Imprinting modulates processing of visual information in the visual wulst of chicks

Assessing preferences for adult versus juvenile features in young animals: Newly hatched chicks spontaneously approach red and large stimuli

Young precocial birds benefit from staying close to both their mother and their siblings, while prioritising adults, which provide better care. Which features of the stimuli are used by young birds to prioritise approach and eventually attachment to adults over siblings is unknown. We started to address this question in newly hatched domestic chicks (Gallus gallus), focusing on their spontaneous preferences for visual features that systematically vary between adult and juvenile chickens, and that had previously been identified as attractive: size (larger in adults than in juveniles) and colour (darker and redder in adults than in juveniles). Overall, chicks at their first visual experience, that had never seen a conspecific beforehand, were most attracted to the red and large stimuli (two adult features) and spent more time in close proximity with red stimuli than with yellow stimuli. When tested with red large versus small objects (Exp. 1), chicks preferred the large shape. When tested with yellow large and small objects (Exp. 2), chicks did not show a preference. Chicks had a stronger preference for large red stimuli (vs. small yellow objects) than for small red stimuli (vs. a large yellow object) (Exp. 3). These results suggest that the combination of size and colour form the predisposition that helps chicks to spontaneously discriminate between adult and juvenile features from the first stages of life, in the absence of previous experience, exhibiting a preference to approach stimuli with features associated with the presence of adult conspecifics.

Spontaneous biases enhance generalization in the neonate brain

Inductive generalization is adaptive in novel contexts for both biological and artificial intelligence. Spontaneous generalization in inexperienced animals raises questions on whether predispositions (evolutionarily acquired biases, or priors) enable generalization from sparse data, without reinforcement. We exposed neonate chicks to an artificial social partner of a specific color, and then looked at generalization on the red-yellow or blue-green ranges. Generalization was inconsistent with an unbiased model. Biases included asymmetrical generalization gradients, some preferences for unfamiliar stimuli, different speed of learning, faster learning for colors infrequent in the natural spectrum. Generalization was consistent with a Bayesian model that incorporates predispositions as initial preferences and treats the learning process as an update of predispositions. Newborn chicks are evolutionarily prepared for generalization, via biases independent from experience, reinforcement, or supervision. To solve the problem of induction, biological and artificial intelligence can use biases tuned to infrequent stimuli, such as the red and blue colors.

Functional MRI of imprinting memory: a new avenue for neurobiology of early learning

Filial imprinting, a crucial ethological paradigm, provides insights into the neurobiology of early learning and its long-term impact on behaviour. To date, only invasive techniques, such as autoradiography or lesion, have been employed to understand this behaviour. The primary limitation of these methods lies in their constrained access to the entire brain, impeding the exploration of brain networks crucial at various stages of this paradigm. Recently, advances in functional magnetic resonance imaging (fMRI) in the avian brain have opened new windows to explore bird's brain function at the network level. Here, we developed a ground-breaking non-invasive functional MRI technique for awake, newly hatched chicks that record whole-brain BOLD signal changes throughout imprinting experiments. While the initial phases of memory acquisition imprinting behaviour have been unravelled, the long-term storage and retrieval components of imprinting memories are still unknown. Our findings identified potential long-term storage of imprinting memories across a neural network, including the hippocampal formation, the medial striatum, the arcopallium, and the prefrontal-like nidopallium caudolaterale. This platform opens up new avenues for exploring the broader landscape of learning and memory processes in neonatal vertebrates, contributing to a more comprehensive understanding of the intricate interplay between behaviour and brain networks.

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.

Temporal hampering of thyroid hormone synthesis just before hatching impeded the filial imprinting in domestic chicks

