If you feel overwhelmed in a crowd, spare a thought for electric fish. In their underwater world, the electric organ discharges (EOD) of each passing stranger can interfere with a fish's own EOD-generated view of its surroundings. Intrigued by this problem, Angel Caputi and his team from the Instituto de Investigaciones Biológicas Clemente Estable in Uruguay set out to investigate how electric fish stop the signals of neighbouring fish from jamming their own signals(p. 1122).
`An electric fish “illuminates” its surroundings in an electrical way,' Caputi explains. The electroreceptors in its skin detect changes in the fish's self-generated electric field when it passes nearby objects. Nerve fibres run from the electroreceptors to the electrosensory lobe in the fish's brain, and nestled deep inside this lobe are spherical cells. Spherical cells are highly specialized neurons that transmit information using a latency code: they fire only once, and the latency of this single spike is directly related to the electrical input that caused it. From previous work,the team had good reason to suspect that spherical cells enable electric fish to block jamming signals from other fish.
To take a closer look at the role of spherical cells in the avoidance of jamming, the team caught Gymnotus fish in lakes and creeks around Montevideo, Uruguay. Back in the lab, they inserted two tiny wires into a fish's brain to record the spherical cells' activity. Placing this fish in an aquarium, they restrained other live fish in various locations in the tank and recorded the fish's responses to its own EOD and those generated by the other fish. They discovered that after the fish responded to a self-generated EOD,it no longer responded to an EOD generated soon afterwards by a neighbouring fish. In other words, activation of a fish's electrosensory pathway by its own EOD blocks responses to subsequent EODs for a period of time.
But is this ability to block potential interference from other fish conferred by the fish's spherical cells? To find out, the team stimulated spherical cells in in vitro brain slices with stepped current at different delays and examined the cells' responses. Constructing voltage versus current plots, the team saw that the cells responded just as the live fish had: after responding to a pulse, spherical cells did not react to further stimuli for a long period. This suggests that the spherical cells enable Gymnotus fish to avoid jamming signals from other fish. The team also found that the potassium conductance of the spherical cell membrane increases after a spike, which causes the reduced excitability of the cell.
The team concludes that the intrinsic properties of the spherical cells'cause their low responsiveness to subsequent stimuli shortly after they have fired. `These cells create a time window that blocks interfering stimuli,'Caputi says. Cleverly, electric fish repeat their self-generated EODs at slightly longer intervals than the cells' low responsiveness window, allowing them to enjoy a continuous stream of self-generated information about their surroundings – even when they're in a crowd.