Comprehensive Study Guide: Neuropharmacology and Brain Signaling Systems

Major Categories of Neurotransmitters

• Two Primary Categories:
      * Small Molecule Neurotransmitters (Classical Neurotransmitters): These are primarily modified amino acids. A notable exception is acetylcholine, which is synthesized from the B-vitamin nutrient known as choline.
      * Neuropeptides: These are small protein molecules consisting of sequences of several amino acids. They frequently function as hormones that transmit signals over long distances.
• Taxonomy of Small Molecule Neurotransmitters:
      * Amino Acid Transmitters:
          * Gamma aminobutyric acid (GABA): Inhibitory.
          * Glycine: Inhibitory.
          * Glutamate: Excitatory.
          * Aspartate: Excitatory.
      * Monoamines (Derived from Aromatic Amino Acids):
          * Dopamine: Modulatory.
          * Adrenalin/Noradrenaline: Modulatory.
          * Serotonin: Modulatory.
          * Histamine: Modulatory.
      * Acetlycholine: Excitatory/Modulatory.

Amino Acid Synthesis and Metabolic Pathways

• General Biology of Amino Acids:
      * There are $20$ amino acids occurring in nature.
      * These acids link together to form chained molecules called proteins.
      * Peptide: A small protein composed of only a few amino acids.
      * Large proteins can consist of thousands of chained amino acids.
• Specific Synthesis Chains:
      * Glutamine $\rightarrow$ Glutamate $\rightarrow$ GABA.
      * Glycine $\rightarrow$ Glycine.
      * Tyrosine $\rightarrow$ Dopamine $\rightarrow$ Noradrenalin $\rightarrow$ Adrenalin.
      * Tryptophan $\rightarrow$ Serotonin.
      * Histidine $\rightarrow$ Histamine.
• Glutamate Synthesis:
      * Glutamate is the dominant excitatory neurotransmitter in the brain and is synthesized in many neurons.
      * The enzyme glutamate synthase is responsible for converting glutamine into glutamate.
• GABA Synthesis:
      * Gamma-amino-butyric acid (GABA) is the dominant inhibitory neurotransmitter in the central nervous system (CNS).
      * The enzyme glutamate decarboxylase (GAD) converts glutamate into GABA.
      * Distinction: Both excitatory and inhibitory neurons produce glutamate; the presence and expression of glutamate decarboxylase is what specifically characterizes GABAergic neurons.

Fundamental Principles of Postsynaptic Receptors

• Determinants of Response: The response of the postsynaptic neuron is determined entirely by the properties of the postsynaptic receptor, not the presynaptic neurotransmitter molecule itself.
• Mechanism: Binding to receptors triggers physiological or chemical responses within the neuron.
• Two Major Receptor Categories:
      * Ionotropic Receptors: Also known as ligand-gated ion channels. These are opened directly by neurotransmitter binding to allow the passage of cations ($Na^{+}$, $K^{+}$, $Ca^{++}$) or anions ($Cl^{-}$).
      * Metabotropic Receptors: Also known as G-protein coupled receptors (GPCRs). These activate internal signaling pathways when a neurotransmitter binds to an extracellular domain.

Detailed Mechanism of G-Protein Coupled Receptors (GPCRs)

• Structural Composition: GPCR proteins possess $7$ membrane-spanning segments.
• The Activation Cycle:
      1. At rest (no ligand), G-protein subunits ($\alpha, \beta, \gamma$) are bound to the intracellular side of the GPCR.
      2. An extracellular ligand binds to the receptor, inducing a conformational change.
      3. This change facilitates the exchange of GDP (guanosine diphosphate) for GTP (guanosine triphosphate) on the $\alpha$ subunit of the heterotrimeric complex.
      4. The GTP-bound $G\alpha$ subunit and the released $G\beta\gamma$ dimer dissociate to stimulate downstream effectors.
      5. The cycle terminates when the GTP on $G\alpha$ is hydrolyzed back to GDP, restoring the original receptor state.
• Protein Families and Selectivity:
      * The extracellular structure determines which ligand (e.g., neurotransmitter) will bind.
      * The intracellular structure determines which type of G-protein ($G_{q}, G_{s}, \text{ or } G_{i}$) binds, which subsequently determines the specific signaling pathway activated.

