Karienn Souza Montgomery

Cognitive flexibility declines early in Alzheimer’s disease, yet the underlying circuit mechanisms remain unknown. Here, we report that young 5xFAD mice exhibit deficits in instrumental reversal learning prior to spatial memory impairment. This behavioral inflexibility is associated with abnormal neuronal reactivation in the medial prefrontal cortex and dorsomedial striatum. Electrophysiological recordings reveal that medial prefrontal cortex neurons are hyperexcitable and receive increased excitatory input. Furthermore, glutamatergic transmission from the medial prefrontal cortex to striatal direct-pathway medium spiny neurons is enhanced and coincides with strengthened inhibitory transmission onto striatal cholinergic interneurons, reduced spontaneous firing, and diminished striatal acetylcholine release. Critically, sustained chemogenetic inhibition of this corticostriatal circuit attenuates cortical amyloid accumulation, reduces glutamatergic transmission, and increases acetylcholine levels. This also rescues reversal learning deficits in 5xFAD mice. Here, we show that pathological corticostriatal hyperactivity contributes to early cognitive inflexibility in a mouse model of Alzheimer’s disease.

Background: Disturbances of the sleep-wake cycle and other circadian rhythms typically precede the age-related deficits in learning and memory, suggesting that these alterations in circadian timekeeping may contribute to the progressive cognitive decline during aging. The present study examined the role of immune cell activation and inflammation in the link between circadian rhythm dysregulation and cognitive impairment in aging. Methods: C57Bl/6J mice were exposed to shifted light-dark (LD) cycles (12 h advance/5d) during early adulthood (from ≈ 4-6mo) or continuously to a “fixed” LD12:12 schedule. At middle age (13-14mo), the long-term effects of circadian rhythm dysregulation on cognitive performance, immune cell regulation and hippocampal microglia were analyzed using behavioral, flow cytometry and immunohistochemical assays. Results: Entrainment of the activity rhythm was stable in all mice on a fixed LD 12:12 cycle but was fully compromised during exposure to shifted LD cycles. Even during “post-treatment” exposure to standard LD 12:12 conditions, re-entrainment in shifted LD mice was marked by altered patterns of entrainment and increased day-to-day variability in activity onset times that persisted into middle-age. These alterations in light-dark entrainment were closely associated with dramatic impairment in the Barnes maze test for the entire group of shifted LD mice at middle age, well before cognitive decline was first observed in aged (18-22mo) animals maintained on fixed LD cycles. In conjunction with the effects of circadian dysregulation on cognition, shifted LD mice at middle age were distinguished by significant expansion of splenic B cells and B cell subtypes expressing the activation marker CD69 or inflammatory marker MHC Class II Invariant peptide (CLIP), differential increases in CLIP+, 41BB-Ligand+, and CD74 + B cells in the meningeal lymphatics, alterations in splenic T cell subtypes, and increased number and altered functional state of microglia in the dentate gyrus. In shifted LD mice, the expansion in splenic B cells was negatively correlated with cognitive performance; when B cell numbers were higher, performance was worse in the Barnes maze. These results indicate that disordered circadian timekeeping associated with early exposure to shift work-like schedules alone accelerates cognitive decline during aging in conjunction with altered regulation of immune cells and microglia in the brain.

Preclinical quantitative models of cognitive performance are necessary for translation from basic research to clinical studies. In rodents, non-cognitive factors are a potential influence on testing outcome and high variability in behavior requires multiple time point testing for better assessment of performance in more sophisticated tests. Thus, these models have limited translational value as most human cognitive tests characterize cognition using single digit scales to distinguish between impaired and unimpaired function. To address these limitations, we developed a cognitive index for learning based on previously described scores for strategies used by mice to escape the Barnes maze. We compared the cognitive index and circadian patterns of light-dark entrainment in young (4–6 months), middle-aged (13–14 months), and aged (18–24 months) mice as cognitive changes during aging are often accompanied by pronounced changes in sleep-wake cycle. Following continuous analysis of circadian wheel-running activity (30–40 days), the same cohorts of mice were tested in the Barnes maze. Aged mice showed significant deficits in the learning and memory portions of the Barnes maze relative to young and middle-aged animals, and the cognitive index was positively correlated to the memory portion of the task (probe) in all groups. Significant age-related alterations in circadian entrainment of the activity rhythm were observed in the middle-aged and aged cohorts. In middle-aged mice, the delayed phase angle of entrainment and increased variability in the daily onsets of activity preceded learning and memory deficits observed in aged animals. Interestingly, learning-impaired mice were distinguished by a positive relationship between the extent of Barnes-related cognitive impairment and variability in daily onsets of circadian activity. While it is unclear whether changes in the sleep-wake cycle or other circadian rhythms play a role in cognitive impairment during aging, our results suggest that circadian rhythm perturbations or misalignment may nevertheless provide an early predictor of age-related cognitive decline.

