M2 muscarinic receptor subtype in the feline medial pontine reticular formation modulates the amount of rapid eye movement sleep

Rapid eye movement (REM) sleep is generated, in part, by activating muscari nic cholinergic receptors (mAChRs) in the medial pontine reticular formatio n (mPRF). Molecular cloning has identified five mAChR subtypes, and this st udy tested the hypothesis that the M2 subtype in the mPRF modulates the amo unt of REM sleep. This hypothesis cannot be tested directly, due to lack of subtype selective muscarinic agonists. However, the amount of REM sleep ca n be enhanced by mPRF microinjection of a muscarinic agonist, and the relat ive potencies of muscarinic antagonists to block the REM sleep enhancement can be determined. Two muscarinic antagonists, methoctramine and 4-DAMP, we re studied. Six concentrations of each antagonist were microinjected into t he mPRF of conscious cat 15 min prior to the agonist bethanechol. Nonlinear regression analysis was used to calculate the dose of antagonist that caus ed a 50% inhibition (ID50) of bethanechol-induced REM sleep. Bethanechol si gnificantly increased (442%) the amount of time spent in REM sleep. Both me thoctramine and 4-DAMP significantly blocked the bethanechol-induced REM sl eep increase, with an ID50 of 1.8 mu M and 0.6 mu M, respectively. The ID50 ratio for methoctramine-to-4-DAMP (3.0) was similar to the affinity ratio of methoctramine-to-4-DAMP only at the M2 subtype (3.5), suggesting that th e M2 subtype in the mPRF modulates the amount of REM sleep. This study also tested the null hypothesis that sleep-dependent respiratory depression evo ked by mPRF cholinomimetics would not be antagonized by pretreatment of the mPRF with muscarinic antagonists, Neither methoctramine nor 4-DAMP antagon ized the bethanechol-induced decrease in respiratory rate.