DIFFERENTIAL-EFFECTS OF SPINALIZATION ON DISCHARGE PATTERNS AND DISCHARGE RATES OF SIMULTANEOUSLY RECORDED NOCICEPTIVE AND NONNOCICEPTIVE SPINAL DORSAL HORN NEURONS

Recordings were made simultaneously from 2-5 neurons at the same site in the lumbar spinal dorsal horn of pentobarbital-anesthetized rats. N eurons were classified as low-threshold (LT) or multireceptive (MR) ac cording to their responses to non-noxious mechanical or noxious radian t heat stimuli of the skin. At the same recording sites neurons could be encountered which belong to different classes and/or which had mech anoreceptive fields which did not overlap. Cold blocks of the upper or lower thoracic cord or transsections of the upper cervical cord were made to evaluate the effects of spinalization on both the rate and pat tern of background activity and/or noxious heat-evoked responses of di fferent dorsal horn neurons under identical experimental conditions. A t 24 of 27 recording sites, spinalization had qualitatively or quantit atively different effects on the rate of background activity of simult aneously recorded neurons. Interspike interval (ISI) means of backgrou nd activity were significantly reduced in 29 of 65 (44.6%) neurons, pr olonged in 23 of 65 (35.4%) neurons, or unchanged in 13 of 65 (20%) ne urons. MR neurons displayed a significantly higher incidence of decrea sed background activity 17 of 45 (37.8%) and a lower incidence of incr eased background activity (18 of 45, 40%) during spinalization than th e LT neurons from which 1 of 12 (8.3%) decreased and 8 of 12 (66.6%) i ncreased background activity. Almost all (95.4%) neurons changed their discharge patterns after spinalization. At 9 of 27 recording sites, t he discharge patterns of simultaneously recorded neurons were affected differently by spinalization as reveled by the coefficient of dispers ion of the interspike intervals (ISI), indicating changes in the tende ncy to discharge action potential in clusters (bursts). At the same re cording sites the level of noxious heat-evoked responses of simultaneo usly recorded MR neurons was also differentially affected by spinaliza tion. Nociceptive responses were significantly enhanced in 19 of 37 (5 1.4%) neurons (137.8 +/- 142.6% of control, mean +/- SD), reduced in 1 3 of 37 neurons (35.1%) (by 58.9 +/- 20.9%) and/or unchanged in 5 of 3 7 (13.5%) neurons. It is concluded that no general 'tone' of descendin g antinociception exists and that tonic descending excitatory and inhi bitory systems may be active simultaneously modulating both the level and pattern of neuronal discharges.