LTP induced by TBS in the ACC of female mice
LTP plays an important role in cortical excitation in chronic pain [8, 18, 19]. In our previous work, the intercellular network plasticity in the ACC was studied focuses on the adult male mice, and the LTP induced by TBS was mainly recorded in the ACC of male mice by MED64 recording system [17, 20, 21]. Here we performed the same protocol to study the network plasticity in the ACC of adult female mice. We placed a slice containing the ACC of female mice on the MED64 chamber and ensured the recording microelectrodes covered the ACC area (Figure 1a). One channel located in the deep layer of ACC was chosen as stimulation site (Figure 1a, red circle). The fEPSP slope of all activated channels were recorded from superficial to deep layer around the stimulation site for 30 min as a baseline (Figure 1b). After recording 0.5 h stable baseline, we applied TBS to evoke network LTP in the ACC. Consistent with previous reports from male mice, we found that TBS induced L-LTP (late-phase LTP) in the ACC of female mice (n=14 slices/9 female mice, Figure 1c-1e). In one typical female sample slice, there were 11 activated channels, among which there were 9 channels showed L-LTP (165.2 ± 11.6% of the baseline, Figure 1c) and 1 channel showed E-LTP (early-phase LTP) (Figure 1d), while 1 channel showed none-LTP (Figure 1e). The final averaged slope of all 11 channels from the typical sample slice of the female mouse was 152.4 ± 13.8% of the baseline at 3 h after applying TBS (Figure 1f). These results are similar with that in our previous work performed in male mice [14, 20].
No sex difference of LTP in the ACC between adult female and male mice
To investigate whether there is any sex difference in LTP within ACC between male and female mice, we recorded LTP in both female and male mice by performing MED64 recording system. In all 194 activated channels that we recorded from 14 slices from 9 female mice, we found that there were 108 channels with L-LTP, 17 channels with E-LTP and 69 channels showed none-LTP and the induction rates of three different types of channels were 54.9 ± 10.4%, 10.3 ± 4.0% and 34.8 ± 9.2%, respectively (Figure 2a-2c). In the male mice, there were 101 channels activated from 8 slices in 5 male mice. We found there were 56 channels with L-LTP, 17 channels with E-LTP and 28 channels showed none-LTP and the induction rates of three different types of channels were 52.8 ± 14.2%, 22.1 ± 7.8% and 25.1 ± 9.9%, respectively (Figures 2d-2f).
The final averaged fEPSP slope of all activated channels was 131.6 ± 9.3% of the baseline at 3 h in female mice and 134.5 ± 11.5% of the baseline at 3 h in male mice (Figures 2g and 2h). Meanwhile, the final averaged fEPSP slope of all channels with L-LTP was 160.4 ± 4.4% of the baseline at 3 h in female mice and 150.4 ± 5.1% of the baseline at 3 h in male mice (Figure 2h). There was no difference in fEPSP slope either in all activated channels or channels with L-LTP between the female mice and male mice. Further analysis of the number of channels with L-LTP, E-LTP and none-LTP respectively from 14 slices from 9 female mice showed that there were 7.3 ± 1.6 channels with L-LTP, 1.6 ± 0.4 channels with E-LTP, and 2.0 ± 1.7 channels with none-LTP in each slice of female mice on average (Figure 2i). In male mice, there were 7.0 ± 1.9 channels with L-LTP, 3.8 ± 1.6 channels with E-LTP and 3.5 ± 1.1 channels with none-LTP in each slice of male mice on average (Figure 2i). There was no statistical difference in the number of different types of channels between female and male mice. Taken together, our results suggested that there was no sex difference of LTP in the ACC between female and male adult mice.
Silent synapses were recruited after the induction of LTP in the ACC of female and male mice
Previous studies showed that some silent responses were recruited after L-LTP induction. To determine whether there was any sex difference in recruited synapses between female and male mice, we analyzed all recruited channels from female and male mice, respectively. In figure 3, it showed a spatial distribution and the change of fEPSP amplitude of recruited channels in a typical slice of female mice. The recruited channels mainly located on the edge of the activated area of female mice (Figure 3a and 3b) and the amplitude which was about 0 μV during baseline would increase over time after applying TBS protocol (Figure 3c and 3d).
