Orexin system increases energy expenditure by brown adipose tissue activity

Adipose tissue is a biological caloric reservoir that expands itself in response to overnutrition and releases lipids in response to energy deficit. It comprises white adipose tissue, the main energy storage, and brown adipose tissue (BAT), a key site of thermogenesis that dissipates chemical energy as heat. BAT is richly innervated by sympathetic nerve efferent fibers, and its development and activation are mediated by the sympathetic nervous system (SNS). Furthermore, substantial evidences show that BAT activation leads to increased thermogenesis. Thus, the regulation of the SNS tone provides a complex mechanism able to specifically coordinate the function of the organs involved in energy homeostasis. On the other hand, adipose tissue acts as an endocrine organ by producing various signaling cytokines and interacting with some neuropeptides as orexins. Orexins are produced by the lateral hypothalamus and evidences have suggested that orexins promote energy expenditure (EE) through modulation of locomotor activity and BAT thermogenesis. Recent data also suggest that orexins are required for BAT development, differentiation, and function. In the light of this, the orexin neuropeptides are part of a network able to increase EE through modulation of BAT thermogenesis which can be a target to new strategies to reduce the incidences of overweight and obesity. Aim of this review is to report our evidences showing that the autonomic nervous system influences food intake and energy consumption under various conditions, such as injection of orexins, which change the sympathetic and/or parasympathetic activities.