The development of spatial behaviour and the hippocampal neural representation of space

Behavioral comorbidities of early-life seizures: Insights from developmental studies in rats

Childhood epilepsy is frequently associated with neurobehavioral comorbidities such as depression, anxiety, cognitive impairments, and social dysfunction, as revealed by both clinical and experimental studies. Despite extensive neurophysiological research, behavioral studies in developing animals remain limited and underreported. Here, we review the behavioral impact of early-life seizures (ELSs) in commonly used rat models in developmental studies. We outline suitable tests and provide guidance on how traditional tests should be adapted and interpreted in this context. Finally, we examine factors influencing behavioral analysis in developmental studies, exploring confounding variables and offering strategies to minimize their impact.

Effects of cypermethrin exposure on learning and memory functions and anxiety-like behavior in rats

BACKGROUND

Cypermethrin (CYP), a synthetic pyrethroid widely used to control plant pests, has been associated with various diseases in humans exposed to pesticides, either directly or indirectly. This study aimed to examine the effects of CYP on learning and memory functions, as well as anxiety-like behavior.

METHODS

Forty male Wistar rats (8 weeks old) were randomly assigned to 4 groups: The first group served as the control, while the other three groups received different doses of CYP (5, 20, and 80 mg/kg) via gavage once daily for one month. Passive avoidance learning (PAL) and memory were assessed using the shuttle box test, cognitive memory was evaluated using the novel object recognition (NOR) test, and spatial memory was measured with the Morris water maze (MWM) test. The elevated plus-maze (EPM) and open field tests were used to assess locomotor activity and anxiety levels.

RESULTS

In the PAL test, significant differences were observed in the time spent in the dark compartment (TDC) and step-through latency in the retention trial (STLr) in rats receiving 80 mg/kg of CYP. MWM results indicated memory impairment in rats treated with 20 and 80 mg/kg of CYP. Additionally, rats treated with the highest dose of CYP (80 mg/kg) showed a reduction in the number of entries into the open arms of the EPM compared to the control group.

CONCLUSION

This study demonstrates that CYP negatively affects learning and memory retention. Further research is needed to explore the precise mechanisms by which this toxin impacts cognitive functions.

Early and late place cells during postnatal development of the hippocampus

A proportion of hippocampal CA1 neurons function as place cells from the onset of navigation, which are referred to as early place cells. It is not clear whether this subset of neurons is predisposed to become place cells during early stages, or if all neurons have this potential. Here, we longitudinally imaged the activity of CA1 neurons in developing male rats during navigation with both one-photon and two-photon microscopy. Our results suggested that a largely consistent population of cells functioned as early place cells, demonstrating higher spatial coding abilities across environments and a tendency to form more synchronous cell assemblies. Early place cells were present in both deep and superficial layers of CA1. Cells in the deep layer exhibited greater synchrony than those in the superficial layer during early ages. These results support the theory that an initial cognitive map is primarily shaped by a predetermined set of hippocampal cells.

A behavioral roadmap for the development of agency in the rodent

Behavioral interactions within the nuclear family play a pivotal role in the emergence of agency: the capacity to regulate physiological, psychological and social needs. While behaviors may develop over days or weeks in line with nervous system maturation, individual behaviors can occur on sub-second time scales making it challenging to track development in lab studies with brief observation periods, or in field studies with limited temporal precision and animal identification. Here we study development in families of gerbils, a highly social rodent, collecting tens of millions of behavior time points and implementing machine learning methods to track individual subjects. We provided maturing gerbils with a large, undisturbed environment between postnatal day 15 and the age at which they would typically disperse from the family unit (day 30). We identified complex and distinct developmental trajectories for food and water acquisition, solitary exploration, and social behaviors, some of which displayed sex differences and diurnal patterns. Our work supports the emergence of well-delineated autonomous and social behavior phenotypes that correlate with specific periods and loci of neural maturation.

Egocentric processing of items in spines, dendrites, and somas in the retrosplenial cortex

Egocentric representations of external items are essential for spatial navigation and memory. Here, we explored the neural mechanisms underlying egocentric processing in the retrosplenial cortex (RSC), a pivotal area for memory and navigation. Using one-photon and two-photon calcium imaging, we identified egocentric tuning for environment boundaries in dendrites, spines, and somas of RSC neurons (egocentric boundary cells) in the open-field task. Dendrites with egocentric tuning tended to have similarly tuned spines. We further identified egocentric neurons representing landmarks in a virtual navigation task or remembered cue location in a goal-oriented task, respectively. These neurons formed an independent population with egocentric boundary cells, suggesting that dedicated neurons with microscopic clustering of functional inputs shaped egocentric boundary processing in RSC and that RSC adopted a labeled line code with distinct classes of egocentric neurons responsible for representing different items in specific behavioral contexts, which could lead to efficient and flexible computation.

Septotemporal Variation of Information Processing in the Hippocampus of Fmr1 KO Rat

INTRODUCTION

Fragile X messenger ribonucleoprotein (FMRP) is a protein involved in many neuronal processes in the nervous system including the modulation of synaptic transmission. The loss of FMRP produces the fragile X syndrome (FXS), a neurodevelopmental disorder affecting synaptic and neuronal function and producing cognitive impairments. However, the effects of FXS on short-term processing of synaptic inputs and neuronal outputs in the hippocampus have not yet been sufficiently clarified. Furthermore, it is not known whether dorsal and ventral hippocampi are affected similarly or not in FXS.

METHOD

We used an Fmr1 knockout (KO) rat model of FXS and recordings of evoked field potentials from the CA1 field of transverse slices from both the dorsal and the ventral hippocampi of adult rats.

RESULTS

Following application of a frequency stimulation protocol consisting of a ten-pulse train and recordings of fEPSP, we found that the dorsal but not ventral KO hippocampus shows altered short-term synaptic plasticity. Furthermore, applying the frequency stimulation protocol and recordings of population spikes, both segments of the KO hippocampus display altered short-term neuronal dynamics.

CONCLUSIONS

These data suggest that short-term processing of synaptic inputs is affected in the dorsal, not ventral, FXS hippocampus, while short-term processing of neuronal output is affected in both segments of the FXS hippocampus in a similar way. These FXS-associated changes may have significant impact on the functions of the dorsal and ventral hippocampi in individuals with FXS.

