Neurotrophin-3 restores synaptic plasticity in the striatum of a mouse model of Huntington's disease

Abstract

Aims: Neurotrophin-3 (NT-3) is expressed in the mouse striatum; however, it is not clear the NT-3 role in striatal physiology. The expression levels of mRNAs and immune localization of the NT-3 protein and its receptor TrkC are altered in the striatum following damage induced by an in vivo treatment with 3-nitropropionic acid (3-NP), a mitochondrial toxin used to mimic the histopathological hallmarks of Huntington's disease (HD). The aim of this study was to evaluate the role of NT-3 on corticostriatal synaptic transmission and its plasticity in both the control and damaged striatum.

Methods: Corticostriatal population spikes were electrophysiologically recorded and striatal synaptic plasticity was induced by high-frequency stimulation. Further, the phosphorylation status of Trk receptors was tested under conditions that imitated electrophysiological experiments.

Results: NT-3 modulates both synaptic transmission and plasticity in the striatum; nonetheless, synaptic plasticity was modified by the 3-NP treatment, where instead of producing striatal long-term depression (LTD), long-term potentiation (LTP) was obtained. Moreover, the administration of NT-3 in the recording bath restored the plasticity observed under control conditions (LTD) in this model of striatal degeneration.

Conclusion: NT-3 modulates corticostriatal transmission through TrkB stimulation and restores striatal LTD by signaling through its TrkC receptor.

Keywords: Huntington's disease; NT-3; TrkC; neurodegeneration; neurotrophins.

Figure 1

NT‐3 modulates corticostriatal transmission. A, Time course of the population spike amplitude in the presence of NT‐3. The amplitude of the spike in response to the first stimulation (S1) increased 57% ± 6% compared to the control condition (t₁₆ = −38.31; P < 0.001) in the control tissue, whereas in slices from animals treated with 3‐NP, the increase in the amplitude of S1 was 30% ± 5.7% (t₁₇ = −22.6; P < 0.001). The line in A indicates the time of administration and NT‐3 concentration (50 ng/mL). B, Representative traces of population spikes in slices from control and (C) 3‐NP‐treated mice. D, PPR (S2/S1) in each experimental condition. Significant differences in the PPR were not observed. Dashed lines are the individual values for PPR in each experiment. The solid line illustrates average values (n = 6; Mean ± SE)

Figure 2

Corticostriatal LTD is modulated by NT‐3. A, Time course of the population spike amplitude in control and NT‐3‐treated slices after HFS (arrow). Note that LTD is obtained under both conditions but to different extents. Control slices without NT‐3 (57.9% ± 3.7%, t₁₇ = 73.09; P < 0.001) vs slices treated with NT‐3 (28.8% ± 2.4%, t₂₉ = 65.23; P < 0.001). B, Representative traces of population spikes in control slices before and after HFS. C, Representative traces of population spikes in control slices in the presence of NT‐3 before and after HFS. D, PPR (S2/S1) in untreated control slices and in the presence of NT‐3. Note that presynaptic mechanisms are involved in the LTD generated in control slices; however, a postsynaptic mechanism mediates the effects of NT‐3 on LTD (n = 6; Mean ± SE)

Figure 3

NT‐3 reestablished corticostriatal LTD in an animal model of HD. A, Time course of the population spike amplitude in slices from 3‐NP‐treated mice after HFS. 3‐NP‐treated slices exhibited an increase in the spike amplitude after HFS (38.8% ± 3.5%, t₁₉ = −48.7, P < 0.001); however, in the presence of NT‐3, 3‐NP‐treated slices exhibited a decrease in amplitude after HFS, (36.3% ± 4.1%, t₁₉ = 38.81; P < 0.001). B, Representative traces of population spikes in slices from 3‐NP‐treated mice before and after HFS (arrow). HFS triggers LTP in 3‐NP‐treated slices. C, Representative traces of population spikes in slices from 3‐NP‐treated mice before and after HFS and in the presence of NT‐3. Note that NT‐3 reestablished LTD to the levels observed in control tissues (Fig. 2). D, PPR (S2/S1) in 3‐NP slices and 3‐NP slices + NT‐3. There were no significant differences in both experimental conditions. Note that postsynaptic mechanism mediates the effects of NT‐3 on the reestablishment of LTD in slices from 3‐NP‐treated mice (n = 6; Mean ± SE)

