Clip-induced analgesia and immobility in the mouse: pharmacological characterization

A pinch to the nape of the neck of mice, by application of a noxious clip, produces analgesia and immobility. Because both opiate and dopaminergic systems are usually implicated in analgesia and immobility, the pharmacological profile of clip-induced effects was compared to those elicited by the dopamine antagonist haloperidol, and by morphine. In addition, the effects of a series of pharmacological agents on clip-induced effects was examined. Haloperidol, but not morphine, produced immobility similar to that seen after application of the clip to the neck. Application of clip completely inhibited righting in all tests utilized. Haloperidol inhibited righting in all tests, except for inversion from a supine position. Righting from this position could also be inhibited in mice treated with haloperidol when a mild pinch, ineffective in a naive animal, was applied. Clip-induced immobility, but not analgesia, was reversed by amphetamine. Administration of the cholinergic antagonist scopolamine, but not methylscopolamine, reversed both the analgesia and immobility. Pinch-induced analgesia was as marked as that elicited by morphine but could not be reversed by the opiate antagonist naloxone. It is proposed that pinch-induced immobility is mediated by both dopaminergic and cholinergic systems. Additional unidentified systems are also involved. Analgesia, induced by a noxious pinch, can be dissociated pharmacologically from the immobilizing effect, is non-opiate in nature, and involves activation of central cholinergic synapses.