Glucose is a crucial substrate essential for cell survival and function. Changes in glucose levels impact neuronal activity and glucose deprivation increases feeding. Several brain regions have been shown to respond to glucoprivation, including the nucleus of the solitary tract (NTS) in the brain stem. The NTS is the primary site in the brain that receives visceral afferent information from the gastrointestinal tract. The catecholaminergic (CA) subpopulation within the NTS modulates many homeostatic functions including cardiovascular reflexes, respiration, food intake, arousal, and stress. However, it is not known if they respond to changes in glucose. Here we determined whether NTS-CA neurons respond to changes in glucose concentration and the mechanism involved. We found that decreasing glucose concentrations from 5 mM to 2 mM to 1 mM, significantly decreased action potential firing in a cell-attached preparation, whereas increasing it back to 5 mM increased the firing rate. This effect was dependent on glutamate release from afferent terminals and required presynaptic 5-HTRs. Decreasing the glucose concentration also decreased both basal and 5-HTR agonist-induced increase in the frequency of spontaneous glutamate inputs onto NTS-CA neurons. Low glucose also blunted 5-HT-induced inward currents in nodose ganglia neurons, which are the cell bodies of vagal afferents. The effect of low glucose in both nodose ganglia cells and in NTS slices was mimicked by the glucokinase inhibitor glucosamine. This study suggests that NTS-CA neurons are glucosensing through a presynaptic mechanism that is dependent on vagal glutamate release, 5-HTR activity, and glucokinase.

Activation of the leptin receptor, LepRb, by the adipocytokine/neurotrophic factor leptin in the central nervous system has procognitive and antidepressive effects. Leptin has been shown to increase glutamatergic synaptogenesis in multiple brain regions. In contrast, mice that have a mutation in the LepRb gene show abnormal synapse development in the hippocampus as well as deficits in cognition and increased depressive-like symptoms. Leptin increases glutamatergic synaptogenesis, in part, through enhancement of N-methyl-D-aspartic acid (NMDA) receptor function; yet the underlying signaling pathway is not known. In this study, we examine how leptin regulates surface expression of NR2B-containing NMDA receptors in hippocampal neurons. Leptin stimulation increases NR2BY1472 phosphorylation, which is inhibited by the Src family kinase inhibitor, PP1. Moreover, we show that Fyn, a member of the Src family kinases, is required for leptin-stimulated NR2BY1472 phosphorylation. Furthermore, inhibiting Y1472 phosphorylation with either a dominant negative Fyn mutant or an NR2B mutant that lacks the phosphorylation site (NR2BY1472F) blocks leptin-stimulated synaptogenesis. Additionally, we show that LepRb forms a complex with NR2B and Fyn. Taken together, these findings expand our knowledge of the LepRb interactome and the mechanisms by which leptin stimulates glutamatergic synaptogenesis in the developing hippocampus. Comprehending these mechanisms is key for understanding dendritic spine development and synaptogenesis, alterations of which are associated with many neurological disorders.

Leptin has neurotrophic actions in the hippocampus to increase synapse formation and stimulate neuronal plasticity. Leptin also enhances cognition and has antidepressive and anxiolytic-like effects, two hippocampal-dependent behaviors. In contrast, mice lacking leptin or the long form of the leptin receptor (LepRb) have lower cortical volume and decreased memory and exhibit depressive-like behaviors. A number of the signaling pathways regulated by LepRb are known, but how membrane LepRb levels are regulated in the central nervous system is not well understood. Here, we show that the lysosomal inhibitor chloroquine increases LepRb expression in hippocampal cultures, suggesting that LepRb is degraded in the lysosome. Furthermore, we show that leptin increases surface expression of its own receptor by decreasing the level of ubiquitinated LepRbs. This decrease is mediated by the deubiquitinase ubiquitin-specific protease 8 (USP8), which we show is in complex with LepRb. Acute leptin stimulation increases USP8 activity. Moreover, leptin stimulates USP8 gene expression through cAMP response element–binding protein (CREB)–dependent transcription, an effect blocked by expression of a dominant-negative CREB or with short hairpin RNA knockdown of CREB. Increased expression of USP8 causes increased surface localization of LepRb, which in turn enhances leptin-mediated activation of the MAPK kinase/extracellular signal–regulated kinase pathway and CREB activation. Lastly, increased USP8 expression increases glutamatergic synapse formation in hippocampal cultures, an effect dependent on expression of LepRbs. Leptin-stimulated synapse formation also requires USP8. In conclusion, we show that USP8 deubiquitinates LepRb, thus inhibiting lysosomal degradation and enhancing surface localization of LepRb, which are essential for leptin-stimulated synaptogenesis in the hippocampus.

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Developing hippocampal neurons undergo rapid synaptogenesis in response to neurotrophic signals to form and refine circuit connections. The adipokine leptin is a satiety factor with neurotrophic actions, which potentiates both glutamatergic and GABAergic synaptogenesis in the hippocampus during neonatal development. Brief exposure to leptin enhances GABAA receptor–dependent synaptic currents in hippocampal neurons. Here, using molecular and electrophysiological techniques, we found that leptin increased the surface localization of GABAA receptors and the number of functional GABAergic synapses in hippocampal cultures from male and female rat pups. Leptin increased the interaction between GABAA receptors and the Rho guanine exchange factor β-PIX (a scaffolding protein at GABAergic postsynaptic sites) in a manner dependent on the kinase CaMKK. We also found that the leptin receptor and β-PIX formed a complex, the amount of which transiently increased upon leptin receptor activation. Furthermore, Tyr985 in the leptin receptor and the SH3 domain of β-PIX are crucial for this interaction, which was required for the developmental increase in GABAergic synaptogenesis. Our results suggest a mechanism by which leptin promotes GABAergic synaptogenesis in hippocampal neurons and reveal further complexity in leptin receptor signaling and its interactome.