Environment geometry alters subiculum boundary vector cell receptive fields in adulthood and early development

Boundaries to movement form a specific class of landmark information used for navigation: Boundary Vector Cells (BVCs) are neurons which encode an animal's location as a vector displacement from boundaries. Here we characterise the prevalence and spatial tuning of subiculum BVCs in adult and developing male rats, and investigate the relationship between BVC spatial firing and boundary geometry. BVC directional tunings align with environment walls in squares, but are uniformly distributed in circles, demonstrating that environmental geometry alters BVC receptive fields. Inserted barriers uncover both excitatory and inhibitory components to BVC receptive fields, demonstrating that inhibitory inputs contribute to BVC field formation. During post-natal development, subiculum BVCs mature slowly, contrasting with the earlier maturation of boundary-responsive cells in upstream Entorhinal Cortex. However, Subiculum and Entorhinal BVC receptive fields are altered by boundary geometry as early as tested, suggesting this is an inherent feature of the hippocampal representation of space.

Postnatal persistence of hippocampal Cajal-Retzius cells has a crucial role in the establishment of the hippocampal circuit

Cajal-Retzius (CR) cells are a transient neuron type that populate the postnatal hippocampus. To understand how the persistence of CR cells influences the maturation of hippocampal circuits, we combined a specific transgenic mouse line with viral vector injection to selectively ablate CR cells from the postnatal hippocampus. We observed layer-specific changes in the dendritic complexity and spine density of CA1 pyramidal cells. In addition, transcriptomic analysis highlighted significant changes in the expression of synapse-related genes across development. Finally, we were able to identify significant changes in the expression levels of latrophilin 2, a postsynaptic guidance molecule known for its role in the entorhinal-hippocampal connectivity. These findings were supported by changes in the synaptic proteomic content in CA1 stratum lacunosum-moleculare. Our results reveal a crucial role for CR cells in the establishment of the hippocampal network.

Sharpening the blades of the dentate gyrus: how adult-born neurons differentially modulate diverse aspects of hippocampal learning and memory

For decades, the mammalian hippocampus has been the focus of cellular, anatomical, behavioral, and computational studies aimed at understanding the fundamental mechanisms underlying cognition. Long recognized as the brain's seat for learning and memory, a wealth of knowledge has been accumulated on how the hippocampus processes sensory input, builds complex associations between objects, events, and space, and stores this information in the form of memories to be retrieved later in life. However, despite major efforts, our understanding of hippocampal cognitive function remains fragmentary, and models trying to explain it are continually revisited. Here, we review the literature across all above-mentioned domains and offer a new perspective by bringing attention to the most distinctive, and generally neglected, feature of the mammalian hippocampal formation, namely, the structural separability of the two blades of the dentate gyrus into "supra-pyramidal" and "infra-pyramidal". Next, we discuss recent reports supporting differential effects of adult neurogenesis in the regulation of mature granule cell activity in these two blades. We propose a model for how differences in connectivity and adult neurogenesis in the two blades can potentially provide a substrate for subtly different cognitive functions.

How Do You Build a Cognitive Map? The Development of Circuits and Computations for the Representation of Space in the Brain

In mammals, the activity of neurons in the entorhinal-hippocampal network is modulated by the animal's position and its movement through space. At multiple stages of this distributed circuit, distinct populations of neurons can represent a rich repertoire of navigation-related variables like the animal's location, the speed and direction of its movements, or the presence of borders and objects. Working together, spatially tuned neurons give rise to an internal representation of space, a cognitive map that supports an animal's ability to navigate the world and to encode and consolidate memories from experience. The mechanisms by which, during development, the brain acquires the ability to create an internal representation of space are just beginning to be elucidated. In this review, we examine recent work that has begun to investigate the ontogeny of circuitry, firing patterns, and computations underpinning the representation of space in the mammalian brain.

Bilirubin-Induced Transcriptomic Imprinting in Neonatal Hyperbilirubinemia

Recent findings indicated aberrant epigenetic control of the central nervous system (CNS) development in hyperbilirubinemic Gunn rats as an additional cause of cerebellar hypoplasia, the landmark of bilirubin neurotoxicity in rodents. Because the symptoms in severely hyperbilirubinemic human neonates suggest other regions as privileged targets of bilirubin neurotoxicity, we expanded the study of the potential impact of bilirubin on the control of postnatal brain development to regions correlating with human symptoms. Histology, transcriptomic, gene correlation, and behavioral studies were performed. The histology revealed widespread perturbation 9 days after birth, restoring in adulthood. At the genetic level, regional differences were noticed. Bilirubin affected synaptogenesis, repair, differentiation, energy, extracellular matrix development, etc., with transient alterations in the hippocampus (memory, learning, and cognition) and inferior colliculi (auditory functions) but permanent changes in the parietal cortex. Behavioral tests confirmed the presence of a permanent motor disability. The data correlate well both with the clinic description of neonatal bilirubin-induced neurotoxicity, as well as with the neurologic syndromes reported in adults that suffered neonatal hyperbilirubinemia. The results pave the way for better deciphering the neurotoxic features of bilirubin and evaluating deeply the efficacy of new therapeutic approaches against the acute and long-lasting sequels of bilirubin neurotoxicity.

Optogenetic modulation of hippocampal oscillations ameliorates spatial cognition and hippocampal dysrhythmia following early-life seizures

There is increasing human and animal evidence that brain oscillations play a critical role in the development of spatial cognition. In rat pups, disruption of hippocampal rhythms via optogenetic stimulation during the critical period for memory development impairs spatial cognition. Early-life seizures are associated with long-term deficits in spatial cognition and aberrant hippocampal oscillatory activity. Here we asked whether modulation of hippocampal rhythms following early-life seizures can reverse or improve hippocampal connectivity and spatial cognition. We used optogenetic stimulation of the medial septum to induce physiological 7 Hz theta oscillations in the hippocampus during the critical period of spatial cognition following early-life seizures. Optogenetic stimulation of the medial septum in control and rats subjected to early-life seizures resulted in precisely regulated frequency-matched hippocampal oscillations. Rat pups receiving active blue light stimulation performed better than the rats receiving inert yellow light in a test of spatial cognition. The improvement in spatial cognition in these rats was associated with a faster theta frequency and higher theta power, coherence and phase locking value in the hippocampus than rats with early-life seizures receiving inert yellow light. These findings indicate that following early life seizures, modification of hippocampal rhythms may be a potential novel therapeutic modality.

