Morphology and ontogeny of rat perirhinal cortical neurons

Developmental trajectory of episodic-like memory in rats

Introduction

Episodic memory formation requires the binding of multiple associations to a coherent episodic representation, with rich detail of times, places, and contextual information. During postnatal development, the ability to recall episodic memories emerges later than other types of memory such as object recognition. However, the precise developmental trajectory of episodic memory, from weaning to adulthood has not yet been established in rats. Spontaneous object exploration tasks do not require training, and allow repeated testing of subjects, provided novel objects are used on each trial. Therefore, these tasks are ideally suited for the study of the ontogeny of episodic memory and its constituents (e.g., object, spatial, and contextual memory).

Methods

In the present study, we used four spontaneous short-term object exploration tasks over two days: object (OR), object-context (OCR), object-place (OPR), and object-place-context (OPCR) recognition to characterise the ontogeny of episodic-like memory and its components in three commonly used outbred rat strains (Lister Hooded, Long Evans Hooded, and Sprague Dawley).

Results

In longitudinal studies starting at 3-4 weeks of age, we observed that short term memory for objects was already present at the earliest time point we tested, indicating that it is established before the end of the third week of life (consistent with several other reports). Object-context memory developed during the fifth week of life, while both object-in-place and the episodic-like object-place-context memory developed around the seventh postnatal week. To control for the effects of previous experience in the development of associative memory, we confirmed these developmental trajectories using a cross-sectional protocol.

Discussion

Our work provides robust evidence for different developmental trajectories of recognition memory in rats depending on the content and/or complexity of the associations and emphasises the utility of spontaneous object exploration tasks to assess the ontogeny of memory systems with high temporal resolution.

Ontogeny of spontaneous recognition memory in rodents

Spontaneous recognition memory tasks explore thewhat,whereandwhencomponents of recognition memory. These tasks are widely used in rodents to assess cognitive function across the lifespan. While several neurodevelopmental and mental disorders present symptom onset in early life, very little is known about how memories are expressed in early life, and as a consequence how they may be affected in pathological conditions. In this review, we conduct an analysis of the studies examining the expression of spontaneous recognition memory in young rodents. We compiled studies using four different tasks: novel object recognition, object location, temporal order recognition and object place. First, we identify major sources of variability between early life spontaneous recognition studies and classify them for later comparison. Second, we use these classifications to explore the current knowledge on the ontogeny of each of these four spontaneous recognition memory tasks. We conclude by discussing the possible implications of the relative time of onset for each of these tasks and their respective neural correlates. In compiling this research, we hope to advance on establishing a developmental timeline for the emergence of distinct components of recognition memory, while also identifying key areas of focus for future research. Establishing the ontogenetic profile of rodent spontaneous recognition memory tasks will create a necessary blueprint for cognitive assessment in animal models of neurodevelopmental and mental disorders, a first step towards improved and earlier diagnosis as well as novel intervention strategies.

Developmental onset distinguishes three types of spontaneous recognition memory in mice

Spontaneous recognition memory tasks build on an animal’s natural preference for novelty to assess the what, where and when components of episodic memory. Their simplicity, ethological relevance and cross-species adaptability make them extremely useful to study the physiology and pathology of memory. Recognition memory deficits are common in rodent models of neurodevelopmental disorders, and yet very little is known about the expression of spontaneous recognition memory in young rodents. This is exacerbated by the paucity of data on the developmental onset of recognition memory in mice, a major animal model of disease. To address this, we characterized the ontogeny of three types of spontaneous recognition memory in mice: object location, novel object recognition and temporal order recognition. We found that object location is the first to emerge, at postnatal day (P)21. This was followed by novel object recognition (24 h delay), at P25. Temporal order recognition was the last to emerge, at P28. Elucidating the developmental expression of recognition memory in mice is critical to improving our understanding of the ontogeny of episodic memory, and establishes a necessary blueprint to apply these tasks to probe cognitive deficits at clinically relevant time points in animal models of developmental disorders.

