Structural plasticity in the entorhinal and perirhinal cortices following hippocampal lesions in rhesus monkeys

Immature neurons expressing the Bcl2 protein are present in various regions of the mammalian brain, including the amygdala and the entorhinal and perirhinal cortices. Their functional role is unknown but we have previously shown that neonatal and adult hippocampal lesions increase their differentiation in the monkey amygdala. Here, we assessed whether hippocampal lesions similarly affect immature neurons in the entorhinal and perirhinal cortices. Since Bcl2-positive cells were found mainly in areas Eo, Er, and Elr of the entorhinal cortex and in layer II of the perirhinal cortex, we also used Nissl-stained sections to determine the number and soma size of immature and mature neurons in layer III of area Er and layer II of area 36 of the perirhinal cortex. We found different structural changes in these regions following hippocampal lesions, which were influenced by the time of the lesion. In neonate-lesioned monkeys, the number of immature neurons in the entorhinal and perirhinal cortices was generally higher than in controls. The number of mature neurons was also higher in layer III of area Er of neonate-lesioned monkeys but no differences were found in layer II of area 36. In adult-lesioned monkeys, the number of immature neurons in the entorhinal cortex was lower than in controls but did not differ from controls in the perirhinal cortex. The number of mature neurons in layer III of area Er did not differ from controls, but the number of small, mature neurons in layer II of area 36 was lower than in controls. In sum, hippocampal lesions impacted populations of mature and immature neurons in discrete regions and layers of the entorhinal and perirhinal cortices, which are interconnected with the amygdala and provide major cortical inputs to the hippocampus. These structural changes may contribute to some functional recovery following hippocampal injury in an age-dependent manner.

Structural differences in the hippocampus and amygdala of behaviorally inhibited macaque monkeys

Behavioral inhibition is a temperamental disposition to react warily when confronted by unfamiliar people, objects, or events. Behaviorally inhibited children are at greater risk of developing anxiety disorders later in life. Previous studies reported that individuals with a history of childhood behavioral inhibition exhibit abnormal activity in the hippocampus and amygdala. However, few studies have investigated the structural differences that may underlie these functional abnormalities. In this exploratory study, we evaluated rhesus monkeys exhibiting a phenotype consistent with human behavioral inhibition. We performed quantitative neuroanatomical analyses that cannot be performed in humans including estimates of the volume and neuron number of distinct hippocampal regions and amygdala nuclei in behaviorally inhibited and control rhesus monkeys. Behaviorally inhibited monkeys had larger volumes of the rostral third of the hippocampal field CA3, smaller volumes of the rostral third of CA2, and smaller volumes of the accessory basal nucleus of the amygdala. Furthermore, behaviorally inhibited monkeys had fewer neurons in the rostral third of CA2. These structural differences may contribute to the functional abnormalities in the hippocampus and amygdala of behaviorally inhibited individuals. These structural findings in monkeys are consistent with a reduced modulation of amygdala activity via prefrontal cortex projections to the accessory basal nucleus. Given the putative roles of the amygdala in affective processing, CA3 in associative learning and CA2 in social memory, increased amygdala and CA3 activity, and diminished CA2 structure and function, may be associated with increased social anxiety and the heritability of behavioral inhibition. The findings from this exploratory study compel follow-up investigations with larger sample sizes and additional analyses to provide greater insight and more definitive answers regarding the neurobiological bases of behavioral inhibition.

Profound seasonal changes in brain size and architecture in the common shrew

The seasonal changes in brain size of some shrews represent the most drastic reversible transformation in the mammalian central nervous system known to date. Brain mass decreases 10-26% from summer to winter and regrows 9-16% in spring, but the underlying structural changes at the cellular level are not yet understood. Here, we describe the volumetric differences in brain structures between seasons and sexes of the common shrew (Sorex araneus) in detail, confirming that changes in different brain regions vary in the magnitude of change. Notably, shrews show a decrease in hypothalamus, thalamus, and hippocampal volume and later regrowth in spring, whereas neocortex and striatum volumes decrease in winter and do not recover in size. For some regions, males and females showed different patterns of seasonal change from each other. We also analyzed the underlying changes in neuron morphology. We observed a general decrease in soma size and total dendrite volume in the caudoputamen and anterior cingulate cortex. This neuronal retraction may partially explain the overall tissue shrinkage in winter. While not sufficient to explain the entire seasonal process, it represents a first step toward understanding the mechanisms beneath this remarkable phenomenon.

Hippocampal adult neurogenesis: Its regulation and potential role in spatial learning and memory

Adult neurogenesis, defined here as progenitor cell division generating functionally integrated neurons in the adult brain, occurs within the hippocampus of numerous mammalian species including humans. The present review details various endogenous (e.g., neurotransmitters) and environmental (e.g., physical exercise) factors that have been shown to influence hippocampal adult neurogenesis. In addition, the potential involvement of adult-generated neurons in naturally-occurring spatial learning behavior is discussed by summarizing the literature focusing on traditional animal models (e.g., rats and mice), non-traditional animal models (e.g., tree shrews), as well as natural populations (e.g., chickadees and Siberian chipmunk).

