Thalamocortical projections of the anteroventral thalamic nucleus in the rabbit

Convergence of location, direction, and theta in the rat anteroventral thalamic nucleus

The thalamus and cortex are anatomically interconnected, with the thalamus providing integral information for cortical functions. The anteroventral thalamic nucleus (AV) is reciprocally connected to retrosplenial cortex (RSC). Two distinct AV subfields, dorsomedial (AVDM) and ventrolateral (AVVL), project differentially to granular vs. dysgranular RSC, respectively. To probe if functional responses of AV neurons differ, we recorded single neurons and local field potentials from AVDM and AVVL in rats during foraging. We observed place cells (neurons modulated by spatial location) in both AVDM and AVVL. Additionally, we characterized neurons modulated by theta oscillations, heading direction, and a conjunction of these. Place cells and conjunctive Theta-by-Head direction cells were more prevalent in AVVL; more non-conjunctive theta and directional neurons were prevalent in AVDM. These findings add further evidence that there are two thalamocortical circuits connecting AV and RSC, and reveal that the signaling involves place information in addition to direction and theta.

Evidence for two distinct thalamocortical circuits in retrosplenial cortex

Retrosplenial cortex (RSC) lies at the interface between sensory and cognitive networks in the brain and mediates between these, although it is not yet known how. It has two distinct subregions, granular (gRSC) and dysgranular (dRSC). The present study investigated how these subregions differ with respect to their electrophysiology and thalamic connectivity, as a step towards understanding their functions. The gRSC is more closely connected to the hippocampal formation, in which theta-band local field potential oscillations are prominent. We, therefore, compared theta-rhythmic single-unit activity between the two RSC subregions and found, mostly in gRSC, a subpopulation of non-directional cells with spiking activity strongly entrained by theta oscillations, suggesting a stronger coupling of gRSC to the hippocampal system. We then used retrograde tracers to test for differential inputs to RSC from the anteroventral thalamus (AV). We found that gRSC and dRSC differ in their afferents from two AV subfields: dorsomedial (AVDM) and ventrolateral (AVVL). Specifically: (1) as a whole AV projects more strongly to gRSC; (2) AVVL targets both gRSC and dRSC, while AVDM provides a selective projection to gRSC, (3) the gRSC projection is layer-specific: AVDM targets specifically gRSC superficial layers. These same AV projections are topographically organized with ventral AV neurons innervating rostral RSC and dorsal AV neurons innervating caudal RSC. These combined results suggest the existence of two distinct but interacting RSC subcircuits: one connecting AVDM to gRSC that may comprise part of the cognitive hippocampal system, and the other connecting AVVL to both RSC regions that may link hippocampal and perceptual regions. We suggest that these subcircuits are distinct to allow for differential weighting during integration of converging sensory and cognitive computations: an integration that may take place in thalamus, RSC, or both.

Cellular components and circuitry of the presubiculum and its functional role in the head direction system

Orientation in space is a fundamental cognitive process relying on brain-wide neuronal circuits. Many neurons in the presubiculum in the parahippocampal region encode head direction and each head direction cell selectively discharges when the animal faces a specific direction. Here, we attempt to link the current knowledge of afferent and efferent connectivity of the presubiculum to the processing of the head direction signal. We describe the cytoarchitecture of the presubicular six-layered cortex and the morphological and electrophysiological intrinsic properties of principal neurons and interneurons. While the presubicular head direction signal depends on synaptic input from thalamus, the intra- and interlaminar information flow in the microcircuit of the presubiculum may contribute to refine directional tuning. The interaction of a specific interneuron type, the Martinotti cells, with the excitatory pyramidal cells may maintain the head direction signal in the presubiculum with attractor-like properties.

Brain anatomy of the 4-day-old European rabbit

The European rabbit (Oryctolagus cuniculus) is a widely used model in fundamental, medical and veterinary neurosciences. Besides investigations in adults, rabbit pups are relevant to study perinatal neurodevelopment and early behaviour. To date, the rabbit is also the only species in which a pheromone - the mammary pheromone (MP) - emitted by lactating females and active on neonatal adaptation has been described. The MP is crucial since it contributes directly to nipple localisation and oral seizing in neonates, i.e. to their sucking success. It may also be one of the non-photic cues arising from the mother, which stimulates synchronisation of the circadian system during pre-visual developmental stages. Finally, the MP promotes neonatal odour associative and appetitive conditioning in a remarkably rapid and efficient way. For these different reasons, the rabbit offers a currently unique opportunity to determine pheromonal-induced brain processing supporting adaptation early in life. Therefore, it is of interest to create a reference work of the newborn rabbit pup brain, which may constitute a tool for future multi-disciplinary and multi-approach research in this model, and allow comparisons related to the neuroethological basis of social and feeding behaviour among newborns of various species. Here, in line with existing experimental studies, and based on original observations, we propose a functional anatomical description of brain sections in 4-day-old rabbits with a particular focus on seven brain regions which appear important for neonatal perception of sensory signals emitted by the mother, circadian adaptation to the short and single daily nursing of the mother in the nest, and expression of specific motor actions involved in nipple localisation and milk intake. These brain regions involve olfactory circuits, limbic-related areas important in reward, motivation, learning and memory formation, homeostatic areas engaged in food anticipation, and regions implicated in circadian rhythm and arousal, as well as in motricity.