Electrical Stimulation of the Lateral Entorhinal Cortex Causes a Frequency-Specific BOLD Response Pattern in the Rat Brain

Abstract

Although deep brain stimulation of the entorhinal cortex has recently shown promise in the treatment of early forms of cognitive decline, the underlying neurophysiological processes remain elusive. Therefore, the lateral entorhinal cortex (LEC) was stimulated with trains of continuous 5 Hz and 20 Hz pulses or with bursts of 100 Hz pulses to visualize activated neuronal networks, i.e., neuronal responses in the dentate gyrus and BOLD responses in the entire brain were simultaneously recorded. Electrical stimulation of the LEC caused a wide spread pattern of BOLD responses. Dependent on the stimulation frequency, BOLD responses were only triggered in the amygdala, infralimbic, prelimbic, and dorsal peduncular cortex (5 Hz), or in the nucleus accumbens, piriform cortex, dorsal medial prefrontal cortex, hippocampus (20 Hz), and contralateral entorhinal cortex (100 Hz). In general, LEC stimulation caused stronger BOLD responses in frontal cortex regions than in the hippocampus. Identical stimulation of the perforant pathway, a fiber bundle projecting from the entorhinal cortex to the dentate gyrus, hippocampus proper, and subiculum, mainly elicited significant BOLD responses in the hippocampus but rarely in frontal cortex regions. Consequently, BOLD responses in frontal cortex regions are mediated by direct projections from the LEC rather than via signal propagation through the hippocampus. Thus, the beneficial effects of deep brain stimulation of the entorhinal cortex on cognitive skills might depend more on an altered prefrontal cortex than hippocampal function.

Keywords: BOLD fMRI; amygdala; in vivo electrophysiology; limbic system; piriform cortex; prefrontal cortex.

FIGURE 1

Concurrent measurements of electrophysiological and fMRI data. (A) Electrophysiological recordings in the right dentate gyrus during stimulation of the right LEC (black line) or right dentate gyrus (red line). Neuronal responses to LEC stimulation were smaller and delayed when compared to perforant pathway stimulation. Arrows indicate stimulation artifacts (pulses), and the red [perforant pathway (pp) stimulation] or black (LEC stimulation) line indicates the location of the scanner-induced gradient artifacts. (B) Scheme of the general stimulation protocol. In all experiments, 20 identical 8 s long stimulation periods (indicated by red lines) were applied. (C) Anatomical images (top) reveal the presence of the stimulation electrode (red arrow), the recording electrode (green arrow), and the grounding electrode (blue arrow). The artifacts induced by the electrodes are more pronounced in the corresponding gradient-echo EPI (middle) that are used for fMRI. Overlay of the BOLD activation map on the anatomical image revealed significant BOLD activation in long distance from the stimulation and recording electrode (bottom).

FIGURE 2

Summary of BOLD responses induced by stimulation of the right LEC with repetitive 5 Hz pulse trains. Spatial distribution of significantly activated voxels revealed the presence of three main clusters of activation. BOLD time series of each individual cluster and event related averaging of all responses to train 3–20 indicate that the strongest BOLD response was induced in the lateral entorhinal cortex, whereas the smallest BOLD response was induced in ventral frontal cortical regions. The gray boxes indicate the time periods of stimulation.

FIGURE 3

Summary of BOLD activation pattern induced by stimulation of the right perforant pathway (top) and LEC (bottom) with different stimulation protocols. Significant differences in BOLD activation between continuous (5 or 20 Hz pulses) and high-frequency pulse burst (5 or 20 pulses) stimulation of the right LEC (bottom). The same number of pulses applied as bursts with an inter-pulse interval of 10 ms (100 Hz) triggers an expanded BOLD response in frontal regions.

FIGURE 4

Summary of BOLD responses induced by stimulation of the right LEC with bursts of five high-frequency (100 Hz) pulses. Note that the spatial distribution of significantly activated voxels as well as the magnitude of BOLD responses is increased when compared to continuous 5 Hz pulse stimulation (Figure 2, 3). The magnitude of the BOLD responses in the amygdala and frontal cortex region were similar.

FIGURE 5

Development of significant BOLD responses during repetitive stimulation of the right LEC with different protocols. Independent of the stimulation protocol, the spatial distribution of significantly activated voxels varied during consecutive trains.

FIGURE 6

Summary of BOLD responses induced by stimulation of the right LEC with repetitive 20 Hz pulse trains. Under this condition, the spatial distribution of significantly activated voxels in the frontal cortex was increased compared to continuous 5 Hz stimulation (see Figure 2).

FIGURE 7

Summary of BOLD responses induced by stimulation of the right LEC with bursts of 20 high-frequency (100 Hz) pulses. Although the same number of pulses were applied as in Figure 5, the spatial distribution of significantly activated voxels increased as well as the magnitude in activation clusters 1 and 2.

FIGURE 8

Summary of BOLD responses in individual VOIs during stimulation of the LEC with different stimulation protocols. VOIs of interest are depicted in the 3D rat brain template. Event-related BOLD responses represent average BOLD signal changes of all voxels in the appropriate VOI. Whereas BOLD responses in the septum and hippocampus mainly depended on the number of pulses, BOLD responses in the entorhinal cortex, piriform cortex, and basolateral amygdala mainly depended on the pulse frequency.

FIGURE 9

Development of BOLD signal changes in the right hippocampus during repetitive stimulation of the right LEC (red graphs) or the right perforant pathway (blue graphs) with different stimulation protocols. Note that the strongest BOLD responses were always induced by perforant pathway stimulation.

FIGURE 10

Development of BOLD signal changes in the prefrontal cortex during repetitive stimulation of the right LEC (red graphs) or the right perforant pathway (blue graphs) with different stimulation protocols. Note that the strongest BOLD responses were always induced by LEC stimulation.