Licking during forced spout alternation in rats: resetting the pacemaker or disconnecting the motor output?

Summary Rats trained to alternate between two adjacent drinking spouts (each spout is retracted after a single lick and presented again after the animal has licked at the other spout) emit 2.5 licks instead of the optimum 1 lick per spout presentation. The question whether the pacemaker of licking is reset or continues to run during transition between spouts was addressed in three experiments performed in 10 highly overtrained rats. Videorecording analysis of oro-facial activities showed that the rat's mouth was firmly closed during the 500 ms transition between spouts. EEG was recorded from the region of hypoglossal nucleus with implanted bipolar electrodes during continued licking at one spout and during spout alternation. Averaging (n = 256, 1024 ms) centered around the onset of the first lick at the newly contacted spout showed 2–3 lick related potentials in the post-transitional interval whereas only one such wave occasionally appeared at the end of the pre-transition period. Computer plotted distributions of post-transition licks (timed with respect to the last pre-transition lick) were examined while the horizontal or vertical distance between spouts was increased. Growing spout separation changed the height but not the timing of the modes of the post-transition lick distribution. This phase-locked synchronization of pre- and post-transition licks indicates that the central timing network of the lick generator is not reset but continues to run during transition between spouts and that the cessation of tongue movements is due to inhibition of the intracycle pattern generator and motor output. It is concluded that more sensitive recording techniques are required to detect the activity of the central timing network in absence of overt licking.