Hippocampal electrical stimulation disrupts associative learning when targeted at dentate spikes

Abstract

Key points: Dentate spikes are fast fluctuations of hilar local-field potentials that take place during rest and are thought to reflect input arriving from the entorhinal cortex to the hippocampus. During dentate spikes, neuronal firing in hippocampal input (dentate gyrus) and output (CA1/CA3) regions is uncoupled. To date, the behavioural significance of dentate spikes is unknown. Here, we provide evidence that disrupting the dentate spike-related uncoupling of the dentate gyrus and the CA1/CA3 subregions for 1 h after training retards associative learning. We suggest dentate spikes play a significant role in memory consolidation.

Abstract: Hippocampal electrophysiological oscillations, namely theta and ripples, have been implicated in encoding and consolidation of new memories, respectively. According to existing literature, hippocampal dentate spikes are prominent, short-duration (<30 ms), large-amplitude (∼2-4 mV) fluctuations in hilar local-field potentials that take place during awake immobility and sleep. Interestingly, previous studies indicate that during dentate spikes dentate gyrus granule cells increase their firing while firing of CA1 pyramidal cells are suppressed, thus resulting in momentary uncoupling of the two hippocampal subregions. To date, the behavioural significance of dentate spikes is unknown. Here, to study the possible role of dentate spikes in learning, we trained adult male Sprague-Dawley rats in trace eyeblink classical conditioning. For 1 h immediately following each conditioning session, one group of animals received hippocampal stimulation via the ventral hippocampal commissure (vHC) contingent on dentate spikes to disrupt the uncoupling between the dentate gyrus and the CA1 subregions. A yoked control group was stimulated during immobility, irrespective of brain state, and another control group was not stimulated at all. As a result, learning was impaired only in the group where vHC stimulation was administered contingent on dentate spikes. Our results suggest dentate spikes and/or the associated uncoupling of the dentate gyrus and the CA1 play a significant role in memory consolidation. Dentate spikes could possibly reflect reactivation and refinement of a memory trace within the dentate gyrus triggered by input from the entorhinal cortex.

Keywords: dentate gyrus; hippocampus; learning.

Figure 1. Stimulation electrodes were placed in the ventral hippocampal commissure (vHC) (A and C) and recording electrodes were placed in the hilus (B and D) to study the role of dentate spikes (E and F) in learning with the help of electrical stimulation of the hippocampus via the vHC (G and H)

A, stimulation electrodes were placed in the vHC (grey shading). Black indicates ventricles. B, recording electrodes were targeted to the hilus (grey shading). Black circle indicates placement of a recording electrode in the CA1. C, example photomicrograph of stimulation electrode placement. Arrows point to electrode tips. D, example photomicrograph of recording electrode placement in the hilus. Arrows point to electrode tips. Horizontal scale bars in C and D are 500 μm in length. E and F, examples of spontaneous local‐field potentials (LFPs) illustrating a dentate spike (starting at time point 0) recorded from the CA1 (upper panel) and hilus (lower panel) in one rat belonging to the yoked control group. F is a magnification of E. Note the absence of a major deflection in CA1 during a dentate spike in hilus. G and H, LFPs recorded during vHC stimulation (at time point 0, bipolar pulse of 0.2 ms in total duration). H is a magnification of G. Note the clear response in CA1 when compared to the spontaneously generated dentate spike illustrated in E and F. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 2. Trace eyeblink conditioning in rats was used to study the effect of disrupting dentate spike‐related hippocampal neural activity on learning

The outline of the experiment is presented in A and an example of a typical conditioned eyeblink response in B. Examples of hippocampal stimulation via the vHC are shown in panels C–F. A, rats were trained in trace eyeblink conditioning using a 500 ms trace period. The conditioned stimulus (CS) was a 200 ms, 75 dB white noise auditory stimulus. The unconditioned stimulus (US) was a 100 ms electrical stimulation of the eyelid that elicited a robust blink of the eye. A total of 60 trials were presented during each daily session, with an intertrial interval (ITI) of 30–60 s. Conditioning was followed immediately by a 1 h rest period. During the rest period, animals were either left alone (normal control, NC) or the hippocampus stimulated via the ventral hippocampal commissure (vHC) contingent on dentate spikes (EXP). For animals in the yoked control group (YC) vHC stimulation was administered to a random brain state but during immobility. B, an example of electromyogram (EMG) from the stimulated eyelid during a trial in which the animal emitted a typical conditioned response. The pale grey box indicates the time period during which blinking was categorized as a conditioned response. C, an example of hilar local‐field potential during hippocampal stimulation (at 0 ms) via the vHC, immediately following a dentate spike (DS). D, same data as in C but with a higher time resolution. E, an example of hilar local‐field potential during hippocampal stimulation (at 0 ms) via the vHC, to a random brain state but during immobility. F, same data as in E but with a higher time resolution. Note the ∼10 ms delay in the hippocampal‐evoked response to vHC stimulation.

Figure 3. Hippocampal stimulation via the ventral hippocampal commissure impaired learning if it took place contingent on hippocampal dentate spikes

A, learned responses across two‐session blocks of trace eyeblink classical conditioning. Only animals in the non‐stimulated normal control (NC) and the stimulated yoked control (YC) groups learned. Animals in the experimental group (EXP) did not show an increase in learned responding. Line 1. refers to the result of a repeated measures ANOVA comparing the NC and the YC groups. Line 2. refers to the result of repeated measures ANOVA comparing the control groups (NC + YC) to the EXP group. Line 3. refers to results of separate repeated measures ANOVAs conducted on control (NC + YC) and experimental groups to study the change in learned responding across conditioning. B, highest percentage of learned responding per session (last 4 sessions). At best, animals in the control groups performed more learned responses than animals in the experimental group. The difference was significant according to a one‐way ANOVA. Asterisks refer to P values: ^*** P ≤ 0.001, ^* P < 0.05. Vertical lines indicate standard error of mean.