NK1 (or tachykinin receptor TACR1) is a selective receptor for substance P with relatively higher affinity for neurokinin A compared to neurokinin B (100- and 500-fold lower). Neurons in lamina I of the spinal dorsal horn express the NK1 receptor for substance P and mediate hyperalgesia, an enhanced sensitivity to painful stimuli. Although substance P does not mediate the signaling of acute pain or hyperalgesia, it is essential for the full development of stress-induced analgesia and for an aggressive response to territorial challenge. It is also suggested that substance P is important for orchestrating the response of the animal to major stressors such as pain, injury or invasion of territory. Modulation of substance P activity offers a radical new approach to the management of depression, anxiety and stress. The substance P receptor is highly expressed in areas of the brain that are implicated in these behaviors.