Navigational roots of spatial and temporal memory structure

Our minds are constantly in transit, from the present to the past to the future, across places we have and have not directly experienced. Nevertheless, memories of our mental time travel are not organized continuously and are adaptively chunked into contexts and episodes. In this paper, I will review evidence that suggests that spatial boundary representations play a critical role in providing structure to both our spatial and temporal memories. I will illustrate the intimate connection between hippocampal spatial mapping and temporal sequencing of episodic memory to propose that high-level cognitive processes like mental time travel and conceptual mapping are rooted in basic navigational mechanisms that we humans and nonhuman animals share. Our neuroscientific understanding of hippocampal function across species may provide new insight into the origins of even the most uniquely human cognitive abilities.

Influence of early social isolation on general activity and spatial learning in adult WAG/Rij rats

The study identifies the critical period of early ontogeny, during which social factors have the greatest influence on the operant behavior with positive reinforcement in adult WAG/Rij rats. Individual social isolation of rats from dam and siblings was performed daily for 3 h during postnatal day (PND) 2-8, 9-15, and 16-22. General activity and water consumption were examined using the IntelliCage (IC) in adulthood. The operant behavior training was performed in four consecutive sessions: free exploration of the IC environment (adaptation), learning to retrieve water by nosepoking (nosepoke adaptation), spatial learning to retrieve water in the specific corner (place learning), and retraining with a change of a place preference (reversal learning). Social isolation during PND16-22 led to the greatest behavioral changes in all sessions of the experiment. These rats were more active, consumed more water, demonstrated a higher ratio of visits with drinking to the total number, and relearned faster after changing the location of the rewarded corner. Thus, the postnatal period between days 16 and 22 in WAG/Rij rat pups is more sensitive to social isolation for change of adaptive behavior in the IC in adulthood.

Maternal gastrointestinal nematode infection enhances spatial memory of uninfected juvenile mouse pups

The developing brain is particularly vulnerable to factors including maternal infection during pregnancy. Establishment of neural networks critical for memory and cognition begins during the perinatal period, when Heligmosomoides bakeri, a gastrointestinal (GI) nematode restricted to the maternal mouse intestine, has been shown to upregulate expression of long-term potentiation genes in the young rodent pup brain. We explored the impact of maternal infection during pregnancy and early lactation on the spatial behavior of uninfected male and female juvenile mice. Pre-weaned pups of H. bakeri infected dams exhibited less exploratory behaviour compared to pups of uninfected dams on postnatal day (PD) 16 but not PD 17, possibly reflecting a transient fear of an unfamiliar environment and/or a brief neurodevelopmental delay. Our two spatial memory tests show for the first time an enhancement of spatial memory in response to maternal nematode infection regardless of pup sex. At PD 17, pups of infected dams expressed object location memories after 3 h in the Object Location Test whereas offspring of uninfected mothers did not. In addition, at PD 34, juveniles of infected mothers retained their ability to find the escape hole in the Barnes Maze Test for one week whereas offspring from uninfected mothers did not. This finding is even more striking given that spatial memory was positively associated with pup length, yet this maternal infection impaired linear growth of pups. Thus, the positive impact of maternal infection on spatial memory countered any impairment associated with the shorter length of the pups. Overall, these novel findings indicate that a maternal GI nematode infection during pregnancy and lactation positively influences the spatial memory of uninfected juvenile offspring with potential fitness implications for the next generation.

Extensive Structural Remodeling of the Axonal Arbors of Parvalbumin Basket Cells during Development in Mouse Neocortex

Parvalbumin-containing (PV+) basket cells are specialized cortical interneurons that regulate the activity of local neuronal circuits with high temporal precision and reliability. To understand how the PV+ interneuron connectivity underlying these functional properties is established during development, we used array tomography to map pairs of synaptically connected PV+ interneurons and postsynaptic neurons from the neocortex of mice of both sexes. We focused on the axon-myelin unit of the PV+ interneuron and quantified the number of synapses onto the postsynaptic neuron, length of connecting axonal paths, and their myelination at different time points between 2 weeks and 7 months of age. We find that myelination of the proximal axon occurs very rapidly during the third and, to a lesser extent, fourth postnatal weeks. The number of synaptic contacts made by the PV+ interneuron on its postsynaptic partner meanwhile is significantly reduced to about one-third by the end of the first postnatal month. The number of autapses, the synapses that PV+ interneurons form on themselves, however, remains constant throughout the examined period. Axon reorganizations continue beyond postnatal month 2, with the postsynaptic targets of PV+ interneurons gradually shifting to more proximal locations, and the length of axonal paths and their myelin becoming conspicuously uniform per connection. These continued microcircuit refinements likely provide the structural substrate for the robust inhibitory effects and fine temporal precision of adult PV+ basket cells.SIGNIFICANCE STATEMENT The axon of adult parvalbumin-containing (PV+) interneurons is highly specialized for fast and reliable neurotransmission. It is myelinated and forms synapses mostly onto the cell bodies and proximal dendrites of postsynaptic neurons for maximal impact. In this study, we follow the development of the PV+ interneuron axon, its myelination and synapse formation, revealing a rapid sequence of axonal reorganization, myelination of the PV+ interneuron proximal axon, and pruning of almost two-thirds of the synapses in an individual connection. This is followed by a prolonged period of axon refinement and additional myelination leading to a remarkable precision of connections in the adult mouse cortex, consistent with the temporal precision and fidelity of PV+ interneuron action.

Differential prioritization of intramaze cue and boundary information during spatial navigation across the human lifespan

Spatial learning can be based on intramaze cues and environmental boundaries. These processes are predominantly subserved by striatal- and hippocampal-dependent circuitries, respectively. Maturation and aging processes in these brain regions may affect lifespan differences in their contributions to spatial learning. We independently manipulated an intramaze cue or the environment's boundary in a navigation task in 27 younger children (6-8 years), 30 older children (10-13 years), 29 adolescents (15-17 years), 29 younger adults (20-35 years) and 26 older adults (65-80 years) to investigate lifespan age differences in the relative prioritization of either information. Whereas learning based on an intramaze cue showed earlier maturation during the progression from younger to later childhood and remained relatively stable across adulthood, maturation of boundary-based learning was more protracted towards peri-adolescence and showed strong aging-related decline. Furthermore, individual differences in prioritizing intramaze cue- over computationally more demanding boundary-based learning was positively associated with cognitive processing fluctuations and this association was partially mediated by spatial working memory capacity during adult, but not during child development. This evidence reveals different age gradients of two modes of spatial learning across the lifespan, which seem further influenced by individual differences in cognitive processing fluctuations and working memory, particularly during aging.

