Biphasic effects of orexin-A on autonomic nerve activity and lipolysis

Previously, we showed that orexin-A, a 33-aa peptide, influences renal sympathetic nerve activity. Because the autonomic nervous system plays an important role in the regulation of lipid metabolism, we investigated the in vivo effects of orexin-A on the sympathetic nerve activity innervating white adipose tissue (WAT-SNA) and lipolysis. We found that intracerebroventricular (icv) administration of orexin-A at doses of 1 μg/rat and 10 ng/rat elevated and suppressed WAT-SNA, respectively. The effect of the high dose of orexin-A (1 μg/rat) was eliminated by pretreatment with diphenhydramine hydrochloride, a histamine H 1 receptor antagonist. In contrast, the effect of the low dose of orexin-A (10 ng/rat) was suppressed by thioperamide maleate salt, a histamine H 3 receptor antagonist. Moreover, icv administration of 1 μg/rat and 10 ng/rat of orexin-A increased and decreased the levels of plasma free fatty acids (FFAs), respectively. The effect of 1 μg/rat of orexin-A on plasma FFA was eliminated by propranolol hydrochloride, a β-adrenergic receptor blocker, and also by diphenhydramine. The effect of orexin-A at dose of 10 ng/rat disappeared by pretreatment with atropine sulfate, a muscarinic receptor blocker, and thioperamide maleate salt. Our results suggest that high doses of orexin-A may regulate the lipolytic processes in adipose tissue through facilitation of the sympathetic nervous system, which is driven by histamine neurons through the H 1 receptor, and that the β 3-receptor may be involved in this enhanced lipolytic response. Low doses of orexin-A, on the other hand, may lower lipolysis by suppressing sympathetic nerve activity via the H 3-receptor, and the muscarinic receptor may be related to this response.