The Effects of Hyperventilation upon the Spinal Pain Modulating System

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The purpose of this study is to investigate the effects of hyperventilation on the wide dynamic range (WDR) cell of the dorsal horn of the feline lumbar spinal cord.Under enflurane anesthesia, cats were prepared with midcollicular decerebration and lumbar laminectomy. The spinal cord was transected at T12-L1. WDR cells, responding primarily to noxious peripheral stimuli, were sampled with a microelectrode at the depth of 1500-2000 um from the cord dorsum. Spontaneous activity and noxiously evoked activity by pinching the receptive field on the hind paw were measured.Two series of experiments were carried out to characterize the effects of hyperventilation. In the first, following the control period with PaCO_2 of 35-45 mHg, respiratory rate and tidal volume were adjusted to make a level of hypocapnia as low as 25-30 and 20-25 mmHg. The recovery of cell activity was followed when normocapnia was restored. In the second, after activities of WDR cells were well suppressed by hypocapnia of … More PaCO_2 20-25mmHg, naloxone 0.1mg/kg or phentolamine 0.5mg/kg with normal saline 1.0 ml was given intravenously. Changes of firings of WDR cells were investigated. Retuening to normocapnia, recovery of firings was followed.Hypocapnia with PaCO_2 of 25-30 mmHg and 20-25 mmHg suppressed the activity of WDR cell significantly. At PaCO_2 of 25-30 mmHg, spontaneous activity was suppressed for about 20 % and evoked activity for 27 %. Ar PaCO_2 of 25-30 mmHg the spontaneous activity was suppressed for. about 50 % and evoked activity for 33 %. The results suggest that hyperventilation has suppressive effects on single-unit activity of WDR cell. Both naloxone and phentolamine significantly antagonized the suppressive effects of hyperventilation upon the activity of WDR cell.Our results suggest that hyperventilation has suppressive effects on single-unit activity of WDR cell and the mechanisms of those suppressive effects are related to not only endogeneous opiate but also adrenergic pain modulating system. Less