All rats were treated in accordance with federal (National Institutes of Health Publications No. 8023, revised 1978) and University of Florida Institutional Animal Care and Use Committee (IACUC) policies regarding the ethical use of animals for research. The University of Florida IACUC committee approved the animal protocols that we employed. Young (4 mo; n = 32), middle-aged (12 mo; n = 32) and aged (18 mo; n = 34) male Fischer 344 rats were obtained in 6 batches of 4–7 rats per age group (due to monthly order limits) from the National Institute of Aging (NIA) colony at Harlan Laboratories. Upon arrival, rats were pair-housed in corn cob bedding-lined shoebox cages in a colony room maintained at 24 ± 1°C on a 12:12 h light:dark cycle and given Harlan Teklad Rodent Diet #8604 and water ad libitum for the duration of the experiment. Body masses were recorded every other day to monitor potential NSAID-induced gastrointestinal side effects and general health was assessed daily. We humanely euthanized 1 middle-aged rat and 3 aged rats that exhibited age-related health problems (i.e., excessive weight loss and/or tumor growth) during the study.

Figure 1 shows the experiment timeline and the number of rats obtained and included in each analysis. A week after arrival, the rats were trained to locate a visible platform and then a hidden platform 3 d later followed by immediate and 24 h delayed probe trials in the first water maze session. Two young rats were removed from the study after the first water maze session because they exhibited thigmotaxic behavior and immediate probe discrimination index (DI) scores < 0 that reflect failure to learn the spatial maze strategy. The next day, the rats were assigned uniformly by their combined probe trial DI scores (ranked 1–6 by batch) and age to treatment groups and began twice daily feedings (12 h apart) of frozen strawberry milk vehicle (500 μl), indomethacin (2.0 mg/ml) or rosiglitazone (8.0 mg/ml) treats that continued through the experiment endpoint. A week later, the rats were injected intraperitoneally (i.p.) once daily over 3 d with BrdU (50 mg/kg, i.p.) to label dividing cells. A week after the final BrdU injection, rats were trained to locate a new hidden platform position, tested on an immediate and a 24 h delayed probe trial and then killed 24 h after the final probe trial.

Water maze training and testing was conducted as described previously (Foster et al., 2003; Foster and Kumar, 2007; Carter et al., 2009; Speisman et al., 2013a,b; Scheinert et al., 2015) in a well-lit room containing a black water maze (1.7 m d) filled with water (27 ± 2°C) such that the water surface was ~8–10 cm below the tank rim. A platform (13 cm d) protruded 1.5 cm above the water surface on visible platform trials or was submerged 1.5 cm below the water surface on hidden platform trials. Swim times, distances, and speeds were recorded with a Columbus Instruments tracking system. Rats were towel dried and warm air was blown over their holding cages between trial blocks.

All rats voluntarily consumed frozen Nestle Nesquick Strawberry milk™ vehicle treats (500 μl) 2x per day in their home cages before and during the first water maze session (~7 days). Beginning 24 h after the first water maze session, rats voluntarily consumed frozen vehicle (500 μl), indomethacin (2.0 mg/ml; Sigma Aldrich, St. Louis, MO, USA) or rosiglitazone (8.0 mg/ml; Cayman Chemical Company, Ann Arbor, MI, USA) treats 2x per day (12 h apart) in individual feeding cages according to their drug treatment group assignment (Figure 1): vehicle (n = 10 young, n = 11 middle-aged, and n = 10 aged), indomethacin (n = 9 young, n = 10 middle-aged, and n = 11 aged) and rosiglitazone (n = 11 young, n = 10 middle-aged, and n = 10 aged). Each rat was returned to its home cage immediately after the entire frozen vehicle, indomethacin or rosiglitazone treat was consumed. Drug treatment continued throughout the duration of the experiment (Days 5–24; 20 days).