Thyroid hormones play a critical role in the initiation of the sensitive period of filial imprinting. The amount of thyroid hormones in the brains of chicks increases intrinsically during the late embryonic stages and peaks immediately before hatching. After hatching, a rapid imprinting-dependent inflow of circulating thyroid hormones into the brain occurs via vascular endothelial cells during imprinting training. In our previous study, inhibition of hormonal inflow impeded imprinting, indicating that the learning-dependent inflow of thyroid hormones after hatching is critical for the acquisition of imprinting. However, it remained unclear whether the intrinsic thyroid hormone level just before hatching affects imprinting. Here, we examined the effect of temporal thyroid hormone decrease on embryonic day 20 on approach behavior during imprinting training and preference for the imprinting object. To this end, methimazole (MMI; a thyroid hormone biosynthesis inhibitor) was administered to the embryos once a day on days 18-20. Serum thyroxine (T₄) was measured to evaluate the effect of MMI. In the MMI-administered embryos, the T₄ concentration was transiently reduced on embryonic day 20 but recovered to the control level on post-hatch day 0. At the beginning of imprinting training on post-hatch day 1, control chicks approached the imprinting object only when the object was moving. In the late phase of training, control chicks subsequently approached towards the static imprinting object. On the other hand, in the MMI-administered chicks, the approach behavior decreased during the repeated trials in the training, and the behavioral responses to the imprinting object were significantly lower than those of control chicks. This indicates that their persistent responses to the imprinting object were impeded by a temporal thyroid hormone decrease just before hatching. Consequently, the preference scores of MMI-administered chicks were significantly lower than those of control chicks. Furthermore, the preference score on the test was significantly correlated with the behavioral responses to the static imprinting object in the training. These results indicate that the intrinsic thyroid hormone level immediately before hatching is crucial for the learning process of imprinting.

Mother-young bond in non-human mammals: Neonatal communication pathways and neurobiological basis

Mother-young bonding is a process by which the young establish social preferences for their mother. It fosters reproductive success and the survival of offspring by providing food, heat, and maternal care. This process promotes the establishment of the mother-young bond through the interaction of olfactory, auditory, tactile, visual, and thermal stimuli. The neural integration of multimodal sensory stimuli and attachment is coordinated into motor responses. The sensory and neurobiological mechanisms involved in filial recognition in precocial and altricial mammals are summarized and analyzed in this review.

The power of innate: Behavioural attachment and neural activity in responses to natural and artificial objects in filial imprinting in chicks

Newly hatched domestic chicks are known to orient preferentially toward naturalistic stimuli, resembling a conspecific. Here, we examined to what extent this behavioral preference can be transcended by an artificial imprinting stimulus in both short-term and long-term tests. We also compared the expression maps of the plasticity-associated c-fos gene in the brains of chicks imprinted to naturalistic (rotating stuffed jungle fowl) and artificial (rotating illuminated red box) stimuli. During training, the approach activity of chicks to a naturalistic object was always higher than that to an artificial object. However, the induction of c-fos mRNA was significantly higher in chicks imprinted to a box than to a fowl, especially in the intermediate medial mesopallium, hyperpallium apicale, arcopallium, and hippocampus. Initially, in the short-term test (10 min after the end of training), chicks had a higher preference for a red box than for a stuffed fowl. However, in the long-term test (24 h after imprinting), the response to an artificial object decreased to the level of preference for a naturalistic object. Our results thus show that despite the artificial object causing a stronger c-fos novelty response and higher behavioral attachment in the short term, this preference was less stable and fades away, being overtaken by a more stable innate predisposition to the naturalistic social object.

Imprintability of Newly Hatched Domestic Chicks on an Artificial Object: A Novel High Time-Resolution Apparatus Based on a Running Disc

In filial imprinting, newly hatched chicks repeatedly approach a conspicuous object nearby and memorize it, even though it is an artificial object instead of their mother hen. Imprinting on an artificial object in a laboratory setting has a clear sensitive period from post hatch days 1–3 in the case of domestic chicks. However, the establishment of imprintability are difficult to investigate because of the limitations of the behavioral apparatus. In this study, we developed a novel behavioral apparatus, based on a running disc, to investigate the learning processes of imprinting in newly hatched domestic chicks. In the apparatus, the chick repeatedly approaches the imprinting object on the disc. The apparatus sends a transistor-transistor-logic signal every 1/10 turn of the disc to a personal computer through a data acquisition system following the chick’s approach to the imprinting object on the monitor. The imprinting training and tests were designed to define the three learning processes in imprinting. The first process is the one in which chicks spontaneously approach the moving object. The second is an acquired process in which chicks approach an object even when it is static. In the third process, chicks discriminate between the differently colored imprinting object and the control object in the preference test. Using the apparatus, the difference in the chicks’ behavior during or after the sensitive period was examined. During the sensitive period, the chicks at post hatch hour 12 and 18 developed the first imprinting training process. The chicks at post hatch hour 24 maintained learning until the second process. The chicks at post hatch hour 30 reached the discrimination process in the test. After the sensitive period, the chicks reared in darkness until post hatch day 4 exhibited poor first learning process in the training. Thus, this apparatus will be useful for the detection of behavioral changes during neuronal development and learning processes.

