Seizures accelerate functional integration of adult-generated granule cells - PubMed (original) (raw)
Seizures accelerate functional integration of adult-generated granule cells
Linda S Overstreet-Wadiche et al. J Neurosci. 2006.
Abstract
In humans and experimental animals, structural and functional changes in neural circuits can accompany the development of epilepsy. In the dentate gyrus, seizures enhance adult neurogenesis, but it is unclear to what extent newborn granule cells participate in seizure-induced synaptic reorganization. During the first weeks of their existence, mouse newborn granule cells labeled with enhanced green fluorescent protein have only short dendrites that lack excitatory input. We report that pilocarpine-induced seizures accelerated the morphological development of labeled granule cells, causing their dendrites to extend through the molecular layer. In whole-cell recordings 5-16 d after seizure induction, perforant-path stimulation now evoked glutamatergic input to newborn granule cells. These synaptic responses were mediated by monosynaptic as well as recurrent polysynaptic input. Thus, seizures facilitated functional integration of adult-generated granule cells. One month later, subsequent generations of newborn cells also showed alterations in dendrite morphology, suggesting persistent effects of seizures on granule cell maturation. The sensitivity of newborn granule cells to seizures could contribute to hyperexcitability during the latent period.
Figures
Figure 1.
Seizures enhance dendrite outgrowth of newborn granule cells. a, Two weeks after pilocarpine-induced seizures, many EGFP-labeled newborn granule cells had dendrites that extended into the middle and outer molecular layer. b, In control mice, the dendrites of all newborn granule cells terminated in the inner molecular layer. Some newborn granule cells after seizures remained unaffected (a, asterisk). Seizures also appeared to increase the number of labeled newborn cells. Images are maximum intensity _z_-projections through 50 μm sections from perfusion-fixed POMC-EGFP brains. Image intensity was increased to visualize the layers. Scale bar, 100 μm. gcl, Granule cell layer; iml, inner molecular layer; mml, middle molecular layer; oml, outer molecular layer. c, Seizures increased the total dendritic length and number of branch points of EGFP-labeled newborn granule cells. The cumulative probability distribution of measured values is shown. The _x_-axis indicates the percentage of values that fall below and above each measurement, such that the median falls at 50%. Twenty-five cells from 15 sections were measured in control mice (n = 10), and 27 cells from 12 sections were measured in mice that had level III–V seizures after pilocarpine injections (n = 3).
Figure 2.
EGFP labels newborn granule cells after seizures. a, Diagram of the BrdU experiment to birth date newborn granule cells before seizure induction. b, EGFP and BrdU immunolabeling from control (top) and seizure (bottom) mice perfused 16 d after BrdU incorporation. After seizures, some EGFP-expressing cells with elongated dendrites were labeled with BrdU. Colabeled cells are indicated with arrows. The percentage of BrdU-positive cells that expressed EGFP was the same in seizure and control mice, indicating there was no change in the temporal pattern of EGFP expression after seizures. c, After seizures, almost all EGFP-labeled granule cells expressed PSA-nCAM, indicating EGFP was selective for newborn granule cells. Scale bars: b, 50 μm; c, 20 μm.
Figure 3.
Seizures promote functional integration of newborn granule cells. a, MPP stimulation failed to evoke EPSCs in newborn granule cells in slices from control mice. After seizures, single MPP stimuli evoked EPSCs in newborn granule cells. At a holding potential of −70 mV, inward EPSCs were blocked by NBQX. At +40 mV in NBQX, outward EPSCs were blocked by AP-5, even after multiple stimuli (arrows). The asterisk indicates asynchronous synaptic events on the decay of the EPSC at −70 mV. Calibration: 10 pA, 50 ms. These synaptic responses were recorded in the biocytin-filled cell shown in supplemental Figure 1 (available at
as
supplemental material
). b, The percentage of mature and newborn granule cells with MPP-evoked EPSCs in control (con) and seizure mice. There were significant differences among groups (p < 0.0001; χ2 test). Post hoc comparisons showed that newborn granule cells in control were different from the other two groups (p < 0.001; Fisher’s exact test). c, After seizures, newborn granule cells had dendritic spines that were not present in control mice. Scale bar, 5 μm.
Figure 4.