Strawberry milk treats were prepared as described previously (Ormerod et al., 2013). Briefly, ethanol was added to Nestle Nesquick Strawberry milk™ at a concentration of 50 μl/ml to produce vehicle. Powdered indomethacin (2.0 mg) and crushed rosiglitazone (8.0 mg) was dissolved in 50 μl of ethanol and the drug ethanol solution was added to strawberry milk at a concentration of 50 μl/ml. Each milk solution was prepared at the beginning of the experiment and frozen in 500 μl aliquots as treats in small plastic petri dishes stored at −20°C. Given our assumption that the ~400 g body mass recorded in the first batch of rats received would remain consistent across subsequent batches, these concentrations approximated clinical indomethacin (2.5 mg/kg/day) and rosiglitazone (10 mg/kg/day) dosages (Monje et al., 2003; Norris et al., 2007) and produced a 0.005 g/kg ethanol dosage that neither impacts neurogenesis nor spatial behavior (Nixon and Crews, 2002; Crews et al., 2006; Ormerod et al., 2013). Final mean (±S.E.M.) body masses were 354.5 ± 33.0 g in young, 463.6 ± 24.0 g in middle-aged, and 454.1 ± 34.1 g in aged rats making final indomethacin dosages range from 2.6 to 3.1 mg/kg in young, 2.1 to 2.3 mg/kg in middle-aged, and 2.1 to 2.4 mg/kg in aged rats and rosiglitazone dosages range from 10.4 to 15.2 mg/kg in young, 8.1 to 9.2 mg/kg in middle-aged and 8.1 to 9.6 mg/kg in aged rats, which were all within clinical ranges.

Rats were injected i.p. with BrdU (Sigma Aldrich) daily over 3 d beginning a week after the onset of drug treatment, which was a week after the 1st water maze session and a week before the 2nd water maze session to minimize the effects of spatial behavior on neural progenitor cell (NPC) proliferation and new neuron survival (Gould et al., 1999a; Epp et al., 2010). BrdU was dissolved in freshly prepared 0.9% sterile saline at a concentration of 20 mg/mL and injected at a volume of 2.5 ml/kg (50 mg/kg).

The day after the final probe trial in the second water maze session, rats were anesthetized with isoflurane (Halocarbon Laboratories, River Edge, NJ) and decapitated. Serum supernatant harvested from trunk blood was refrigerated for 2 h at 4°C, centrifuged at 1,000 × g for 10 min and then stored at −86°C for future analysis. Brains were extracted and dissected laterally. Hippocampi dissected from left side of the brain were flash frozen and stored at −86°C for future work. The right side of the brain was post-fixed overnight in fresh 4% paraformaldehyde (Electron Microscopy Sciences; Hatfield, PA) and then equilibrated in 30% sucrose (~4 days) at 4°C. Postfixed ½ brains were sectioned coronally at 40 μm intervals on a freezing stage sledge microtome (Model 860; American Optical Corporation; IMEB Inc., San Marcos, CA). Sets of every six sections through the subependymal zone including the rostral migratory stream (RMS) and olfactory bulb (OB) and through the hippocampal dentate gyrus were stored at −20°C in cryoprotectant solution (30% ethylene glycol, 25% glycerin, and 45% 0.1 M PBS).

Immunohistochemistry was conducted as described previously (Bruijnzeel et al., 2011; Chen et al., 2011; Lee et al., 2013; Ormerod et al., 2013; Asokan et al., 2014) on free floating sections washed repeatedly between steps with tris-buffered saline (TBS; pH 7.4). Blocking and antibody solution was 3% normal donkey serum and 0.1% Triton-X in TBS. BrdU⁺ cells and ionized calcium binding adaptor molecule 1 (Iba1)⁺ microglia were revealed on separate sets of dentate gyrus sections and BrdU⁺ cells were revealed on RMS and OB sections using 3,3′-diaminobenzidine tetrahydrochloride (DAB) to calculate total cell numbers and densities under light microscopy. BrdU⁺ cell and Iba1⁺ microglia phenotypes were confirmed on separate sets of sections co-labeled with phenotypic makers revealed by fluorophore-conjugated secondary antibodies under confocal microscopy.

All statistical analyses were conducted with Version 13 STATISTICA software (http://onthehub.com/). Analyses of variance (ANOVA) explored the effects of the independent variables (age and drug treatment) on water maze performances (latencies, pathlengths and swim speeds across trial blocks, % time spent in goal and opposite quadrants), BrdU⁺ cells (total numbers, percentages of new neurons and glia and total numbers of new neurons and glia) and Iba1⁺ microglia (total numbers, percentages of resting, activated, and phagocytic microglia and total numbers of resting, activated, and phagocytic microglia) and group differences were revealed by Newman Keuls post-hoc tests. Kruskal–Wallis ANOVAs tested the effects of the independent variables (age and drug treatment) on non-continuous behavioral variables (DI scores and difference scores) and group differences were revealed by Mann–Whitney U-tests. Dependent t-tests confirmed differences in performance across binned hidden platform training blocks (1st and 2nd combined vs. 3rd and 4th combined) and Pearson Product Moment correlation coefficient analyses tested the strength of relationships between variables. All data are shown as group means (± S.E.M.) or as individual probe trial DI scores. The α-level was set at p < 0.05.

