The distribution of neuropeptides in the dorsomedial telencephalon of the pigeon (Columba livia): a basis for regional subdivisions

A bird's eye view of the hippocampus beyond space: Behavioral, neuroanatomical, and neuroendocrine perspectives

Although the hippocampus is one of the most-studied brain regions in mammals, research on the avian hippocampus has been more limited in scope. It is generally agreed that the hippocampus is an ancient feature of the amniote brain, and therefore homologous between the two lineages. Because birds and mammals are evolutionarily not very closely related, any shared anatomy is likely to be crucial for shared functions of their hippocampi. These functions, in turn, are likely to be essential if they have been conserved for over 300 million years. Therefore, research on the avian hippocampus can help us understand how this brain region evolved and how it has changed over evolutionary time. Further, there is a strong research foundation in birds on hippocampal-supported behaviors such as spatial navigation, food caching, and brood parasitism that scientists can build upon to better understand how hippocampal anatomy, network circuitry, endocrinology, and physiology can help control these behaviors. In this review, we summarize our current understanding of the avian hippocampus in spatial cognition as well as in regulating anxiety, approach-avoidance behavior, and stress responses. Although there are still some questions about the exact number of subdivisions in the avian hippocampus and how that might vary in different avian families, there is intriguing evidence that the avian hippocampus might have complementary functional profiles along the rostral-caudal axis similar to the dorsal-ventral axis of the rodent hippocampus, where the rostral/dorsal hippocampus is more involved in cognitive processes like spatial learning and the caudal/ventral hippocampus regulates emotional states, anxiety, and the stress response. Future research should focus on elucidating the cellular and molecular mechanisms - including endocrinological - in the avian hippocampus that underlie behaviors such as spatial navigation, spatial memory, and anxiety-related behaviors, and in so doing, resolve outstanding questions about avian hippocampal function and organization.

Topography of inputs into the hippocampal formation of a food-caching bird

The mammalian hippocampal formation (HF) is organized into domains associated with different functions. These differences are driven in part by the pattern of input along the hippocampal long axis, such as visual input to the septal hippocampus and amygdalar input to the temporal hippocampus. HF is also organized along the transverse axis, with different patterns of neural activity in the hippocampus and the entorhinal cortex. In some birds, a similar organization has been observed along both of these axes. However, it is not known what role inputs play in this organization. We used retrograde tracing to map inputs into HF of a food-caching bird, the black-capped chickadee. We first compared two locations along the transverse axis: the hippocampus and the dorsolateral hippocampal area (DL), which is analogous to the entorhinal cortex. We found that pallial regions predominantly targeted DL, while some subcortical regions like the lateral hypothalamus (LHy) preferentially targeted the hippocampus. We then examined the hippocampal long axis and found that almost all inputs were topographic along this direction. For example, the anterior hippocampus was preferentially innervated by thalamic regions, while the posterior hippocampus received more amygdalar input. Some of the topographies we found bear a resemblance to those described in the mammalian brain, revealing a remarkable anatomical similarity of phylogenetically distant animals. More generally, our work establishes the pattern of inputs to HF in chickadees. Some of these patterns may be unique to chickadees, laying the groundwork for studying the anatomical basis of these birds' exceptional hippocampal memory.

An entorhinal-like region in food-caching birds

The mammalian entorhinal cortex routes inputs from diverse sources into the hippocampus. This information is mixed and expressed in the activity of many specialized entorhinal cell types, which are considered indispensable for hippocampal function. However, functionally similar hippocampi exist even in non-mammals that lack an obvious entorhinal cortex or, generally, any layered cortex. To address this dilemma, we mapped extrinsic hippocampal connections in chickadees, whose hippocampi are used for remembering numerous food caches. We found a well-delineated structure in these birds that is topologically similar to the entorhinal cortex and interfaces between the hippocampus and other pallial regions. Recordings of this structure revealed entorhinal-like activity, including border and multi-field grid-like cells. These cells were localized to the subregion predicted by anatomical mapping to match the dorsomedial entorhinal cortex. Our findings uncover an anatomical and physiological equivalence of vastly different brains, suggesting a fundamental nature of entorhinal-like computations for hippocampal function.

Neuronal circuits within the homing pigeon hippocampal formation

The current study aimed to reveal in detail patterns of intrahippocampal connectivity in homing pigeons (Columba livia). In light of recent physiological evidence suggesting differences between dorsomedial and ventrolateral hippocampal regions and a hitherto unknown laminar organization along the transverse axis, we also aimed to gain a higher-resolution understanding of the proposed pathway segregation. Both in vivo and high-resolution in vitro tracing techniques were employed and revealed a complex connectivity pattern along the subdivisions of the avian hippocampus. We uncovered connectivity pathways along the transverse axis that started in the dorsolateral hippocampus and continued to the dorsomedial subdivision, from where information was relayed to the triangular region either directly or indirectly via the V-shaped layers. The often-reciprocal connectivity along these subdivisions displayed an intriguing topographical arrangement such that two parallel pathways could be discerned along the ventrolateral (deep) and dorsomedial (superficial) aspects of the avian hippocampus. The segregation along the transverse axis was further supported by expression patterns of the glial fibrillary acidic protein and calbindin. Moreover, we found strong expression of Ca²⁺ /calmodulin-dependent kinase IIα and doublecortin in the lateral but not medial V-shape layer, indicating a difference between the two V-shaped layers. Overall, our findings provide an unprecedented, detailed description of avian intrahippocampal pathway connectivity, and confirm the recently proposed segregation of the avian hippocampus along the transverse axis. We also provide further support for the hypothesized homology of the lateral V-shape layer and the dorsomedial hippocampus with the dentate gyrus and Ammon's horn of mammals, respectively.

Topography of inputs into the hippocampal formation of a food-caching bird

The mammalian hippocampal formation (HF) is organized into domains associated with different functions. These differences are driven in part by the pattern of input along the hippocampal long axis, such as visual input to the septal hippocampus and amygdalar input to temporal hippocampus. HF is also organized along the transverse axis, with different patterns of neural activity in the hippocampus and the entorhinal cortex. In some birds, a similar organization has been observed along both of these axes. However, it is not known what role inputs play in this organization. We used retrograde tracing to map inputs into HF of a food-caching bird, the black-capped chickadee. We first compared two locations along the transverse axis: the hippocampus and the dorsolateral hippocampal area (DL), which is analogous to the entorhinal cortex. We found that pallial regions predominantly targeted DL, while some subcortical regions like the lateral hypothalamus (LHy) preferentially targeted the hippocampus. We then examined the hippocampal long axis and found that almost all inputs were topographic along this direction. For example, the anterior hippocampus was preferentially innervated by thalamic regions, while posterior hippocampus received more amygdalar input. Some of the topographies we found bear resemblance to those described in the mammalian brain, revealing a remarkable anatomical similarity of phylogenetically distant animals. More generally, our work establishes the pattern of inputs to HF in chickadees. Some of these patterns may be unique to chickadees, laying the groundwork for studying the anatomical basis of these birds ’ exceptional hippocampal memory.

