From unspecific to adjusted, how the BOLD response in the rat hippocampus develops during consecutive stimulations

Abstract

To determine the possibility to deconvolve measured BOLD responses to neuronal signals, the rat perforant pathway was electrically stimulated with 10 related stimulation protocols. All stimulation protocols were composed of low-frequency pulse sequences with superimposed high-frequency pulse bursts. Because high-frequency pulse bursts trigger only one synchronized spiking of granular cells, variations of the stimulation protocol were used: (a) to keep the spiking activity similar during the presentation of different numbers of pulses, (b) to apply identical numbers of pulses to induce different amounts of spiking activity, and (c) to concurrently vary the number of applied electrical pulses and resultant spiking activity. When complex pulse sequences enter the hippocampus, an unspecific default-like BOLD response is first generated, which relates neither to the number of incoming pulses nor to the induced spiking activity. Only during subsequent stimulations does the initial unspecific response adjust to a more adequate response, which in turn either strongly related to spiking activity when low-frequency pulses were applied or depended on the incoming activity when high-frequency pulse bursts were presented. Thus, only the development of BOLD responses during repetitive stimulations can predict the underlying neuronal activity and deconvolution analysis should not be performed during an initial stimulation period.

Keywords: Functional magnetic resonance imaging; dentate gyrus; electrophysiology; field excitatory postsynaptic potential; neurovascular coupling.

Figure 1.

Summary of the applied stimulation protocols. During all experiments, the perforant pathway was stimulated with 20 consecutive stimulation trains. The first stimulation train (indicated by red boxes) was applied 2 min after starting fMRI. Each stimulation train lasts for 8 s and was followed by 52 s rest (indicated by the gray boxes). During the first set of experiments, continuous 1, 5, 10, or 20 Hz pulses were applied during each stimulation train. During these stimulation frequencies, each applied pulse elicited one population spike; consequently, under these conditions, the input (i.e. applied pulses) and output activity (number of population spikes) vary in a similar way (Experiment 1). In a second set of experiments, high frequency pulses (5, 10, or 20) were superimposed on a 1 Hz stimulation protocol. Because only one population spike was generated to the first pulse of the burst during these conditions, whereas all subsequent pulses only generated fEPSPs, the number of population spikes was similar. Consequently, only the incoming activity varied, whereas the output activity remained similar (Experiment 2). During the third set of experiments, the perforant pathway was additionally stimulated with 5 Hz-4 pulses and 10 Hz-2 pulses; thus, there were four different experimental conditions in which 20 pulses were applied but different numbers of population spikes were generated (Experiment 3).

Figure 2.

Summary of the measured BOLD responses in the right hippocampus during electrical stimulation of the right perforant pathway. Here, averaged maximal BOLD signal intensities during each BOLD response measured in experiment 1 (a), experiment 2 (b), and experiment 3 (c) are depicted. The averaged BOLD response during the first, fourth, and 18th–20th stimulation trains are summarized on the right. Asterisks indicate significantly increased BOLD response during train 4 when compared with train 1 (black: 20 Hz-1-pulse, dark blue: 1 Hz-50-pulse, turquoise: 10 Hz-2-pulse).

Figure 3.

Summary of the electrophysiological responses in the right dentate gyrus during electrical stimulation of the right perforant pathway. Here, averaged population spike amplitudes during individual stimulus trains recorded in experiment 1 (a), experiment 2 (b), and experiment 3 (c) are depicted. The development of individual population spike amplitudes during the first, fourth, and 18th–20th stimulation trains are summarized on the right.

Figure 4.

Distribution of significantly activated voxels depends on the actual stimulation protocol. The applied stimulation protocol does not only determine the consistency of significant BOLD responses between early (i.e. train 5–11) and late (i.e. train 14–20) trains, but also the spatial distribution.

Figure 5.

The relation between spiking activity (i.e. averaged population spike amplitude, see Figure 3) and the resultant BOLD response (see Figure 2) depends on the applied stimulation pattern. Strong correlations between these two factors were only found after an initial default-like BOLD response (i.e. induced by the first stimulation train, red dot) and followed by a short period of functional reconfiguration (train 2–4, pale dots). During stimulation with ≥ 10 high-frequency burst pulses, this correlation is nonexistent.

Figure 6.

Application of an alternative stimulation protocol affects the development of BOLD responses to low frequency (5 Hz, green color) but not to high-frequency (1 Hz-20-pulse, blue color) stimulation. (a) Left panel: BOLD time series measured during alternate stimulations starting with a 5 Hz (protocol 1, top) or a 1 Hz-20-pulse (protocol 2, bottom) protocol. Middle panel: Summary of individual BOLD responses during stimulation and their respective decline over time (calculate between trains 5 and 20). Right panel: Averaged BOLD responses to 5 Hz and 1 Hz-20-pulse stimulation (between trains 5 and 20). (b) Left panel: BOLD time series measured during repetitive 5 Hz- and 1 Hz-20-pulse stimulations. Middle panel: Decline of BOLD responses during repetitive stimulations; top graph depicts all measuring points that correspond to protocol 1 and the lower graph depicts all measuring points that correspond to protocol 2. Right panel: average BOLD responses calculated from all responses between trains 5 and 20). (c) Comparison of BOLD responses observed during application of alternating or single stimulation protocols. An asterisk indicates a significant difference.