Figure 4

CTX‐B blocks synaptic modulation but not NT‐3 effects on corticostriatal synapse plasticity of healthy slices. A, Time course of population spikes of slices from healthy mice recorded in the presence of the TrkB blocker CTX‐B. In the presence of CTX‐B, NT‐3 did not modulate the synaptic transmission of the corticostriatal pathway (100 ± 0.82 vs 97.97 ± 0.52). However, CTX‐B did not prevent striatal LTD after HFS and in the presence of NT‐3 (100 ± 0.82 vs 31.5 ± 1.91; t ₂₈ = 51.5, P < 0.001). B, Representative traces in the presence of CTX‐B (black), in the presence of CTX‐B + NT‐3 (gray) and after HFS (red). The block of TrkB impedes NT‐3 modulation but not the plasticity of corticostriatal synapses. C, PPR analysis of NT‐3 effect on corticostriatal transmission or (D) striatal LTD did not exhibit significant changes

Figure 5

CTX‐B blocks synaptic modulation but not NT‐3 effects on corticostriatal synapse plasticity of 3‐NP‐treated mice. A, Time course of population spikes of slices from 3‐NP‐treated mice recorded in the presence of the TrkB blocker CTX‐B. In the presence of CTX‐B, NT‐3 did not modulate the synaptic transmission of the corticostriatal pathway. B, Representative traces in the presence of CTX‐B (black), in the presence of CTX‐B + NT‐3 (red), and the overlap. Note that in the presence of the TrkB blocker, NT‐3 did not modulate corticostriatal transmission. C, Changes in PPR were not observed between the CTX‐B and CTX‐B + NT‐3 groups of slices from 3‐NP‐treated mice. D, Time course of the population spike amplitude of slices from 3‐NP‐treated mice incubated with the TrkB blocker and subjected to the HFS protocol. Note that the TrkB blocker did not impede LTD triggered by HFS in the presence of NT‐3. The spike amplitude was significantly decreased by 41.31% after HFS (100.16 ± 0.73 vs 58.78 ± 0.62, t₄₀ = 43.094, P < 0.001). E, Representative traces of population spikes in slices from 3‐NP‐treated mice in the presence of CTX‐B (black), CTX‐B + NT‐3 (gray), after HFS in the CTX‐B + NT‐3 group (red) and the overlap. In 3‐NP‐treated slices, the TrkB blocker did not prevent LTD induced by HFS in the presence of NT‐3. F, PPR (S2/S1) in 3‐NP slices in the presence of CTX‐B + NT‐3 before and after HFS. No significant differences were observed indicating that a postsynaptic mechanism mediates the effect of NT‐3 on reestablishing striatal plasticity (n = 5; Mean ± SE)

Figure 6

Phosphorylation levels of TrkB and TrkC in the presence of NT‐3. A, Representative images showing Western blot analyses of protein extracts from slices of 3‐NP‐treated mice maintained under conditions that imitated electrophysiological recordings. In the absence of CTX‐B (lines 1‐3), increased levels of p‐Tyr816‐TrkB were observed in response to NT‐3 and NT+ HFS (high K⁺) treatments. In the presence of CTX‐B (lines 4‐6), p‐Tyr816‐TrkB levels decreased in all experimental conditions. In contrast, p‐Tyr820‐TrkC expression level evaluated under the same conditions was not affected by CTX‐B and increased in the presence of NT‐3 and high K⁺. Independent slices from 3‐NP‐treated mice were used for each experimental condition. B, Illustrates in box plot the percentage of p‐TrkB induction (p‐TrkB/total TrkB) relative to control condition. The effect of NT‐3 treatment in the band intensities was normalized to that for the untreated condition for each experimental group. The statistical analysis was performed using a Kruskal‐Wallis one‐way analysis of variance on ranks test (H₅ = 19.274; P = 0.002); multiple comparisons were assessed using the Student‐Newman‐Keuls test. * P < 0.05. P‐Tyr820‐TrkC activation (p‐Tyr820‐TrkC/TrkC) increased in high K⁺ in the absence of CTX‐B (lines 1‐3); the CTX‐B did not affect its expression (line 4 ‐6). There was a nonsignificant increase in p‐Tyr820‐TrkC after the addition of CTX‐B and in the presence of NT‐3+ high K⁺

Figure 7

Phosphorylation levels of TrkA in the presence of NT‐3. A, Western blot analyses of protein extracts from slices of 3‐NP‐treated mice maintained under conditions that imitated electrophysiological recordings. Lines 1‐3 in the first row show the expression of p‐Tyr674/675‐TrkA in control and in response to NT‐3 and NT+ HFS (high K⁺) treatments. Lines 4‐6 exhibit phospho‐TrkA in the presence of CTX‐B. There were no changes in the phosphorylation level in any of the experimental conditions. B, Illustrates in box plot the percentage of p‐TrkA expression (p‐TrkA/Total TrkB) relative to control condition