Modeling Behavior by Coastal River Otter (Lontra Canadensis) in Response to Prey Availability in Prince William Sound, Alaska: A Spatially-Explicit Individual-Based Approach

Effects of climate change on animal behavior and cascading ecosystem responses are rarely evaluated. In coastal Alaska, social river otters (Lontra Canadensis), largely males, cooperatively forage on schooling fish and use latrine sites to communicate group associations and dominance. Conversely, solitary otters, mainly females, feed on intertidal-demersal fish and display mutual avoidance via scent marking. This behavioral variability creates "hotspots" of nutrient deposition and affects plant productivity and diversity on the terrestrial landscape. Because the abundance of schooling pelagic fish is predicted to decline with climate change, we developed a spatially-explicit individual-based model (IBM) of otter behavior and tested six scenarios based on potential shifts to distribution patterns of schooling fish. Emergent patterns from the IBM closely mimicked observed otter behavior and landscape use in the absence of explicit rules of intraspecific attraction or repulsion. Model results were most sensitive to rules regarding spatial memory and activity state following an encounter with a fish school. With declining availability of schooling fish, the number of social groups and the time simulated otters spent in the company of conspecifics declined. Concurrently, model results suggested an elevation of defecation rate, a 25% increase in nitrogen transport to the terrestrial landscape, and significant changes to the spatial distribution of "hotspots" with declines in schooling fish availability. However, reductions in availability of schooling fish could lead to declines in otter density over time.

Seasonal change in the avian hippocampus

The hippocampus plays an important role in cognitive processes, including memory and spatial orientation, in birds. The hippocampus undergoes seasonal change in food-storing birds and brood parasites, there are changes in the hippocampus during breeding, and further changes occur in some species in association with migration. In food-storing birds, seasonal change in the hippocampus occurs in fall and winter when the cognitively demanding behaviour of caching and retrieving food occurs. The timing of annual change in the hippocampus of food-storing birds is quite variable, however, and appears not to be under photoperiod control. A variety of factors, including cognitive performance, exercise, and stress may all influence seasonal change in the avian hippocampus. The causal processes underlying seasonal change in the avian hippocampus have not been extensively examined and the more fully described hormonal influences on the mammalian hippocampus may provide hypotheses for investigating the control of hippocampal seasonality in birds.

The size of non-hippocampal brain regions varies by season and sex in Richardson's ground squirrel

Sex- and season-specific modulation of hippocampal size and function is observed across multiple species, including rodents. Other non-hippocampal-dependent behaviors exhibit season and sex differences, and whether the associated brain regions exhibit similar variation with sex and season remains to be fully characterized. As such, we examined the brains of wild-caught Richardson's ground squirrels (RGS; Urocitellus richardsonii) for seasonal (breeding, non-breeding) and sex differences in the volumes of specific brain areas, including: total brain volume, corpus callosum (CC), anterior commissure (AC), medial prefrontal cortex (mPFC), total neocortex (NC), entorhinal cortex (EC), and superior colliculus (SC). Analyses of variance and covariance revealed significant interactions between season and sex for almost all areas studied, primarily resulting from females captured during the breeding season exhibiting larger volumes than females captured during the non-breeding season. This was observed for volumes of the AC, mPFC, NC, EC, and SC. Where simple main effects of season were observed for males (the NC and the SC), the volume advantage favoured males captured during the NBr season. Only two simple main effects of sex were observed: males captured in the non-breeding season had significantly larger total brain volume than females captured in the non-breeding season, and females captured during the breeding season had larger volumes of the mPFC and EC than males captured in the breeding season. These results indicate that females have more pronounced seasonal differences in brain and brain region sizes. The extent to which seasonal differences in brain region volumes vary with behaviour is unclear, but our data do suggest that seasonal plasticity is not limited to the hippocampus and that RGS is a useful mammalian species for understanding seasonal plasticity in an ecologically relevant context.

Photoperiodic regulation of hippocampal neurogenesis in adult male white-footed mice (Peromyscus leucopus)

Photoperiodic organisms monitor environmental day length to engage in seasonally appropriate adaptions in physiology and behavior. Among these adaptations are changes in brain volume and neurogenesis, which have been well described in multiple species of birds, yet few studies have described such changes in the brains of adult mammals. White-footed mice (Peromyscus leucopus) are an excellent species in which to investigate the effects of day length on adult hippocampal neurogenesis, as males, in addition to having reduced hippocampal volume in short days (SD) with concomitant impairments in hippocampus-mediated behaviors, have photoperiod-dependent changes in olfactory bulb neurogenesis. We performed the current experiment to assess the effects of photoperiod on hippocampal neurogenesis longitudinally, using the thymidine analog bromodeoxyuridine at multiple time points across 10 weeks of SD exposure. Compared with counterparts held in long day (LD) lengths, across the first 8 weeks of SD exposure hippocampal neurogenesis was reduced. However, at 10 weeks in SD lengths neurogenic levels in the hippocampus were elevated above those levels in mice held in LD lengths. The current findings are consistent with the natural photoperiodic cycle of hippocampal function in male white-footed mice, and may help to inform research on photoperiodic plasticity in neurogenesis and provide insight into how the complex interplay among the environment, genes and adaptive responses to changing day lengths affects brain structure, function and behavior at multiple levels.

The effects of season and sex on dentate gyrus size and neurogenesis in a wild rodent, Richardson's ground squirrel (Urocitellus richardsonii)

Sex and reproductive status affect hippocampal neurogenesis and dentate gyrus (DG) size in rodents. Relatively few studies, however, address these two effects simultaneously and even fewer studies address this issue in wild populations. Here, we examined seasonal and sex differences in neurogenesis and DG size in a wild, polygynous and social rodent, Richardson's ground squirrel (Uriocitellus richardsonii). Based on the behavioral ecology of this species, we predicted that both neurogenesis and DG size would be sexually dimorphic and the degree of dimorphism would be greatest in the breeding season. Using unbiased stereology and doublecortin (DCX) immunohistochemistry, we found that brain volume, DG size and number of DCX cells varied significantly between breeding and non-breeding seasons, but only brain volume and the number of DCX labeled cells differed between the sexes. Both sex and seasonal differences likely reflect circulating hormone levels, but the extent to which these differences relate to space use in this species is unclear. Based on the degree of seasonal differences in neurogenesis and the DG, we suggest that ground squirrels could be considered model species in which to examine hippocampal plasticity in an ecologically valid context.

