Regulation of the subcellular distribution of m4 muscarinic acetylcholine receptors in striatal neurons in vivo by the cholinergic environment: Evidence for regulation of cell surface receptors by endogenous and exogenous stimulation

Our aim was to determine how the cholinergic environment influences, in viv o, the membrane abundance and the intracellular trafficking of the muscarin ic receptor m4 (m4R). Immunohistochemistry at light and electron microscopi c level was used to detect the subcellular localization of m4R in several p opulations of striatal cholinoceptive neurons, including cholinergic neuron s and medium spiny neurons. (1) In control rats, in cholinergic neurons, m4R is mostly restricted to in tracytoplasmic sites involved in its synthesis, especially endoplasmic reti culum. In contrast, m4R is preferentially located at the plasma membrane in cell bodies and dendritic shafts and spines of medium spiny neurons. The d ensity of m4R was greater at the membrane of perikarya in patches (striatal areas with low acetylcholine activity) than in matrix (striatal areas with high acetylcholine activity). (2) Stimulation of muscarinic receptor with oxotremorine provokes translocation of m4R from the membrane to endosomes i n perikarya and dendrites of medium spiny neurons but has no influence on t he localization of m4R in the cytoplasm of cholinergic cell bodies. Our results suggest that the intraneuronal trafficking and the abundance of membrane-bound m4R in vivo is under regulation of the cholinergic environm ent. The m4R subcellular compartmentalization depends on the phenotype of t he cholinoceptive neuron and on its neurochemical environment. Such regulat ion, by modulating availability of receptor for endogenous and exogenous li gands, may play key roles in the response of target neurons.