Decreased rhythmic GABAergic septal activity and memory-associated theta oscillations after hippocampal amyloid-beta pathology in the rat

Abstract

The memory deficits associated with Alzheimer's disease result to a great extent from hippocampal network dysfunction. The coordination of this network relies on theta (symbol) oscillations generated in the medial septum. Here, we investigated in rats the impact of hippocampal amyloid beta (Abeta) injections on the physiological and cognitive functions that depend on the septohippocampal system. Hippocampal Abeta injections progressively impaired behavioral performances, the associated hippocampal theta power, and theta frequency response in a visuospatial recognition test. These alterations were associated with a specific reduction in the firing of the identified rhythmic bursting GABAergic neurons responsible for the propagation of the theta rhythm to the hippocampus, but without loss of medial septal neurons. Such results indicate that hippocampal Abeta treatment leads to a specific functional depression of inhibitory projection neurons of the medial septum, resulting in the functional impairment of the temporal network.

Figure 1.

Parallel progressive recognition memory and θ rhythm power impairments during test sessions after hippocampal Aβ injection. A, Representative diagrams of two successive sessions in the visuospatial recognition task. Sessions were composed of 5 min training and test trials, spaced 1 h apart. Six different flat items and two empty positions were placed over a circular area as follows: long term stimuli, LT, were always in the same position for a given rat; new stimuli, NS, were changed for each trial; the empty position, EP, was changed for each trial. The two latter classes were pooled into a single group (novel) because they share systematic spatial novelty. Short term stimuli, ST, were conserved during the course of a session but changed in subsequent sessions. They were used as second-level recognition discriminators, but no differences in visits to these items were observed in this study, and they were not taken into consideration in further analyses. B, C, Exploration of familiar (LT) items (B), and novel (NS and EP) items (C) for vehicle-treated (n = 7, open circle) and Aβ-treated animals (n = 9, black square) from 1 d before to 21 d after injections. Dashed gray lines indicate the chance level to visit familiar items (25%) and novel items (50%) in B and C, respectively. D–G, Power spectrum from LFP sequences (7 s epochs) (1 s for illustration with raw and Butterworth-filtered LFP traces) recorded during visits to items on the 21st day after vehicle (D) or Aβ (E) injections. F, G, Quantification of normalized power, from 1 d before to 21 d after injection. Normalized power (percentage) of 6–9 Hz (F) and 9–50 Hz (G) bands for vehicle group (n = 8, open circle) and Aβ group (n = 9, black square). Data are represented as mean ± SEM. The dependence of the activity with respect to stimulus class was determined by linear regression analyses; *p < 0.05. Vh-LT: F_(1,89) = 9.7, p < 0.05; Vh-NS: F_(1,89) = 4.27, p < 0.05; Aβ-LT: F_(1,115) = 15.77, p < 0.05; Aβ-NS: F_(1,115) = 8.43, p < 0.05. Repeated measures ANOVA with group and time as factors (LT items: F_(12,168) = 3.22; NS items: F_(12,168) = 1,75; ^#p < 0.05). H, Quantification of normalized θ power 21 d after injection in rats that achieved stimuli recognition task (left) for vehicle (n = 8, white bar) and amyloid (n = 9, black bar) and in rats with no learning (i.e., no stimuli recognition) (right) for vehicle (n = 3, white striped bar) and amyloid (n = 4, black striped bar) (D21, unpaired t test, no learning: t_(1–5) = −8.357; p < 0.5). Hippocampal Aβ injections induced progressive recognition memory and θ rhythm power impairments independently of learning per se.

Figure 2.

Loss of task-related θ frequency modulation after hippocampal Aβ injection. A, Examples of raw LFP (3 s epochs) for vehicle-treated (top) and Aβ-treated animals (bottom) during exploration of familiar (black line) and new (gray line) items, on the 21st day after the injections. B, Averaged power spectra in the 0–10 Hz band, analyzed from LFP epochs during exploration of familiar or new items on the 21st day after the injections. C, Progressive evolution of peak power spectrum frequency during exploration of familiar (black diamond) or new (gray diamond) items for vehicle-treated (top) and Aβ-treated animals (bottom) (n = 8 to 12 sequences per class). Data are represented as mean ± SEM, *p < 0.05; ANOVA with group and day as factors. θ frequency is modulated by novelty in vehicle but not in Aβ-treated animals.

