Challenges to body fluid homeostasis differentially recruit phasic dopamine signaling in a taste-selective manner

Abstract

The internal environment of an organism must remain stable to ensure optimal performance and ultimately survival. The generation of motivated behaviors is an adaptive mechanism for defending homeostasis. Although physiological state modulates motivated behaviors, the influence of physiological state on phasic dopamine signaling, an underlying neurobiological substrate of reward-driven behavior, is underexplored. Here, we use sodium depletion and water restriction, manipulations of body fluid homeostasis, to determine the flexibility and specificity of dopamine responses. Changes in dopamine concentration were measured using fast-scan cyclic voltammetry in the nucleus accumbens shell of male rats in response to intraoral infusions of fluids that either satisfied or did not satisfy homeostatic need. Increases in dopamine concentration during intraoral infusions were observed only under conditions of physiological deficit. Furthermore, dopamine increases were selective and limited to those that satisfied the need state of the animal. Thus, dopamine neurons track fluid balance and respond to salt and water stimuli in a state-and taste-dependent manner. Using Fluoro-Gold tracing and immunohistochemistry for c-Fos and Foxp2, a marker of sodium-deprivation responsive neurons, we revealed brainstem populations of neurons that are activated by sodium depletion and project directly to the ventral tegmental area. The identified projections may modulate dopamine neuron excitability and consequently the state-specific dopamine release observed in our experiments. This work illustrates the impact of physiological state on mesolimbic dopamine signaling and a potential circuit by which homeostatic disruptions are communicated to mesolimbic circuitry to drive the selective reinforcement of biologically-required stimuli under conditions of physiological need.