Evolution of the brain: Just how related are reptilian and mammalian brains? Tosches et al. used single-cell transcriptomics to study turtle, lizard, mouse, and human brain samples. They assessed how the mammalian six-layered cortex might be derived from the reptilian three-layered cortex. Despite a lack of correspondence between layers, mammalian astrocytes and adult neural stem cells shared evolutionary origins. General classes of interneuron types were represented across the evolutionary span, although subtypes were species-specific. Pieces of the much-folded mammalian hippocampus were represented as adjacent fields in the reptile brains. Science , this issue p. 881

Entorhinal Cell Clusters: There is considerable interest in understanding the function of neurons in layer 2 of the medial entorhinal cortex and how they generate their unique firing patterns, which are important in the recall of facts and past events (see the Perspective by Blair ). Ray et al. (p. 891 , published online 23 January) investigated principal cells in layer 2 by immunoreactivity, projection patterns, microcircuit analysis, and assessment of temporal discharge properties in awake, freely moving animals. In tangential sections, pyramidal neurons were clustered into patches arranged in a hexagonal grid—very similar to the patterns observed in grid cell spatial firing. These patches received selective cholinergic innervation, which is critical for sustaining grid cell activity. Kitamura et al. (p. 896 , published online 23 January) found that these cells drive a hippocampal circuit by projecting directly to the hippocampal CA1 area and synapsing with a distinct class of inhibitory neurons. This circuit provides feed-forward inhibition in combination with excitatory inputs from layer 3 cells of the medial entorhinal cortex, projecting to CA1 pyramidal cells to determine the strength and time window of temporal associative inputs.

A Primer on the reptile brain, in particular the light it sheds on the structural and functional evolution of vertebrate neural circuits.

In medial entorhinal cortex, layer 2 principal cells divide into pyramidal neurons (mostly calbindin positive) and dentate gyrus-projecting stellate cells (mostly calbindin negative). We juxtacellularly labeled layer 2 neurons in freely moving animals, but small sample size prevented establishing unequivocal structure-function relationships. We show, however, that spike locking to theta oscillations allows assigning unidentified extracellular recordings to pyramidal and stellate cells with ∼83% and ∼89% specificity, respectively. In pooled anatomically identified and theta-locking-assigned recordings, nonspatial discharges dominated, and weakly hexagonal spatial discharges and head-direction selectivity were observed in both cell types. Clear grid discharges were rare and mostly classified as pyramids (19%, 19/99 putative pyramids versus 3%, 3/94 putative stellates). Most border cells were classified as stellate (11%, 10/94 putative stellates versus 1%, 1/99 putative pyramids). Our data suggest weakly theta-locked stellate border cells provide spatial input to dentate gyrus, whereas strongly theta-locked grid discharges occur mainly in hexagonally arranged pyramidal cell patches and do not feed into dentate gyrus.

The parasubiculum is a major input structure of layer 2 of medial entorhinal cortex, where most grid cells are found. Here we investigated parasubicular circuits of the rat by anatomical analysis combined with juxtacellular recording/labeling and tetrode recordings during spatial exploration. In tangential sections, the parasubiculum appears as a linear structure flanking the medial entorhinal cortex mediodorsally. With a length of ∼5.2 mm and a width of only ∼0.3 mm (approximately one dendritic tree diameter), the parasubiculum is both one of the longest and narrowest cortical structures. Parasubicular neurons span the height of cortical layers 2 and 3, and we observed no obvious association of deep layers to this structure. The “superficial parasubiculum” (layers 2 and 1) divides into ∼15 patches, whereas deeper parasubicular sections (layer 3) form a continuous band of neurons. Anterograde tracing experiments show that parasubicular neurons extend long “circumcurrent” axons establishing a “global” internal connectivity. The parasubiculum is a prime target of GABAergic and cholinergic medial septal inputs. Other input structures include the subiculum, presubiculum, and anterior thalamus. Functional analysis of identified and unidentified parasubicular neurons shows strong theta rhythmicity of spiking, a large fraction of head-direction selectivity (50%, 34 of 68), and spatial responses (grid, border and irregular spatial cells, 57%, 39 of 68). Parasubicular output preferentially targets patches of calbindin-positive pyramidal neurons in layer 2 of medial entorhinal cortex, which might be relevant for grid cell function. These findings suggest the parasubiculum might shape entorhinal theta rhythmicity and the (dorsoventral) integration of information across grid scales.SIGNIFICANCE STATEMENTGrid cells in medial entorhinal cortex (MEC) are crucial components of an internal navigation system of the mammalian brain. The parasubiculum is a major input structure of layer 2 of MEC, where most grid cells are found. Here we provide a functional and anatomical characterization of the parasubiculum and show that parasubicular neurons display unique features (i.e., strong theta rhythmicity of firing, prominent head-direction selectivity, and output selectively targeted to layer 2 pyramidal cell patches of MEC). These features could contribute to shaping the temporal and spatial code of downstream grid cells in entorhinal cortex.