Abstract

A portion of the cDNA sequence corresponding to the third intracellular loop of either the m4 or m5 muscarinic cholinergic receptor was ligated into the pRIT23 or pET-3a expression vector, respectively. The expressed fusion proteins were purified and used to develop selective polyclonal antisera to the m4 and m5 muscarinic receptors. These antisera were used in an immunoprecipitation protocol to examine quantitatively the distribution of receptor subtypes in regions of rat brain. The density of m4 receptors in rat brain increased in the caudal to rostral direction. The highest levels of m4 receptors were detected in the striatum (1280 fmol/mg) and olfactory tubercle (750 fmol/mg). Low levels of m5 receptors were detected in several brain regions (< 25 fmol/mg). By combining the previously determined receptor densities for the m1, m2, and m3 receptors and results obtained with the newly developed antisera to m4 and m5 receptors, it was determined that 86-99% of the [3H]quinuclidinyl benzilate binding sites in several brain regions were immunoprecipitated. In addition to measuring receptor densities in rat brain, the immunoprecipitation protocol was used to quantify muscarinic receptor levels in tissues reported to express mRNA encoding the m4 receptor. Thus, although only m4 mRNA has been detected in rabbit lung, NG108-15 cells, and N1E-115 cells, both rabbit lung and NG108-15 cells possess both m2 (rabbit lung, 27%; NG108-15 cells, 31%) and m4 (rabbit lung, 55%; NG108-15, 42%) receptors, whereas N1E-115 cells were found to have both m1 (15%) and m4 (65%) receptors. These antisera will be useful in studies of receptor regulation and in determining alterations in density that may occur after pharmacological or physiological manipulations and in various disease states.