Notes on Pharmacokinetics, Psychoactive Drugs, and the Blood-Brain Barrier

Pharmacokinetics, Psychoactive Drugs, and the Brain

• Purpose of today: set the stage for understanding what drugs are, what psychoactive drugs are, how they relate to food, and where definitions can break down. This will underpin what we discuss in upcoming weeks, including the brain, neurons, and specific drugs.
• Quiz note: first quiz next week will cover material from syllabus chapter 2 (brains and neurons).
• Core framework introduced: pharmacokinetics — how drugs enter the body, how they distribute, how they are metabolized, and how they are excreted. Today is a broad, general view; details for specific drugs will come later (e.g., half-life, metabolism, distribution, brain effects).
• Key takeaway: concentrations of drug in the bloodstream can be measured easily, but that doesn’t directly tell us what a person is experiencing.

What is a drug? definitions, scope, and the blur with food

• Broad definition of a drug used here: an exogenous substance (a xenobiotic) that comes from outside the body and can affect bodily function.
• Psychoactive drug: a drug that directly affects behavior and/or cognition by interacting with the brain.
  • Direct interaction in the brain is a key criterion.
  • Must be bioavailable (can reach the brain) and must produce a measurable effect (behavioral or physiological).
  • Not all drugs that affect the body are psychoactive (e.g., many analgesics or antibacterials affect peripheral systems but not brain function in a way we classify as psychoactive).
• Food as a potential psychoactive agent is tricky and somewhat arbitrary; the line between food/nutrition and drug is not always clear-cut.
• Examples and discussion points (as in the lecture):
  • Vitamin C tablet could be considered a drug by the basic definition (affects bodily function by preventing scurvy) but typically not psychoactive because it lacks direct brain effects.
  • Caffeine in energy drinks (e.g., Red Bull, Celsius) is considered a drug because it has psychoactive effects via brain mechanisms.
  • Sucrose (sugar) also has indirect effects on the brain (via energy metabolism, insulin/leptin/cortisol signaling) and has subjective, pleasurable effects, raising questions about whether it could be considered psychoactive in some contexts.
  • Glucose directly affects brain function through brain receptors and energy metabolism, and indirectly via liver-pancreas signaling (insulin, leptin, cortisol).
  • Aspirin and ibuprofen are drugs that affect peripheral pathways (inflammation, pain) and may or may not be considered psychoactive depending on whether there is a direct brain effect at typical usage levels.
• The speaker emphasizes the arbitrariness and complexity of the boundary between drugs and foods, and he uses this to motivate studying how these substances interact with the brain via pharmacokinetics and neurochemistry.
• Key examples of plant-derived psychoactive compounds (introduced in lecture):
  • Bitter alkaloids and common plant sources: caffeine (coffee), nicotine (tobacco), cocaine (from coca leaves), morphine (opium), heroin (a morphine dimer), psilocybin mushrooms, mescaline (peyote cactus), cannabis (marijuana), arecoline from areca nut ( Southeast Asia) – arecoline is structurally similar to caffeine in some respects and can have stimulant-like effects.
  • Note: Coca leaves and coca tea historically provide a low, slow release of cocaine via buccal absorption when chewed; cocaine is metabolized quickly via first-pass metabolism when ingested orally.
  • Psychedelics like psilocybin and mescaline are highlighted as drugs of therapeutic and research interest (psychedelic research has faced regulatory barriers in the past but is re-emerging).
• The instructor also highlights the idea of “natural” vs. synthetic drugs: many drugs humans use recreationally or medicinally are chemically related to natural plant compounds, and synthetic chemistry has refined or altered these compounds for research, therapy, or abuse.
• The role of evolution and biology: many plants produce these compounds as defense, and humans have evolved preferences and cultural practices around these substances.
• A few concrete points about substances mentioned in the talk:
  • Fentanyl is a synthetic opioid related to morphine; it is a synthetic version of an opiate produced in nature (morphine).
  • Cocaine is highlighted as having rapid cardiovascular and CNS effects; its pharmacokinetics include fast on/off dynamics when administered in ways that bypass first-pass metabolism.
  • Ether of tobacco: nicotine can be absorbed via inhalation, oral ingestion, and other routes; different routes change onset and intensity.
  • Opium and laudanum: historically used as alcohol-based opium extracts; the bitter alkaloids are soluble in alcohol.
• A guiding theme: drugs are not magical; they are molecules that interact with neural receptors and transport systems, often by mimicking or modulating endogenous neurotransmitters or neuromodulators.
• The lecturer foreshadows later topics: receptors, synapses, allosteric modulators, agonists/antagonists, and how plant-derived molecules help reveal brain chemistry.

