exam 2 Flashcards
Discovery of Acetylcholine
Identified by Otto Loewi (1920)
Loewi referred to it as “Vagusstoff”
In a dream, designed a frog heart experiment confirming chemical transmission at synapses
First clear demonstration of neurotransmission
ACh synthesis (the process of building complex molecules or compounds from simpler ones)
Choline (Your brain and nervous system need it to regulate memory, mood, muscle control, and other functions) + Acetyl-CoA (metabolite) → via Choline Acetyltransferase (ChAT)
ChAT is found only in cholinergic neurons (a nerve cell that primarily uses acetylcholine (ACh) as its neurotransmitter to send signals)
Rate of synthesis depends on:
Precursor availability (choline from diet)
Rate of neuronal firing
Synthesized in the mitochondria and cytoplasm
ACh storage
Packaged into vesicles by VAChT (Vesicular ACh Transporter)
Vesamicol (drug)blocks VAChT → reduced ACh in vesicles → decreased release
ACh Release
ACh released upon calcium influx from synaptic vesicles
Toxins affecting ACh release:
Black widow spider venom (α-latrotoxin):
Causes massive ACh release
Symptoms: muscle pain, tremors, nausea, sweating
Mechanism:
Interacts with neurexins → α-LTX inserted → Ca²⁺ influx
Activates latrophilin → Gq pathway → Ca²⁺ release
Botulinum toxin:
Blocks ACh release at neuromuscular junction
Causes muscle paralysis
Most potent toxin known, but also used medically (e.g., Botox)
ACh inactivation
AChE (Acetylcholinesterase) breaks ACh into:
Choline (recycled via transporter)
Acetic acid
Forms of AChE
G4 form: in cholinergic neurons and post-synaptic targets
A12 form: in neuromuscular junction, attached to extracellular matrix via collagen tail
Choline transporters: return ~50% of choline for new ACh synthesis
Not ACh-specific; transport choline only
Anticholinesterases (AChE Inhibitors)
Reversible: reversible inhibitors forming non-covalent or easily hydrolyzed bonds
Used to treat Myasthenia Gravis (autoimmune disease)
Antibodies attack ACh receptors at NMJ
Inhibitors increase ACh at synapse
Do not cross the blood-brain barrier
Irreversible: form permanent covalent bonds, leading to prolonged effects
Organophosphates: weak inhibitors used in insecticides (chemicals used to control insects by killing them or preventing)
Nerve gases (Sarin, VX, Novichok):
Cause excessive ACh → overstimulation → paralysis and asphyxiation
Gulf War Syndrome:
Soldiers took pyridostigmine bromide (PB) as a protective measure
Intended to block AChE and shield against Sarin
Stressed mice studies show PB crosses BBB and increases AChE inhibition
VA states no conclusive evidence of PB as cause
Organization & Function of the Cholinergic System
ACh is found in:
NMJ
CNS (cortex, hippocampus, striatum)
ANS: a part of the peripheral nervous system that controls involuntary functions like heart rate, breathing, and digestion (parasympathetic and sympathetic divisions)
CNS Cholinergic Nuclei
Striatum:
ACh-DA balance regulates movement
Anticholinergics used in Parkinson’s disease
Basal Forebrain Cholinergic System (BFCS):
Projects to hippocampus, cortex → memory and attention
Degeneration associated with Alzheimer’s
Dorsolateral pons:
Arousal, REM sleep, sensory processing
Part of the reticular activating system
Cholinergic Receptors
Nicotinic Receptors (nAChRs)
Ionotropic (ligand-gated)
Composed of 5 subunits (α, β, γ, δ, ε); 17 subunits exist
Found at:
NMJ
Autonomic ganglia
CNS
3 functional states:
Open
Closed
Desensitized (prolonged agonist exposure)
Depolarization block: overactivation leads to loss of resting potential
Clinical use:
Succinylcholine: nAChR agonist used for muscle relaxation during surgery
Varenicline (Chantix): partial agonist for smoking cessation
α7 agonists: improve cognition; potential use in Alzheimer’s
Muscarinic Receptors (mAChRs)
Metabotropic (G-protein coupled)
