Studies involving carbachol (100 microM) 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 microM 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.