Fox squirrels match food assessment and cache effort to value and scarcity

Scatter hoarders must allocate time to assess items for caching, and to carry and bury each cache. Such decisions should be driven by economic variables, such as the value of the individual food items, the scarcity of these items, competition for food items and risk of pilferage by conspecifics. The fox squirrel, an obligate scatter-hoarder, assesses cacheable food items using two overt movements, head flicks and paw manipulations. These behaviors allow an examination of squirrel decision processes when storing food for winter survival. We measured wild squirrels' time allocations and frequencies of assessment and investment behaviors during periods of food scarcity (summer) and abundance (fall), giving the squirrels a series of 15 items (alternating five hazelnuts and five peanuts). Assessment and investment per cache increased when resource value was higher (hazelnuts) or resources were scarcer (summer), but decreased as scarcity declined (end of sessions). This is the first study to show that assessment behaviors change in response to factors that indicate daily and seasonal resource abundance, and that these factors may interact in complex ways to affect food storing decisions. Food-storing tree squirrels may be a useful and important model species to understand the complex economic decisions made under natural conditions.

Mice selectively bred for high voluntary wheel running have larger midbrains: support for the mosaic model of brain evolution

Increased brain size, relative to body mass, is a primary characteristic distinguishing the mammalian lineage. This greater encephalization has come with increased behavioral complexity and, accordingly, it has been suggested that selection on behavioral traits has been a significant factor leading to the evolution of larger whole-brain mass. In addition, brains may evolve in a mosaic fashion, with functional components having some freedom to evolve independently from other components, irrespective of, or in addition to, changes in size of the whole brain. We tested whether long-term selective breeding for high voluntary wheel running in laboratory house mice results in changes in brain size, and whether those changes have occurred in a concerted or mosaic fashion. We measured wet and dry brain mass via dissections and brain volume with ex vivo magnetic resonance imaging of brains that distinguished the caudate-putamen, hippocampus, midbrain, cerebellum and forebrain. Adjusting for body mass as a covariate, mice from the four replicate high-runner (HR) lines had statistically larger non-cerebellar wet and dry brain masses than those from four non-selected control lines, with no differences in cerebellum wet or dry mass or volume. Moreover, the midbrain volume in HR mice was ~13% larger (P<0.05), while volumes of the caudate-putamen, hippocampus, cerebellum and forebrain did not differ statistically between HR and control lines. We hypothesize that the enlarged midbrain of HR mice is related to altered neurophysiological function in their dopaminergic system. To our knowledge, this is the first example in which selection for a particular mammalian behavior has been shown to result in a change in size of a specific brain region.

On the nature of three-dimensional encoding in the cognitive map: Commentary on Hayman, Verriotis, Jovalekic, Fenton, and Jeffery

A recent article by Hayman, Verriotis, Jovalekic, Fenton, and Jeffery titled Anisotropic encoding of three-dimensional space by place cells and grid cells (2011) explored how place and grid cells respond when rats locomote vertically above the ground. From their results the authors concluded a number of points about rats' abilities to orient and navigate in three dimensions. Here, we review evidence revolving around several issues including: (1) what reference frame rats use when locomoting vertically, (2) whether rats can perceive their height above the ground, (3) whether rats can estimate vertical distance and have a cognitive map in the vertical domain, (4) whether rats can path integrate in the vertical domain, and (5) does processing 3-dimensional representations require a large number of neurons. We argue that the Hayman et al. results can be accounted for by considering the reference frame the animals used in the tasks. Had the rats been facing inward with their limbs in contact with the vertical surface when moving, it is possible that different patterns of place and grid cell activity would have been observed. Further, there is good evidence to indicate that rats can orient and navigate effectively in the vertical domain.

Seasonal and sex differences in the hippocampus of a wild rodent

Studies across and within species suggest that hippocampus size is sexually dimorphic in polygamous species, but not in monogamous species. Although hippocampal volume varies with sex, season and mating system, few studies have simultaneously tested for sex and seasonal differences. Here, we test for sex and seasonal differences in the hippocampal volume of wild Richardson's ground squirrels (Urocitellus richardsonii), a polygamous species that lives in matrilineal, kin-based social groups and has profound sex differences in behavior. Based on the behavior and ecology of this species, we predicted that males would have a significantly larger hippocampus than females and that the hippocampus would be largest in males during the breeding season. Analyses of both absolute and relative volumes of the hippocampus yielded a significant difference between the sexes and seasons as well as an interaction between the two such that non-breeding males have significantly larger hippocampal volumes than breeding males or females from either season. Dentate gyrus, CA1 and CA3 subfield volumes were generally larger in the non-breeding season and in males, but no significant interaction effects were detected. This sex and seasonal variation in hippocampal volume is likely the result of their social organization and male-only food caching behavior during the non-breeding season. The demonstration of a sex and seasonal variation in hippocampal volume suggests that Richardson's ground squirrel may be a useful model for understanding hippocampal plasticity within a natural context.

