1 INSERM U480, Université Claude Bernard, Lyon, France ,
2 Institut des Neurosciences CNRS URA 1488, Université Pierre et Marie Curie, Paris, France and
3 INSERM U257, Institut Cochin de Génétique MolŽculaire, Paris, France

Correspondence: Jean-Louis Valatx, INSERM U480, University Claude Bernard, 8 Avenue Rockefeller, 96373 LYON Cedex 08, France.

SUMMARY

Recently, insulin synthesis and the presence of an insulin receptor have been demonstrated in the brain. Intracerebroventricular infusion of insulin causes a selective increase in the amount of slow-wave sleep. In the present study, the sleep-wake cycle of transgenic mice, with or without habenular neuronal expression of the human insulin gene, was studied to investigate the possible role of brain insulin as a sleep modulator. Slow-wave sleep duration was increased in those mice expressing human insulin in the habenula. However, it is possible that this effect was not due to expression of the insulin transgene, but to the genetic background of one of the parental strains (CBA) used for insertion of the transgene. Users of transgenic mice should be aware of this possibility and be cautious in interpreting results when hybrid embryos are used as transgene recipients.

KEYWORDS

human insulin, paradoxical sleep, slow-wave sleep, transgenic mice

INTRODUCTION

Insulin or insulin-related peptides are found in both vertebrates and invertebrates (Smit et al. 1998). Their classical role in energy (glucose) metabolism has now been extended to include many behavioural functions, such as food intake, motor activity, and memory (Mayer et al. 1990; Douhet et al. 1997; Wickelgreen 1998). The existence of insulin receptors in the brain (Pezzino et al. 1996; DorŽ et al. 1997) and the transport of insulin across the blood-brain barrier may explain these behavioural effects. However, using RT-PCR, amplification and in situ hybridization, both preproinsulin 11 and insulin messenger RNA have been shown to be present in the hypothalamus (Young 1986; Devaskar et al. 1993, 1994; Tsuji et al. 1996), and central insulin may therefore be involved in the regulation of several aspects of behaviour. In terms of sleep, Danguir and Nicola•dis (1984) showed, in the rat, that intracerebroventricular insulin infusion results in a specific increase in the amount of slow-wave sleep (SWS). In transgenic mice bearing a human insulin transgene (HIg) with a modified 5'-flanking sequence (transcription unit), the length of the sequence (258, 168, or 58 nucleotides upstream of the transcription starting point; DELTA-258, DELTA-168, or DELTA-58, respectively) determines whether the transgene is expressed in the pancreatic P-islets, in the brain, or not at all (Fromont-Racine et al. 1990). In DELTA-168 mice, human insulin is preferentially expressed in the brain (Douhet et al. 1993). Cerebral insulin expression has been observed in only one structure, the median habenular nucleus. Using double immunohistochemical staining, human insulin was shown to be colocalized in cholinergic habenular neurons (Douhet et al. 1995). Given that the habenular nucleus is connected to the sleep regulating network, the aim of the present work was to study the effects of extra brain insulin on the sleep-wake cycle.

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