An entorhinal-like region in food-caching birds

The mammalian entorhinal cortex routes inputs from diverse sources into the hippocampus. This information is mixed and expressed in the activity of many specialized entorhinal cell types, which are considered indispensable for hippocampal function. However, functionally similar hippocampi exist even in non-mammals that lack an obvious entorhinal cortex, or generally any layered cortex. To address this dilemma, we mapped extrinsic hippocampal connections in chickadees, whose hippocampi are used for remembering numerous food caches. We found a well-delineated structure in these birds that is topologically similar to the entorhinal cortex and interfaces between the hippocampus and other pallial regions. Recordings of this structure revealed entorhinal-like activity, including border and multi-field grid-like cells. These cells were localized to the subregion predicted by anatomical mapping to match the dorsomedial entorhinal cortex. Our findings uncover an anatomical and physiological equivalence of vastly different brains, suggesting a fundamental nature of entorhinal-like computations for hippocampal function.

Spatial cognition and the avian hippocampus: Research in domestic chicks

In this review, we discuss the functional equivalence of the avian and mammalian hippocampus, based mostly on our own research in domestic chicks, which provide an important developmental model (most research on spatial cognition in other birds relies on adult animals). In birds, like in mammals, the hippocampus plays a central role in processing spatial information. However, the structure of this homolog area shows remarkable differences between birds and mammals. To understand the evolutionary origin of the neural mechanisms for spatial navigation, it is important to test how far theories developed for the mammalian hippocampus can also be applied to the avian hippocampal formation. To address this issue, we present a brief overview of studies carried out in domestic chicks, investigating the direct involvement of chicks’ hippocampus homolog in spatial navigation.

Neural Substrates of Homing Pigeon Spatial Navigation: Results From Electrophysiology Studies

Over many centuries, the homing pigeon has been selectively bred for returning home from a distant location. As a result of this strong selective pressure, homing pigeons have developed an excellent spatial navigation system. This system passes through the hippocampal formation (HF), which shares many striking similarities to the mammalian hippocampus; there are a host of shared neuropeptides, interconnections, and its role in the storage and manipulation of spatial maps. There are some notable differences as well: there are unique connectivity patterns and spatial encoding strategies. This review summarizes the comparisons between the avian and mammalian hippocampal systems, and the responses of single neurons in several general categories: (1) location and place cells responding in specific areas, (2) path and goal cells responding between goal locations, (3) context-dependent cells that respond before or during a task, and (4) pattern, grid, and boundary cells that increase firing at stable intervals. Head-direction cells, responding to a specific compass direction, are found in mammals and other birds but not to date in pigeons. By studying an animal that evolved under significant adaptive pressure to quickly develop a complex and efficient spatial memory system, we may better understand the comparative neurology of neurospatial systems, and plot new and potentially fruitful avenues of comparative research in the future.

Dissociation of spatial and object memory in the hippocampal formation of Japanese quail

The mammalian temporal cortex can be functionally segregated into regions that encode spatial information and others that are predominantly responsible for object recognition. In the present study, we report comparable functional segregation in the avian brain. Using Japanese quail, we find that bilateral lesions of the hippocampus (Hp) produce robust deficits in performance in a foraging array (FA) spatial memory task, while sparing spontaneous object recognition (SOR). In contrast, lesions to the adjacent area parahippocampalis (APH) compromise both SOR and FA. These observations demonstrate a functional dissociation between Hp and APH that is comparable to the distinctions seen in mammals between the hippocampus and surrounding temporal cortex.

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Are spatial and object information separable in the avian hippocampal formation?

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Quail with lesions to the hippocampus are impaired in a spatial foraging task

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Lesions to area parahippocampalis also selectively impair object recognition

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Like mammals, bird hippocampus shows functional gradients in information processing

Acute stress-induced neuronal plasticity in the corticoid complex of 15-day-old chick, Gallus domesticus

Several studies conducted on chicken have shown that a single stress exposure may impair or improve memory as well as learning processes. However, to date, stress effects on neuronal morphology are poorly investigated wherefore it was of interest to evaluate this further in chicks. Thus, the present study aims to investigate the role of single acute stress (AS) of 24 h food and water deprivation in neuronal plasticity in terms of spine density of the corticoid complex (CC) in 15-day-old chick, Gallus domesticus, by using three neurohistological techniques: Cresyl Violet, Golgi Colonnier, and Golgi Cox technique. The dorsolateral surface of the cerebral hemisphere is occupied by CC which can be differentiated into two subfields: an intermediate corticoid (CI) subfield (arranged in layers) and a dorsolateral corticoid (CDL) subfield. Based on different criteria such as soma shape, dendritic branching pattern, and dendritic spine density, two main moderately spinous groups of neuronal cells were observed in the CC, namely, projection neurons (comprising of multipolar and pyramidal neurons) and stellate neurons. In the present study, the stellate neurons have shown a significant decrease as well as an increase in their spine density in both CI and CDL subfields, whereas the multipolar neurons had shown a significant increase in their spine density in the CDL region only. The present study shows that AS induces neuronal plasticity in terms of spine density in both CI and CDL neurons. The morphological changes in the form of decreased dendritic branches due to stress have been observed in the CI region in comparison to CDL region, which could be linked to more effect of stress in this region. The avian CDL corresponds to the entorhinal cortex of mammals on the basis of neuronal morphology and bidirectional connections between adjacent areas. The projection neurons increase their branches and also their spine number to cope with the stress effects, while the stellate neurons show contrasting effect in their spine density. Therefore, this study will establish that slight modifications in natural stimuli or environmental changes faced by the animal may affect their dorsolateral forebrain which shows neuronal plasticity that help in the development of an adaptive capacity of the animal to survive under changing environmental conditions.

The effects of hippocampal and area parahippocampalis lesions on the processing and retention of serial-order behavior, autoshaping, and spatial behavior in pigeons

We examined the role of the avian hippocampus and area parahippocampalis in serial-order behavior and a variety of other tasks known to be sensitive to hippocampal damage in mammals. Damage to the hippocampus and area parahippocampalis caused impairments in autoshaping and performance on an analogue of a radial-arm maze task, but had no effect on acquisition of 2-item, 3-item, and 4-item serial-order lists. Additionally, the lesions had no effect on the retention of 3-items lists, or on the ability to perform novel derived lists composed of elements from lists they had previously learned. The impairments in autoshaping and spatial behavior are consistent with the findings in mammals. The absence of impairments on the serial-order task may also be consistent once one considers that damage to the hippocampus in mammals seems to affect more internally-organized rather than externally-organized serial-order tasks. Together, the findings support the view that the avian hippocampal complex serves a function very similar to the mammalian hippocampus, a finding that is interesting given that the architecture of the avian hippocampus differs dramatically from that of the mammalian hippocampus.

