Licking Behavior in the Rat: Measurement and Situational Control of Licking Frequency

Abstract

WEIJNEN, JAWM. Licking behavior in the rat: Measurement and situational control of licking frequency. NEUROSCI BIOBEHAV REV 22(6) 751–760, 1998—Lick sensors are versatile instruments that are routinely used in behavioral and neuroscience research involving fluid ingestion in the rat. There are three different types of sensors: electrical, optical and force sensors. They differ in the exact moment of activation during the tongue protrusion/retraction cycle. Precautions in the use of each type of sensor are discussed. Adequate lick detection requires restriction of access to the water source to the tongue of the animal. It appears that drinking configurations that fulfill this task may affect the modal licking frequency. Increasing the amount of tongue travel that is needed to reach the drinking tube or water surface, decreases the licking frequency. The licking frequency can be manipulated between about 7.5 and 4 Hz. Therefore, if `invariant' licking/lapping frequencies are observed, this is not so much the manifestation of a rigid output of a central pattern generator, but more the consequence of similarity in the effects of the employed drinking configurations. Applications of lick sensors in behavioral and neuroscience research are briefly discussed.

Introduction

Stellar and Hill [44]wanted to study drinking behavior in detail, and not limit themselves to recording the total water intake. To this end they constructed an `electronic drinkometer' that could record every lick the animal makes while drinking from an inverted water bottle [20]. This instrument is also known as a `lickometer'. However, the present author favors the term lick sensor over drinkometer and lickometer. The affix `-meter' refers to a possible application; lick sensor is a purely descriptive term.

Lick sensors do not give an accurate account of the volume of the liquid that is ingested. The volume/lick can vary within and between experiments; values of 4–8 μl/lick are common, the maximum volume per lick that a rat can handle is about 10 μl [58]. Variables affecting the lick volume can be: the drinking configuration, diameter of the orifice of the watering tube, liquid characteristics like surface tension and viscosity, amount of air in the inverted water bottle, etc. Within a test session most of these variables are kept constant. An accurate record of fluid ingestion can be obtained if the sensor is used to operate a pump that supplies the liquid at a constant rate during periods of uninterrupted licking [58]. Ingested volume versus time curves can then be computed from the lick data.

In most experimental situations animals lick a drinking tube. Occasionally, drinking from an open pool of water is investigated. This is called `lapping'. Licking and lapping have much in common, but the tongue-movement patterns are not identical. These patterns are further influenced by restrictions imposed on the access to the drinking tube or water surface [16].

The stability of the licking frequency has struck investigators from the beginning. Stellar and Hill reported that independent of the level of water deprivation the rats licked at a constant rate (6–7 licks/s), or not at all. The title of a paper by Corbitt and Luschei [4]: `Invariance of the rat's rate of drinking' further promoted this concept. This notion still persists in the present literature [21]. However, a stable licking frequency is not the reflection of a rigid output of a central rhythm generator that is either active or not. It is the result of holding all aspects of the licking situation constant 16, 32. The principal factor being a fixed position of the rat relative to the watering tube or water surface. Corbitt and Luschei already noted that increasing the distance between the rat and the drinking tube reduced their licking rates. Others have confirmed these results (e.g. Refs 11, 27, 51). It will be shown later that the licking/lapping frequency can easily be modulated in the range of 4–7.5 Hz, by changing the drinking configuration. If this configuration is kept constant, then the licking frequency can only be influenced to a minor degree by factors such as deprivation level, type of solution, phase of the session, etc. 2, 3, 16, 29.

Major effects on licking frequency can be obtained by drug administration, as Knowler and Ukena have already shown [27]. A slowing down of the licking frequency can, amongst other things, be due to a drug-induced reduction of the force of tongue protrusion [13]. It is difficult to exclude the possibility that the results of drug administration are influenced by drug-induced changes in the position of the rat, especially if substances are used that are sedating or causing muscle relaxation.

Good knowledge of the properties of lick sensors is important for setting up proper experiments. Much attention is given in this review to this subject and to the contribution of the configuration of the drinking environment to the frequency of tongue movements in the drinking rat.