5 types:
M1, M3, M5: Gq ➔ activate PLC
M2, M4: Gi ➔ inhibit adenylyl cyclase
Found in:
Brain (widespread)
Postganglionic parasympathetic targets
M5 subtype:
Enhances dopamine in nucleus accumbens
M5 KO mice → decreased drug reward (e.g., morphine, cocaine)
Potential use: M5 antagonists for addiction treatment
Clinical Relevance of Cholinergic Receptors
Schizophrenia Treatment:
KarXT (xanomeline + trospium)
Muscarinic agonist and antagonist combo
Acts centrally, avoiding peripheral side effects
Does not rely on dopamine/serotonin systems
Glutamate Synthesis, Release & Inactivation
Glutamate = ionized form of glutamic acid
Found in all neurons and glia, but glutamatergic neurons use it as a transmitter
Must be segregated from metabolic glutamate
Synthesis & Transport
Loaded into vesicles by VGLUT1-3 (vesicular glutamate transporters)
Only in glutamatergic neurons (specific marker)
glutamate inactivation
Inactivation
EAATs (1–5) remove glutamate from synapse (neurons + astrocytes)
EAAT2: in astrocytes, handles ~90% of uptake
EAAT1: cerebellar glia
EAAT4: Purkinje cells
EAAT5: retina
EAAT3: post-synaptic buffering, synaptic plasticity
EAAT2 Details
KO mice: seizures, shortened lifespan
ALS: EAAT2 downregulation may cause excitotoxicity
Riluzole (NMDA antagonist) approved for ALS
Experimental drugs upregulate EAAT2 ➔ improve motor function/lifespan
Glutamate Recycling: Metabolic Partnership
Glutamate → Glutamine (in astrocytes via glutamine synthetase) → Back to neurons via glutamine transporters
Safe storage of excess glutamate
Glutamatergic System Organization & Function
Two Families of Glutamate receptors:
▪ Ionotropic—fast signaling
Ionotropic: AMPA, Kainate, NMDA
All are tetramers (4 subunits)
▪ Metabotropic—second
messengers»> slower signaling
Metabotropic (mGluRs): mGluR1-8
mGluR1 & 5 (Gq): ↑ PLC, post-synaptic
mGluR2,3,4,6,7,8 (Gi): ↓ AC, mostly presynaptic (autoreceptors)
NMDA Receptor Specifics:
Requires glutamate + glycine or D-serine (co-agonists)
Requires depolarization to remove Mg2+ block
Coincidence detector
Serine racemase: produces D-serine (in astrocytes)
Clinical Connection: Schizophrenia
NMDA hypofunction hypothesis:
PCP & ketamine ➔ induce or worsen schizophrenia-like symptoms
NMDA receptor deficits in schizophrenia patients
NMDA-KO mice: show similar behaviors
Learning & Memory: LTP, NMDA, AMPA
LTP (Long-Term Potentiation) = synaptic strengthening via Ca2+ influx through NMDA receptors (Long-Term Potentiation)
Activates CaMKII ➔ phosphorylates & recruits more AMPA receptors
Doogie Mouse: genetically engineered mice with enhanced learning and memory abilities
Overexpresses NR2B NMDA subunit
Enhanced memory, LTP, and pain sensitivity
Morris Water Maze & Object Recognition used in learning/memory testing
Excitotoxicity
Excess glutamate ➔ prolonged depolarization ➔ Ca2+ influx ➔ cell death
MSG: damages hypothalamic neurons
Stroke/TBI: glutamate release + NMDA overactivation
Necrosis from membrane degradation
GABA Synthesis, Release & Inactivation
Found only in CNS, only in GABAergic neurons
Synthesized from glutamate via glutamic acid decarboxylase (GAD)
Vesicular transport via VGAT
Reuptake Transporters:
GAT-1, GAT-2: neurons + astrocytes
GAT-3: astrocytes only
Metabolism
GABA-T (GABA aminotransferase): converts GABA → glutamate + succinate
In astrocytes: glutamate → glutamine → sent back to neuron
GABA Co-Transmission
Some neurons release GABA + another NT:
ACh neurons (BF): store ACh & GABA separately
DA neurons: co-release GABA via VMAT2
Some entopeduncular neurons: co-release GABA + glutamate (VGLUT2 + VGAT)
GABA Receptors
GABAA (Ionotropic)
Cl– influx causes hyperpolarization
Pentameric receptor (20+ subunits = heterogeneity)
Drugs:
BDZs (e.g., Valium): ➔ increase GABA potency (PAMs)
Barbiturates: high dose = direct agonist (lethal)
