<Dd> The central location of action (effect) of the fat cell - specific hormone leptin is the hypothalamus, a part of the brain, which is a part of the central nervous system . Non-hypothalamic targets of leptin are referred to as peripheral targets . There is a different relative importance of central and peripheral leptin interactions under different physiologic states, and variations between species . </Dd> <Dl> <Dt> Function </Dt> <Dd> The primary function of the hormone leptin is the regulation of adipose tissue mass through central hypothalamus mediated effects on hunger, food energy use, physical exercise and energy balance . Outside the brain, in the periphery of the body, leptin's secondary functions are: modulation of energy expenditure, modulation between fetal and maternal metabolism, and that of a permissive factor in puberty, activator of immune cells, activator of beta islet cells, and growth factor . </Dd> </Dl> <Dd> The primary function of the hormone leptin is the regulation of adipose tissue mass through central hypothalamus mediated effects on hunger, food energy use, physical exercise and energy balance . Outside the brain, in the periphery of the body, leptin's secondary functions are: modulation of energy expenditure, modulation between fetal and maternal metabolism, and that of a permissive factor in puberty, activator of immune cells, activator of beta islet cells, and growth factor . </Dd> <P> In vertebrates, the nervous system consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The primary effect of leptins is in the hypothalamus, a part of the central nervous system . Leptin receptors are expressed not only in the hypothalamus but also in other brain regions, particularly in the hippocampus . Thus some leptin receptors in the brain are classified as central (hypothalamic) and some as peripheral (non-hypothalamic). </P>

​on which organ or tissue does leptin primarily act