5-HT1A receptor-mediated attenuation of synaptic transmission in rat medial vestibular nucleus impacts on vestibular-related motor function

KEY POINTS

Chemogenetic activation of medial vestibular nucleus-projecting 5-HT neurons resulted in deficits in vestibular-mediated tasks, including negative geotaxis, balance beam and rota-rod tests. The 5-HT_1A receptor mediates the vestibular-related behavioural effects of 5-HT in the vestibular nucleus. 5-HT_1A receptor activation attenuated evoked excitatory postsynaptic currents and evoked inhibitory postsynaptic currents via a presynaptic mechanism in the vestibular nucleus.

ABSTRACT

While the anxiolytic effects of serotonergic neuromodulation are well studied, its role in sensorimotor coordination and postural control is unclear. In this study, we show that an increase of serotonin (5-hydroxytryptamine, 5-HT) at the medial vestibular nucleus (MVN), a brainstem centre for vestibulospinal coordination, by either direct cannula administration or chemogenetic stimulation of MVN-projecting serotonergic neurons, adversely affected performance of rats in vestibular-mediated tasks, including negative geotaxis, balance beam and rota-rod tests. Application of the 5-HT₁ and 5-HT₇ receptor co-agonist 8-hydroxy-2-(di-n-propylamino) tetralin recapitulated the effect of 5-HT, while co-administration of the specific 5-HT_1A receptor antagonist WAY 100135 effectively abolished all 5-HT-induced behavioural deficits. This indicated that 5-HT_1A receptors mediated the effects of 5-HT in the rat MVN. Using whole-cell patch-clamp recording, we demonstrated that 5-HT_1A receptor activation attenuated both evoked excitatory and evoked inhibitory postsynaptic currents through a presynaptic mechanism in the rat MVN. The results thus highlight the 5-HT_1A receptor as the gain controller of vestibular-related brainstem circuits for posture and balance.

Phospholipase Cβ3 in the hippocampus may mediate impairment of memory by long-term blockade of orexin 1 receptors assessed by the Morris water maze

Orexin-A is an endogenous peptide with receptors throughout the brain. According to some recent research, learning and memory are affected by the central administration of orexin; however, no study so far has investigated the long-term inhibition of the orexinergic system. The present study has evaluated the effect of pretraining administration of orexin 1 receptor (OXR1) antagonist, SB-334867, on the acquisition of memory. The Morris water maze (MWM) task was used for training and trial purposes in all groups. Memory performance was analyzed by measuring escape latency, traveled distance, and time spent in the target quadrant. Moreover, the effect of SB-334867 on phospholipase Cβ3 (PLCβ3) levels in the CA1 region of hippocampus slices was examined. Hippocampus slices were prepared using an immunohistochemistry (IHC) approach. SB-334867 (20 mg/kg) increased escape latency in SB-treated rats compared to SB-vehicle group (P < 0.01). SB-treated rats spent less time in the target quadrant compared to the SB-vehicle group (P < 0.001). Distance traveled in the target quadrant was significantly more in SB-treated rats compared to the SB-vehicle group (P < 0.001). Furthermore, SB-334867 decreased PLCβ3 levels in the CA1 of the hippocampus (P < 0.01 and P < 0.05, respectively). Put together, our results suggest that the long-term inhibition of OXR1 plays a prominent role in spatial learning and memory, probably by attenuating PLCβ3 in CA1 neurons.

Disruption of hippocampal rhythms via optogenetic stimulation during the critical period for memory development impairs spatial cognition

BACKGROUND

Hippocampal oscillations play a critical role in the ontogeny of allocentric memory in rodents. During the critical period for memory development, hippocampal theta is the driving force behind the temporal coordination of neuronal ensembles underpinning spatial memory. While known that hippocampal oscillations are necessary for normal spatial cognition, whether disrupted hippocampal oscillatory activity during the critical period impairs long-term spatial memory is unknown. Here we investigated whether disruption of normal hippocampal rhythms during the critical period have enduring effects on allocentric memory in rodents.

OBJECTIVE/HYPOTHESIS

We hypothesized that disruption of hippocampal oscillations via artificial regulation of the medial septum during the critical period for memory development results in long-standing deficits in spatial cognition.

METHODS

After demonstrating that pan-neuronal medial septum (MS) optogenetic stimulation (465 nm activated) regulated hippocampal oscillations in weanling rats we used a random pattern of stimulation frequencies to disrupt hippocampal theta rhythms for either 1Hr or 5hr a day between postnatal (P) days 21-25. Non-stimulated and yellow light-stimulated (590 nm) rats served as controls. At P50-60 all rats were tested for spatial cognition in the active avoidance task. Rats were then sacrificed, and the MS and hippocampus assessed for cell loss. Power spectrum density of the MS and hippocampus, coherences and voltage correlations between MS and hippocampus were evaluated at baseline for a range of stimulation frequencies from 0.5 to 110 Hz and during disruptive hippocampal stimulation. Unpaired t-tests and ANOVA were used to compare oscillatory parameters, behavior and cell density in all animals.

RESULTS

Non-selective optogenetic stimulation of the MS in P21 rats resulted in precise regulation of hippocampal oscillations with 1:1 entrainment between stimulation frequency (0.5-110 Hz) and hippocampal local field potentials. Across bandwidths MS stimulation increased power, coherence and voltage correlation at all frequencies whereas the disruptive stimulation increased power and reduced coherence and voltage correlations with most statistical measures highly significant (p < 0.001, following correction for false detection). Rats receiving disruptive hippocampal stimulation during the critical period for memory development for either 1Hr or 5hr had marked impairment in spatial learning as measured in active avoidance test compared to non-stimulated or yellow light-control rats (p < 0.001). No cell loss was measured between the blue-stimulated and non-stimulated or yellow light-stimulated controls in either the MS or hippocampus.

CONCLUSION

The results demonstrated that robust regulation of hippocampal oscillations can be achieved with non-selective optogenetic stimulation of the MS in rat pups. A disruptive hippocampal stimulation protocol, which markedly increases power and reduces coherence and voltage correlations between the MS and hippocampus during the critical period of memory development, results in long-standing spatial cognitive deficits. This spatial cognitive impairment is not a result of optogenetic stimulation-induced cell loss.

