Dorsal hippocampal lesions boost performance in the rat sequential reaction time task

mTORC1 Signaling Pathway Mediates Chronic Stress-Induced Synapse Loss in the Hippocampus

Background: The mechanistic target of rapamycin complex 1 (mTORC1) signaling has served as a promising target for therapeutic intervention of major depressive disorder (MDD), but the mTORC1 signaling underlying MDD has not been well elucidated. In the present study, we investigated whether mTORC1 signaling pathway mediates synapse loss induced by chronic stress in the hippocampus. Methods: Chronic restraint stress-induced depression-like behaviors were tested by behavior tests (sucrose preference test, forced swim test and tail suspension test). Synaptic proteins and alternations of phosphorylation levels of mTORC1 signaling-associated molecules were measured using Western blotting. In addition, mRNA changes of immediate early genes (IEGs) and glutamate receptors were measured by RT-PCR. Rapamycin was used to explore the role of mTORC1 signaling in the antidepressant effects of fluoxetine. Results: After successfully establishing the chronic restraint stress paradigm, we observed that the mRNA levels of some IEGs were significantly changed, indicating the activation of neurons and protein synthesis alterations. Then, there was a significant downregulation of glutamate receptors and postsynaptic density protein 95 at protein and mRNA levels. Additionally, synaptic fractionation assay revealed that chronic stress induced synapse loss in the dorsal and ventral hippocampus. Furthermore, these effects were associated with the mTORC1 signaling pathway-mediated protein synthesis, and subsequently the phosphorylation of associated downstream signaling targets was reduced after chronic stress. Finally, we found that intracerebroventricular infusion of rapamycin simulated depression-like behavior and also blocked the antidepressant effects of fluoxetine. Conclusion: Overall, our study suggests that mTORC1 signaling pathway plays a critical role in mediating synapse loss induced by chronic stress, and has part in the behavioral effects of antidepressant treatment.

Behavioral facilitation after hippocampal lesion: A review

When parts of the brain suffer from damage, certain functional deficits or impairments are the expected and typical outcome. A myriad of examples show such negative consequences, which afford the daily tasks of neurologists, neuropsychologists, and also behavioral neuroscientists working with experimental brain lesions. Compared to lesion-induced deficits, examples for functional enhancements or facilitation after brain lesions are rather rare and usually not well studied. Here, the mammalian hippocampus seems to provide an exception, since substantial evidence shows that its damage can have facilitatory behavioral effects under certain conditions. This review will address these effects and their possible mechanisms. It will show that facilitatory effects of hippocampal lesions, although mostly studied in rats, can be found in many mammalian species, that is, they are apparently not species-specific. Furthermore, they can be found with various lesion techniques, from tissue ablation, to neurotoxic damage, and from damage of hippocampal structure itself to damage of fiber systems innervating it. The major emphasis of this review, however, lies on the behavioral effects and their interpretations. Thus, facilitatory effects can be found in several learning paradigms, especially active avoidance, and some forms of Pavlovian and instrumental conditioning. These will be discussed in light of pertinent theories of hippocampal function, such as inhibition, spatial cognition, and multiple memory systems theories, which state that facilitatory effects of hippocampal lesions may reflect the loss of interference between hippocampal spatial and striatal procedural cognition. Using the example of the rat sequential reaction time task, it will also be discussed how such lesions can have direct and indirect consequences on certain behavioral readouts. A final note will advocate considering possible functional facilitation also in neurologic patients, especially those with hippocampal damage, since such a strategy might provide new avenues for therapeutic treatments.

Decoupling Actions from Consequences: Dorsal Hippocampal Lesions Facilitate Instrumental Performance, but Impair Behavioral Flexibility in Rats

The present study is part of a series of experiments, where we analyze why and how damage of the rat's dorsal hippocampus (dHC) can enhance performance in a sequential reaction time task (SRTT). In this task, sequences of distinct visual stimulus presentations are food-rewarded in a fixed-ratio-13-schedule. Our previous study (Busse and Schwarting, 2016) had shown that rats with lesions of the dHC show substantially shorter session times and post-reinforcement pauses (PRPs) than controls, which allows for more practice when daily training is kept constant. Since sequential behavior is based on instrumental performance, a sequential benefit might be secondary to that. In order to test this hypothesis in the present study, we performed two experiments, where pseudorandom rather than sequential stimulus presentation was used in rats with excitotoxic dorsal hippocampal lesions. Again, we found enhanced performance in the lesion-group in terms of shorter session times and PRPs. During the sessions we found that the lesion-group spent less time with non-instrumental behavior (i.e., grooming, sniffing, and rearing) after prolonged instrumental training. Also, such rats showed moderate evidence for an extinction impairment under devalued food reward conditions and significant deficits in a response-outcome (R-O)-discrimination task in comparison to a control-group. These findings suggest that facilitatory effects on instrumental performance after dorsal hippocampal lesions may be primarily a result of complex behavioral changes, i.e., reductions of behavioral flexibility and/or alterations in motivation, which then result in enhanced instrumental learning.

A rodent model of the human psychomotor vigilance test: Performance comparisons

BACKGROUND

The human Psychomotor Vigilance Test (PVT) is commonly utilized as an objective risk assessment tool to quantify fatigue and sustained attention in laboratory, clinical, and operational settings.

