SNI induced mechanical allodynia, hippocampus-dependent memory impairment and decreased GLP-1andGLP-1R in mice
To evaluate development of mechanical allodynia after SNI surgery, the MPWT was tested on day 0(baseline),day 3,day 7,day 10,day 14,day 21 after the surgeries(Fig. 1B).The results showed that MPWT was significantly decreased in SNI mice ,compared to sham mice.
Next, we evaluated the novel-object recognition memory via the NORT. In the NORT, no significant difference was found in the total time spent on identical objects between the Sham and SNI groups during the training phase (Fig. 1C). In the testing stage, the mice spent more time on the novel object than on the familiar object in the Sham group (Fig. 1D).However, the time spent on the novel and familiar objects was similar in the SNI group. Furthermore, the discrimination ratio in the Sham group was higher than that in the SNI group (Fig. 1E).These results demonstrate that recognition memory was impaired in the mice with neuropathic pain.
In addition to this, we uncovered that the GLP-1/GLP-1R axis was inhibited in the murine hippocampus induced by neuropathic pain (Fig. 1F, G). West Blot showed that the levels of GLP-1 and GLP-1R were decreased in murine hippocampus.
SNI caused disturbance of inflammatory signaling, neuroinflammation, and decreased synaptic associated proteins in the murine hippocampus
We found that the AMPK/NF-κB signaling was disturbed in the murine hippocampus with cognitive dysfunction. Neuropathic pain could inhibit the activation of AMPK by inhibiting its phosphorylation at the Thr172 site (Fig. 2A) but activate its downstream NF-κB p65 by increasing the level of p- NF-κB p65 (Fig. 2B) compared with the Sham group. Furthermore, activating NF-κB p65 could induce neuroinflammation by up-regulating the expression of pro-inflammatory cytokines, including mature IL-1β (IL-1β p17) (Fig. 2C) in the hippocampus of mice. Also, over expressed IL-1β could decrease the level of synaptic associated proteins, such as PSD95 (Fig. 2D) and Arc (Fig. 2E), thus inducing memory impairment, compared with Sham mice.
Ex-4, a GLP-1R agonists, improved memory impairment caused by SNI in mice
First, we evaluated the effects of Ex-4 on mechanical allodynia and memory deficit in mice with neuropathic pain after SNI. In the MPWT test, we found that both vehicle and Ex-4 application had no effect on allodynia in the SNI mice compared with that in the Sham + Veh and Sham + Ex-4 groups (Fig. 3B).
In the training phase of the NORT, no difference was found in the total time spent on identical objects among the four groups (Fig. 3C). In the testing stage, the mice spent more time on the novel object than on the familiar object in the Sham + Veh, Sham + Ex-4 and SNI + Ex-4groups.However, no difference was found in the time spent with novel and familiar objects in the SNI + Veh group(Fig. 3D). In addition, the DR in the SNI + Veh group was lower than that in the Sham + Veh group, but Ex-4 application could increase the DR in SNI mice compared with that in SNI + Veh mice(Fig. 3E).Taken together, these results suggested that Ex-4 can ameliorate deficits in the recognition memory caused by neuropathic pain.
Ex-4 improved inflammatory signaling, neuroinflammation, and upregulated synaptic proteins in mice hippocampus
Next, we explored whether Ex-4 treatment could regulate the inflammatory signaling pathway and improve inflammation, the decreased synaptic associated-proteins. It was found that, in the murine hippocampus with memory deficits, Ex-4 could reverse the down-regulated levels of p-AMPKThr172 compared with Veh group (Fig. 4A), and could also inhibit the phosphorylation of NF-κB p65 downstream of AMPK (Fig. 4B).Furthermore, Ex-4 could attenuate neuroinflammation by down-regulating the secretion of IL-1β p17 (Fig. 4C).The results suggested that Ex-4 restored the decreased levels of PSD95 (Fig. 4D) and Arc (Fig. 4E) in the murine hippocampus, which was consistent with the memory protection of EX-4.
Ex(9–39),a GLP-1R antagonist, reversed the memory protection of Ex-4 in SNI mice
We further verified the memory protective effect of activating GLP-1R in SNI mice via administration with Ex-4 and Ex(9–39).In the MPWT ,the results showed both Ex-(9–39) and Ex-4 + Ex(9–39) treatment did not affect the decrease of MPWT in SNI mice compared with that in the SNI + Veh group(Fig. 5B).In the NORT, the four groups spent a similar total time exploring two identical objects during the training phase(Fig. 5C). However, in the testing stage, the SNI + Ex-4 + Veh group spent more time on the novel object than on the familiar object compared with the SNI + Veh and SNI + Ex-4 + Ex(9–39) groups(Fig. 5D).It was found that Ex(9–39) could abrogate the increase in DR induced by Ex-4 in mice(Fig. 5E).
Ex(9–39) abolished the anti-inflammatory effect of Ex-4 in thehippocampus of SNI mice
First, we reported that Ex-4 could activate AMPK by increasing the level of p-AMPKThr172, however, Ex(9–39) could inhibit the activation of AMPK induced by Ex-4 in SNI mice(Fig. 6A). Similarly, Ex(9–39) could also upregulate the decreased level of p-NF-κBp65 downstream of AMPK caused by Ex-4 in SNI mice(Fig. 6B).Moreover, Ex(9–39) could reverse the anti-inflammatory effect of Ex-4 by promoting the expression of IL-1β p17 (Fig. 6C).We also uncovered that Ex(9–39) could reversed the increased synaptic associated proteins PDS95 (Fig. 6D) and Arc (Fig. 6E) induced by Ex-4 in mice with memory impairment caused by neuropathic pain.
CC, an AMPK inhibitor, reversed improved recognition memory caused by Ex-4 in SNI mice
In order to further explore the mechanism of activating GLP-1R to improve recognition memory dysfunction caused by neuropathic pain, we applied Ex-4 and CC in SNI mice. We found that both CC and Ex-4 + CC did not influence the MPWT in SNI mice as compared to that in the SNI + Veh mice (Fig. 7B). In the NORT, during the training phase, no significant difference was found in the total time spent on identical objects among the four groups (Fig. 7C).However, the SNI + Ex-4 group spent significantly more time on the novel object than on the familiar object compared to the SNI + Veh or SNI + Ex-4 + CC groups(Fig. 7D).About DR, the results demonstrated that application with CC could reverse the increased the DR induced by Ex-4 in SNI mice (Fig. 7E).
CC weakened the neuroprotection of Ex-4 in SNI mice
We used CC to verify whether the improved recognition memory effect via activation of GLP-1R was mediated by regulating the AMPK/NF-κB signaling pathway. We discovered that activating GLP-1R could inhibit the activation of NF-κB via AMPK. On the contrary, when an AMPK inhibitor was administered in mice, it was found that the protein level of p- NF-κB p65 in the SNI+Ex-4+CC group was significantly higher than that in the SNI+Ex-4+Veh group(Fig 8A).Furthermore, activation of GLP-1R by Ex-4 treatment in the mice with memory dysfunction induced by SNI could ameliorate neuroinflammation, but CC could abolish this effect (Fig 8B).We also tested synaptic-associated proteins, the results suggested that the combined application of Ex-4 and CC could downregulate the contents of PSD95(Fig 8C) and Arc(Fig 8D) proteins compared that of Ex-4 alone in SNI mice. The results suggested that activating GLP-1R might rescue the decreased level of synaptic associated proteins via improving neuroinflammation through regulation of AMPK/NF-κB signaling.