Effects of analgesic-antipyretics on the spinal reflex potentials in cats: an analysis of the excitatory action of aminopyrine

Abstract

The effects of some analgesic-antipyretics on the spinal reflex potentials were studied in spinal cats. Aminopyrine at 25-100 mg/kg, i.v. produced a marked increase in mono- and poly-synaptic reflex potentials (MSR and PSR), and a decrease in dorsal root reflex potentials (DRR) in a dose-dependent manner. The amplitude of DRR decreased by aminopyrine was reversed by diazepam at 0.2 mg/kg, i.v.; however, the increased amplitudes of MSR and PSR were not affected by diazepam. Pretreatment of semicarbazide at 200 mg/kg, i.v. did not influence the increasing effect of aminopyrine on MSR and PSR. DL-5-Hydroxytryptophan produced facilitation of the MSR and PSR. In DL-5-hydroxytryptophan-treated cats, the amplitude of MSR was further increased by aminopyrine. Methysergide at 1 mg/kg, i.v. antagonized this increasing effect of aminopyrine on MSR and PSR. These observations suggest that the excitatory action of aminopyrine may be partly related to 5-hydroxytryptamine and not connected to the GABAergic mechanism. Other pyrazolone derivatives were also studied. Isopropylantipyrine at 50 mg/kg, i.v. produced increases in MSR and PSR. Intravenous sulpyrine at 500 mg/kg, antipyrine at 50 mg/kg or 4-aminoantipyrine at 50 mg/kg did not affect the reflex potentials. The non-pyrazolones, acetaminophen and indomethacin, did not increase the MSR and PSR. These results suggest that the N-dimethyl or isopropyl residue at the 4 position of the pyrazolone structure plays an important role in the excitatory action of analgesic-antipyretics in cat spinal cord.