Cellular correlates of stages of memory formation in the chick following passive avoidance training

The role of intracellular calcium stores in synaptic plasticity and memory consolidation

Memory processing requires tightly controlled signalling cascades, many of which are dependent upon intracellular calcium (Ca(2+)). Despite this, most work investigating calcium signalling in memory formation has focused on plasma membrane channels and extracellular sources of Ca(2+). The intracellular Ca(2+) release channels, ryanodine receptors (RyRs) and inositol (1,4,5)-trisphosphate receptors (IP3Rs) have a significant capacity to regulate intracellular Ca(2+) signalling. Evidence at both cellular and behavioural levels implicates both RyRs and IP3Rs in synaptic plasticity and memory formation. Pharmacobehavioural experiments using young chicks trained on a single-trial discrimination avoidance task have been particularly useful by demonstrating that RyRs and IP3Rs have distinct roles in memory formation. RyR-dependent Ca(2+) release appears to aid the consolidation of labile memory into a persistent long-term memory trace. In contrast, IP3Rs are required during long-term memory. This review discusses various functions for RyRs and IP3Rs in memory processing, including neuro- and glio-transmitter release, dendritic spine remodelling, facilitating vasodilation, and the regulation of gene transcription and dendritic excitability. Altered Ca(2+) release from intracellular stores also has significant implications for neurodegenerative conditions.

Hippocampal memory consolidation during sleep: a comparison of mammals and birds

The transition from wakefulness to sleep is marked by pronounced changes in brain activity. The brain rhythms that characterize the two main types of mammalian sleep, slow-wave sleep (SWS) and rapid eye movement (REM) sleep, are thought to be involved in the functions of sleep. In particular, recent theories suggest that the synchronous slow-oscillation of neocortical neuronal membrane potentials, the defining feature of SWS, is involved in processing information acquired during wakefulness. According to the Standard Model of memory consolidation, during wakefulness the hippocampus receives input from neocortical regions involved in the initial encoding of an experience and binds this information into a coherent memory trace that is then transferred to the neocortex during SWS where it is stored and integrated within preexisting memory traces. Evidence suggests that this process selectively involves direct connections from the hippocampus to the prefrontal cortex (PFC), a multimodal, high-order association region implicated in coordinating the storage and recall of remote memories in the neocortex. The slow-oscillation is thought to orchestrate the transfer of information from the hippocampus by temporally coupling hippocampal sharp-wave/ripples (SWRs) and thalamocortical spindles. SWRs are synchronous bursts of hippocampal activity, during which waking neuronal firing patterns are reactivated in the hippocampus and neocortex in a coordinated manner. Thalamocortical spindles are brief 7-14 Hz oscillations that may facilitate the encoding of information reactivated during SWRs. By temporally coupling the readout of information from the hippocampus with conditions conducive to encoding in the neocortex, the slow-oscillation is thought to mediate the transfer of information from the hippocampus to the neocortex. Although several lines of evidence are consistent with this function for mammalian SWS, it is unclear whether SWS serves a similar function in birds, the only taxonomic group other than mammals to exhibit SWS and REM sleep. Based on our review of research on avian sleep, neuroanatomy, and memory, although involved in some forms of memory consolidation, avian sleep does not appear to be involved in transferring hippocampal memories to other brain regions. Despite exhibiting the slow-oscillation, SWRs and spindles have not been found in birds. Moreover, although birds independently evolved a brain region--the caudolateral nidopallium (NCL)--involved in performing high-order cognitive functions similar to those performed by the PFC, direct connections between the NCL and hippocampus have not been found in birds, and evidence for the transfer of information from the hippocampus to the NCL or other extra-hippocampal regions is lacking. Although based on the absence of evidence for various traits, collectively, these findings suggest that unlike mammalian SWS, avian SWS may not be involved in transferring memories from the hippocampus. Furthermore, it suggests that the slow-oscillation, the defining feature of mammalian and avian SWS, may serve a more general function independent of that related to coordinating the transfer of information from the hippocampus to the PFC in mammals. Given that SWS is homeostatically regulated (a process intimately related to the slow-oscillation) in mammals and birds, functional hypotheses linked to this process may apply to both taxonomic groups.

