Effects on sleep of the inhibition of the lateral hypothalamus in the rat

Introduction: The antagonism of hypocretins (HCRT) has been proposed as a new approach to the treatment of insomnia. Since HCRT is exclusively synthesized by neurons located within the lateral hypothalamus (LH) [1], in the present study, we have evaluated the effects on sleep induced by a prolonged pharmacological inhibition of the LH at both normal laboratory (25C) and low (10C) ambient temperature (Ta), a condition which is known to dysfacilitate sleep ocurrence [2]. Methods: Twelve male Sprague-Dawley rats (300–350 g) were surgically implanted, under general anaesthesia (Diazepam, 5mg/ kg, i.m., ketamine, 100 mg/kg, i.p.), with electrodes for chronic EEG recording, a thermistor for the detection of the hypothalamic temperature, a catheter for arterial blood presssure recording and two microcannulas for drug delivery within the LH. Two groups of animals were studied under a 6-day (D1–D6) protocol. Animals from group 1 (n = 5) were injected with either the GABAA agonist Muscimol (100 nl, 1 mM, 1 injection/h for 6 h, from 11:00 h to 16:00 h) or vehicle (saline, 0.9%; according to the former protocol) during D2 and D5, respecitvely. In group 2 (n = 7), the same experimental protocol was carried out, but Ta was lowered to 10C in both D2 and D5, from 9:00 h to 17:00 h. Results: In the six experimental hours, muscimol injection induced a significant increase in NREM sleep amount at both Ta 25C (+6099 ± 574 s) and Ta 10C (+4896 ± 408 s) compared to what observed after vehicle (P < 0.01, for both comparisons). In both groups, a concomitant suppression of REM sleep was obsereved after muscimol injection Conclusion: The acute inhibition of LH neurons in free behaving rats produces a long bout of NREM sleep, even in environmental condition unfavourable to sleep occurence. This effect can be explained by the inhibition of the activity of HCRT neurons, which would favor the activation of the circuits promoting NREM sleep occurrence. The suppression of REM sleep may be due to the concomitant inhibithion of a group of melanin concentrating hormone (MCH) neurons located in the LH, which have been shown to be involved in REM sleep regulation [3]. 1. Sakurai T, Mieda M, Tsujino N. Ann N Y Acad Sci 2010, 1200: 149–61. 2. Cerri, M. Ocampo-Garces, A. Amici R, Baracchi F, Capitani P, Jones CA, Luppi M, Perez E, Parmeggiani PL, and Zamboni G. Sleep 2005, 28: 694:705. 3. Sapin E, Berod A, Leger L, Herman PA, Luppi PH, Peyron C. PLoS One 2010, 5: e11766.