In the myenteric plexus of rat ileum, NK(1) and NK(3) receptors are co-located almost exclusively on neurons of a single population. This study compares endocytosis of NK(1) and NK(3) receptors in these neurons. In the absence of agonist, 26.2+/-2.8% of NK(1) receptor and 29.1+/-1.1% of NK(3) receptor was located in the cytoplasm of the neurons; the remaining receptor was on the surface. The tachykinin neurotransmitters, substance P (10 pM-10 microM) and neurokinin A (10 pM-100 microM), both induced concentration-dependent endocytosis of NK(1) and NK(3) receptors. The selective NK(1) receptor agonist, [Sar(9),Met(O(2))(11)]-substance P (1 microM), induced endocytosis of NK(1) receptor (64.2+/-1.5% in cytoplasm) but not NK(3) receptor (32.9+/-5.0%). The NK(1) receptor endocytosis was reduced by the selective NK(1) receptor antagonist, CP-99994 (100 nM), but not by the selective NK(3) receptor antagonist, SR-142801 (1 microM). The selective NK(3) receptor agonist, senktide (10 nM), induced endocytosis of NK(3) receptor (61.2+/-5.4%) but not NK(1) receptor (34.0+/-4.5%). The NK(3) receptor endocytosis was blocked by SR-142801 but not by CP-99994. We also investigated the effects of monensin, which generally blocks recycling of endocytosed receptor. In the absence or presence of exogenous agonist, monensin caused a build-up of NK(1) receptor, but not NK(3) receptor, in the cytoplasm of neurons.The results demonstrate independent, agonist-induced endocytosis of NK(1) and NK(3) receptors in neurons of the myenteric plexus of rat ileum and suggest that the mechanisms of recycling of NK(1) and NK(3) receptors differ.