The internal brain dynamics that link sensation and action are arguably better studied during natural animal behaviors. Here, we report on a novel volume imaging and 3D tracking technique that monitors whole brain neural activity in freely swimming larval zebrafish (Danio rerio). We demonstrated the capability of our system through functional imaging of neural activity during visually evoked and prey capture behaviors in larval zebrafish.

The hyperpolarization-activated cation channels (Ih) play a distinct role in rhythmic activities in a variety of tissues, including neurons and cardiac cells. In the present study, we investigated whether Ca 2؉ can permeate through the hyperpolarization-activated pacemaker channels (HCN) expressed in HEK293 cells and Ih channels in dorsal root ganglion (DRG) neurons. Using combined measurements of whole-cell currents and fura-2 Ca 2؉ imaging, we found that there is a Ca 2؉ influx in proportion to Ih induced by hyperpolarization in HEK293 cells. The Ih channel blockers Cs ؉ and ZD7288 inhibit both HCN current and Ca 2؉ influx. Measurements of the fractional Ca 2؉ current showed that it constitutes 0.60 ؎ 0.02% of the net inward current through HCN4 at ؊120 mV. This fractional current is similar to that of the low Ca 2؉ -permeable AMPA-R (␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor) channels in Purkinje neurons. In DRG neurons, activation of Ih for 30 s also resulted in a Ca 2؉ influx and an elevated action potentialinduced secretion, as assayed by the increase in membrane capacitance. These results suggest a functional significance for Ih channels in modulating neuronal secretion by permitting Ca 2؉ influx at negative membrane potentials.