ENDOCYTOSIS AND RECYCLING OF NEUROKININ-1 RECEPTORS IN ENTERIC NEURONS

Neurotransmission depends on the availability of transmitter and on th e presence of functional, high-affinity receptors at the plasma membra ne that are capable of binding ligand. The pathway, mechanism and func tion of endocytosis and recycling of the substance P or neurokinin 1 r eceptor in enteric neurons were studied using fluorescent substance P, receptor antibodies and confocal microscopy. In both the soma and neu rites, substance P induced rapid, clathrin-mediated internalization of the neurokinin 1 receptor into early endosomes, which also contained the transferrin receptor. After 4-8 h, there was a return in surface n eurokinin 1 receptor immunoreactivity in the soma, which was not preve nted by cycloheximide, and was thus independent of new protein synthes is. This return was prevented by acidotropic agents, and therefore req uired endosomal acidification. This suggests that the neurokinin 1 rec eptor recycles in the soma. In contrast, in neurites, substance P and the neurokinin 1 receptor remained in endosomes and recycling was not detected. Neurons of the myenteric plexus were heavily innervated by s ubstance P-containing nerve fibers, and K+-stimulated release of endog enous substance P from cultured neurons induced internalization of the neurokinin 1-receptor. Therefore, endogenous substance P may induce e ndocytosis of the neurokinin 1 receptor. In the soma, endocytosis and recycling correlated with loss and recovery of functional binding site s for substance P, suggesting that this process contributes to the reg ulation of peptidergic neurotransmission. Thus, ligand-induced endocyt osis of the neurokinin 1 receptor in myenteric neurons is associated w ith a loss of surface receptors and functional binding sites. Since re lease of endogenous substance P induces neurokinin 1 receptor internal ization, and neurokinin 1 receptor neurons are innervated by substance P-containing fibers, endocytosis of neuropeptide receptors may regula te neurotransmission. Copyright (C) 1996 IBRO. Published by Elsevier S cience Ltd.