Fluid licking in mice is an example of a rhythmic behavior thought to be under the control of a central pattern generator. Inbred strains of mice have been shown to differ in mean or modal interlick interval (ILI) duration, suggesting a genetic-based variation. We investigated water licking in the commonly used inbred strains C57BL/6J (B6) and DBA/2J (D2), using a commercially available contact lickometer. Results from 20-min test sessions indicated that D2 mice lick at a faster rate than B6 mice (10.6 licks/s vs. 8.5 licks/s), based on analysis of the distribution of short-duration ILIs (50-160 ms). This strain difference was independent of sex, extent of water deprivation or total number of licks. D2 mice also displayed a faster lick rate when the strains were tested with a series of brief (5 s) trials. However, when ingestion over the entire 20-min session was analyzed, it was evident that D2 mice had an overall slower rate of ingestion than B6 mice. This was because of the tendency for D2 mice to have more very long pauses (>30 s) between sequences of licking bursts. Overall, it appeared that D2 mice licked more efficiently, ingesting more rapidly during excursions to the spout that were fewer and farther between. Fluid licking is a highly stereotyped behavior in rats, mice and many other mammals that involves the rhythmic co-ordination of muscle groups involved in tongue protrusion and retraction, jaw opening and closing and swallowing. The coordination of these oromotor movements is thought to be under the control of central pattern generators (CPGs), motor 'programs' extant among premotor neuron networks that send rhythmic inputs to motor neuron pools in cranial nerve nuclei V, VII and XII (for reviews see Nakamura & Katakura 1995;Travers et al. 1997). Current evidence suggests a substrate for rhythmic licking organized among premotor neurons in the medullary reticular formation (RF). Premotor neurons associated with intrinsic and extrinsic tongue muscles are located in a number of medullary and pontine RF cell groups (Travers & Rinaman 2002;Travers et al. 2005). Neurons rhythmically active during licking are found in both the parvocellular and intermediate zones of the RF (e.g. Travers et al. 1997), and reversible lesion studies in awake rat preparations suggest a necessary role for the rostrolateral medullary RF (Chen & Travers 2003). However, the specific identity and physiological properties of the neurons and networks that underlie the CPG for licking are unknown.The species of choice for investigating the physiological and anatomical substrates of licking has been the rat (Weijnen 1998) but the study of strains of mice with different lick or ingestion rates holds substantial promise for genetic approaches to the study of oromotor CPGs (e.g. Okayasu et al. 2003;Tomiyama et al. 2004). Horowitz et al. (1977) examined ad lib fluid licking over a series of 20-h periods in undeprived C57BL/6 (B6) and DBA/2 (D2) mice, and their F 1 progeny, using an infrared-beam lickometer. Local lick rate, as defined by ...

Rodents consume water by performing stereotypic, rhythmic licking movements which are believed to be controlled by brainstem pattern-generating circuits. Previous work has shown that synchronized population activity of inferior olive neurons was phase locked to the licking rhythm in rats, suggesting a cerebellar involvement in temporal aspects of licking behavior. However, what role the cerebellum has in licking behavior and whether licking is represented in the high frequency simple spike output of Purkinje cells remains unknown. We recorded Purkinje cell simple and complex spike activity in awake mice during licking and determined the behavioral consequences of loss of cerebellar function. Mouse cerebellar cortex contained a multifaceted representation of licking behavior encoded in the simple spike activities of Purkinje cells distributed across Crus I, Crus II and lobus simplex of the right cerebellar hemisphere. Lick-related Purkinje cell simple spike activity was modulated rhythmically, phase-locked to the lick rhythm, or non-rhythmically. A subpopulation of lick-related Purkinje cells differentially represented lick interval duration in their simple spike activity. Surgical removal of the cerebellum or temporary pharmacological inactivation of the cerebellar nuclei significantly slowed the licking frequency. Fluid licking was also less efficient in mice with impaired cerebellar function, indicated by a significant decline in the volume per lick fluid intake. The gross licking movement appeared unaffected. Our results suggest a cerebellar role in modulating the frequency of the central pattern generating circuits controlling fluid licking and in the fine coordination of licking, while contributing little to the coordination of the gross licking movement.

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.