Expression of the RNA methyltransferase Nsun5 is essential for developing cerebral cortex

Nsun5 gene, encoding a cytosine-5 RNA methyltransferase, is deleted in about 95% patients with Williams-Beuren syndrome (WBS). WBS is a neurodevelopmental disorder and characterized by cognitive disorder. We generated single-gene Nsun5 knockout (Nsun5-KO) mice and reported that the Nsun5 deletion leads to deficit in spatial cognition. This study focused on investigating the influence of Nsun5 deficiency in the development of cerebral cortex. In comparison with wild-type littermates, the cortical thickness in postnatal day 10 Nsun5-KO mice was obviously reduced with an abnormal laminar organization, and the processes of pyramidal cells were shorter and finer. Nsun5 was selectively expressed in radial glial cells (RGCs) of cerebral cortex from embryonic day (E) 12.5 to E16.5, but not in intermediate progenitor cells (IPCs) or neocortical neurons. The Nsun5 deletion did not alter proliferation of RGCs or differentiation of RGCs into IPCs. Notably, the ablation of Nsun5 disrupted the growth of radial glial scaffolds, thus numerous basal processes of RGCs failed to reach pial basement membrane. Level of cell polarity regulator Cdc42 protein in radial glial scaffolds of E14.5 Nsun5-KO mice was reduced, but the level of Cdc42 mRNA was unchanged. The dysfunction of glial scaffolds impeded the radial migration of upper-layer and deeper-layer neurons to cause their subcortical accumulation and apoptosis, resulting in an obvious thinness of the cortical plate in E18.5 Nsun5-KO mice. These findings establish a critical role of Nsun5 in development of cerebral cortex through regulating radial glial scaffolds of RGCs to control migration of neocortical neurons.

Using diffusion anisotropy to study cerebral cortical gray matter development

Diffusion-weighted magnetic resonance imaging (diffusion MRI) is being used to characterize morphological development of cells within developing cerebral cortical gray matter. Abnormal morphology is a shared characteristic of cerebral cortical neurons for many neurodevelopmental disorders, and therefore diffusion MRI is potentially of high value for monitoring growth-related anatomical changes of relevance to brain function. Here, the theoretical framework for analyzing diffusion MRI data is summarized. An overview of quantitative methods for validating the interpretations of diffusion MRI data using light microscopy is then presented. These theoretical modeling and validation methods have been used to precisely characterize changes in water diffusion anisotropy with development in the context of several animal model systems. Further, in diffusion MRI studies of several preclinical models of neurodevelopmental disorders, the ability is demonstrated of diffusion MRI to detect abnormal morphological neural development. These animal model studies are reviewed along with recent initial efforts to translate the findings into an approach for studies of human subjects. This body of data indicates that diffusion MRI has the requisite sensitivity to detect abnormal cellular development in the context of several models of neurodevelopmental disorders, and therefore may provide a new strategy for detecting abnormalities in early stages of brain development in humans.

Limbic system development underlies the emergence of classical fear conditioning during the third and fourth weeks of life in the rat

Classical fear conditioning creates an association between an aversive stimulus and a neutral stimulus. Although the requisite neural circuitry is well understood in mature organisms, the development of these circuits is less well studied. The current experiments examine the ontogeny of fear conditioning and relate it to neuronal activation assessed through immediate early gene (IEG) expression in the amygdala, hippocampus, perirhinal cortex, and hypothalamus of periweanling rats. Rat pups were fear conditioned, or not, during the third or fourth weeks of life. Neuronal activation was assessed by quantifying expression of FBJ osteosarcoma oncogene (FOS) using immunohistochemistry (IHC) in Experiment 1. Fos and early growth response gene-1 (EGR1) expression was assessed using qRT-PCR in Experiment 2. Behavioral data confirm that both auditory and contextual fear continue to emerge between PD 17 and 24. The IEG expression data are highly consistent with these behavioral results. IHC results demonstrate significantly more FOS protein expression in the basal amygdala of fear-conditioned PD 23 subjects compared to control subjects, but no significant difference at PD 17. qRT-PCR results suggest specific activation of the amygdala only in older subjects during auditory fear expression. A similar effect of age and conditioning status was also observed in the perirhinal cortex during both contextual and auditory fear expression. Overall, the development of fear conditioning occurring between the third and fourth weeks of life appears to be at least partly attributable to changes in activation of the amygdala and perirhinal cortex during fear conditioning or expression. (PsycINFO Database Record

