Spontaneous firing rate and regularity are modulated by HCN channel activity. (A and B) Representative traces of spontaneously active calyx terminals in the presence of ZD7288 or cAMP, respectively. The broken line indicates 0 mV. (C) The percentage of calyx terminals that fired spontaneous action potentials. The number of terminals is indicated for each condition. Note that inhibition of Ih resulted in fewer spontaneously active calyx terminals. (D) The mean spike rate plotted for four conditions revealed that cAMP increased the mean rate. *, P < 0.05. The number of samples is shown inside the bars for each condition. Error bars indicate mean ± SEM. (E) Wild-type, control calyx terminals fired either regularly or irregularly. The broken line indicates a CV of 0.25. Inhibition of Ih with ZD7288 resulted in irregular firing, whereas cAMP resulted in only regular firing. This cAMP effect was the result of modulation of Ih, as it could be blocked by ZD7288. (F) A plot of Gh at rest versus spike rate shows a positive correlation. The linear fit indicates a statistically significant correlation (P < 0.01). (G) A plot of Gh at rest versus CV shows a similar statistically significant correlation (P < 0.05).
