Immobility accompanies the antinociception mediated by the rostral ventromedial medulla of the rat

The rostral ventromedial medulla (RVM) is part of a descending pain modulat ory system that runs from the periaqueductal gray (PAG) to the spinal cord. The objective of the present study was to determine whether the antinocice ption mediated by the RVM is associated with locomotor changes as has been reported for the PAG [42]. Kainate (4, 20, or 40 pmol), morphine (1, 5, or 10 mu g), or saline (0.2 or 0.5 mu l) was injected into the RVM and locomot ion and nociception assessed. Microinjections of kainate and morphine that produced antinociception almost invariably inhibited locomotor activity. In some rats this immobility consisted of no movements when placed in the cen ter of the open field chamber. These data are consistent with the immobilit y and antinociception produced by activation of the ventrolateral PAG and i ndicate that the descending ventrolateral PAG/RVM system integrates a behav ioral response of which antinociception is only one component. (C) 2000 Els evier Science B.V. All rights reserved.