Base Recording: A Technique for Analyzing Responses of Taste Neurons in Drosophila

A rarely used method of electrophysiological recording, base recording, allows analysis of features of taste coding that cannot be examined by conventional recording methods. Base recording also allows the analysis of taste responses to hydrophobic stimuli that cannot be studied using traditional electrophysiological methods.

Insects taste the external world through taste hairs, or sensilla, that have pores at their tips. When a sensillum comes into contact with a potential food source, compounds from the food source enter through the pore and activate neurons within. For over 50 years, these responses have been recorded using a technique called tip recording. However, this method has major limitations, including the inability to measure neural activity before or after stimulus contact and the requirement for tastants to be soluble in aqueous solutions. We describe here a technique that we call base recording, which overcomes these limitations. Base recording allows the measurement of taste neuron activity before, during, and after the stimulus. Thus, it allows extensive analysis of OFF responses that occur after a taste stimulus. It can be used to study hydrophobic compounds such as long-chain pheromones that have very low solubility in water. In summary, base recording offers the advantages of single-sensillum electrophysiology as a means of measuring neuronal activity - high spatial and temporal resolution, without the need for genetic tools - and overcomes key limitations of the traditional tip recording technique.

Insects, including drosophilid flies, are endowed with a sophisticated taste system that enables them to extract complex chemical information from their surroundings. This system allows them to discern the chemical composition of various substances, distinguishing between those that are nutritious and those that are harmful^1,2.

At the core of this system are specialized structures known as taste hairs or sensilla, strategically located on various body parts. In drosophilid flies, these sensilla are located on the labellum, which is the major taste organ of the fly head

The following protocol complies with all the animal care guidelines of Yale University.

1. Flies

1. Place 10-15 newly emerged flies in fresh standard culture vials at 25 °C and 60% relative humidity in a 12:12 h light-dark cycle.
2. Use flies when 3-7 days old.

2. Chemosensory stimuli

1. Obtain chemosensory stimuli of the highest available purity. Store them as recommended by the vendor until use.......

Figure 4A shows spontaneous spikes that arise from a sensillum. They fall into two classes based on amplitude, with the larger spikes deriving from the neuron that is sensitive to bitter compounds and the smaller spikes from the neuron that responds to sugars. The relationship between spike amplitude and functional specificity has been corroborated by genetic experiments^4,14,37,

In recordings from some types of sensilla, it can be challenging to differentiate the spikes of different neurons. For example, the sugar neurons and mechanosensory neurons of S and I sensilla produce spikes of similar amplitudes, making it difficult to distinguish them^4,14. We find that the use of a very sharp tungsten recording electrode reduces the firing of the mechanosensory neuron, as does the judicious placement of the recording electrode. Insertion of the.......

We thank Zina Berman for support, Lisa Baik for comments on the manuscript, and other members of the Carlson laboratory for discussion. This work was supported NIH grant K01 DC020145 to H.K.M.D; and NIH grants R01 DC02174, R01 DC04729, and R01 DC011697 to J.R.C.