Recurrent polysynaptic input to mature granule cells. a, Morphological evidence for mossy fiber sprouting 5 d after seizures. An axon collateral (small arrowheads) of a biocytin-filled mature granule cell (red, top cell) appears to terminate on the dendritic shaft of a second biocytin-filled mature granule cell (large arrowhead, enlarged in inset). EGFP-labeled newborn granule cells (green) are also shown. The axon collateral is enlarged in the bottom panel. Scale bar, 10 μm. b, After seizures, MPP-evoked EPSCs in mature granule cells had asynchronous events detectable in individual traces (−70 mV, asterisk, bottom traces; calibration: 50 pA, 10 ms) that prolonged the duration of the averaged EPSC (top trace; calibration: 100 pA, 10 ms). When polysynaptic activity was blocked by NBQX, the amplitude and duration of the NMDA receptor-mediated EPSC (+40 mV) was reduced (right traces; calibration: 100 pA, 10 ms), indicating recurrent polysynaptic input contributed to the synaptic response. Normalized traces at +40 mV are also shown. Scale bar, 50 ms. Stimulus artifacts are blanked for clarity. c, In contrast, polysynaptic activity did not contribute to EPSCs in control slices. At −70 mV, individual (bottom) and averaged (top) traces had monotonic decay phases. Blocking polysynaptic activity with NBQX did not alter the NMDA-EPSC at +40 mV (right and normalized traces). Calibration: 100 pA, 10 ms.
Figure 5.
Newborn granule cells receive direct and recurrent EPSCs after seizures. a, MPP-evoked EPSCs in newborn granule cells were recorded at −70 and +40 mV. Calibration: 10 pA, 50 ms. Blocking polysynaptic activity with NBQX reduced the half-width of NMDA receptor-mediated EPSCs (normalized traces; scale bar, 100 ms), indicating polysynaptic activity contributed to the synaptic response. The remaining NMDA receptor-mediated EPSC (right) revealed monosynaptic input from the MPP. Stimulus artifacts are blanked. The histogram illustrates the relative contribution of polysynaptic activity in each condition. NBQX had no effect on EPSCs in mature (mat) granule cells in control (con) slices (n = 7), whereas after seizures (seiz), polysynaptic activity contributed to EPSCs in both mature (n = 11) and newborn (new; n = 10) granule cells. b, Synapses on newborn granule cells had prolonged and large NMDA receptor-mediated components. Examples of monosynaptic NMDA receptor-mediated EPSCs in a newborn and neighboring mature granule cell are shown normalized to the peak currents (left). Scale bar, 100 ms. The ratio of NMDA/AMPA receptor-mediated components was larger in newborn cells (right). Ratios were measured as the peak current at +40 mV (in NBQX)/the peak current at −70 mV. Calibration: 50 pA, 20 ms. c, Exclusively polysynaptic responses were recorded in a subset of newborn neurons after seizures (n = 5). These EPSCs had long and variable latencies that were completely blocked by NBQX at both voltages. Calibration: 5 pA, 50 ms. The insets show individual events that were aligned and averaged, with kinetics characteristic of NMDA (+40 mV) and AMPA (−70 mV) receptor-mediated responses. Calibration: top: 10 pA, 100 ms; bottom: 4 pA, 5 ms.
Figure 6.
Accelerated development persists for subsequent generations of newborn granule cells. a, Examples of newborn granule cells with elongated dendrites in mice perfused 30–40 d after seizure induction. At this time, all labeled cells were generated after the initial seizure event. Seven of nine mice had obvious dendrite elongation compared with control mice. In affected mice, some sections of the dentate had large numbers of labeled newborn cells (right), whereas other sections had fewer or no newborn cells (left). b, Mice that displayed dendrite outgrowth 30–40 d after seizure induction (left) also had reduced numbers of calretinin-positive hilar interneurons (arrowhead) compared with the two of nine mice with dendrites similar to controls (right).
References
- Austin JE, Buckmaster PS (2004). Recurrent excitation of granule cells with basal dendrites and low interneuron density and inhibitory postsynaptic current frequency in the dentate gyrus of macaque monkeys. J Comp Neurol 476:205–218. - PubMed
- Behr J, Heinemann U, Mody I (2001). Kindling induces transient NMDA receptor-mediated facilitation of high-frequency input in the rat dentate gyrus. J Neurophysiol 85:2195–2202. - PubMed
- Bernard C, Anderson A, Becker A, Poolos NP, Beck H, Johnston D (2004). Acquired dendritic channelopathy in temporal lobe epilepsy. Science 305:532–535. - PubMed
- Carmignoto G, Vicini S (1992). Activity-dependent decrease in NMDA receptor responses during development of the visual cortex. Science 258:1007–1011. - PubMed
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