Stability and individual variability of social attachment in imprinting

Filial imprinting has become a model for understanding memory, learning and social behaviour in neonate animals. This mechanism allows the youngs of precocial bird species to learn the characteristics of conspicuous visual stimuli and display affiliative response to them. Although longer exposures to an object produce stronger preferences for it afterwards, this relation is not linear. Sometimes, chicks even prefer to approach novel rather than familiar objects. To date, little is known about how filial preferences develop across time. This study aimed to investigate filial preferences for familiar and novel imprinting objects over time. After hatching, chicks were individually placed in an arena where stimuli were displayed on two opposite screens. Using an automated setup, the duration of exposure and the type of stimuli were manipulated while the time spent at the imprinting stimulus was monitored across 6 days. We showed that prolonged exposure (3 days vs 1 day) to a stimulus produced robust filial imprinting preferences. Interestingly, with a shorter exposure (1 day), animals re-evaluated their filial preferences in functions of their spontaneous preferences and past experiences. Our study suggests that predispositions influence learning when the imprinting memories are not fully consolidated, driving animal preferences toward more predisposed stimuli.

Connectivity between nidopallium caudolateral and visual pathways in color perception of zebra finches

Researchers demonstrated an elegant ability for red discrimination in zebra finches. It is interested to understand whether red activates exhibit much stronger response than other colors in neural network levels. To reveal the question, local field potentials (LFPs) was recorded and analyzed in two visual pathways, the thalamofugal and the tectofugal pathways, of zebra finches. Human studies demonstrate visual associated telencephalons communicate with higher order brain areas such as prefrontal cortex. The present study determined whether a comparable transmission occurs in zebra finches. Telencephalic regions of the thalamofugal (the visual Wulst) and the tectofugal pathway (the entopallium) with their higher order telencephalon, nidopallium caudolateral (NCL) were simultaneously recorded. LFPs of relay nuclei (the nucleus rotundus, ROT) of tectofugal pathway were also acquired. We demonstrated that LFP powers in the tectofugal pathway were higher than those in the thalamofugal pathway when illuminating blue lights. In addition, the LFP synchronization was stronger between the entopallium and NCL. LFPs also revealed a higher Granger causality from the direction of entopallium to NCL and from ROT to entopallium. These results suggest that zebra finches' tectofugal pathway predominately processing color information from ROT to NCL, relayed by entopallium, and blue could trigger the strongest response.

Combined predisposed preferences for colour and biological motion make robust development of social attachment through imprinting

To study how predisposed preferences shape the formation of social attachment through imprinting, newly hatched domestic chicks (Gallus gallus domesticus) were simultaneously exposed to two animations composed of comparable light points in different colours (red and yellow), one for a walking motion and another for a linear motion. When a walking animation in red was combined with a linear one in yellow, chicks formed a learned preference for the former that represented biological motion (BM). When the motion-colour association was swapped, chicks failed to form a preference for a walking in yellow, indicating a bias to a specific association of motion and colour. Accordingly, experiments using realistic walking chicken videos revealed a preference for a red video over a yellow one, when the whole body or the head was coloured. On the other hand, when the BM preference had been pre-induced using an artefact moving rigidly (non-BM), a clear preference for a yellow walking animation emerged after training by the swapped association. Even if the first-seen moving object was a nonbiological artefact such as the toy, the visual experience would induce a predisposed BM preference, making chicks selectively memorize the object with natural features. Imprinting causes a rapid inflow of thyroid hormone in the telencephalon leading to the induction of the BM preference, which would make the robust formation of social attachment selectively to the BM-associated object such as the mother hen.

Spontaneous Learning of Visual Structures in Domestic Chicks

Simple Summary

Our aim is to investigate the recognition of the structure of multi-element configurations; one mechanism that supports communicative functions in different species. Cognitive mechanisms involved in this ability might not have evolved specifically for communicative use, but derive from other functions. Thus, it is crucial to study these abilities in species that are not vocal learners and with stimuli from other modalities. We know already that domestic chicks can learn the temporal statistical structure of sequences of visual shapes, however their abilities to encode the spatial structure of visual patterns (configurations composed of multiple visual elements presented simultaneously side-by-side) is much less known. Using filial imprinting learning, we showed that chicks spontaneously recognize the structure of their imprinting stimulus, preferring it to one composed of the same elements in different configurations. Moreover, we found that in their affiliative responses chicks give priority to information located at the stimulus edges, a phenomenon that was so far observed only with temporal sequences. This first evidence of a spontaneous edge bias with spatial stimuli further stresses the importance of studying similarities and differences between the processing of linguistic and nonlinguistic stimuli and of stimuli presented in various sensory modalities.