The pharmacokinetics framework (ADME) and why it matters

• Pharmacokinetics (PK) describes how a drug moves through the body: Absorption, Distribution, Metabolism, and Excretion (ADME).
• Today’s focus is broad and conceptual; later weeks will apply these concepts to specific drugs and their PK properties, such as half-life and distribution.
• Core idea: the amount of drug that reaches the bloodstream, and then the brain, depends on route of administration and formulation; this is what determines onset, intensity, and duration of effect.
• Important implication: measuring blood concentrations alone provides limited information about subjective experience; brain exposure and receptor engagement are the decisive factors for psychoactive effects.
• Definitions and terms frequently used:
  • Bioavailability: the fraction of the administered dose that reaches systemic circulation in an active form. Typically discussed as a fraction between 0 and 1.
  • Biodistribution: how the drug distributes throughout body compartments after entering circulation.
  • Bioactive: the substance has biological activity at its target (often a receptor or transporter) in the tissue of interest.
  • Blood-brain barrier (BBB): physiological barrier that regulates and restricts the passage of substances from blood into brain tissue.
• Practical consequences: different routes lead to different bioavailability profiles and timing, which is critical for designing therapeutic regimens and for understanding patterns of recreational use.
• Example to connect concepts: direct brain access requires the molecule to be small enough, sufficiently lipophilic, and not pumped out or metabolized before crossing barriers; many psychoactives exploit transporters or passively diffuse if they satisfy these criteria.
• The half-life and metabolism of a drug affect duration of action and how often the drug needs to be taken to maintain effect; metabolites can be active (e.g., fluoxetine and its active metabolite norfluoxetine).
• A note on metabolism and metabolites:
  • Nicotine metabolites (e.g., cotinine) have psychoactive effects and contribute to the overall pharmacological profile.
  • Fluoxetine (Prozac) is active itself via inhibition of the serotonin transporter; its metabolite norfluoxetine also has psychoactive effects.
• Practical question posed by the instructor: how to determine if a substance like sugar directly affects brain function as a psychoactive agent? The answer involves measuring brain entry and then, ideally, linking that brain presence to measurable changes in neural activity and behavior.

Routes of administration and how they shape bioavailability

• A central point: there are many routes to administer drugs (oral, intramuscular, intravenous, subcutaneous, transdermal, intranasal, vaginal, rectal, etc.). Each route has unique implications for onset, peak concentration, and duration.
• Key reasons for multiple routes:
  • Different drugs have different stability and solubility profiles.
  • Some drugs require rapid onset (e.g., IV or intranasal) for acute effects or safety considerations.
  • Some formulations are designed for convenience or extended exposure (e.g., transdermal patches).
  • First-pass metabolism reduces oral bioavailability for many drugs, especially those that are extensively metabolized by the liver prior to reaching systemic circulation.
• Basic pharmacokinetic intuition:
  • After administration, the drug must be absorbed into systemic circulation, circulates, and then distributes to tissues.
  • The amount that reaches the target tissue (e.g., brain) depends on solubility, transport mechanisms, binding to plasma proteins, and barriers like the BBB.
• Examples of routes and their implications:
  • Intravenous: straight into the bloodstream; rapid onset; often 100% bioavailability but risk of overdose.
  • Intramuscular: slower diffusion from injection site into capillaries; variable onset.
  • Subcutaneous: diffusion from tissue; slower, more prolonged absorption.
  • Oral: absorption through GI tract; subject to stomach/intestinal conditions and first-pass hepatic metabolism; commonly lower bioavailability and slower onset.
  • Buccal/transmucosal (buccal lining of the cheek), intranasal: absorption through mucous membranes; bypasses some first-pass metabolism; faster onset than oral in many cases.
  • Transdermal: skin absorption; useful for lipophilic drugs; provides steady, slow release (e.g., nicotine patch, estrogen patches).
• The lecture emphasizes the practical reason for different routes: maximizing bioavailability to the brain while balancing safety, onset, dose control, and patient comfort.
• Important note on a common misconception: some drugs administered orally can be effectively absorbed and crossed into brain tissue, while others may be degraded too quickly; different drugs require different routes for optimal effect.
• The concept of bioavailability (F) in practice:
  • For a given route, F is the fraction of the administered dose that ultimately reaches the systemic circulation in an active form. A simplified way to think about it: $F = \frac{ ext{Amount reaching systemic circulation}}{ ext{Amount administered}}.$
  • Route, formulation, and first-pass metabolism all influence F.
• Distribution and high-affinity sites:
  • After entering circulation, drugs distribute to tissues; psychoactive effects are typically mediated by high-affinity binding at brain targets (e.g., mu-opioid receptors for morphine, dopamine-related targets for cocaine).
  • Different drugs have different target regions in the brain (e.g., morphine acts broadly but with high density of opioid receptors in certain brain regions; cocaine affects dorsal/ventral striatum).
• Biotransformation (metabolism) and the role of metabolites:
  • Some drugs produce active metabolites that contribute to the overall effect or duration (e.g., nicotine → cotinine; fluoxetine → norfluoxetine).
  • Metabolism can alter potency, duration, and toxicity.
• A practical example from the lecture: the pharmacokinetics of cocaine are dramatically affected by the route of administration due to first-pass metabolism; buccal or intranasal administration can yield different brain exposure than oral ingestion.
• Measurement approaches mentioned: autoradiography, fluorescence labeling, PET scans to visualize where labeled drug molecules go in the brain and how they accumulate over time.