Attenuation of dendritic spine density in the perirhinal cortex following 17β-Estradiol replacement in the rat

Intraperirhinal cortex infusion of 17-β estradiol (E2) impairs object-recognition memory. However, it is not currently known whether this hormone modulates synaptic plasticity in this structure. Most excitatory synapses in the central nervous system are located on dendritic spines, and elevated E2 levels influence the density of these spines in several brain areas. The goal of the present study was to determine whether differences in dendritic spine density in the perirhinal cortex are observed following high E2 replacement in ovariectomized rats. The density of total spines, and mushroom-shaped (i.e. mature) spines were compared between a high E2 replacement (10 µg/kg/day, s.c.) and a no replacement condition. The perirhinal cortex is subdivided into Broadmann's area 35 and 36 and so group comparisons were made within each sub-region separately. High E2 replacement resulted in lower density of mushroom-shaped spines in area 35 relative to no replacement. There was no effect of high E2 replacement on dendritic spine density in area 36. These findings are consistent with the idea that higher E2 levels reduce dendritic spine density in area 35, which may result from spine shrinkage, or reduced synapse formation. This study provides preliminary evidence for a mechanism through which E2 may impair object-recognition memory.

Ontogeny of object-in-context recognition in the rat

The object-in-context recognition (OiC) task [19] is a spontaneous exploration task that serves as an index of incidental contextual learning and memory. During the test phase, rats prefer to explore the object mismatched to the testing context based on previous object-context pairings experienced during training. The mechanisms of OiC memory have been explored in adult rats [12,35]; however, little is known about its determinants during development. Thus, the present study examined the ontogeny of the OiC task in preweanling through adolescent rats. We demonstrate that postnatal day (PD) 17, 21, 26, and 31 rats can perform the OiC task (Experiment 1) and that preference for the novel target is eliminated when rats are tested in an alternate context not encountered during training (Experiment 2). Lastly, we show that PD26 but not PD17 rats can perform the OiC task when the training contexts only differed by distal spatial cues (Experiment 3). These data demonstrate for the first time that PD17 rats can acquire and retain short-term OiC memory, which involves associative learning of object and context information. However, we also provide evidence that preweanling rats' ability to utilize certain aspects of a context (i.e., distal spatial cues) in the OiC task is not equivalent to that of their older counterparts. Implications for the development of contextual memory and its related neural substrates are discussed.

Spatially distributed dendritic resonance selectively filters synaptic input

An important task performed by a neuron is the selection of relevant inputs from among thousands of synapses impinging on the dendritic tree. Synaptic plasticity enables this by strenghtening a subset of synapses that are, presumably, functionally relevant to the neuron. A different selection mechanism exploits the resonance of the dendritic membranes to preferentially filter synaptic inputs based on their temporal rates. A widely held view is that a neuron has one resonant frequency and thus can pass through one rate. Here we demonstrate through mathematical analyses and numerical simulations that dendritic resonance is inevitably a spatially distributed property; and therefore the resonance frequency varies along the dendrites, and thus endows neurons with a powerful spatiotemporal selection mechanism that is sensitive both to the dendritic location and the temporal structure of the incoming synaptic inputs.

