Spike-wave discharges in adult Sprague-Dawley rats and their implications for animal models of temporal lobe epilepsy

Abstract

Spike-wave discharges (SWDs) are thalamocortical oscillations that are often considered to be the EEG correlate of absence seizures. Genetic absence epilepsy rats of Strasbourg (GAERS) and Wistar Albino Glaxo rats from Rijswijk (WAG/Rij) exhibit SWDs and are considered to be genetic animal models of absence epilepsy. However, it has been reported that other rat strains have SWDs, suggesting that SWDs may vary in their prevalence, but all rats have a predisposition for them. This is important because many of these rat strains are used to study temporal lobe epilepsy (TLE), where it is assumed that there is no seizure-like activity in controls. In the course of other studies using the Sprague-Dawley rat, a common rat strain for animal models of TLE, we found that approximately 19% of 2- to 3-month-old naive female Sprague-Dawley rats exhibited SWDs spontaneously during periods of behavioral arrest, which continued for months. Males exhibited SWDs only after 3 months of age, consistent with previous reports (Buzsáki et al., 1990). Housing in atypical lighting during early life appeared to facilitate the incidence of SWDs. Spike-wave discharges were often accompanied by behaviors similar to stage 1-2 limbic seizures. Therefore, additional analyses were made to address the similarity. We observed that the frequency of SWDs was similar to that of hippocampal theta rhythm during exploration for a given animal, typically 7-8 Hz. Therefore, activity in the frequency of theta rhythm that occurs during frozen behavior may not reflect seizures necessarily. Hippocampal recordings exhibited high frequency oscillations (>250 Hz) during SWDs, suggesting that neuronal activity in the hippocampus occurs during SWDs, i.e., it is not a passive structure. The data also suggest that high frequency oscillations, if rhythmic, may reflect SWDs. We also confirmed that SWDs were present in a common animal model of TLE, the pilocarpine model, using female Sprague-Dawley rats. Therefore, damage and associated changes to thalamic, hippocampal, and cortical neurons do not prevent SWDs, at least in this animal model. The results suggest that it is possible that SWDs occur in rodent models of TLE and that investigators mistakenly assume that they are stage 1-2 limbic seizures. We discuss the implications of the results and ways to avoid the potential problems associated with SWDs in animal models of TLE.

Keywords: Absence seizures; Female; Limbic seizures; Pilocarpine; Thalamocortical oscillations; Video-EEG.

Figure 1. SWDs in adult Sprague-Dawley rats

A. A schematic illustrates the location of implanted electrodes. There were four epidural (screw) electrodes, two over frontal cortex (one/hemisphere) and two over occipital cortex (one/hemisphere). Two twisted bipolar electrodes were placed in dorsal hippocampus (one in each hemisphere). Reference (Ref) and ground (Gnd) electrodes were epidural screws. B. The percentage of naïve rats with SWDs is shown. Differences were not significant (p=0.090). Samples sizes are above each bar. C. SWDs are interrupted by noise. An example of a SWD during behavioral arrest from a naïve rat. A noise occurred at the arrow and interrupted the SWD episode, which resumed spontaneously. When the SWD was disrupted, the animal moved, turning towards the noise. When the SWD resumed, the rat froze, exhibiting the behavior typical of behavioral arrest. In this figure and other figures, OC= occipital cortex; FC= frontal cortex; HC= hippocampus. D. 1–3. A representative example of SWDs before (Pre), after ethosuximide administration (approximately 30 min after; Post), and 24 hrs later (Recovery). In 2, the transition between exploration and behavioral arrest is marked by the arrowhead. The bar marks the period of hippocampal theta oscillations occurring before behavioral arrest. 4. SWDs are quantified as the duration of SWDs in 2 min of spontaneous behavioral arrest, expressed as a percent. The differences in SWDs were significant (n=3 rats/group; p<0.05).

Figure 2. SWD patterns vary across animals but are consistent within each animal

A. EEG (top) and spectrogram (bottom) for two consecutive days of recording from a naïve female rat. For A and B, only 4 of the 8 channels used for the recording are shown. B. Same as A, but recordings were from a different rat. The calibrations are the same as in A.

Figure 3. SWDs in naïve rats resemble seizure-like discharges in the hippocampal recording

A. 1–4. EEG is shown for a period of exploration (1) and behavioral arrest (2–4) from the same recording session. 3. The part of the SWD episode boxed in red is shown with greater temporal resolution as indicated by the arrow. 4. One of the poles of the left hippocampal recording is expanded to show that the recording in hippocampus resembles a seizure. Calibration for 1–2 is shown in 2; calibration for 3 is shown in 3; calibration for 4 is shown to the right of the trace. B. 1–4. EEG is shown from a different animal, with a recording during a period of exploration on the left (1), a recording during behavioral arrest with a SWD in the center (2), and expansion of the area in the red box on the far right (3). 4. The recording in the blue box is expanded below 2. Same calibrations as A.

Figure 4. Hippocampal neuronal activity during SWDs

A. An example of a SWD is shown. B. The boxed area of A is shown with greater temporal resolution. Below the left FC trace is the same recording after filtering at 250–500 Hz. Similarly, the HC recording is shown from B and then below it is the same recording after filtering at 250–500 Hz. Arrows point to examples of high frequency oscillations (>250 Hz) in the hippocampus that occur at the same time as the spike of the spike-wave pattern. The results suggest that there are high frequency oscillations of hippocampal principal cells. C. SWD frequencies during spontaneous episodes of behavioral arrest are plotted in relation to the frequencies of hippocampal theta rhythm, recorded during spontaneous periods of exploration. For a given animal, its SWD frequency and its theta frequency are used for each point in the graph. One animal is excluded because theta rhythm was not robust in the hippocampal recordings, most likely because the tips of the electrodes were not in an optimal layer [45].

Figure 5. SWDs are longer in duration when head movement occurs

A. SWD frequency was determined for SWDs with movement and without movement. Movement included small head movements (e.g., head nodding and mastications) that accompanied frozen posture. Frequency was similar regardless of movements that occurred during SWDs (n=9; Student’s t-test, p>0.05). B. Animals were divided into those that showed head movement during SWDs and those that did not exhibit detectable head movement. The duration of SWDs, expressed as a percentage of time in behavioral arrest, was greatest in animals that exhibited movement (p<0.05). To assess SWDs similarly in all animals, the first two minutes of behavioral arrest in a given recording session was used. C. There was a correlation between SWD duration (defined as described in A-B) and movement of the head in females (1; n=5) and males (2; n=3). Movements were scored as follows: 1, no movement; 2, movement of the nose; 3, vigorous movement of nose and vibrissae; 4, lowering the head to the floor; 5, head bobbing; 6, head lowering with head bobbing. Two animals are excluded because they were not scored.

Figure 6. SWDs are increased in rats after pilocarpine-induced seizures

A–B. A. A recording from a rat during behavioral arrest after pilocarpine-induced SE. The boxed area is expanded in B as indicated by the arrow. Calibration, 250 μV (A); 500 μV (B). C. The percent of animals with SWDs are shown for the naïve rats (same data as Fig. 1), rats that had pilocarpine but did not have SE (“pilocarpine”) and rats that had pilocarpine and SE in the subsequent hrs (“pilocarpine + SE”). Differences were significant (Fisher’s exact test, p=0.005). D. The frequencies of spike-wave oscillations were not different among the groups (p>0.05).