Characterization of a decrease in muscarinic m2 mRNA in cerebellar granule cells by carbachol

Studies involving carbachol (100 μM) treatment of cerebellar granule cells for 1, 3, 6, 9, 12 and 24 hr show a decrease in the mRNA encoding for the muscarinic m₂ receptor. The response was transient, decreasing m₂ mRNA by 25 to 50% in 6 and 9 hr, respectively. The data presented in this work were quantified by ribonuclease protection assay, using a [³²P]-cRNA probe corresponding to nucleotide +1138 to 1650 of the rat m₂ muscarinic receptor. Because cerebellar granule cells express muscarinic m₂ and m₃ receptors, we tested whether the carbachol-mediated decrease in m₂ mRNA resulted from a homologous or heterologous activation of muscarinic receptors. At a 1 μM concentration, methoctramine specifically blocked the muscarinic m₂ receptor and reversed carbachol's action. These data suggested that carbachol acts via a possible homologous activation of muscarinic m₂ receptors. The half-life of the receptor mRNA measured in the presence of actinomycin D with and without carbachol were similar. Because carbachol treatments decrease the steady-state levels of m₂ mRNA without changing the half-life of the message, we suggest that a carbachol treatment induces a decrease in the transcription of the gene for the muscarinic m₂ receptor.