Neurobiology Research, 151A3 VAMC
Sepulveda CA 91343, Dept. of Psychiatry, UCLA Medical School
Los Angeles CA 90024
Univ. of Queensland, Brisbane, Australia
ammals can be subdivided into three orders, the placentals, marsupials and monotremes. All extensively studied placental and marsupial mammals have been found to have REM sleep. Although monotremes date to the origin of the mammalian line, only three monotreme species exist, two species of echidna and the platypus. The first study of sleep in monotremes by Allison et al found that the echidna (Tachyglossus anatinus) did not have REM sleep. This finding led to the hypothesis that REM sleep in mammals had evolved after the divergence of the monotreme line from the placental and marsupials.
We have analyzed the nature of sleep in the echidna using neuronal recording from the brainstem to characterize state organization. Our findings agree with Allison's in that we did not see REM sleep with EEG desynchrony and phasic motor activity. However, brainstem units fired in REM sleep-like bursts during sleep Wit]l slow waves. Thus the echidna has a mixture of nonREM and REM components in sleep.
Using telemetry and video recording techniques, we have now completed the first study of sleep in the platypus (Ornithorhynchus anatinus). The platypus is generally thought to be the most plesiomorphic mammal (i.e., having properties of the common ancestor of all mammals). We found clear evidence for substantial amounts of REM sleep in the platypus. REM sleep in the platypus was characterized by rapid movements and twitching of the bill, head and eyes. We are currently in the process of analyzing recorded EEG, EOG and EKG changes during this state.
Our findings support the hypothesis that REM sleep was present in the earliest mammals. They further suggest that REM sleep evolved only once, appearing in the reptilian common ancestor of birds and mammals.
