A low-cost solution to measure mouse licking in an electrophysiological setup with a standard analog-to-digital converter

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Abstract

Licking behavior in rodents is widely used to determine fluid consumption in various behavioral contexts and is a typical example of rhythmic movement controlled by internal pattern-generating mechanisms. The measurement of licking behavior by commercially available instruments is based on either tongue protrusion interrupting a light beam or on an electrical signal generated by the tongue touching a metal spout. We report here that licking behavior can be measured with high temporal precision by simply connecting a metal sipper tube to the input of a standard analog/digital (A/D) converter and connecting the animal to ground (via a metal cage floor). The signal produced by a single lick consists of a 100–800 mV dc voltage step, which reflects the metal-to-water junction potential and persists for the duration of the tongue–spout contact. This method does not produce any significant electrical artifacts and can be combined with electrophysiological measurements of single unit activity from neurons involved in the control of the licking behavior.

Introduction

Fluid licking in rodents is a rhythmic behavior that is probably driven by a central pattern generator (CPG): it is a highly stereotyped behavior characterized by repetitive tongue and jaw movements, which are in turn under the control of one or more rhythmically active neural networks (for review, see Travers et al., 1997). Monitoring licking activity is also a very useful method for rating the dose-related behavioral effects of acute or chronic drug treatment (e.g. Genn et al., 2003, Hsiao and Spencer, 1983, Peachey et al., 1976).

Licking in rodents has typically been measured with “lickometers” utilizing either electrical, optical or force sensors (for review, see Weijnen, 1998). Electric sensors are most commonly used, and typically depend on a high-frequency ac contact circuit. Commercially available lickometers designed for use with rats and mice come at a cost of a few hundred to several thousand dollars, and vary widely in application, from those used to monitor licks from a single spout/bottle to those capable of switching between multiple spouts/bottles (Glendinning et al., 2002, Hill and Stellar, 1951, Smith, 2001). Here we describe a method that allows the reliable and temporally precise measurement of the licking behavior at virtually no cost, provided that a standard analog-to-digital converter is available. An advantage of our method is that it is easily combined with electrophysiological measurement of neuronal spike activity during licking behavior. A standard stainless steel sipper tube attached to a small drinking bottle is used, occupying minimal space and compatible with almost any existing in vivo recording setup. Most importantly, lick contact produces no significant electrical artifact during extracellular recording of single-unit brain activity.

Section snippets

Animals

Adult male and female mice (>60 days old) from three common inbred strains (C57BL/6J, DBA/2J, BALB/cByJ) were individually housed in plastic shoebox cages in a temperature and humidity-controlled vivarium on a 12:12 h light-dark cycle. Animals were treated according to a protocol approved by the University of Tennessee Health Science Institutional Center Animal Care and Use Committee. Ad libitum water was removed from the cages of mice approximately 23 h prior to training or testing in the

Results

Junction potentials occur wherever dissimilar conductors are in contact. The voltage generated by the contact of the mouse tongue with the spout is due to the metal–water junction potential for the following reasons: (i) connecting the BNC core and shield directly with each other, i.e. short-circuiting the input to ground or via resistors of different magnitude, reduced the 60 Hz cycle noise in the acquired signal but produced no voltage change; (ii) connecting both poles of the BNC via a

Discussion

Lick sensors are valuable tools to study licking and drinking behavior (Weijnen, 1989). Commercially available lickometers come at a cost of a few to several hundred dollars (e.g., TSE systems, model 2.07, Midland, MI; Columbus Instruments drinkometer, Columbus, OH; Stoelting, model 57450, Wood Dale, IL; Med Associates Inc., model ENV-250, St. Albans, VT; Coulbourn Instruments, model H24-01, Allentown, PA; MS-160 Lickometer, DiLog Instruments, Tallahassee, FL). There are three different types

Acknowledgements

This work was supported by startup-funds to Dr. Heck provided by the University of Tennessee Health Science Center Department of Anatomy and Neurobiology. We would like to thank Dr. Randall J. Nelson for comments on earlier versions of the manuscript.

References (20)

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