Cognitive performance in aged rats is associated with differences in distinctive neuronal populations in the ventral tegmental area and altered synaptic plasticity in the hippocampus

Abstract

Introduction: Deterioration of cognitive functions is commonly associated with aging, although there is wide variation in the onset and manifestation. Albeit heterogeneity in age-related cognitive decline has been studied at the cellular and molecular level, there is poor evidence for electrophysiological correlates. The aim of the current study was to address the electrophysiological basis of heterogeneity of cognitive functions in cognitively Inferior and Superior old (19-20 months) rats in the ventral tegmental area (VTA) and the hippocampus, having Young (12 weeks) rats as a control. The midbrain VTA operates as a hub amidst affective and cognitive facets, processing sensory inputs related to motivated behaviours and hippocampal memory. Increasing evidence shows direct dopaminergic and non-dopaminergic input from the VTA to the hippocampus.

Methods: Aged Superior and Inferior male rats were selected from a cohort of 88 animals based on their performance in a spatial learning and memory task. Using in vivo single-cell recording in the VTA, we examined the electrical activity of different neuronal populations (putative dopaminergic, glutamatergic and GABAergic neurons). In the same animals, basal synaptic transmission and synaptic plasticity were examined in hippocampal slices.

Results: Electrophysiological recordings from the VTA and hippocampus showed alterations associated with aging per se, together with differences specifically linked to the cognitive status of aged animals. In particular, the bursting activity of dopamine neurons was lower, while the firing frequency of glutamatergic neurons was higher in VTA of Inferior old rats. The response to high-frequency stimulation in hippocampal slices also discriminated between Superior and Inferior aged animals.

Discussion: This study provides new insight into electrophysiological information underlying compromised cerebral ageing. Further understanding of brain senescence, possibly related to neurocognitive decline, will help develop new strategies towards the preservation of a high quality of life.

Keywords: LTP; VTA GABAergic and VTA glutamatergic neurons; dementia; dopamine; in vivo electrophysiology; learning and memory.

Figure 1

(A) Schematic demonstration of hole-board maze. Animals have to learn and remember the position of four baited holes during four test days and 13 trials. (B) From a cohort of 88 male Sprague–Dawley rats (19–20 months old) “Superior” (green area) and “Inferior” (red area) performers were selected based on their mean reference memory index (RMI) derived from trial 6, 10, and 13 ± 1 standard deviation. Ten aged rats were excluded from the analysis because they did not fulfill the criteria. Individual points represent specific animals and the line indicates the mean of mean RMIs. Performance of Young, Old (Mean of All), Superior old and Inferior old rats in the hole-board test as expressed by RMIs (C) and latency (D) to find all pellets. Two-way RM-ANOVA followed by Tukey’s post-hoc tests, # Superior old vs. Young, * Superior old vs. Inferior old. Total numbers of hole dips in the first trial (E) and over four days of training (F). (G) Food-deprived animals were habituated to hole-board in two sessions (H1 and H2; one session per day) and distance traveled was recorded. Two-way ANOVA followed by Sidak’s multiple comparison test. Values are expressed as mean ± SEM; n = 9 per group; ^#/*p < 0.05, ^##p < 0.01, ^###/***p < 0.001, ****p < 0.0001.

Figure 2

Spontaneous locomotor activity and sucrose-preference test in young and aged male Sprague–Dawley rats. (A) Distance travelled, (B) number of entries to the inner zone, (C) time immobile, and (D1-2) representative traces of locomotor activity recorded in novel open field (NOF) session and familiar open field (FOF) session. Two-way RM-ANOVA followed by Sidak’s post-hoc test for multiple comparisons. (E) Water intake (averaged values over three days when only water was provided), (F) water and 2% sucrose intake when two bottles presented, (G) preference for sucrose. One-way ANOVA with Dunnett’ post-hoc test. Data are expressed as a mean with individual data points, n = 9 per group, *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001.

