Brain-derived serotonin synthesis is initiated by hydroxylation of tryptophan, a rate-limiting reaction performed by the enzyme tryptophan hydroxylase 2 (Tph2) in the neurons of the dorsal and median raphe nuclei in the brainstem. Leptin, an adipocyte-derived hormone, can directly inhibit serotonin production and release by the raphe nuclei neurons of the brainstem. The action of leptin is mediated by LepR, also known as the leptin/obese receptor ObRb, expressed on these neurons. The asterisks (*) represent the genes that were cell-specifically inactivated.

The serotonergic neurons of the brainstem project to the ventromedial (VMH) and arcuate (Arc) neurons of the hypothalamus. Brain-derived serotonin regulates bone mass accrual positively after binding to Htr2c receptors in neurons of VMH, whereas serotonin binding to the Htr1a and Htr2b receptors on neurons of the arcuate nuclei (Arc) favors appetite. The inactivation of Htr1a receptor signaling in arcuate neurons by a selective antagonist can inhibit the positive regulation of appetite by serotonin. Asterisks (*) represent the genes that were cell-specifically inactivated.

The answer to the first two questions came from the use of the same Cre driver mouse described above. Deletion of the leptin receptor from serotonergic neurons in six-week-old mice using the Tph2–CreER^T2 mice resulted in a massive increase of appetite and a decrease in energy expenditure [26,36]. These results established that the leptin–serotonin axis operates in adult mice. Addressing the third question of whether inhibition of serotonin signaling could rescue the orexigenic phenotype of ob/ob mice was more delicate because serotonin acts to inhibit appetite through two receptors on arcuate neurons, Htr1a and Htr2b. On treating ob/ob mice with 20 mg/kg of an Htr1a inhibitor it was observed that food intake by the mutant mice was 20–25% lower than in those treated with vehicle (Figure 3). Moreover, when ob/ob mice were administered this compound daily for over one month, a 30% decrease of food intake and body weight was consistently reported [36]. Taken together, these data demonstrated firmly an important mechanism in which leptin inhibits appetite by decreasing brain-derived serotonin synthesis and/or release by the serotonergic neurons of the brainstem, which signals in arcuate neurons through Htr1a receptor (Figures 2 and 3).