Researchers from the Wellcome Trust Sanger Institute demonstrate how action potentials are recorded from brain slices, and how long-term potentiation is measured.

Learning and memory are strongly associated with electrical activity in neurons, particularly in the hippocampus. We can study learning and memory in hippocampal neurons by examining how they respond to different patterns of electrical input. In this demonstration, researchers from the Wellcome Trust Sanger Institute will demonstrate how neurons are prepared for analysis, the equipment used to record electrical activity, and examine how different patterns of electrical activity are thought to induce long-term potentiation. We will begin with a slice of brain tissue that contains thousands of hippocampal neurons. The slice is being removed from a storage chamber that replicates the physiology of the brain in the body… The storage chamber Here we can see a storage chamber, which is used to store brain slices. It is important to store the slices in an environment that replicates the physiology of the brain in the body. The storage chamber contains the necessary salts and glucose to simulate these conditions. It has a constant flow of fresh artificial cerebrospinal fluid, and is pre-gassed by a mixture of 95% oxygen and 5% carbon dioxide. Neurons need oxygen to breathe and carbon dioxide to maintain proper acidity levels. The temperature in the chamber is close to body temperature – 30 ° C. We have removed one of the slices from the chamber, and we will transfer it to a biochip. Once the brain slice is inside the biochip, we can record the electrical activity of neurons. The biochip Here we see the Biochip, which has a glass well in the centre. 64 electrodes are imprinted on the glass surface of the Biochip, and they allow us to record tiny electrical potentials from our brain slice. You can see the hippocampal slice sitting on top of the electrodes. Then we put the Biochip inside an amplifier box, which will amplify the electrical signals recorded by electrodes. Next, we place a nylon grid attached to a flattened platinum wire on the top of the brain slice, which will hold it firmly in place. If we look at the lid of the amplifier, we can see that it has a series of pins around the inner surface. Each of these pins makes contact with each of the electrodes on our biochip. Once the lid is in place, we can connect the amplifier to a computer, and we are almost ready to record. Recording the electrical activity of neurons The biochip is connected to our computer. As well as recording electrical output from hippocampal neurons, the biochip allows us to provide input by stimulating neuronal pathways. In this recording, every little picture represents responses of neurons in different areas of the hippocampal slice to a voltage pulse applied to electrode 26. Let us take a closer look at what happens to electrical activity recorded by one particular electrode. If we record electrical activity in response the absence of stimulation, it will essentially be a flat line reflecting negligible activity in the hippocampal slice. When I stimulate the neurons, I can see that the potential shows a big change. The first little wave here is a stimulus artefact. It is a marker for when the electrical stimulation is applied. The broad waveform here is the excitatory postsynaptic potential, which is a measure of the postsynaptic potentials coming from thousands of neurons in the vicinity of the electrode. If we stimulate the slice infrequently, say once every few seconds, the size of the response will not change appreciably. However, we will see in a moment that by altering the stimulation pattern to the neurons we can produce major differences in the postsynaptic response. Tetanic stimulation Next, I will apply a tetanic stimulation to the neuron. This is a series of electrical pulses given in close succession on a millisecond timescale. We can see that electrical responses generated by neurons change quite dramatically. Re-applying original stimulation – the neurons have learned! Now, let’s go back and re-apply the original low-frequency stimulation. We can see that the neurons’ response to the original stimulation has changed considerably. The neurons now show a much larger response to the same stimulation as before. This means that they are communicating better – they have learned. Long term effects If we revisit the brain slice an hour later by exciting it with the same stimulation, let’s see what happens… We can see that the neurons’ response to the stimulation is still much larger than before we applied the tetanic stimulation. Long-term potentiation The large burst of stimulation provided by the tetanic stimulation excitation of the neurons somehow made the neurons better communicators. Before tetanic stimulation, the neurons showed a relatively modest response to electrical input. After tetanic stimulation, the same input produced a large response. This phenomenon is called long-term potentiation and is thought to be critical to many types of learning.

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