Involvement of Amylin and Leptin in the Development of Projections from the Area Postrema to the Nucleus of the Solitary Tract

Abstract

The area postrema (AP) and the nucleus of the solitary tract (NTS) are important hindbrain centers involved in the control of energy homeostasis. The AP mediates the anorectic action and the inhibitory effect on gastric emptying induced by the pancreatic hormone amylin. Amylin's target cells in the AP project to the NTS, an integrative relay center for enteroceptive signals. Perinatal hormonal and metabolic factors influence brain development. A postnatal surge of the adipocyte-derived hormone leptin represents a developmental signal for the maturation of projections between hypothalamic nuclei controlling energy balance. Amylin appears to promote neurogenesis in the AP in adult rats. Here, we examined whether amylin and leptin are required for the development of projections from the AP to the NTS in postnatal and adult mice by conducting neuronal tracing studies with DiI in amylin- (IAPP^-/-) and leptin-deficient (ob/ob) mice. Compared to wild-type littermates, postnatal (P10) and adult (P60) IAPP^-/- mice showed a significantly reduced density of AP-NTS projections. While AP projections were also reduced in postnatal (P14) ob/ob mice, AP-NTS fiber density did not differ between adult ob/ob and wild-type animals. Our findings suggest a crucial function of amylin for the maturation of neuronal brainstem pathways controlling energy balance and gastrointestinal function. The impaired postnatal development of neuronal AP-NTS projections in ob/ob mice appears to be compensated in this experimental model during later brain maturation. It remains to be elucidated whether an amylin- and leptin-dependent modulation in neuronal development translates into altered AP/NTS-mediated functions.

Keywords: energy homeostasis; hindbrain; neuronal development; neuronal programming; obesity.

Figure 1

Neuroanatomical localization of the region that was used for quantification (rectangle). Approximate rostrocaudal level of the nucleus of the solitary tract (NTS): bregma −7.8 mm (caudal of the DiI crystal implantation site in the area postrema). Representative image (10-fold magnification) of DiI staining illustrating that labeling was only present in fibers, but not in neuronal cell bodies. 10N, vagus nerve nucleus; CC, central canal; Gr, gracile nucleus; sol, solitary tract; SolDL, solitary nucleus, dorsomedial part; SolM, solitary nucleus, medial part; SolV, solitary nucleus, ventral part; SolVL, solitary nucleus, ventrolateral part. Schematic drawings: modified and reproduced with permission from Ref. (19).

Figure 2

Quantification and representative images of DiI staining of area postrema–NTS projections in P10 (upper panel) and P60 (lower panel) IAPP^+/+ wild-type and amylin-deficient IAPP^−/− mice. Amylin deficiency resulted in reduced fiber densities at both time points compared to controls (*p < 0.05, Student’s t-test).

Figure 3

Quantification and representative images of DiI staining of area postrema–NTS projections in P14 (upper panel) and P60 (lower panel) wt/wt and leptin-deficient ob/ob mice. Leptin deficiency resulted in reduced fiber densities compared to controls in P14 but not P60 mice (**p < 0.01, Student’s t-test).

Figure 4

Postnatal amylin plasma levels in wild-type mice did not differ significantly between P6 and P9. During this time window amylin levels were similar to values reported for adult mice [dashed line and dotted lines represent mean ± SEM reported in Ref. (20)].