Retrieval of contextual aversive memory and induction of Zenk expression in the hippocampus of pigeons

The hippocampus has a fundamental role in many learning and memory processes, which include the formation and retrieval of context-fear associations, as evidenced by studies in rodents and birds. The present paper has analyzed contextual memory and Zenk expression in the hippocampus of the pigeon after fear conditioning. Pigeons were trained under four conditions: with 3 tone-shock associations (Paired), with shock and tone presented randomly (Unpaired), with exposure to the experimental chamber without stimulation (Control) and with only daily handling (Naive). The testing was conducted 24 h after training. All sessions were digitally recorded. The level of freezing expressed by the Paired and Unpaired groups differed significantly from that of the control group during both training and test sessions. Pigeons from the Paired group revealed a significantly greater density of Zenk positive nuclei in the ventromedial region of the hippocampus than did the Unpaired, Control and Naive groups. These data suggest that Zenk-mediated processes of synaptic plasticity in the hippocampus are induced during the retrieval of conditioned fear memory in the pigeon.

Heterogeneous organization and connectivity of the chicken auditory thalamus (Gallus gallus)

The auditory ascending system contains parallel pathways in vertebrate brains. In chickens (Gallus gallus), three pathways arise from nucleus laminaris (NL), nucleus angularis (NA), and regio intermedius (RI) in the brainstem, innervating three subdivisions of the nucleus mesencephalicus lateralis pars dorsalis (MLd) in the midbrain. The current study reveals the segregation of these pathways in their subsequent projections to the nucleus ovoidalis (Ov) in the thalamus. Based on cytoarchitecture and myelin distribution, we identified seven Ov subregions, including five neuronal clusters within the Ov proper, the nucleus semilunaris parovoidalis (SPO), and the circum-ovoidalis (cOv). Immunocytochemistry further revealed that a ventromedial cluster of the Ov proper (Ovvm) contains unique cell types expressing α8 subunit nicotinic acetylcholine receptor, while SPO and cOv are characterized with expression of calcitonin-gene-related peptide and cholecystokinin. Tract tracing studies demonstrated that Ovvm is a major target of the NL-recipient zone of MLd, while the RI-recipient zone of MLd predominantly projects to a ventrolateral cluster of the Ov proper. Afferent inputs to the remaining regions of the Ov proper mostly arise from the NA-recipient zone of MLd. SPO and cOv receive a projection from the surrounding areas of MLd, named the nucleus intercollicularis. Importantly, the Ov proper, SPO and cOv all project to the Field L2 in the forebrain, the avian auditory cortex. Taken together, these results demonstrate that the avian auditory thalamus is a structurally and functionally heterogeneous structure, implicating an important role in generating novel representations for specific acoustic features.

Context-Dependent Egr1 Expression in the Avian Hippocampus

In mammals, episodic memory and spatial cognition involve context-specific recruitment of unique ensembles in the hippocampal formation (HF). Despite their capacity for sophisticated spatial (e.g., for migration) and episodic-like (e.g., for food-caching) memory, the mechanisms underlying contextual representation in birds is not well understood. Here we demonstrate environment-specific Egr1 expression as male brown-headed cowbirds (Molothrus ater) navigate environments for food reward, showing that the avian HF, like its mammalian counterpart, recruits distinct neuronal ensembles to represent different contexts.

Neurochemical compartmentalization within the pigeon basal ganglia

The goals of this study were to use multiple informative markers to define and characterize the neurochemically distinct compartments of the pigeon basal ganglia, especially striatum and accumbens. To this end, we used antibodies against 12 different neuropeptides, calcium-binding proteins or neurotransmitter-related enzymes that are enriched in the basal ganglia. Our results clarify boundaries between previously described basal ganglia subdivisions in birds, and reveal considerable novel heterogeneity within these previously described subdivisions. Sixteen regions were identified that each displayed a unique neurochemical organization. Four compartments were identified within the dorsal striatal region. The neurochemical characteristics support previous comparisons to part of the central extended amygdala, somatomotor striatum, and associational striatum of mammals, respectively. The medialmost part of the medial striatum, however, has several unique features, including prominent pallidal-like woolly fibers and thus may be a region unique to birds. Four neurochemically distinct regions were identified within the pigeon ventral striatum: the accumbens, paratubercular striatum, ventrocaudal striatum, and the ventral area of the lateral part of the medial striatum that is located adjacent to these regions. The pigeon accumbens is neurochemically similar to the mammalian rostral accumbens. The pigeon paratubercular and ventrocaudal striatal regions are similar to the mammalian accumbens shell. The ventral portions of the medial and lateral parts of the medial striatum, which are located adjacent to accumbens shell-like areas, have neurochemical characteristics as well as previously reported limbic connections that are comparable to the accumbens core. Comparisons to neurochemically identified compartments in reptiles, mammals, and amphibians indicate that, although most of the basic compartments of the basal ganglia were highly conserved during tetrapod evolution, uniquely avian compartments may exist as well.

Distribution of vasotocin- and vasoactive intestinal peptide-like immunoreactivity in the brain of blue tit (Cyanistes coeruleus)

Blue tits (Cyanistes coeruleus) are songbirds, used as model animals in numerous studies covering a wide field of research. Nevertheless, the distribution of neuropeptides in the brain of this avian species remains largely unknown. Here we present some of the first results on distribution of Vasotocine (AVT) and Vasoactive intestinal peptide (VIP) in the brain of males and females of this songbird species, using immunohistochemistry mapping. The bulk of AVT-like cells are found in the hypothalamic supraoptic, paraventricular and suprachiasmatic nuclei, bed nucleus of the stria terminalis, and along the lateral forebrain bundle. Most AVT-like fibers course toward the median eminence, some reaching the arcopallium, and lateral septum. Further terminal fields occur in the dorsal thalamus, ventral tegmental area and pretectal area. Most VIP-like cells are in the lateral septal organ and arcuate nucleus. VIP-like fibers are distributed extensively in the hypothalamus, preoptic area, lateral septum, diagonal band of Broca. They are also found in the bed nucleus of the stria terminalis, amygdaloid nucleus of taenia, robust nucleus of the arcopallium, caudo-ventral hyperpallium, nucleus accumbens and the brainstem. Taken together, these results suggest that both AVT and VIP immunoreactive structures show similar distribution to other avian species, emphasizing evolutionary conservatism in the history of vertebrates. The current study may enable future investigation into the localization of AVT and VIP, in relation to behavioral and ecological traits in the brain of tit species.

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.

Seasonal dynamics within the neurons of the hippocampus in adult female Indian Ring neck Parrots (Psittacula krameri) and Asian Koels (Eudynamys scolopaceus)

In birds, a narrow strip of tissue found on the dorsomedial surface of the telencephalon and separated from the rest of the hemisphere by a ventricle is termed the hippocampal complex. Two neurohistological techniques, namely the cresyl-violet method and the Golgi–Colonnier technique, have been employed in the present study to observe seasonal dynamics within the neuronal classes of hippocampus in female Indian Ring neck Parrots (Psittacula krameri (Scopoli, 1769)) and Asian Koels (Eudynamys scolopaceus (L., 1758)). Hippocampus is known to play a central role in a variety of behaviors such as homing, visual discrimination, learning, and sexual behavior. Therefore, changes in sexual behavior during the breeding period contribute to plasticity in the hippocampus in terms of fluctuations in neuronal characteristics thereby helping the bird cope with changing conditions. A significant increase in dendritic thickness, neuronal spacing, spine morphology, and spine density were identified within the hippocampal neurons during the breeding period of the studied birds. This study establishes an overall account of seasonal dynamics occurring within the neurons of all fields of the hippocampus of birds in terms of increased dendritic thickness, spine density, spine morphology, and neuronal spacing thereby favoring the view that morphological fluctuations in neuronal characteristics during the breeding period are likely to have consequences for hippocampal neuronal function.

