Explore the system
Change the mechanism, watch the synapse
Start with normal transmission. Select a molecular target and compare its acute and chronic consequences.
Sources and model limits
Original schematic. Positions show relationships; structures and concentrations are not to scale.
- Julien's Primer of Drug Action · 15 · 3 · Change the mechanism, watch the synapse · PDF 34, 35, 36, 37, 38 · 3.3, 3.7, 3.10
- Professor deck 3 · Monoamine clearance / transporters · PDF 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31
- IUPHAR recommendations · Supplement: inverse agonism and allosteric terminology
- Assay Guidance Manual · Supplement: agonists, antagonists and inverse agonists
- Monoamine transporter research · Supplement: amphetamine-related vesicular and membrane transport
Course notes and existing practice
Learning objective
Distinguish reduced response from adaptation to continued exposure.
More than one source of reduced response
The lecture defines tolerance as a decreasing response to the same dose and distinguishes metabolic, cellular-adaptive and behavioral contributions. Metabolic tolerance concerns altered drug handling; cellular adaptation concerns changed responsiveness; learned context can influence observed responses. These are explanatory categories rather than proof that one mechanism accounts for every change.
Adaptation and removal
Physical dependence concerns adaptation to continued exposure such that reducing or stopping exposure can produce withdrawal or discontinuation symptoms. Tolerance and dependence are related concepts but are not interchangeable, and neither alone defines addiction. The slide's statements that dependence follows tolerance and withdrawal simply reverses drug effects are treated as simplifications, not universal rules. This lesson teaches the distinctions without proposing a taper or stop/start schedule.
- Tolerance
- A reduced response to the same dose with repeated exposure.
- Physical dependence
- Adaptation to continued drug exposure that can produce withdrawal when exposure decreases or stops.
Professor deck 04, PDF pages 50. Course material is paraphrased. Qualifications and excluded clinical generalizations are documented in the Pharmacokinetics editorial record. Source review does not establish clinical validation.
Examples and practice explanations are original study aids, not professor quotations, exam requirements or medication instructions.
Julien connection: a distinction to explain on your own
Compare response at matched exposure to separate pharmacodynamic adaptation from lower exposure due to metabolism. Withdrawal can indicate physical dependence, but neither tolerance nor dependence alone establishes addiction; Chapter 4 adds impaired control and continued use despite harm.
Book connection: see the Julien Chapters 1–4 · Midterm study section in the Library for worked examples and recall practice. The cited source uses consolidated PDF page numbers.