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. 2021 Apr 26;10:e62639. doi: 10.7554/eLife.62639

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© 2021, Boeri et al

This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited.

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Figure 1. — (A) Representative traces of voltage responses showing single spiking (SS) activity in E12.5 SS V1^R (A1), repetitive action potential firing in repetitive spiking (RS) V1^R (A2), mixed plateau potential activity (PP) and repetitive action potential firing in mixed event (ME) V1^R (A3), and PP activity in PP V1^R (A4). (B) Representative examples of the total outward K⁺ currents (IKV total) obtained from $V_{H}$ = –100 mV (left traces), of $I_{K d r}$ ( $V_{H}$ = –30 mV, middle traces), and of isolated $I_{A}$ (left traces) recorded at E12.5 in SS V1^R (B1), RS V1^R (B2), ME V1^R (B3), and PP V1^R (B4). Voltage-dependent potassium currents were evoked in response to 10 mV membrane potential steps (200 ms) from –100 or from –30 mV to +40 mV (10 s interval between pulses). V1^R were voltage clamped at $V_{H}$ = –60 mV. A prepulse of –40 mV (300 ms) was applied to activate both $I_{A}$ and $I_{K d r}$ . $I_{K d r}$ was isolated by applying a prepulse of -30 mV (300 ms) to inactivate $I_{A}$ (B1 inset). $I_{A}$ was isolated by subtracting step-by-step the currents obtained using a prepulse of -30 mV ( $V_{H}$ = –30 mV) from the currents obtained using a prepulse of –40 mV ( $V_{H}$ = –100 mV). (C) Current-voltage relationship ( $I - V$ curves) of $I_{K d r}$ (filled circles) and of $I_{A}$ (open circles) recorded in SS V1^R (C1), RS V1^R (C2), ME V1^R (C3), and PP V1^R (C4). $I - V$ curves were obtained from currents shown in (B1), (B2), (B3), and (B4). Note that $I - V$ curves are similar between SS V1^R, RS V1^R, ME V1^R, and PP V1^R. (D) Bar graph showing maximal $G_{K d r}$ value (Max $G_{K d r}$ ) in SS V1^R at E12.5 (n = 9; N = 9; gray bar) and at E14.5 (n = 10; N = 10 gray bar), and in RS V1^R (n = 7; N = 7; red bar), ME V1^R (n = 3; N = 3 purple bar), and PP V1^R at E12.5 (n = 7; N = 7 blue bar) was calculated from $I_{K d r}$ at $V_{H}$ = + 20 mV, assuming a K⁺ equilibrium potential of –96 mV. There is no significant difference in $G_{K d r}$ between SS V1^R and RS V1^R, while $G_{K d r}$ is significantly smaller in PP V1^R as compared to $G_{K d r}$ SS V1^R and RS V1^R was significantly higher in SS V1^R at E14.5 than in SS V1^R, RS V1^R, and PP V1^R at E12.5 (Kruskal–Wallis test p<0.0001; SS V1^R versus RS V1^R at E12.5, p=0.5864; SS V1^R versus PP V1^R at E12.5, p=0.0243; RS V1^R versus PP V1^R at E12.5, p=0.0086; E14.5 SS V1^R versus E12.5 SS V1^R, p=0.0048; E14.5 SS V1^R versus E12.5 RS V1^R, p=0.0384, E14.5 SS V1^R versus E12.5 PP V1^R, p<0.0001). The increase in $G_{K d r}$ density between E12.5 and E14.5 is likely to be due to the increase in neuronal size (input capacitance; Figure 2A). Indeed, there was no significant difference (Mann–Whitney test, p=0.133) in $G_{N a p}$ between SS V1^R at E12.5 (n = 9; N = 9 gray bar) and at E14.5 (n = 10; N = 10 gray bar). (E) Bar graph showing the maximal Max $G_{N a p}$ value ( $M a x G_{N a p}$ ) in SS V1^R at E12.5 (n = 9; N = 9 gray bar) and E14.5 (n = 10; N = 10 gray bar), and in RS V1^R (n = 8; N = 8 red bar), ME V1^R (n = 3; N = 3 purple bar), and PP V1^R (n = 6; N = 6 blue bar) at E12.5. Max $G_{N a p}$ was calculated from maximal $I_{N a p}$ value measured on current evoked by assuming a Na⁺ equilibrium potential of +60 mV. There was no difference in $G_{N a p}$ between RS V1^R and PP V1^R. On the contrary, $G_{N a p}$ measured in SS V1^R at E12.5 or at E14.5 was significantly smaller as compared to $G_{N a p}$ measured at E12.5 in RS V1^R or in PP V1^R measured at E12.5 and E14.5 in SS V1^R were not significantly different (Kruskal–Wallis test p<0.0001; E12.5 SS V1^R versus E12.5 RS V1^R, p=0.0034; E12.5 SS V1^R versus E12.5 PP V1^R, p=0.0006; E12.5 RS V1^R versus E12.5 PP V1^R, p=0.5494; E14.5 SS V1^R versus E12.5 SS V1^R, p=0.5896; E14.5 SS V1^R versus E12.5 RS V1^R, p=0.0005; E14.5 SS V1^R versus E12.5 PP V1^R, p<0.0001). (F) Histograms showing the $G_{N a p}$ / $G_{K d r}$ ratio in SS V1^R at E12.5 (n = 9; gray bar) and E14.5 (n = 10; green bar) and in RS V1^R (n = 8; red bar), ME V1^R (n = 3; purple bar), and PP V1^R (n = 6; blue bar) at E12.5. Note that the $G_{N a p}$ / $G_{K d r}$ ratio differs significantly between SS V1^R, RS V1^R, and PP V1^R at E12.5, while it is not different between SS V1^R recorded at E12.5 and at E14.5 (Kruskal–Wallis test p<0.0001; SS V1^R versus RS V1^R at E12.5, p=0.0367; SS V1^R versus PP V1^R at E12.5, p<0.0001; RS V1^R versus PP V1^R at E12.5, p=0.0159; E14.5 SS V1^R versus E12.5 SS V1^R, p=0.2319; E14.5 SS V1^R versus E12.5 RS V1^R, p=0.0017; E14.5 SS V1^R versus E12.5 PP V1^Rp<0.0001). Data shown in (A) and (B) were used to calculate $G_{N a p}$ / $G_{K d r}$ ratio shown in (C) (*p<0.05, **p<0.01, ***p<0.001).