LOCALIZATION OF MUSCARINIC RECEPTOR MESSENGER-RNAS IN RAT-HEART AND INTRINSIC CARDIAC GANGLIA BY IN-SITU HYBRIDIZATION

Although the heart is considered a relatively pure source of m2 muscar inic receptors, the possible expression of other muscarinic receptor g enes at discrete sites within the myocardium or by intrinsic cardiac g anglia had not been evaluated. Accordingly, the present study used in situ hybridization histochemistry with S-35-labeled oligonucleotide pr obes to address this issue. Initial experiments demonstrated that the localization of m2 mRNA was similar to that reported for muscarinic re ceptors labeled with the nonselective muscarinic antagonist quinuclidi nyl benzilate; however, there were two important exceptions. The condu cting system contained less message than expected, whereas the intrins ic cardiac ganglia contained more. The mismatch between muscarinic rec eptor and m2 mRNA densities in the conducting system could not be expl ained by the local expression of other muscarinic receptor genes, sinc e m1, m3, and m4 mRNAs were not detected at this or any other site wit hin the myocardium. However, the presence of a high density of prejunc tional muscarinic receptors in the conducting system would be consiste nt with such a mismatch. Surprisingly, the intrinsic cardiac ganglia c ontained more than four times as much m2 mRNA as found in the atria. T his level of message may be necessary for the production of prejunctio nal receptors on cholinergic nerve fibers within the heart and recepto rs localized to the ganglion cell bodies. The ganglia also contained s maller amounts of m1 and m4 mRNAs. These observations suggest that pre junctional muscarinic receptors could have a prominent role in regulat ing cholinergic neurotransmission in the conducting system and that mu ltiple muscarinic receptors are present in the intrinsic cardiac gangl ia.