Abstract
Two thalamic sites are of especial significance for understanding hippocampal – diencephalic interactions: the anterior thalamic nuclei and nucleus reuniens. Both nuclei have dense, direct interconnections with the hippocampal formation, and both are directly connected with many of the same cortical and subcortical areas. These two thalamic sites also contain neurons responsive to spatial stimuli while lesions within these two same areas can disrupt spatial learning tasks that are hippocampal dependent. Despite these many similarities, closer analysis reveals important differences in the details of their connectivity and the behavioural impact of lesions in these two thalamic sites. These nuclei play qualitatively different roles that largely reflect the contrasting relative importance of their medial frontal cortex interactions (nucleus reuniens) compared with their retrosplenial, cingulate, and mammillary body interactions (anterior thalamic nuclei). While the anterior thalamic nuclei are critical for multiple aspects of hippocampal spatial encoding and performance, nucleus reuniens contributes, as required, to aid cognitive control and help select correct from competing memories.
Highlights
Within the thalamus, the anterior thalamic nuclei and nucleus reuniens stand out because of their dense, direct interactions with the hippocampus and frontal cortices
While lesions in both nucleus reuniens and the anterior thalamic nuclei can impair the learning and retention of spatial tasks that depend on the hippocampus (Aggleton and Nelson, 2015; Dolleman-van der Weel et al, 2019), their respective connections suggest that the out comes will be qualitatively different
Subsequent studies have shown that nucleus reuniens and its medial prefrontal afferents are required for fear conditioning and its normal extinction in rats (Ramanathan and Maren, 2019; Ramanathan et al, 2018), while other evidence points to a role in regulating fear memory intensity and maintenance (Troyner et al, 2018)
Summary
The anterior thalamic nuclei and nucleus reuniens stand out because of their dense, direct interactions with the hippocampus and frontal cortices. Electrophysiological recordings show that both thalamic areas contain spatially-responsive neurons Reflecting these shared properties, lesions in both sites disrupt spatial tasks known to depend on the integrity of the rodent hippocampal formation. Despite these similarities, this review highlights how these two thalamic sites are, quite dissimilar. The behavioural analysis largely concentrates on rat spatial learning, given the numbers of relevant studies In view of this focus we should, at the outset, acknowledge one well-established difference between the anterior thalamic nuclei and nucleus reuniens. It should be remembered that the rodent ventral (or ‘temporal’) hip pocampus is homologous with the primate anterior hippocampus, while the dorsal (or ‘septal’) hippocampus is homologous with the primate posterior hippocampus
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