Figure 6. 600 μM 4-aminopiridine (4-AP) changed the firing pattern of single spiking (SS) embryonic V1^R recorded at E14.5.

The firing pattern of embryonic V1^R was evoked by 2 s suprathreshold depolarizing current steps. (A) Representative traces showing the effect of 4-AP application (600 μM) on the firing pattern of SS V1^R recorded at E14.5. Note that the applications of 600 μM 4-AP evoked either a plateau potential (PP, A1) or repetitive spiking (RS, A2), both fully blocked by tetrodotoxin. (B) Bar plots showing the proportions of the different firing patterns observed in the presence of 600 μM 4-AP versus control recorded in SS V1^R at E14.5 (n = 14; N = 14). SS V1^R (gray), RS V1^R (red), mixed Events (ME) V1^R (purple), and PP V1^R (blue).

Figure 6—figure supplement 1. ${\mathit{I}}_{\mathit{N}\mathit{a}\mathit{p}}$ is present in embryonic V1^R recorded at E14.5.

(A) Representative trace of $I_{Nap}$ evoked by a slow depolarizing voltage ramp (70 mV/s, upper inset) in single spiking (SS) embryonic V1^R (lower inset). ${\displaystyle I_{Nap}}$ was isolated by subtracting currents evoked by depolarizing ramps in the presence of 1 μM tetrodotoxin (TTX) to the control current evoked in the absence of TTX (upper inset). (B) Voltage dependence of G_Nap conductance calculated from the trace shown in (A). The activation curve was obtained by transforming the current evoked by a depolarizing voltage ramp from –100 mV to 20 mV (70 mV/s) using the following equation: G_NaP = –I_Nap/((–Vh) + E_Na), where Vh is the holding potential at time t during a depolarizing voltage ramp and E_Na is the equilibrium potential for sodium (E_Na = 60 mV). The G_NaP/Vh curve was fitted with the following Boltzmann function: G = G_MAX/(1 + exp(–(V–V_HALF)/k)) (Boeri et al., 2018), where V_half is the Vh value for G_Nap half activation, k the slope factor of the curve, and G_max the maximum conductance. We found no significant difference between the values of V_half (Mann–Whitney test: p=0.8518) and of k (Mann–Whitney test: p=0.7546) obtained at E12.5 (Boeri et al., 2018) and those obtained at E14.5. At E14.5, V_half = –27 ± 5.1 mV and k = 7.73 ± 0.78 (n = 6).

Figure 6—figure supplement 2. ${\displaystyle I_{Kdr}}$ was inhibited by 4-aminopiridine (4-AP) in V1^R recorded at E14.5.

(A1) Representative examples of the total outward K⁺ currents obtained from ${\displaystyle V_H}$ = –100 mV (left traces), ${\displaystyle I_{Kdr}}$ (${\displaystyle V_H}$ = –30 mV, middle traces), and isolated ${\displaystyle I_A}$ (left traces) recorded in single spiking (SS) V1^R at E14.5. (A2) Current-voltage relationship of ${\displaystyle I_{Kdr}}$ (filled circle) and ${\displaystyle I_A}$ (open circle) in SS V1^R at E14.5. ${\displaystyle I-V}$ curves were obtained from currents shown in (A1). (B1) Representative example of the effect of 4-AP at 600 μM in V1^R at E14.5. (B2) Current-voltage curves in control condition and in the presence of 600 μM 4-AP. (B3) Bar plots showing the percentage of $I_{Kdr}$ inhibition evoked by 300 μM 4-AP application (n = 8) and 600 μM 4-AP application (n = 7). The percentages of ${\displaystyle I_{Kdr}}$ inhibition evoked by 300 μM 4-AP and 600 μM 4-AP applications were not significantly different (p=0.574). (C1) Representative example of the effect of 600 μM 4-AP on ${\displaystyle I_A}$ in V1^R recorded at E14.5. (C2) ${\displaystyle I-V}$ curves in control conditions and in the presence of 600 μM 4-AP. These curves were obtained from the traces shown in (B1). (C3) Bar graph showing the percentage of $I_A$ block elicited by 4-AP. 4-AP did not significantly block $I_A$ (Wilcoxon test p=0.11, n = 6).