Blood-brain barrier (BBB): structure, function, and relevance for psychoactive drugs

• The BBB is a specialized barrier that protects the brain by regulating what can enter from the bloodstream into neural tissue.
• Key structural components:
  • Endothelial cells line capillaries throughout the body; in the brain, these endothelial cells are tightly joined and form a barrier to many molecules.
  • Basal lamina (basement membrane): extracellular matrix surrounding the capillaries in the brain; contributes to barrier integrity.
  • Astrocytic endfeet: astrocyte processes that envelop brain capillaries and modulate exchange; important for active transport and regulatory control of what enters the brain.
  • Tight junctions between endothelial cells in brain capillaries reduce paracellular diffusion, making brain capillaries relatively non-permeable to many substances.
• Functional consequence:
  • The BBB restricts passage of large or hydrophilic molecules, requiring specific transport mechanisms (e.g., diffusion for small lipophilic molecules, transporter-mediated uptake for certain nutrients, and active transport for others).
  • Many drugs must be lipophilic enough to diffuse through endothelial cell membranes or rely on specific transport systems to reach brain tissue.
• Experimental illustration of BBB integrity:
  • A classic dye-permeability test uses methylene blue and compares dye presence in brain tissue after systemic injection.
  • In animals with intact BBB, dye remains out of the brain; when BBB integrity is compromised (e.g., due to encephalitis, bacterial meningitis, or experimental viral infection), dye can enter the brain, indicating increased permeability.
  • The figure demonstrates that the same dye distributes throughout the body but is excluded from the brain in a normal BBB and becomes detectable in brain tissue when barrier integrity is disrupted.
• Clinical and research implications:
  • Drugs with poor BBB permeability are unlikely to have central (psychoactive) effects unless they are delivered by routes that bypass the BBB (e.g., intranasal, transdermal, or direct CNS administration) or are designed to utilize transport systems.
  • Conditions that disrupt the BBB (infections, inflammation, certain diseases) can alter drug distribution to the brain, potentially changing efficacy and toxicity.
• Takeaway: The BBB is a major determinant of whether and how a drug can act in the brain; understanding BBB permeability is essential for predicting psychoactive potential and safety.

Indirect brain effects via food, interoception, and the reward concept

• The brain is constantly exposed to circulating substances; even nondrug foods can modulate brain function via indirect mechanisms.
• Indirect pathways include:
  • Gut hormones and energy signals (e.g., leptin, insulin, cortisol) that influence brain circuits controlling appetite, mood, and energy balance.
  • Gut-to-brain signaling through the vagus nerve (an afferent pathway that reports on gut state, digestion, and metabolic status).
  • Stomach distension itself can affect brain function and mood through interoceptive signaling (internal bodily state awareness).
• Interoception: subjective feelings that originate from inside the body (e.g., fullness, nausea, gut sensations). The term is used to describe how internal bodily states contribute to experiences and perception.
• Chocolate cake example (subjective description activity):
  • Participants describe the experience of eating cake and relate it to brain processes.
  • Common themes: pleasure, dopamine release, a sense of reward, potential after-effects like fullness or mild discomfort after overconsumption.
  • The discussion connects subjective pleasure to neural circuits (e.g., reward circuitry) and to pharmacokinetic concepts (calories, metabolite effects, hormone changes).
• Reward circuitry, dopamine, and psychoactivity:
  • Eating pleasurable foods can activate the brain’s reward circuits, producing dopamine release and corresponding learning signals.
  • Similarities and differences exist between natural rewards (food, sex) and drugs of abuse, often converging on shared neural pathways (e.g., mesolimbic dopamine system).
• Important caveat: interpreting “sugar addiction” or “fat addiction” is controversial; the lecturer notes these terms can be misleading because sugar/fats are not classic psychoactive drugs with direct brain receptor targets, but they do influence brain function indirectly via energy metabolism and hormonal signaling.
• Mechanisms by which the brain can be influenced by food and beverages include:
  • Direct effects of glucose on brain function (brain uses glucose as fuel; glucose interacts with glutamate transporters and can influence insulin and hormone regulation).
  • Indirect hormonal signaling (insulin, leptin, cortisol) that affects appetite and mood, with downstream neural consequences.
  • The stomach-brain axis and vagal signaling reporting on caloric intake and energy status.
• The lecturer emphasizes the distinction between direct brain action (true psychoactive effect) and indirect modulation via peripheral metabolism and signaling; both can alter cognition and behavior, but only the former is a direct brain interaction.