Figure 5A shows representative examples of 10–12 d old BrdU⁺ cells revealed by DAB (in brown) in the dentate gyrus of a young vehicle-treated rat. Table 1 shows the total number of BrdU⁺ cells in the dentate gyri of young, middle-aged and aged rats in each drug treatment group. Total new cell numbers varied by age [F_{(2, 38)} = 28.71; p < 0.001] and treatment [F_{(2, 38)} = 9.39; p < 0.001] but not by the overall interaction [F_{(4, 38)} = 0.053; p = 0.99]. Young rats had more new cells than middle-aged (p < 0.001) and aged (p < 0.001) rats and indomethacin-treated rats had more new cells than vehicle- (p < 0.001) and rosiglitazone- (p < 0.05) treated rats.

Figure 5 also shows representative examples of BrdU⁺ cells (in red) co-labeled with the (Figure 5B) the immature neuronal marker DCX (in blue) and/or the mature neuronal marker NeuN (in green) or (Figure 5C) the astrocyte marker GFAP (in blue) or the oligodendrocyte precursor marker NG2 (in green) in the dentate gyrus of a young vehicle-treated rat. Table 1 shows the mean (±S.E.M) % of BrdU⁺ cells expressing neuronal or glial phenotypes across age and treatment groups. These percentages varied by phenotype [F_{(4, 152)} = 204.31; p < 0.0001] and the interactions between age and treatment [F_{(4, 38)} = 2.94; p < 0.05], age and phenotype [F_{(8, 152)} = 3.55; p < 0.001] and phenotype and treatment [F_{(8, 152)} = 3.02; p < 0.01] but not by age, treatment, or other interactions between the independent variables (all p ≥ 0.14). Overall, the greatest % of BrdU⁺ cells expressed a transitioning (p < 0.0001 vs. all other phenotypes) followed by an immature (p < 0.0001 vs. mature neurons and glia) neuronal phenotype. However, the % of new cells expressing an immature neuronal phenotype was lower in middle-aged and aged vs. young rats (p < 0.01^a and p < 0.05^a, respectively) and the % expressing a transitioning neuronal phenotype was lower in aged vs. young rats (p < 0.01^a). In young and aged rats, the greatest % of new cells expressed an immature or transitioning neuronal phenotype (both p < 0.0001 vs. mature neurons and glia^c) while in middle-aged rats the greatest % expressed a transitioning (p < 0.01 vs. all other phenotypes^d) followed by an immature (p < 0.0001 vs. mature neurons and glia^c) neuronal phenotype. Relative to vehicle, rosiglitazone and indomethacin increased the % of BrdU⁺ cells expressing a transitioning neuronal phenotype while decreasing the % expressing an immature neuronal phenotype (all p ≤ 0.05^b). In vehicle-treated rats, the greatest percentages of BrdU⁺ cells expressed immature or transitioning neuronal phenotypes (both p < 0.001^c) and in rosiglitazone and indomethacin-treated rats, greater percentages of cells expressed transitioning (both p < 0.001^d vs. all other phenotypes) followed by immature (both p < 0.001^c vs. mature neurons and glia) neuronal phenotypes.

