3-acetylpyridine reduces tongue protrusion force but does not abolish lick rhythm in the rat

Data from other laboratories suggest that neurons in the inferior olivary nucleus (IO) may play a role in the modulation of rhythmic tongue movements in rats. Because of its known harmful effects on neurons of the IO, it was suspected that administration of the neurotoxin 3-acetylpyridine (3AP) would affect subsequent tongue dynamics during rat licking. In the present study, the task of licking water from a force-transducing disk was investigated in water-restricted rats that received systemic administration of 3AP (12.5, 25, and 50 mg/kg). After recovery from the acute toxic effects of 3AP, tongue dynamics were assessed by measuring lick force, lick rhythm, variability of timing within bursts of licking, and number of licks per 2-min session. At 50 mg/kg, 3AP resulted in: (1) reduced lick force; (2) reduced number of licks; and (3) increased variance in the timing within bursts. Lick rhythm was not significantly affected by any dose of 3AP. All 3AP treatment groups and the vehicle control group displayed slowing of lick rhythm after harmaline challenge. Compared to vehicle controls, rats receiving lower and mid-range doses of 3AP displayed indistinguishable lick behaviors, with one exception--when the lick task was made incrementally more difficult by extending the distance required to make contact with the lick-disk, rats that had received 25 mg/kg 3AP persevered at the task more than all other rats. The various changes in lick dynamics may be due to the detrimental effects of 3AP at the IO, and possibly at the hypoglossal nucleus and other sites.