The development of the nociceptive responses in neurokinin-1 receptor knockout mice

An important yet unanswered question is how neonates respond to painful stimuli, given the immaturity of their neural pathways. We examined the development of the neurokinin system using a novel approach, examining changes of this system by observing the pain responses of mice lacking the NK1 receptor at different stages of development We show that the NK1 receptor is not involved in nociception to heat, mechanical or chemical stimuli, at 3 days. In contrast, the NK1 receptor is involved in nociceptive responses to high intensity heat and mechanical stimuli, and mediates the second phase of the formalin response in 21-day-old mice. This indicates that nociception in neonates does not require the NK1 receptor and that the functional maturation of the NK1 receptor allows diversity in both the type of stimuli that activate the pain system and the types of responses elicited by nociceptive stimuli.