Table 1 shows net neurogenesis or total mean (±S.E.M.) numbers of new neurons and glia and Figures 5D,E show the neuronal data separately. The number of new differentiated cells varied by age [F_{(2, 38)} = 35.02; p < 0.0001], treatment [F_{(2, 38)} = 10.57; p < 0.001] and phenotype [F_{(4, 152)} = 179.80; p < 0.0001] and numbers of neurons and glia varied by age [F_{(8, 152)} = 10.26; p < 0.0001] and by treatment [F_{(8, 152)} = 4.75; p < 0.0001] but not by other interactions between the independent variables (all p ≥ 0.23). Generally, more new differentiated cells were found in young vs. middle-aged (p < 0.0001) and aged (p < 0.0001) rats and in indomethacin-treated vs. vehicle- (p < 0.001) and rosiglitazone- (p < 0.01) treated rats. Most new cells were transitioning (p < 0.0001 vs. other phenotypes) followed by immature (p < 0.0001 vs. mature neurons and glia) neurons. Young rats had more new immature and transitioning neurons than middle-aged and aged rats (all p < 0.0001). In young and middle-aged rats, most new cells were transitioning neurons (p < 0.01 vs. all other cell types) followed by immature neurons (p < 0.001 vs. mature neurons and glia) and in aged rats, most new cells were immature or transitioning neurons (p < 0.0001 vs. mature neurons and glia). Table 1 and Figure 5E show that total new neuron numbers varied by age [F_{(2, 38)} = 42.52; p < 0.0001] and treatment [F_{(2, 38)} = 9.81; p < 0.001] but not by the overall interaction [F_{(4, 38)} = 0.46; p = 0.77]. Young rats had more new neurons than middle-aged (p < 0.001) and aged (p < 0.001) rats and indomethacin-treated rats had more new neurons than vehicle- (p < 0.001) and rosiglitazone- (p < 0.01) treated rats.

Dentate gyrus volumes (±S.E.M.) were calculated for young vehicle- (1.83 ± 0.03 mm³), indomethacin- (1.76 ± 0.02 mm³), and rosiglitazone- (1.73 ± 0.02 mm³) treated rats, middle-aged vehicle- (1.89 ± 0.03 mm³), indomethacin- (1.88 ± 0.07 mm³) and rosiglitazone- (1.89 ± 0.03 mm³) treated rats and aged vehicle- (1.86 ± 0.04 mm³), indomethacin- (1.88 ± 0.05 mm³) and rosiglitazone- (1.86 ± 0.03 mm³) treated rats. Age [F_{(2, 38)} = 6.60; p < 0.01] but not treatment [F_{(2, 38)} = 0.62; p = 0.54] or the overall interaction [F_{(4, 38)} = 0.07; p = 0.60] affected volumes. Young rats had smaller dentate gyri than middle-aged and aged rats (p < 0.01).

New RMS neuroblasts and OB neuron densities were quantified to test whether age-related declines occurred concomitantly with hippocampal neurogenesis and whether the declines could be reversed by drug treatment. The schematic in Figure 6A shows that neuroblasts chain migrate through the RMS_VL, RMS_HL and then RMS_OB before migrating radially into OB_GCL. Figure 6B shows representative examples of DAB-stained BrdU⁺ cells (in brown) in the RMS_HL and Figure 6C shows representative examples of BrdU⁺ cells (in red) expressing the immature neuronal protein DCX (in blue) and/or the mature neuronal protein NeuN (in green) in the RMS_OB and OB_GCL. Note that BrdU⁺ cells expressing a mature DCX⁻/NeuN⁺ neuronal phenotype were not detected in the RMS or OB_GCL at the time point examined. The % of BrdU⁺/DCX⁺ was consistent across age [F_{(2, 28)} = 2.4; p = 0.11] and drug treatment [F_{(2, 28)} = 0.5; p = 0.63] and although a significant interaction was detected [F_{(4, 28)} = 4.1; p = 0.01], a Newman-Keuls post hoc test revealed that the % only tended to be higher in indomethacin- vs. vehicle-treated middle-aged rats (p = 0.10). Therefore, we simply report analyses of BrdU⁺/DCX⁺ densities in subependymal regions to avoid redundancy because > 95% of BrdU⁺ cells expressed DCX⁺ neuroblast or neuronal phenotypes and because BrdU⁺ cell densities correlated with BrdU⁺/DCX⁺ neuronal densities in the RMS_vl[r₍₃₇₎ = 0.998; p < 0.0001], RMS_hl[r₍₃₇₎ = 0.998; p < 0.0001], RMS_ob [r₍₃₇₎ = 0.998; p < 0.0001], GCL_ob [r₍₃₇₎ = 0.998; p < 0.0001] and hippocampal dentate gyrus [r₍₃₇₎ = 0.920; p < 0.0001].

Table 2 and Figure 6D shows that migrating neuroblast densities in the RMS_VL varied by age [F_{(2, 28)} = 3.51; p < 0.04] and drug treatment [F_{(2, 28)} = 9.24; p < 0.001] but not by the overall interaction [F_{(4, 28)} = 1.02; p = 0.41]. Densities were higher in the RMS_VL of young (p < 0.05) and tended to higher in the RMS_VL of middle-aged (p = 0.07) vs. aged rats and were higher in the RMS_VL of indomethacin- (p < 0.05) and rosiglitazone- (p < 0.01) treated vs. vehicle-treated rats.

