Cholinergic systems play an important role in generating EEG as well as in regulating the vigilance states. Classically, the muscarinic cholinergic agonists (i.e. arecholine, RS86, pilocarpine, etc.) shortened REM latency, increased REM time and reduced slow wave sleep (SWS). These results led to the proposal of a cholinergic control for REM.

Pilocarpine is a partial cholinergic agonist with a moderate affinity for M1 muscarinic receptors and higher for M5 ones. Its affinity for M2 receptors, which are believed to play a role in REM induction, is of the same order as that of arecoline and RS86. Rapid REM induction was achieved with a high arecholine dose, but dose-response studies for pilocarpine REM sleep induction are lacking. This study aims at analysing the effects of pilocarpine on sleep architecture in rats.

Nine male Wistar rats weighing 410 ± 10 g (S.E.M.) were used. Under isoflurane anaesthesia, each rat was implanted with two epidural cortical electrodes (AP +2.0, ML +2.5 and AP -6.0, ML +4.0), and a reference one over the cerebellum. Two additional electrodes were used for neck EMG recording. After recovery from surgery, bipolar EEG and EMG were recorded during dark time in freely moving animals for 120 min, starting 30 min after pilocarpine injection. Recordings were made in three randomly ordered conditions: I.P. saline and pilocarpine at both 1 mg kg^-1 and 3 mg kg^-1. Latencies from injection time and duration of (1) active waking, (2) SWS, (3) REM and (4) theta rhythm periods, were calculated. The study was performed in accordance with the guidelines of the local ethical committee for animal studies. After the experiments, the animals were killed by an overdose of barbiturate anaesthetic.

Significant differences in duration of the behavioral states between saline and low dose pilocarpine were not found. On the contrary, the latencies increased with the low dose for REM (ANOVA, P < 0.05) and caused an almost total REM loss with the high dose. For SWS, latencies increased from 570 ± 60 to 694 ± 75 and 5400 ± 1300 s for saline, low and high doses respectively (means ± S.E.M., P < 0.0001). As the increase in latencies was so large, the total amount of the two phases was significantly reduced (P < 0.001 for SWS and P < 0.05 for REM). Instead, the theta rhythm time was increased for every dosage (P < 0.0003), remaining without change the active waking.

In conclusion, the effect of pilocarpine in rat sleep is a strong reduction in both SWS and REM and a high increase in theta rhythm time. However, these results contradict previous ones where the activation of cholinergic systems greatly increased REM duration and decreased its latency.