Chapter 20 — Exercises

Endorphins, Enkephalins, and the Opioid Peptides

Items marked † are extension exercises: they require you to combine this chapter with earlier material, to reason past what the chapter states explicitly, or to sit with genuine uncertainty rather than resolve it. They are the ones worth doing slowly.

No answers are provided here. Several items have no single correct answer, and the ones that do are answerable from the chapter text.

A standing constraint for every item in this file: none of these exercises asks you to design, recommend, or evaluate anyone's treatment. Where an item describes a clinical situation, you are analyzing the reasoning, not making a decision. Decisions belong with a clinician.

Section A — The reframe and the three families

A1. In one sentence, state why it is misleading to describe opioid receptors as "the receptors that morphine acts on," and give the more accurate formulation.

A2. The receptors were characterized before the endogenous ligands were found. Explain why that sequence of discovery was itself a scientific argument — what did the existence of a specific, saturable binding site for a plant alkaloid predict?

A3. Fill in the table from memory, then check it against §20.2.

A4. Write out the first four residues shared by met-enkephalin, leu-enkephalin, β-endorphin, and dynorphin A. Explain what the "message" and "address" portions of an endogenous opioid peptide each determine.

A5. Nociceptin begins Phe-Gly-Gly-Phe rather than Tyr-Gly-Gly-Phe. State two consequences of that single substitution, one structural and one experimental.

A6. † Proopiomelanocortin yields β-endorphin, ACTH, and α-MSH, among others. Chapter 13 treated α-MSH as part of the appetite system; this chapter treats β-endorphin as part of the pain system. Argue, in a short paragraph, that "the POMC system" is not a useful unit of analysis — and then argue the opposite. Which argument do you find stronger, and what evidence would settle it?

A7. Enkephalins are five residues long and are degraded within seconds. β-endorphin is 31 residues and lasts substantially longer. Using Chapter 1's material on peptide size and Chapter 4's on clearance, give at least two reasons why length correlates with survival time.

Section B — Three receptors, three jobs

B1. All three classical opioid receptors couple to inhibitory G proteins and reduce neuronal excitability. Name the three cellular consequences of that coupling described in §20.3.

B2. For each of the following effects, name the receptor primarily responsible: euphoria; dysphoria; respiratory depression; constipation; miosis.

B3. A colleague says, "If we could just make a mu agonist that was selective enough, we'd have an analgesic with no respiratory depression." Explain in two sentences why selectivity for mu does not solve that problem.

B4. † Kappa agonists are analgesic, produce little respiratory depression, and have low abuse liability — the exact profile the field has wanted for decades. Explain why they are nonetheless not in wide clinical use as analgesics, and then explain what design change made a kappa-targeting peptide approvable for a different indication (§20.9).

B5. Different effects of a mu agonist develop tolerance at different rates. Name one effect that develops tolerance quickly and one that develops it slowly or incompletely, and explain why that asymmetry is a safety problem rather than a curiosity.

B6. Opioid-induced itch is mu-mediated. Explain why calling it an "allergy" is both incorrect and consequential for the patient's future care.

B7. † Chapter 2's Case Study 2 examined biased agonism at the mu receptor. Without retelling that case study, state the general principle it tested, and then say what §20.3's account of receptor distribution predicts about the ceiling on how much any single-receptor strategy can achieve.

Section C — Descending inhibition

C1. Draw the descending inhibitory pathway from memory: name the two brain regions, the spinal target, and the direction of information flow. Then annotate where endogenous opioid peptides act.

C2. Explain what is meant by "pain is not a passive readout of tissue damage." Give the mechanism, not the slogan.

C3. The periaqueductal gray receives input from the amygdala, hypothalamus, and prefrontal cortex. Explain why that anatomical fact is the structural precondition for everything in §20.7.

C4. † Descending modulation includes facilitation as well as inhibition. Explain how that fact relates to opioid-induced hyperalgesia, and why a purely inhibitory model of the descending system would make that phenomenon look paradoxical.

C5. A patient's reported pain is much greater than their imaging findings would predict. Write two sentences a clinician could say to that patient that are consistent with §20.4 — one describing the biology, one making clear what the finding does not imply about them.

C6. † Electrical stimulation of the periaqueductal gray produces analgesia; naloxone reduces that analgesia. Write out the inference chain from those two facts to the conclusion "the analgesia is mediated by endogenous opioid transmission," naming every step. Then identify the weakest link.

