Neonatal neuronal circuitry shows hyperexcitable disturbance in a mouse model of the adult-onset neurodegenerative disease amyotrophic lateral sclerosis - PubMed (original) (raw)

Neonatal neuronal circuitry shows hyperexcitable disturbance in a mouse model of the adult-onset neurodegenerative disease amyotrophic lateral sclerosis

Brigitte van Zundert et al. J Neurosci. 2008.

Abstract

Distinguishing the primary from secondary effects and compensatory mechanisms is of crucial importance in understanding adult-onset neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS). Transgenic mice that overexpress the G93A mutation of the human Cu-Zn superoxide dismutase 1 gene (hSOD1(G93A) mice) are a commonly used animal model of ALS. Whole-cell patch-clamp recordings from neurons in acute slice preparations from neonatal wild-type and hSOD1(G93A) mice were made to characterize functional changes in neuronal activity. Hypoglossal motoneurons (HMs) in postnatal day 4 (P4)-P10 hSOD1(G93A) mice displayed hyperexcitability, increased persistent Na(+) current (PC(Na)), and enhanced frequency of spontaneous excitatory and inhibitory transmission, compared with wild-type mice. These functional changes in neuronal activity are the earliest yet reported for the hSOD1(G93A) mouse, and are present 2-3 months before motoneuron degeneration and clinical symptoms appear in these mice. Changes in neuronal activity were not restricted to motoneurons: superior colliculus interneurons also displayed hyperexcitability and synaptic changes (P10-P12). Furthermore, in vivo viral-mediated GFP (green fluorescent protein) overexpression in hSOD1(G93A) HMs revealed precocious dendritic remodeling, and behavioral assays revealed transient neonatal neuromotor deficits compared with controls. These findings underscore the widespread and early onset of abnormal neural activity in this mouse model of the adult neurodegenerative disease ALS, and suggest that suppression of PC(Na) and hyperexcitability early in life might be one way to mitigate or prevent cell death in the adult CNS.

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Figures

Figure 1.

Figure 1.

Intrinsic excitability is increased in HMs from presymptomatic hSOD1G93A ALS mice. HMs in acutely prepared brainstem slices of mSOD1WT and hSOD1G93A mice were recorded. A, Membrane potential sample traces in mSOD1WT (top) and hSOD1G93A HMs (middle) showing APs evoked by rectangular depolarizing current pulses (bottom; 10–120 pA, 300 ms, 0.5 Hz). B, Mean AP frequency plotted against injected current show that the intrinsic excitability in hSOD1G93A HMs (n = 6) is significantly increased compared with mSOD1WT HMs (n = 12). Statistical significance is indicated.

Figure 2.

Figure 2.

PCNa is increased in HMs from presymptomatic hSOD1G93A ALS mice. A, AP sample traces in a mSOD1WT HM without (A1) and with (A2) riluzole (20 μ

m

). Note that riluzole is unable to prevent AP generation at the current step onset but abolishes repetitive AP firing without changing subthreshold responses (gray traces). B, PCNa current traces generated by a slow (total 8 s) triangular voltage-clamp command (B5) from a holding potential of −60 to 10 mV and recorded from mSOD1WT HMs in the absence (gray traces) and presence of riluzole (20 μ

m

, black traces; B1) or TTX (500 n

m

, black traces; B2) or from hSOD1G93A HMs (riluzole, B3; TTX, B4). Note that PCNa (which is the TTX-sensitive inward current activated at voltages more than −40 mV) is significantly larger in the hSOD1G93A HM. C, Current–voltage relationship of mean PCNa current normalized to cell capacitance (current density) plotted against voltage ramp membrane potential (MP) for mSOD1WT HMs (n = 9) and hSOD1G93A HMs (n = 12), showing that the voltage dependence of PCNa is unchanged in hSOD1G93A HMs.

Figure 3.

Figure 3.

Intrinsic excitability is increased in SC interneurons from presymptomatic hSOD1G93A ALS mice. SC interneurons of acutely prepared midbrain slices of mSOD1WT and hSOD1G93A mice were recorded. A, Membrane potential sample traces in mSOD1WT (top) and hSOD1G93A SC interneurons (middle) evoked by rectangular depolarizing current pulses (bottom; 10–120 pA, 300 ms, 0.5 Hz). B, Mean AP frequency plotted against injected current show that the intrinsic excitability in hSOD1G93A SC interneurons is significantly increased compared with mSOD1WT. Statistical significance is indicated.

