Introduction

Neurons use electrical signals to process and transmit information. These electrical signals are part of the “currency” of neural processing, and appear in multiple forms that can be recorded in a variety of ways. The simplest recordings are measurements of extracellular potentials, which are generated by current flow between different parts of individual neurons. These recordings can reflect either the spiking of neurons (as in single-unit or multi-unit recordings), or can be field potentials that are composed of a mixture of time-locked synaptic currents and cell spiking in a population of cells near the recording electrode. Neither of these types of recordings require access to the interior of the cells.

The electrical signals recorded outside of single neurons are generally very small, in the range of microvolts (μV) to a few millivolts (mV). While these are useful for understanding the coding of information in spike trains, or the spatial and temporal organization of synaptic inputs in laminar structures, they do not readily reveal the underlying mechanisms of synaptic integration or spike generation. A modern electrophysiological method called patch clamp, permits the measurement of transmembrane voltage and current with high resolution and low noise. What sets patch clamp apart from other methods, such as sharp-electrode intracellular recording, is its use of a tight seal between the recording electrode and the cell membrane. This seal essentially blocks external currents associated with the electrical activity of surrounding cells, allowing the experimenter to record even small subthreshold events in a single neuron. The development of these techniques to create high-resistance seals with the micrometer-sized pipette tips needed to record from small vertebrate neurons (Neher et al., 1978; Hamill et al., 1981) revolutionized electrophysiology.

There are several variants of the patch clamp method. The most commonly used method is best described as whole-cell recording. Of the other variants on the patch clamp method, the next most commonly used configurations are the cell-attached patch, followed by outside-out and inside-out patch recording. In this chapter, we will focus on the technical aspects involved in making whole-cell patch clamp recordings, and only briefly touch on cell-attached and outside-out patches.

When studying electrical signaling in neurons, there are two widely-used recording methods. Current clamp refers to recording the voltage across the membrane of individual cells. Voltage clamp refers to measuring the current that is associated with conductance changes in the membrane in response to voltage changes. Current clamp and whole-cell voltage clamp require electrical access to the interior of the cell. Both current clamp and voltage clamp can be very informative about the mechanisms by which cells fire particular patterns of action potentials or by which synaptic inputs change with time and are integrated. These recording methods also permit a variety of manipulations that can yield insight into the cellular physiology and biology of specific neurons, revealing mechanisms underlying processes such as learning, memory, decision making, hormonal regulation, the construction of motor activity patterns, and perception.

In this chapter, we focus on the use of brain slices when making patch clamp recordings. Brain slices are an in vitro preparation, created by sectioning fresh brain tissue (Yamamoto and McIlwain, 1966). Slices offer a number of advantages for analysis of cellular mechanisms and small networks, including excellent optical access to even small cellular elements such as dendritic spines, mechanical stability, and the ability to control the extracellular environment for ionic or pharmacological manipulations. Slices have an advantage over dispersed neuronal cultures in that they can retain much of the in vivo network structure and connection specificity, as well as the normal complement of cells. However they have disadvantages as well. Brain slices can only be used for a few hours after preparation, and they lack many of the normal activity patterns that can be observed in vivo. Regardless, over the past 30 years, slices have been instrumental in deepening our understanding of cellular and synaptic physiology, and have significantly contributed to understanding local neuronal networks.