Z-drugs (e.g., Ambien): chemically different but act at same site
GABAB (Metabotropic)
GPCR; requires two subunits
Autoreceptors: inhibit Ca2+ channels, reduce NT release
Postsynaptic: ↓ cAMP or ↑ K+ efflux
Drugs:
Baclofen: muscle relaxant, trialed for ASD
GHB (Xyrem): used for narcolepsy
Gabapentinoids (e.g., gabapentin, pregabalin)
GABA analogues, but don’t act on GABA receptors
Block Ca2+ channels (VGCCs)
Used in epilepsy, pain, GAD, bipolar
Drug Use Sta; ts & Overdose Trends
war on drugs
Over 1 million overdose deaths since 2000
From June 2023 to June 2024: 14.5% decrease in overdose deaths (CDC)
The War on Drugs
Controlled Substances Act (1970): categorized drugs, increased penalties, Oregon Measure 110: example of decriminalization
DSM-5 Substance Use Disorders
9 substance classes:
Alcohol
Caffeine
Cannabis
Hallucinogens
Inhalants
Opioids
Sedatives/hypnotics/anxiolytics
Stimulants
Tobacco
gambling
Modern Conceptions of Addiction
Addiction = chronic, relapsing disorder (Koob & Volkow, 2016)
Compulsion to use
Loss of control
Negative emotional state when denied access
Key contributors to harm:
Overdose potential (margin of safety)
Long-term physical harm (e.g., HIV, lung cancer)
Cognitive/motor impairment
Dependence potential (capture ratio: Tobacco 33%, Heroin 23%, Cocaine 17%)
Social impact (violence, medical costs, crime)
why Do Some People Get Addicted?
Paradox: addiction persists despite harm
Route of administration matters:
IV/inhalation = rapid onset → higher risk
Oral = slower absorption
Reinforcement, Reward, and Craving
Positive reinforcement: euphoric effect strengthens drug-taking behavior
Craving: strong urge for drug
Withdrawal → Negative reinforcement: removes aversive symptoms
Example: “dope sick” in opioid users
Development of Addiction (Koob & Le Moal)
Impulsive stage:
Goal = positive reinforcement (euphoria)
Behavior = goal-directed
Compulsive stage:
Goal = negative reinforcement (relief from withdrawal)
Behavior = habit-like
Conditioned withdrawal: can be triggered by drug-paired environments even years later
fMRI: drug cues activate ventral striatum/medial PFC in meth users
Disease Model of Addiction
Addiction = brain disease due to long-term changes in structure/function
Key advocates: Benjamin Rush, Magnus Huss, E.M. Jellinek
DSM-5: maladaptive pattern for 12+ months, 2+ symptoms, severity graded
Criticisms:
No definitive lab test
Vietnam War: environment change led to quitting
Personal agency: “disease made me do it”
Rat Park study: isolated rats use more drugs than socially enriched rats
What is the criteria to determine how harmful a drug is?
Lethality
▪ How common is overdose?»_space;>Margin of safety
2. Long-term health effects
* HIV, hepatitis, infection
* Lung cancer
3. Impaired brain function
▪ ability to perform cognitive or motor tasks
4. Dependence liability
▪ How likely is the user to become addicted: capture ratio
▪ Tobacco: 33%; Heroin: 23%; Cocaine: 17%; Alcohol: 15%
5. Harm to the family and community
* domestic violence, theft, medical costs
Animal Models of Addiction
CPP (Conditioned Place Preference)
ICSS (Intracranial self-stimulation): threshold drops with acute use, rises with withdrawal
Self-Administration:
Fixed Ratio schedule: inverted U-shaped curve
Higher dose = fewer infusions (satiety, aversion, side effects)
Breaking point: highest response ratio tolerated
Relapse triggers: stress, priming dose, or environmental cues
Neurobiology of Addiction
Mesolimbic DA Pathway (VTA → NAcc): core of reward system
Not all drugs depend on DA (e.g., alcohol, opioids)
Dopamine Hypotheses:
Pleasure transmitter (hedonic)
Rejected: APTD doesn’t affect euphoria
Incentive Salience (“wanting”)
DA = craving, not liking
Sensitization: more craving over time, no increase in pleasure
Reward Prediction Error
DA encodes if outcome is better/worse than expected
Neuroadaptations:
