THE EFFECTS OF STRYCHNINE, BICUCULLINE, AND KETAMINE ON IMMERSION-INHIBITED DORSAL HORN CONVERGENT NEURONS IN INTACT AND SPINALIZED RATS

In both intact and spinalized rats, this study examined the effects of strychnine (a glycine antagonist), bicuculline (a GABA, antagonist), and ketamine (a non-competitive NMDA receptor antagonist) on one parti cular class of lumbar dorsal horn convergent neurons. This group of co nvergent neurons are inhibited when a rat's entire ipsilateral hindpaw is immersed in 50 degrees C water. A strong neuronal afterdischarge f ollows as soon as the paw is removed from the water. Strychnine (2 mg/ kg, iv) increased ongoing activity and blocked the 'inhibition phase' in both intact and spinalized rats demonstrating that a spinal-related glycine mechanism was involved in the inhibition. However, only in in tact rats did the firing rate of the 'afterdischarge phase' increase s ignificantly from pre-drug levels, suggesting that supraspinal sites m ay be involved in modulating this phase. Ketamine (15 mg/kg, iv) depre ssed ongoing activity and the firing rate during the afterdischarge ph ase of these neurons. Additionally, ketamine reversed the strychnine-i nduced increase in ongoing activity. Bicuculline (2 mg/kg, iv) had no effect on the activity of this cell class. As shown previously, and re plicated here, these 'immersion-inhibited' neurons invariably have bot h inhibitory and excitatory mechano-receptive fields on the ipsilatera l hindpaw. Thus, the response of this class of convergent neurons to n oxious stimulation may be a function of relative inputs of glycine and EAA's,each triggered by the stimulation of different receptive fields /regions on the same paw. Furthermore, when both fields are co-stimula ted during noxious immersion of the entire paw, glycine has a stronger influence on activity than does the EAA's. (C) 1998 Elsevier Science B.V.