MODULATION OF SPINAL REFLEX ACTIVITIES IN ACUTE SPINAL RATS WITH α-ADRENERGIC AGONISTS AND ANTAGONISTS

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Abstract
The effects of clonidine, a centrally acting .alpha.-adrenergic agonist, on the tail-flick reflex in acutely spinalized rats were investigated and differential potentials of several .alpha.-adrenergic blockers in modulating the effect of clonidine were compared. In order to observe the interactions of clonidine with 5 .alpha.-blockers, the animals were pretreated with 1 of the .alpha.-blockers 10 min prior to clonidine injection. The inhibitory effect of clonidine upon the tail-flick reflex was completely antagonized by pretreatment with yohimbine (3 mg/kg), tolazoline (20 mg/kg) and phentolamine (10 mg/kg); clonidine-induced reflex potentiation was not influenced by pretreatment with higher doses of these 3 .alpha.-blockers (doses up to 8 mg/kg of yohimbine, 50 mg/kg of tolazoline and 20 mg/kg of phentolamine were without effect). Pretreatment with phenoxybenzamine (5 mg/kg) or chlorpromazine (0.5 mg/kg) did not produce any antagonism on clonidine-induced reflex inhibition, but rather potentiated the inhibition, and 0.5 mg/kg of clonidine, which usually enhanced reflex activities in untreated animals, showed a marked inhibition after phenoxybenzamine or chlorpromazine treatment. Chlorpromazine or phenoxybenzamine alone had little inhibitory effect on the reflex at the dose used here, although it inhibited significantly the tonic component which had been previously elevated by 0.5 mg/kg of clonidine. In phenoxybenzamine- or chlorpromazine-treated animals, the reflex inhibition by 0.5 mg/kg of clonidine was completely restored following an injection of yohimbine or tolazoline. Clonidine affects the tail-flick reflex in the acutely spinalized rat in 2 ways, one involving the inhibitory mechanism and the other the excitatory one, and each .alpha.-blocker interacts specifically with either one or the other or both adrenoceptive mechanisms. The .alpha.-blockers should be carefully selected when central adrenergic mechanisms are being investigated.

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