Biased M1positive allosteric modulators reveal novel role of phospholipase D in M1-dependent rodent cortical plasticity

Highly selective positive allosteric modulators (PAMs) of the M 1 subtype of muscarinic acetylcholine receptor have emerged as an exciting new approach for the potential improvement of cognitive function in patients suffering from Alzheimer’s disease and schizophrenia. M 1 PAM discovery programs have produced a structurally diverse range of M 1 PAMs with distinct pharmacological properties, including different levels of agonist activity and differences in signal bias. This includes the recent discovery of novel biased M 1 PAMs that can potentiate coupling of M 1 to activation of phospholipase C but not phospholipase D (PLD). However, little is known about the role of PLD in M 1 signaling in native systems and it is not clear whether biased M 1 PAMs will display differences in modulating M 1 -mediated responses in native tissue. We now report a series of studies using novel PLD inhibitors and PLD knockout mice to show that PLD is necessary for the induction of M 1 -dependent long-term depression (LTD) in the prefrontal cortex (PFC). Importantly, biased M 1 PAMs that do not couple to PLD not only fail to potentiate orthosteric agonist-induced LTD but also block M 1 -dependent LTD in the PFC. In contrast, biased and non-biased M 1 PAMs act similarly in potentiating M 1 -dependent electrophysiological responses that are PLD-independent. These findings demonstrate that PLD plays a critical role in the ability of M 1 PAMs to modulate certain CNS functions and that biased M 1 PAMs function differently in brain regions implicated in cognition. Summary We demonstrate a novel role of phospholipase D in M 1 -dependent rodent cortical plasticity and M 1 PAMs that do not couple to phospholipase D have functionally distinct effects on cortical plasticity than non-biased M 1 PAMs.