Recordings of Neural Circuit Activation in Freely Behaving Animals

Summary

Non-invasive measurements of neural activity patterns in freely behaving animals are obtained by combining neurophysiological recordings with high speed videography.

Abstract

The relationship between patterns of neural activity and corresponding behavioral expression is difficult to establish in unrestrained animals. Traditional non-invasive methods require at least partially restrained research subjects, and they only allow identification of large numbers of simultaneously activated neurons. On the other hand, small ensembles of neurons or individual neurons can only be measured using single-cell recordings obtained from largely reduced preparations. Since the expression of natural behavior is limited in restrained and dissected animals, the underlying neural mechanisms that control such behavior are difficult to identify.

Here, I present a non-invasive physiological technique that allows measuring neural circuit activation in freely behaving animals. Using a pair of wire electrodes inside a water-filled chamber, the bath electrodes record neural and muscular field potentials generated by juvenile crayfish during natural or experimentally evoked escape responses. The primary escape responses of crayfish are mediated by three different types of tail-flips which move the animals away from the point of stimulation. Each type of tail-flip is controlled by its own neural circuit; the two fastest and most powerful escape responses require activation of different sets of large “command” neurons. In combination with behavioral observations, the bath electrode recordings allow unambiguous identification of these neurons and the associated neural circuits. Thus activity of neural circuitry underlying naturally occurring behavior can be measured in unrestrained animals and in different behavioral contexts.

Protocol

Part 1: Recording chamber

1. The recording chamber is of rectangular shape and is made of thin walled glass. Chamber dimensions are 8.5 cm x 2 cm x 5 cm (length x width x height) for animals of 2.5 – 3.5 cm total lengths (measured from rostrum to telson).
   See Fig. 1 for an example of a chamber used in our experiments.
2. Alternatively, recording chambers can be made from other materials (e.g., non-toxic clear plastic). Chambers size may vary according to experimental procedure and chambers should be customized for each experimental series. For best results, chamber size should be as small as possible without restraining the animals in their natural be....

Discussion

Non-invasive recordings of single neuron activity or neural circuit activation are difficult to obtain in unrestrained animals. The method described here provides a means to identify patterns of neural activation underlying naturally occurring behavior.

In the past, we successfully used this technique to measure patterns of activity in neural escape circuits of juvenile crayfish during the formation of social dominance hierarchies¹, during attacks from natural predators²,.......