DIFFERENTIAL-EFFECTS OF MORPHINE ON CORNEAL-RESPONSIVE NEURONS IN ROSTRAL VERSUS CAUDAL REGIONS OF SPINAL TRIGEMINAL NUCLEUS IN THE RAT

The initial processing of corneal sensory input in the rat occurs in t wo distinct regions of the spinal trigeminal nucleus, at the subnucleu s interpolaris/caudalis transition (Vi/Vc) and in laminae I-Il at the subnucleus caudalis/spinal cord transition (Vc/C1). Extracellular reco rding was used to compare the effects of morphine on the evoked activi ty of corneal-responsive neurons located in these two regions. Neurons also were characterized by cutaneous receptive field properties and p arabrachial area (PBA) projection status. Electrical corneal stimulati on-evoked activity of most (10/13) neurons at the Vi/Vc transition reg ion was increased [146 +/- 16% (mean +/- SE) of control, P < 0.025] af ter systemic morphine and reduced after naloxone. None of the Vi/Vc co rneal units were inhibited by morphine. By contrast, all corneal neuro ns recorded at the Vc/C1 transition region displayed a naloxone-revers ible decrease (55 +/- 10% of control, P < 0.001) in evoked activity af ter morphine. None of 13 Vi/Vc corneal units and 7 of 8 Vc/C1 corneal units tested projected to the PEA. To determine if the Vc/C1 transitio n acted as a relay for the effect of intravenous morphine on corneal s timulation-evoked activity of Vi/Vc units, morphine was applied topica lly to the dorsal brain stem surface overlying the Vc/C1 transition. L ocal microinjection of morphine at the Vc/C1 transition increased the evoked activity of 4 Vi/Vc neurons, inhibited that of 2 neurons, and d id not affect the remaining 12 corneal neurons tested. In conclusion, the distinctive effects of morphine on Vi/Vc and Vc/C1 neurons support the hypothesis that these two neuronal groups contribute to different aspects of corneal sensory processing such as pain sensation, autonom ic reflex responses, and recruitment of descending controls.