Elsevier

Neuropharmacology

Volume 197, 1 October 2021, 108720
Neuropharmacology

Attenuated NMDAR signaling on fast-spiking interneurons in prefrontal cortex contributes to age-related decline of cognitive flexibility

https://doi.org/10.1016/j.neuropharm.2021.108720Get rights and content

Highlights

  • Aged rats are impaired on a set-shifting test of cognitive flexibility

  • Age-related set-shifting impairment correlates with lower expression of NR1 in PFC

  • Age increases AMPA/NMDA ratio on fast-spiking interneurons but not pyramidal neurons

  • Expression of AMPAR subunits in PFC is not associated with cognition in aging

  • Active and passive properties of PFC neurons are generally conserved with age

Abstract

Ionotropic glutamate receptors of the NMDA and AMPA subtypes transduce excitatory signaling on neurons in the prefrontal cortex (PFC) in support of cognitive flexibility. Cognitive flexibility is reliably observed to decline at advanced ages, coinciding with changes in PFC glutamate receptor expression and neuronal physiology. However, the relationship between age-related impairment of cognitive flexibility and changes to excitatory signaling on distinct classes of PFC neurons is not known. In this study, one cohort of young adult (4 months) and aged (20 months) male F344 rats were characterized for cognitive flexibility on an operant set-shifting task. Expression of the essential NMDAR subunit, NR1, was correlated with individual differences in set-shifting abilities such that lower NR1 in the aged PFC was associated with worse set-shifting. In contrast, lower expression of two AMPAR subunits, GluR1 and GluR2, was not associated with set-shift abilities in aging. As NMDARs are expressed by both pyramidal cells and fast-spiking interneurons (FSI) in PFC, whole-cell patch clamp recordings were performed in a second cohort of age-matched rats to compare age-associated changes on these neuronal subtypes. Evoked excitatory postsynaptic currents were generated using a bipolar stimulator while AMPAR vs. NMDAR-mediated components were isolated using pharmacological tools. The results revealed a clear increase in AMPA/NMDA ratio in FSIs that was not present in pyramidal neurons. Together, these data indicate that loss of NMDARs on interneurons in PFC contributes to age-related impairment of cognitive flexibility.

Introduction

Executive functions encompass higher-order cognitive processes that guide goal-directed behavior. Cognitive flexibility, the ability to modify behavioral strategies in accord with shifting contingencies, is an integral aspect of executive function that normally attains its maximal capacity in early adulthood, contemporaneous with the maturation of the prefrontal cortex (PFC; reviewed in Diamond, 2013). Advanced aging is characterized by a decline in cognitive flexibility, which can be assessed across species via “set-shifting” tasks (Barense et al., 2002; Beas et al., 2013; Boone et al., 1993; Floresco et al., 2008; Hernandez et al., 2017; Lacreuse et al., 2018; Moore et al., 2003; Nieves-Martinez et al., 2012; Rhodes, 2004; Ridderinkhof et al., 2002; Tomm et al., 2018; and reviewed in Bizon et al., 2012; McQuail et al., 2018). Attenuated N-methyl-d-aspartate receptor (NMDAR) signaling in aging may be one contributor to age-related cognitive deficits as acutely blocking NMDARs with MK-801, phencyclidine or ketamine reliably impairs set-shifting in young adult rats (Blot et al., 2015; Darrah et al., 2008; Egerton et al., 2005; Jett et al., 2017; Nikiforuk et al., 2010; Stefani et al., 2003; Stefani and Moghaddam, 2005, 2010). Consistent with this view, NMDARs are known to decline in PFC aging across humans and rats (Dickstein et al., 2013; Dyall et al., 2007; Hellström-Lindahl and Court, 2000; Magnusson, 1998; Magnusson et al., 2005, Magnusson et al., 2007; Magnusson and Cotman, 1993; McQuail et al., 2016; Migani et al., 2000; Mitchell and Anderson, 1998; Piggott et al., 1992; Wenk et al., 1991). Critically, activation of PFC NMDARs during cognitively demanding tasks depends on permissive contributions from α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs), which may also diminish with age and are implicated in cognitive flexibility (Jett et al., 2017; Magnusson and Cotman, 1993; McQuail et al., 2016; Stefani et al., 2003; Wang et al., 2013). While these data make a compelling case that age-related changes in glutamatergic NMDARs and AMPARs contribute to cognitive dysfunction, relatively little is known about whether such changes are localized to specific types of neurons.