Distribution of neurotransmitter receptors and zinc in the pigeon (Columba livia) hippocampal formation: A basis for further comparison with the mammalian hippocampus

The avian hippocampal formation (HF) and mammalian hippocampus share a similar functional role in spatial cognition, but the underlying neuronal mechanisms allowing the functional similarity are incompletely understood. To understand better the organization of the avian HF and its transmitter receptors, we analyzed binding site densities for glutamatergic AMPA, NMDA, and kainate receptors; GABAA receptors; muscarinic M1 , M2 and nicotinic (nACh) acetylcholine receptors; noradrenergic α1 and α2 receptors; serotonergic 5-HT1A receptors; dopaminergic D1/5 receptors by using quantitative in vitro receptor autoradiography. Additionally, we performed a modified Timm staining procedure to label zinc. The regionally different receptor densities mapped well onto seven HF subdivisions previously described. Several differences in receptor expression highlighted distinct HF subdivisions. Notable examples include 1) high GABAA and α1 receptor expression, which rendered distinctive ventral subdivisions; 2) high α2 receptor expression, which rendered distinctive a dorsomedial subdivision; 3) distinct kainate, α2 , and muscarinic receptor densities that rendered distinctive the two dorsolateral subdivisions; and 4) a dorsomedial region characterized by high kainate receptor density. We further observed similarities in receptor binding densities between subdivisions of the avian and mammalian HF. Despite the similarities, we propose that 300 hundred million years of independent evolution has led to a mosaic of similarities and differences in the organization of the avian HF and mammalian hippocampus and that thinking about the avian HF in terms of the strict organization of the mammalian hippocampus is likely insufficient to understand the HF of birds.

Combinatorial expression of Lef1, Lhx2, Lhx5, Lhx9, Lmo3, Lmo4, and Prox1 helps to identify comparable subdivisions in the developing hippocampal formation of mouse and chicken

We carried out a study of the expression patterns of seven developmental regulatory genes (Lef1, Lhx2, Lhx9, Lhx5, Lmo3, Lmo4, and Prox1), in combination with topological position, to identify the medial pallial derivatives, define its major subdivisions, and compare them between mouse and chicken. In both species, the medial pallium is defined as a pallial sector adjacent to the cortical hem and roof plate/choroid tela, showing moderate to strong ventricular zone expression of Lef1, Lhx2, and Lhx9, but not Lhx5. Based on this, the hippocampal formation (indusium griseum, dentate gyrus, Ammon's horn fields, and subiculum), the medial entorhinal cortex, and part of the amygdalo-hippocampal transition area of mouse appeared to derive from the medial pallium. In the chicken, based on the same position and gene expression profile, we propose that the hippocampus (including the V-shaped area), the parahippocampal area (including its caudolateral part), the entorhinal cortex, and the amygdalo-hippocampal transition area are medial pallial derivatives. Moreover, the combinatorial expression of Lef1, Prox1, Lmo4, and Lmo3 allowed the identification of dentate gyrus/CA3-like, CA1/subicular-like, and medial entorhinal-like comparable sectors in mouse and chicken, and point to the existence of mostly conserved molecular networks involved in hippocampal complex development. Notably, while the mouse medial entorhinal cortex derives from the medial pallium (similarly to the hippocampal formation, both being involved in spatial navigation and spatial memory), the lateral entorhinal cortex (involved in processing non-spatial, contextual information) appears to derive from a distinct dorsolateral caudal pallial sector.

Neuronal classes and their specialization in the corticoid complex of a food-storing bird, the Indian House Crow (Corvus splendens)

Neuronal classes and their specialization in the corticoid complex of a food-storing bird (the Indian House Crow, Corvus splendens Vieillot, 1817) have been investigated using Golgi and Cresyl-violet methods. The aim of present study is to observe the neuronal characteristics of corticoid complex of the House Crow (food-storing bird) and to compare them with that of a nonfood-storing bird (the Strawberry Finch, Estrilda amandava = Amandava amandava (L., 1758)). Three main neuronal classes, viz. projection neurons, local circuit neurons, and stellate neurons, have been identified in both intermediate corticoid area (CI) and dorsolateral corticoid area (CDL) based on soma shape, arrangement of dendrites around the soma, and axonal projection. Projection neurons have four neuronal subtypes: multipolar, pyramidal, pyramidal-like, and horizontal cells. It seems that the specialization in pyramidal, local circuit, and pyramidal-like neurons show advantages in the House Crow as a food-storing bird for better memory, cognition, and connectivity in corticoid complex. This is the first study of its kind that provides information regarding neuronal classes within the corticoid complex of a food-storing bird and a comparison between a food-storing bird (House Crow) and the only available study on a nonfood-storing bird (Strawberry Finch).

Gonadotropin releasing hormone (GnRH) and gonadotropin inhibitory hormone (GnIH) in the songbird hippocampus: regional and sex differences in adult zebra finches

Hypothalamic gonadotropin releasing hormone (GnRH) and gonadotropin inhibitory hormone (GnIH) are vital to reproduction in all vertebrates. These neuropeptides are also present outside of the hypothalamus, but the roles of extra-hypothalamic GnRH and GnIH remain enigmatic and widely underappreciated. We used immunohistochemistry and PCR to examine whether multiple forms of GnRH (chicken GnRH-I (GnRH1), chicken GnRH-II (GnRH2) and lamprey GnRH-III (GnRH4)) and GnIH are present in the hippocampus (Hp) of adult zebra finches (Taeniopygia guttata). Using immunohistochemistry, we provide evidence that GnRH1, GnRH2 and GnRH4 are present in hippocampal cell bodies and/or fibers and that GnIH is present in hippocampal fibers only. There are regional differences in hippocampal GnRH immunoreactivity, and these vary across the different forms of GnRH. There are also sex differences in hippocampal GnRH immunoreactivity, with generally more GnRH1 and GnRH2 in the female Hp. In addition, we used PCR to examine the presence of GnRH1 mRNA and GnIH mRNA in micropunches of Hp. PCR and subsequent product sequencing demonstrated the presence of GnRH1 mRNA and the absence of GnIH mRNA in the Hp, consistent with the pattern of immunohistochemical results. To our knowledge, this is the first study in any species to systematically examine multiple forms of GnRH in the Hp or to quantify sex or regional differences in hippocampal GnRH. Moreover, this is the first demonstration of GnIH in the avian Hp. These data shed light on an important issue: the sites of action and possible functions of GnRH and GnIH outside of the HPG axis.