Time-dependent lateralization of social learning in the domestic chick (Gallus gallus domesticus): Effects of retention delays in the observed lateralization pattern

Day-old chicks have been shown capable of learning to avoid pecking by observation only of a conspecific showing a disgust reaction after pecking a bitter-tasting bead. This learning is lateralized: access to the right hemisphere appears necessary for successful performance 30 min after training. This is in contrast to the non-social learning version of this learning task, in which the left hemisphere appears to play the dominant role, although both the left and right hemispheres are, in turn, subject to brief windows of "enhanced recall" during memory formation. In our present work we wished to investigate whether such recall events are also prevalent in the social learning task. We investigated 3 such windows; 25 min (a right hemisphere event), 32 min (left-hemisphere event) and 64 min (a possible left-hemisphere event following an interconnection of both hemispheres allowing memory transfer between the two hemispheres). At 32 and 64 min after training we found no evidence of functional lateralization. At 25 min, however, we found right hemisphere dominance. We suggest that a lateralization effect occurs in the social version of the PAL (passive avoidance learning) only at time points associated with right hemisphere dominance. It seems that lateralization is not able to emerge at time points where a left-hemisphere event is expected, because the two "lateralization effects" (right hemisphere dominance associated with the social task and left-hemisphere dominance associated with the PAL task) are in conflict.

Dopamine receptors in a songbird brain

Dopamine is a key neuromodulatory transmitter in the brain. It acts through dopamine receptors to affect changes in neural activity, gene expression, and behavior. In songbirds, dopamine is released into the striatal song nucleus Area X, and the levels depend on social contexts of undirected and directed singing. This differential release is associated with differential expression of activity-dependent genes, such as egr1 (avian zenk), which in mammalian brain are modulated by dopamine receptors. Here we cloned from zebra finch brain cDNAs of all avian dopamine receptors: the D1 (D1A, D1B, D1D) and D2 (D2, D3, D4) families. Comparative sequence analyses of predicted proteins revealed expected phylogenetic relationships, in which the D1 family exists as single exon and the D2 family exists as spliced exon genes. In both zebra finch and chicken, the D1A, D1B, and D2 receptors were highly expressed in the striatum, the D1D and D3 throughout the pallium and within the mesopallium, respectively, and the D4 mainly in the cerebellum. Furthermore, within the zebra finch, all receptors, except for D4, showed differential expression in song nuclei relative to the surrounding regions and developmentally regulated expression that decreased for most receptors during the sensory acquisition and sensorimotor phases of song learning. Within Area X, half of the cells expressed both D1A and D2 receptors, and a higher proportion of the D1A-only-containing neurons expressed egr1 during undirected but not during directed singing. Our findings are consistent with hypotheses that dopamine receptors may be involved in song development and social context-dependent behaviors. J. Comp. Neurol. 518:741–769, 2010. © 2009 Wiley-Liss, Inc.

Role of beta-adrenoceptors in memory consolidation: beta3-adrenoceptors act on glucose uptake and beta2-adrenoceptors on glycogenolysis

