title: Differential effects of morphine on corneal-responsive neurons in rostral versus caudal regions of spinal trigeminal nucleus in the rat.
abstract: 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.
authors.affiliation: Department of Neuroscience, Brown University/Rhode Island Hospital, Providence, Rhode Island 02903, USA.
authors.forename: I D
authors.initials: ID
authors.lastname: Meng
authors.forename: J W
authors.initials: JW
authors.lastname: Hu
authors.forename: D A
authors.initials: DA
authors.lastname: Bereiter
mesh_terms: D000287:Administration, Topical; D000344:Afferent Pathways / Q000502:physiology; D000818:Animals; D003315:Cornea / Q000294:innervation*; D004558:Electric Stimulation; D007275:Injections, Intravenous; D008297:Male; D008845:Microinjections; D009020:Morphine / Q000008:administration & dosage / Q000494:pharmacology*; D009270:Naloxone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D009475:Neurons, Afferent / Q000187:drug effects* / Q000502:physiology; D009619:Nociceptors / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D013314:Stress, Mechanical; D014278:Trigeminal Nuclei / Q000166:cytology / Q000187:drug effects*
chemical_list: D009292:Narcotic Antagonists; D009270:Naloxone; D009020:Morphine