Central tachykinins: mediators of defence reaction and stress reactions.

The tachykinins substance P, neurokinin A, and neurokinin B are natural agonists for NK1, NK2, and NK3 receptors, respectively. Evidence from biochemical, neurophysiological, pharmacological, and molecular biology studies indicates that the tachykinin-containing pathways within the brain contribute to central cardiovascular and endocrine regulation and to the control of motor activity. The hypothalamus, which represents a site for the integration of central neuroendocrine and autonomic processes, is rich in tachykinin nerve endings and tachykinin receptors. Stimulation of periventricular or hypothalamic NK1 receptors in conscious rats induces an integrated cardiovascular, behavioural, and endocrine response. The cardiovascular response is associated with increased sympathoadrenal activity and comprises an increase in blood pressure and heart rate, mesenteric and renal vasoconstriction, and hind-limb vasodilatation. The behavioural response consists of increased locomotion and grooming behaviour. This response pattern is consistent with an integrated stress response to nociceptive stimuli and pain in rodents. Several studies have demonstrated rapid changes in substance P levels and its receptors in distinct brain areas following acute stress. These data indicate that substance P and other tachykinins, in addition to serving as nociceptive and pain transmitters in the spinal cord, may act in the brain as neurotransmitters--neuromodulators within the neuronal circuits mediating central stress responses.