Sex differences in offspring neurodevelopment, cognitive performance and microglia morphology associated with maternal diabetes: Putative targets for insulin therapy

Diabetes during pregnancy has been shown to affect the central nervous system (CNS) of the offspring, resulting in short- and long-term adverse effects. Children of diabetic mothers are more likely to develop cognitive impairment, also having increased susceptibility to psychiatric disorders. Microglia, the immune cells of the CNS, work as sensors of environmental changes, namely metabolic challenges, as early as the intrauterine period. During this period, microglia is actively involved in processes of neurogenesis, synaptic pruning and detection of any environmental alteration that may impact brain development. The remarkable sex dimorphism in neurodevelopment, as well as sex differences in the morphology and immune function of microglia during development, led us to clarify if maternal diabetes affects specific behavioral traits and microglia morphology during infancy in a sex-specific manner. Another important goal of this study was to clarify if insulin, the gold standard treatment of diabetes during gestation, could prevent maternal diabetes-induced behavioral changes, as well as microglia morphology, also considering sex specificities. Other molecular and cellular players potentially involved in the link between changes in metabolism and behavior were also analyzed in the hippocampus, a brain region implicated in cognition and other behavioral outcomes. Diabetes during pregnancy globally delayed female and male offspring development and was associated with impairments in recognition memory, but only in female offspring. In line with these results, at early and late infancy, some molecular and cellular markers were altered in offspring hippocampus in a sex-specific manner. The strict control of glycemia by insulin during pregnancy prevented most of the negative effects induced by uncontrolled hyperglycemia. Notably, insulin administration to diabetic dams may also modulate offspring development in a way that differs from what is observed in physiological conditions, since it promoted the expedited acquisition of developmental milestones and of discrimination ability at memory test, also inducing a hyper-ramification of male and female hippocampal microglia. Importantly, this study highlights the importance of analyzing the impact of maternal diabetes and insulin therapy, taking into account sex differences, since male and female present different vulnerabilities to hyperglycemia in this critical period of life.

Using hippocampal-dependent eyeblink conditioning to predict individual differences in spatial reorientation strategies in 3- to 6-year-olds

The hippocampus is a subcortical structure in the medial temporal lobe involved in cognitive functions such as spatial navigation and reorientation, episodic memory, and associative learning. While much is understood about the role of hippocampal function in learning and memory in adults, less is known about the relations between the hippocampus and the development of these cognitive skills in young children due to the limitations of using standard methods (e.g., MRI) to examine brain structure and function in developing populations. This study used hippocampal-dependent trace eyeblink conditioning (EBC) as a feasible approach to examine individual differences in hippocampal functioning as they relate to spatial reorientation and episodic memory performance in young children. Three- to six-year-old children (N = 50) completed tasks that measured EBC, spatial reorientation, and episodic memory, as well as non-hippocampal-dependent processing speed abilities. Results revealed that when age was held constant, individual differences in EBC performance were significantly related to individual differences in performance on the spatial reorientation test, but not on the episodic memory or processing speed tests. When the relations between hippocampal-dependent EBC and different reorientation strategies were explored, it was found that individual differences in hippocampal function predicted the use of geometric information for reorienting in space as opposed to a combined strategy that uses both geometric information and salient visual cues. The utilization of eyeblink conditioning to examine hippocampal function in young populations and its implications for understanding the dissociation between spatial reorientation and episodic memory development are discussed.

Postnatal Development of Functional Projections from Parasubiculum and Presubiculum to Medial Entorhinal Cortex in the Rat

Neurons in parasubiculum (PaS), presubiculum (PrS), and medial entorhinal cortex (MEC) code for place (grid cells) and head direction. Directional input has been shown to be important for stable grid cell properties in MEC, and PaS and PrS have been postulated to provide this information to MEC. In line with this, head direction cells in those brain areas are present at postnatal day 11 (P11), having directional tuning that stabilizes shortly after eye opening, which is before premature grid cells emerge in MEC at P16. Whether functional connectivity between these structures exists at those early postnatal stages is unclear. Using anatomical tracing, voltage-sensitive dye imaging and single-cell patch recordings in female and male rat brain slices between P2 and P61, we determined when the pathways from PaS and PrS to MEC emerge, become functional, and how they develop. Anatomical connections from PaS and PrS to superficial MEC emerge between P4 and P6. Monosynaptic connectivity from PaS and PrS to superficial MEC was measurable from P9 to P10 onward, whereas connectivity with deep MEC was measurable from P11 to P12. From P14/P15 on, reactivity of MEC neurons to parasubicular and presubicular inputs becomes adult-like and continues to develop until P28-P30. The maturation of the efficacy of both inputs between P9 and P21 is paralleled by maturation of morphological properties, changes in intrinsic properties of MEC principal neurons, and changes in the GABAergic network of MEC. In conclusion, synaptic projections from PaS and PrS to MEC become functional and adult-like before the emergence of grid cells in MEC.SIGNIFICANCE STATEMENT Head direction information, crucial for grid cells in medial entorhinal cortex (MEC), is thought to enter MEC via parasubiculum (PaS) and presubiculum (PrS). Unraveling the development of functional connections between PaS, PrS, and MEC is key to understanding how spatial navigation, an important cognitive function, may evolve. To gain insight into the development, we used anatomical tracing techniques, voltage-sensitive dye imaging, and single-cell recordings. The combined data led us to conclude that synaptic projections from PaS and PrS to MEC become functional and adult-like before eye opening, allowing crucial head direction information to influence place encoding before the emergence of grid cells in rat MEC.

Synergistic effects of combined vaccination with BCG and influenza vaccines on spatial cognition and hippocampal plasticity in rats

Previous study has demonstrated the neurobeneficial role of BCG and influenza vaccines. Based on this, our study concentrated on the synergistic effects on development of central nervous system by combined vaccination with BCG and influenza vaccines in rats. Our results displayed that pups combinedly vaccinated with BCG and influenza vaccines showed a significant enhance in spatial cognition, induction of LTP, hippocampal neurogenesis and morphology of dendritic spines compared with pups vaccinated with BCG solely. Furthermore, combined vaccination with BCG and influenza vaccines showed higher expression of BDNF, IGF-1, IL-4, IFN-γ and lower IL-1β, TNF-α and IL-6 than BCG. Taken together, combined vaccination with BCG and influenza vaccines presented synergistic effects on spatial cognition and hippocampal plasticity in rats.