Postnatal development of the amygdala: A stereological study in macaque monkeys

Abnormal development of the amygdala has been linked to several neurodevelopmental disorders, including schizophrenia and autism. However, the postnatal development of the amygdala is not easily explored at the cellular level in humans. Here we performed a stereological analysis of the macaque monkey amygdala in order to characterize the cellular changes underlying its normal structural development in primates. The lateral, basal, and accessory basal nuclei exhibited the same developmental pattern, with a large increase in volume between birth and 3 months of age, followed by slower growth continuing beyond 1 year of age. In contrast, the medial nucleus was near adult size at birth. At birth, the volume of the central nucleus was half of the adult value; this nucleus exhibited significant growth even after 1 year of age. Neither neuronal soma size, nor neuron or astrocyte numbers changed during postnatal development. In contrast, oligodendrocyte numbers increased substantially, in parallel with an increase in amygdala volume, after 3 months of age. At birth, the paralaminar nucleus contained a large pool of immature neurons that gradually developed into mature neurons, leading to a late increase in the volume of this nucleus. Our findings revealed that distinct amygdala nuclei exhibit different developmental profiles and that the amygdala is not fully mature for some time postnatally. We identified different periods during which pathogenic factors might lead to the abnormal development of distinct amygdala circuits, which may contribute to different human neurodevelopmental disorders associated with alterations of amygdala structure and functions.

Stereological analysis of the rat and monkey amygdala

The amygdala is part of a neural network that contributes to the regulation of emotional behaviors. Rodents, especially rats, are used extensively as model organisms to decipher the functions of specific amygdala nuclei, in particular in relation to fear and emotional learning. Analysis of the role of the nonhuman primate amygdala in these functions has lagged work in the rodent but provides evidence for conservation of basic functions across species. Here we provide quantitative information regarding the morphological characteristics of the main amygdala nuclei in rats and monkeys, including neuron and glial cell numbers, neuronal soma size, and individual nuclei volumes. The volumes of the lateral, basal, and accessory basal nuclei were, respectively, 32, 39, and 39 times larger in monkeys than in rats. In contrast, the central and medial nuclei were only 8 and 4 times larger in monkeys than in rats. The numbers of neurons in the lateral, basal, and accessory basal nuclei were 14, 11, and 16 times greater in monkeys than in rats, whereas the numbers of neurons in the central and medial nuclei were only 2.3 and 1.5 times greater in monkeys than in rats. Neuron density was between 2.4 and 3.7 times lower in monkeys than in rats, whereas glial density was only between 1.1 and 1.7 times lower in monkeys than in rats. We compare our data in rats and monkeys with those previously published in humans and discuss the theoretical and functional implications that derive from our quantitative structural findings.

Influence of photoperiod on hormones, behavior, and immune function

Photoperiodism is the ability of plants and animals to measure environmental day length to ascertain time of year. Central to the evolution of photoperiodism in animals is the adaptive distribution of energetically challenging activities across the year to optimize reproductive fitness while balancing the energetic tradeoffs necessary for seasonally-appropriate survival strategies. The ability to accurately predict future events requires endogenous mechanisms to permit physiological anticipation of annual conditions. Day length provides a virtually noise free environmental signal to monitor and accurately predict time of the year. In mammals, melatonin provides the hormonal signal transducing day length. Duration of pineal melatonin is inversely related to day length and its secretion drives enduring changes in many physiological systems, including the HPA, HPG, and brain-gut axes, the autonomic nervous system, and the immune system. Thus, melatonin is the fulcrum mediating redistribution of energetic investment among physiological processes to maximize fitness and survival.

Postnatal development of the hippocampal formation: a stereological study in macaque monkeys

We performed a stereological analysis of neuron number, neuronal soma size, and volume of individual regions and layers of the macaque monkey hippocampal formation during early postnatal development. We found a protracted period of neuron addition in the dentate gyrus throughout the first postnatal year and a concomitant late maturation of the granule cell population and individual dentate gyrus layers that extended beyond the first year of life. Although the development of CA3 generally paralleled that of the dentate gyrus, the distal portion of CA3, which receives direct entorhinal cortex projections, matured earlier than the proximal portion of CA3. CA1 matured earlier than the dentate gyrus and CA3. Interestingly, CA1 stratum lacunosum-moleculare, in which direct entorhinal cortex projections terminate, matured earlier than CA1 strata oriens, pyramidale, and radiatum, in which the CA3 projections terminate. The subiculum developed earlier than the dentate gyrus, CA3, and CA1, but not CA2. However, similarly to CA1, the molecular layer of the subiculum, in which the entorhinal cortex projections terminate, was overall more mature in the first postnatal year compared with the stratum pyramidale in which most of the CA1 projections terminate. Unlike other hippocampal fields, volumetric measurements suggested regressive events in the structural maturation of presubicular neurons and circuits. Finally, areal and neuron soma size measurements revealed an early maturation of the parasubiculum. We discuss the functional implications of the differential development of distinct hippocampal circuits for the emergence and maturation of different types of "hippocampus-dependent" memory processes, including spatial and episodic memories.

Sexually dimorphic neural phenotypes in golden-collared manakins (Manacus vitellinus)

Male golden-collared manakins (Manacus vitellinus) perform a high-speed acrobatic courtship display punctuated by loud 'snaps' produced by the wings. Females join males on display courts to select individuals for copulation; females follow displaying males but do not perform acrobatics or make wing snaps. Sexually dimorphic courtship displays such as those performed by manakins are the result of intense sexual selection and suggest that differences between sexes exist at neural levels as well. We examined sex differences in the volume of brain areas that might be involved in the male manakin courtship display and in the female assessment of this display. We found that males had a larger hippocampus (HP, spatial learning) and arcopallium (AP, motor and limbic areas) than females when adjusted for the size of the telencephalon (TELE) minus the target area. Females had a larger ventrolateral mesopallium (MVL) both when adjusting for the size of the remaining TELE and by direct comparison. The entopallium (E) was not sexually dimorphic. The E is part of the avian tectofugal pathway and the MVL is linked to this pathway by reciprocal connections. The MVL likely modulates visually guided behavior via descending brainstem pathways. We found no sex differences in the volume of the cerebellum or cerebellar nuclei. We speculate that the HP is important to males for cross-season site fidelity and for local spatial memory, the AP for sexually driven motor patterns that are complex in males, and that the MVL facilitates female visual processing in selecting male display traits. These results are consistent with the idea that sexual selection has acted to select sex-specific behaviors in manakins that have neural correlates in the brain.

