Independent endocytosis of the NK1 and NK3 tachykinin receptors in neurons of the rat myenteric plexus

Abstract 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 μM) and neurokinin A (10 pM–100 μM), 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 μM), 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 μM). 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.