Differential ability of tachykinin NK-1 and NK-2 agonists to produce scratching and grooming behaviours in mice

Potent and selective NK-1 and NK-2 agonists as well as compounds with lower selectivity and affinity for NK-1 binding sites were compared in their ability to produce scratching and grooming behaviours when injected intracerebroventricularly in mice. Septide, an agonist with a low affinity for NK-1 binding sites, [Sar 9, Met(O 2) 11]SP and to a lesser extent [Pro 9]SP, two potent and selective NK-1 agonists were the most effective drugs in stimulating these behaviours. Only high doses of [Apa 9,10]SP and [Lys 5, Tyr 7, Pro 8]NKA(4–10), two agonists with low affinity for NK-1 binding sites, produced scratching and grooming responses. Similarly, only high doses of [Lys 5, MeLeu 9, NLe 10]NKA(4–10), a potent NK-2 agonist, produced grooming behaviour. When coinjected with the endopeptidase enzyme inhibitor phosphoramidon, the effects of [Apa 9,10]SP, [Lys 5, Tyr 7, Pro 8]NKA(4–10) and [Pro 9]SP were markedly enhanced. Analyses of the potency of the different agents to displace 3H-SP binding in mouse subcortical structures revealed that the affinities of the agonists for NK-1 receptors are similar to those previously reported in rat brain. The efficacy of the agonists at producing behavioural responses was not equivalent to their potency to bind to central NK-1 receptors. These findings therefore suggest that a stimulation of NK-1 but also non classical NK-1 receptors are involved in the induction of scratching and grooming behaviours.