Glucocorticoid-sensitive ventral hippocampal-orbitofrontal cortical connections support goal-directed action - Curt Richter Award Paper 2019

Abstract

In an ever-changing and often ambiguous environment, organisms must use previously learned associations between antecedents and outcomes to predict future associations and make optimal choices. Chronic stress can impair one's ability to flexibly adjust behaviors when environmental contingencies change, particularly in cases of early-life stress. In mice, exposure to elevated levels of the primary stress hormone, corticosterone (CORT), during early adolescence is sufficient to impair response-outcome decision making later in life, biasing response strategies towards inflexible habits. Nevertheless, neurobiological mechanisms are still being defined. Here, we report that exposure to excess CORT in adolescence causes a loss of dendritic spines on excitatory pyramidal neurons in the lateral, but not medial, orbital prefrontal cortex (loPFC) of mice, and spine loss correlates with the severity of habit biases in adulthood. Excess CORT also reduces the presence of ventral hippocampal (vHC) axon terminals in the loPFC. To identify functional consequences, we inactivated vHC→loPFC projections in typical healthy mice during a period when mice must update response-outcome expectations to optimally acquire food reinforcers. Inactivation impaired the animals' subsequent ability to sustainably choose actions based on likely outcomes, causing them to defer to habit-based response strategies. Thus, vHC→loPFC projections are necessary for response-outcome expectancy updating and a target of excess glucocorticoids during early-life development. Their degradation is likely involved in long-term biases towards habit-based behaviors following glucocorticoid excess in adolescence.

Keywords: Action-outcome; Contingency degradation; DREADDs; Habit; Orbital frontal; Response-outcome.

Fig. 1.. Excess CORT exposure during adolescence impairs the ability of mice to update outcome expectancies in adulthood.

(A) Mice were trained to nose poke for food reinforcers in operant conditioning chambers. Next, the contingency between one response and its outcome was ‘degraded’ by providing food pellets independently of the mouse’s responses. The ability to update expectancies and select responses based on anticipated outcomes was then assessed in a brief choice test conducted in extinction. (B) Experimental timeline is above. First, mice exposed to excess CORT during adolescence were trained to nose poke as adults. All mice acquired the nose poke responses. “FR1” and “RI” denote the schedules of reinforcement used throughout. (C) A probe test following instrumental contingency degradation revealed that control mice preferentially engaged the response most likely to be reinforced (“non-degraded” vs. “degraded”), a goal-directed response strategy. Meanwhile, CORT-exposed mice generated habit-based response strategies, failing to differentiate between behaviors that were or were not likely to be reinforced. (D) Response rates were converted to preferences scores (“non-degraded”/“degraded”), again highlighting goal-directed responding in control mice (scores > 1) and non-preferential, habit-based responding following excess CORT (scores ~1). n = 8/group. (E) Separate mice were exposed to excess CORT in adulthood. In this case, CORT was insufficient to alter preference ratios. n = 6/group. Symbols in B represent means + SEMs, symbols in C–E represent individual mice, bars represent means ± SEMs. *p ≤ 0.05. B and C are reprinted from Barfield et al. (2017), and D and E were generated from a dataset originally reported in that manuscript. Experiments were conducted twice.

Fig. 2.. Excess CORT in adolescence induces dendritic spine loss in the loPFC: Correlations with decision-making abnormalities.

(A) Experimental time (left), thy1-YFP-expressing mice from instrumental conditioning experiments in Fig. 1 were euthanized in adulthood after behavioral testing, and basilar dendrites on excitatory neurons in the loPFC (and, for comparison, moPFC) were imaged. These regions are highlighted on images from the Mouse Brain Library (Rosen et al., 2000). (B) Adolescent CORT exposure eliminated dendritic spines in the loPFC (n = 6–8/group), (C) but not moPFC (n = 4–5/group; less than loPFC due to sparseness of YFP signal in this region). (D) In the rostral loPFC, the density of mature, mushroom-shaped spines correlated with response preference ratios from Fig. 1D. n = 6–7/group. (E) Representative dendrites from the loPFC (unprocessed images at left, reconstructions at right). Bars represent means ± SEMs, symbols represent individual mice. *p ≤ 0.05. Dendritic spines were imaged and reconstructed by a single, blinded rater.

Fig. 3.. Excess CORT during adolescence degrades vHC→loPFC input.

(A) Mice exposed to CORT from P31–42 received infusions of the anterograde tracer fluoro-ruby into the vHC to characterize vHC projections to the oPFC. (B) At left: The spread of fluoro-ruby in the vHC is transposed onto an image from the Mouse Brain Library (Rosen et al., 2000). At right: Representative image of fluoro-ruby in the vHC. (C) Unprocessed images of fluoro-ruby-positive axon terminal punctae in the loPFC. Comparator regions are also highlighted on an image from the Mouse Brain Library (Rosen et al., 2000). (D) Adolescent CORT exposure reduced the presence of vHC terminals in the loPFC, (E) but not the moPFC (F) or PL. n = 9–11/group. Bars represent means ± SEMs, symbols represent individual mice. **p ≤ 0.001. Punctae were imaged and enumerated by a single, blinded rater; 2 independent cohorts of mice contributed to the dataset.

Fig. 4.. Inactivating vHC→loPFC projections weakens goal-directed response selection.

(A) Experimental timeline. Note that the DREADDs ligand CNO was administered immediately following instrumental contingency degradation. Response preferences were tested over the following 2 days when mice were drug-free. (B) Retro-Cre in the loPFC (blue), combined with a Cre-dependent Gi-DREADD in the vHC (red), was used to inactivate vHC→oPFC projections. Spread of viral vectors in the loPFC and vHC is drawn in the left hemispheres on images from the Mouse Brain Library (Rosen et al., 2000), and representative infusions are shown at right. (C) Mice were trained to nose poke for food reinforcers. “FR1” and “RI” denote the schedules of reinforcement. All mice acquired the responses during training without group differences. (D) Inactivation of vHC→loPFC projections impaired the ability of mice to generate response preferences based on the likelihood of reinforcement. (E) Impairments were persistent, detectable across several time bins and 2 test days, n = 10–11/group. Bars represent means ± SEMs. Symbols in C and E represent means + SEMs; otherwise, symbols represent individual mice. *p = 0.03 main effect of group; #p = 0.01 vs. 1 (1 reflects no change). This experiment was conducted in 2 independent cohorts of mice (for interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).