Is bigger always better? A critical appraisal of the use of volumetric analysis in the study of the hippocampus

A well-developed spatial memory is important for many animals, but appears especially important for scatter-hoarding species. Consequently, the scatter-hoarding system provides an excellent paradigm in which to study the integrative aspects of memory use within an ecological and evolutionary framework. One of the main tenets of this paradigm is that selection for enhanced spatial memory for cache locations should specialize the brain areas involved in memory. One such brain area is the hippocampus (Hp). Many studies have examined this adaptive specialization hypothesis, typically relating spatial memory to Hp volume. However, it is unclear how the volume of the Hp is related to its function for spatial memory. Thus, the goal of this article is to evaluate volume as a main measurement of the degree of morphological and physiological adaptation of the Hp as it relates to memory. We will briefly review the evidence for the specialization of memory in food-hoarding animals and discuss the philosophy behind volume as the main currency. We will then examine the problems associated with this approach, attempting to understand the advantages and limitations of using volume and discuss alternatives that might yield more specific hypotheses. Overall, there is strong evidence that the Hp is involved in the specialization of spatial memory in scatter-hoarding animals. However, volume may be only a coarse proxy for more relevant and subtle changes in the structure of the brain underlying changes in behaviour. To better understand the nature of this brain/memory relationship, we suggest focusing on more specific and relevant features of the Hp, such as the number or size of neurons, variation in connectivity depending on dendritic and axonal arborization and the number of synapses. These should generate more specific hypotheses derived from a solid theoretical background and should provide a better understanding of both neural mechanisms of memory and their evolution.

Using ecology to guide the study of cognitive and neural mechanisms of different aspects of spatial memory in food-hoarding animals

Understanding the survival value of behaviour does not tell us how the mechanisms that control this behaviour work. Nevertheless, understanding survival value can guide the study of these mechanisms. In this paper, we apply this principle to understanding the cognitive mechanisms that support cache retrieval in scatter-hoarding animals. We believe it is too simplistic to predict that all scatter-hoarding animals will outperform non-hoarding animals on all tests of spatial memory. Instead, we argue that we should look at the detailed ecology and natural history of each species. This understanding of natural history then allows us to make predictions about which aspects of spatial memory should be better in which species. We use the natural hoarding behaviour of the three best-studied groups of scatter-hoarding animals to make predictions about three aspects of their spatial memory: duration, capacity and spatial resolution, and we test these predictions against the existing literature. Having laid out how ecology and natural history can be used to predict detailed cognitive abilities, we then suggest using this approach to guide the study of the neural basis of these abilities. We believe that this complementary approach will reveal aspects of memory processing that would otherwise be difficult to discover.

Characterization of spatial performance in male and female Long-Evans rats by means of the Morris water task and the ziggurat task

Sex differences are prominent influences on spatial performance. One of the most common tasks to assess sex differences in spatial navigation in rodents is the Morris water task (MWT). In this task rats swim in a pool of water to locate a hidden platform employing the topographical relationships among the distal visual cues, pool wall, and goal location. Some evidence suggests that male rats display superior performance relative to females in the MWT. It is unknown, however, to what extent the sex difference in rats is task-dependent. This study compared the performance of male and female Long-Evans rats in the wet-land MWT versus the dry-land ziggurat task (ZT). The ZT represents a new dry-land task in which rats explore an arena with 16 ziggurat pyramids to locate food rewards. Several behavioural parameters, including latency, path length, path speed, probe trial performance, errors, and the number of returns were used as indices of spatial learning and memory. While males and females did not display significant differences in the traditional measures of spatial navigation within MWT, they displayed a robust sex difference in all measures of the ZT. These results indicate task-specific sex differences in spatial performance. Our findings suggest that males and females may employ different learning strategies in the MWT and ZT and that the latter task provides a more favourable task for assessing sex differences in rats.

Quantitative analysis of postnatal neurogenesis and neuron number in the macaque monkey dentate gyrus

The dentate gyrus is one of only two regions of the mammalian brain where substantial neurogenesis occurs postnatally. However, detailed quantitative information about the postnatal structural maturation of the primate dentate gyrus is meager. We performed design-based, stereological studies of neuron number and size, and volume of the dentate gyrus layers in rhesus macaque monkeys (Macaca mulatta) of different postnatal ages. We found that about 40% of the total number of granule cells observed in mature 5-10-year-old macaque monkeys are added to the granule cell layer postnatally; 25% of these neurons are added within the first three postnatal months. Accordingly, cell proliferation and neurogenesis within the dentate gyrus peak within the first 3 months after birth and remain at an intermediate level between 3 months and at least 1 year of age. Although granule cell bodies undergo their largest increase in size during the first year of life, cell size and the volume of the three layers of the dentate gyrus (i.e. the molecular, granule cell and polymorphic layers) continue to increase beyond 1 year of age. Moreover, the different layers of the dentate gyrus exhibit distinct volumetric changes during postnatal development. Finally, we observe significant levels of cell proliferation, neurogenesis and cell death in the context of an overall stable number of granule cells in mature 5-10-year-old monkeys. These data identify an extended developmental period during which neurogenesis might be modulated to significantly impact the structure and function of the dentate gyrus in adulthood.