Coordinated Emergence of Hippocampal Replay and Theta Sequences during Post-natal Development

Hippocampal place cells encode an animal’s current position in space during exploration [1]. During sleep, hippocampal network activity recapitulates patterns observed during recent experience: place cells with overlapping spatial fields show a greater tendency to co-fire (“reactivation”) [2], and temporally ordered and compressed sequences of place cell firing observed during wakefulness are reinstated (“replay”) [3, 4, 5]. Reactivation and replay may underlie memory consolidation [6, 7, 8, 9, 10]. Compressed sequences of place cell firing also occur during exploration: during each cycle of the theta oscillation, the set of active place cells shifts from those signaling positions behind to those signaling positions ahead of an animal’s current location [11, 12]. These “theta sequences” have been linked to spatial planning [13]. Here, we demonstrate that, before weaning (post-natal day [P]21), offline place cell activity associated with sharp-wave ripples (SWRs) reflects predominantly stationary locations in recently visited environments. By contrast, sequential place cell firing, describing extended trajectories through space during exploration (theta sequences) and subsequent rest (replay), emerge gradually after weaning in a coordinated fashion, possibly due to a progressive decrease in the threshold for experience-driven plasticity. Hippocampus-dependent learning and memory emerge late in altricial mammals [14, 15, 16, 17], appearing around weaning in rats and slowly maturing thereafter [14, 15]. In contrast, spatially localized firing is observed 1 week earlier (with reduced spatial tuning and stability) [18, 19, 20, 21]. By examining the development of hippocampal reactivation, replay, and theta sequences, we show that the coordinated maturation of offline consolidation and online sequence generation parallels the late emergence of hippocampal memory in the rat.

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Hippocampal activity encoding single places is reactivated during sleep in young rats

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The threshold for plasticity-driven reactivation is higher during early development

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Sequential firing linking contiguous places emerges gradually during development

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Maturation of online and offline sequential activity and memory are coordinated

Regulatory roles of perineuronal nets and semaphorin 3A in the postnatal maturation of the central vestibular circuitry for graviceptive reflex

Perineuronal nets (PN) restrict neuronal plasticity in the adult brain. We hypothesize that activity-dependent consolidation of PN is required for functional maturation of behavioral circuits. Using the postnatal maturation of brainstem vestibular nucleus (VN) circuits as a model system, we report a neonatal period in which consolidation of central vestibular circuitry for graviception is accompanied by activity-dependent consolidation of chondroitin sulfate (CS)-rich PN around GABAergic neurons in the VN. Postnatal onset of negative geotaxis was used as an indicator for functional maturation of vestibular circuits. Rats display negative geotaxis from postnatal day (P) 9, coinciding with the condensation of CS-rich PN around GABAergic interneurons in the VN. Delaying PN formation, by removal of primordial CS moieties on VN with chondroitinase ABC (ChABC) treatment at P6, postponed emergence of negative geotaxis to P13. Similar postponement was observed following inhibition of GABAergic transmission with bicuculline, in line with the reported role of PN in increasing excitability of parvalbumin neurons. We further reasoned that PN-CS restricts bioavailability of plasticity-inducing factors such as semaphorin 3A (Sema3A) to bring about circuit maturation. Treatment of VN explants with ChABC to liberate PN-bound Sema3A resulted in dendritic growth and arborization, implicating structural plasticity that delays synapse formation. Evidence is thus provided for the role of PN-CS-Sema3A in regulating structural and circuit plasticity at VN interneurons with impacts on the development of graviceptive postural control.

Arc/Arg3.1 mediates a critical period for spatial learning and hippocampal networks

During early postnatal development, sensory regions of the brain undergo periods of heightened plasticity which sculpt neural networks and lay the foundation for adult sensory perception. Such critical periods were also postulated for learning and memory but remain elusive and poorly understood. Here, we present evidence that the activity-regulated and memory-linked gene Arc/Arg3.1 is transiently up-regulated in the hippocampus during the first postnatal month. Conditional removal of Arc/Arg3.1 during this period permanently alters hippocampal oscillations and diminishes spatial learning capacity throughout adulthood. In contrast, post developmental removal of Arc/Arg3.1 leaves learning and network activity patterns intact. Long-term memory storage continues to rely on Arc/Arg3.1 expression throughout life. These results demonstrate that Arc/Arg3.1 mediates a critical period for spatial learning, during which Arc/Arg3.1 fosters maturation of hippocampal network activity necessary for future learning and memory storage.

Local Cues Establish and Maintain Region-Specific Phenotypes of Basal Ganglia Microglia

Microglia play critical roles in tissue homeostasis and can also modulate neuronal function and synaptic connectivity. In contrast to astrocytes and oligodendrocytes, which arise from multiple progenitor pools, microglia arise from yolk sac progenitors and are widely considered to be equivalent throughout the CNS. However, little is known about basic properties of deep brain microglia, such as those within the basal ganglia (BG). Here, we show that microglial anatomical features, lysosome content, membrane properties, and transcriptomes differ significantly across BG nuclei. Region-specific phenotypes of BG microglia emerged during the second postnatal week and were re-established following genetic or pharmacological microglial ablation and repopulation in the adult, indicating that local cues play an ongoing role in shaping microglial diversity. These findings demonstrate that microglia in the healthy brain exhibit a spectrum of distinct functional states and provide a critical foundation for defining microglial contributions to BG circuit function.

Effects of sevoflurane and clonidine on acid base status and long-term emotional and cognitive outcomes in spontaneously breathing rat pups

Background

Numerous experiments in rodents suggest a causative link between exposure to general anaesthetics during brain growth spurt and poor long-lasting neurological outcomes. Many of these studies have been questioned with regard of their translational value, mainly because of extremely long anaesthesia exposure. Therefore, the aim of the present study was to assess the impact of a short sevoflurane anaesthesia, alone or combined with clonidine treatment, on respiratory function in spontaneously breathing rat pups and overall effects on long-lasting emotional and cognitive functions.

Methods

At postnatal day (PND) 7, male Sprague Dawley rat pups were randomized into four groups and exposed to sevoflurane for one hour, to a single dose of intraperitoneal clonidine or to a combination of both and compared to a control group. Blood gas analysis was performed at the end of sevoflurane anaesthesia and after 60 minutes from clonidine or saline injection. Emotional and cognitive outcomes were evaluated in different group of animals at infancy (PND12), adolescence (PND 30–40) and adulthood (PND 70–90).

Results

Rat pups exposed to either sevoflurane or to a combination of sevoflurane and clonidine developed severe hypercapnic acidosis, but maintained normal arterial oxygenation. Emotional and cognitive outcomes were not found altered in any of the behavioural task used either at infancy, adolescence or adulthood.

Conclusions

Sixty minutes of sevoflurane anaesthesia in newborn rats, either alone or combined with clonidine, caused severe hypercapnic acidosis in spontaneously breathing rat pups, but was devoid of long-term behavioural dysfunctions in the present setting.

