SUPRASPINAL INFLUENCE ON HINDLIMB WITHDRAWAL THRESHOLDS AND MUSTARD OIL-INDUCED SECONDARY ALLODYNIA IN RATS

We examined the role of supraspinal structures in secondary allodynia induced by mustard oil in awake rats. To produce allodynia (=unpleasen t sensation evoked by innocuous stimuli), mustard oil (50%) was applie d for 2 min to the skin of the ankle of one hindlimb. Mechanical hyper sensitivity of the skin was tested by determining the hindlimb withdra wal threshold to a series of monofilaments applied to the glabrous foo t pad (=distal to the mustard oil-treated ankle). In intact rats, must ard oil produced a secondary allodynia in the mustard oil-treated hind limb as indicated by a decreased withdrawal threshold to mechanical te st stimuli applied to the glabrous skin (=outside the mustard oil-trea ted ankle), whereas the withdrawal threshold in the contralateral (=co ntrol) hindlimb was not changed. Following spinalization, mustard oil treatment produced no secondary allodynia, but the interpretation of t his finding was complicated by a concomitant bilateral elevation of hi ndlimb withdrawal thresholds to mechanical skin stimulation. However, the spinalized rats had shorter tail-flick latencies to radiant heat t han intact rats. Administration of an opioid antagonist, naloxone (1 m g/kg, SC), had no effect on withdrawal thresholds in spinalized animal s. Importantly, microinjection of lidocaine (4%) into the nucleus raph e magnus in rats with an intact spinal cord had a selective antiallody nic effect when the injection volume was 1.0 mu l but not when it was 0.5 mu l Lidocaine (4%, 0.5 Gel) in the lateral reticular nucleus of t he medulla also attenuated the spinal hypersensitivity, however, conco mitantly with motor side effects, due to which this finding maybe arti ficial. It is concluded that brain stem spinal pathways, originating a djacent to but not within the raphe magnus, contribute to the behavior al expression of secondary allodynia induced by neurogenic inflammatio n of the skin. Furthermore, there is a differential tonic control of v arious spinal reflexes by the brain stem as indicated by the dissociat ive effects of spinalization on mechanically induced hindlimb withdraw al vs. heat-induced tail-flick reflex. (C) 1997 Elsevier Science Inc.