Adult hippocampal neurogenesis of mammals: evolution and life history

Substantial production of new neurons in the adult mammalian brain is restricted to the olfactory system and the hippocampal formation. Its physiological and behavioural role is still debated. By comparing adult hippocampal neurogenesis (AHN) across many mammalian species, one might recognize a common function. AHN is most prominent in rodents, but shows considerable variability across species, being lowest or missing in primates and bats. The latter finding argues against a critical role of AHN in spatial learning and memory. The common functional denominator across all species investigated thus far is a strong decline of AHN from infancy to midlife. As predicted by Altman and colleagues in 1973, this implies a role in transforming juvenile unpredictable to predictable behaviour, typically characterizing mammalian behaviour once reproductive competence has been attained. However, as only a fraction of mammalian species has been investigated, further comparative studies are necessary in order to recognize whether AHN has a common unique function, or whether it mediates species-specific hippocampal functions.

Size variation and cell proliferation in chemosensory brain areas of a lizard (Podarcis hispanica): effects of sex and season

Many lizards rely on chemoreception for crucial aspects of their biology, including exploration, prey and predator detection, and intraspecific communication. Here we investigate sex and seasonal variation in size and proliferative activity in chemosensory areas of the lizard brain. We captured adult Iberian wall lizards (Podarcis hispanica) of either sex in the breeding (April) and non-breeding (November) season, injected them with 5-bromo-2'-deoxyuridine (BrdU) and killed them 3 weeks later. We removed the brains, measured the length of the olfactory bulbs, and counted BrdU-labelled cells in the main and accessory olfactory bulbs (MOB, AOB), lateral cortex (LC) and nucleus sphericus (NS). Our results show that, relative to body size, males have larger MOBs and AOBs than females; however, relative to brain size, males have larger AOBs, but not larger MOBs than females. Additionally, males produce more new cells than females in the olfactory bulbs, LC and NS. We failed to detect significant seasonal changes or sex x season interaction in size or proliferative activity in these areas. Sex differences in the addition of newly generated cells--mainly neurons--may be partly responsible for the size differences in chemosensory brain areas. The presence of sexual dimorphism in AOB is expected given the available behavioural evidence, which suggests that males of P. hispanica are more responsive than females to socially relevant chemical stimuli. This is the first demonstration of sexual dimorphism in size and proliferative activity in chemosensory areas of a non-mammalian species.

Flexibility of cue use in the fox squirrel (Sciurus niger)

Recent work on captive flying squirrels has demonstrated a novel degree of flexibility in the use of different orientation cues. In the present study, we examine to what extent this flexibility is present in a free-ranging population of another tree squirrel species, the fox squirrel. We trained squirrels to a rewarded location within a square array of four feeders and then tested them on transformations of the array that either pitted two cue types against one cue type, the majority tests, or all cue types against each other, the forced-hierarchy test. In Experiment 1, squirrels reoriented to the two-cue-type location in all majority tests and to the location indicated by the visual features of the feeders in the forced-hierarchy test. This preference for visual features runs contrary to previous studies that report the use of spatial cues over visual features in food-storing species. In Experiments 2-5 we tested squirrels with different trial orders (Experiments 2 and 3), a different apparatus (Experiment 4) and at different times of the year (Experiment 5) to determine why these squirrels had chosen to orient using visual features in the first experiment. Like captive flying squirrels, free-ranging fox squirrels showed a large degree of flexibility in their use of cues. Furthermore, their cue use appeared to be sensitive both to changes in the test apparatus and the season in which we tested. Altogether our results suggest that the study of free-ranging animals over a variety of conditions is necessary for understanding spatial cognition.

Postnatal development of the primate hippocampal formation

The hippocampal formation is a multicomponent region of the medial temporal lobe preferentially involved in declarative and relational memory processing. Behavioral studies have suggested a protracted functional maturation of these structures in primates, and postnatal developmental abnormalities in the hippocampal formation are thought to contribute to neurodevelopmental disorders, such as autism, schizophrenia, epilepsy and Down syndrome. Despite all that we know about the functional organization of the adult hippocampal formation, notably absent is a systematic study of its postnatal maturation in primates. In this article, we review current knowledge of the structural development of the primate hippocampal formation and present new data on its postnatal neuroanatomical development. We summarize what is known about the neurobiological processes, such as the addition of new neurons, the establishment and elaboration of connectivity, and the neurochemical changes, that underlie the structural development and functional maturation of the primate hippocampal formation. We conclude that there is yet insufficient information to identify distinct developmental windows during which different hippocampal regions undergo specific maturational processes. For this reason, it is currently impossible to determine the ages at which specific hippocampal circuits become structurally mature and potentially capable of supporting defined, age-specific functional processes. Together with work in rodents, systematic studies of the structural development and functional maturation of the monkey hippocampal formation will be necessary to gain insight not only into the types of information processing that it subserves, but also into the specific maturational processes that might be affected in human neurodevelopmental disorders.

Annual cycle of the black-capped chickadee: seasonality of food-storing and the hippocampus

Previous research presents a mixed picture of seasonal variation in the hippocampus of food-storing black-capped chickadees. One field study has shown an October peak in hippocampus volume, although laboratory studies conducted to determine whether photoperiod regulates this seasonal growth have failed to find changes in the size of the hippocampus. To resolve the discrepancy between field and lab reports we examined caching activity, hippocampal volume, and neurogenesis in adult male black-capped chickadees at four times over the annual cycle: October, January, April and July. We found that more birds stored food in October than at other times of year, but did not observe a significant change in the size of the hippocampus over the annual cycle. Telencephalon volume, however, was larger in October than in July. Hippocampal neuronal recruitment showed a significant peak in January, but there was no seasonal change in neuronal recruitment in the adjacent hyperpallium apicale. These results indicate that there might be seasonal variation in the recruitment of new neurons into the hippocampus of chickadees without overall seasonal change in hippocampal size.