Within-system: downregulation in reward circuit
Between-system: anti-reward system (stress-related systems)
ΔFosB: transcription factor; epigenetically increases drug sensitivity
Low D2 receptor levels: linked to poor impulse control, PFC dysfunction
Treatment & Rehabilitation
Criticisms of rehab industry: profit-driven, unregulated, sometimes ineffective
MAT (Medication-Assisted Treatment): stigmatized despite evidence
Neurostimulation:
TMS of medial PFC shows promise
Stroke patients with certain brain damage quit smoking effortlessly
Suggests addiction is circuit-based
The Opioid Epidemic – Public Health & Social Context
Headline case (HealthRight 360): Patient found unresponsive in treatment bed; death ruled fentanyl intoxication
U.S. opioid overdose deaths remain high: major public health crisis
Shift in overdose drugs: From prescription opioids → heroin → fentanyl
Epidemiology
2021: >100,000 drug overdose deaths in U.S.; 75% involved opioids
Sources of misuse: friends/family prescriptions, dealer/street sources, less so from direct physician prescribing
Fentanyl prevalence: synthetic opioids dominate mortality stats
Political & Legal Landscape
War on Drugs: criminalization of use, mandatory minimums → racial disparities in sentencing (e.g., crack vs. powder cocaine)
Shift to harm reduction: Oregon Measure 110 decriminalized small possession; redirected funds to treatment
Needle exchanges, safe consumption sites, and overdose prevention education increasingly considered
Opioid Pharmacology – Molecular & Cellular Overview
Opioid Classes
Opiates: natural (morphine, codeine)
Semi-synthetic: heroin, oxycodone
Synthetic: fentanyl, methadone
Endogenous: endorphins, enkephalins, dynorphins
Opioid Receptors
G-protein-coupled receptors (GPCRs)
Subtypes:
μ (mu): analgesia, euphoria, respiratory depression, dependence
κ (kappa): dysphoria, spinal analgesia
δ (delta): modulates mood
NOP-R (nociceptin): anti-analgesic, mood
Mechanism of Action
Gi/o-coupled:
↓ adenylyl cyclase → ↓ cAMP
↑ K⁺ channel opening → hyperpolarization
↓ Ca²⁺ influx → ↓ neurotransmitter release
Neurophysiology of Opioid Action
Three main modes of neuronal inhibition:
Postsynaptic (K⁺ efflux)
Axoaxonic (Ca²⁺ channel inhibition)
Presynaptic autoreceptor modulation
Brain regions affected:
Pain: PAG, spinal cord, raphe, thalamus
Reward: VTA → Nucleus Accumbens (DA disinhibition)
Loperamide: μ-opioid agonist, doesn’t cross BBB (P-glycoprotein efflux)
Drug Use, Reinforcement & Tolerance
Phases of Use:
Rush: Immediate euphoria (IV heroin)
High: General well-being
Nod: Sedated, detached state
Being Straight: Baseline, no withdrawal
Tolerance & Dependence:
Chronic use → receptor desensitization, compensatory upregulation of cAMP
Withdrawal symptoms = rebound hyperactivity (diarrhea, vomiting, chills, etc.)
Noradrenergic hyperactivity (locus coeruleus) → managed with clonidine (α2-agonist)
Neuroadaptation:
Reward circuitry downregulation
ΔFosB accumulation in NAcc → sensitization to drugs
Low D2 receptor levels = high vulnerability to compulsive use
Overdose & Emergency Response
Fentanyl: up to 50x stronger than heroin; lethal at microgram doses
Naloxone (Narcan): opioid antagonist, reverses overdose
Delivered intranasally, IM, or IV
Widely distributed as public health intervention
Treatment: Medication-Assisted Therapy (MAT)
Methadone:
Full agonist at μOR
Oral dosing, long half-life
Requires daily administration at clinics
Buprenorphine:
Partial μOR agonist, κ antagonist
Ceiling effect on respiratory depression
Sublingual dosing 1–3x/week
Suboxone:
Buprenorphine + naloxone
Naloxone has no effect when taken orally/sublingually, but blocks effects if injected (abuse deterrent)
Naltrexone:
Long-acting opioid antagonist
Blocks reward and analgesia
Vivitrol: monthly injectable form
Only used post-detox (precipitates withdrawal otherwise)
