Cocaine, Amphetamines & Related Stimulants – Mechanisms, Effects & Addiction

Lecture Overview & Key Objectives

  • Focus substances: cocaine (primary), fentanyl (parallels only briefly noted), amphetamines (comparison/contrast)
  • Learning targets
    • Mechanisms that underlie cocaine reinforcement & reward
    • Role of dopamine transporter (DAT) occupancy in subjective high
    • Receptor-level contributions (D1, D2, D3)
    • Animal models of self-administration, escalation, craving, relapse
    • Differences & similarities between cocaine, amphetamine, cathinone derivatives
    • Basic therapeutic & treatment considerations

Historical Context & Epidemiology of Cocaine

  • Naturally occurring alkaloid in coca leaves (South American Andes)
  • Indigenous use for ≥ 2000 yrs: ritual, social, altitude labor stamina
  • Spanish conquest
    • Catholic church tried to ban chewing; productivity dropped → ban lifted
    • Leaves shipped to Europe but degraded – low popularity
  • 1850s: cocaine isolated as water-soluble HCl salt
    • Became ingredient in tonics, wines, patent medicines
    • Sigmund Freud: heavy advocate, prescribed widely (“cocaine to kill a small horse” anecdote)
  • Regulation
    • USA Harrison Narcotic Act (1914) curtailed medical/recreational sales
  • Resurgence waves
    • 1970s powder use
    • 1980s crack cocaine (free-base, smoked) ⇢ cheaper, more addictive

Crack vs Powder Cocaine

  • Powder (HCl salt): usually snorted, oral, IV
  • Crack (free-base)
    • Produced by dissolving HCl salt in water + ammonia/baking soda → “rocks”
    • Smoked → very rapid brain entry, audible “crack” when heated

Pharmacokinetics of Cocaine

  • Highly lipophilic ⇒ fast BBB penetration
  • Half-life ≈ t1/2≈1 ht_{1/2}\approx1\text{ h}
  • Route-dependent plasma peaks
    • IV & smoking: seconds–minutes (strongest rush)
    • Intranasal: a few minutes
    • Oral: slowest onset, lowest peak
  • Short half-life ⇒ users take repeated “bumps” to maintain effect

Subjective & Physiological Effects (Acute)

  • Desired (“awesome”) effects
    • Euphoria, empowerment, hyper-vigilance, grandiosity, psychomotor activation
    • Talkativeness, fidgeting, stereotyped sniffing/scratching
    • Duration ≈ 30 min
  • Body effects via catecholamine surge (NE ↑)
    • Anorexia, insomnia (↓ sleep need), ↑ HR, ↑ BP, ↑ respiration, hyperthermia
  • High-dose / sensitive users: irritability, intense anxiety, incoherent rapid speech, flight of ideas, delusions of grandeur, motor stereotypies

Acute Toxicity & Overdose

  • Excess sympathetic drive ⇒ myocardial infarction most common lethal event
  • Severe vasoconstriction → seizures, heart failure, cerebral hemorrhage
  • Lethal temperature / BP spikes possible

Chronic Use: Crash, Withdrawal & Toxicities

  • No classic “physical withdrawal” (shakes, vomiting) like alcohol/opiates
  • Psychological “crash”
    • Profound fatigue, malaise, anhedonia, depressive & anxiety symptoms
    • Craving both for positive effects & to normalize mood
  • Long-term organ/tissue damage
    • Cognitive deficits: verbal memory, attention, motor
    • Multi-organ harm: kidneys, heart, lungs, GI tract
    • Chronic vasoconstriction → ischemic necrosis (e.g., nasal septum perforation photo)
    • Paranoid psychosis, hallucinations, panic attacks

Pharmacodynamics of Cocaine

  • Primary action: blocks monoamine reuptake transporters
    • Highest affinity = SERT > DAT > NET, yet reward mainly DAT-driven
    • Result: transmitter accumulation proportional to neuron firing rate
  • Secondary action: local anesthetic (voltage-gated Na+Na^+ channel blocker) at high doses ⇒ medicinal precursor to lidocaine/procaine
Dopamine-Centric Evidence for Reinforcement
  • All species self-administer cocaine vigorously; extinction only with competing rewards/conditions
  • Mesolimbic / nigrostriatal DA critical
    • Accumbens micro-infusion of cocaine is reinforcing; 6-OHDA lesions of accumbens abolish IV self-admin
    • Knock-in mice with DAT insensitive to cocaine show minimal intake
  • D-receptor specificity
    • D1: essential (D1 KO ⇒ no locomotor activation or self-admin)
    • D2: modulatory
    • D3: complex; antagonism ↑ locomotion but ↓ reinstatement
PET Findings in Humans
  • Subjective “high” emerges when ≈ 50–60 % DAT sites occupied
  • Intensity modulated by
    • Baseline DA neuron activity (stimulating environment ↑)
    • Rate of occupancy (IV/smoke > snort > oral)
  • Dopamine correlates with craving rather than pleasure per se
    • Cue-induced DA release in striatum ∝ self-reported craving

Transition to Dependence & Binge Patterns

  • Only ~5–15 % of experimenters develop daily/compulsive use
  • Risk factors
    • Crack (smoked) users ≈ 22 % transition vs 5 % for intranasal only
  • Binge model: hours–days of heavy intake → crash
  • Incubation of craving (rat & human data)
    • Drug seeking increases with abstinence interval (1 d < 7 d < 60 d)
    • Linked to synaptic/neuroplastic changes in DA pathways
Long- vs Short-Access Animal Model
  • 1-h/day (ShA) vs 6-h/day (LgA)
    • ShA: stable intake
    • LgA: escalation across days
  • Intracranial self-stimulation (ICSS) thresholds
    • LgA rats show elevated reward thresholds ⇒ reward circuitry hypo-function ⇒ compensate by taking more drug (tolerance to hedonic effect)