Abstract

Effective communication crucially depends on the ability to produce and recognize structured signals, as apparent in language and birdsong. Although it is not clear to what extent similar syntactic-like abilities can be identified in other animals, recently we reported that domestic chicks can learn abstract visual patterns and the statistical structure defined by a temporal sequence of visual shapes. However, little is known about chicks’ ability to process spatial/positional information from visual configurations. Here, we used filial imprinting as an unsupervised learning mechanism to study spontaneous encoding of the structure of a configuration of different shapes. After being exposed to a triplet of shapes (ABC or CAB), chicks could discriminate those triplets from a permutation of the same shapes in different order (CAB or ABC), revealing a sensitivity to the spatial arrangement of the elements. When tested with a fragment taken from the imprinting triplet that followed the familiar adjacency-relationships (AB or BC) vs. one in which the shapes maintained their position with respect to the stimulus edges (AC), chicks revealed a preference for the configuration with familiar edge elements, showing an edge bias previously found only with temporal sequences.

Regulation of visual Wulst cell responsiveness by imprinting causes stimulus-specific activation of rostral cells

Imprinting behaviour in chicks can be induced exclusively during a short period after hatching. During this period, visual information on the imprinting stimulus is conveyed to the visual Wulst (VW) in the telencephalon, which corresponds to the visual cortex of mammals, and then to the memory-storing region known as the intermediate medial mesopallium. These two regions are indispensable for imprinting. We previously showed that imprinting training altered the response pattern of the VW to the imprinting stimulus; however, the precise distribution of cells and the mechanism involved with this altered response remains unclear. Here we showed that a specific population of rostral VW cells responded to the imprinting stimulus by analysing the subcellular localization of Arc/arg3.1 transcripts in VW cells. GABAergic parvalbumin (PV) cells are abundant in the dorsal region of this area, and imprinting training doubled the number of activated PV-positive neurons. An injection of bicuculline, a GABA(A) receptor antagonist, in the dorsal VW disturbed the rostral distribution of responsive cells and thus resulted in a lack of imprinting. These results suggest that activated PV cells restrict VW cells response to dorsal area to form a specific imprinting pathway.

Translational control of auditory imprinting and structural plasticity by eIF2α

The formation of imprinted memories during a critical period is crucial for vital behaviors, including filial attachment. Yet, little is known about the underlying molecular mechanisms. Using a combination of behavior, pharmacology, in vivo surface sensing of translation (SUnSET) and DiOlistic labeling we found that, translational control by the eukaryotic translation initiation factor 2 alpha (eIF2α) bidirectionally regulates auditory but not visual imprinting and related changes in structural plasticity in chickens. Increasing phosphorylation of eIF2α (p-eIF2α) reduces translation rates and spine plasticity, and selectively impairs auditory imprinting. By contrast, inhibition of an eIF2α kinase or blocking the translational program controlled by p-eIF2α enhances auditory imprinting. Importantly, these manipulations are able to reopen the critical period. Thus, we have identified a translational control mechanism that selectively underlies auditory imprinting. Restoring translational control of eIF2α holds the promise to rejuvenate adult brain plasticity and restore learning and memory in a variety of cognitive disorders.

DOI:http://dx.doi.org/10.7554/eLife.17197.001

Shortly after hatching, a chick recognizes the sight and sound of its mother and follows her around. This requires a type of learning called imprinting, which only occurs during a short period of time in young life known as the “critical period”. This process has been reported in a variety of birds and other animals where long-term memory formed during a critical period guides vital behaviors. In order to form imprinted memories, neurons must produce new proteins. However, it is not clear how new experiences trigger the production of these proteins during imprinting. Unraveling such mechanisms may help us to develop drugs that can recover plasticity in the adult brain, which could help individuals with brain injuries relearn skills after critical periods are closed.

