Several hundred species of fish have evolved the ability to generate electric fields, which they use to navigate, communicate and home in on prey. But this ability comes at a cost – the electric field is generated continuously throughout life, so consumes a great deal of energy, and it can also attract predators which are sensitive to it. Electrogenic fish species therefore utilize various strategies to save energy and to minimize the likelihood of being detected. Some generate irregular pulses of electrical discharges whose rate can be modulated; others can also modulate the strength of the electric field.
The cellular and molecular mechanisms underlying one of these behavioural adaptations are now revealed in a beautiful study published in the open access journal PLoS Biology. It shows that in one species of electric fish, circadian cues and social encounters regulate the movements of proteins called voltage-gated sodium channels – which are crucial for generating the electric field – in cells of the electric organ. At night, low light levels and social interactions drive the insertion of sodium channels into the cell membranes, leading to a dramatic increase in the strength of the electric field.
Trafficking of membrane proteins is critical for proper cell function. It occurs continuously, because the proteins have a short life-span, and so need to be recycled, or “turned over”. All proteins are synthesized in a structure called the endoplasmic reticulum, then transferred to another, called the Golgi apparatus, where they are modified in various ways. Ion channels and neurotransmitter receptors are then packaged into vesicles, which are stored in reservoirs near the cell membrane. When needed, the vesicles fuse with the cell membrane, by a process called exocytosis, so that the proteins it contains can be inserted into it.
Ion channels generate the electrical impulses produced by neurons and muscle cells, so their insertion into the membrane, and removal from it, is one way of modulating the electrical properties of the cells. For example, one mechanism underlying synaptic plasticity, the process which is widely believed to underly learning and memory, is the insertion of AMPA receptors into the membrane. Receptor molecules are quickly shuttled from the cytoplasmic reservoir to the membrane, and inserted into it, in response to neuronal activity. This strengthens the connections between cells, by enhancing the response to subsequent synaptic signals.
To gain a better understanding of how environmental factors might lead to remodelling of cell membranes and the electrical properties of cells, neurobiologist Michael Markham of the University of Texas at Austin and his colleagues performed a series of experiments on the longtail knifefish (Sternopygus macrurus, below). This is an electric fish which inhabits the freshwater bodies of South America; it is largely nocturnal, engaging in social activities only as nightfall approaches. Like other electrogenic fish species, S. macrurus can modulate its electric field, and so is well suited to investigations of the physiological basis of behaviour.
The electric organ is located in the tail, and consists of specialized neurons or muscle cells called electrocytes. These cells can generate far stronger electrical impulses than those of normal nerve or muscle cells, but the impulses are initiated in the same way, by the flow of sodium ions into the cell through voltage-gated sodium channels. The organ discharges when all the electrocytes generate impulses simultaneously, and this coordinated activity produces an electric field which surrounds the fish. Some species can discharge fields of up to 500 volts, which are powerful enough to stun prey. Others, like S. macrurus, generate a much weaker discharge, of less than 1 volt. This is too weak to stun prey, but can be used for navigation, because nearby objects distort the field, and for communicating, because the discharge can be broadcast to others of the same species.
Markham and his colleagues first recorded the electrical discharges of S. macrurus, which were swimming freely in a large aquarium. The aquarium was equipped with electrodes at each end, and divided into two compartments connected by a narrow tube. This set-up enabled the researchers to record the electrical discharges both when the fish were alone and when they were in close contact with others. This initial experiment showed that there were large and regular fluctuations in the size of the discharge: it was at its weakest during the day, but its strength increased by approximately 40% at nighttime. It was also found that the fish abruptly increased the strength of their electrical signal when they encountered one another in the tank.
The researchers hypothesized that the stronger nighttime discharges occur because of an increase in the size of the impulses generated by the cells in the electrical organ. To test this, they dissected small sections of the fishes’ tails, both during the day and at night. The skin was then peeled back to expose the electric organ, and individual cells in it were impaled with microelectrodes so that the size of the electrical currents could be measured. Sure enough, they found that cells in the electrical organs isolated at night generated larger impulses than those in organs isolated during the day.
Another set of experiments established that the electrocytes in organs isolated at night generate larger impulses because their membranes contain more sodium channels than those of cells in organs isolated during the day. Because there are more channels, the number of ions flowing across the membrane is greater. As a result, each electrocyte produces larger nervous impulses, the organ discharge is bigger, and the electric field is stronger. This effect was found to occur in response to adrenocorticotropic hormone (ACTH), which is normally released into circulation by the pituitary gland. Injections of the hormone given during the day increased the size of the electrical discharge, whereas control injections of a salt solution did not.
Finally, the researchers carried out pharmacological experiments to confirm that ACTH increases electric feld strength by triggering an increase in the rate at which sodium channels are trafficked to the electrocyte membrane. The size of impulses generated by isolated electrocytes was found to increase with the addition of ACTH. The effect of the hormone was abolished by the anti-malarial drug chloroquine, which interferes with sodium channel trafficking and fusion of vesicles with the membrane, but not by Brefeldin A, which disrupts transport of proteins from the endoplasmic reticulum to the Golgi apparatus.
This study elucidates a hitherto unknown role for membrane protein trafficking, as well as a completely novel mechanism by which trafficking is regulated. Low light levels and social cues trigger the release of ACTH, which causes an increase in the rate at which sodium channels are inserted into the membranes of cells in the electrical organ. This leads to an increase in the strength of the electrical discharge, thus enabling the electrical signals to be transmitted over greater distances. This sequence of events occurs within minutes, and affects behaviour by triggering changes at the level of the whole organism. Such close links between molecules and behaviour are rare – a few have been worked out in simpler organisms such as fruit flies and nematode worms, but they are much more difficult to establish in vertebrate species.