DIFFERENTIAL-EFFECTS OF MORPHINE ON NOXIOUS STIMULUS-EVOKED FOS-LIKE IMMUNOREACTIVITY IN SUBPOPULATIONS OF SPINOPARABRACHIAL NEURONS

In previous studies we reported that although morphine dose dependentl y inhibits noxious stimulus-evoked expression of the c-fos proto-oncog ene in the rat spinal cord, morphine was without effect in certain pop ulations of presumed nociresponsive neurons, even under conditions of complete behavioral analgesia. To determine whether the neurons that c ontinue to express the c-fos gene include projection neurons, we evalu ated the effect of morphine on noxious stimulus-evoked c-fos expressio n in spinoparabrachial neurons retrogradely labeled with Fluoro-gold. In the formalin test, we found that morphine analgesia was associated with a significant reduction in the number of Fos-like-immunoreactive spinoparabrachial projection neurons in the lateral reticulated area o f the neck of the dorsal horn. Morphine, however, did not reduce the n umber of Fos-like-immunoreactive spinoparabrachial projection neurons either in the superficial dorsal horn or in the area around the centra l canal. These results indicate that under conditions of morphine anal gesia two distinct populations of spinoparabrachial neurons can be rec ognized on the basis of their expression of the c-fos gene in response to noxious stimulation. Since the expression of the c-fos gene has be en correlated with neuronal activity, these data suggest that activity , and central transmission of nociceptive information, persists in cer tain nociresponsive projection neurons during morphine analgesia. Alte rnatively, if activity has, in fact, been blocked in these neurons, ou r results indicate that injury can produce significant molecular chang es in neurons even though the neuronal activity and pain associated wi th the injury is blocked by morphine.