Tail-pinch induced analgesia and immobility: altered responses to noxious tail-pinch by prior pinch of the neck.

Noxious pinch to the scruff of the neck using a metal clip produces profound immobility and analgesia. Noxious pinch delivered to the tail fails to induce immobility and results in nociceptive behavior directed at the pinched tail. However, when administered shortly after neck-clip removal, noxious tail-pinch reinstated immobility without any nociceptive response. Prior neck-clip also enhanced the antinociception induced by the tail-pinch as measured by nociceptive response to a leg pinch. Immobility, as well as antinociception, decreased as the time interval between neck-clip removal and the tail-pinch application increased. Pharmacological manipulations which reduce nociception produced a similar alteration in the response to tail-pinch. Thus, following local injections antinociceptive doses of lidocaine to the base of the tail and systemic morphine administration tail-pinch produced marked immobility. Transection of the brain at the intercollicular level provides evidence for supraspinal involvement in post-neck pinch effects. Not only was the ability of prior neck-pinch to confer antinociceptive properties on tail-pinch abolished, but increased responsiveness to noxious tail-pinch was seen. We, therefore, propose that prior neck-pinch confers new stimulus properties on noxious pinch of other body regions resulting in an enhanced antinociceptive effect, which affects both remote regions and the site of stimulation, and the ability to induce immobility.