Examination of the role of the pedunculopontine tegmental nucleus in radial maze tasks with or without a delay.

Two radial maze tasks, random foraging and delayed spatial win-shift, have been used to investigate, in rats, the functions and inter-relationships of structures connected through the corticostriatal loops, such as the prelimbic cortex, nucleus accumbens, ventral pallidum and mediodorsal thalamus. The random foraging task is designed to investigate animals' ability to use spatial information to guide foraging on-line. The delayed spatial win-shift task requires, in addition, that animals hold spatially relevant information in working memory across a delay period. The pedunculopontine tegmental nucleus receives direct output from ventral striatal systems and might therefore be expected to share functional properties with them. In the present experiments we have examined the performance of rats bearing bilateral excitotoxic lesions of the pedunculopontine tegmental nucleus on both of these tasks. In acquisition tests rats were given bilateral lesions before any training took place, while in retention tests appropriate training to predetermined criterion levels of performance took place before lesions were made. In both tasks, and in both acquisition (no prelesion training) and retention (prelesion training) tests, rats with pedunculopontine lesions made significantly more errors in selecting arms to enter than did control rats. There was no motor impairment present in pedunculopontine tegmental nucleus-lesioned rats - on the contrary, on measures of speed (latency to make first arm choice and the mean time for subsequent choices) pedunculopontine-lesioned rats were slightly faster than control rats. We suggest that the pedunculopontine tegmental nucleus shares functional properties with frontostriatal systems and that it forms part of a brainstem-directed stream of striatal outflow different to the cortical re-entrant system via the thalamus.