Table 2 and Figure 6E shows that migrating neuroblasts densities in the RMS_HL varied by treatment [F_{(2, 28)} = 12.51; p < 0.001] but not by age [F_{(2, 28)} = 1.02; p = 0.38] or the overall interaction [F_{(4, 28)} = 2.25; p = 0.09]. Indomethacin increased densities relative to rosiglitazone (p < 0.05) and vehicle (p < 0.001) and rosiglitazone increased densities relative to vehicle (p < 0.05).

Table 2 and Figure 6F shows that migrating neuroblast/new neuron densities in the RMS_OB varied by treatment [F_{(2, 28)} = 19.25; p < 0.001] and the overall interaction [F_{(4, 28)} = 3.02; p < 0.05] but not by age [F_{(2, 28)} = 0.22; p = 0.80]. Specifically, indomethacin (p < 0.001) and rosiglitazone (p < 0.001) increased densities relative to vehicle primarily because indomethacin increased densities in middle-aged rats (p < 0.05 vs. vehicle) and indomethacin and rosiglitazone increased densities in aged rats (p < 0.05 and p < 0.001 vs. vehicle, respectively).

Table 2 and Figure 6G shows that new neuron densities in the OB_GCL varied by age [F_{(2, 28)} = 16.61; p < 0.001], treatment [F_{(2, 28)} = 5.74; p < 0.01] and the overall interaction [F_{(4, 28)} = 2.67; p ≤ 0.05]. Generally, young rats had higher densities than middle-aged (p < 0.001) and aged (p < 0.001) rats and middle-aged rats tended to have higher densities than aged rats (p = 0.07). Indomethacin and rosiglitazone reduced densities relative to vehicle (p < 0.01), primarily because lower densities were observed in indomethacin- (p < 0.001) and rosiglitazone- (p < 0.05) vs. vehicle-treated young rats.

Finally, new dentate gyrus neuron densities were affected by age [F_{(2, 28)} = 56.44; p < 0.0001] and drug treatment [F_{(2, 28)} = 10.44; p < 0.001] but not the overall interaction [F_{(4, 28)} = 0.91; p = 0.47]. Higher new neuron densities were found in the dentate gyri of young vs. middle-aged (p < 0.001) and aged (p < 0.001) rats and following treatment with indomethacin (p < 0.05 vs. rosiglitazone and p < 0.001 vs. vehicle) or rosiglitazone (p < 0.05 vs. vehicle; Table 2).

While new dentate gyrus neuron densities neither correlated with RMS_VL, RMS_HL, or RMS_OB neuroblast densities nor new OB_GCL neuron densities in young and middle-aged rats, new DG neuron densities tended to correlate positively with RMS_OB neuroblast densities [r₍₁₁₎ = 0.58; p = 0.06] in aged rats. Interestingly, RMS_OB densities correlated negatively with immature OB_GCL neuron densities [r₍₁₄₎ = −0.73; p < 0.01] in young rats, RMS_HL densities correlated positively with RMS_OB neuroblast densities [r₍₁₂₎ = 0.88; p < 0.001] in middle-aged rats and RMS_OB neuroblast densities correlated positively with both RMS_VL [r₍₁₁₎ = 0.71; p < 0.05] and RMS_HL [r₍₁₁₎ = 0.70; p < 0.05] neuroblast densities in aged rats, suggesting that age impacts neuroblast migration.

Figure 7A shows representative examples of DAB-stained Iba1⁺ microglia and Figure 7B shows representative examples of Iba1⁺ microglia (in blue) expressing the activation marker CD11b (in red) or the phagocytosis marker CD68 (in green) in the dentate gyrus of an aged vehicle-treated rat. Table 3 shows total Iba1⁺ microglia numbers, the percentages of Iba1⁺ microglia expressing CD11b⁻/CD68⁻ (resting), CD11b⁺/CD68⁻ (activated) and CD11b⁺/CD68⁺ (phagocytic) phenotypes and along with Figure 7C the total number of resting, activated and phagocytic microglia in the dentate gyri of young, middle-aged, and aged rats in each treatment group. The total number of Iba-1⁺ microglia varied by age [F_{(2, 38)} = 6.35; p < 0.01] but not by treatment [F_{(2, 38)} = 0.32; p = 0.73] or the overall interaction [F_{(4, 38)} = 1.16; p = 0.34]. More Iba1⁺ microglia were found in the dentate gyri of middle-aged (p < 0.01) and aged (p < 0.01) vs. young rats.