Sensory feedback synchronizes motor and sensory neuronal networks in the neonatal rat spinal cord

Early stages of sensorimotor system development in mammals are characterized by the occurrence of spontaneous movements. Whether and how these movements support correlated activity in developing sensorimotor spinal cord circuits remains unknown. Here we show highly correlated activity in sensory and motor zones in the spinal cord of neonatal rats in vivo. Both during twitches and complex movements, movement-generating bursts in motor zones are followed by bursts in sensory zones. Deafferentation does not affect activity in motor zones and movements, but profoundly suppresses activity bursts in sensory laminae and results in sensorimotor uncoupling, implying a primary role of sensory feedback in sensorimotor synchronization. This is further supported by largely dissociated activity in sensory and motor zones observed in the isolated spinal cord in vitro. Thus, sensory feedback resulting from spontaneous movements is instrumental for coordination of activity in developing sensorimotor spinal cord circuits.

Spontaneous movements are important for mammalian development but how network activity underlies the generation of these actions remains unclear. Here the authors show that both spontaneous twitches and complex movements enable correlated activity in motor and sensory networks of the rat spinal cord in vivo, and that sensory feedback is instrumental in this synchronization.

Place Cell Networks in Pre-weanling Rats Show Associative Memory Properties from the Onset of Exploratory Behavior

Place cells are hippocampal pyramidal cells that are active when an animal visits a restricted area of the environment, and collectively their activity constitutes a neural representation of space. Place cell populations in the adult rat hippocampus display fundamental properties consistent with an associative memory network: the ability to 1) generate new and distinct spatial firing patterns when encountering novel spatial contexts or changes in sensory input (“remapping”) and 2) reinstate previously stored firing patterns when encountering a familiar context, including on the basis of an incomplete/degraded set of sensory cues (“pattern completion”). To date, it is unknown when these spatial memory responses emerge during brain development. Here, we show that, from the age of first exploration (postnatal day 16) onwards, place cell populations already exhibit these key features: they generate new representations upon exposure to a novel context and can reactivate familiar representations on the basis of an incomplete set of sensory cues. These results demonstrate that, as early as exploratory behaviors emerge, and despite the absence of an adult-like grid cell network, the developing hippocampus processes incoming sensory information as an associative memory network.

Structural development and dorsoventral maturation of the medial entorhinal cortex

We investigated the structural development of superficial-layers of medial entorhinal cortex and parasubiculum in rats. The grid-layout and cholinergic-innervation of calbindin-positive pyramidal-cells in layer-2 emerged around birth while reelin-positive stellate-cells were scattered throughout development. Layer-3 and parasubiculum neurons had a transient calbindin-expression, which declined with age. Early postnatally, layer-2 pyramidal but not stellate-cells co-localized with doublecortin - a marker of immature neurons - suggesting delayed functional-maturation of pyramidal-cells. Three observations indicated a dorsal-to-ventral maturation of entorhinal cortex and parasubiculum: (i) calbindin-expression in layer-3 neurons decreased progressively from dorsal-to-ventral, (ii) doublecortin in layer-2 calbindin-positive-patches disappeared dorsally before ventrally, and (iii) wolframin-expression emerged earlier in dorsal than ventral parasubiculum. The early appearance of calbindin-pyramidal-grid-organization in layer-2 suggests that this pattern is instructed by genetic information rather than experience. Superficial-layer-microcircuits mature earlier in dorsal entorhinal cortex, where small spatial-scales are represented. Maturation of ventral-entorhinal-microcircuits - representing larger spatial-scales - follows later around the onset of exploratory behavior.

Neuromodulation of excitatory synaptogenesis in striatal development

Dopamine is released in the striatum during development and impacts the activity of Protein Kinase A (PKA) in striatal spiny projection neurons (SPNs). We examined whether dopaminergic neuromodulation regulates activity-dependent glutamatergic synapse formation in the developing striatum. Systemic in vivo treatment with Gα_s-coupled G-protein receptors (GPCRs) agonists enhanced excitatory synapses on direct pathway striatal spiny projection neurons (dSPNs), whereas rapid production of excitatory synapses on indirect pathway neurons (iSPNs) required the activation of Gα_s GPCRs in SPNs of both pathways. Nevertheless, in vitro Gα_s activation was sufficient to enhance spinogenesis induced by glutamate photolysis in both dSPNs and iSPNs, suggesting that iSPNs in intact neural circuits have additional requirements for rapid synaptic development. We evaluated the in vivo effects of enhanced glutamate release from corticostriatal axons and postsynaptic PKA and discovered a mechanism of developmental plasticity wherein rapid synaptogenesis is promoted by the coordinated actions of glutamate and postsynaptic Gα_s-coupled receptors.

DOI:http://dx.doi.org/10.7554/eLife.10111.001

The brain is composed of intricate circuits of connected neurons that communicate via a combination of electrical and chemical signals. Some signals (referred to as excitatory signals) increase the probability that the neuron receiving the chemical message will produce an electrical impulse. On the other hand, inhibitory messages decrease the likelihood of this activity. Both of these kinds of signals are fast, and act over milliseconds. There is also a diverse set of slower signals, referred to as neuromodulation, which regulates the faster signals. A signaling chemical called dopamine is involved in neuromodulation and is essential for rewarding behavior and complex motor actions. The importance of dopamine is clear from the profound lack of movement seen in individuals with Parkinson’s disease, which is caused by the death of dopamine producing brain cells.

Many nerve endings from dopamine-releasing neurons connect to a part of the brain’s reward system called the striatum. The neurons in this region are organized into two pathways that have opposing impacts on behavior. Dopamine activates different kinds of receptors called “G protein-coupled dopamine receptors” on neurons from each pathway. This allows dopamine to alter the activity of a protein called Protein Kinase A (or PKA) and alter the signaling state of these neurons.

The impact of dopamine on neural circuits in adults has been extensively studied. However it was unknown whether dopamine might influence how neural circuits are wired during brain development. Because the nerve endings from dopamine-releasing neurons reach the striatum before most excitatory connections between the neurons are formed, dopamine stands to influence the development of connections in the striatum.

Kozorovitskiy et al. have now investigated the role of neuromodulation in brain development in young mice. This involved measuring the formation of excitatory connections or synapses and the electrical activity of different striatal neurons during the maturation of brain circuits that occurs after birth. This analysis revealed that turning on dopamine receptors that increase PKA activity rapidly enhances the number of excitatory synapses on the neurons that express this receptor.