Noradrenaline, acting via beta(2)- and beta(3)-adrenoceptors (AR), enhances memory formation in single trial-discriminated avoidance learning in day-old chicks by mechanisms involving changes in metabolism of glucose and/or glycogen. Earlier studies of memory consolidation in chicks implicated beta(3)- rather than beta(2)-ARs in enhancement of memory consolidation by glucose, but did not elucidate whether stimulation of glucose uptake or of glycolysis was responsible. This study examines the role of glucose transport in memory formation using central injection of the nonselective facilitative glucose transporter (GLUT) inhibitor cytochalasin B, the endothelial/astrocytic GLUT-1 inhibitor phloretin and the Na(+)/energy-dependent endothelial glucose transporter (SGLT) inhibitor phlorizin. Cytochalasin B inhibited memory when injected into the mesopallium (avian cortex) either close to or between 25 and 45 min after training, whereas phloretin and phlorizin only inhibited memory at 30 min. This suggested that astrocytic/endothelial (GLUT-1) transport is critical at the time of consolidation, whereas a different transporter, probably the neuronal glucose transporter (GLUT-3), is important at the time of training. Inhibition of glucose transport by cytochalasin B, phloretin, or phlorizin also interfered with beta(3)-AR-mediated memory enhancement 20 min posttraining, whereas inhibition of glycogenolysis interfered with beta(2)-AR agonist enhancement of memory. We conclude that in astrocytes (1) activities of both GLUT-1 and SGLT are essential for memory consolidation 30 min posttraining; (2) neuronal GLUT-3 is essential at the time of training; and (3) beta(2)- and beta(3)-ARs consolidate memory by different mechanisms; beta(3)-ARs stimulate central glucose transport, whereas beta(2)-ARs stimulate central glycogenolysis.

Remembering that things have changed: a review of the cellular mechanisms of memory re-consolidation in the day-old chick

It has been one of the unshakeable orthodoxies of memory research that memory is initially laid down in a labile form for a short period following the experience and that over time the memory is "fixed" or "consolidated" into the physical structure of the brain. Over the last decade a large body of data has gathered which demonstrates that a "consolidated" memory can be returned to a labile state following retrieval of material from the store, which can then be re-consolidated, incorporating the newly acquired information into the representation of the world. The process of re-consolidation thus provides a sensible means for the crucial process of memory updating to occur. The paper focuses on pharmaco-behavioural experiments undertaken in our laboratories as well as in those of other groups which use the day-old chick as subject and the passive avoidance learning (PAL) task to examine the behavioural and metabolic parameters of re-consolidation. The data indicate that the consolidation and the re-consolidation processes are similar but not identical physiological processes. The re-processing of the memory following a re-consolidation involves each of the glutamatergic, adrenergic and dopaminergic neurotransmitter systems as well as re-activation of protein synthesis associated with the respective traces. In the chick model system, the ability to undertake re-consolidation is transient, and is observed only for a maximum of 24-48 h following the initial training event. Controversy persists as to whether the re-consolidated memory represents a new memory or whether it is a modification of the original memory processing.

Aposematic colouration enhances memory formation in domestic chicks trained in a weak passive avoidance learning paradigm

The one-trial passive avoidance learning task is commonly used in avian research to explore anatomical, cellular and molecular parameters of learning and memory. Many factors are known to influence the effectiveness and/or duration of such learning events. Combinations of novel odours, such as pyrazine, and aposematic colours, such as brig ht yellow or red, have been shown to induce a long-lasting aversion to food crumbs in 'visual' predators, including birds such as the domestic chick (1). The aim of this study was to (a) examine whether visual complexity played a role in the generation of an aversive response to a novel visual stimulus and (b) to establish whether the duration of memory of an aversive experience could be modified by altering the visual properties of the stimulus. In the first experiment, naive domestic chicks were trained on a weakly aversive one-trial passive avoidance bead task, in which chicks were allowed to peck at a bead coated with a 10% solution of the bitter-tasting and odorous substance methylanthranilate (MeA). The chicks were trained with (allowed to peck) one of four differently coloured beads dipped in 10% MeA. Chrome, black, yellow or black-and-yellow striped beads were used. 'Recall' of the aversive bead was examined by presenting the (clean) training bead 24h after training and monitoring avoidance to it compared to a 'neutral' white bead. A high proportion (63%) of chicks trained with the black and yellow striped bead avoided it 24h after training, whereas little or no avoidance was seen in response to chrome, yellow or black beads. In a second experiment naive domestic chicks were all trained once only with a black and yellow striped bead coated in a 10% MeA solution, but this time, were tested 24h later, once only, with either a black, a yellow or a black and yellow striped bead. Nearly 60% of chicks tested with a black and yellow striped bead showed avoidance of the bead, whereas only 23% of those tested with a black bead and 14% tested with a yellow bead showed avoidance. These results confirm the importance of complex warning colouration, when paired with a novel olfactory cue and a bitter taste, in avoidance learning. We conclude that the chicks' response to monochromatic colours (e.g. yellow or black) is not affected by their previous experience with a conspicuously patterned stimulus (yellow and black stripes). Moreover, it suggests a predisposition for chicks to attend to aversive cues associated with 'naturalistic' high contrast colour cue combinations such as black and yellow.

