Class I HDAC inhibition improves object recognition memory consolidation through BDNF/TrkB pathway in a time-dependent manner

The weekend warrior effect: Consistent intermittent exercise induces persistent cognitive benefits

Exercise provides a range of cognitive benefits, including improved memory performance. Previously, we demonstrated that 14 days of continuous voluntary wheel-running exercise enables learning in a hippocampus-dependent Object Location Memory (OLM) task under insufficient, subthreshold training conditions in adult mice. Whether similar exercise benefits can be obtained from consistent intermittent exercise as continuous exercise is unknown. Here, we examine whether intermittent exercise (the weekend warrior effect: 2 days of exercise a week for 7 weeks) displays similar or distinct cognitive benefits as previously examined with 14 days of continuous exercise. We find that both continuous and intermittent exercise parameters similarly enable hippocampus-dependent OLM compared to the 2-day exercise control group. Mice receiving intermittent exercise maintained cognitive benefits following a 7-day sedentary delay, whereas mice that underwent 14 continuous days of exercise showed diminished cognitive benefits as previously reported. Further, compared to continuous exercise, intermittent exercise mice exhibited persistently elevated levels of the genes Acvr1c and Bdnf which we know to be critically involved in hippocampus-dependent long-term memory in the dorsal hippocampus. Together findings suggest that consistent intermittent exercise persistently enables hippocampal-dependent long-term memory. Understanding the optimal parameters for persistent cognitive function and the mechanisms mediating persistent effects will aid in therapeutic pursuits investigating the mitigation of cognitive ailments.

Salience to remember: VTA-IC dopaminergic pathway activity is necessary for object recognition memory formation

Previous studies have shown that dopaminergic activity modulates the salience of novel stimuli enabling the formation of recognition memories. In this work, we hypothesize that dopamine released into the insular cortex (IC) from the ventral tegmental area (VTA) inputs enables the acquisition to consolidate object recognition memory. It has been reported that short training produces weak recognition memories; on the contrary, longer training produces lasting and robust recognition memories. Using a Cre-recombinase under the tyrosine hydroxylase (TH+) promoter mouse model, we photostimulated the VTA-IC dopaminergic pathway during short training or photoinhibited the same pathway during long training while mice explored objects. Our results showed that the photostimulation of the VTA-IC pathway during a short training enables the acquisition of recognition memory. Conversely, photoinhibition of the same pathway during a long training prevents the acquisition of recognition memory. Interestingly, the exploration time of the objects under photoinhibition or photostimulation of the dopaminergic VTA-IC pathway was not altered. Significantly, this enhancement of acquisition of the object recognition memory through the photostimulation of the VTA dopaminergic neurons could be impaired by the blockage of the D1-like receptors into the IC, either before or after the photostimulation. Altogether, our results suggest that dopamine released by the VTA is required during the acquisition to consolidate the object recognition memory through D1-like receptors into the IC without affecting the activity or the motivation to explore objects.

Neuronal circuitry for recognition memory of object and place in rodent models

Rats and mice are used for studying neuronal circuits underlying recognition memory due to their ability to spontaneously remember the occurrence of an object, its place and an association of the object and place in a particular environment. A joint employment of lesions, pharmacological interventions, optogenetics and chemogenetics is constantly expanding our knowledge of the neural basis for recognition memory of object, place, and their association. In this review, we summarize current studies on recognition memory in rodents with a focus on the novel object preference, novel location preference and object-in-place paradigms. The evidence suggests that the medial prefrontal cortex- and hippocampus-connected circuits contribute to recognition memory for object and place. Under certain conditions, the striatum, medial septum, amygdala, locus coeruleus and cerebellum are also involved. We propose that the neuronal circuitry for recognition memory of object and place is hierarchically connected and constructed by different cortical (perirhinal, entorhinal and retrosplenial cortices), thalamic (nucleus reuniens, mediodorsal and anterior thalamic nuclei) and primeval (hypothalamus and interpeduncular nucleus) modules interacting with the medial prefrontal cortex and hippocampus.