| Summary: | The role of the pedunculopontine tegmental nucleus (PPTg) in motivation and cognitive functions is controversial. In order to clarify the involvement of this nucleus in learning, rats with N-methyl-D-Aspartate (NMDA) lesions to the PPTg were tested on the acquisition of a delayed non-matching to position task (DNMP) performed in a T-maze. Unlike sham-lesioned rats, animals with PPTg lesions did not learn the task. Analysis of the behavior displayed by the lesions animals, however, suggested that these rats suffered from elevated emotionality or arousal, rather than from learning deficits. This hypothesis was confirmed by demonstrating that the anxiolytic compound diazepam (1 mg/kg) normalized the performance of the PPTg-lesioned rats on the DNMP task and reduced the indices of anxiety displayed by animals with PPTg lesions when tested on the elevated plus maze. === These results suggested the possibility that the motivational impairments reported to be induced by PPTg lesions, could also be an artifact of lesion-induced elevation of anxiety or arousal. Thus, in order to verify this hypothesis, it was tested whether diazepam would modify the expression of conditioned place preference (CPP) to morphine and amphetamine in animals with NMDA-induced lesions to the PPTg. Diazepam reversed the effects of the lesion on a morphine CPP but not on an amphetamine CPP. A series of experiments, aimed at characterizing the effects of diazepam on morphine and amphetamine reinforcement in normal rats, showed that diazepam, either systemic or injected in the nucleus accumbens, blocks the reinforcing effects of amphetamine but has no effect on the reinforcing effects of morphine. These results suggest that impairments in CPP learning caused by PPTg lesions do not result from motivational deficits, but are caused by elevated emotionality or abnormal arousal.
|
|---|
