Expression of the immediate-early genes (IEG) c-FOS, NGF1-A and c-JUN was induced hy noxious thermal stimulation in neurons of the rat spinal cord dorsal horn. Intravenous injection of Kelatorphan (5, 10 and 20 mg/kg), an inhibitor of multiple enkephalin-degrading enzymes, 20 min before noxious stimulation reduced the overall number of dorsal horn neurons expressing c-FOS and NGF1-A by up to 20–30%. While c-FOS expression was suppressed in superficial and deep laminae of the spinal cord, NGF1-A and c-JUN was only suppressed in superficial laminae. Morphine (5, 7.5 and 10 mg/kg) produced a dose-dependent reduction of c-FOS expression by up to 70% only when injected before noxious stimulation. Morphine injected K) min after the noxious treatment was virtually ineffective. The depressant effect of Kelatorphan and morphine could be prevented by prior application of the opioid antagonist naloxone. Naloxone itself slightly increased the overall number of c-FOS-positive neurons in all laminae of the spinal cord.
The present data support the existence of a tonic release of endogenous opioid peptides at the spinal level and show that inhibition of their peptidase-induced degradation modulates IEG expression in dorsal horn neurons of the rat. The finding that opioid agonists were ineffective when applied after stimulation underline the necessity of pre-emptive analgesia to prevent long-term activity-dependent changes in spinal cord neurons.