Learning is a complex process, involving subtle changes in the properties of neurons and the connections between them. But trying to determine what changes occur in the nervous system during learning, and relating a specific neural network to a behaviour, is difficult in intricate vertebrate brains. So Brian Burrell and Christie Sahley, from Purdue University, turned their attention to learning processes in a much simpler animal - the medicinal leech. These little bloodsuckers' nervous systems are far less complex than those of vertebrates, so it is easier to investigate how transmission at the connection between two neurons - the synapse - changes and may contribute to learning.
Burrell and Sahley chose a circuit that receives inputs from touch and pressure neurons on the leech's skin. The circuit is involved in the body shortening reflex, which the leech uses when it is touched and feels threatened. The touch and pressure cells transmit most of their signal via a connecting cell to the S-cell, a neuron in the central nervous system that is important for different forms of learning. Some of the branches from the touch and pressure cell bypass the connecting cell and link directly to the S-cell.
The team wanted to investigate the conditions under which the synapses between pressure and S-cells, and touch and S-cells, showed long-term potentiation - a maintenance or strengthening of a signal across a synapse over time - or long-term depression, which is a weakening of a signal across a synapse over time. Long-term potentiation and long-term depression are thought to contribute to learning and memory by reinforcing or weakening synaptic connections, respectively.
The team chose two touch or two pressure cells and sent each a different signal. One of the signals was tetanising, so the synapse was working at its maximum rate, and the other was not. They then measured the output that was occurring in the S-cell, and again 1h later, to see if long-term potentiation or long-term depression had occurred. For both types of synapse, either pressure to S-cell or touch to S-cell, they found that the changes depended on the signal from the touch or pressure cell.
At the synapse between pressure and S-cells, long-term potentiation occurred only when the signal was tetanising and the individual pulses in the signal were small in amplitude. They also found that the signal in the S-cell was greater after 1h. And when they blocked a type of receptor called the NMDA receptor with AP-5 in the neuron's membrane, long-term potentiation was also prevented. This shows that the leech's synapses can change their properties according to the signal they receive; similar to vertebrate neurons.
At the touch-S-cell synapse, a tetanising pulse caused long-term potentiation but only on one subset of the synapses tested. The signal size had no effect. Long-term depression occurred when the input was non-tetanising, but on a different subset of synapses. It was also blocked by AP-5. The leech's synapses were showing similar long-term potentiation and long-term depression to that seen in vertebrates. This means that researchers can study these processes far more easily in the simple leech to help understand how vertebrates might learn.
Burrell and Sahley don't yet know whether the synapse changes contribute to learning in the leech but hope that they will make more tantalising discoveries about the workings of neurons in our brains thanks to these little bloodsuckers.
- © The Company of Biologists Limited 2004