The telencephalon is a neuronal substrate for systemic inflammatory responses in teleosts via polyamine metabolism

Abstract Systemic inflammation elicits sickness behaviors and fever by engaging a complex neuronal circuitry that begins in the preoptic area of the hypothalamus. Ectotherms such as teleost fish display sickness behaviors in response to infection or inflammation, seeking warmer temperatures to enhance survival via behavioral fever responses. To date, the hypothalamus is the only brain region implicated in sickness behaviors and behavioral fever in teleosts. Yet, the complexity of neurobehavioral manifestations underlying sickness responses in teleosts suggests engagement of higher processing areas of the brain. Using two in vivo models of systemic inflammation in rainbow trout, we find canonical pyrogenic cytokine responses in the hypothalamus whereas in the telencephalon il-1b and tnfa expression is decoupled from il-6 expression. Polyamine metabolism changes, characterized by accumulation of putrescine and decreases in spermine and spermidine, are recorded in the telencephalon but not hypothalamus upon systemic injection of bacteria. While systemic inflammation causes canonical behavioral fever in trout, blockade of bacterial polyamine metabolism prior to injection abrogates behavioral fever, polyamine responses and telencephalic but not hypothalamic cytokine responses. Combined, our work identifies the telencephalon as a neuronal substrate for brain responses to systemic inflammation in teleosts and uncovers the role of polyamines as critical chemical mediators in sickness behaviors. Significance Statement Systemic inflammation induces neuroimmune responses in the brain resulting in sickness behaviors and fever. In endotherms, sickness behaviors and fever are initiated in the hypothalamus but also engage a complex neuronal circuitry in higher areas of the brain. In ectotherms, only the hypothalamus has been linked to sickness behaviors and behavioral fever. Here we demonstrate that the telencephalon, a critical region of the teleost brain responsible for higher order processing, mounts pyrogenic cytokine responses to systemic inflammation in teleosts that are different from those of the hypothalamus. We identify polyamine metabolism as a core response of the teleost telencephalon to systemic inflammation and report that bacterial polyamines are triggers of behavioral fever in rainbow trout. Our work uncovers a previously unrecognized role for the telencephalon and polyamine metabolism in sickness behaviors and behavioral fever in teleosts with implications in fish health and fish conservation.