The function of the pedunculopontine nucleus in the preparation and execution of an externally-cued bar pressing task in the rat

In the present study the role of the pedunculopontine nucleus (PPN) in the preparation and execution of an externally-cued rewarded motor act was inve stigated. Animals were instructed to press down a lever at the presentation of a combined visual and acoustic signal and were required to hold down th e lever until a trigger stimulus occurred after an unpredictable delay rang ing from 2 to 4 s. The trigger stimulus required animals to release the lev er and to press a second lever for food reinforcement. The time between ins truction and trigger signals represented the preparation phase preceding mo vement. Unilateral ibotenic acid-induced focal degeneration of pedunculopon tine neurons did not influence either reaction and movement times, or capac ity of the animals to correctly respond to presentation of stimuli of behav ioral significance. On the contrary, bilateral lesions increased both react ion and movement times, and dramatically reduced the percentage of correct responses. The analysis of incorrect responses suggested that the most stri king deficit exhibited by the animals following the bilateral lesion was a lack of conditioned response to the signal initiating each trial. However, the animals retained the capability to respond correctly in some trials, an d were able to collect the reward when delivered outside the behavioral con text. Histological analysis of lesions showed that in addition to loss of n eurons within the pedunculopontine region, reduction of tyrosine-hydroxylas e positive neurons had occurred in the pars compacta of the substantia nigr a. The data suggest that the PPN is involved in the preparation and executi on of externally-cued movements, and demonstrate that its destruction mimic s the main effects produced by the dopaminergic denervation of the dorsal s triatum. (C) 1999 Elsevier Science B.V. All rights reserved.