The medullary dorsal reticular nucleus as a pronociceptive centre of the pain control system

The endogenous pain control system has long been considered as engaged in pain depression through the commitment of multiple descending actions that reduce the response capacity of spinal dorsal horn nociceptive neurones. Such a pure inhibitory antinociceptive nature was lately questioned by the observation of pronociceptive effects from areas classically regarded as anti nociceptive. The thereby raised hypothesis of a more versatile functional arrangement that dynamically adjusts the pain modulatory effect to multiple conditions by balancing several excitatory and inhibitory actions found strong support on the recent discovery of a medullary area particularly dedicated to pain facilitation. Lesioning the medullary dorsal reticular nucleus (DRt) depresses nociceptive responses to acute and inflammatory pain, whereas stimulation produces the inverse effect. The decrease in formalin-induced pain behaviour following DRt lesioning is accompanied by a decrease of spinal noxious-evoked c-fos neuronal activation. DRt blocking by lidocaine results in a decrease of the nociceptive activity of spinal dorsal horn neurones, whereas stimulation by glutamate has the opposite effect. A reciprocal disynaptic putative excitatory circuit that links the DRt and the spinal dorsal horn and conveys nociceptive input through the ascending branch was described, indicating that the DRt pain facilitating action is mediated by a reverberating spino-DRt circuit that promotes the enhancement of the response capacity of spinal neurones to noxious stimulation. The demonstration of a primary pronociceptive centre in the endogenous pain control system brings new important data to the emerging concept of pain modulation as a dynamic and flexible process that integrates nociceptive processing by balancing multiple excitatory and inhibitory actions as the way of adapting to the various unsteady pain determinants. (C) 2002 Elsevier Science Ltd. All rights reserved.