The Glutamatergic and GABAergic Systems

• Glutamate Receptors:
      * Ionotropic (Fast EPSPs): Includes AMPA, NMDA (also permeable to $Ca^{++}$), and Kainate.
      * Metabotropic (Slow EPSPs):
          * Group I: mGluR1, mGluR5.
          * Group II: mGluR2, mGluR3.
          * Group III: mGluR4, mGluR6, mGluR7, mGluR8.
• GABA Receptors:
      * Ionotropic GABA-A: Ligand-gated $Cl^{-}$ channels composed of $5$ subunits categorized into three types ($\alpha, \beta, \gamma$). Their activation produces fast IPSPs (Inhibitory Post-Synaptic Potentials) that hyperpolarize the cell.
      * Ionotropic GABA-C: Also ligand-gated chloride channels.
      * Metabotropic GABA-B: G-protein coupled receptors that activate $K^{+}$ channels to produce slow IPSPs.

Pharmacology of GABA-A Agonists

• Mechanisms: These drugs increase inhibition, thereby depressing nervous system activity.
      * Low Doses: Produce relaxation and relieve anxiety (anxiolytic effects).
      * Medium/Higher Doses: Induce sleep and loss of consciousness (anesthetic effects).
      * Very High Doses: Can lead to toxic effects, including cessation of breathing or heartbeat.
• Common GABA-A Agonists:
      * Benzodiazepines: Includes Valium, Ambien, Xanax, and Versed.
      * Barbiturates.
      * Alcohol (ethanol).
      * Anti-seizure drugs.

Ligand Interactions: Endogenous and Exogenous

• Endogenous Ligands: Chemicals naturally found in the body that bind to specific receptors.
• Exogenous Ligands: External compounds introduced to the body that affect receptors.
      * Agonists: Bind to and activate the receptor, mimicking or enhancing the endogenous ligand's effect.
      * Antagonists: Bind to the receptor and prevent activation by the endogenous ligand.
• Modes of Antagonism:
      * Competitive Binding: The drug attaches directly to the normal ligand binding site, blocking it.
      * Noncompetitive Binding: The drug attaches to a different site on the receptor, preventing activation even if the natural ligand binds.

The Catecholamine Family: Synthesis and Signaling

• Catecholamine Synthesis: Derived from the amino acid tyrosine.
      1. Tyrosine is catalyzed by tyrosine hydroxylase into DOPA.
      2. DOPA is converted by L-amino acid decarboxylase into Dopamine.
      3. Dopamine is converted by dopamine \beta-hydroxylase into Norepinephrine (Noradrenalin).
      4. Norepinephrine is converted by phenylethanolamine-N-methyltransferase into Epinephrine (Adrenalin).
• Adenylyl Cyclase (AC) and cAMP Pathway:
      * Adenylyl cyclases are enzymatic proteins on the intracellular membrane.
      * AC catalyzes the conversion of ATP into cyclic AMP (cAMP) and pyrophosphate (PP).
      * cAMP regulates cAMP-dependent kinases, specifically protein kinase A (PKA).
      * Caffeine inhibits cAMP phosphodiesterase (PDE), the enzyme that deactivates cAMP.
• G-Protein Classification in the Adenylyl Cyclase Pathway:
      * $G_{s}$ type (Stimulatory): Separated $\alpha$ subunit stimulates AC to increase cAMP and PKA activation.
      * $G_{i}$ type (Inhibitory): Separated $\alpha$ subunit inhibits AC, suppressing cAMP and PKA activation.

Dopamine Receptor Families and Downstream Effects

• D1-Like Family (Excitatory):
      * Coupled to $G_{s}$ proteins that excite cAMP production via adenylyl cyclase.
      * cAMP activates Protein Kinase A (PKA).
      * Kinase Function: Adds phosphorous atoms to proteins (phosphorylation), often switching on enzymatic functions.
      * Phosphatase Function: Removes phosphorous atoms (dephosphorylation), often switching off enzymatic functions.
      * Downstream Effects of PKA: Calcium channel activation, potassium channel inhibition, and enhancement of NMDA receptor currents.
• D2-Like Family (Inhibitory):
      * Coupled to $G_{i}$ proteins that inhibit cAMP production.
      * Reduces PKA activation, blocking excitatory effects.
      * $G\beta\gamma$ subunits exert independent inhibitory effects: Opening potassium channels and closing calcium channels.