It is possible to imprint newly hatched chicks to arbitrary sounds and visual stimuli by placing the chicks in running wheels and exposing them to repeated noises and videos. Later on, the chicks respond to these stimuli by running towards the screen, mimicking how they would naturally follow their mother. This system allows researchers to measure imprinting in a carefully controlled laboratory setting.

A protein called elF2α plays a major role in regulating the production of new proteins and has been shown to be required for the formation of long-term memories in adult rodents. Batista et al. found that elF2α is required to imprint newly hatched chicks to sound. During the critical period, this factor mediates an increase in “memory-spines”, which are small bumps on neurons that are thought to be involved in memory storage. On the other hand, elF2α was not required to imprint newly hatched chicks to visual stimuli, suggesting that there are different pathways involved in regulating imprinting to different senses. Batista et al. also demonstrate that using drugs to increase the activity of eIF2α in older chicks could allow these chicks to be imprinted to new sounds.

The next steps following on from this work are to identify proteins that eIF2α regulates to form memories, and to find out why eIF2α is only required to imprint sounds. Future research will investigate the mechanisms that control visual imprinting and how it differs from imprinting to sounds.

DOI:http://dx.doi.org/10.7554/eLife.17197.002

Avian Test Battery for the Evaluation of Developmental Abnormalities of Neuro- and Reproductive Systems

Most of the currently used toxicity assays for environmental chemicals use acute or chronic systemic or reproductive toxicity endpoints rather than neurobehavioral endpoints. In addition, the current standard approaches to assess reproductive toxicity are time-consuming. Therefore, with increasing numbers of chemicals being developed with potentially harmful neurobehavioral effects in higher vertebrates, including humans, more efficient means of assessing neuro- and reproductive toxicity are required. Here we discuss the use of a Galliformes-based avian test battery in which developmental toxicity is assessed by means of a combination of chemical exposure during early embryonic development using an embryo culture system followed by analyses after hatching of sociosexual behaviors such as aggression and mating and of visual memory via filial imprinting. This Galliformes-based avian test battery shows promise as a sophisticated means not only of assessing chemical toxicity in avian species but also of assessing the risks posed to higher vertebrates, including humans, which are markedly sensitive to nervous or neuroendocrine system dysfunction.

Distribution of calcium-binding proteins in the pigeon visual thalamic centers and related pretectal and mesencephalic nuclei. Phylogenetic and functional determinants

Multichannel processing of environmental information constitutes a fundamental basis of functioning of sensory systems in the vertebrate brain. Two distinct parallel visual systems - the tectofugal and thalamofugal exist in all amniotes. The vertebrate central nervous system contains high concentrations of intracellular calcium-binding proteins (CaBPrs) and each of them has a restricted expression pattern in different brain regions and specific neuronal subpopulations. This study aimed at describing the patterns of distribution of parvalbumin (PV) and calbindin (CB) in the visual thalamic and mesencephalic centers of the pigeon (Columba livia). We used a combination of immunohistochemistry and double labeling immunofluorescent technique. Structures studied included the thalamic relay centers involved in the tectofugal (nucleus rotundus, Rot) and thalamofugal (nucleus geniculatus lateralis, pars dorsalis, GLd) visual pathways as well as pretectal, mesencephalic, isthmic and thalamic structures inducing the driver and/or modulatory action to the visual processing. We showed that neither of these proteins was unique to the Rot or GLd. The Rot contained i) numerous PV-immunoreactive (ir) neurons and a dense neuropil, and ii) a few CB-ir neurons mostly located in the anterior dorsal part and associated with a light neuropil. These latter neurons partially overlapped with the former and some of them colocalized both proteins. The distinct subnuclei of the GLd were also characterized by different patterns of distribution of CaBPrs. Some (nucleus dorsolateralis anterior, pars magnocellularis, DLAmc; pars lateralis, DLL; pars rostrolateralis, DLAlr; nucleus lateralis anterior thalami, LA) contained both CB- and PV-ir neurons in different proportions with a predominance of the former in the DLAmc and DLL. The nucleus lateralis dorsalis of nuclei optici principalis thalami only contained PV-ir neurons and a neuropil similar to the interstitial pretectal/thalamic nuclei of the tectothalamic tract, nucleus pretectalis and thalamic reticular nucleus. The overlapping distribution of PV and CB immunoreactivity was typical for the pretectal nucleus lentiformis mesencephali and the nucleus ectomamillaris as well as for the visual isthmic nuclei. The findings are discussed in the light of the contributive role of the phylogenetic and functional factors determining the circuits׳ specificity of the different CaBPr types.