Ontogeny of object versus location recognition in the rat: acquisition and retention effects

Novel object and location recognition tasks harness the rat's natural tendency to explore novelty (Berlyne, 1950) to study incidental learning. The present study examined the ontogenetic profile of these two tasks and retention of spatial learning between postnatal day (PD) 17 and 31. Experiment 1 showed that rats ages PD17, 21, and 26 recognize novel objects, but only PD21 and PD26 rats recognize a novel location of a familiar object. These results suggest that novel object recognition develops before PD17, while object location recognition emerges between PD17 and PD21. Experiment 2 studied the ontogenetic profile of object location memory retention in PD21, 26, and 31 rats. PD26 and PD31 rats retained the object location memory for both 10-min and 24-hr delays. PD21 rats failed to retain the object location memory for the 24-hr delay, suggesting differential development of short- versus long-term memory in the ontogeny of object location memory.

Quantitative investigations of axonal and dendritic arbors: development, structure, function, and pathology

The branching structures of neurons are a long-standing focus of neuroscience. Axonal and dendritic morphology affect synaptic signaling, integration, and connectivity, and their diversity reflects the computational specialization of neural circuits. Altered neuronal morphology accompanies functional changes during development, experience, aging, and disease. Technological improvements continuously accelerate high-throughput tissue processing, image acquisition, and morphological reconstruction. Digital reconstructions of neuronal morphologies allow for complex quantitative analyses that are unattainable from raw images or two-dimensional tracings. Furthermore, digitized morphologies enable computational modeling of biophysically realistic neuronal dynamics. Additionally, reconstructions generated to address specific scientific questions have the potential for continued investigations beyond the original reason for their acquisition. Facilitating multiple reuse are repositories like NeuroMorpho.Org, which ease the sharing of reconstructions. Here, we review selected scientific literature reporting the reconstruction of axonal or dendritic morphology with diverse goals including establishment of neuronal identity, examination of physiological properties, and quantification of developmental or pathological changes. These reconstructions, deposited in NeuroMorpho.Org, have since been used by other investigators in additional research, of which we highlight representative examples. This cycle of data generation, analysis, sharing, and reuse reveals the vast potential of digital reconstructions in quantitative investigations of neuronal morphology.

The impact of development and sensory deprivation on dendritic protrusions in the mouse barrel cortex

Dendritic protrusions (spines and filopodia) are structural indicators of synapses that have been linked to neuronal learning and memory through their morphological alterations induced by development and experienced-dependent activities. Although previous studies have demonstrated that depriving sensory experience leads to structural changes in neocortical organization, the more subtle effects on dendritic protrusions remain unclear, mostly due to focus on only one specific cell type and/or age of manipulation. Here, we show that sensory deprivation induced by whisker trimming influences the dendritic protrusions of basilar dendrites located in thalamocortical recipient lamina (IV and VI) of the mouse barrel cortex in a layer-specific manner. Following 1 month of whisker trimming after birth, the density of dendritic protrusions increased in layer IV, but decreased in layer VI. Whisker regrowth for 1 month returned protrusion densities to comparable level of age-matched controls in layer VI, but not in layer IV. In adults, chronic sensory deprivation led to an increase in protrusion densities in layer IV, but not in layer VI. In addition, chronic pharmacological blockade of N-methyl-d-aspartate receptors (NMDARs) increased protrusion density in both layers IV and VI, which returned to the control level after 1 month of drug withdrawal. Our data reveal that different cortical layers respond to chronic sensory deprivation in different ways, with more pronounced effects during developmental critical periods than adulthood. We also show that chronically blocking NMDARs activity during developmental critical period also influences the protrusion density and morphology in the cerebral cortex.