Inhibition of mGluR1 and IP3Rs impairs long-term memory formation in young chicks

Calcium (Ca(2+)) is involved in a myriad of cellular functions in the brain including synaptic plasticity. However, the role of intracellular Ca(2+) stores in memory processing remains poorly defined. The current study explored a role for glutamate-dependent intracellular Ca(2+) release in memory processing via blockade of metabotropic glutamate receptor subtype 1 (mGluR1) and inositol (1,4,5)-trisphosphate receptors (IP(3)Rs). Using a single-trial discrimination avoidance task developed for the young chick, administration of the specific and potent mGluR1 antagonist JNJ16259685 (500nM, immediately post-training, ic), or the IP(3)R antagonist Xestospongin C (5microM, immediately post-training, ic), impaired retention from 90min post-training. These findings are consistent with mGluR1 activating IP(3)Rs to release intracellular Ca(2+) required for long-term memory formation and have been interpreted within an LTP2 model. The consequences of different patterns of retention loss following ryanodine receptor (RyR) and IP(3)R inhibition are discussed.

Receipt of vaginal-cervical stimulation modifies synapsin content in limbic areas of the female rat

Female rats require a sufficient amount and pattern of vaginal-cervical stimulation to initiate neuroendocrine changes required for the successful implantation of a fertilized ovum in the uterus. These changes are characterized by twice daily prolactin surges that last 10-12 days. Following a sterile mating, the endocrine changes are still observed, and are termed pseudopregnancy (PSP). The mating stimulation required to initiate these changes prior to pregnancy or PSP has a neural representation, which we have termed the intromission mnemonic. We sought to examine if the formation of the intromission mnemonic is accompanied by alterations in the number or density of synapses in limbic areas by immuno-labeling a pre-synaptic protein, synapsin. Groups of cycling female rats on proestrus day received either 15 or 5 intromissions or mounts-without intromissions from a vasectomized male; an additional time-matched control group was left in the home cage. All females were perfused after 90 min or 8 h. The brains were removed and sliced, and the amygdala and hippocampus immunostained for synapsin, then imaged by confocal microscopy. We found that 90 min after mating sufficient for PSP, the number of synapsin puncta (points of immunoreactivity equivalent to a synapse) was decreased and the intensity of the synapsin staining was increased in the posterodorsal medial amygdala (MePD). A similar reduction of puncta was observed in the CA1 region of the hippocampus, and an increase of intensity occurred in the basolateral amygdala. Spaced intromissions had no effect on synapsin expression anywhere examined. Intensity reductions unrelated to receipt of vaginal-cervical stimulation were observed in the hippocampus. None of these effects were observed after 8 h. Together, these results raise the possibility that synapses in the MePD may be pruned after mating stimulation, resulting in pathway-specific stabilization that contributes to the intromission mnemonic associated with the establishment of PSP.

d-Lactate inhibition of memory in a single trial discrimination avoidance task in the young chick

L-Lactate is a metabolite possibly able to meet some neuronal energy demands. However, a clear role for L-lactate in behaviour remains elusive. Administration of the inactive isomer D-lactate (1.75 mM; ic), immediately post-training, resulted in a persistent retention loss from 40 min post-training when used in conjuction with a single trial discrimination avoidance task designed for the young chick. Furthermore, 1mM noradrenaline (ic) administered 20 min post-training overcame the retention loss induced by D-lactate. Although not directly demonstrated in the current study, it is plausible that D-lactate inhibited memory processing by competing with L-lactate for uptake into neurons. The time of onset of the retention loss induced by D-lactate is in accord with findings where the action of noradrenaline is inhibited. The successful challenge of D-lactate inhibition by a high concentration of noradrenaline may suggest a relationship by some unidentified mechanism.

