Selank

Selank is a synthetic heptapeptide (Thr-Lys-Pro-Arg-Pro-Gly-Pro) built by extending the immunoglobulin-derived tetrapeptide tuftsin with a Pro-Gly-Pro tail that resists enzymatic degradation. The proposed mechanism is genuinely multimodal — three converging lines of mechanistic evidence have accumulated across two decades of Russian and Western primary research.

The first line is opioidergic: Kost and colleagues established in 2001 that Semax and Selank inhibit human serum enkephalin-degrading enzymes with IC50 = 20 µM for Selank, three orders of magnitude more potent than the standard inhibitor puromycin, with the inhibitory activity localized to the C-terminal Pro-Gly-Pro extension that is the structural innovation common to both peptides. The Sokolov 2002 in-vivo follow-up showed that 100 µg/kg Selank produced both anxiolytic effects and prolonged plasma leu-enkephalin half-life in BALB/c mice (the high-anxiety phenotype) but had no detectable effect in C57BL/6 mice (the low-anxiety phenotype) — a clean strain-by-treatment interaction consistent with the opioidergic mechanism keyed to baseline enkephalinergic tone.

The second line is GABAergic: Volkova et al. 2016 showed in rat frontal cortex that a single intranasal 300 µg/kg dose of Selank produced a gene-expression signature at 1 hour with strong positive correlation (Pearson's r = 0.86) to GABA's own expression-change pattern across an 84-gene array, consistent with positive allosteric modulation of the GABAergic system. At 3 hours the patterns diverged — Selank uniquely drove a 128-fold increase in hypocretin (Hcrt) expression alongside a reversal of the early downregulation of GABA-A receptor epsilon (16-fold up) and theta (13-fold up) subunits. The Hcrt upregulation is the candidate molecular explanation for why Selank does not produce the sedation that benzodiazepines do at the same GABAergic engagement.

The third line is cognitive / learning facilitation: Kozlovskii and Danchev 2003 showed in rats that Selank 300 µg/kg significantly improved acquisition of a conditioned active avoidance reflex, with the effect front-loaded onto day 1 in animals with initially poor learning ability and emerging on day 3 in normal-learning animals. The differential time-course profile distinguished Selank from piracetam in the same paradigm.

Unlike benzodiazepines, Selank does not appear to bind the benzodiazepine site of the GABA-A receptor directly — the receptor engagement appears allosteric and at non-classical sites, which is the hypothesis underlying the reported non-sedating, non-dependency-forming profile. In Russian clinical use it is administered intranasally; subcutaneous and intravenous routes appear in the experimental rodent literature.