The Journal of Neurophysiology Vol. 79 No. 5 May 1998, pp. 2593-2602
Copyright ©1998 The American Physiological Society

Differential Effects of Morphine on Corneal-Responsive Neurons in Rostral Versus Caudal Regions of Spinal Trigeminal Nucleus in the Rat

Ian D. Meng¹, James W. Hu³, and David A. Bereiter^{1, 2}

¹ Department of Neuroscience and ² Department of Surgery, Brown University/Rhode Island Hospital, Providence, Rhode Island 02903; and ³ Faculty of Dentistry, University of Toronto, Toronto, Ontario M5G 1G6, Canada

Meng, Ian D., James W. Hu, and David A. Bereiter. Differential effects of morphine on corneal-responsive neurons in rostral versus caudal regions of spinal trigeminal nucleus in the rat. J. Neurophysiol. 79: 2593-2602, 1998. The initial processing of corneal sensory input in the rat occurs in two distinct regions of the spinal trigeminal nucleus, at the subnucleus interpolaris/caudalis transition (Vi/Vc) and in laminae I-II at the subnucleus caudalis/spinal cord transition (Vc/C1). Extracellular recording was used to compare the effects of morphine on the evoked activity of corneal-responsive neurons located in these two regions. Neurons also were characterized by cutaneous receptive field properties and parabrachial area (PBA) projection status. Electrical corneal stimulation-evoked activity of most (10/13) neurons at the Vi/Vc transition region was increased [146 ± 16% (mean ± SE) of control, P < 0.025] after systemic morphine and reduced after naloxone. None of the Vi/Vc corneal units were inhibited by morphine. By contrast, all corneal neurons recorded at the Vc/C1 transition region displayed a naloxone-reversible decrease (55 ± 10% of control, P < 0.001) in evoked activity after morphine. None of 13 Vi/Vc corneal units and 7 of 8 Vc/C1 corneal units tested projected to the PBA. To determine if the Vc/C1 transition acted as a relay for the effect of intravenous morphine on corneal stimulation-evoked activity of Vi/Vc units, morphine was applied topically to the dorsal brain stem surface overlying the Vc/C1 transition. Local microinjection of morphine at the Vc/C1 transition increased the evoked activity of 4 Vi/Vc neurons, inhibited that of 2 neurons, and did 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, autonomic reflex responses, and recruitment of descending controls.

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