Effect of gestational ethanol exposure on long-term memory formation in newborn chicks

Fetal alcohol syndrome (FAS), a condition occurring in some children of mothers who have consumed alcohol during pregnancy, is characterized by craniofacial malformations, and physical and mental retardation. It is significant that even children with history of gestational ethanol exposure but relatively unaffected overall IQ performance, often exhibit learning difficulties and behavioral problems, suggestive of impaired memory formation. Hence, the specific aim of this study was to examine memory formation in chicks exposed to ethanol during early gestation toward the understanding of neurobehavioral disturbances in FAS. Chicks were exposed to alcohol on gestational days 1-3 by injection of ethanol into the airspace of freshly fertilized eggs. The effects of prenatal ethanol on physical growth and development, and memory formation were studied. The one-trial passive avoidance learning paradigm in 1-day-old chicks was used to study memory formation in these chicks. It was observed that chick embryos exposed to 10% ethanol on gestational days 1-3 had significant reduction in all body parameters when compared with appropriate controls. Further, ethanol-exposed chick embryos had significantly impaired (P<.05) long-term memory (LTM) formation after training, though short-term or intermediate-term memory formation was unimpaired. Thus, the findings of the current study demonstrate the detrimental effects of ethanol exposure during early pregnancy on developing chick embryos in general and on memory formation in particular. Hence, it is suggested that impairment in LTM could be a fundamental mechanism for learning disorders and neurobehavioral abnormalities observed in FAS.

In-vivo gene transfer into newly hatched chick brain by electroporation

Newly hatched domestic chicks serve as ideal models for studies of the neural basis of behavioral plasticity, particularly for understanding the mechanisms of learning such as filial imprinting. To elucidate the molecular basis and gene functions involved in learning, we developed an in-vivo gene-transfer system in the brain of a living chick using electroporation. When green fluorescent protein-encoding plasmids were transfected to a chick brain, green fluorescence was clearly observed, and expression at the protein level was confirmed by immunoblotting. Most of the transfected brain cells were neuronal cells with dendrites. This neuron-selective electroporation system will facilitate the analysis of gene functions in the living chick brain and provide further clues as to the molecular mechanisms of avian learning.

A role for eukaryotic translation initiation factor 2B (eIF2B) in taste memory consolidation and in thermal control establishment during the critical period for sensory development

All species exhibit critical periods for sensory development, yet very little is known about the molecules involved in the changes in the network wiring that underlies this process. Here the role of transcription regulation of the translation machinery was determined by evaluating the expression of eIF2Bepsilon, an essential component of translation initiation, in both taste-preference development and thermal control establishment in chicks. Analysis of the expression pattern of this gene after passive-avoidance training revealed clear induction of eIF2Bepsilon in both the mesopallium intermediomediale (IMM) and in the striatum mediale (StM). In addition, a correlation was found between the concentration of methylanthranilate (MeA), which was the malaise substrate in the passive-avoidance training procedure, the duration of memory, and the expression level of eIF2Bepsilon. Training chicks on a low concentration of MeA induced short-term memory and low expression level of eIF2Bepsilon, whereas a high concentration of MeA induced long-term memory and a high expression level of eIF2Bepsilon in both the IMM and StM. Furthermore, eIF2Bepsilon-antisense "knock-down" not only reduced the amount of eIF2Bepsilon but also attenuated taste memory formation. In order to determine whether induction of eIF2Bepsilon is a general feature of neuronal plasticity, we checked whether it was induced in other forms of neuronal plasticity, with particular attention to its role in temperature control establishment, which represents hypothalamic-related plasticity. It was established that eIF2Bepsilon-mRNA was induced in the preopotic anterior hypothalamus during heat conditioning. Taken together, these results correlate eIF2Bepsilon with sensory development.

