Leptin, the product of the ob gene, plays a key role in the regulation of food intake via a cross-talk between hypothalamic leptin receptors and neuropeptides that affect feeding behaviour. Recent studies have shown a synergistic interaction between leptin and cholecystokinin (CCK) leading to suppression of food intake, which involves CCK-1 receptors and capsaicin-sensitive vagal fibres. In this study, we have investigated the presence of leptin receptors in afferent and efferent neurons of the vagus nerve. By using reverse transcription-polymerase chain reaction, mRNAs encoding long (Ob-Rb) and short (Ob-Ra) leptin receptor isoforms were detected in the rat nodose ganglion, which contains the cell bodies of the vagal afferent neurons. Western blot analysis confirmed the presence of leptin receptor-immunoreactive proteins in extracts from the vagal trunk. Immunohistochemistry showed the presence of leptin receptors and the leptin-induced transcription factor STAT3 in the cytoplasm of nodose ganglion cells. In cervical vagal segments, levels of leptin receptor protein displayed physiological regulation, with decreased amounts after feeding and increased levels after food restriction. In addition, leptin receptor and STAT3 immunoreactivities were detected in neurons of the nucleus of tractus solitarius (NTS) and the dorsal motor nucleus of the vagus nerve (DMNX) by immunofluorescence histochemistry. Furthermore, direct double-labelling demonstrated colocalization of Ob-Rb and STAT3 immunoreactivities in cholinergic vagal efferent cell bodies of the DMNX. It is speculated that vagal leptin receptors, apart from being activated by adipocyte-derived leptin, may also be influenced by leptin produced by the stomach. This may explain the synergistic action of leptin and CCK on neuronal activity in the NTS and on food intake.

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In the present study, we investigated whether cholecystokinin (CCK) or its structurally related peptide gastrin participates in long term regulation of adipocyte leptin secretion. The levels of circulating leptin observed after 2 and 6 h of refeeding in 18-h fast rats were significantly lowered by injection of the specific gastrin/CCK-B receptor antagonist YM022 at doses that did not affect feeding behavior. Moreover, in normally fed animals, circulating leptin was markedly decreased by chronic injection of YM022 (from 4 +/- 0.6 to 2.1 +/- 0.5 ng/ml). Consistent with these observations, YM022 treatment decreased leptin messenger RNA (mRNA) levels and increased the leptin content in rat epididymal fat tissue. Rat adipocytes exclusively contain gastrin/CCK-B receptor mRNA, but not CCK-A receptor mRNA. Furthermore, adipocyte membranes bound [125I]CCK-8 in a saturable manner, with kinetics consistent with a single class of high affinity sites with a Kd of 0.2 nM. These data argue for a physiological role for the CCK-B/gastrin receptor in adipocyte leptin regulation. We therefore propose that gastrin is involved in long term regulation of leptin expression and secretion in rat fat tissues through activation of an adipocyte gastrin/CCK-B receptor.