Lesions of the thalamic reuniens cause impulsive but not compulsive responses

On account of its strong efferent projections to the hippocampus, recent animal studies have emphasized an important role for the nucleus reuniens (NRe) of the midline thalamus in spatial memory. However, by virtue of its reciprocal connections with the orbital and ventromedial prefrontal cortex, the NRe may also be involved in aspects of executive inhibition. To date, there has been no systematic attempt to examine the role of the NRe in inhibitory mechanisms of response control. Accordingly, we compared rats with neurotoxic lesions of the NRe with sham surgery controls on performance of the 5-choice reaction time task, a test of visuospatial attention and inhibitory control. When tested post-operatively, rats with NRe lesions were unable to actively inhibit premature responses when the intertrial interval was varied. However, the same rats with NRe lesions showed normal inhibition of perseverative responses, and under some conditions were less perseverative than shams. The NRe lesion was also associated with a reduction in omissions and fast reward collection latencies, which persisted 2 months following surgery. The NRe lesion did not affect response accuracy or latency to respond correctly throughout the course of experimental testing. Together, these results signify the important role of the NRe in impulse inhibition, especially when slight changes are made to the temporal demands of the environment, and reveal the potential contribution of the NRe in motivational processes.