Perirhinal cortex and temporal lobe epilepsy

The perirhinal cortex—which is interconnected with several limbic structures and is intimately involved in learning and memory—plays major roles in pathological processes such as the kindling phenomenon of epileptogenesis and the spread of limbic seizures. Both features may be relevant to the pathophysiology of mesial temporal lobe epilepsy that represents the most refractory adult form of epilepsy with up to 30% of patients not achieving adequate seizure control. Compared to other limbic structures such as the hippocampus or the entorhinal cortex, the perirhinal area remains understudied and, in particular, detailed information on its dysfunctional characteristics remains scarce; this lack of information may be due to the fact that the perirhinal cortex is not grossly damaged in mesial temporal lobe epilepsy and in models mimicking this epileptic disorder. However, we have recently identified in pilocarpine-treated epileptic rats the presence of selective losses of interneuron subtypes along with increased synaptic excitability. In this review we: (i) highlight the fundamental electrophysiological properties of perirhinal cortex neurons; (ii) briefly stress the mechanisms underlying epileptiform synchronization in perirhinal cortex networks following epileptogenic pharmacological manipulations; and (iii) focus on the changes in neuronal excitability and cytoarchitecture of the perirhinal cortex occurring in the pilocarpine model of mesial temporal lobe epilepsy. Overall, these data indicate that perirhinal cortex networks are hyperexcitable in an animal model of temporal lobe epilepsy, and that this condition is associated with a selective cellular damage that is characterized by an age-dependent sensitivity of interneurons to precipitating injuries, such as status epilepticus.

Using diffusion anisotropy to characterize neuronal morphology in gray matter: the orientation distribution of axons and dendrites in the NeuroMorpho.org database

Accurate mathematical modeling is integral to the ability to interpret diffusion magnetic resonance (MR) imaging data in terms of cellular structure in brain gray matter (GM). In previous work, we derived expressions to facilitate the determination of the orientation distribution of axonal and dendritic processes from diffusion MR data. Here we utilize neuron reconstructions available in the NeuroMorpho database (www.neuromorpho.org) to assess the validity of the model we proposed by comparing morphological properties of the neurons to predictions based on diffusion MR simulations using the reconstructed neuron models. Initially, the method for directly determining neurite orientation distributions is shown to not depend on the line length used to quantify cylindrical elements. Further variability in neuron morphology is characterized relative to neuron type, species, and laboratory of origin. Subsequently, diffusion MR signals are simulated based on human neocortical neuron reconstructions. This reveals a bias in which diffusion MR data predict neuron orientation distributions to have artificially low anisotropy. This bias is shown to arise from shortcomings (already at relatively low diffusion weighting) in the Gaussian approximation of diffusion, in the presence of restrictive barriers, and data analysis methods involving higher moments of the cumulant expansion are shown to be capable of reducing the magnitude of the observed bias.

Dual functions of perirhinal cortex in fear conditioning

The present review examines the role of perirhinal cortex (PRC) in Pavlovian fear conditioning. The focus is on rats, partly because so much is known, behaviorally and neurobiologically, about fear conditioning in these animals. In addition, the neuroanatomy and neurophysiology of rat PRC have been described in considerable detail at the cellular and systems levels. The evidence suggests that PRC can serve at least two types of mnemonic functions in Pavlovian fear conditioning. The first function, termed "stimulus unitization," refers to the ability to treat two or more separate items or stimulus elements as a single entity. Supporting evidence for this perceptual function comes from studies of context conditioning as well as delay conditioning to discontinuous auditory cues. In a delay paradigm, the conditional stimulus (CS) and unconditional stimulus (US) overlap temporally and co-terminate. The second PRC function entails a type of "transient memory." Supporting evidence comes from studies of trace cue conditioning, where there is a temporal gap or trace interval between the CS offset and the US onset. For learning to occur, there must be a transient CS representation during the trace interval. We advance a novel neurophysiological mechanism for this transient representation. These two hypothesized functions of PRC are consistent with inferences based on non-aversive forms of learning.