Tolerance vs Sensitization

  • Tolerance
    • Heavy users report diminished euphoria ⇒ escalate dose
    • PET: blunted DA release in dorsal striatum after stimulant challenge
  • Sensitization
    • Enhanced locomotor response & cue-induced craving
    • Context-dependent; greater when drug taken in familiar environment
  • Both processes co-exist; effect measured depends on response domain & abstinence duration

Amphetamines & Methamphetamine

Chemical & Historical Notes
  • Parent compound: amphetamine (first synthesized 1887)
  • Structurally resembles dopamine (see ring + ethylamine side chain)
  • Naturally in Ephedra (ephedrine) ⇒ basis for synthetic variants
  • WWII: military “pep pills” to stay awake/fight
  • 1970s abuse peak → decline during cocaine wave → 1990s meth labs resurgence
Common Forms & Routes
  • d-Amphetamine (“Dexies”): oral, IV, sub-Q
  • Methamphetamine (meth, crank, ice, crystal): smoked, snorted, IV; much more potent & lipophilic
  • Meth half-life ∼10 h\sim10\,\text{h} (vs cocaine 1 h) ⇒ single large dose sustains high for many hours
Cellular Mechanism (vs Cocaine)
  1. Enters terminal via DAT/NET
  2. Displaces vesicular monoamines ⇒ cytoplasmic DA↑
  3. Reverses transporter (“carrier-mediated efflux”)
  4. Meth ± other analogs inhibit MAO ⇒ further transmitter surplus
  5. Outcome: massive, action-potential-independent DA (& NE, 5-HT) release; far greater magnitude than cocaine
Acute & Chronic Effects
  • Acute psychological: similar to cocaine (euphoria, confidence, psychomotor activation)
  • Binge pattern: 3–6 day runs with no sleep/food
  • Withdrawal (“crash”): stronger, longer (weeks) than cocaine; severe depression, anxiety, lethargy
  • Sex difference: women slightly higher prevalence/progression; estradiol modulation of DA hypothesized
  • Chronic neurotoxicity
    • Meth psychosis (paranoid, hallucinations)
    • Persistent deficits: impulse control, working memory, verbal learning, social cognition
    • Oxidative stress & excitotoxic death of DA & 5-HT axons

Synthetic Cathinones (“Bath Salts”)

  • Examples: mephedrone, methylone, 4-APB; legal evasions under labels “plant food,” “room odorizer,” “bath salts”
  • Routes: typically smoked, snorted, ingested (NOT effective as actual bath additive)
  • Mechanism & effects mirror meth/MDMA: DAT/SERT/NET substrate + releaser
  • Rats self-admin at doses comparable to cocaine/amphetamine → high abuse liability

Legitimate Therapeutic Uses of Stimulants

  • Narcolepsy
    • NE↑⇒NE\uparrow \Rightarrow REM suppression ⇒ fewer cataplexy episodes (modafinil, amphetamines)
  • Anti-obesity (historical): strong anorectic; pulled from OTC due to abuse risk
  • ADHD
    • Core problem: hypoactive PFC catecholamine tone ⇒ distractibility/hyperactivity
    • Low oral doses ↑ DA/NE in PFC (D1 & α2A\alpha_{2A}) ⇒ improved focus, ↓ motor restlessness
    • Medications
    • Adderall: mixed d-amphetamine salts, slow oral absorption, low abuse risk when taken as prescribed
    • Methylphenidate (Ritalin): DAT/NET blocker; abuse potential if crushed/snorted/IV

Treatment Strategies for Stimulant Use Disorder

  • Pharmacotherapies
    • Limited success; many pre-clinically promising agents (e.g., D3 antagonists) ineffective clinically
    • Substitution approaches
    • Modafinil: mild stimulant reducing cocaine craving
    • Prescribed oral amphetamine (Adderall) as controlled maintenance (analogous to methadone)
  • Psychosocial / behavioral
    • Cognitive-behavioral therapy (CBT)
    • Contingency management
    • Tangible rewards (cash, vouchers, snacks) contingent on negative urine screens
    • Mirrors animal data where alternative reinforcers (sweets, money) can compete with cocaine
    • Major barriers: patient insight, cost, access, high relapse due to incubation of craving

Miscellaneous Details, Examples & Anecdotes

  • “Everything is awesome” (Lego Movie) referenced to illustrate manic cocaine state
  • Local anesthetic property may numb nasal mucosa, facilitating intranasal use
  • Rats sometimes prefer saccharin to cocaine; preference modulated by prior exposure, experimental conditions
  • Money vs drug choice in humans: casual users prefer $50\$50 over a hit; crack-dependent users choose drug
  • Sex separate aspects
    • Estradiol ↑ stimulant sensitivity in females (animal & some human data)
  • Half-life / dosing interaction question clarified: short t1/2t_{1/2} drives rapid redosing but not direct pharmacokinetic tolerance

Conceptual Take-Home Connections

  • Cocaine & amphetamines converge on final common pathway: rapid, supraphysiological dopamine elevation, especially in nucleus accumbens ⇒ reinforcement
  • Speed of brain entry (pharmacokinetics) + magnitude of DA surge (pharmacodynamics) = addiction liability
  • Dopamine signaling is necessary for incentive motivation & craving, not equivalent to pleasure; subjective high involves additional, still-unresolved processes (possible opioid contribution)
  • Chronic high-dose exposure produces both tolerance (reward blunting) and sensitization (cue reactivity) – a dual adaptation that sustains compulsive use & relapse risk.