A New Insight into the Role of CART Peptide in Serotonergic Function and Anxiety

Abstract

Cocaine- and amphetamine-regulated transcript (CART) peptide has been implicated in stress-related behaviors that are regulated by central serotonergic (5-HT) systems in the dorsal raphe nucleus (DRN). Here, we aimed to investigate the interaction between CART and DRN 5-HTergic systems after initially observing CART axonal terminals in the DRN. We found that microinfusion of CART peptide _(55–102) into the DRN-induced anxiogenic effects in male C57BL/6J mice, while central administration of CART reduced c-Fos in 5-HT^DRN neurons. This inhibitory effect of exogenous CART on 5-HT^DRN activity and local 5-HT release was also demonstrated via in vivo fiber photometry coupled with calcium and 5-HT biosensors. CART inputs to the DRN were observed in various subcortical nuclei, but only those in the centrally projecting Edinger–Westphal nucleus (EWcp) were highly responsive to stress. Chemogenetic activation of these DRN-projecting CART^EWcp neurons recapitulated the effects of intra-DRN CART infusion on anxiety-like behavior in males, but not in females, suggesting a sex-specific role for this pathway. Interestingly, CART^EWcp projections to the DRN made direct synaptic contact primarily with non-5-HT neurons, which were also found to express putative CART receptors. Furthermore, chemogenetic stimulation of this CART^EWcp->DRN pathway inhibited 5-HT neurons while increasing activity in local GABAergic neurons. In summary, this study establishes for the first time a neuromodulatory role for CART^EWcp neurons in 5-HT^DRN neurotransmission and suggests that CART may drive anxiety-like behavior by promoting feedforward inhibition of 5-HT neurons.