Muscarinic receptor activation enables persistent firing in pyramidal neurons from superficial layers of dorsal perirhinal cortex

Persistent-firing neurons in the entorhinal cortex (EC) and the lateral nucleus of the amygdala (LA) continue to discharge long after the termination of the original, spike-initiating current. An emerging theory proposes that endogenous persistent firing helps support a transient memory system. This study demonstrated that persistent-firing neurons are also prevalent in rat perirhinal cortex (PR), which lies immediately adjacent to and is reciprocally connected with EC and LA. Several characteristics of persistent-firing neurons in PR were similar to those previously reported in LA and EC. Persistent firing in PR was enabled by the application of carbachol, a nonselective cholinergic agonist, and it was induced by injecting a suprathreshold current or by stimulating suprathreshold excitatory synaptic inputs to the neuron. Once induced, persistent firing lasted for seconds to minutes. Persistent firing could always be terminated by a sufficiently large and prolonged hyperpolarizing current; it was prevented by antagonists of muscarinic cholinergic receptors (mAChRs); and it was blocked by flufenamic acid. The latter has been suggested to inhibit a Ca(2+) -activated nonspecific cation conductance (G(CAN) ) that normally furnishes the sustained depolarization during persistent firing. In many PR neurons, the discharge rate during persistent firing was a graded function of depolarizing and/or hyperpolarizing inputs. Persistent firing was not prevented by blocking fast excitatory and inhibitory synaptic transmission, demonstrating that it can be generated endogenously. We suggest that persistent-firing neurons in PR, EC, LA, and certain other brain regions may cooperate in support of a transient-memory system.

Aging-related changes in calcium-binding proteins in rat perirhinal cortex

Dysregulation of intracellular calcium homeostasis has been linked to neuropathological symptoms observed in aging and age-related disease. Alterations in the distribution and relative frequency of calcium-binding proteins (CaBPs), which are important in regulating intracellular calcium levels, may contribute to disruption of calcium homeostasis. Here we examined the laminar distribution of three CaBPs in rat perirhinal cortex (PR) as a function of aging. Calbindin-D28k (CB), parvalbumin (PV), and calretinin (CR) were compared in adult (4 mo.), middle-aged (13 mo.) and aged (26 mo.) rats. Results show an aging-related and layer-specific decrease in the number of CB-immunoreactive (-ir) neurons, beginning in middle-aged animals. Dual labeling suggests that the age-related decrease in CB reflects a decrease in neurons that are not immunoreactive for the inhibitory neurotransmitter GABA. In contrast, no aging-related differences in PV- or CR-immunoreactivity were observed. These data suggest that selective alterations in CB-ir neurons may contribute to aging-related learning and memory deficits in tasks that depend upon PR circuitry.

Ontogeny and neural substrates of the context preexposure facilitation effect

Contextual fear conditioning emerges around post-natal day (PD) 23 in the rat. This is thought to reflect hippocampus-dependent conjunctive learning, which binds the individual features of the context into a unified representation (Rudy, 1993). However, context conditioning can also be supported by hippocampus-independent, feature-based simple associations (Rudy, 2009) and these may operate at PD 23-24 (Pugh & Rudy, 1996). To address this issue, we studied the ontogeny of a variant of contextual fear conditioning, termed the context-preexposure-facilitation-effect (CPFE), in which exposure to context and (immediate) foot shock occur on successive occasions. This variant requires conjunctive as opposed to feature-based simple associations (Rudy, 2009). We tested PD 17, 24, and 31 rats on the CPFE vs. conventional context conditioning (Exp. 1) and on the CPFE with stronger reinforcement (Exp. 2). The CPFE emerged on PD 24 regardless of reinforcer strength and in parallel with context conditioning. Infusions of the NMDA-receptor antagonist, MK-801, into the dorsal hippocampus just before pre-exposure on PD 24 eliminated the CPFE, whereas infusions occurring after pre-exposure had no effect (Exp. 3). These findings demonstrate a role of hippocampal NMDA receptors in the CPFE as early as PD 24 and implicate conjunctive learning mechanisms in the ontogeny of contextual fear conditioning.