A Multi-disciplinary Approach to Understanding Hippocampal Function in Foodhoarding Birds

Spatial memory and the hippocampal formation (HF) of food-hoarding birds have been put forward as a prime example of how natural selection has shaped a cognitive system and its neural underpinnings. Here, I review what we know about the HF of hoarding birds and lay out the work that is currently underway to use this system to obtain a better understanding of hippocampal function in general. This interdisciplinary programme includes evolutionary, ecological, psychological, ethological, and neuroscientific approaches to the study of behaviour and cognition. Firstly, we need to understand the behaviour of the birds in their natural environment, and identify the aspects of cognition and behaviour that may be especially valuable for the species under study. Secondly, these cognitive and behavioural traits are compared to closely-related non-hoarding species. Thirdly, we also compare HF anatomy between closely-related hoarding and non-hoarding species, identifying possible neural mechanisms underlying behavioural differences. Finally, behavioural and neuroscientific approaches are combined in experiments directly investigating the involvement of the HF or any of its anatomical and physiological aspects in the behaviours under study. This process loops back upon itself in many different ways, with all the different approaches informing each other. In this way we are making progress in understanding the functioning of the HF, not only in food-hoarding birds, but in all vertebrates.

Relative Medial and Dorsal Cortex Volume in Relation to Sex Differences in Spatial Ecology of a Snake Population

In non-avian reptiles the medial and dorsal cortices are putative homologues of the hippocampal formation in mammals and birds. Studies on mammals and birds commonly report neuro-ecological correlations between hippocampal volume and aspects of spatial ecology. We examined the relationship between putative homologous cortical volumes and spatial use in a population of the squamate reptile, Agkistrodon piscivorus, that exhibits sex differences in spatial use. Do male A. piscivorus that inhabit larger home ranges than females also have larger putative hippocampal volumes? Male and female brains were sectioned and digitized to quantify regional cortical volumes. Although sex differences in dorsal cortex volume were not observed, males had a significantly larger medial cortex relative to telencephalon volume. Similar to studies on mammals and birds, relative hippocampal or medial cortex volume was positively correlated with patterns of spatial use. We demonstrate volumetric sex differences within a reptilian putative hippocampal homologue.

Sex differences in the stereological parameters of the hippocampal dentate gyrus of the guinea-pig before puberty

Studies in rats and mice have shown several sex-dependent functional and structural differences in the hippocampal region, a brain structure playing a key role in learning and memory. The aim of the present study was to establish whether sex differences exist prior to puberty in the stereological parameters of the dentate gyrus in the guinea-pig, a long-gestation rodent, whose brain is at a more advanced stage of maturation at birth than the rat and mouse. The number of granule cells and volumes of the granule cell layer, molecular layer and hilus were evaluated in Nissl-stained brains of neonatal (15-16 days old) and peripubescent (45-46 days old) guinea-pigs. Based on a pilot study, the optical disector method was preferred to the optical fractionator method to estimate cell number. For volume (Vref) estimation with the Cavalieri principle, contour tracing was preferred to the point counting method, as the latter appeared to underestimate volumes. The results showed that neonatal males had more granule cells than females in both the dorsal and ventral dentate gyrus and a larger volume in all layers. Peripubescent males had a larger volume of the granule cell layer than females in both the dorsal and ventral dentate gyrus, more granule cells in the ventral dentate gyrus, a larger volume of the hilus in both the dorsal and ventral dentate gyrus and a larger volume of the molecular layer in the ventral dentate gyrus. The results show that sex differences are present in the guinea-pig dentate gyrus prior to puberty and go in the same direction at both investigated ages, with males exhibiting more granule cells and larger volumes than females. The widespread distribution of these sex differences suggests that in the guinea-pig, similarly to other rodents, hippocampus-dependent functions may be sexually dimorphic.

Where's my dinner? Adult neurogenesis in free-living food-storing rodents

Postnatal hippocampal neurogenesis in wild mammals may play an essential role in spatial memory. We compared two species that differ in their reliance on memory to locate stored food. Yellow-pine chipmunks use a single cache to store winter food; eastern gray squirrels use multiple storage sites. Gray squirrels had three times the density of proliferating cells in the dentate gyrus (determined by Ki-67 immunostaining) than that found in chipmunks, but similar density of young neurons (determined by doublecortin immunostaining). Three explanations may account for these results. First, the larger population of young cells in squirrels may increase the flexibility of the spatial memory system by providing a larger pool of cells from which new neurons can be recruited. Second, squirrels may have a more rapid cell turnover rate. Third, many young cells in the squirrels may mature into glia rather than neurons. The densities of young neurons were higher in juveniles than in adults of both species. The relationship between adult age and cell density was more complex than that has been found in captive populations. In adult squirrels, the density of proliferating cells decreased exponentially with age, whereas in adult chipmunks the density of young neurons decreased exponentially with age.

Neuron production, neuron number, and structure size are seasonally stable in the hippocampus of the food-storing black-capped chickadee (Poecile atricapillus)

Neuronal recruitment in the black-capped chickadee (Poecile atricapillus) hippocampus occurs at a higher rate in the fall than at other times of the year. As a means of determining whether this increase in recruitment results from greater neuron production, chickadees were caught in the wild between October and March and injected with the cell-birth marker 5-bromo-2'-deoxyuridine. Two weeks later, birds were killed by overdose, and hippocampal neuron production, apoptosis, neuron number, and hippocampal volume were determined. Chickadees collected in October, November, January, February, and March did not differ in neuron production, apoptosis, hippocampal volume, or neuron number. These findings indicate that increases in neuronal recruitment in the chickadee hippocampus in the fall do not result from increased neuron production, but instead, enhanced survival of new neurons.

