Muscarinic antagonist control of myopia: a molecular search for the M1 receptor in chick

PURPOSE: Pirenzepine, an M1 selective muscarinic antagonist, is effective in slowing the progression of myopia in both humans and experimental animals, including chick. As an M1 selective antagonist, pirenzepine is considered to mediate its effect through M1 receptors. However, there is currently no report of the M1 receptor in chicken. Therefore, if the mechanism of action of pirenzepine is similar across species, either the drug mediates its effect through a non-M1 mechanism, or M1 muscarinic receptors are present in chicken. The aim of the present study was to determine whether a genetic template for the M1 receptor was expressed, or even present, in chick. METHODS: Polymerase Chain Reaction (PCR), and Southern and northern blotting analyses were used to search for M1 mRNA in chick ocular and brain tissues. PCR and Southern analyses were then used for searching the chick M1 gene and promoter. Appropriate rat positive controls were included throughout the study. RESULTS: Direct mRNA detection by northern analysis showed no evidence of M1 mRNA expression in the chick ocular and brain tissues studied. Identical results were obtained from PCR amplification and were further confirmed by Southern analysis. Similarly, no M1 gene or promoter sequences were detected by PCR or Southern analyses. Our methods were validated in every case by a positive finding in equivalent rat tissue and by detection of M2 and M4 mRNA expression in chick retina. CONCLUSIONS: Findings in this study suggest that the chick does not possess an M1 receptor. This finding is of primary interest to vision researchers in that it suggests pirenzepine is unlikely to mediate its inhibitory effect on the progression of myopia through an M1 receptor in chick. Alternative mechanisms of action are discussed.