Short-term frequency-dependent plasticity at recurrent mossy fiber synapses of the epileptic brain

Abstract

The recurrent mossy fiber pathway of the dentate gyrus expands dramatically in human temporal lobe epilepsy and in animal models of this disorder, creating monosynaptic connections among granule cells. This novel granule cell network can support reverberating excitation but is difficult to activate with low-frequency stimulation. This study used hippocampal slices from pilocarpine-treated rats to explore the dependence of synaptic transmission in this pathway on stimulus frequency. Minimal electrically evoked EPSCs exhibited a high failure rate ( approximately 60%). Stimulus trains delivered at a frequency of <1 Hz depressed synaptic transmission, as evidenced by an increase in response failures. Conversely, stimulus trains delivered at higher frequencies reduced the percentage of response failures and increased the amplitude of compound EPSCs, including pharmacologically isolated NMDA receptor-mediated EPSCs. Short-term frequency-dependent facilitation was of modest size compared with mossy fiber synapses on other neuronal types. Facilitation depended on the activation of kainate receptors by released glutamate and was inhibited by feedback activation of type II metabotropic glutamate receptors. These results suggest that the recurrent mossy fiber pathway may be functionally silent during baseline asynchronous granule cell activity in vivo attributable, in part, to progressive transmission failure. The pathway may synchronize granule cell firing and may promote seizure propagation most effectively during the brief periods of high-frequency granule cell firing that occur during normal behavior, during the periods of hypersynchronous fast activity characteristic of epileptic brain and, most importantly, during the period of increasing granule cell activity that precedes a spontaneous seizure.

Figure 1.

Frequency-dependent short-term facilitation of the compound recurrent mossy fiber EPSC during a train of 10 near-maximal electrical stimuli. Each train was presented 10 times, and corresponding responses were averaged. Recordings were made in the presence of 30 μm bicuculline at a holding potential of -80 mV. Top, Representative averaged traces illustrating facilitation during stimulation at 2 Hz but not at 0.2 or 10 Hz. Bottom, Mean values ± SEM for 11–17 cells. Only stimulus trains delivered at 1 or 2 Hz facilitated synaptic transmission significantly (p < 0.005 for 1 Hz; p < 0.001 for 2 Hz by one-way repeated measures ANOVA; for the other frequencies, p > 0.4).

Figure 2.

Frequency-dependent short-term facilitation of the compound recurrent mossy fiber EPSP during a train of 10 near-maximal electrical stimuli. Recordings were made in the presence of 30 μm bicuculline. Representative single traces illustrate facilitation during stimulation at 2 Hz but not at 0.2 or 10 Hz. In this example, the fourth and each succeeding stimulus in the 2 Hz train drove the firing of an action potential (shown truncated). Resting membrane potentials were -75 mV (0.2 Hz), -75 mV (2 Hz), and -72 mV (10 Hz). Similar results were obtained in recordings from six granule cells. ^*Stimulus artifacts.

Figure 3.

Electrically evoked minimal recurrent mossy fiber EPSC. Top, Representative trace showing an average of the 15 responses recorded during a train of 50 minimal electrical stimuli delivered at a frequency of 0.2 Hz. In this example, the peak amplitude was 15 pA; the 10–90% rise time was 3.4 msec; the decay time constant was 40.5 msec; and charge transfer was 167 pC. Bottom, Means ± SEM for 40 granule cells.

Figure 4.

Percentage of response failures during a train of 50 minimal electrical stimuli. Recordings were made in the presence of 30 μm bicuculline at a holding potential of -80 mV. Values are means ± SEM for 11 (0.4–0.8 Hz), 20 (1 Hz), 27 (10 Hz), or 39 (0.2 and 2 Hz) cells. Stimulation at 1–10 Hz reduced the percentage of failures compared with each of the lower frequencies (p < 0.01 by Newman–Keuls test after one-way ANOVA yielded p < 0.001). Although the failure rate tended to be lower during 2 and 10 Hz trains, the mean difference from 1 Hz trains was not statistically significant (p > 0.2). ^*Result obtained with minimal laser photostimulation (Molnár and Nadler, 1999).

Figure 5.

Short-term synaptic plasticity developed progressively during a train of 50 minimal electrical stimuli. A, Traces from representative experiments. In these examples, only 4 of the first 10 stimuli presented at a frequency of 0.2 or 2 Hz evoked a minimal EPSC. In contrast, only 1 of the last 10 stimuli presented at a frequency of 0.2 Hz evoked a synaptic response compared with 7 of the last 10 stimuli presented at a frequency of 2 Hz. B, Mean values ± SEM for the number of cells given in Figure 4. From stimuli 21–30 onward, repeated stimulation at a frequency of 0.2 Hz significantly increased the percentage of response failures, and repeated stimulation at a frequency of 2 or 10 Hz significantly reduced the percentage of response failures (p < 0.01 compared with stimuli 1–10 by Dunnett's test after one-way repeated measures ANOVAs yielded p < 0.001). For other details, see Figure 4.

