The mechanisms by which natural rewards such as sugar affect synaptic transmission and behavior are largely unexplored. Here, we investigate regulation of nucleus accumbens synapses by sucrose intake. Previous studies have shown that AMPA receptor trafficking is a major mechanism for regulating synaptic strength, and that in vitro, trafficking of AMPA receptors containing the GluA1 subunit takes place by a two-step mechanism involving extrasynaptic and then synaptic receptor transport. We report that in rat, repeated daily ingestion of a 25% sucrose solution transiently elevated spontaneous locomotion and potentiated accumbens core synapses through incorporation of Ca2+-permeable AMPA receptors (CPARs), which are GluA1-containing, GluA2-lacking AMPA receptors. Electrophysiological, biochemical and quantitative electron microscopy studies revealed that sucrose training (7 days) induced a stable (>24 hr) intraspinous GluA1 population, and that in these rats a single sucrose stimulus rapidly (5 min) but transiently (<24 hr) elevated GluA1 at extrasynaptic sites. CPARs and dopamine D1 receptors were required in vivo for elevated locomotion after sucrose ingestion. Significantly, a 7-day protocol of daily ingestion of a 3% solution of saccharin, a non-caloric sweetener, induced synaptic GluA1 similarly to 25% sucrose ingestion. These findings identify multi-step GluA1 trafficking, previously described in vitro, as a mechanism for acute regulation of synaptic transmission in vivo by a natural orosensory reward. Trafficking is stimulated by a chemosensory pathway that is not dependent on the caloric value of sucrose.

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Activation, differentiation and expansion of alloreactive CD8 + T cells, the dominant effectors that mediate murine heart allograft rejection, requires allorecognition, costimulation, and cytokineinitiated signals. While previous work showed that alloreactive CD4 + T cell immunity entails immune cell-produced and locally activated complement, whether and how C3a receptor 1 (C3aR1) signaling impacts transplant outcomes and the mechanisms linking C3aR1 to alloreactive CD8 + T cell activation/expansion remain unclear. Herein we show that recipient C3aR1 deficiency or pharmacological C3aR1 blockade synergizes with tacrolimus to significantly prolong allograft survival vs. tacrolimus-treated controls (median survival time 21 vs. 14 days, p<0.05). Recipient C3aR1-deficiency reduced the frequencies of post-transplant, donor-reactive CD8 + T cells 2-fold. Reciprocal adoptive transfers of naive WT or C3ar1 −/− CD8 + T cells into syngeneic WT or C3ar1 −/− allograft recipients showed that T cell-expressed C3aR1 induces CD8 + T proliferation, mTOR activation and transcription factor T-bet expression. Host C3aR1 indirectly facilitates alloreactive CD8 + T cell proliferation/expansion by amplifying antigen presenting cell costimulatory molecule expression and innate cytokine production. In addition to expanding mechanistic insight, our findings identify C3aR1 as a testable therapeutic target for future studies aimed at improving human transplant outcomes.

TNF ligation of TNF receptor 1 (TNFR1) promotes either inflammation and cell survival by (a) inhibiting RIPK1’s death-signaling function and activating NF- κ B or (b) causing RIPK1 to associate with the death-inducing signaling complex to initiate apoptosis or necroptosis. The cellular source of TNF that results in RIPK1-dependent cell death remains unclear. To address this, we employed in vitro systems and murine models of T cell–dependent transplant or tumor rejection in which target cell susceptibility to RIPK1-dependent cell death could be genetically altered. We show that TNF released by T cells is necessary and sufficient to activate RIPK1-dependent cell death in target cells and thereby mediate target cell cytolysis independently of T cell frequency. Activation of the RIPK1-dependent cell death program in target cells by T cell–derived TNF accelerates murine cardiac allograft rejection and synergizes with anti-PD1 administration to destroy checkpoint blockade–resistant murine melanoma. Together, the findings uncover a distinct immunological role for TNF released by cytotoxic effector T cells following cognate interactions with their antigenic targets. Manipulating T cell TNF and/or target cell susceptibility to RIPK1-dependent cell death can be exploited to either mitigate or augment T cell–dependent destruction of allografts and malignancies to improve outcomes.

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