Figure 6. I_NaR amplitude strongly correlates with I_NaT amplitude, but I_NaR expression and amplitude also show intrinsic variability in GCs.

A, scatter plot of I_NaR amplitude as a function of I_NaT amplitude. I_NaR and I_NaT amplitudes were measured at the peak of the respective I–V relationships (normally −40/−35 mV and −20 mV, respectively). The straight line is the linear regression to data points (slope factor = 35 pA nA⁻¹; R = 0.69, P < 1 × 10⁻⁴). B, scatter plot of I_NaR amplitude as a function of the postnatal age of the animal (in days after birth). The straight line is the linear regression to data points (slope factor = 0.51 pA day⁻¹; R = 0.062, P = 0.62). Data in A and B are from 67 cells. In both cases, I_NaR amplitude values are ‘net’ values, calculated as peak current minus I_end (see the text). C and D, I_NaR and I_NaT, respectively, recorded, at the peak of the I–V relationships, in two exemplary neurones (Ca and Da, cell B3428, representative of 7 cells; Cb and Db, cell B3327; both cells were classified as group 1 cells). Note the similar size of I_NaT, the presence of a clear I_NaR in Cb, and its almost complete absence in Ca. The two cells illustrated here are identified by the two filled symbols in A. Ca, inset, average tracing obtained from 7 cells (four group 1 cells, three group 2 cells) that were judged as devoid of a measurable I_NaR (same voltage protocol as in the main panel). Note that, at the time point at which an I_NaR peak would be expected (short horizontal bar), current amplitude was not different from that of the late I_NaP component (long horizontal bar). In the same 7 cells, the average I_NaT amplitude at the peak of the I–V relationship was −301.0 ± 47.0 pA. Calibration bars in the inset, 20 ms, 15 pA.