Effects of narcotic and non-narcotic analgesics on the abdominal or tail stimulation-induced struggling in rats.

As a model, we used struggling induced by a repetitive stimulation of the tail or abdomen and two types of pseudoaffective responses were evoked in rats. The effects of narcotic and non-narcotic analgesics on these responses were alternately assessed. Narcotics were markedly effective in inhibiting struggling elicited by tail stimulation and slightly less effective in suppressing the abdominal stimulation-induced struggling. These effects on two types of struggling differed quantitatively but not qualitatively. On the other hand, non-narcotics such as aspirin (200 mg/kg, i.p.), aminopyrine (160 mg/kg, i.p.) and indomethacin (20 mg/kg, i.p.) had an inhibitory action on the struggling elicited by the tail stimulation but showed equivocal and far less inhibition on the abdominal stimulation-induced response. These two effects were quantitatively and qualitatively different because the slope of the dose-response curves were not in parallel. The action of baclofen appears to be ambiguous since it had distinctive inhibitory actions on both typed of struggling. However, baclofen can be classified into the latter group in that it exerted an inhibitory action on two types of struggling, in a quantitatively different manner. These results suggest that two types of struggling in rats provide a convenient means to assess the potency of analgesics.