Elevated β-amyloid and impaired synaptic function in hippocampus are among the earliest manifestations of Alzheimer's disease (AD). Most cognitive assessments employed in both humans and animal models, however, are insensitive to this early disease pathology. One critical aspect of hippocampal function is its role in episodic memory, which involves the binding of temporally coincident sensory information (e.g., sights, smells, and sounds) to create a representation of a specific learning epoch. Flexible associations can be formed among these distinct sensory stimuli that enable the "transfer" of new learning across a wide variety of contexts. The current studies employed a mouse analog of an associative "transfer learning" task that has previously been used to identify risk for prodromal AD in humans. The rodent version of the task assesses the transfer of learning about stimulus features relevant to a food reward across a series of compound discrimination problems. The relevant feature that predicts the food reward is unchanged across problems, but an irrelevant feature (i.e., the context) is altered. Experiment 1 demonstrated that C57BL6/J mice with bilateral ibotenic acid lesions of hippocampus were able to discriminate between two stimuli on par with control mice; however, lesioned mice were unable to transfer or apply this learning to new problem configurations. Experiment 2 used the APPswe PS1 mouse model of amyloidosis to show that robust impairments in transfer learning are evident in mice with subtle β-amyloid-induced synaptic deficits in the hippocampus. Finally, Experiment 3 confirmed that the same transfer learning impairments observed in APPswePS1 mice were also evident in the Tg-SwDI mouse, a second model of amyloidosis. Together, these data show that the ability to generalize learned associations to new contexts is disrupted even in the presence of subtle hippocampal dysfunction and suggest that, across species, this aspect of hippocampal-dependent learning may be useful for early identification of AD-like pathology.

Variation in dopamine receptor levels has been associated with different facets of impulsivity. To further delineate the neural substrates underlying impulsive action (inability to withhold a prepotent motor response) and impulsive choice (delay aversion), we characterised rats in the Differential Reinforcement of Low Rates of Responding task and a delay discounting task. We also measured performance on an effort‐based discounting task. We then assessed D1 and D2 dopamine receptor mRNA expression in subregions of the prefrontal cortex and nucleus accumbens using in situ hybridisation, and compared these data with behavioral performance. Expression of D1 and D2 receptor mRNA in distinct brain regions was predictive of impulsive action. A dissociation within the nucleus accumbens was observed between subregions and receptor subtypes; higher D1 mRNA expression in the shell predicted greater impulsive action, whereas lower D2 mRNA expression in the core predicted greater impulsive action. We also observed a negative correlation between impulsive action and D2 mRNA expression in the prelimbic cortex. Interestingly, a similar relationship was present between impulsive choice and prelimbic cortex D2 mRNA, despite the fact that behavioral indices of impulsive action and impulsive choice were uncorrelated. Finally, we found that both high D1 mRNA expression in the insular cortex and low D2 mRNA expression in the infralimbic cortex were associated with willingness to exert effort for rewards. Notably, dopamine receptor mRNA in these regions was not associated with either facet of impulsivity. The data presented here provide novel molecular and neuroanatomical distinctions between different forms of impulsivity, as well as effort‐based decision‐making.

Ethanol exposure during perinatal development can cause cognitive abnormalities including difficulties in learning, attention, and memory, as well as heightened impulsivity. The purpose of this study was to assess performance in spatial learning and impulsive choice tasks in rats subjected to an intragastric intubation model of binge ethanol exposure during human third trimester-equivalent brain development. Male and female Sprague–Dawley rat pups were intubated with ethanol (5.25 g/kg/day) on postnatal days 4–9. At adolescence (between postnatal days 35–38), these rats and sham intubated within-litter controls were trained in both spatial and cued versions of the Morris water maze. A subset of the male rats was subsequently tested on a delay-discounting task to assess impulsive choice. Ethanol-exposed rats were spatially impaired relative to controls, but performed comparably to controls on the cued version of the water maze. Ethanol-exposed rats also showed greater preference for large delayed rewards on the delay discounting task, but no evidence for altered reward sensitivity or perseverative behavior. These data demonstrate that early postnatal intermittent binge-like ethanol exposure has prolonged, detrimental, but selective effects on cognition, suggesting that even relatively brief ethanol exposure late in human pregnancy can be deleterious for cognitive function.

Many psychiatric disorders are characterized by abnormal risky decision-making and dysregulated dopamine receptor expression. The current study was designed to determine how different dopamine receptor subtypes modulate risk-taking in young adult rats, using a “Risky Decision-making Task” that involves choices between small “safe” rewards and large “risky” rewards accompanied by adverse consequences. Rats showed considerable, stable individual differences in risk preference in the task, which were not related to multiple measures of reward motivation, anxiety, or pain sensitivity. Systemic activation of D2-like receptors robustly attenuated risk-taking, whereas drugs acting on D1-like receptors had no effect. Systemic amphetamine also reduced risk-taking, an effect which was attenuated by D2-like (but not D1-like) receptor blockade. Dopamine receptor mRNA expression was evaluated in a separate cohort of drug-naive rats characterized in the task. D1 mRNA expression in both nucleus accumbens shell and insular cortex was positively associated with risk-taking, while D2 mRNA expression in orbitofrontal and medial prefrontal cortex predicted risk preference in opposing nonlinear patterns. Additionally, lower levels of D2 mRNA in dorsal striatum were associated with greater risk-taking. These data strongly implicate dopamine signaling in prefrontal cortical-striatal circuitry in modulating decision-making processes involving integration of reward information with risks of adverse consequences.