The % of Iba1⁺ cells expressing activation markers varied by phenotype [F_{(2, 76)} = 3026.47; p < 0.0001] and the interactions between phenotype and age [F_{(4, 76)} = 151.61; p < 0.0001] and phenotype, age and drug treatment [overall interaction: F_{(8, 76)} = 3.19; p < 0.01] but was not affected by age, drug treatment or other interactions between the independent variables (all p ≥ 0.09). The greatest % of Iba1⁺ microglia exhibited a resting (p < 0.001 vs. phagocytic and activated) followed by a phagocytic (p < 0.001 vs. activated) phenotype. The % of resting microglia was higher while the percentages of activated and phagocytic microglia were lower in young vs. middle-aged rats and aged rats and in middle-aged vs. aged rats (all p < 0.001). These percentages were unaffected by drug treatment in young rats but rosiglitazone increased the % of resting microglia in middle-aged rats relative to vehicle (p < 0.01) and in aged rats relative to vehicle (p < 0.05) and indomethacin (p < 0.05).

These effects translated into group differences in total resting, activated and phagocytic microglia numbers. Microglial numbers varied by age [F_{(2, 38)} = 6.25; p < 0.01], phenotype [F_{(2, 76)} = 1148.66; p < 0.0001] and the interaction between phenotype and age [F_{(4, 76)} = 41.41; p < 0.0001] but not by treatment or other interactions between the dependent variables (all p ≥ 0.12). Overall, more resting microglia were detected than activated (p < 0.001) or phagocytic (p < 0.001) microglia and more phagocytic microglia were detected than activated microglia (p < 0.05). The latter effect was largely due to the increase phagocytic microglia numbers detected in aged rats (young rats: resting microglia > activated and phagocytic microglia; p < 0.001; middle-aged rats: resting microglia > activated and phagocytic microglia; p < 0.001; aged rats: resting microglia > phagocytic microglia > activated microglia; all p < 0.001). In fact, fewer resting microglia were detected in aged vs. young (p < 0.001) and middle-aged rats (p < 0.001), more activated microglia were detected in aged vs. young rats (p < 0.001), and the number of phagocytic microglia was highest in aged rats (vs. middle-aged and young rats; p < 0.001) followed by middle-aged rats (vs. young rats; p < 0.02).

Table 4 shows correlations between new neurons, microglia, and behavioral measures obtained from the 2nd water maze session within each age group. In young rats, new neuron numbers correlated positively with immediate probe DI scores [r₍₁₆₎ = 0.56; p < 0.05] and the % time spent in the goal quadrant [r₍₁₆₎ = 0.50; p ≤ 0.05]. Total activated microglia numbers correlated positively with DI scores [r₍₁₆₎ = 0.62; p < 0.01] and the % time spent in the target quadrant [r₍₁₆₎ = 0.54; p < 0.05] on the delayed probe trial and with the difference in DI scores obtained from the delayed probe trial in the 2nd vs. 1st water maze session [r₍₁₆₎ = 0.51; p < 0.05]. Phagocytic microglia numbers correlated negatively with the % time spent in the target quadrant on the immediate probe trial [r₍₁₆₎ = −0.65; p < 0.01] and delayed probe trial [r₍₁₆₎ = −0.51; p < 0.01]. In middle-aged rats, new neuron number correlated negatively with average pathlength across hidden trial blocks [r₍₁₅₎ = −0.78; p < 0.001] and phagocytic microglia number [r₍₁₅₎ = −0.52; p < 0.05]. In aged rats, resting microglia numbers correlated negatively with DI scores [r₍₁₅₎ = −0.64; p < 0.01] and % time spent in the target quadrant [r₍₁₅₎ = −0.77; p < 0.001] on the immediate probe trial and resting microglia numbers correlated negatively with the difference in % time spent in the target quadrant on the delayed probe trial in the 2nd vs. 1st session [r₍₁₅₎ = −0.56; p < 0.05].

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.