Neurotrophins are a compact family of secreted growth factors comprising NGF, BDNF, NT-3 and NT-4/5. All four share a cystine-knot fold, all form non-covalent homodimers, and all are produced as pro-forms that are cleaved by furin or proconvertases. They govern neuronal survival, axonal growth, synaptic strength and, in sensory neurons, nociceptor sensitisation.
Receptor logic
Two receptor systems operate in parallel. The Trk receptor tyrosine kinases give the family its selectivity: NGF prefers TrkA, BDNF and NT-4/5 prefer TrkB, NT-3 prefers TrkC with some TrkA/TrkB cross-reactivity. The p75 neurotrophin receptor, a TNF-receptor superfamily member, binds all four with similar affinity and, with sortilin, preferentially binds the uncleaved pro-forms — where it signals apoptosis and growth-cone collapse. Whether a neurotrophin supports or eliminates a neuron therefore depends on processing state and on which receptors that cell expresses.
For catalog purposes: neurotrophins themselves are recombinant proteins, not synthetic peptides. Small peptides marketed in the cognitive category are studied as modulators of neurotrophin expression or as Trk-directed mimetics, a distinction worth preserving in any experimental write-up.
Related terms and material
nootropic · recombinant · in vitro. Reference material: neurotrophin research family, nootropic peptides. Further reading: brain research hub.