|Budget Amount *help
¥1,100,000 (Direct Cost : ¥1,100,000)
Fiscal Year 1997 : ¥600,000 (Direct Cost : ¥600,000)
Fiscal Year 1996 : ¥500,000 (Direct Cost : ¥500,000)
The phasic activity of wide dynamic range (WDR) neurons in the spinal dorsal horn in response to noxious stimulation is suppressed by anesthetics. However, the effects of anesthetics on the activity of WDR neurons that is evoked by a tonic pain state, as in the formalin test, are not well understood. Therefore, we examined the effects of intravenous anesthetics, ketamine and thiamyla1 (3, l0 and 30 mg.kg^<-1> inhalational anesthetics, isoflurane and halothane (0.5% and l.5%) on the activity of WDR neurons that is evoked by the formalin test. In addition, dose-response curves for thiamylal and ketamine were compared between preemptive administration and later administration.
Methods and Results : In decerebrate, spinal cord-transected cats (L1-2), a subcutaneous (s.c.) injection of formalin (0.05 ml ; 5.0%) was placed in the receptive fields of spinal WDR neurons in the hind paw. This injection elicited an immediate and continuous discharge or burst of activity in the neurons that lasted
for at least 60 minutes. Intravenous thiamylal and ketamine, administered 5 min prior to the injection of formalin inhibited this neuronal discharge in a dose-dependent manner. However, when thiamylal and ketamine was administered 5 min after the injection of formalin, the inhibitory effect on the neuronal discharge was significantly smaller. 1n contrast, preemptive administerted inhalatinal anesthetics, isoflurane and halothane did not different from the posttreatment administration. An s.c. injection of saline (0.05 ml) in the same receptive field elicited only transient neuronal excitation and the activity returned to the pre-injection level within l-2 min.
Conclusions : The results suggest that an injection of formalin reliably produces a continuous burst of neuronal activity in the spinal dorsal horn of cats and, moreover, that this activity can be markedly reduced by the preemptive intravenous administration of thiamylal and ketamine. The ability of thiamylal and ketamine, but not inhalational anesthetics, to inhibit this neuronalresponse was found to be related to the timing of the formalin injection. Less