Figure 6.

Activation of type II metabotropic glutamate receptors inhibited transmission at recurrent mossy fiber synapses. Recordings were made in the absence and then in the presence of bath-applied 1 μm DCG-IV. A, The NMDA receptor-mediated EPSC was recorded in the presence of 10 μm NBQX and 30 μm bicuculline at a holding potential of -20 mV. These responses from a representative experiment are averages of six traces. Stimulus artifacts have been removed. DCG-IV reduced the size of the response. B, DCG-IV significantly increased the percentage of response failures during minimal electrical stimulation (50 stimuli) at frequencies of 0.2, 2, and 10 Hz. Values are means ± SEM for nine cells. ^*p < 0.05; ^***p < 0.001; ^**p < 0.02 by paired t test. C, Analysis of failure rates at different times during the stimulus train demonstrated that DCG-IV abolished the frequency-dependent facilitation normally observed during stimulation at 2 Hz and at least delayed the appearance of facilitation during stimulation at 10 Hz. For other details, see Figure 4.

Figure 7.

Block of type II metabotropic glutamate receptors enhanced the frequency-dependent reduction in response failures during minimal electrical stimulation. Recordings were made in the absence and then in the presence of bath-applied 100 nm LY 341495. A, LY 341495 did not affect the percentage of response failures during stimulation at a frequency of 0.2 Hz but reduced the percentage of failures during 2 and 10 Hz stimulus trains. Values are means ± SEM for nine cells. ^*p < 0.05 by paired t test. B, Analysis of failure rates at different times during the stimulus train suggested no obvious time or use dependence in the action of LY 341495; its onset of action must have occurred within the first 10 stimuli. For other details, see Figure 4.

Figure 8.

NMDA receptor-mediated recurrent mossy fiber EPSC isolated pharmacologically with 30 μm bicuculline and either 10 μm NBQX or 40 μm SYM 2206. Recordings shown are from representative experiments in which a train of 10 near-maximal stimuli was presented at a frequency of 2 Hz. The stimulus train was repeated 10 times, and corresponding traces were averaged. Then 50 μm d-AP-5 was added to the superfusion medium, and the stimulation protocol was repeated. Stimulus artifacts have been removed. Top, During superfusion with NBQX, d-AP-5 abolished the response recorded at a holding potential of -20 mV. Note the smaller response to the ninth stimulus in the train. Middle, During superfusion with SYM 2206, a small, presumably kainate receptor-mediated, response (dashed arrow) remained after addition of d-AP-5 to the superfusion medium. Note the larger response to the ninth stimulus in the train. Bottom, The kainate receptor-mediated component of the response to the ninth stimulus in the train (+d-AP-5) is scaled to the peak amplitude of the response recorded before addition of d-AP-5 to the superfusion medium. Note the change in the time course of the evoked response after application of d-AP-5.

Figure 9.

Unblocking kainate receptors converts short-term depression into facilitation. The NMDA receptor-mediated EPSC was isolated pharmacologically with 30 μm bicuculline and 10 μm NBQX, 30 μm GYKI 53655, or 40 μm SYM 2206. Left, Recordings from representative experiments performed as described in Figure 8, except that d-AP-5 was not used. In the presence of NBQX the amplitude of the response declined during the stimulus train, whereas response amplitude increased when SYM 2206 was used. Right, Mean values ± SEM for 10 cells in each group. The synaptic response exhibited statistically significant depression when the mossy fibers were stimulated at a frequency of 1 or 2 Hz in the presence of NBQX (p < 0.001 in each case by one-way repeated measures ANOVA). In contrast, the response facilitated during stimulation at these same frequencies in the presence of GYKI 53655 or SYM 2206 (p < 0.005 for 1 Hz; p < 0.001 for 2 Hz).

Figure 10.

Block of type II metabotropic glutamate receptors modified short-term plasticity at recurrent mossy fiber synapses in response to a 2 Hz stimulus train. Values are means ± SEM for 10 cells in each group. Top, LY 341495 (100 nm) reduced and eventually abolished response depression observed when the NMDA receptor-mediated EPSC was isolated with 30 μm bicuculline and 10 μm NBQX [p < 0.001 by Newman–Keuls test after two-way ANOVA (treatment × stimulus number)]. Bottom, When the NMDA receptor-mediated component was isolated with 30 μm bicuculline and 40 μm SYM 2206, facilitation usually appeared earlier in the train during application of LY 341495. However, this effect was not statistically significant [p > 0.1, two-way ANOVA (treatment × stimulus number)].