Functional evidence for multiple receptor activation by κ-ligands in the inhibition of spinal nociceptive reflexes in the rat

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Abstract
1 The evidence for κ-receptor heterogeneity is equivocal. We have now investigated this question by comparing the effects of five putatively selective κ-agonists. The parameters examined were: the relative potencies in depressing hindlimb flexor muscle reflexes to noxious pinch stimuli in both spinalized and sham-spinalized rats; the reversibility of these effects by naloxone; and the effects on blood pressure. 2 Two types of drug effect was discriminated. One drug group, represented by U-50,488, U-69,593 and PD-117,302, had a potency ratio between sham and spinalized rats approximately 10 fold lower than the other group, which comprised GR103545 and CI-977. 3 Under sham-spinalized conditions, CI-977 and GR103545 at high doses caused only sub-maximal reductions of spinal reflexes. U-50,488 was still active when superimposed on these high doses of GR103545. 4 Naloxone reversed all effects, but different doses were required between compounds, with GR103545 taking some 20 times higher doses of naloxone to cause reversal than did U-50,488. 5 The effects on mean arterial pressure were opposite between groups. 6 The results imply that more than one type of naloxone-sensitive non-μ opioid receptor must be involved in mediating these complex actions of ligands that have been claimed to be selective for κ-receptors.