The concept of psychoactivity: criteria, measurement, and gray areas

• The speaker outlines three core criteria for a molecule to be considered psychoactive in the strict sense:
  1) Bioavailability: the molecule must be accessible to the tissues where it has an effect, including the brain. This implies it must be soluble in blood and able to cross membranes (including the BBB).
  2) Biodistribution and brain access: there must be a mechanism by which the drug gets from the circulatory system into neural tissue.
  3) Direct neural effect: once in the brain, the molecule must bind to targets (receptors, transporters, or other molecular structures) and produce a measurable effect on neural function and behavior.
• The drug does not have to be hallucinogenic or stimulatory; any direct alteration of brain activity qualifies as psychoactive under this framework.
• Non-psychoactive examples given to illustrate boundaries:
  • Aspirin: primarily anti-inflammatory and analgesic; may not be considered psychoactive unless there is a direct brain effect at typical doses.
  • Ibuprofen: similar category as NSAID; not primarily a brain-acting psychoactive drug.
  • Penicillin: antibacterial agent; effects are peripheral unless toxicity or off-target brain effects occur.
• The lecturer emphasizes that these criteria are deliberately broad and somewhat arbitrary, reflecting the real-world difficulty of categorizing substances along a single line.
• The discussion also introduces the idea that some drugs are psychoactive at certain concentrations or routes but not others, and some substances may have both peripheral and central actions.
• A final practical point: in this course, you should think about how a molecule’s ability to reach the brain and interact with neural targets differentiates a psychoactive drug from typical foods.

A closer look at an everyday example: chocolate cake, sugar, and brain function

• The group dialogue around chocolate cake is used to illustrate how everyday foods can influence brain function through multiple, interacting pathways:
  • Direct effects: glucose and related metabolites can influence brain activity and neuromodulation.
  • Indirect effects: caloric intake changes hormonal milieu (insulin, leptin, cortisol) and energy balance, which in turn influence brain circuits involved in mood, motivation, and cognition.
  • Subjective experience: notes on pleasure, mood improvement, subsequent “crash” or fullness sensations; relates to dopamine and reward circuits.
• The discussion acknowledges that terms like “sugar addiction” or “fat addiction” are controversial, and it cautions that these concepts do not map cleanly onto classic, brain-receptor mediated psychoactive drug actions.
• Important concept: the distinction between direct brain action of a molecule and the indirect brain effects driven by energy metabolism and hormonal signaling from the gut and periphery.
• The example underscores the complexity of measuring true psychoactivity for foods: to show direct brain action, one would need to demonstrate that the sugar molecule or its metabolite actually enters the brain and binds to neural targets, distinct from peripheral metabolic effects.
• Experimental approach hinted: autoradiography with radiolabeled sugars, or modern methods like PET imaging, to trace whether sugar or other molecules reach the brain after ingestion; the key criterion for psychoactivity remains direct brain action.

Mechanisms of entry into the brain and the role of bioavailability in psychoactivity

