Male Goal-Tracker and Sign-Tracker Rats Do Not Differ in Neuroendocrine or Behavioral Measures of Stress Reactivity

Abstract

Environmental cues attain the ability to guide behavior via learned associations. As predictors, cues can elicit adaptive behavior and lead to valuable resources (e.g., food). For some individuals, however, cues are transformed into incentive stimuli and elicit motivational states that can be maladaptive. The goal-tracker (GT)/sign-tracker (ST) animal model captures individual differences in cue-motivated behaviors, with reward-associated cues serving as predictors of reward for both phenotypes but becoming incentive stimuli to a greater degree for STs. While these distinct phenotypes are characterized based on Pavlovian conditioned approach (PavCA) behavior, they exhibit differences on a number of behaviors relevant to psychopathology. To further characterize the neurobehavioral endophenotype associated with individual differences in cue-reward learning, neuroendocrine and behavioral profiles associated with stress and anxiety were investigated in male GT, ST, and intermediate responder (IR) rats. It was revealed that baseline corticosterone (CORT) increases with Pavlovian learning, but to the same degree, regardless of phenotype. No significant differences in behavior were observed between GTs and STs during an elevated plus maze (EPM) or open field test (OFT), nor were there differences in CORT response to the OFT or physiological restraint. Upon examination of central markers associated with stress reactivity, we found that STs have greater glucocorticoid receptor (GR) mRNA expression in the ventral hippocampus, with no phenotypic differences in the dorsal hippocampus or prelimbic cortex (PrL). These findings demonstrate that GTs and STs do not differ on stress-related and anxiety-related behaviors, and suggest that differences in neuroendocrine measures between these phenotypes can be attributed to distinct cue-reward learning styles.

Keywords: corticosterone; glucocorticoid receptors; incentive salience; stress reactivity.

Figure 1.

Experimental timelines. A, “Baseline” tail nicks were performed for blood collection Pre-PavCA, and after the rats had acquired a conditioned response (Post-PavCA). Rats were subsequently tested on the elevated plus maze (EPM) and the open field test (OFT), followed by physiological restraint, with a 10-d rest period before each. CORT response to the OFT and acute restraint was captured with time course blood sampling. B, A separate group of rats underwent five sessions of PavCA training and were subsequently euthanized to assess GR expression in the hippocampus and PrL using in situ hybridization.

Figure 2.

Acquisition of sign-tracking and goal-tracking behavior. Sign-tracking (i.e., lever-directed, left panels) and goal-tracking (i.e., food-cup directed, right panels) behavioral measures were assessed across five PavCA sessions. Mean + SEM for probability to contact (A) the lever or (B) the food-cup; total number of contacts with (C) the lever or (D) the food-cup; and latency to contact (E) the lever or (F) the food-cup. Rats with a sign-tracking conditioned response were classified as STs (n = 32), those with a goal-tracking conditioned response as GTs (n = 11), and those that vacillated between the two conditioned responses as IRs (n = 17).

Figure 3.

“Baseline” CORT levels before and after PavCA training. Mean + SEM for baseline plasma CORT levels before (Pre-PavCA) and following (Post-PavCA) PavCA training experience. Basal plasma CORT levels increased with Pavlovian training experience (*p = 0.001; n = 60; GT n = 10, IR = 17, ST = 32).

Figure 4.

EPM. A, Heat map representations for the average time spent in each zone during the 5-min EPM test for each phenotype. B, Mean + SEM for the time spent in each zone of the EPM for GTs (n = 11), IRs (n = 13), and STs (n = 14). All rats spent significantly more time in the closed arms compared with the open arms and center of the maze (*p < 0.001). There was not a significant difference between GTs and STs in the amount of time spent in either the center of the arena or the open or closed arms. IRs spent significantly less time in the open arms, relative to GTs (#p < 0.05).

Figure 5.

OFT. A, Heat map representations for the average time spent in each zone (outer edge vs center) during the 5-min OFT for each phenotype. B, Mean + SEM for time spent in the outer edge or center of the arena for GTs (n = 11), IRs (n = 13), and STs (n = 14). All rats spent significantly more time on the outer edge of the arena compared with the center. Time spent in the center of the arena is shown as an inset on a different scale for illustration purposes. There was not a significant difference between phenotypes for the amount of time spent in the center of the arena.

Figure 6.

CORT response to the OFT and acute physiological restraint. A, Mean + SEM for plasma CORT levels 0, 20, 40, 60, and 80 min postonset of the OFT for GTs (n = 11), IRs (n = 13), and STs (n = 14). There was a significant increase in CORT induced by the OFT at 20-, 40-, 60-, and 80-min time points (*p < 0.001), but no significant difference between phenotypes. B, Mean + SEM for plasma CORT levels 0, 30, 90, and 120 min postonset of acute restraint for GTs (n = 11), IRs (n = 13), and STs (n = 12). There was a significant increase in CORT induced by restraint at 30- and 90-min time points (*p < 0.001), but no significant difference between phenotypes.

Figure 7.

GR mRNA expression in the dorsal and ventral hippocampus. A, Coronal brain sections representing bregma coordinates used to quantify GR mRNA expression (adapted from Paxinos and Watson, 2007). B, Representative in situ images for a GT, IR, and ST rat with tracing selections of the region of interest (ROI; in red) on the right hemisphere, including hippocampal subregions demarcated as CA1, CA2, CA3, and DG. C, D, Mean + SEM optical density for GR mRNA in subregions of the (C) dorsal and (D) ventral hippocampus for GTs (n = 10), IRs (n = 10) and STs (n = 10). In the ventral hippocampus, GR mRNA varied between subregions (*p < 0.001 vs CA1, #p < 0.001 vs DG). Relative to GTs and IRs, STs show greater GR mRNA density across subregions.

Figure 8.

GR mRNA expression in the PrL. A, Coronal brain sections representing bregma coordinates used to quantify GR mRNA expression (adapted from Paxinos and Watson, 2007). B, Representative in situ images for a GT, IR, and ST rat with tracing selections of the region of interest (ROI; in red) on the right hemisphere. C, Mean + SEM optical density for GR mRNA in the PrL for GTs (n = 10), IRs (n = 10), and STs (n = 10).