NEW METHOD

Recent studies have employed a rodent version of the PVT (rPVT) to measure various aspects of attention (lapses in attention, reaction times) under varying experimental conditions.

RESULTS

Data are presented here to evaluate the short- and long-term utility of the rPVT adapted for laboratory rats designed to track the same types of performance variables as the human PVT-i.e., motor speed, inhibitory control ("impulsivity"), and attention/inattention. Results indicate that the rPVT is readily learned by rats and requires less than two weeks of training to acquire the basic procedure. Additional data are also presented on the effects of radiation exposure on these performance measures that indicate the utility of the procedure for assessing changes in neurobehavioral function in rodents across their lifespans.

COMPARISON WITH EXISTING METHOD(S)

Once stable performances are obtained, rats evidence a high degree of similarity to human performance measures, and include similarities in terms of lapses and reaction times, in addition to percent correct and premature responding. Similar to humans, rats display both a vigilance decrement across time on task and a response-stimulus interval effect.

CONCLUSIONS

The rPVT is a useful tool in the investigation of the effects of a wide range of variables on vigilance performance that compares favorably to the human PVT and for developing potential prophylactics, countermeasures, and treatments for neurobehavioral dysfunctions.

Differential effects of social and physical environmental enrichment on brain plasticity, cognition, and ultrasonic communication in rats

Environmental enrichment (EE) exerts beneficial effects on brain plasticity, cognition, and anxiety/depression, leading to a brain that can counteract deficits underlying various brain disorders. Because the complexity of the EE commonly used makes it difficult to identify causal aspects, we examined possible factors using a 2 × 2 design with social EE (two vs. six rats) and physical EE (physically enriched vs. nonenriched). For the first time, we demonstrate that social and physical EE have differential effects on brain plasticity, cognition, and ultrasonic communication. Expectedly, physical EE promoted neurogenesis in the dentate gyrus of the hippocampal formation, but not in the subventricular zone, and, as a novel finding, affected microRNA expression levels, with the activity-dependent miR-124 and miR-132 being upregulated. Concomitant improvements in cognition were observed, yet social deficits were seen in the emission of prosocial 50-kHz ultrasonic vocalizations (USV) paralleled by a lack of social approach in response to them, consistent with the intense world syndrome/theory of autism. In contrast, social EE had only minor effects on brain plasticity and cognition, but led to increased prosocial 50-kHz USV emission rates and enhanced social approach behavior. Importantly, social deficits following physical EE were prevented by additional social EE. The finding that social EE has positive whereas physical EE has negative effects on social behavior indicates that preclinical studies focusing on EE as a potential treatment in models for neuropsychiatric disorders characterized by social deficits, such as autism, should include social EE in addition to physical EE, because its lack might worsen social deficits.

Comparison of interval timing behaviour in mice following dorsal or ventral hippocampal lesions with mice having δ-opioid receptor gene deletion

Mice with cytotoxic lesions of the dorsal hippocampus (DH) underestimated 15 s and 45 s target durations in a bi-peak procedure as evidenced by proportional leftward shifts of the peak functions that emerged during training as a result of decreases in both 'start' and 'stop' times. In contrast, mice with lesions of the ventral hippocampus (VH) displayed rightward shifts that were immediately present and were largely limited to increases in the 'stop' time for the 45 s target duration. Moreover, the effects of the DH lesions were congruent with the scalar property of interval timing in that the 15 s and 45 s functions superimposed when plotted on a relative timescale, whereas the effects of the VH lesions violated the scalar property. Mice with DH lesions also showed enhanced reversal learning in comparison to control and VH lesioned mice. These results are compared with the timing distortions observed in mice lacking δ-opioid receptors (Oprd1(-/-)) which were similar to mice with DH lesions. Taken together, these results suggest a balance between hippocampal-striatal interactions for interval timing and demonstrate possible functional dissociations along the septotemporal axis of the hippocampus in terms of motivation, timed response thresholds and encoding in temporal memory.

Dissociating hippocampal and striatal contributions to sequential prediction learning

Behavior may be generated on the basis of many different kinds of learned contingencies. For instance, responses could be guided by the direct association between a stimulus and response, or by sequential stimulus-stimulus relationships (as in model-based reinforcement learning or goal-directed actions). However, the neural architecture underlying sequential predictive learning is not well understood, in part because it is difficult to isolate its effect on choice behavior. To track such learning more directly, we examined reaction times (RTs) in a probabilistic sequential picture identification task in healthy individuals. We used computational learning models to isolate trial-by-trial effects of two distinct learning processes in behavior, and used these as signatures to analyse the separate neural substrates of each process. RTs were best explained via the combination of two delta rule learning processes with different learning rates. To examine neural manifestations of these learning processes, we used functional magnetic resonance imaging to seek correlates of time-series related to expectancy or surprise. We observed such correlates in two regions, hippocampus and striatum. By estimating the learning rates best explaining each signal, we verified that they were uniquely associated with one of the two distinct processes identified behaviorally. These differential correlates suggest that complementary anticipatory functions drive each region's effect on behavior. Our results provide novel insights as to the quantitative computational distinctions between medial temporal and basal ganglia learning networks and enable experiments that exploit trial-by-trial measurement of the unique contributions of both hippocampus and striatum to response behavior.