3.1. Increased sharp wave-ripple (SW-R) incidence in B6N and 129 mouse strain
Rodent acute brain slices obtained from the ventral-to-mid portion of the hippocampus spontaneously generate SW-R in the CA3-CA1 axis of the hippocampus (Figure 1a) (Maier et al. , 2003; Çalişkan et al. , 2016). Simultaneous local field potential recordings from CA3 and CA1 regions in slices of mouse strains B6J, B6N and 129 revealed substantial differences in the incidence and CA3-CA1 network interactions during SW-R (Figure 1b-f). We detected an increased incidence of CA3 SW (Figure 1c: F(2,57)=4.457, p=0.016) in the B6N substrain in comparison to 129 strain (p=0.005) while CA1 SW incidence (Figure 1d: H(2)=13.413, p=0.001) was significantly higher in both B6N (p<0.05) and 129 (p<0.05) mouse strains compared to B6J indicating a potential difference in CA3-CA1 communication during SW-R events. Indeed, propagation of SW events across CA3-CA1 regions (Figure 1e: F(2,57)=6.419, p=0.003) was more efficient in the 129 strain which exhibited much less propagation failures compared to other two strains (129 vs. B6J: p<0.001 and 129 vs. B6N: p=0.049). Moreover, correlation of co-occurring SW events in the CA3 and CA1 (Figure 1f: F(2,57)=3.615, p=0.033) were higher in the 129 (p=0.016) and B6N (p=0.038) strains in comparison to B6J strain. These results indicate that CA3-CA1 network interactions during SW-R are less efficient in the B6J substrain compared to other two strains evident by lower CA1 SW incidence and CA3-CA1 SW correlation.
Further analysis of SW-R characteristics revealed region-specific strain effects (Figure 1g-o). In the CA3, SW area (Figure 1h: H(2)=12.837, p=0.002) was significantly increased in B6N strain in comparison to other two strains (B6J vs. B6N: p<0.05, B6N vs. 129: p<0.05). No alterations were evident for signal-to-noise ratios (Figure 1i: H(2)=0.290, p=0.865). Ripple amplitudes (Figure 1j: H(2)=7.390, p=0.025) were significantly enhanced in the B6N compared to 129 (p<0.05) whereas ripple frequencies (Figure 1k: H(2)=13.060, p=0.001) were significantly lower in comparison to B6J (p<0.05) and 129 (p<0.05). In the CA1, SW area (Figure 1l: H(2)=6.477, p=0.039), signal-to-noise ratios (Figure 1m: H(2)=7.119, p=0.028) and ripple amplitudes (Figure 1n: H(2)=12.452, p=0.002) were significantly increased in B6N in comparison to B6J substrain (p<0.05 for all parameters). Lastly, no differences were detected in ripple frequencies (Figure 1o: H(2)=3.084, p=0.214). Together, both B6N and 129 strain appear to have increased propensity for SW-R generation/propagation whereas slices obtained from B6N seem to generate SW and ripples with larger amplitudes.