Kozorovitskiy et al. then used a variety of approaches to investigate whether there is cooperation between G protein-coupled receptors, PKA activity and a signaling molecule called glutamate in striatal development. This revealed a more general mechanism by which the activation of G-protein-coupled receptors interacts with glutamate (the primary excitatory signal sent between neurons) in order to produce new synapses. These results reveal a previously unknown role for neuromodulation in “wiring up” the brain and open the possibility of new therapies to treat neurodevelopmental and neurodegenerative disorders.

DOI:http://dx.doi.org/10.7554/eLife.10111.002

A new biomarker to examine the role of hippocampal function in the development of spatial reorientation in children: a review

Spatial navigation is an adaptive skill that involves determining the route to a particular goal or location, and then traveling that path. A major component of spatial navigation is spatial reorientation, or the ability to reestablish a sense of direction after being disoriented. The hippocampus is known to be critical for navigating, and has more recently been implicated in reorienting in adults, but relatively little is known about the development of the hippocampus in relation to these large-scale spatial abilities in children. It has been established that, compared to school-aged children, preschool children tend to perform poorly on certain spatial reorientation tasks, suggesting that their hippocampi may not be mature enough to process the demands of such a task. Currently, common techniques used to examine underlying brain activity, such as electroencephalography (EEG) and functional magnetic resonance imaging (fMRI), are not suitable for examining hippocampal development in young children. In the present paper, we argue instead for the use of eyeblink conditioning (EBC), a relatively under-utilized, inexpensive, and safe method that is easy to implement in developing populations. In addition, EBC has a well defined neural circuitry, which includes the hippocampus, making it an ideal tool to indirectly measure hippocampal functioning in young children. In this review, we will evaluate the literature on EBC and its relation to hippocampal development, and discuss the possibility of using EBC as an objective measure of associative learning in relation to large-scale spatial skills. We support the use of EBC as a way to indirectly access hippocampal function in typical and atypical populations in order to characterize the neural substrates associated with the development of spatial reorientation abilities in early childhood. As such, EBC is a potential, simple biomarker for success in tasks that require the hippocampus, including spatial reorientation.

Finding minimal action sequences with a simple evaluation of actions

Animals are able to discover the minimal number of actions that achieves an outcome (the minimal action sequence). In most accounts of this, actions are associated with a measure of behavior that is higher for actions that lead to the outcome with a shorter action sequence, and learning mechanisms find the actions associated with the highest measure. In this sense, previous accounts focus on more than the simple binary signal of "was the outcome achieved?"; they focus on "how well was the outcome achieved?" However, such mechanisms may not govern all types of behavioral development. In particular, in the process of action discovery (Redgrave and Gurney, 2006), actions are reinforced if they simply lead to a salient outcome because biological reinforcement signals occur too quickly to evaluate the consequences of an action beyond an indication of the outcome's occurrence. Thus, action discovery mechanisms focus on the simple evaluation of "was the outcome achieved?" and not "how well was the outcome achieved?" Notwithstanding this impoverishment of information, can the process of action discovery find the minimal action sequence? We address this question by implementing computational mechanisms, referred to in this paper as no-cost learning rules, in which each action that leads to the outcome is associated with the same measure of behavior. No-cost rules focus on "was the outcome achieved?" and are consistent with action discovery. No-cost rules discover the minimal action sequence in simulated tasks and execute it for a substantial amount of time. Extensive training, however, results in extraneous actions, suggesting that a separate process (which has been proposed in action discovery) must attenuate learning if no-cost rules participate in behavioral development. We describe how no-cost rules develop behavior, what happens when attenuation is disrupted, and relate the new mechanisms to wider computational and biological context.

Know your limits: the role of boundaries in the development of spatial representation

In this issue of Neuron,Bjerknes et al. (2014) show that cells responding to environmental boundaries (border/boundary cells) are present as soon as rat pups can independently explore their environment. These boundary-based representations may thus provide a scaffold for other, later emerging, spatial representations.

Developmental studies of the hippocampus and hippocampal-dependent behaviors: insights from interdisciplinary studies and tips for new investigators

The hippocampus is not fully developed at birth and, with respect to spatial cognition, only begins to show signs of adult-like function at three postnatal weeks in rodents. Studying the developmental period spanning roughly two to four weeks of age permits an understanding of the neural framework necessary for the emergence of spatial navigation and, quite possibly, human episodic memory. However, due to developmental factors, behavior data collection and interpretation can be severely compromised if inappropriate designs are applied. As such, we propose methodological considerations for the behavioral assessment of hippocampal function in developing rats that take into account animal size, growth rate, and sensory and motor ability. We further summarize recent key interdisciplinary studies that are beginning to unravel the molecular machinery and physiological alterations responsible for hippocampal maturation. In general, hippocampal development is a protracted process during which unique contributions to spatial cognition and complex recognition memory come "on line" at different postnatal ages creating a unique situation for elucidating the neural bases of specific components of higher cognitive abilities.

Space in the brain: how the hippocampal formation supports spatial cognition

Over the past four decades, research has revealed that cells in the hippocampal formation provide an exquisitely detailed representation of an animal's current location and heading. These findings have provided the foundations for a growing understanding of the mechanisms of spatial cognition in mammals, including humans. We describe the key properties of the major categories of spatial cells: place cells, head direction cells, grid cells and boundary cells, each of which has a characteristic firing pattern that encodes spatial parameters relating to the animal's current position and orientation. These properties also include the theta oscillation, which appears to play a functional role in the representation and processing of spatial information. Reviewing recent work, we identify some themes of current research and introduce approaches to computational modelling that have helped to bridge the different levels of description at which these mechanisms have been investigated. These range from the level of molecular biology and genetics to the behaviour and brain activity of entire organisms. We argue that the neuroscience of spatial cognition is emerging as an exceptionally integrative field which provides an ideal test-bed for theories linking neural coding, learning, memory and cognition.

The development of the hippocampal neural representation of space

The hippocampal formation (HF) contains a neural representation of the environment, based on the activity of several classes of neurons whose firing is tuned to an animal's position and orientation in space. Recently, work has begun on understanding when and how this neural map of space emerges during development. Different classes of spatially tuned neurons emerge at different ages, some of them very early during development, before animals have started exploring their environment. The developmental timeline thus far uncovered has yielded insights into both the mechanisms of the ontogeny of the neural code for space, as well as how this system functions in the adult.