Previous studies have identified potential antidepressant effects of buprenorphine (BPN), a drug with high affinity for mu opioid receptor (MORs) and kappa opioid receptors (KORs) and some affinity at delta opioid receptor (DOR) and opioid receptor-like 1 (ORL-1) receptors. Therefore, these studies examined which opioid receptors were involved in BPN's effects on animal behavior tests sensitive to antidepressant drugs. The acute effects of BPN were tested in the forced swim test (FST) using mice with genetic deletion of individual opioid receptors or after pharmacological blockade of receptors. For evaluating the effects of BPN on chronic stress, separate groups of mice were exposed to unpredictable chronic mild stress (UCMS) for 3 weeks and treated with BPN for at least 7 days before behavioral assessment and subsequent measurement of Oprk1, Oprm1, and Pdyn mRNA expression in multiple brain regions. BPN did not reduce immobility in mice with KOR deletion or after pretreatment with norbinaltorphimine, even though desipramine remained effective. In contrast, BPN reduced immobility in MOR and DOR knockout mice and in mice pretreated with the ORL-1 antagonist JTC-801. UCMS reduced sucrose preference, decreased time in the light side of the light/dark box, increased immobility in the FST and induced regionspecific alterations in Oprk1, Oprm1, and PDYN mRNA expression in the frontal cortex and striatum. All of these changes were normalized following BPN treatment. The KOR was identified as a key player mediating the effects of BPN in tests sensitive to antidepressant drugs in mice. These studies support further development of BPN as a novel antidepressant.

Experimental models and experimental designsAll procedures were approved by the Institutional Care and Use Committee of the University of Pennsylvania and Eli Lilly and Company. Adult male C57BL/6 mice (Taconic) weighing ~20g at arrival (n=84) were housed under a 12-hour:12-hour light/dark cycle in a temperature-and humidity-controlled vivarium. Mice were individually housed in standard cages with ad libitum access to chow diet (2014, Research Diets) and tap water for all experiments except when noted.Adult male Sprague-Dawley rats (Charles River) weighing ~250-270 g (n=93) at arrival were housed under a 12-hour:12-hour light/dark cycle in a temperature-and humidity-controlled vivarium. Rats were individually housed in hanging wire-bottomed cage with ad libitum access to chow diet (Purina Lab Diet 5001), tap water and also had ad libitum access to kaolin pellets (Research Diets, K50001). Rats were exposed to kaolin for at least 5 days prior to measuring kaolin consumption in pica testing.Adult male shrews (Suncus murinus) weighing ~50-80 g (n=118 total), where bred and

The neurobiological substrates that mediate the anorectic effects of both endogenous glucagon-like peptide-1 (GLP-1) and exogenous GLP-1 receptor (GLP-1R) agonists are an active area of investigation. As the lateral dorsal tegmental nucleus (LDTg) expresses the GLP-1R and represents a potential neuroanatomical hub connecting the nucleus tractus solitarius (NTS), the major central source of GLP-1, with the other nuclei in the midbrain and forebrain, we tested the hypothesis that GLP-1R signaling in the LDTg controls food intake. Direct activation of LDTg GLP-1R suppresses food intake through a reduction in average meal size and independent of nausea/malaise. Immunohistochemical data show that GLP-1-producing neurons in the NTS project to the LDTg, providing anatomical evidence of endogenous central GLP-1 in the LDTg. Pharmacological blockade of LDTg GLP-1Rs with exendin-(9-39) dose-dependently increases food intake and attenuates the hypophagic effects of gastric distension. As GLP-1 mimetics are administered systemically in humans, we evaluated whether peripherally administered GLP-1R agonists access the LDTg to affect feeding. Immunohistochemical data show that a systemically administered fluorescent GLP-1R agonist accesses the LDTg and is juxtaposed with neurons. Additionally, blockade of LDTg GLP-1Rs attenuates the hypophagic effects of a systemic GLP-1R agonist. Together, these data indicate that LDTg GLP-1R signaling controls energy balance and underscores the role of the LDTg in integrating energy balance-relevant signals to modulate feeding.

The overlap in neurobiological circuitry mediating the physiological and behavioral response to satiation and noxious/stressful stimuli are not well understood. The interaction between serotonin (5-HT) and glucagon-like peptide-1 (GLP-1) could play a role as upstream effectors involved in mediating associations between anorectic and noxious/stressful stimuli. We hypothesize that 5-HT acts as an endogenous modulator of the central GLP-1 system to mediate satiation and malaise in rats. Here, we investigate whether interactions between central 5-HT and GLP-1 signaling are behaviorally and physiologically relevant for the control of food intake and pica (i.e., behavioral measure of malaise). Results show that the anorexia and body weight changes induced by administration of exogenous hindbrain 5-HT are dependent on central GLP-1 receptor signaling. Furthermore, anatomical evidence shows mRNA expression of 5-HT2C and 5-HT3 receptors on GLP-1-producing preproglucagon (PPG) neurons in the medial nucleus tractus solitarius by fluorescent in situ hybridization, suggesting that PPG neurons are likely to express both of these receptors. Behaviorally, the hypophagia induced by the pharmacological activation of both of these receptors is also dependent on GLP-1 signaling. Finally, 5-HT3, but not 5-HT2C receptors, are required for the anorectic effects of the interoceptive stressor LiCl, suggesting the hypophagia induced by these 5-HT receptors may be driven by different mechanisms. Our findings highlight 5-HT as a novel endogenous modulator of the central GLP-1 system and suggest that the central interaction between 5-HT and GLP-1 is involved in the control of food intake in rats.

Glucagon-like peptide-1 receptor (GLP-1R) agonists decrease body weight and improve glycemic control in obesity and diabetes. Patient compliance and maximal efficacy of GLP-1 therapeutics are limited by side effects including nausea and emesis. In three different species (i.e., mice, rats, and musk shrews), we show that glucose-dependent insulinotropic polypeptide receptor (GIPR) signaling blocks emesis and attenuates illness behaviors elicited by GLP-1R activation, while maintaining reduced food intake, body weight loss, and improved glucose tolerance. The area postrema and nucleus tractus solitarius (AP/NTS) of the hindbrain are required for food intake and body weight suppression by GLP-1R ligands and processing of emetic stimuli. Utilizing single-nuclei RNA-sequencing, we identified the cellular phenotypes of AP/NTS GIPR- and GLP-1R-expressing cells on distinct populations of inhibitory and excitatory neurons, with the greatest expression of GIPR in GABAergic neurons. This work suggests that combinatorial pharmaceutical targeting of GLP-1R and GIPR will increase efficacy in treating obesity and diabetes by reducing nausea and vomiting.