VPAC receptor signaling modulates grouping behavior and social responses to contextual novelty in a gregarious finch: a role for a putative prefrontal cortex homologue

In both mammals and birds, vasoactive intestinal polypeptide (VIP) neurons and fibers are present in virtually every brain area that is important for social behavior. VIP influences aggression in birds, social recognition in rodents, and prolactin secretion in both taxa, but other possible functions in social modulation remain little explored. VIP effects are mediated by VPAC receptors, which bind both VIP and pituitary adenylate cyclase activating peptide. Within the lateral septum and medial bed nucleus of the stria terminalis, VPAC receptors are found at higher densities in gregarious finch species relative to territorial species, suggesting that VPAC receptor activation promotes social contact and/or preference for larger groups. Here we here test this hypothesis in zebra finches (Taeniopygia guttata), and also examine the relevance of VPAC receptors to anxiety-like processes. Intraventricular infusions of the VPAC receptor antagonist, neurotensin6-11 mouseVIP7-28, strongly reduce social contact when animals are tested in a novel environment, and exert sex-specific effects on grouping behavior. Specifically, VPAC receptor antagonism reduces gregariousness in females but increases gregariousness in males. Interestingly, VPAC antagonism in the medial pallium (putative prefrontal cortex homologue) significantly reduces gregariousness in both sexes, suggesting site-specific effects of VIP signaling. However, VPAC antagonism does not modulate novel-familiar social preferences in a familiar environment or general anxiety-like behaviors. The current results suggest that endogenous activation of VPAC receptors promotes social contact under novel environmental conditions, a function that may be accentuated in gregarious species. Moreover, endogenous VIP modulates gregariousness in both males and females.

Differences in relative hippocampus volume and number of hippocampus neurons among five corvid species

The relative size of the avian hippocampus (Hp) has been shown to be related to spatial memory and food storing in two avian families, the parids and corvids. Basil et al. [Brain Behav Evol 1996;47:156-164] examined North American food-storing birds in the corvid family and found that Clark's nutcrackers had a larger relative Hp than pinyon jays and Western scrub jays. These results correlated with the nutcracker's better performance on most spatial memory tasks and their strong reliance on stored food in the wild. However, Pravosudov and de Kort [Brain Behav Evol 2006;67:1-9] raised questions about the methodology used in the 1996 study, specifically the use of paraffin as an embedding material and recalculation for shrinkage. Therefore, we measured relative Hp volume using gelatin as the embedding material in four North American species of food-storing corvids (Clark's nutcrackers, pinyon jays, Western scrub jays and blue jays) and one Eurasian corvid that stores little to no food (azure-winged magpies). Although there was a significant overall effect of species on relative Hp volume among the five species, subsequent tests found only one pairwise difference, blue jays having a larger Hp than the azure-winged magpies. We also examined the relative size of the septum in the five species. Although Shiflett et al. [J Neurobiol 2002;51:215-222] found a difference in relative septum volume amongst three species of parids that correlated with storing food, we did not find significant differences amongst the five species in relative septum. Finally, we calculated the number of neurons in the Hp relative to body mass in the five species and found statistically significant differences, some of which are in accord with the adaptive specialization hypothesis and some are not.

Birdsong: is it music to their ears?

Since the time of Darwin, biologists have wondered whether birdsong and music may serve similar purposes or have the same evolutionary precursors. Most attempts to compare song with music have focused on the qualities of the sounds themselves, such as melody and rhythm. Song is a signal, however, and as such its meaning is tied inextricably to the response of the receiver. Imaging studies in humans have revealed that hearing music induces neural responses in the mesolimbic reward pathway. In this study, we tested whether the homologous pathway responds in songbirds exposed to conspecific song. We played male song to laboratory-housed white-throated sparrows, and immunolabeled the immediate early gene product Egr-1 in each region of the reward pathway that has a clear or putative homologue in humans. We found that the responses, and how well they mirrored those of humans listening to music, depended on sex and endocrine state. In females with breeding-typical plasma levels of estradiol, all of the regions of the mesolimbic reward pathway that respond to music in humans responded to song. In males, we saw responses in the amygdala but not the nucleus accumbens – similar to the pattern reported in humans listening to unpleasant music. The shared responses in the evolutionarily ancient mesolimbic reward system suggest that birdsong and music engage the same neuroaffective mechanisms in the intended listeners.

Distribution of prosaposin mRNA in the central nervous system of the pigeon (Columba livia)

Bioassay and immunohistochemical studies have detected the presence of prosaposin in the central nervous system (CNS) of mammals. Here, first time, we have determined the partial cDNA sequence of pigeon prosaposin and mapped the distribution of its mRNA in the pigeon CNS. The predicted amino acid sequence of pigeon prosaposin showed 93 and 60% identity to chicken and human prosaposin, respectively. In situ hybridization, autoradiograms showed that the prosaposin mRNA expression was found in the olfactory bulb, prepiriform cortex, Wulst, mesopallium, nidopallium, hippocampal formation, thalamus, tuberis nucleus, pre-tectal nucleus, nucleus mesencephalicus lateralis, pars dorsalis, nucleus isthmi, pars parvocellularis and magnocellularis, Edinger-Westphal nucleus, optic tectum, cerebellar cortex and nuclei, vestibular nuclei and gray matter of the spinal cord. These results suggest that the cDNA sequence of pigeon prosaposin is comparable to other vertebrates, and the general distribution pattern of prosaposin mRNA resembles those are found in mammals.

A direct hippocampo-cerebellar projection in chicken

Our previous studies suggested that a direct hippocampo-cerebellar projection might exist in the chicken. To confirm such a presumption of hippocampo-cerebellar interactions, horseradish peroxidase (HRP) was used as a retrograde tracer to be injected into the white matter of the folia VI-VIII of the cerebellum in young and adult chickens. In another set of experiments, young chickens were subjected to electrolytic lesions of the hippocampal formation (HF), especially the ventromedial portion, and the cerebellum was observed with the electron microscope to find neuronal degeneration in the HF. Following injections of HRP into the cerebellum, a large number of labeled neurons were found in the area APHm-APHim of the HF in the young and adult chickens. As a result of the electrolytic lesions of the APHm-APHim in the HF, many large degenerated nerve fibers were found in the white matter in the vicinity of the lateral nucleus of the cerebellum, and some small degenerated fibers were found in the white matter of the folia VI-VIII. In the cerebellar cortex of folia VI-VIII, degenerated axonal terminals occurred in both the molecular and Purkinje layers, but not in the granular layer. In the lateral nucleus, some dark degenerating axonal terminals were recognized to connect with the perikarya of neurons of this nucleus. The present experiments demonstrate that the APHm-APHim of the HF directly projected to the cortex of folia VI-VIII and the lateral nucleus of the cerebellum in young chickens.

