Effects of activators and antagonists of the neuropeptides substance P and substance K on cell proliferation in planarians.

Substance P and substance K (Neurokinin A) are mammalian peptides belonging to the tachykinin family. Both have been studied extensively, are widely distributed in both central and peripheral mammalian nervous systems, and seem to be involved in pain reactions and inflammatory responses. We report here that substance P and substance K, as well as Epidermal Growth Factor (EGF), are potent mitogens, at micro and nanomolar concentrations, for planarian cells. This stimulation is inhibited by the substance P and substance K antagonist spantide, while capsaicin, a pungent agent of capsicum peppers that destroys sensory neurons, stimulates cell division, probably through release of substance P. These results, jointly with the reported stimulation of cell division by naloxone and its inhibition by Met-Enkephalin (Baguña, 1986), both probably acting on tachykinin release, suggest that target cells, the neoblasts, must have in their cell membranes numerous receptors for growth hormones and neuropeptides analogous to their mammalian counterparts.