Positive feedback of NR2B-containing NMDA receptor activity is the initial step toward visual imprinting: a model for juvenile learning

Imprinting in chicks is a good model for elucidating the processes underlying neural plasticity changes during juvenile learning. We recently reported that neural activation of a telencephalic region, the core region of the hyperpallium densocellulare (HDCo), was critical for success of visual imprinting, and that N-Methyl-D-aspartic (NMDA) receptors containing the NR2B subunit (NR2B/NR1) in this region were essential for imprinting. Using electrophysiological and multiple-site optical imaging techniques with acute brain slices, we found that long-term potentiation (LTP) and enhancement of NR2B/NR1 currents in HDCo neurons were induced in imprinted chicks. Enhancement of NR2B/NR1 currents as well as an increase in surface NR2B expression occurred even following a brief training that was too weak to induce LTP or imprinting behavior. This means that NR2B/NR1 activation is the initial step of learning, well before the activation of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors which induces LTP. We also showed that knockdown of NR2B/NR1 inhibited imprinting, and inversely, increasing the surface NR2B expression by treatment with a casein kinase 2 inhibitor successfully reduced training time required for imprinting. These results suggest that imprinting stimuli activate post-synaptic NR2B/NR1 in HDCo cells, increase NR2B/NR1 signaling through up-regulation of its expression, and induce LTP and memory acquisition. The study investigated the neural mechanism underlying juvenile learning. In the initial stage of chick imprinting, NMDA receptors containing the NMDA receptor subunit 2B (NR2B) are activated, surface expression of NR2B/NR1 (NMDA receptor subunit 1) is up-regulated, and consequently long-term potentiation is induced in the telencephalic neurons. We suggest that the positive feedback in the NR2B/NR1 activation is a unique process of juvenile learning, exhibiting rapid memory acquisition.

Impaired imprinting and social behaviors in chicks exposed to mifepristone, a glucocorticoid receptor antagonist, during the final week of embryogenesis

The effects of glucocorticoid receptor dysfunction during embryogenesis on the imprinting abilities and social behaviors of hatchlings were examined using "fertile hen's egg-embryo-chick" system.

METHODS AND RESULTS

Of embryos treated with mifepristone (0.4μmol/egg) on day 14, over 75% hatched a day later than the controls (day 22) without external anomalies. The mifepristone-treated hatchlings were assayed for imprinting ability on post-hatching day 2 and for social behaviors on day 3. The findings were as follows: imprinting ability (expressed as preference score) was significantly lower in mifepristone-treated hatchlings than in controls (0.65±0.06 vs. 0.92±0.02, P<0.005). Aggregation tests to evaluate the speed (seconds) required for four chicks, individually isolated with cardboard dividers in a box, to form a group after removal of the barriers showed that aggregation was significantly slower in mifepristone-treated hatchlings than in controls (8.7±1.1 vs. 2.6±0.3, P<0.001). In belongingness tests to evaluate the speed (seconds) for a chick isolated at a corner to join a group of three chicks placed at the opposite corner, mifepristone-treated hatchlings took significantly longer than controls (4.5±0.4/40 cm vs. 2.4±0.08/40 cm, P<0.001). In vocalization tests, using a decibel meter to measure average decibel level/30s (chick vocalization), mifepristone-treated hatchlings had significantly weaker vocalizations than controls (14.2±1.9/30s vs. 26.4±1.3/30s P<0.001). In conclusion, glucocorticoid receptor dysfunction during the last week embryogenesis altered the programming of brain development, resulting in impaired behavioral activities in late life.

Impaired social behavior in chicks exposed to sodium valproate during the last week of embryogenesis

RATIONALE AND OBJECTIVES

To evaluate direct exposure to sodium valproate (VPA) during embryogenesis, we administered VPA to chick embryos and examined their social behaviors after hatching.