Dopamine Pathways and Associated Pathologies

• Origin Points: Most dopamine is produced in the ventral tegmental area (VTA) and the substantia nigra (as well as the retina and hypothalamus).
• Major Projections:
      * Mesolimbocortical Projection: Originates in the VTA; projects to the cortex (especially frontal), hippocampus, and nucleus accumbens.
      * Mesostriatal Projection: Originates in the substantia nigra; projects to the caudate nucleus (striatum) and globus pallidus.
• Dopamine-Related Disorders:
      * Parkinson’s Disease: Death of dopamine neurons in the substantia nigra leads to degeneration of the mesostriatal pathway.
      * Schizophrenia: Linked to dysregulation of dopamine inputs to the prefrontal cortex; treated with antipsychotics targeting dopamine receptors.
      * Tourette’s Syndrome: Characterized by uncontrolled motor tics and swearing; involves overactivation of dopamine pathways.
      * Addiction: Many drugs of abuse stimulate the mesolimbocortical dopamine pathway.
      * Major Depression: Growing evidence suggests an inhibition of dopamine signaling.

The Adrenergic System and Gq-Protein Signaling

• Adrenoreceptor Families:
      * $\alpha$ Family (Metabotropic):
          * $\alpha_{1}$ Receptors: Coupled to $G_{q}$ G-proteins.
          * $\alpha_{2}$ Receptors: Coupled to $G_{i}$ G-proteins (inhibits AC, similar to D2).
      * $\beta$ Family ($\beta_{1}, \beta_{2}, \beta_{3}$) (Metabotropic): Coupled to $G_{s}$ proteins (stimulates AC, similar to D1); typically produce excitation.
• $G_{q}$ Signaling Pathway Detailed Mechanism:
      * Ligand binds to $\alpha_{1}$ receptor.
      * Phospholipase C (PLC) is activated by the $G\alpha_{q}$ subunit.
      * PLC acts on membrane lipids (PIP2) to create two second messengers:
          1. Diacyl-glycerol (DAG): Activates protein kinase C (PKC).
          2. IP3: Binds to IP3 receptors (IP3R) on the endoplasmic reticulum (ER), releasing stored $Ca^{2+}$ into the cytoplasm.

Functional Role of Norepinephrine and Adrenoreceptors

• Pharmacology of Adrenoreceptors:
      * $\alpha$-Blockers: Anxiolytic; used for anxiety and PTSD.
      * $\alpha$-Agonists: Decongestants, pupil dilators; can be anxiogenic (anxiety-producing).
      * $\beta$-Blockers: Lower blood pressure (hypertension); used as anxiolytics.
      * $\beta$-Agonists: Used in albuterol inhalers for asthma; side effects include hyperactivity and panic attacks.
• Physiological Effects of Epinephrine/Norepinephrine:
      * Stimulation of $\beta$ receptors leads to increased heart rate, dilation of skeletal muscle blood vessels, and breakdown of glycogen to glucose.
• Anatomical Distribution: Produced in the locus coeruleus ("blue region") and lateral tegmental areas.
• Arousal States:
      * Low NE: Associated with sleep.
      * Medium NE: Associated with waking and vigilance.
      * High NE: Associated with anxiety and distractibility.

Membrane Transporters and Reuptake Inhibition

• Transporters vs. Ion Channels:
      * Ion Channels: Passive; molecules flow through based on external forces.
      * Transporters: Active; use energy (fuel source) to push molecules across the membrane. Examples: Na-K ATPase, vesicle loading transporters, and reuptake transporters.
• Catecholamine Reuptake Inhibitors (e.g., Amphetamine, Ritalin, Cocaine):
      * Competitive Antagonism: Transporter takes up the drug instead of the neurotransmitter.
      * Non-competitive Antagonism: Drug causes the transporter to be internalized into the presynaptic membrane.
      * Reversal: Drug reverses the transporter direction.
          * Vesicular transporters dump transmitter into the cytoplasm.
          * Synaptic transporters dump transmitter into the synapse.
• Methamphetamine Effects: Chronic abuse leads to motor disorders, cognitive impairment, psychosis, accelerated tooth decay ("meth mouth"), skin pathology, and excessive weight loss.