Neural distribution of vasotocin receptor mRNA in two species of songbird

The neurohypophyseal hormones vasopressin and oxytocin are produced and released within the mammalian brain, where they act via multiple receptor subtypes. The neural distributions of these receptors, for example, V1a and oxytocin receptors, have been well described in many mammals. In birds, the distribution of binding sites for the homologous neuropeptides, vasotocin (VT) and mesotocin, has been studied in several species by using synthetic radioligands designed to bind to mammalian receptors. Such binding studies, however, may not reveal the specific distributions of each receptor subtype. To identify and map the receptors likely to bind VT and mesotocin, we generated partial cDNA sequences for four VT receptor subtypes, VT1, VT2 (V1b), VT3 (oxytocin-like), and VT4 (V1a), in white-throated sparrow (Zonotrichia albicollis) and zebra finch (Taeniopygia guttata). These genes shared high sequence identity with the homologous avian and mammalian neurohypophyseal peptide receptors, and we found evidence for VT1, VT3, and VT4 receptor mRNA expression throughout the brains of both species. As has been described in rodents, there was striking interspecific and intraspecific variation in the densities and distribution of these receptors. For example, whereas the VT1 receptor mRNA was more widespread in zebra finch brain, the VT3 (oxytocin-like) receptor mRNA was more prevalent in the sparrow brain. Although VT2 (V1b) receptor mRNA was abundant in the pituitary, it was not found in the brain. Because of their association with brain regions implicated in social behavior, the VT1, VT3, and VT4 receptors are all likely candidates for mediating the behavioral effects of VT.

Sites of origin and developmental dynamics of the neurons in the core and shell regions of torus semicircularis in the Chinese softshell turtle (Pelodiscus sinensis)

To know the embryogenesis of the core and shell regions of the midbrain auditory nucleus, a single dose of [(3)H]-thymidine was injected into the turtle embryos at peak stages of neurogenesis in the shell and core of the torus semicircularis. Following sequential survival times, labeled neurons and the dynamics of cell proliferation were examined. The expression of vimentin (VM), reelin, calbindin, parvalbumin, and substance P were also studied. The results showed that: 1) progenitor cells for the core and shell regions were generated in different sites of the ventricular zone; 2) the length of the cell cycle or S-phase for the shell region were both longer than those for the core region (4.7 and 3.2 hours longer, respectively), suggesting that mitotic activity in the core region is higher than it is in the shell region; 3) the elongated cell bodies of the labeled core and shell cells had close apposition to VM fibers, suggesting that the migration of these cells is guided by VM fibers; 4) the germinal sites of the core and shell constructed by projecting the orientation of radial VM fibers back to the ventricular zone was consistent with those obtained by short and sequential survival [(3)H]-thymidine radiography; and 5) the beginning of positive staining for parvalbumin in the core region was interposed between those for calbindin and substance P in the shell regions. This study contributes to the understanding of how auditory nuclei are organized and how their components developed and evolved.

Localization of cerebellin-2 in late embryonic chicken brain: implications for a role in synapse formation and for brain evolution

Cerebellin-1 (Cbln1), the most studied member of the cerebellin family of secreted proteins, is necessary for the formation and maintenance of parallel fiber-Purkinje cell synapses. However, the roles of the other Cblns have received little attention. We previously identified the chicken homolog of Cbln2 and examined its expression in dorsal root ganglia and spinal cord (Yang et al. [2010] J Comp Neurol 518:2818-2840). Interestingly, Cbln2 is expressed by mechanoreceptive and proprioceptive neurons and in regions of the spinal cord where those afferents terminate, as well as by preganglionic sympathetic neurons and their sympathetic ganglia targets. These findings suggest that Cbln2 may demonstrate a tendency to be expressed by synaptically connected neuronal populations. To further assess this possibility, we examined Cbln2 expression in chick brain. We indeed found that Cbln2 is frequently expressed by synaptically connected neurons, although there are exceptions, and we discuss the implications of these findings for Cbln2 function. Cbln2 expression tends to be more common in primary sensory neurons and in second-order sensory regions than it is in motor areas of the brain. Moreover, we found that the level of Cbln2 expression for many regions of the chicken brain is very similar to that of the mammalian homologs, consistent with the view that the expression patterns of molecules playing fundamental roles in processes such as neuronal communication are evolutionarily conserved. There are, however, large differences in the pattern of Cbln2 expression in avian as compared to mammalian telencephalon and in other regions that show the most divergence between the two lineages.

Brain expression and song regulation of the cholecystokinin gene in the zebra finch (Taeniopygia guttata)

The gene encoding cholecystokinin (Cck) is abundantly expressed in the mammalian brain and has been associated with such functions as feeding termination and satiety, locomotion and self-stimulation, the modulation of anxiety-like behaviors, and learning and memory. Here we describe the brain expression and song regulation of Cck in the brain of the adult male zebra finch (Taeniopygia guttata), a songbird species. Using in situ hybridization we demonstrate that Cck is highly expressed in several discrete brain regions, most prominently the caudalmost portion of the hippocampal formation, the caudodorsal nidopallial shelf and the caudomedial nidopallium (NCM), the core or shell regions of dorsal thalamic nuclei, dopaminergic cell groups in the mesencephalon and pons, the principal nucleus of the trigeminal nerve, and the dorsal raphe. Cck was largely absent in song control system, a group of nuclei required for vocal learning and song production in songbirds, although sparse labeling was detected throughout the striatum, including song nucleus area X. We also show that levels of Cck mRNA and the number of labeled cells increase in the NCM of males and females following auditory stimulation with conspecific song. Double labeling further reveals that the majority of Cck cells, excluding those in the reticular nucleus of the thalamus, are non-GABAergic. Together, these data provide the first comprehensive characterization of Cck expression in a songbird, and suggest a possible involvement of Cck regulation in important aspects of birdsong biology, such as perceptual processing, auditory memorization, and/or vocal-motor control of song production.

Germinal sites and migrating routes of cells in the mesencephalic and diencephalic auditory areas in the African clawed frog (Xenopus laevis)

There is a clear core-shell organization in the auditory nuclei of amniotes. However, such organization only exists in the mesencephalic, but not in the diencephalic auditory regions of amphibians. To gain insights into how this core-shell organization developed and evolved, we injected a small dose of [(3)H]-thymidine into tadpoles of Xenopus laevis at peak stages of neurogenesis in the mesencephalic and diencephalic auditory areas. Following different survival times, the germinal sites and migrating routes of cells were examined in the shell (laminar nucleus, Tl; magnocellular nucleus, Tmc) and core (principal nucleus, Tp) regions of the mesencephalic auditory nucleus, torus semicircularis (Ts), as well as in the diencephalic auditory areas (posterior thalamic nucleus, P; central thalamic nucleus, C). Double labeling for [(3)H]-thymidine autoradiography and immunohistochemistry for vimentin was also performed to help determine the routes of cell migration. We found three major results. First, the germinal sites of Tp were intercalated between Tl and Tmc, arising from those of the shell regions. Second, although the germinal sites of Tl, Tmc, and Tp were located in the same brain levels (at rostromedial or caudomedial levels of Ts), neurogenesis in Tl or Tmc started earlier than that in Tp. Finally, the P and C were also generated in different ventricle sites. However, unlike Ts their neurogenesis showed no obvious temporal differences. These data demonstrate that a highly differentiated auditory region, such as Tp in Ts, is lacking in the diencephalon of amphibian. Our data are discussed from the view of the constitution and evolutionary origins of auditory nuclei in vertebrates.

