Objectives: 

Tricyclic Antidepressants (TA) belong to a pharmacological group that causes almost a total suppression of REM sleep in mammals. However, the search of REM in submammalian vertebrates rendered no results. Therefore, searching for changes in the reptilian behavioral states after TA administration, could provide with cues on the phylogenetic ancestor of the REM sleep.

Materials: 

A group (n=9) of Gallotia galloti lizards were surgically implanted with chronic EEG, ECG and breathing recording electrodes. In a first experimental series, each animal received a single i.p. injection of saline and their EEG was recorded. After a week for washout, the same animals received imipramine. Taps applied to the walls of the recording chamber were used to provoke startles. EEG segments (15 s) were digitized and computer stored for off line absolute and relative frequency EEG power analysis.

Results: 

When compared with saline treatment, imipramine caused a significant reduction in the spontaneous slow wave EEG power from 0.5 to 2.5 Hz and a significant increase in high frequency power, from 3 to 11Hz. On the other hand, the sensory stimulation caused no changes in the EEG frequency power distribution in imipramine treated animals, while those treated with saline showed a clear reactivity, with reduced power below 3 Hz and significant increases between 3-11 Hz

Conclusions: 

These results provide support for the hypothesis of the evolution from the active reptilian waking to the mammalian REM (Rial, et al, 2010) as Imipramine blocks REM in mammals and diminishes the reactivity to significant stimulus in reptiles, that is, blocks their active wakefulness. Therefore, the two states may be homologous, i.e, they may have a common phylogenetic origin.