Stimulation Protocol

Twenty hertz stimulus trains (1 msec biphasic pulses) are delivered through the CA3 stimulating electrode for a maximum of 30 s. Initially, trains are administered every 2 to 3 min with increasing stimulus intensities (in 40- to 100-pA steps, beginning with 400 pA) to determine the threshold for maximal dentate activation. Maximal dentate activation is defined by the appearance of large-amplitude population spikes in the extracellular recording (10 to 40 mV) associated with a rapid decrease in the DC potential (Figure 4.8A). The stimulus threshold for elicitation of maximal dentate activation is usually 500 to 800 pA. If the animal requires higher stimulus intensities, then either the stimulating electrode is not positioned correctly or the animal has received too much urethane. A stimulus train sufficient to produce maximal dentate activation is then administered every 5 min until an afterdischarge appears and then every 10 min. An afterdischarge is defined as at least two bursts of population spikes after the end of a stimulus train. For every stimulus train, the stimulus is stopped 2 to 3 s after maximal dentate activation begins (Figure 4.9). This causes most of the paroxysmal discharges of the granule cells to be in the form of an afterdischarge.

Afterdischarge Duration

FIGURE 4.9

Measurements of the parameters of maximal dentate activation. One chart recording of the extracellular DC potential during and after the stimulus train to the CA3 region is shown. The stimulus duration is indicated. The time to onset of maximal dentate activation (MDA) is defined as the time from the beginning of the stimulus train to the midpoint of the positive-to-negative DC shift. The duration of maximal dentate activation is defined from the midpoint of the positive-to-negative DC shift to the midpoint of the return of the DC shift to baseline. The afterdischarge duration is defined as the time from the end of the stimulus train to the return of the DC potential back to baseline. Normally the stimulus duration is stopped 2 to 3 s after the onset of maximal dentate activation, so the afterdischarge duration is 2 to 3 s shorter than the duration of maximal dentate activation.

Afterdischarge Duration

FIGURE 4.9

Measurements of the parameters of maximal dentate activation. One chart recording of the extracellular DC potential during and after the stimulus train to the CA3 region is shown. The stimulus duration is indicated. The time to onset of maximal dentate activation (MDA) is defined as the time from the beginning of the stimulus train to the midpoint of the positive-to-negative DC shift. The duration of maximal dentate activation is defined from the midpoint of the positive-to-negative DC shift to the midpoint of the return of the DC shift to baseline. The afterdischarge duration is defined as the time from the end of the stimulus train to the return of the DC potential back to baseline. Normally the stimulus duration is stopped 2 to 3 s after the onset of maximal dentate activation, so the afterdischarge duration is 2 to 3 s shorter than the duration of maximal dentate activation.

This repeated elicitation of reverberatory seizures in the hippocampal circuit is thought to closely mimic the changes seen in afterdischarge duration during kindling and, thus, could be referred to as electrographic kindling.

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