Figure 3.

Decreased rhythmic bursting activity in medial septal neurons after hippocampal Aβ injection. A, Examples of medial septal neurons electrophysiologically classified as slow, rhythmic, or tonic for vehicle- and Aβ-treated animals during hippocampal θ epochs (top traces). B, Corresponding interspike interval histograms for each class of cells in vehicle-treated (left-hand graph for each cell type) and Aβ-treated animals (right-hand graph for each cell type). C, Mean spontaneous firing rate for each class of medial septal neurons from vehicle-treated (white bars, n = 49) and Aβ-treated animals (black bars, n = 53); *p < 0.05, ANOVA with group and cell class as factors. D, Examples of rhythmic bursts in rhythmic neurons from vehicle-treated (left) and Aβ-treated animals (right). The rhythmic bursting activity in medial septal neurons is specifically altered in Aβ-treated rats.

Figure 4.

Increased occurrence of θ phase-locked slow neurons after hippocampal Aβ injection. A–C, Examples of spike distribution within the θ cycle for slow (A), rhythmic (B), and tonic (C) neurons. The phase distribution of spikes was fitted with a von Mises distribution and its concentration parameter (κ) provides a quantitative measure of the phase-locking. The goodness-of-fit was tested against the null hypothesis of uniformity by a Rayleigh test. D, Histogram of the distribution into class of phase-locked medial septal neurons (Rayleigh test, p < 0.05 and κ > 0.5) from vehicle (white bar; total phase-locked neurons, n = 19) and Aβ-treated animals (black bar; total phase-locked neurons, n = 21). *p < 0.05; χ² test for population variance followed by a post hoc binomial test for cell class. E, Firing rate of classified septal neurons as a function of their κ value from vehicle (left panel) and Aβ-treated animals (right panel). Cells with a black outline are the cells with p < 0.05 in the test for uniformity. Light gray triangles, Tonic-firing cells; dark gray circles, slow-firing cells; black squares, rhythmic firing cells. The number of medial septum slow-firing neurons that are phase-correlated to θ waves increased in Aβ-treated rats.

Figure 5.

Neurochemical identification of slow and rhythmic firing septal neurons after hippocampal injections. A, C, Examples of juxtacellular labeling in vivo under anesthesia of rhythmic (A) and slow (C) firing septal neurons from vehicle (top) and Aβ-treated (bottom) rats that were recorded during θ hippocampal activity, identified by neurobiotin staining, and counterstained for ChAT and GAD. A, two rhythmic GABAergic parvalbumin-positive neurons. C, A cholinergic neuron in a vehicle-treated rat and a GABAergic neuron in one Aβ-treated rat. Scale bars, 20 μm. B, D, Corresponding spike activity and hippocampal LFP epochs. E, Histograms of autocorrelation of B traces with the corresponding rhythmicity index (RI) (see Materials and Methods). F, ChAT immunoreactivity in the medial septum after hippocampal Aβ injection. Shown is an example of analyzed sections of the medial septum-diagonal band corresponding to the major part of the vertical limb (0.6 mm² fields, >60% of medial septum-diagonal band between level 9.7 and 9.3 anterior to the interaural level) from vehicle-treated (left) and Aβ-treated rats (right). Scale bar, 200 μm. G, Parvalbumin immunoreactivity in the medial septum from vehicle-treated (left) and Aβ-treated rats (right). Scale bar, 200 μm. Hippocampal Aβ injections induced a reduction in GABAergic cell rhythmic firing rate and an increase in the probability for slow-firing cells to display GAD immunoreactivity in the absence of cell loss in the medial septum (see also supplemental Table 1, available at www.jneurosci.org as supplemental material).