Substance P is a tachykinin family 11 amino acid neuropeptide involved in neurotransmission and neuromodulation. Substance P acts via the neurokinin-1 receptor (NK1-receptor, NK1R). [Tyr8]-Substance P is used as a biologically active analogue of Substance P. [Tyr8] SP is often used as a radioligand tracer in studies of Substance P binding and distribution.