Captivity reduces hippocampal volume but not survival of new cells in a food-storing bird

In many naturalistic studies of the hippocampus wild animals are held in captivity. To test if captivity itself affects hippocampal integrity, adult black-capped chickadees (Poecile atricapilla) were caught in the fall, injected with bromodeoxyuridine to mark neurogenesis, and alternately released to the wild or held in captivity. The wild birds were recaptured after 4-6 weeks and perfused simultaneously with their captive counterparts. The hippocampus of captive birds was 23% smaller than wild birds, with no hemispheric differences in volume within groups. Between groups there was no statistically significant difference in the size of the telencephalon, or in the number and density of surviving new cells. Proximate causes of the reduced hippocampal volume could include stress, lack of exercise, diminished social interaction, or limited caching opportunity-a hippocampal-dependent activity. The results suggest the avian hippocampus-a structure essential for rapid, complex relational and spatial learning-is both plastic and sensitive, much as in mammals, including humans.

Evolutionary significance of delayed neurogenesis in the core versus shell auditory areas of Mus musculus

Early comparative embryogenesis can reflect the organization and evolutionary origins of brain areas. Neurogenesis in the auditory areas of sauropsids displays a clear core-to-shell distinction, but it remains unclear in mammals. To address this issue, [3H]-thymidine was injected into pregnant mice on consecutive embryonic (E) days (E10-E19) to date neuronal birthdays. Immunohistochemistry for substance P, calbindin, and parvalbumin was conducted to distinguish the core and shell auditory regions. The results showed that: 1) cell generation began at E13 in the external or dorsal nucleus of the inferior colliculus (IC), but it did not start in the caudomedial portion of the central nucleus of IC, and significantly fewer cells were produced in the medial and rostromedial portions of the central nucleus of IC; 2) cells were generated at E11 in the dorsal and medial divisions of the medial geniculate complex (MGd and MGm, respectively), whereas cell generation was absent in the medial and rostromedial portions of the ventral medial geniculate complex (MGv), and fewer cells were produced in the caudomedial portion of MGv; 3) in the telencephalic auditory cortex, cells were produced at E11 or E12 in layer I and the subplate, which receive projections from the MGd and MGm. However, cell generation occurred at E13-E18 in layers II-VI, including the area receiving projections from the MGv. The core-to-shell distinction of neurogenesis is thus present in the mesencephalic to telencephalic auditory areas in the mouse. This distinction of neurogenesis is discussed from an evolutionary perspective.

Prenatal auditory stimulation alters the levels of CREB mRNA, p-CREB and BDNF expression in chick hippocampus

Prenatal auditory stimulation influences the development of the chick auditory pathway and the hippocampus showing an increase in various morphological parameters as well as expression of calcium-binding proteins. Calcium regulates the activity of cyclic adenosine monophosphate-response element binding (CREB) protein. CREB is known to play a role in development, undergo phosphorylation with neural activity as well as regulate transcription of BDNF. BDNF is important for the survival of neurons and regulates synaptic strength. Hence in the present study, we have evaluated the levels of CREB mRNA and protein along with p-CREB protein as well as BDNF mRNA and protein levels in the chick hippocampus at embryonic days (E) 12, E16, E20 and post-hatch day (PH) 1 following activation by prenatal auditory stimulation. Fertilized eggs were exposed to species-specific sound or sitar music (frequency range: 100-6300Hz) at 65dB levels for 15min/h over 24h from E10 till hatching. The control chick hippocampus showed higher CREB mRNA and p-CREB protein in the early embryonic stages, which later decline whereas BDNF mRNA and BDNF protein levels increase until PH1. The CREB mRNA and p-CREB protein were significantly increased at E12, E16 and PH1 in the auditory stimulated groups as compared to control group. A significant increase in the level of BDNF mRNA was observed from E12 and the protein expression from E16 onwards in both auditory stimulated groups. Therefore, enhanced phosphorylation of CREB during development following prenatal sound stimulation may be responsible for cell survival. Increased levels of p-CREB again at PH1 may trigger synthesis of proteins necessary for synaptic plasticity. Further, the increased levels of BDNF may also help in regulating synaptic plasticity.

Neural distribution of nonapeptide binding sites in two species of songbird

Vasotocin (VT) and its mammalian homologue, vasopressin (VP), modulate many social behaviors in a variety of vertebrate species. In songbirds, the effects of centrally administered VT vary according to species, which may reflect species-specific distributions of VT binding sites. Different radioligands used to map receptors in previous autoradiographical studies have revealed nonoverlapping distributions of VT binding, suggesting a heterogeneous population of more than one type of VT receptor. For two model songbird species, the white-throated sparrow (Zonotrichia albicollis) and zebra finch (Taeniopygia guttata), we labeled putative VT receptors with two radioligands, [(125)I]ornithine vasotocin analog ([(125)I]OVTA) and [(125)I]linear VP antagonist ([(125)I]HO-LVA). Competitive binding assays in the lateral septum showed that both ligands were effectively displaced by both VT and a related nonapeptide, mesotocin (MT), showing that these radioligands, which were developed to label mammalian nonapeptide receptors, label at least one population of related receptors in songbirds. [(125)I]OVTA labeled receptors throughout the telencephalon, diencephalon, midbrain, and brainstem, with a similar distribution in both species. In contrast, the binding of [(125)I]HO-LVA was restricted to the septal area, dorsal arcopallium, and optic tectum in sparrow and was essentially undetectable in zebra finch. Because the avian brain is likely to express multiple types of VT receptors, we hypothesize that the binding patterns of these radioligands represent a heterogeneous receptor population.

Effect of prenatal auditory stimulation on numerical synaptic density and mean synaptic height in the posthatch Day 1 chick hippocampus

Previous studies on prenatal auditory stimulation by species-specific sound or sitar music showed enhanced morphological and biochemical changes in chick hippocampus, which plays an important role in learning and memory. Changes in the efficiency of synapses, synaptic morphology and de novo synapse formation affects learning and memory. Therefore, in the present study, we set out to investigate the mean synaptic density and mean synaptic height at posthatch Day 1 in dorsal and ventral part of chick hippocampus following prenatal auditory stimulation. Fertilized 0 day eggs of domestic chick incubated under normal conditions were exposed to patterned sounds of species-specific and sitar music at 65 dB levels for 15 min/h round the clock (frequency range: 100-6300 Hz) from embryonic Day 10 till hatching. The synapses identified under transmission electron microscope were estimated for their numerical density by physical disector method and also the mean synaptic height calculated. Our results demonstrate a significant increase in mean synaptic density with no alterations in the mean synaptic height following both types of auditory stimulation in the dorsal as well as ventral part of the hippocampus. The observed increase in mean synaptic density suggests enhanced synaptic substrate to strengthen hippocampal function.

