Differential distribution of muscarinic receptor subtypes and their regulation by G-protein in rat brain

1. 3H]quinuclidinyl benzilate (3H]QNB) binding in rat cerebral and cerebellar synaptosomes had different Bmax values, but similar Kd values. 2. These bindings could be displaced by classic muscarinic agents: pilocarpine (partial agonist), and atropine (antagonist), which both had similar binding affinities in rat cerebral and cerebellar synaptosomes. 3. The new muscarinic M1 selective agents: McN-A-343 (agonist), pirenzepine and trihexyphenidyl (antagonists) and higher affinities for receptor sites in the cerebrum than in the cerebellum. 4. The muscarinic M2 selective agents: carbachol, oxotremorine (agonists), and AF-DX-116 (antagonist) had higher affinities for receptor sites in the cerebellum than in the cerebrum. 5. GPP(NH)p (40 microM) decreased the binding affinities of carbachol and oxotremorine in the cerebellum, but not in the cerebrum. However, it did not decrease the binding affinities of all the antagonists studied in both brain regions. 6. These results reveal that more muscarinic M1 sites are present in the cerebrum than in the cerebellum, while the opposite is true for M2 sites. Furthermore, the regulatory role of G-protein on these muscarinic receptor subtypes in the brain is different.