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We refer readers to Supplement 1: Prologue-Overall Conceptual Issues and Rationale (hereafter, Supplement 1) in the Supplemental Material available online for a complete explanation of our use of terms, such as affects, emotions, and feelings, and our rationale for going to a level of brain-mind analysis in animals, a strategy that has generally been deemed undoable and, thus, scientifically inappropriate. Other background literature and issues relevant for understanding our novel preclinical psychiatric strategies are summarized in four additional supplements in the Supplemental Material. Because they are critical for contextualizing our arguments (but space limitations of Clinical Psychological Science articles did not allow their incorporation into the main text), they should be consulted for a fuller development of some of the arguments. An Affective Neuroscience Strategy for Discovery of New Treatments for DepressionOur specific goal is to introduce new affective neuroscience approaches to facilitate the treatment of depression.

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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.

injection of CCK reduces food intake and triggers a behavioral pattern similar to natural satiation. Reduction of food intake by CCK is mediated by vagal afferents that innervate the stomach and small intestine. These afferents synapse in the hindbrain nucleus of the solitary tract (NTS) where gastrointestinal satiation signals are processed. Previously, we demonstrated that intraperitoneal (IP) administration of either competitive or noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists attenuates reduction of food intake by CCK. However, because vagal afferents themselves express NMDA receptors at both central and peripheral endings, our results did not speak to the question of whether NMDA receptors in the brain play an essential role in reduction of feeding by CCK. We hypothesized that activation of NMDA receptors in the NTS is necessary for reduction of food intake by CCK. To test this hypothesis, we measured food intake following IP CCK, subsequent to NMDA receptor antagonist injections into the fourth ventricle, directly into the NTS or subcutaneously. We found that either fourth-ventricle or NTS injection of the noncompetitive NMDA receptor antagonist MK-801 was sufficient to inhibit CCK-induced reduction of feeding, while the same antagonist doses injected subcutaneously did not. Similarly fourth ventricle injection of D-3-(2-carboxypiperazin-4-yl)-1-propenyl-1-phosphoric acid (D-CPPene), a competitive NMDA receptor antagonist, also blocked reduction of food intake following IP CCK. Finally, DCPPene injected into the fourth ventricle attenuated CCK-induced expression of nuclear c-Fos immunoreactivity in the dorsal vagal complex. We conclude that activation of NMDA receptors in the hindbrain is necessary for the reduction of food intake by CCK. Hindbrain NMDA receptors could comprise a critical avenue for control and modulation of satiation signals to influence food intake and energy balance.vagus; satiation; gut-brain peptides SATIATION IS THE PROCESS by which food entering the gastrointestinal tract gradually reduces food intake, eventually resulting in termination of a meal. Previous reports from our group and others indicate that systemic administration of antagonists of N-methyl-D-aspartate-type glutamate receptors (NMDAr antagonists) delays satiation and increases meal size (see, for example, Refs. 8, 9, and 25). Additionally, we demonstrated that injecting NMDAr antagonists directly into the nucleus of the solitary tract (NTS), where vagal afferents from the gastrointestinal tract synapse, increases meal size (19,23,46) and that lesions of the NTS abolish this effect (47). These observations suggest that NMDA receptors, perhaps in the NTS, participate in vagally mediated control of food intake.The hypothesis that NMDA receptors participate in the process of satiation is supported by our prior reports that peripherally administered NMDA receptor antagonists attenuate reduction of food intake induced by CCK (11, 19), a gut peptide known to reduce food intake by activating abdominal vagal a...