Only a portion of slices would successfully recruit silent channels after L-LTP induction. Therefore, in the figure 4, the spatial distribution of all recruited channels was showed from 7 slices of 6 female mice and 5 slices of 3 male mice. In both of female and male mice, the recruited channels were mainly located on the edge of the basal active area (Figure 4a-4d). Then, we also did an in-depth analysis of the number and fEPSP amplitude of recruited channels to see whether there was any sex difference between female and male mice. The final averaged fEPSP amplitude of all recruited channels was 9.3 ± 2.5 μV and 9.9 ± 2.6 μV of the baseline at 3 h in female mice and male mice, respectively (Figure 4e). There was no statistical difference in the fEPSP amplitude of recruited channels between female and male mice. After L-LTP induction, 2.0 ± 0.6 channels would be recruited at 3 h in each slice of female mice on average which was similar to that in male mice (2.6 ± 1.0 recruited channels, Figure 4f). These results showed that both the recruited fEPSP amplitude and the number of the recruited channels had no general difference in female and male mice.
LTD in male and female mice
LTD plays an important physiological role in pain by weakening the synaptic connections [21, 22]. In order to study possible sex-related difference in LTD, we recorded LTD in the ACC of adult female and male mice. We recorded the fEPSPs baseline of all activated channels for 15 min before LTD inducing protocol wad applied (LFS: 1 Hz, 900 pulses, with the same intensity as baseline recording). As shown in Figure 5, we got two different types of synaptic plasticity in all activated channels: channels with a slope decrease of more than 20% at 30 min represented LTD and channels that the fEPSP slope did not change after LFS represented none-LTD (Figure 5a). In a typical sample slice of a female mouse, there were 11 channels being activated, including 4 channels with LTD and 7 channels with none-LTD after LFS (Figure 5b and 5c). The final averaged slope of all 11 channels from the typical female sample slice was 83.3 ± 7.6% of the baseline at 30 min after applying LFS (Figure 5d).
Next, we compared the sex difference of LTD in female and male mice. There were 199 activated channels from 13 slices of 6 female mice and 266 activated channels from 16 slices of 6 male mice, respectively. In these activated channels, the induction rate of channels with LTD was 33.7 ± 8.5% in female mice and 53.5 ± 7.7 in male mice (P<0.05, Figure 6a-6d). Although there was no difference in the final averaged slope of all channels with LTD between male and female mice after applying LFS on stimulation site (Figure 6f), the final averaged slope of all activated channels from 16 slices of 6 male mice (78.8 ± 2.8% of the baseline) was not as high as that from 13 slices of 6 female mice (87.6 ± 4.3% of the baseline, P<0.05, female mice vs male mice, Figure 6e and 6f).
In addition, we found that when there was no difference in the total number of activated channels between female and male mice, the number of channels with LTD in each slice of male mice on average (9.2 ± 0.4 channels) was significantly higher than that in the female mice (4.5 ± 1.2 channels, P<0.05, Figure 6g). These results suggested that LFS was more likely to induce LTD in the ACC of male mice than that in female mice. This difference was mainly concluded from the number of channels with LTD.
Silent synapses appeared in the ACC of adult mice after LFS
There are hypotheses indicate that functional synapses can be transformed into silent synapses by endocytosis and inactivation of postsynaptic membrane α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor [23-26]. In the present study, we found that some of active channels became silent after LFS. As shown in figure 7a and 7b, the activated area was reduced after LFS in one typical slice from female mice. At 30 min after LFS, the amplitudes of the channels that were activated during baseline were decreased to about 0 μV (Figure 7c). Next, we compared the number of silent channels that appeared after LFS in the ACC between male and female mice. There are 3.3 ± 1.5 silent channels in each slice of female mice on average and this result was similar to that in male mice (3.3 ± 0.8 silent channels, Figure 7d).