Serotonin (5-HT) and Tryptamines

• Synthesis: Synthesized from the amino acid tryptophan.
• Anatomy: Produced in the raphe nuclei; axons project throughout the brain.
• Receptor Families: There are $7$ families of 5-HT receptors. All are metabotropic except the 5-HT3 receptor, which is a ligand-gated cation channel ($Na^{+}$ and $K^{+}$).
• Pharmacology:
      * Buspirone: 5-HT1A agonist used for anxiety/depression.
      * LSD: Nonselective psychedelic agonist.
      * SSRIs (Prozac/fluoxetine): Selective serotonin reuptake inhibitors used as antidepressants.
      * MDMA (Ecstasy/Molly): Nonselective serotonin reuptake inhibitor; psychedelic.

The Cholinergic System: Acetylcholine

• Synthesis: Combined from choline and acetate (acetyl-CoA) via choline acetyltransferase.
• Anatomy: Released from the basal forebrain (nucleus basalis and medial septum).
• Receptor Subtypes:
      * Nicotinic Receptors: Ionotropic, 5-subunit ligand-gated cation channels. Found at the neuromuscular junction where they activate muscle fibers.
          * Agonist: Nicotine.
          * Antagonist: Curare (paralytic); muscle relaxants.
      * Muscarinic Receptors: Metabotropic, 5 subtypes ($M_{1}-M_{5}$) often linked to voltage-gated potassium channels.
          * Agonist: Muscarine (toxin from poisonous mushrooms).
          * Antagonist: Atropine (anaesthesia/bradycardia treatment); Scopolamine (motion sickness/amnesia).

Endogenous Opiates and Opioid Receptors

• Endogenous Opiates: Peptides including \beta-Endorphin, [Met]enkephalin, [Leu]enkephalin, Metorphamide, Dynorphin A/B, Nociceptin, and Endomorphin-1/2.
• Opioid Receptor Subtypes (All GPCRs):
      * Mu ($\mu$) Receptor: Activation causes analgesia, euphoria, and addiction.
          * Agonists: Endorphins, Morphine, Fentanyl, Oxycodone.
          * Antagonists: Naloxone (acute overdose); Naltrexone (chronic addiction).
      * Kappa ($\kappa$) Receptor: Activation causes analgesia, sedation, and dysphoria.
          * Agonists: Dynorphins.
      * Delta ($\delta$) Receptor: May contribute to analgesia.
          * Agonists: Enkephalins, Codeine, Hydrocodone.

The Opioid Epidemic and Public Health Data

• Prescription Trends: Since $1991$, prescriptions for opiate pain medications like oxycodone and hydrocodone have risen from approximately $76$ million to over $219$ million in $2011$.
• Mortality Statistics:
      * Unintentional drug poisoning deaths for opioid analgesics rose from near zero in $1999$ to over $16,000$ in $2011$.
      * Total U.S. drug overdose deaths reached $107,941$ in $2022$.
      * A $10\%$ decline in deaths was noted between the $12$ months ending in April $2023$ and April $2024$, falling from roughly $115k$ toward $100k$.

Endocannabinoids and Retrograde Signaling

• Nature of Endocannabinoids: Lipid molecules (fatty acids) synthesized in neurons. Major types include Anandamide (AEA) and 2-Arachidonoylglycerol (2-AG).
• Exogenous Ligand: THC (tetrahydrocannabinol) from cannabis.
• Retrograde Transmission: Signals are sent backwards across the synapse from the postsynaptic cell to the presynaptic cell.
• Mechanism: They are not packaged into vesicles (described as paracrines).
• Receptors: CB1 and CB2 (GPCRs). Typically found on presynaptic terminals.
      * Activation of CB1 inhibits calcium influx into the terminal, which inhibits neurotransmitter release from the presynaptic terminal, reducing overall neurotransmission.