An inverse approach for elucidating dendritic function

We outline an inverse approach for investigating dendritic function-structure relationships by optimizing dendritic trees for a priori chosen computational functions. The inverse approach can be applied in two different ways. First, we can use it as a "hypothesis generator" in which we optimize dendrites for a function of general interest. The optimization yields an artificial dendrite that is subsequently compared to real neurons. This comparison potentially allows us to propose hypotheses about the function of real neurons. In this way, we investigated dendrites that optimally perform input-order detection. Second, we can use it as a "function confirmation" by optimizing dendrites for functions hypothesized to be performed by classes of neurons. If the optimized, artificial, dendrites resemble the dendrites of real neurons the artificial dendrites corroborate the hypothesized function of the real neuron. Moreover, properties of the artificial dendrites can lead to predictions about yet unmeasured properties. In this way, we investigated wide-field motion integration performed by the VS cells of the fly visual system. In outlining the inverse approach and two applications, we also elaborate on the nature of dendritic function. We furthermore discuss the role of optimality in assigning functions to dendrites and point out interesting future directions.

One rule to grow them all: a general theory of neuronal branching and its practical application

Understanding the principles governing axonal and dendritic branching is essential for unravelling the functionality of single neurons and the way in which they connect. Nevertheless, no formalism has yet been described which can capture the general features of neuronal branching. Here we propose such a formalism, which is derived from the expression of dendritic arborizations as locally optimized graphs. Inspired by Ramón y Cajal's laws of conservation of cytoplasm and conduction time in neural circuitry, we show that this graphical representation can be used to optimize these variables. This approach allows us to generate synthetic branching geometries which replicate morphological features of any tested neuron. The essential structure of a neuronal tree is thereby captured by the density profile of its spanning field and by a single parameter, a balancing factor weighing the costs for material and conduction time. This balancing factor determines a neuron's electrotonic compartmentalization. Additions to this rule, when required in the construction process, can be directly attributed to developmental processes or a neuron's computational role within its neural circuit. The simulations presented here are implemented in an open-source software package, the “TREES toolbox,” which provides a general set of tools for analyzing, manipulating, and generating dendritic structure, including a tool to create synthetic members of any particular cell group and an approach for a model-based supervised automatic morphological reconstruction from fluorescent image stacks. These approaches provide new insights into the constraints governing dendritic architectures. They also provide a novel framework for modelling and analyzing neuronal branching structures and for constructing realistic synthetic neural networks.

More than a century has passed since Ramón y Cajal presented a set of fundamental biological laws of neuronal branching. He described how the shape of the core elements of the neural circuitry – axons and dendrites – are constrained by physical parameters such as space, cytoplasmic volume, and conduction time. The existence of these laws enabled him to organize his histological observations, to formulate the neuron doctrine, and to infer directionality in signal flow in the nervous system. We show that Cajal's principles can be used computationally to generate synthetic neural circuits. These principles rigorously constrain the shape of real neuronal structures, providing direct validation of his theories. At the same time, this strategy provides us with a powerful set of tools for generating synthetic neurons, as well as a model-based approach for automated reconstructions of neuronal trees from confocal image stacks.

The anterior olfactory nucleus: quantitative study of dendritic morphology

The anterior olfactory nucleus (AON) occupies a crucial position within the olfactory circuit, as it is able to influence function in nearly every major synaptic processing stage of both the ipsilateral and the contralateral pathways. Nevertheless, very little is known about the region's internal organization and circuitry. The present study provides basic quantitative and qualitative data on the morphology of several cell types within the two major regions of the AON, pars externa and pars principalis. In pars externa two types of cells are analyzed, the "classical" cell (type I), containing only apically directed dendrites with large spines, and a previously unreported cell with basilar dendrites and complex, spiny apical processes (type II). In pars principalis the characteristic pyramidal cell is described both on the basis of the depth of the cell bodies in the cell layer comprising the structure and on the basis of their radial location. Several other nonpyramidal neurons are also described. The findings provide useful basic information necessary for understanding and modeling the circuitry of the AON.

