The rostral ventromedial medulla mediates the facilitatory effect of microinjected orphanin FQ in the periaqueductal gray on spinal nociceptive transmission in rats.
Single unit extracellular recordings from spinal dorsal horn neurons were obtained with glass micropipettes in pentobarbital-anesthetized rats. A total of 50 wide dynamic range (WDR) neurons were studied in 25 rats. Microinjection of orphanin FQ (OFQ, 0.1 microg/0.1 microl) (a potent endogenous ligand of the opioid receptor-like receptor (ORL-1)) into the ipsilateral ventrolateral parts of periaqueductal gray (vlPAG) significantly increased C-response and post-discharge activity in most of the WDR neurons. Pre-microinjection of lidocaine (4%) into the nucleus raphe magnus (NRM) (0.5 microl), ipsilateral nucleus reticularis gigantocellularis (NGC) (0.6 microl), or nucleus gigantocellularis pars alpha (NGCalpha) and nucleus reticularis paragigantocellularis lateralis (NPGL) (0.5 microl) markedly reduced intra-vlPAG microinjection of OFQ-induced facilitatory effects on nociceptive responses of WDR neurons. Furthermore, if the NRM and ipsilateral NGC were simultaneously pre-microinjected with lidocaine, the intra-vlPAG microinjection of OFQ-induced facilitation on nociceptive responses of WDR neurons was eliminated. Also, a similar effect was observed when all the NRM, ipsilateral NGC, NGCalpha and NPGL were blocked with lidocaine. No significant effect on nociceptive responses of WDR neurons per se was found after blocking the NRM, ipsilateral NGC, NGCalpha/NPGL, or all the NRM, ipsilateral NGC, and NGCalpha/NPGL with lidocaine. These results indicate that (1) the facilitatory effect evoked by microinjection of OFQ into the vlPAG on nociceptive responses of WDR neurons in the spinal dorsal horn is primarily mediated by the NRM and ipsilateral NGC; (2) the NRM, ipsilateral NGC, and NGCalpha/NPGL do not mediate tonic descending inhibition of the spinal dorsal horn neurons.
Yang
Zhi-Lan
ZL
State Key Laboratory of Medical Neurobiology, Department of Neurobiology, Medical Center of Fudan University, 138 Yi Xue Yuan Road, Shanghai 200032, China.
Gao
Yong-Jing
YJ
Wu
Gen-Cheng
GC
Zhang
Yu-Qiu
YQ
D000200:Action Potentials / Q000187:drug effects; D000779:Anesthetics, Local / Q000494:pharmacology; D000818:Animals; D004558:Electric Stimulation; D004594:Electrophysiology; D008012:Lidocaine / Q000494:pharmacology; D008297:Male; D008845:Microinjections / Q000379:methods; D018847:Opioid Peptides / Q000494:pharmacology*; D010487:Periaqueductal Gray / Q000187:drug effects* / Q000502:physiology; D020671:Posterior Horn Cells / Q000187:drug effects / Q000502:physiology; D011903:Raphe Nuclei / Q000033:anatomy & histology / Q000187:drug effects; D051381:Rats; D017207:Rats, Sprague-Dawley; D011930:Reaction Time; D012154:Reticular Formation / Q000033:anatomy & histology / Q000187:drug effects; D012867:Skin / Q000294:innervation; D013116:Spinal Cord / Q000166:cytology / Q000187:drug effects* / Q000502:physiology; D014665:Vasodilator Agents / Q000494:pharmacology*; D000097629:Nociceptin
D000779:Anesthetics, Local; D008012:Lidocaine; D018847:Opioid Peptides; D014665:Vasodilator Agents; D000097629:Nociceptin