• Reiteration of a central question: why do we care about routes of administration? Because they determine how much drug reaches the brain and when.
• Bioavailability and routes of administration:
  • Oral administration often leads to partial first-pass metabolism in the liver, reducing the amount that reaches the brain.
  • Non-oral routes (intravenous, intramuscular, subcutaneous, intranasal, buccal, transdermal) can bypass or reduce first-pass metabolism and change the timing of brain exposure.
  • Buccal and intranasal routes can bypass hepatic first-pass metabolism and deliver drug more rapidly to the brain via rich capillary beds in the mucosa.
• The practical consequence: route choice affects onset, intensity, duration, and risk. For example, cocaine may yield different brain exposure and effects depending on whether it’s snorted, smoked, or injected.
• Fundamental pharmacokinetic relationships discussed:
  • Absorption: how a drug enters systemic circulation from the site of administration.
  • Distribution: how the drug moves from the bloodstream to tissues (brain versus peripheral tissues).
  • Metabolism (biotransformation): chemical changes to the drug, producing metabolites; some metabolites are active and contribute to psychoactive effects.
  • Excretion: elimination from the body, ending the drug’s action.
• Concept of high-affinity binding sites and target tissues:
  • After distribution, drugs bind to target receptors and transporters; the brain often has high densities of these targets relative to other tissues, shaping therapeutic and psychoactive effects.
  • Morphine has high affinity for opioid receptors in the brain; cocaine affects dopamine transporters and related pathways in reward-related brain regions (e.g., dorsal and ventral striatum).
• The lecture uses concrete examples to illustrate these concepts:
  • Morphine: natural product with widespread effects; high affinity for mu-opioid receptors.
  • Cocaine: acts primarily as a transporter blocker (dopamine reuptake inhibitor) in reward circuits.
  • Nicotine: both direct receptor interactions (nicotinic acetylcholine receptors) and active metabolites contribute to psychoactivity.
  • Fluoxetine (Prozac): serotonin transporter inhibition; active metabolite norfluoxetine also contributes to activity.
• Key takeaway: the combination of route, metabolism, and receptor interactions determines a drug’s psychoactive profile and duration of action.

The big picture: drugs, food, ethics, and real-world relevance

• The course connects basic neurochemistry to real-world issues: regulation, clinical use, addiction, psychedelic therapy, and public health.
• Psychedelic research: psilocybin, mescaline, and peyote are highlighted as substances under investigation for psychiatric disorders; historical regulatory barriers limited research, but there is renewed interest and clinical trials.
• Policy and ethics: the lecture alludes to how societal decisions about scheduling and legality influence scientific progress and public health outcomes.
• Practical implications for students:
  • Be comfortable with the idea that “what is a drug?” is not a simple yes/no categorization; it depends on route, dose, tissue access, and brain interaction.
  • Appreciate the distinction between direct brain actions (true psychoactives) and indirect effects via peripheral metabolism or gut-brain signaling.
  • Recognize that many everyday substances (caffeine, sugar, glucose) have complex, multi-level effects on the brain and behavior, some direct and some indirect.
• Final thought from the lecturer: the brain does not exist in isolation; it is constantly influenced by circulating substances, hormones, and signals from the gut and body. This integration is central to understanding psychoactive drugs and their effects.

Quick recap of core concepts and formulas (for quick study reference)

• Psychoactive drug criteria (conceptual):
  • Must be bioavailable to reach the brain; must cross into neural tissue; must have a direct effect on neural function; effects are measurable via behavior or physiology.
• Bioavailability (definition):
  • Fraction of administered dose that reaches systemic circulation in an active form: $F = \frac{ ext{Amount reaching systemic circulation}}{ ext{Amount administered}}.$
• Route-dependent bioavailability and first-pass metabolism:
  • Oral: subject to gastric/intestinal absorption and hepatic first-pass metabolism, reducing F.
  • Other routes (intravenous, intramuscular, subcutaneous, intranasal, buccal, transdermal): generally bypass or reduce first-pass effects, altering onset and magnitude of brain exposure.
• Absorption, distribution, metabolism, excretion (ADME): the four pillars of pharmacokinetics that determine a drug’s time course.
• Brain access and receptor engagement: high-affinity brain targets determine the psychoactive effects once the drug reaches the brain.
• Metabolites can be active: example — nicotine → cotinine; fluoxetine → norfluoxetine; these metabolites extend or modify psychoactive effects.
• Blood-brain barrier (BBB): tightly regulated barrier composed of endothelial tight junctions, basement membrane, and astrocytic endfeet; regulates brain exposure to circulating substances; disruption of BBB increases brain permeability (as seen in encephalitis and meningitis models).
• Experimental approaches to study brain entry: autoradiography, fluorescent labeling, PET imaging to track drug distribution and brain entry.
• Interoception and gut-brain signaling: gut hormones (leptin, insulin, cortisol) and vagal signaling influence brain function and mood indirectly via peripheral metabolic states.
• Sugar vs. sugar’s brain effects debate: sugar can produce pleasurable sensations and energy-related signaling that influence mood and cognition, but whether it is a true psychoactive drug (via direct brain action) is a topic of discussion and methodological challenge.