MAPPING OF THE NEURONAL POPULATIONS ACTIVE DURING WAKING AND PARADOXICAL (REM) SLEEP IN THE LATERAL HYPOTHALAMUS: A STUDY USING THE TRAP2 TRANSGENIC MICE

The lateral hypothalamus (LH) regulates homeostatic processes across many physiological functions, including energy metabolism, feeding behavior, thermoregulation and vigilance states. One of the most studied neuronal populations is the Orexin/Hypocretin neurons which are active during waking (W) and orchestrate feeding and energy balance through widespread efferent projections. Besides, our team has previously shown in rats and mice that LH neurons expressing either melanin-concentrating hormone (MCH), GAD2 or Lim homeobox 6 (Lhx6) are active during paradoxical (REM) sleep (PS). We indeed reported many cFos-immunopositive cells within LH, including zona incerta and perifornical area, after a homeostatic PS rebound and showed that several of these cells are GABAergic or contain MCH or Lhx6. In our previous studies, it was impossible to map in the same animal the neurons activated during W and those activated during PS to determine whether these two neuronal populations show differential topography within LH. To address this question, we used the new transgenic mouse strain TRAP2-red to label neurons activated during W (tdtomato red staining) or PS (cFos green staining) in the same mouse brain.TRAP2-red mice (n=6) were kept awake during 4h using an open field and were IP injected with 4-hydroxytamoxifen (50mg/kg) after the first 2h. One week later, the same mice were submitted to an automatic PS deprivation for 48h and then allowed to recover for 2h (PS rebound) before sacrifice. LH sections were treated for the immunofluorescent detection of cFos, hypocretin and MCH. They were then scanned to map the distribution of tdtomato, cFos, MCH, hypocretin and double- and triple-labeled neurons.Many MCH neurons were cFos positive, whereas they did not express tdtomato. Conversely, a large proportion of the hypocretin neurons were tdtomato- positive and only a few were labelled for cFos. Importantly, many cFos-labelled and MCH negative neurons were distributed in the zona incerta and the caudal and ventral LH parts. In addition, a large number of tdtomato positive hypocretin negative neurons were localized in the caudal and lateral part of the LH medial to the subthalamic nucleus.In summary, our results indicate that LH contains other populations of neurons active during W and PS than the hypocretin and MCH neurons. Our aim in the future is to identify specific markers for these neurons to study their function in W or PS