Neurochemicals and Hunger Feeding behavior is influenced by complex interactions among several neurochemicals. Fat cells produce and secrete a substance known as leptin, from leptos, the Greek word for "thin" (Zhang et al. 1994). When fat stores are low, levels of circulating leptin will also be low. As shown in Figure 9.16, initial reports that administering additional leptin to obese rodents reduced their weight led many researchers to believe that leptin could be used to treat human obesity. Unfortunately, obese humans already produce large amounts of leptin, but they seem to be resistant to its effects (Friedman & Halaas, 1998). Providing more leptin is unlikely to help obese individuals lose weight.
Leptin communicates with neurons in the arcuate nucleus of the hypothalamus, shown in Figure 9.17. When leptin levels are low, cells in the arcuate nucleus use neuropeptide Y (NPY) and agouti-related protein (AgRP) to communicate with the LH and with the paraventricular (around the ventricle) nucleus (PvN) of the hypothalamus. Consequently, the parasympathetic division of the autonomic nervous system is activated, and feeding behavior is stimulated
Experimental evidence supports a role for NPY and AgRP in the initiation of feeding. When NPY is applied directly to the hypothalamus,animals will begin eating