Selegiline

Three different drugs in one molecule depending on dose: low oral (1-2.5 mg) is a clean, daily-safe MAO-B preservation tool with neuroprotective animal data; Emsam 6 mg patch is a real antidepressant that bypasses gut MAO-A and is still tyramine-safe; Emsam 9-12 mg patch is a non-selective MAOI where a slice of aged cheese can put you in the ER. For Dylan (20yo, brain-priority), the low-oral tier is a STRONG-CANDIDATE post-baseline; the patch tiers are not a fit at 20 given the diet constraint and the nootropic ceiling already set by the oral microdose. The "Emsam was the best thing ever, slice of cheese could kill them" external-source story is real — but it describes the 9-12 mg patch tier specifically, not selegiline as a category.

What MAO-B does (and what blocking it accomplishes)

Monoamine oxidase B (MAO-B) is an enzyme found mostly in glial cells and serotonergic neurons that breaks down dopamine, phenylethylamine (PEA — an endogenous amphetamine-like trace amine), and benzylamine. It does NOT meaningfully metabolize serotonin or tyramine — that's MAO-A's job. Inhibiting MAO-B raises intrasynaptic and intracellular dopamine + PEA without raising serotonin. Selegiline (R-(-)-deprenyl) is an irreversible suicide-inhibitor: it covalently binds MAO-B and the enzyme is permanently inactivated. New MAO-B has to be transcribed and translated from scratch (~40 day half-life of brain MAO-B inhibition is the cited washout timeline).

Selectivity is dose-dependent — the critical point

• ≤5 mg/day oral: ~90%+ selective for MAO-B. PEA-pressor protection holds. No tyramine restriction.
• 5-10 mg/day oral: Selectivity starting to slip but tyramine restrictions still not formally required at 10 mg in PD patients.
• >10 mg/day oral OR >6 mg/24hr patch: Selectivity lost — meaningful MAO-A inhibition begins. Tyramine restrictions are mandatory. Drug now behaves like a "real" non-selective MAOI (similar liability profile to phenelzine / tranylcypromine, just with a different chemotype).
• 9-12 mg patch: Definitively non-selective at brain + gut MAO-A. This is where serotonin-related mechanisms drive the antidepressant effect AND where the cheese-reaction window opens.

TAAR1 / catecholaminergic activity enhancer (CAE) — the secondary mechanism

Beyond MAO-B inhibition, selegiline is a weak agonist at trace amine-associated receptor 1 (TAAR1). Knoll proposed and later research supports a "catecholaminergic activity enhancer" effect: at very low doses (sub-MAO-inhibitory), selegiline increases the impulse-evoked release of dopamine and norepinephrine without acting as a stimulant in the amphetamine sense. This is the same family of mechanism that BPAP and PPAP target more selectively. CAE activity is part of why ultra-low doses (0.25 mg/kg in rats — Knoll's protocol) produced lifespan extension that didn't track linearly with MAO-B inhibition.

Metabolism — the methamphetamine question

Selegiline is metabolized by CYP2B6 (primary) + CYP2C19 + CYP3A4 (minor) into:

1. N-desmethylselegiline (DMS) — bioactive, weak MAO-B inhibitor itself
2. L-methamphetamine (levomethamphetamine) — the LEVO-rotatory enantiomer, NOT the d-methamphetamine recreational/abuse form. L-methamphetamine has ~10× less CNS stimulant activity than d-methamphetamine (it's the active ingredient in OTC Vicks inhalers). Half-life 14-21 hours.
3. L-amphetamine (levoamphetamine) — same chirality logic. Half-life 16-18 hours.

Bioavailability matters for metabolite load:

• Oral: 4-10% bioavailability → high first-pass → high metabolite exposure. A 5 mg oral dose generates meaningful L-amph + L-meth.
• Zydis ODT (orally disintegrating): 5-8× higher bioavailability than oral tablet via buccal absorption → at 1.25 mg ODT you get the same MAO-B inhibition as 10 mg oral with much LESS metabolite exposure.
• Transdermal patch: 75% bioavailability, bypasses gut entirely → 50× higher selegiline parent concentrations and ~70% LOWER metabolite exposure than equivalent oral. At 9-12 mg patch you're still getting less L-amph than 10 mg oral generates.

Why "no receptor downregulation" is technically correct

Dylan's external source claim — "selegiline doesn't downregulate receptors" — is mechanistically defensible and worth understanding plainly:

Dopamine-receptor downregulation is the homeostatic adaptation neurons make when receptors are over-stimulated by excessive agonist (e.g., chronic high-dose amphetamine releases massive supraphysiologic DA → post-synaptic D2 receptors decrease in number/sensitivity to compensate → tolerance + post-cycle "anhedonia"). Selegiline does NOT pharmacologically agonize dopamine receptors. It does NOT release DA from vesicles like amphetamine. What it does is:

1. Prevent catabolism of endogenously-released DA (so what your neurons release sticks around longer)
2. Slightly enhance impulse-coupled release via TAAR1 (so when neurons WANT to fire, they release a bit more)

The DA increase is physiologically gated — it tracks endogenous neuronal firing rather than overriding it. Result: receptors don't see the kind of supraphysiologic flood that drives downregulation. There is also no evidence in humans of significant DAT (dopamine transporter) downregulation at low-dose selegiline (one PMC study explicitly investigated this question and found no DAT change).

Caveat to the claim: at higher doses (>10 mg oral, 9-12 mg patch) where MAO-A is also inhibited and L-amph metabolite levels rise, you start getting SOME amphetaminergic action, and the "no downregulation" elegance starts to erode. The claim holds cleanest at the 1-2.5 mg oral microdose tier.