METHODS AND RESULTS

Embryos treated with VPA (35 μmol/egg) on day 14 were similar to controls for hatching date (day 21) and hatchlings' abilities, such as motor, imprinting, and surface righting. However, these VPA chicks on posthatching day 3 scored significantly low in the chick's social separation stress (SSS) test as follows. Aggregation test evaluated the speed of four chicks, individually isolated by a cardboard in a box, to aggregate upon removal of the cardboards. Belongingness test evaluated the speed of a chick isolated at a corner to join the group of three chicks placed at the opposite corner. Vocalization test for each chick was performed in an isolated corner by using a sound level meter. The results demonstrated that compared with controls, VPA chicks were significantly slow in aggregation (12.7 ± 2.5 s vs. 2.9 ± 0.9 s, p = 0.006) and belongingness (3.6 ± 0.28 s/40 cm vs. 2.6 ± 0.14 s/40 cm, P = 0.003) and weak in vocalization (13.4 ± 2.8 dB/30 s vs. 26.7 ± 1.3 dB/30 s, P = 0.001), respectively. Weight of cerebellum of VAP chick was 15 % lighter than controls (P = 0.004).

CONCLUSIONS

Chick embryos exposed to VPA during the last week of embryogenesis had impaired social behaviors in spite of normal mortar and imprinting ability. The present method will be a useful animal model for assessing the effects of environment during embryogenesis on social behaviors in later life.

Peer attachment formation by systemic redox regulation with social training after a sensitive period

Attachment formation is the most pivotal factor for humans and animals in the growth and development of social relationships. However, the developmental processes of attachment formation mediated by sensory-motor, emotional, and cognitive integration remain obscure. Here we developed an animal model to understand the types of social interactions that lead to peer-social attachment formation. We found that the social interaction in a sensitive period was essential to stabilise or overwrite the initially imprinted peer affiliation state and that synchronised behaviour with others based on common motivations could be a driver of peer social attachment formation. Furthermore, feeding experience with supplementation of ubiquinol conferred peer social attachment formation even after the sensitive period. Surprisingly, the experience of feeding beyond the cage window was also effective to reduce the required amount ubiquinol, suggesting that peri-personal space modulation may affect socio-emotional cognition and there by lead to attachment formation.

Demonstration of a neural circuit critical for imprinting behavior in chicks

Imprinting behavior in birds is elicited by visual and/or auditory cues. It has been demonstrated previously that visual cues are recognized and processed in the visual Wulst (VW), and imprinting memory is stored in the intermediate medial mesopallium (IMM) of the telencephalon. Alteration of neural responses in these two regions according to imprinting has been reported, yet direct evidence of the neural circuit linking these two regions is lacking. Thus, it remains unclear how memory is formed and expressed in this circuit. Here, we present anatomical as well as physiological evidence of the neural circuit connecting the VW and IMM and show that imprinting training during the critical period strengthens and refines this circuit. A functional connection established by imprint training resulted in an imprinting behavior. After the closure of the critical period, training could not activate this circuit nor induce the imprinting behavior. Glutamatergic neurons in the ventroposterior region of the VW, the core region of the hyperpallium densocellulare (HDCo), sent their axons to the periventricular part of the HD, just dorsal and afferent to the IMM. We found that the HDCo is important in imprinting behavior. The refinement and/or enhancement of this neural circuit are attributed to increased activity of HDCo cells, and the activity depended on NR2B-containing NMDA receptors. These findings show a neural connection in the telencephalon in Aves and demonstrate that NR2B function is indispensable for the plasticity of HDCo cells, which are key mediators of imprinting.

Activation of cholecystokinin neurons in the dorsal pallium of the telencephalon is indispensable for the acquisition of chick imprinting behavior

Chick imprinting behavior is a good model for the study of learning and memory. Imprinting object is recognized and processed in the visual wulst, and the memory is stored in the intermediate medial mesopallium in the dorsal pallium of the telencephalon. We identified chicken cholecystokinin (CCK)-expressing cells localized in these area. The number of CCK mRNA-positive cells increased in chicks underwent imprinting training, and these cells expressed nuclear Fos immunoreactivity at high frequency in these regions. Most of these CCK-positive cells were glutamatergic and negative for parvalbumin immunoreactivity. Semi-quantitative PCR analysis revealed that the CCK mRNA levels were significantly increased in the trained chicks compared with untrained chicks. In contrast, the increase in CCK- and c-Fos-double-positive cells associated with the training was not observed after closure of the critical period. These results indicate that CCK cells in the dorsal pallium are activated acutely by visual training that can elicit imprinting. In addition, the CCK receptor antagonist significantly suppressed the acquisition of memory. These results suggest that the activation of CCK cells in the visual wulst as well as in the intermediate medial mesopallium by visual stimuli is indispensable for the acquisition of visual imprinting.