Dominance-related changes in spatial memory are associated with changes in hippocampal cell proliferation rates in mountain chickadees

It is well established that spatial memory is dependent on the hippocampus in both mammals and birds. As memory capacity can fluctuate on a temporal basis, it is important to understand the mechanisms mediating such changes. It is known that early memory-dependent experiences in young animals result in hippocampal enlargement and in increased neurogenesis, including cell proliferation and neuron survival. It is less clear, however, whether temporal changes in spatial memory are also associated with changes in hippocampal anatomy and cell proliferation in fully grown and experienced adult animals. In a previous study, we experimentally demonstrated that socially subordinate mountain chickadees (Poecile gambeli) showed inferior spatial memory performance compared to their dominant group mates, in the absence of significant differences in baseline corticosterone levels. Here we investigated whether these differences in memory between dominant and subordinate birds were associated with changes in the hippocampus. Following memory tests, chickadees were injected with 5-bromo-2'-deoxyuridine to label dividing cells and sacrificed 2 days after the injections. We found no significant differences in volume or the total number of neurons in the hippocampal formation between dominant and subordinate chickadees, but subordinate birds had significantly lower cell proliferation rates in the ventricular zone adjacent to both the hippocampus and mesopallium compared to the dominants. Individuals, which performed better on spatial memory tests tended to have higher levels of cell proliferation. These results suggest that social status can affect cell proliferation rates in the ventricular zone and support the hypothesis that neurogenesis might be involved in memory function in adult animals.

Life-long stability of neurons: a century of research on neurogenesis, neuronal death and neuron quantification in adult CNS

In this chapter we provide an extensive review of 100 years of research on the stability of neurons in the mammalian brain, with special emphasis on humans. Although Cajal formulated the Neuronal Doctrine, he was wrong in his beliefs that adult neurogenesis did not occur and adult neurons are dying throughout life. These two beliefs became accepted "common knowledge" and have shaped much of neuroscience research and provided much of the basis for clinical treatment of age-related brain diseases. In this review, we consider adult neurogenesis from a historical and evolutionary perspective. It is concluded, that while adult neurogenesis is a factor in the dynamics of the dentate gyrus and olfactory bulb, it is probably not a major factor during the life-span in most brain areas. Likewise, the acceptance of neuronal death as an explanation for normal age-related senility is challenged with evidence collected over the last fifty years. Much of the problem in changing this common belief of dying neurons was the inadequacies of neuronal counting methods. In this review we discuss in detail implications of recent improvements in neuronal quantification. We conclude: First, age-related neuronal atrophy is the major factor in functional deterioration of existing neurons and could be slowed down, or even reversed by various pharmacological interventions. Second, in most cases neuronal degeneration during aging is a pathology that in principle may be avoided. Third, loss of myelin and of the white matter is more frequent and important than the limited neuronal death in normal aging.

Changes in spatial memory mediated by experimental variation in food supply do not affect hippocampal anatomy in mountain chickadees (Poecile gambeli)

Earlier reports suggested that seasonal variation in food-caching behavior (caching intensity and cache retrieval accuracy) might correlate with morphological changes in the hippocampal formation, a brain structure thought to play a role in remembering cache locations. We demonstrated that changes in cache retrieval accuracy can also be triggered by experimental variation in food supply: captive mountain chickadees (Poecile gambeli) maintained on limited and unpredictable food supply were more accurate at recovering their caches and performed better on spatial memory tests than birds maintained on ad libitum food. In this study, we investigated whether these two treatment groups also differed in the volume and neuron number of the hippocampal formation. If variation in memory for food caches correlates with hippocampal size, then our birds with enhanced cache recovery and spatial memory performance should have larger hippocampal volumes and total neuron numbers. Contrary to this prediction we found no significant differences in volume or total neuron number of the hippocampal formation between the two treatment groups. Our results therefore indicate that changes in food-caching behavior and spatial memory performance, as mediated by experimental variations in food supply, are not necessarily accompanied by morphological changes in volume or neuron number of the hippocampal formation in fully developed, experienced food-caching birds.

Localization of estrogen receptor alpha (ER alpha) in pyramidal neurons of the developing rat hippocampus

During development, estrogen has a variety of effects on morphological, biochemical and electrophysiological properties of hippocampal neurons. Correspondingly, estrogen receptor (ER) binding and mRNA increase transiently in the developing hippocampus. In this study, we used immunocytochemistry to determine the localization of the ER alpha subtype in the developing rat hippocampus. Nuclear staining was present in pyramidal cells and some interneurons of the CA1 and CA3 regions of the developing rat hippocampus. Little or no immunoreactivity was observed in postnatal day (P)0 animals (day of birth=P0), however, beginning on P4, ER alpha-immunoreactivity (ER alpha-ir) was visible and reached maximal levels by P10. These levels subsequently declined to low levels so that by P15, levels approximated those of adult females. Western blot analysis confirmed that this antibody recognized a 67 kDa protein, characteristic of the full-length ER alpha protein, in the hippocampus and pituitary. Furthermore, most of the ER alpha-immunopositive cells in the hippocampus were located in the pyramidal cell layer, and did not co-localize appreciably with gamma-aminobutyric acid (GABA) at any age examined. We conclude, based on the immunocytochemical localization of ER alpha, that the effects of estrogen on biochemistry and morphology of the developing hippocampus may be direct through the ER alpha subtype in hippocampal pyramidal cells.