A tripartite view of the posterior cingulate cortex

The posterior cingulate cortex (PCC) is one of the least understood regions of the cerebral cortex. By contrast, the anterior cingulate cortex has been the subject of intensive investigation in humans and model animal systems, leading to detailed behavioural and computational theoretical accounts of its function. The time is right for similar progress to be made in the PCC given its unique anatomical and physiological properties and demonstrably important contributions to higher cognitive functions and brain diseases. Here, we describe recent progress in understanding the PCC, with a focus on convergent findings across species and techniques that lay a foundation for establishing a formal theoretical account of its functions. Based on this converging evidence, we propose that the broader PCC region contains three major subregions - the dorsal PCC, ventral PCC and retrosplenial cortex - that respectively support the integration of executive, mnemonic and spatial processing systems. This tripartite subregional view reconciles inconsistencies in prior unitary theories of PCC function and offers promising new avenues for progress.

Retrosplenial Cortical Connectivity with Frontal Basal Ganglia Networks

Previous studies of the retrosplenial cortex (RSC) have focused on its role in navigation and memory, consistent with its well-established medial temporal connections, but recent evidence also suggests a role for this region in reward and decision making. Because function is determined largely by anatomical connections, and to better understand the anatomy of RSC, we used tract-tracing methods to examine the anatomical connectivity between the rat RSC and frontostriatal networks (canonical reward and decision-making circuits). We find that, among frontal cortical regions, RSC bidirectionally connects most strongly with the anterior cingulate cortex, but also with an area of the central-medial orbito-frontal cortex. RSC projects to the dorsomedial striatum, and its terminal fields are virtually encompassed by the frontal-striatal projection zone, suggestive of functional convergence through the basal ganglia. This overlap is driven by anterior cingulate cortex, prelimbic cortex, and orbito-frontal cortex, all of which contribute to goal-directed decision making, suggesting that the RSC is involved in similar processes.

Native bovine selenocysteine tRNA(Sec) secondary structure as probed by two plant single-strand-specific nucleases

Two single-strand-specific nucleases, discovered in plants, have been used to investigate the secondary and tertiary structures of the native bovine liver selenocysteine tRNA(Sec). To check the possible influence of nucleotide modifications on these structures, we compared the results obtained with the fully modified tRNA to the unmodified transcript prepared by in vitro T7 transcription of the Xenopus laevis tRNA(Sec) gene. We found that the structures in solution of the native tRNA(Sec) and the transcript are very similar despite some differences in accessibility to the enzymatic probes. Indeed, the modified anticodon-loop of native bovine tRNA(Sec), containing 5-methylcarboxymethyluridine (mcm5U34) and N6-isopentenyladenosine (i6A37), is less accessible to Rn nuclease than that of the transcript: the intensity of bands representing cuts at A36 and A38 is much lower as compared to those of the transcript, whereas no cuts were found at the level of i6A37 in the anticodon loop of the native molecule. Surprisingly, the variable arm of the native molecule has been found to be more susceptible to single-strand-specific nuclease action, suggesting a looser structure of the variable arm in native bovine tRNA(Sec) than in the transcript.