An orphan receptor resembling the neurokinin 3 tachykinin receptor (NK3), initially claimed to be an atypical opioid receptor, is shown herein to respond potently to the physiological NK3 receptor ligand, neurokinin B. This ‘NK4’ receptor did not give functional responses in Xenopus oocytes to opioid agonists. However, NK4 receptor activation was inhibited by nanomolar concentrations of dynorphin. The NK4 receptor is therefore a tachykinin receptor which is functionally antagonized by an endogenous opioid peptide.

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