Section D — Evidence, measurement, and study design

D1. State the compartment problem in one sentence, using β-endorphin as the example.

D2. A study reports that a novel intervention "significantly increased plasma β-endorphin." List three separate things that result does not establish.

D3. † The chapter argues that when a peptide cannot be measured where it acts, the antagonist experiment is often the only honest causal test. Design — in outline, not in detail — an antagonist experiment to test whether endogenous opioids mediate the analgesia some people report after a cold water immersion. Name your primary endpoint, your control condition, and the result that would falsify your hypothesis.

D4. Explain why a null result under opioid blockade (as in the human runner's-high experiment) is weaker evidence than a positive result under opioid blockade (as in the placebo experiments). What specific feature of study design determines how much weight a null deserves?

D5. In the placebo work, verbal-expectation-induced analgesia was naloxone-reversible while analgesia conditioned with a non-opioid drug largely was not. Explain what that dissociation establishes about "the placebo effect" as a category.

D6. † Naloxone is not a perfectly clean probe — it can alter pain sensitivity on its own in some conditions. Explain how that complicates the interpretation of a naloxone-reversal result, and name one design feature that addresses the complication.

D7. Nocebo hyperalgesia has been linked to a signaling system other than the opioid system. Explain why that finding strengthens rather than weakens the case made in §20.7.

Section E — Rating practice

For each claim below, assign ✅ / ⚠️ / ❌ / 🔬, write a one-sentence justification, and write one line naming what would change your rating. Then check your rating against the chapter's, if the chapter issued one, and account for any difference.

E1. Endogenous opioid peptides mediate a substantial component of placebo analgesia in adults with postoperative pain.

E2. Endorphins cause the runner's high.

E3. Oral DL-phenylalanine produces clinically meaningful analgesia in adults with chronic pain.

E4. Intrathecal ziconotide produces clinically meaningful analgesia in adults with severe chronic pain refractory to other therapies.

E5. Dual inhibition of neprilysin and aminopeptidase N will produce an approved non-addictive analgesic.

E6. † A supplement company cites E1's ✅ rating in marketing copy for a product claiming to "activate your body's placebo-analgesia pathway." Explain, using the rating rules from Chapter 5, why citing a legitimate ✅ does not transfer any support to their claim. Name the specific rule being violated.

E7. † Write a rating for a claim this chapter did not rate: "Kappa receptor agonism reduces the abuse liability of an analgesic." Note that this is a claim about a receptor mechanism rather than about a clinical endpoint. Decide whether the rating system can handle it as written, and if not, rewrite the claim so that it can.

Section F — Tolerance, dependence, and the language problem

F1. Define tolerance, physical dependence, and addiction in one sentence each, without using any of the three words inside another's definition.

F2. Explain why physical dependence is expected in a patient taking long-term opioid therapy exactly as prescribed.

F3. Name three specific, concrete harms that follow from conflating physical dependence with addiction. For each, say who is harmed.

F4. † Chapter 3 argued that the difference between a hormone and a drug is often the schedule rather than the molecule. Apply that argument to the opioid system: explain why endogenous enkephalin release does not produce tolerance and dependence while a sustained exogenous agonist does, and be specific about which feature of the endogenous pattern is doing the work.

F5. A product claims to "boost your endorphins with no tolerance and no dependence." Explain the internal inconsistency in that claim at the level of receptor biology.

Section G — Synthesis and the dossier

G1. Run the three-joint test from §20.8 — magnitude, duration, existence proof — on a claim of your own choosing from outside this chapter. Show your work at each joint.

G2. Explain the "existence proof" argument to someone with no science background in three sentences, using no term you have not first defined for them.

G3. Ziconotide must be delivered intrathecally. Write a short paragraph explaining why that administration requirement is itself a piece of evidence, and about what.

G4. † Convert one entry in your own Evidence Dossier from the single-molecule Field 2 format to the system-level format given in this chapter. Fill the Opposing effects and Measurement trap rows first, and write a sentence about what filling them changed.

G5. † The endogenous opioid system carries a ✅, a ⚠️, an ❌, and a 🔬 in this chapter. Write the paragraph you would use to explain to a skeptical reader why that is not fence-sitting, and why a single overall rating would be less honest rather than more decisive.

G6. Write the three sentences you would say to a friend who tells you they are buying an "endorphin support" supplement. Constraint: no jargon, no condescension, and each sentence must make a distinct point.