Figure 4.

Figure 4.

The frequency of spontaneous excitatory and inhibitory synaptic transmission is enhanced in SC interneurons from presymptomatic hSOD1G93A mice. A, Representative spontaneous (sAMPA, sGABA, and sNMDA) and quantal (mAMPA) synaptic currents traces recorded from SC interneurons of mSOD1WT (left) and hSOD1G93A (right) mice. Amplitude calibration bars are 10 pA for sAMPA, mAMPA, and sNMDA traces and 50 pA for sGABA traces. Mean amplitude (B), interevent interval (C), rise time (D), and decay time (E) for the populations of sAMPA, sGABA, mAMPA, and sNMDA events recorded from each SC interneuron from mSOD1WT (open circles) and hSOD1G93A (open triangles) mice are shown, together with the overall mean and SEM of these values for each type of current and animal genotype superimposed over the individual cell values. Note that the frequency of sAMPA (p < 0.01) and sGABA (p < 0.05) currents are significantly increased (i.e., interevent interval is decreased) in hSOD1G93A mice, whereas amplitude, rise time, and decay time of these current types are not significantly different. There are no significant differences in any parameters for mAMPA currents, whereas sNMDA currents showed only a significant decrease in decay time (p < 0.05). Student's unpaired t test was used to compare overall means of each parameter for mSOD1WT and hSOD1G93A SC interneurons.

Figure 5.

Figure 5.

The frequency of spontaneous excitatory and inhibitory synaptic transmission is enhanced in HMs from presymptomatic hSOD1G93A mice. A, Representative spontaneous (sAMPA, sIPSC, and sNMDA) and quantal (mAMPA) synaptic currents traces recorded from HMs of mSOD1WT (left) and hSOD1G93A (right) mice. Amplitude calibration bars are 10 pA for sAMPA, mAMPA, and sNMDA traces and 50 pA for sIPSC traces. Mean amplitude (B), interevent interval (C), rise time (D), and decay time (E) for the populations of sAMPA, sIPSC, mAMPA, and sNMDA events recorded from each HM from mSOD1WT (open circles) and hSOD1G93A (open triangles) mice are shown, together with the overall mean and SEM of these values for each type of current and animal genotype superimposed over the individual cell values. Note that the frequency of sAMPA (p < 0.01) and sIPSC (p < 0.05) currents are significantly increased (i.e., interevent interval is decreased) in hSOD1G93A mice, whereas amplitude, rise time, and decay time of these current types are not significantly different. There are no significant differences in any parameters for mAMPA currents, whereas sNMDA currents showed only a significant decrease in decay time (p < 0.05). Student's unpaired t test was used to compare overall means of each parameter for mSOD1WT and hSOD1G93A HMs.

Figure 6.

Figure 6.

HMs dendrite retraction occurs earlier in presymptomatic hSOD1G93A mice (P6). A, Schematic of HSV-GFP p1003 (2 × 108/ml) injection in the tongue muscle at P1 to retrogradely label HMs in the ipsilateral hypoglossal nucleus (nXII). B, C, Low-magnification differential interference contrast (B) and fluorescent confocal (C) images of 350-μm-thick 4% paraformaldehyde-fixed slices of nXII showing GFP expression in HMs. D, E, Higher-magnification images showing GFP-positive mSOD1WT HMs, with labeled dendrites crossing midline (dashed line) at P6 (D) but not present at P9 (E). F, G, At P6, hSOD1G93A HMs (G) had fewer midline-crossing dendrites than mSOD1WT motoneurons (F). Scale bars: D, E, 100 μm; F, G, 50 μm.

Figure 7.

Figure 7.

Transient delays in development of gross locomotor abilities in presymptomatic hSOD1G93A mice are present at P2–P4. The fraction of pups displaying forelimb placing (A, at P3–P4 only) or righting (B, at P2 only) responses was lower in hSOD1G93A than in mSOD1WT and hSOD1WT mice (p < 0.05, χ2 test), whereas development of forepaw (C) and hindpaw (E) grasping, cliff-drop aversion (D), and vibrissae placing (F) were unaltered.

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