Enhancement of long-term memory retention by Colostrinin in one-day-old chicks trained on a weak passive avoidance learning paradigm

Colostrinin (CLN) is a biologically active proline-rich polypeptide which has therapeutic potential for the alleviation of memory deficits in age-related dementias in a number of human conditions, particularly Alzheimer's disease. To examine the efficacy of CLN in other species, day-old domestic chicks were used as a model system to study its effects on retention of memory for a single one-trial learning paradigm--avoidance of a bitter-tasting substance (methylanthranilate, MeA). Birds were presented with a bead coated with either a dilute (10%) solution of MeA or a bead coated with 100% MeA. Those trained on 100% MeA avoided pecking at a similar but dry bead 24 h later, thereby demonstrating long-term memory whereas chicks trained on the 10% solution pecked the bead at 24 h, indicating lack of long term memory for the task. However, when CLN was injected (i.c.) into a region known to be important in memory formation, the mesopallium intermediomediale (IMM), prior to training with 10% MeA, chicks exhibited strong memory retention at 24 h, similar to those trained on 100% MeA. Control chicks trained on 10% MeA but injected i.c. with a 10% saline solution did not show improvement in memory retention. Intraperitoneal (i.p.) injections of CLN were as effective as the i.c. route. These data extend the known efficacy of CLN from mammals demonstrating its widespread efficacy as a cognitive enhancer.

Passive avoidance training decreases synapse density in the hippocampus of the domestic chick

The bird hippocampus (Hp), although lacking the cellular lamination of the mammalian Hp, possesses comparable roles in spatial orientation and is implicated in passive avoidance learning. As in rodents it can be divided into dorsal and ventral regions based on immunocytochemical, tracing and electrophysiological studies. To study the effects of passive avoidance learning on synapse morphometry in the Hp, spine and shaft synapse densities of 1-day-old domestic chicks were determined in dorsal and ventral Hp of each hemisphere by electron microscopy, 6 and 24 h following training to avoid pecking at a bead coated with a bitter-tasting substance, methyl anthranilate (MeA). The density of asymmetric spine and shaft synapses in MeA-trained birds at 6 h post-training was significantly lower in the dorsal and ventral Hp of the right hemisphere relative to control (untrained) chicks, but by 24 h this difference was absent. A hemispheric asymmetry was apparent in the ventral Hp where the water-trained group showed enhanced shaft and spine synapse density in the left hemisphere, whilst in the MeA-trained group only asymmetric shaft synapses follow the same pattern in relation to the right hemisphere. There were no differences in asymmetric shaft synapses in the dorsal Hp at 6 h post-training, but at 24 h post-training there was a reduction in the density of shaft synapses in the right hemisphere in MeA compared with control birds. These data are discussed in relation to the pruning effects of stress and learning on synapse density in chick Hp.

Neural correlates of the proximity and quantity of anticipated food rewards in the ventral striatum of domestic chicks

To identify the neuro-cognitive substrates of valuation and choice, we analysed the neural correlates of anticipated food rewards in the ventral striatum of freely behaving chicks. One-week-old chicks were trained in a color-discrimination task using four color cues (red, yellow, green and blue), each of which was associated with a different food reward. Choosing a red bead was immediately rewarded with a large amount of food, choosing a yellow bead resulted in an immediate-small food reward, and choosing a green bead resulted in a late-large food reward. We selected chicks that consistently chose a large and immediate food reward (red over yellow, and red over green), with the proximity of the food valued higher than the size of the food reward (yellow over green). Of the 47 neurons recorded from the ventral striatum of these chicks, 20 neurons selectively showed cue-period responses to cues associated with food rewards. Five of these 20 neurons responded differentially during the cue period according to the expected delay to reward, and were thus assumed to code for the proximity of the reward. Additionally, three other neurons responded to the quantity of the reward. Furthermore, in the post-operant delay period, many of these 20 neurons showed reward-related activities that were linked to the proximity or presence of the food reward. We therefore propose that impulsive choice and behavioral perseveration observed after lesions of the ventral striatum could be due to impaired anticipation of rewards in the cue and delay periods, respectively.

