Contribution of the "knock-out" technology to understanding the role of serotonin in sleep regulations

Genetic manipulation of the 5-HT system leads to alterations of 5-HT neurotransmission and provides new opportunities to investigate the role of 5-HT in sleep regulations. Indeed, it represents an alternative to the use of pharmacological tools and, to some extent, of localized lesions of the 5-HT system, which have been, from the 1960s until recently, the main approaches to investigate this question. The homologous recombinaison for the knock-out of genes coding for various proteins involved in 5-HT neurotransmission in the mouse, has allowed to explore further the role of the serotonin transporter (5-HTT), the momoamine oxydase A (MAO-A), or the 5-HT_1A, 5-HT _1B and 5-HT₂ receptors in the regulation of sleep. In 5-HT_1A-/- and 5-HT_1B-/- knock-outs, REM sleep (REMs) was enhanced. Pharmacological blockade of these receptors had the same effects in wild-types. Thus, both receptor types exert a tonic inhibitory influence on REMs. In addition, 5-HT_1A-/- and 5-HT_1B-/- mutants exhibited hypersensitivity of other serotoninergic receptor types (notably the 5-HT_1A in 5-HT_1B-/- mice and vice versa), which suggests that adaptive changes at 5-HT neurotransmission develop in knock-outs. In the same manner, 5-HTT-/- knock-outs exhibited increased REMs. This may be accounted for by a decrease of 5-HT_1A and 5-HT_1B receptor-mediated sleep regulations. In contrast, MAO-A-/- knock-outs exhibited decreased REMs, associated with an enhanced response to 5-HTT blockade. Finally, in 5-HT _2A-/- mice, we observed more wakefulness and less SWS than in wild-types. These effects could not be reproduced by 5-HT_2A blockade in wild-types. To conclude, constitutive knock-outs undergo adaptive processes involving other proteins than those coded by the invalidated gene, which render the interpretation of the corresponding sleep phenotype difficult. Inducible knock-outs will probably help to overcome this difficulty. Finally, we believe that association of genetic manipulations with relevant pharmacological ones should allow further progress in the understanding of sleep mechanisms.