization of the postsynaptic membrane that moves the membrane potential further from the action potential threshold.

  • Ionic Mechanism: Neurotransmitter binding to inhibitory receptors opens channels selective for $\text{K}^+$ efflux or $\text{Cl}^-$ influx, increasing the threshold needed to generate an action potential.

  • Key Variables: Membrane potential ($\text{mV}$) measured over time ($\text{ms}$) relative to threshold stimulus.

• Excitation/Inhibition (E/I) Balance:

  • Proper CNS function requires continuous balance between excitatory stimulation (EPSPs) and inhibitory regulation (IPSPs).

  • Neuropsychiatric dysfunction directly originates from signaling damage and the disruption of the excitation/inhibition balance across CNS pathways.

Specific Neurotransmitter Systems and Receptor Modalities

• Receptor Diversity and Synaptopathies:

  • Synaptic defects and receptor-level dysfunctions are termed synaptopathies.

  • Receptor diversity represents the most complex aspect of deciphering central neurotransmission signaling.

• Acetylcholine System:

  • Anatomy: Acetylcholine-releasing neurons are widely dispersed throughout the brain.

  • Function: Critical for mediating cognitive functions, memory, and attention.

  • Therapeutic Targets: Key site of therapeutic intervention in Alzheimer's disease and Parkinson's disease.

  • Receptor Modalities:

  • Muscarinic Receptors: G-protein coupled receptors (GPCRs), divided into subtypes $\text{M}_1$, $\text{M}_2$, $\text{M}_3$, $\text{M}_4$, and $\text{M}_5$.

  • Nicotinic Receptors: Ligand-gated ionotropic receptors.

• Norepinephrine System:

  • Anatomy & Regulation: Regulates vast central regions, controlling sleep, arousal, attention, and learning.

  • Clinical Relevance: Implicated in the pathophysiology and treatment of depression and anxiety.

  • Receptor Modalities: Exclusively GPCRs, categorized into $\alpha$ ($\alpha_1$, $\alpha_2$) and $\beta$ ($\beta_1$, $\beta_2$, $\beta_3$) adrenergic receptors.

• Dopamine System:

  • Receptor Modalities: Exclusively GPCRs, categorized into two primary families:

  • $\text{D}_1\text{-like}$ Receptor Family: Comprises $\text{D}_1$ and $\text{D}_5$ receptors; coupled to $\text{G}_s$ to stimulate adenylate cyclase (AC).

  • $\text{D}_2\text{-like}$ Receptor Family: Comprises $\text{D}_2$, $\text{D}_3$, and $\text{D}_4$ receptors; coupled to $\text{G}_i$ to inhibit adenylate cyclase (AC).

  • Clinical Relevance: Target system for Parkinson's disease, depression, anxiety, and psychosis/schizophrenia.

• Serotonin ($\text{5-HT}$) System:

  • Receptor Subtypes & Indications:

  • $\text{5-HT}_1$ Family (e.g., $\text{5-HT}_{1\text{A}}$): Primary target in treatment of depression and anxiety.

  • $\text{5-HT}_2$ Family: Target in depression management and mechanism of action for atypical antipsychotics.

  • $\text{5-HT}_3$ Family: Targeted by antiemetic agents.

  • Signaling Modality: All serotonin receptors belong to the GPCR superfamily, with the sole exception of $\text{5-HT}_3$, which is an ionotropic ligand-gated ion channel.

Glutamatergic and GABAergic Synaptic Dynamics

• Ubiquitous CNS Distribution:

  • Glutamate and GABA are distributed throughout every region of the brain and CNS, serving as universal excitatory/inhibitory neurotransmitters and neuromodulators.

• Glutamatergic Synapses and Glial Regulation:

  • Neurodegeneration & Duration of Signaling: Surrounding glial cells actively regulate neurotransmitter lifetime inside the synaptic cleft to prevent excitotoxicity, controlling whether signals are too strong or too weak.

  • Glutamine-Glutamate Cycle:

  • Presynaptic Glutamate is released into the synaptic cleft.

  • Surrounding glia take up excess synaptic Glutamate.

  • Glial enzyme Glutamine synthetase converts Glutamate into Glutamine.

  • Glutamine is transported out of glia back into the presynaptic terminal.

  • Presynaptic enzyme Glutaminase converts Glutamine back into Glutamate for vesicular storage.

  • Postsynaptic Density (PSD) Localization: Multiple glutamate receptor classes typically coexist on a single postsynaptic density:

  • Metabotropic Glutamate Receptors (mGluR): GPCR-mediated modulatory responses.