Figure 3

Dopamine neurons in the VTA of young and aged rats. (A) Example of typical action potential (AP) waveform (top left) and extracellular recordings (bottom) from different experimental groups (Young n = 9; Superior old n = 8; Inferior old n = 9 rats). The graph (top right) shows the average action potential (AP) duration as measured from the start to the negative trough. (B) Percentage of responding/not responding dopamine neurons to the contralateral paw pinch. The χ² test on the number of cells (Young n = 35 inhibited, n = 3 stimulated, n = 12 no effect; Superior old n = 18 inhibited, n = 1 stimulated, n = 4 no effect; Inferior old n = 30 inhibited, n = 0 stimulated, n = 4 no effect) showed no difference among groups [χ²(4) = 4.49; p = 0.34]. (C) Bar histograms showing the average number of cells per electrode track [F(2, 153) = 0.75, p = 0.47]. (D) Individual data points and average firing frequency of dopamine neurons [Young n = 58; Superior old n = 35; Inferior old n = 44; F(2, 132) = 4.78, p = 0.009]. Individual data points and the average (E) percentage of spikes in burst [F(2, 132) = 5.119, p = 0.007], (F) burst rate [F(2, 117) = 8.003, p = 0.0006], and (G) coefficient of variation in percentage [F(2, 132) = 1.27, p = 0.28]. Unless differently stated, data are expressed as mean ± s.e.m. and one-way ANOVA was used to assess differences. Tukey’s multiple comparisons test **p < 0.01, and ***p < 0.001 Inferior old vs. Young.

Figure 4

Non-dopaminergic neurons in the VTA of young and aged rats. (A) Example of action potential (AP) waveform (top left) and recordings (bottom) from a putative glutamatergic neuron. The graph (top right) shows the average action potential duration as measured from the start to the negative trough. (B) Bar histograms showing the average number of glutamatergic neurons per electrode track [F(2,151) = 0.49, p = 0.61]. (C) Individual data points and average firing frequency of glutamatergic neurons [Young n = 34; Superior old n = 31; Inferior old n = 34; F(2,96) = 6.09, p = 0.003]. (D) Individual data points and the average of coefficient of variation in percentage [F(2,96) = 1.58, p = 0.21]. (E) Example of action potential waveform (top left) and recordings (bottom) from putative GABAergic neuron. The graph (top right) shows the average action potential duration as measured from the start to the positive trough. (F) Bar histograms showing the average number of GABAergic neurons per electrode track [F(2,148) = 0.58, p = 0.55]. (G) Individual data points and average firing frequency of GABAergic neurons [Young n = 35; Superior old n = 19; Inferior old n = 29; F(2,76) = 5.19, p = 0.007]. (H) Individual data points and the average of coefficient of variation in percentage [F(2,76) = 0.53, p = 0.58]. Data are expressed as mean ± s.e.m. and one-way ANOVA was used to assess differences; Tukey’s multiple comparisons test *p < 0.05 and **p < 0.01 vs. Young; #p < 0.05 vs. Superior old.

Figure 5

(A) For field potential recordings from hippocampal slices, the stimulating electrode was placed in the Schaffer collateral axons of CA3 and a recording electrode in the Stratum radiatum of CA1 (B) Input/output (I/O) responses were significantly shifted downward in hippocampal slices from Superior and Inferior performing aged rats. Two-way RM-ANOVAwith Tukey’s post-hoc test. (C) Enhanced paired-pulse facilitation in aged animals. Data represent the ratio of the second fEPSP slope to the first fEPSP slope. Two-way ANOVA with Tukey’s post-hoc test. (D) Potentiation induced after high-frequency stimulation (HFS) was significantly enhanced immediately after HFS (first 2 min) and over 40 min of recording in Inferior old rats compared to Young. Two timepoints (2 min and 40 min) are shown as a dot plot. Two-way RM-ANOVA. * Superior old vs. Young, # Inferior old vs. Young. Data are expressed as a mean ± s.e.m., n = 9 per group, ^#/*p < 0.05, ^##p < 0.01, ^####/****p < 0.0001.