Age-Related Alterations in Prelimbic Cortical Neuron Arc Expression Vary by Behavioral State and Cortical Layer

Abstract

Prefrontal cortical and medial temporal lobe connectivity is critical for higher cognitive functions that decline in older adults. Likewise, these cortical areas are among the first to show anatomical, functional, and biochemical alterations in advanced age. The prelimbic subregion of the prefrontal cortex and the perirhinal cortex of the medial temporal lobe are densely reciprocally connected and well-characterized as undergoing age-related neurobiological changes that correlate with behavioral impairment. Despite this fact, it remains to be determined how changes within these brain regions manifest as alterations in their functional connectivity. In our previous work, we observed an increased probability of age-related dysfunction for perirhinal cortical neurons that projected to the prefrontal cortex in old rats compared to neurons that were not identified as projection neurons. The current study was designed to investigate the extent to which aged prelimbic cortical neurons also had altered patterns of Arc expression during behavior, and if this was more evident in those cells that had long-range projections to the perirhinal cortex. The expression patterns of the immediate-early gene Arc were quantified in behaviorally characterized rats that also received the retrograde tracer cholera toxin B (CTB) in the perirhinal cortex to identify projection neurons to this region. As in our previous work, the current study found that CTB+ cells were more active than those that did not have the tracer. Moreover, there were age-related reductions in prelimbic cortical neuron Arc expression that correlated with a reduced ability of aged rats to multitask. Unlike the perirhinal cortex, however, the age-related reduction in Arc expression was equally likely in CTB+ and CTB− negative cells. Thus, the selective vulnerability of neurons with long-range projections to dysfunction in old age may be a unique feature of the perirhinal cortex. Together, these observations identify a mechanism involving prelimbic-perirhinal cortical circuit disruption in cognitive aging.

A long-term ketogenic diet in young and aged rats has dissociable effects on prelimbic cortex and CA3 ensemble activity

“…Differences in neuronal activation during the behavioral epochs in relation to age, cortical layers and diet group were statistically evaluated by calculating the mean percentage of Arc positive cells per rat as done previously (Hartzell et al, 2013;Hernandez et al, 2018c;Hernandez et al, 2020b) to avoid inflating statistical power by including multiple measures from one animal and ensure data do not violate the assumption of independent observations (Aarts et al, 2014).…”

“…The present study utilized the cellular compartment analysis of temporal activity by fluorescence in situ hybridization (catFISH) for the immediate early genes (IEGs) Arc and Homer 1a (Guzowski et al, 1999; Vazdarjanova et al, 2002) to quantify neuronal activity during three distinct epochs (Marrone et al, 2008): one prior to behavior, one while rats performed the working memory/biconditional association task (WM/BAT), and another while rats performed a continuous spatial alternation task. Activity was quantified in young and aged rats fed a ketogenic (KD) or calorically equivalent control diet (CD) to test the hypothesis that a KD can reverse aberrant activity dynamics that have been associated with poor cognitive performance in the aged prelimbic cortex (PL) (Hernandez et al, 2018c; Hernandez et al, 2020b) and CA3 subregion of the hippocampus (Wilson et al, 2005; Maurer et al, 2017; Lee et al, 2021; Lee et al, 2022). Previous results were replicated regarding elevated IEG expression in aged CA3 neurons (Maurer et al, 2017), altered ensemble dynamics in PL (Hernandez et al, 2018c; Hernandez et al, 2020b), and a bias for neurons that were active in the home cage prior to behavior to also be active during behavior compared to those cells that did not show activity in the home cage (Mizuseki and Buzsáki, 2013; Grosmark and Buzsaki, 2016).…”

“…Activity was quantified in young and aged rats fed a ketogenic (KD) or calorically equivalent control diet (CD) to test the hypothesis that a KD can reverse aberrant activity dynamics that have been associated with poor cognitive performance in the aged prelimbic cortex (PL) (Hernandez et al, 2018c; Hernandez et al, 2020b) and CA3 subregion of the hippocampus (Wilson et al, 2005; Maurer et al, 2017; Lee et al, 2021; Lee et al, 2022). Previous results were replicated regarding elevated IEG expression in aged CA3 neurons (Maurer et al, 2017), altered ensemble dynamics in PL (Hernandez et al, 2018c; Hernandez et al, 2020b), and a bias for neurons that were active in the home cage prior to behavior to also be active during behavior compared to those cells that did not show activity in the home cage (Mizuseki and Buzsáki, 2013; Grosmark and Buzsaki, 2016). The novel findings observed in the current study are that the KD increased the proportion of active neurons in the superficial layers of the PL during both behaviors in the aged rats.…”

“…Differences in neuronal activation during the behavioral epochs in relation to age, cortical layers and diet group were statistically evaluated by calculating the mean percentage of Arc positive cells per rat as done previously (Hartzell et al, 2013; Hernandez et al, 2018c; Hernandez et al, 2020b) to avoid inflating statistical power by including multiple measures from one animal and ensure data do not violate the assumption of independent observations (Aarts et al, 2014). Potential effects of age, layers/subregion and task were examined using repeated measures ANOVAs (ANOVA-RM) with the within-subject factors of behavioral task, cortical layers, pre-active status, and the between-subjects factors of age and diet.…”

“…In old age, increased activity in hippocampal CA3 is associated with worse cognitive outcomes (Yassa et al, 2011a; Maurer et al, 2017). Conversely, increased activation in the prelimbic cortex (PL) is correlated with better cognition (Hernandez et al, 2020b). While pharmacological agents have been shown to improve either PL (Banuelos et al, 2014) or CA3 function (Koh et al, 2010; Bakker et al, 2012; Bakker et al, 2015; Robitsek et al, 2015), an effective therapeutic approach for optimizing neural activity patterns across these brain regions with divergent relationships between activation and cognitive performance has not been established.…”