  • N-methyl-D-aspartate Receptors (NMDAR): Ionotropic channels permitting influx of $\text{Na}^+$ and $\text{Ca}^{2+}$.

  • $\alpha\text{-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid}$ Receptors (AMPAR): Ionotropic channels permitting rapid $\text{Na}^+$ influx.

• GABAergic Synapses and Mechanisms:

  • Therapeutic Applications: Key target for drugs treating anxiety, depression, and central hyperexcitability states (such as epilepsies).

  • Key Structural & Molecular Components:

  • Presynaptic components: Synaptic vesicle amine transporter (SVA), Voltage-gated calcium channels ($\text{VG-Ca}^{2+}\text{(N)}$), Voltage-gated sodium channels ($\text{VG-Na}^+$), Presynaptic $\text{GABA}_\text{B}$ autoreceptors, Metabotropic glutamate receptor 3 (mGluR3).

  • Clearances & Enzymes: GABA transporter 1 (GAT-1) on presynaptic membrane and glial cells; GABA transaminase (GABA-T) inside glia and mitochondria for degradation.

  • Postsynaptic components: Postsynaptic density (PSD), Postsynaptic cell soma, Voltage-gated calcium channels ($\text{VG-Ca}^{2+}\text{(T)}$).

  • Receptor Class Differences:

  • $\text{GABA}_\text{A}$ Receptors: Ligand-gated ion channels located at the postsynaptic density. Binding drives rapid changes in membrane potential, inducing phasic, fast IPSPs.

  • $\text{GABA}_\text{B}$ Receptors: GPCRs located presynaptically or extrasynaptically. Mediates tonic, slow IPSPs by promoting $\text{K}^+$ channel opening and ion flux.

Key Processes of Synaptic Pharmacology

• Ten Primary Sites of Synaptic Pharmacological Action:

  1. Action potential propagation down the presynaptic axon.

  2. Neurotransmitter synthesis inside the presynaptic neuron.

  3. Neurotransmitter storage into synaptic vesicles.

  4. Metabolic processing of precursors or neurotransmitters.

  5. Release of neurotransmitter into the synaptic cleft via vesicular exocytosis.

  6. Re-uptake of neurotransmitter via presynaptic or glial transporter proteins.

  7. Enzymatic degradation of neurotransmitter in the cleft or cytoplasm.

  8. Neurotransmitter receptor binding on postsynaptic or presynaptic membranes.

  9. Intracellular signaling responses induced post-receptor binding.

  10. Retrograde signaling from the postsynaptic neuron back to the presynaptic terminal.

Pharmacological Strategies for Synaptic Dysfunction and Hyperexcitability

• Rational Drug Design Principle: Target specific molecular sites along the chemical synapse to correct underlying synaptic dysfunction (synaptopathies).

• Hyperexcitability Model (e.g., Epilepsy):

  • Glutamate Neuron Interventions (Targeting Excessive Presynaptic Glutamate Release):

  • Block voltage-gated sodium channels ($\text{Na}_\text{v}$).

  • Block voltage-gated calcium channels ($\text{Ca}_\text{v}$).

  • Stimulate voltage-gated potassium channels ($\text{K}_\text{v}$).

  • Block vesicular glutamate packaging into presynaptic vesicles.

  • Target Postsynaptic Neuron Interventions (Targeting Excessive Excitatory Postsynaptic Response):

  • Block ionotropic glutamate receptors (NMDAR / AMPAR).

  • Block metabotropic glutamate receptors (mGluR).

  • Block voltage-gated calcium channels ($\text{Ca}_\text{v}$).

  • Stimulate postsynaptic inhibitory GABA receptors.

  • GABA Neuron Interventions (Targeting Deficient Inhibitory Tone):

  • Block GABA re-uptake channels (e.g., GAT-1 inhibitor).

  • Block GABA metabolic degradation enzymes (e.g., GABA-T inhibitor).

Core Learning Objectives and Summary Criteria

• Major Neurotransmitters: Identify all major neurotransmitter classes and individual molecules operating in the CNS.

• Receptor Modalities: Master major neurotransmitter receptors at CNS synapses along with their precise signaling mechanisms (GPCRs vs. ionotropic channels).

• Monoaminergic Systems: Demonstrate appreciation for the extensive anatomical distribution and regulatory footprint of monoaminergic synapses in the brain.

• Excitatory and Inhibitory Balance: Articulate the critical roles of Glutamate (primary excitation) and GABA (primary inhibition) in neuronal circuit regulation.

• Synaptic Pharmacotherapy: Define and locate key molecular sites of drug action across chemical synapses to design targeted therapeutic approaches.