Effects of synaptic modulation on beta-amyloid, synaptophysin, and memory performance in Alzheimer's disease transgenic mice

Abstract

Accumulation of β-amyloid (Aβ) and loss of synapses are hallmarks of Alzheimer's disease (AD). How synaptic activity relates to Aβ accumulation and loss of synapses is a current topic of major interest. Synaptic activation promotes Aβ secretion, and chronic reduction of synaptic activity reduced Aβ plaques in an AD transgenic mouse model. This suggested beneficial effects of reducing synaptic activity in AD. We now show that reduced synaptic activity causes detrimental effects on synapses and memory despite reducing plaques using two different models of chronic synaptic inhibition: deafferentation of the barrel cortex and administration of benzodiazepine. An interval of prolonged synaptic inhibition exacerbated loss of synaptophysin compared with synaptically more active brain in AD transgenic but not wild-type mice. Furthermore, an interval of benzodiazepine treatment, followed by a washout period, exacerbated memory impairment in AD transgenic mice. Exacerbation of synaptic and behavioral abnormalities occurred in the setting of reduced Aβ plaques but elevated intraneuronal Aβ immunoreactivity. These data support beneficial effects of synaptic activation on Aβ-related synaptic and behavioral impairment in AD.

Conflict of interest statement

The authors declare that they have no conflicts of interest.

Figures

Figure 1.

Chronic synaptic inhibition reduces amyloid plaques and increases intraneuronal Aβ42 immunoreactivity. A, Top, COX staining shows reduced activity in the deafferented compared with the control barrel cortex. Bottom: adjacent brain sections demonstrated reduced amyloid plaque burden in the deafferented barrel cortex compared with control. Quantification of plaque number and area covered by plaques revealed a 29 ± 5% and a 32 ± 8% decrease, respectively, in the deafferented compared with the control barrel cortex (n = 4, p < 0.05). B, Intraneuronal Aβ42 immunoreactivity was increased by 64 ± 30% in the deafferented compared with the control Tg19959 barrel cortex (n = 4; p < 0.05). Scale bar, 75 μm.

Figure 2.

Chronic synaptic inhibition reduces levels of synaptophysin and number of synapses. A, Left, Fluorescent immunolabeling of synaptophysin in the barrel cortex of Tg19959 mice. The deafferented barrels demonstrated a marked 80 ± 3% reduction in synaptophysin compared with the control barrel cortices (n = 4; p < 0.01). Scale bar, 75 μm. Right, Fluorescent immunolabeling of synaptophysin in the barrel cortex of wild-type mice. No change in synaptophysin immunofluorescence was detectable between the deafferented and control sides (n = 3). Scale bar, 75 μm. B, EM images of Tg19959 barrel cortex: the number of synapses (red arrows) was reduced by 31 ± 12% in the deafferented compared with control barrels (n = 5/group; p < 0.05). Scale bar, 500 nm.

Figure 3.

Chronic diazepam administration reduces amyloid plaques and increases intraneuronal Aβ42 immunofluorescence. A, Quantification of cortical plaque number demonstrated a 32 ± 12% decrease in diazepam-treated (Tg+diazepam) compared with untreated Tg19959 (Tg) mice (n = 5; p < 0.01). Scale bar, 500 μm. B, Quantification of hippocampal plaque number demonstrated a 20 ± 9% decrease in plaques in diazepam-treated compared with untreated Tg19959 mice (n = 5, p < 0.05). Scale bar, 500 μm. C, Confocal microscopy showing intraneuronal Aβ42 immunofluorescence in Tg19959 hippocampus. Diazepam-treated Tg19959 mice show 41 ± 10% higher levels of intraneuronal Aβ42 immunoreactivity compared with untreated Tg19959 mice (n = 5; p < 0.01). Scale bar, 75 μm.

Figure 4.

Chronic diazepam administration reduces levels of synaptophysin and worsens spatial memory in Tg19959 mice. A, Fluorescent immunolabeling of synaptophysin in the hippocampus of Tg19959 and wild-type mice. Compared with wild-type mice, both Tg19959 mice and Tg19959 mice treated with diazepam showed 77 ± 1% and 65 ± 2% decreases, respectively, in synaptophysin. Compared with untreated Tg19959 mice, levels of synaptophysin were also significantly reduced in Tg19959 mice treated with diazepam. n = 5; *p < 0.05; **p < 0.01). Scale bar, 75 μm. B, Percentage of time spent in each quadrant during the first 15 s of the probe trial. Both Tg19959 groups spent less time in the target quadrant compared with wild-type mice. Tg19959 mice treated with diazepam showed greater impairment in memory retention compared with untreated Tg19959 mice (*p < 0.05, **p < 0.01).