Non-competitive NMDAR antagonists that produce set-shifting deficits in rodent models are known to suppress activity of GABAergic interneurons in medial PFC (mPFC), resulting in increased, but desynchronized, activity of pyramidal neurons (Homayoun and Moghaddam, 2007; Kargieman et al., 2007; Hervig et al., 2016). In particular, fast-spiking interneurons (FSIs) target and potently inhibit somatic compartments of pyramidal neurons, contributing to feedforward inhibition, gain control, and the generation of gamma oscillations (Bartos et al., 2007, Cardin et al., 2009, Cruikshank et al., 2012, Delevich et al., 2015, Doischer et al., 2008, Ferguson and Gao, 2018, Isaacson and Scanziani, 2011, Mann and Paulsen, 2007, Sohal et al., 2009). Consistent with a vital role for inhibitory regulation of PFC-dependent cognitive flexibility, blocking GABAARs with their selective antagonist bicuculline impairs set-shifting whereas the GABABR agonist baclofen improves set-shifting in young adult rats (Enomoto et al., 2011; Beas et al., 2016). This relationship is conserved in the aging brain as intra-mPFC infusion of baclofen effectively reverses impaired set-shifting in aged rats (Beas et al., 2017). These cellular and behavioral pharmacology data are suggestive that diminished NMDAR signaling on aging FSIs may be particularly consequential to impaired cognitive flexibility. The goals of the present study were two-fold. Our first goal was to determine the relationship between individual differences in cognitive flexibility of aging rats and PFC protein levels of ionotropic glutamate receptor (iGluR) subunits. The second goal was to determine whether the relevant age-associated changes were specific to either PFC pyramidal neurons or FSIs.

Section snippets

Subjects

Young adult (4 months, n = 23) and aged (22 months, n = 27) male Fischer 344 (F344) rats were obtained from the National Institute on Aging's Aging Rodent Colony maintained by Charles River Laboratories. All animals were housed in the Association for Assessment and Accreditation of Laboratory Animal Care International-accredited vivarium facility in the McKnight Brain Institute at the University of Florida. The facility was maintained at a consistent temperature of 25 °C with a 12-h light/dark

Age-related deficits in cognitive flexibility associated with lower level of NR1 in medial prefrontal cortex

Young adult and aged rats did not differ in number of trials to acquire the initial discrimination (see Methods) compared to young adults (t (21) = −0.392, p = 0.699; Fig. 1B). In contrast, aged rats were impaired relative to young adults in their ability to modify learned behavior as revealed by the greater number of trials needed to reliably shift their response strategy to conform to a new rule (t (21) = -3.075, p = 0.006; Fig. 1C). In this same cohort of rats, mPFC expression of the

Discussion

Cognitive flexibility, or the ability to update behavioral strategies in relation to shifting contingencies in the environment, complements other aspects of executive function, such as attentional control, behavioral inhibition and working memory, to coordinate goal-directed behaviors, which critically depend on the PFC. The present study selectively links age-related deficits in cognitive flexibility to loss of NMDARs from the PFC and, further, reveals an age-related increase in AMPA/NMDA

Conclusions

These new, descriptive data strongly suggest that diminished NMDAR signaling specific to FSIs contributes to later-life decline of PFC-dependent cognition. The apparent specificity of this deficit may explain why non-selective NMDAR potentiators produce modest-to-mixed effects on cognition in aging (Baxter et al., 1994; Billard and Rouaud, 2007; Burgdorf et al., 2011; McQuail et al., 2016; Panizzutti et al., 2014). Consequently, new studies of NMDAR signaling on FSIs may inform the development

CRediT authorship contribution statement

Joseph A. McQuail: Conceptualization, Investigation, Visualization, Formal analysis, Writing – original draft, Writing – review & editing. B. Sofia Beas: Investigation, Visualization, Writing – review & editing. Kyle B. Kelly: Investigation, Visualization, Writing – review & editing. Caesar M. Hernandez: Investigation, Writing – review & editing. Jennifer L. Bizon: Conceptualization, Funding acquisition, Supervision, Writing – review & editing. Charles J. Frazier: Conceptualization,

Declaration of competing interest

The authors declare no competing financial interests.

Acknowledgements

We thank Miranda Schwabe, Kailey Simpson, Shannon Wall, and Lauren Vetere for assistance with behavioral testing, and Brandon Hellbusch for assistance with Western blotting. This work was supported by NIH grants F32AG051371 (JAM), K01AG061263 (JAM), P20GM109091 (JAM), P20GM103641 (JAM), NSF Graduate Research Fellowship Program DGE-0802270 (BSB), NIH/NICHD 2T32HD071866-06 (CMH), a McKnight Predoctoral Fellowship and the Pat Tillman Foundation (CMH), R01AG029421 (JLB), and the McKnight Brain

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