CCK-Induced Reduction of Food Intake and Hindbrain MAPK Signaling Are Mediated by NMDA Receptor Activation

The dorsal vagal complex of the hindbrain, including the nucleus of the solitary tract (NTS), receives neural and humoral afferents that contribute to the process of satiation. The gut peptide, cholecystokinin (CCK), promotes satiation by activating gastrointestinal vagal afferents that synapse in the NTS. Previously, we demonstrated that hindbrain administration of N-methyl-d-aspartate (NMDA)-type glutamate receptor antagonists attenuate reduction of food intake after ip CCK-8 injection, indicating that these receptors play a necessary role in control of food intake by CCK. However, the signaling pathways through which hindbrain NMDA receptors contribute to CCK-induced reduction of food intake have not been investigated. Here we report CCK increases phospho-ERK1/2 in NTS neurons and in identified vagal afferent endings in the NTS. CCK-evoked phospho-ERK1/2 in the NTS was attenuated in rats pretreated with capsaicin and was abolished by systemic injection of a CCK1 receptor antagonist, indicating that phosphorylation of ERK1/2 occurs in and is mediated by gastrointestinal vagal afferents. Fourth ventricle injection of a competitive NMDA receptor antagonist, prevented CCK-induced phosphorylation of ERK1/2 in hindbrain neurons and in vagal afferent endings, as did direct inhibition of MAPK kinase. Finally, fourth ventricle administration of either a MAPK kinase inhibitor or NMDA receptor antagonist prevented the reduction of food intake by CCK. We conclude that activation of NMDA receptors in the hindbrain is necessary for CCK-induced ERK1/2 phosphorylation in the NTS and consequent reduction of food intake.