Inactivation of ventral midline thalamus produces selective spatial delayed conditional discrimination impairment in the rat.

The reuniens (Re) and rhomboid (Rh) nuclei are organized to influence activity in distributed limbic networks involving hippocampus and medial prefrontal cortex (mPFC). To elucidate the role of these nuclei in spatial memory we inactivated Re and Rh in rats with the GABA(A) agonist muscimol and compared effects on two spatial delayed conditional discriminations: delayed nonmatching to position (DNMTP) and varying choice radial maze delayed nonmatching (VC-DNM). DNMTP is trained in operant chambers and requires rats to choose between the same two levers on all trials. VC-DNM is trained in automated radial mazes and requires rats to choose between two arms, randomly selected from eight alternatives on each trial. DNMTP is affected by hippocampal and mPFC lesions while VC-DNM is affected by hippocampal, but not mPFC lesions (Porter et al. (2000) Behav Brain Res 109:69-81). We found evidence of a localized (low dose) effect of ReRh inactivation on DNMTP but not VC-DNM. This was confirmed by comparison with muscimol injections in an anatomical control site. These results are consistent with evidence that Re and Rh affect measures of spatial working memory that depend on interactions between hippocampus and mPFC, but not measures that depend on hippocampus alone.