Spinal laminae I-II neurons in rat recorded in vivo in whole cell, tight seal configuration: Properties and opioid responses

Using the in vivo whole cell recording procedure described previously, we r ecorded 73 neurons in laminae I and II: in the lumbar spinal cord of the ra t. Input impedances averaged 332 M Omega, which indicated that prior sharp electrode recordings contained a significant current shunt. Characterizatio n of the adequate stimuli from the excitatory hindlimb receptive held indic ated that 39 of 73 neurons were nociceptive, 6 were innocuous cooling cells , 20 responded maximally to brush, and 8 cells were not excited by stimulat ion of the skin of the hindlimb. The locations of 15 neurons were marked wi th biocytin. Nociceptive neurons were mostly found in lamina I and outer II , cooling cells in lamina I, and innocuous mechanoreceptive cells were most ly found in inner II or in the overlying white matter. The mu-opioid agonis t [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-Enkephalin (DAMGO) hyperpolarized 7 of 19 tested neurons with a conductance increase. This hyperpolarization was reversed by naloxone in the neurons in which it was applied. DAMGO also dec reased the frequency of spontaneous PSPs in 13 neurons, 7 of which were als o hyperpolarized by DAMGO. Five of the seven hyperpolarized neurons were no ciceptive, responding to both heat and mechanically noxious stimuli, wherea s two responded to slow, innocuous brush. These results indicate that whole cell, tight seal recordings sample a similar population of lamina I and II neurons in the rat as those found with sharp electrode recordings in cat a nd monkey. They further indicate that DAMGO hyperpolarizes a subset of the nociceptive neurons that have input from both heat and mechanical nocicepto rs and that presynaptic DAMGO effects can be observed in nociceptive neuron s that are not hyperpolarized by DAMGO.