Muscarinic cholinergic M₁ and M₄ receptors may participate in schizophrenia’s etiology and have been proposed as targets for antipsychotic medications.

Here, we investigated the involvement of these receptors in behavioral measures pertinent to schizophrenia using knockout mice lacking M₁ receptors (M₁−/−), M₄ receptors (M₄−/−), or both (M₁−/−M₄−/−).

We measured prepulse inhibition (PPI) of startle without drugs and after treatment with scopolamine (0.32–1.8 mg/kg), xanomeline (3.2 mg/kg), oxotremorine (0.032–0.1 mg/kg), clozapine (1.0–5.6 mg/kg), or haloperidol (0.32–3.2 mg/kg).

In female (but not male) mice, combined deletion of both M₁ and M₄ receptors decreased PPI relative to wild-type mice, while knockout of either receptor alone had no significant effect. Scopolamine disrupted PPI in wild-type and M₄−/− mice, but not in female M₁−/−M₄−/− or female M₁−/− mice. When administered before scopolamine, xanomeline restored PPI in wild-type mice and M₁−/− mice, but not in M₄−/− mice. In contrast, pretreatment with oxotremorine increased PPI regardless of genotype. Effects of clozapine and haloperidol on PPI were not hindered by either mutation.

Deletion of both M₁ and M₄ receptors can disrupt PPI, suggesting that (at least partially redundant) M₁ and M₄ receptor-dependent functions are involved in sensorimotor gating mechanisms. PPI-disrupting effects of muscarinic antagonists appeared dependent upon M₁ receptor blockade. Our data also suggest that xanomeline exerts antipsychotic-like effects mainly through M₄ receptor stimulation, while stimulation of non-M₁/M₄ subtypes may also have antipsychotic potential. Finally, our results do not support a role of M₁/M₄ receptors in mediating antipsychotic-like effects of clozapine.