Memory processing in the avian hippocampus involves interactions between beta-adrenoceptors, glutamate receptors, and metabolism

Noradrenaline is known to modulate memory formation in the mammalian hippocampus. We have examined how noradrenaline and selective beta-adrenoceptor (AR) agonists affect memory consolidation and how antagonists inhibit memory consolidation in the avian hippocampus. Injection of selective beta-AR agonists and antagonists at specific times within 30 min of a weakly or strongly reinforced, single-trial, bead discrimination learning test in 1-day-old chicks allowed us to determine the pattern of beta-AR involvement in hippocampal memory processing. Different beta-AR subtypes were recruited in temporal sequence after learning in the order beta(1), beta(3), and beta(2.) We provide evidence that the effect of manipulation of beta(1)-ARs by selective agonists and antagonists within 2.5 min of training parallels the action of NMDA receptor agonists and antagonists. Activation of beta(3)- and beta(2)-ARs facilitated memory but utilized different mechanisms: beta(3)-ARs by stimulating glucose uptake and metabolism, and beta(2)-ARs by increasing the breakdown of glycogen--with both metabolic events occurring in astrocytes and affecting intermediate memory. The different receptors are activated at different times within the lifetime of labile memory and within 30 min of learning. We have defined separate roles for the three beta-ARs in memory and demonstrated that the avian hippocampus is involved in learning and memory in much the same way as the hippocampus in the mammalian brain.

Comparative analysis of neurogenesis between the core and shell regions of auditory areas in the chick (Gallus gallus domesticus)

Early embryogenesis can reflect constituting organizations and evolutionary origins of brain areas. To determine whether a clear core-versus-shell distinction of neurogenesis that occurs from the auditory midbrain to the telencephalon in the reptile also appears in the bird, a single dose of [(3)H]-thymidine was injected into chick (Gallus gallus domesticus) eggs at some successive embryonic days (E) (from E3 to E10). Towards the end of hatching, [(3)H]-thymidine labeling was examined, and the results were as follows: 1) Neuronal generation in the nucleus intercollicularis (ICo) (shell region) began at E3, whereas neurogenesis began at E4 in the nucleus mesencephalicus lateralis pars dorsalis (MLd) (core region); 2) Neurogenesis initiated at E3 in the nucleus ovoidalis (Ov) shell, but initiated at E4 in the rostral Ov core. In the medial or caudal Ov core, the percentage of heavily-labeled neurons with [(3)H]-thymidine was significantly lower at E3 age group than that in the Ov shell; 3) In field L1 and L3, two flanking regions of the primary telencephalic auditory area (field L2a), neurogenesis started at E5, but started at E6 in field L2a. These data indicate that the onset of embryogenesis began earlier in the auditory shell areas than in the core areas from the midbrain to the telencephalon. These findings provide insight into the organization of auditory nuclei and their evolution in amniotes.

Calbindin D-28K and parvalbumin expression in embryonic chick hippocampus is enhanced by prenatal auditory stimulation

Calcium-binding proteins (CaBPs) buffer excess of cytosolic Ca(2+), which accompanies neuronal activity following external stimuli. Prenatal auditory stimulation by species-specific sound and music influences early maturation of the auditory pathway and the behavioral responses in chicks. In this study, we determined the volume, total number of neurons, proportion of calbindin D-28K and parvalbumin-positive neurons along with their levels of expression in the developing chick hippocampus following prenatal auditory stimulation. Fertilized eggs of domestic chicks were exposed to sounds of either species-specific calls or sitar music at 65 dB for 15 min/h round the clock from embryonic day (E) 10 until hatching. Hippocampi of developmental stages (E12, E16 and E20) were examined. With an increase in embryonic age during normal development, the hippocampus showed an increase in its volume, total number of neurons as well as in the neuron proportions and levels of expression of calbindin D-28K and parvalbumin. A significant increase of volume at E20 was noted only in the music-stimulated group compared to that of their age-matched control (p<0.05). On the other hand, both auditory-stimulated groups showed a significant increase in the proportion of immunopositive neurons and the levels of expression of calbindin D-28K and parvalbumin as compared to the control at all developmental stages studied (p<0.003). The increase in proportions of CaBP neurons during development and in the sound-enriched groups suggests an activity-dependent increase in Ca(2+) influx. The enhanced expression of CaBPs may help in cell survival by preventing excitotoxic death of neurons during development and may also be involved in long-term potentiation.

Distinction of neurochemistry between the cores and their shells of auditory nuclei in tetrapod species

The distribution of Met-enkephalin (ENK), substance P (SP) and serotonin (5-HT) differs between the core and shell regions of the mesencephalic and diencephalic auditory nuclei of the turtle [Belekhova et al., 2002]. These neurochemical distinctions are also found in other tetrapods (mammals, birds and amphibians). The distribution of ENK, SP and 5-HT was examined in the core and shell regions of both mesencephalic and diencephalic auditory nuclei, and in the telencephalic auditory areas of Bengalese finches (Lonchura striata) and mice (Mus musculus), as well as in corresponding auditory areas in toads (Bufo bufo). ENK, SP and 5-HT immunoreactive fibers and perikarya were largely absent from the core regions of both mesencephalic and diencephalic auditory nuclei, in comparison with the shell regions of mice and Bengalese finches. In the toad, however, this pattern was observed in the mesencephalic auditory nucleus, but not in the diencephalic auditory areas. ENK and SP immunoreactive perikarya were detected in the telencephalic auditory area of mice, whereas no ENK, SP or 5-HT immunolabeling was observed in the telencephalic auditory area (Field L) of Bengalese finches. These findings are discussed in terms of the evolution of the core-and-shell organization of auditory nuclei of tetrapods.

Passive avoidance training is correlated with decreased cell proliferation in the chick hippocampus

One-trial passive avoidance learning (PAL), where the aversive stimulus is the bitter-tasting substance methylanthranilate (MeA), affects neuronal and synaptic plasticity in learning-related areas of day-old domestic chicks (Gallus domesticus). Here, cell proliferation was examined in the chick forebrain by using 5-bromo-2-deoxyuridine (BrdU) at 24 h and 9 days after PAL. At 24 h post-BrdU injection, there was a significant reduction in labelling in MeA-trained chicks in both the dorsal hippocampus and area parahippocampalis, in comparison to controls. Moreover, double-immunofluorescence labelling for BrdU and the nuclear neuronal marker (NeuN) showed a reduction of neuronal cells in the dorsal hippocampus of the MeA-trained group compared with controls (35 and 49%, respectively). There was no difference in BrdU labelling in hippocampal regions between trained and control groups of chicks at 9 days post-BrdU injection; however, the number of BrdU-labelled cells was considerably lower than at 24 h post-BrdU injection, possibly due to migration of cells within the telencephalon rather than cell loss as apoptotic analyses at 24 h and 9 days post-BrdU injection did not demonstrate differences in cell death between treatment groups. Cortisol levels increased in the chick hippocampus of MeA-trained birds 20 min after PAL, suggesting the possibility of a stress-related mechanism of cell proliferation reduction in the hippocampus. In contrast to hippocampal areas, the olfactory bulb, an area strongly stimulated by the strong-smelling MeA, showed increased cell genesis in comparison to controls at both 24 h and 9 days post-training.