(A), typical voltage recording from a regular pacemaker dopaminergic neuron depicting the parameters extracted to characterize spontaneous activity patterns. Interspike intervals (ISI1, ISI2…ISIi…ISIn) were averaged to calculate the ISIavg while the coefficient of variation of the n ISIs (CVISI) was computed from the standard deviation (SD) of the n ISIs and ISIavg. (B), voltage traces showing the bursting (high CVISI, left, red), irregular (moderate CVISI, center, green) and regular (low CVISI, right, blue) patterns of spontaneous activity observed during the first four postnatal weeks. (C), box and whisker plot representing ISIavg vs postnatal age. (D), box and whisker plot representing CVISI vs postnatal age. Two CVISI threshold values (20 and 80%, horizontal dotted lines) separated three classes of activity patterns: low CV (CVISI < 20%, black circle), medium CV (20% < CVISI < 80%, light gray triangle), high CV (CVISI > 80%, dark gray square). The dark gray square, light gray triangle and black circle on the right indicate the symbols used to represent the three different CV classes (high CV, medium CV and low CV, respectively) in panel E. (E), line and scatter plot representing the evolution of the percentages of high CV (dark gray square), medium CV (light gray triangle) and low CV (black circle) activity patterns vs postnatal age. Scale bars: A, vertical 10 mV, horizontal 500 ms; B, vertical 20 mV, horizontal 2 s. Horizontal dotted lines in A and B indicate −60 mV. For all box and whisker plots, boxes represent the median, first and third quartile, error bars correspond to 10 and 90%, the thick line corresponds to the mean, and all outliers are represented. Colored boxes and symbols in C, D and E match the age and type of firing of the colored traces presented in B (red for P2, bursting; green for P6, irregular; blue for regular, P16).