Morphological heterogeneity of layer VI neurons in mouse barrel cortex

Understanding the basic neuronal building blocks of the neocortex is a necessary first step toward comprehending the composition of cortical circuits. Neocortical layer VI is the most morphologically diverse layer and plays a pivotal role in gating information to the cortex via its feedback connection to the thalamus and other ipsilateral and callosal corticocortical connections. The heterogeneity of function within this layer is presumably linked to its varied morphological composition. However, so far, very few studies have attempted to define cell classes in this layer using unbiased quantitative methodologies. Utilizing the Golgi staining technique along with the Neurolucida software, we recontructed 222 cortical neurons from layer VI of mouse barrel cortex. Morphological analyses were performed by quantifying somatic and dendritic parameters, and, by using principal component and cluster analyses, we quantitatively categorized neurons into six distinct morphological groups. Additional systematic replication on a separate population of neurons yielded similar results, demonstrating the consistency and reliability of our categorization methodology. Subsequent post hoc analyses of dendritic parameters supported our neuronal classification scheme. Characterizing neuronal elements with unbiased quantitative techniques provides a framework for better understanding structure-function relationships within neocortical circuits in general.

Systematic mapping between dendritic function and structure

For many classes of neurons, the relationship between computational function and dendritic morphology remains unclear. To gain insights into this relationship, we utilize an inverse approach in which we optimize model neurons with realistic morphologies and ion channel distributions (of I(KA) and I(CaT)) to perform a computational function. In this study, the desired function is input-order detection: neurons have to respond differentially to the arrival of two inputs in a different temporal order. There is a single free parameter in this function, namely, the time lag between the arrivals of the two inputs. Systematically varying this parameter allowed us to map one axis of function space to structure space. Because the function of the optimized model neurons is known with certainty, their thorough analysis provides insights into the relationship between the neurons' functions, morphologies, ion channel distributions, and electrophysiological dynamics. Finally, we discuss issues of optimality in nervous systems.

Single-unit firing in rat perirhinal cortex caused by fear conditioning to arbitrary and ecological stimuli

Pretraining lesions of rat perirhinal cortex (PR) severely impair pavlovian fear conditioning to a 22 kHz ultrasonic vocalization (USV) cue. However, PR lesions are without significant effect when the cue is a continuous tone at the same or a lower frequency. Here we examined fear-conditioning-produced changes in single-unit firing elicited in rat PR by a 22 kHz tone cue or a 22 kHz USV cue. Chronic recording electrodes were introduced from the lateral surface of the skull. Altogether, 200 well isolated units were studied in 28 rats. Overall, 73% of the recorded single units (145 of 200 units) evidenced statistically significant firing changes in response to the tone or USV conditional stimulus (CS) after it had been paired several times with an aversive unconditional stimulus (US). Interestingly, 33% of units (66 of 200 units) that were initially CS-unresponsive became CS-responsive after conditioning. After conditioning, there were two notable differences between single-unit responses elicited by the USV cue and those elicited by the tone cue. First, 11% of the units (14 of 123 units) recorded from the USV-conditioned group displayed a precisely timed increase in firing rate during the 260 ms interval in which the US had previously occurred. This US-timed response was unique to the USV-conditioned group. Second, the mean latency of cue-elicited firing was approximately 30 ms longer in the USV-conditioned group than in the tone-conditioned group. These cue-specific differences in acquired firing latencies and acquired firing patterns suggest that spectrotemporal properties of a CS can control the essential circuitry or neurophysiological mechanisms underlying fear conditioning.