Information in the brain is thought to be encoded as complex patterns of electrical impulses generated by thousands of neuronal cells. Each impulse, known as an action potential, is mediated by currents of charged ions flowing through a neuron’s membrane. But how the ions pass through the insulated membrane of the neuron remained a puzzle for many years. In 1976, Erwin Neher and Bert Sakmann developed the patch-clamp technique, which showed definitively that currents result from the opening of many channel proteins in the membrane1. Although the technique was originally designed to record tiny currents, it has since become one of the most important tools in neuroscience for studying electrical signals — from those at the molecular scale to the level of networks of neurons.
By the 1970s, current flowing through the cell was generally accepted to result from the opening of many channels in the membrane, although the underlying mechanism was unknown. At that time, current was commonly recorded by impaling tissue with a sharp electrode — a pipette with a very fine point. Unfortunately, however, the signal recorded in this way was excessively noisy, and so only the large, ‘macroscopic’ current — the collective current mediated by many different types of channel — that flows through the tissue could be resolved.
In 1972, Bernard Katz and Ricardo Miledi2, pioneers of the biology of the synaptic connections between cells, managed to infer from the macroscopic current certain properties of the membrane channels, but only after a heroic effort to exclude all possible confounding factors. The problem was that the macroscopic current could be influenced by factors not directly related to channel activity, such as cell geometry and modulatory processes that regulate cell excitability. Also troublesome was that interpretations of macroscopic-current features were based on unverified assumptions about the statistics of individual channel activity2,3. Despite Katz and Miledi’s careful analyses, there was a lingering doubt about whether their conclusions were correct. The crucial data were obtained by Neher and Sakmann using patch clamp.
The patch-clamp technique is conceptually rather simple. Instead of impaling the cells, a pipette with a relatively large diameter is pressed against the cell membrane. Under the right conditions, the pipette tip ‘bonds’ with the membrane, forming a tight seal. This substantially reduces the noise compared with that encountered using sharp electrodes, because the small patch of membrane encompassed by the pipette tip is electrically isolated from the rest of the cell’s membrane and from the environment surrounding the cell (Fig. 1).
The tiny currents passing through the few channels in the patch were thus observed for the first time. The recording confirmed key channel properties: when channels open, there is a step-like jump in the current trace and, when they close, a step-like drop back to baseline. It was now possible to determine details such as the statistics of the opening and closing of channels, the amplitude of the currents they mediate and the optimal stimuli that trigger their opening. For this work, Neher and Sakmann were awarded the 1991 Nobel Prize in Physiology or Medicine.
Improvements in patch clamp made it feasible to study channels in various preparations4 to finally address long-standing questions. There was particular interest in verifying a model for action-potential generation5 proposed by Nobel laureates Alan Hodgkin and Andrew Huxley in the 1950s. Specific predictions of the model could now be tested directly by examining the current through individual channels and by observing the changes in current that occur when the molecular structure of the channel is modified6. Ultimately, the model was shown to be mostly correct and remains the gold standard for computational neuroscientists today.
One of the several variants of patch clamp4 — the whole-cell configuration — found an audience with neuroscientists studying electrical phenomena in neurons beyond the channel level. To achieve whole-cell recording, the patch of membrane under the electrode is ruptured, enabling electrical access to the cell. Compared with the use of sharp electrodes, whole-cell patch clamp allows much more accurate recordings and, crucially, is less damaging to the cell. This allowed systematic investigation of synergistic processes at the cellular level, such as the regulation of macroscopic currents by modulatory molecules, and interactions between the different types of channel in the neuron.
The relatively large opening created in the cell in the whole-cell configuration also provided access to the cell by chemicals, enabling dyes to be delivered for visualizing intricate cell structures, and RNA to be extracted for gene-expression analysis7. Neher’s group examined the sequence of events that underlie the transfer of information between cells by introducing chemicals into the cell and simultaneously tracking the resulting changes in the electrical properties of the cell’s membrane8.
Whole-cell patch clamp proved ideal for studying the collective properties of neurons and neuronal networks in brain slices maintained in vitro. A challenge in working with more-complex systems such as neuronal networks is that the number of possible confounding factors increases. Sakmann’s solution in the 1990s was to carry out simultaneous whole-cell recording using two or three electrodes, which to some seemed excessive because comparable data could be obtained by sequential recordings using fewer electrodes. However, the rationale was that taking time to design the near-perfect experiment mitigated later difficulties in data interpretation analogous to those faced by Katz and Miledi.
Hence, simultaneous recordings from different parts of the neuron definitively confirmed that action potentials are initiated at one part of the main long neuronal protrusion (the axon) and propagate back to the dendrites (clustered protrusions that receive inputs from other neurons)9. The mechanisms that underlie signalling between neurons were directly investigated by placing electrodes on either side of a synaptic connection10. Moreover, triple recordings from neurons of different classes uncovered certain basic principles of network organization11.
The patch-clamp technique is also used to examine cell activities under more natural conditions. To study how sensory stimuli and movements are represented in the brain, experiments must be carried out in living animals. The challenge with this approach, however, is that the slightest movement can dislodge an electrode from the neuron. Whole-cell patch-clamping turns out to be remarkably stable because of the tight seal between the electrode and the membrane. Thus, this technique has permitted recording from dendrites12 and pairs of neurons13 in anaesthetized rodents, and even from animals that are able to walk and run14.
Patch-clamp recording is arguably still the most direct and effective way of studying electrical signals in the brain. The data obtained with this technique essentially represent the ground truth for investigators in many branches of neuroscience, from theorists15 to translational researchers developing drugs for the treatment of certain brain conditions, including epilepsy16 and autism spectrum disorder17.
Moreover, patch clamp complements modern ‘optogenetic’ techniques, which enable control and visualization of the activities of large populations of neurons using light18. Emerging technologies, such as prostheses for vision19, will probably rely heavily on patch-clamp recording to establish the optimal conditions for converting external stimuli into electrical signals. Patch-clamping will clearly remain a vital tool for the neuroscientist in the foreseeable future.
Nature 575, 38-39 (2019)