Localized lesions of ventral striatum, but not arcopallium, enhanced impulsiveness in choices based on anticipated spatial proximity of food rewards in domestic chicks

The effects of bilateral chemical lesions of the ventral striatum (nucleus accumbens and the surrounding areas in the medial striatum) and arcopallium (major descending area of the avian telencephalon) were examined in 1-2-weeks old domestic chicks. Using a Y-maze, we analyzed the lesion effects on the choices that subject chicks made in two tasks with identical economical consequences, i.e., a small-and-close food reward vs. a large-and-distant food reward. In task 1, red, yellow, and green beads were associated with a feeder placed at various distances from the chicks; chicks thus anticipated the spatial proximity of food by the bead's color, whereas the quantity of the food was fixed. In task 2, red and yellow flags on the feeders were associated with various amount of food; the chicks thus anticipated the quantity of food by the flag's color, whereas the proximity of the reward could be directly visually determined. In task 1, bilateral lesions of the ventral striatum (but not the arcopallium) enhanced the impulsiveness of the chicks' choices, suggesting that choices based on the anticipated proximity were selectively changed. In task 2, similar lesions of the ventral striatum did not change choices. In both experiments, motor functions of the chicks remained unchanged, suggesting that the lesions did not affect the foraging efficiency, i.e., objective values of food. Neural correlates of anticipated food rewards in the ventral striatum (but not those in the arcopallium) could allow chicks to invest appropriate amount of work-cost in approaching distant food resources.

Social recognition memory requires two stages of protein synthesis in mice

Olfactory recognition memory was tested in adult male mice using a social discrimination task. The testing was conducted to begin to characterize the role of protein synthesis and the specific brain regions associated with activity in this task. Long-term olfactory recognition memory was blocked when the protein synthesis inhibitor anisomycin was injected 20 min before, immediately after, or 6 h after sampling. No effect was observed when anisomycin was administered 3 h or 18 h after sampling. Immunohistochemical analysis of Fos expression revealed that sampling-like exposure to a juvenile increased the activity of a subset of cells in the accessory olfactory bulb and the brain areas that are associated with it. Additionally, increased Fos expression was measured in the main olfactory bulb and the piriform cortex, whereas no signs of activation were seen in the cortical nucleus of the amygdala, all components of the main olfactory system. No increases in Fos immunoreactivity were observed after 4 h. Our data suggest that long-lasting olfactory recognition memory requires two stages of protein synthesis. The first stage takes place within 1-2 h and the second stage between 6-7 h after sampling. The first but not the second stage is paralleled by an increase in the number of Fos-immunoreactive cells in brain areas associated with both the main and accessory olfactory systems. It therefore appears that the role of the second stage of protein synthesis in recognition memory depends on the integrity of the first stage of protein synthesis.

Glycan analysis of the chicken synaptic plasma membrane glycoproteins--a major synaptic N-glycan carries the LewisX determinant

The majority of synaptic plasma membrane components are glycosylated. It is now widely accepted that this post-translational modification is crucial during the establishment, maintenance and function of the nervous system. Despite its significance, structural information about the glycosylation of nervous system specific glycoproteins is very limited. In the present study the major glycan structures of the chicken synaptic plasma membrane (SPM) associated glycoprotein glycans were determined. N-glycans were released by hydrazinolysis, labelled with 2-aminobenzamide, treated with neuraminidase and subsequently fractionated by size exclusion chromatography. Individual fractions were characterized by the combination of high-pressure liquid chromatography, exoglycosidase treatment or reagent array analysis method (RAAM). In addition to oligomannose-type glycans, core-fucosylated complex glycans with biantennary bisecting glycans carrying the LewisX epitope were most abundant. The overall chicken glycan profile was strikingly similar to the rat brain glycan profile. The presence of the LewisX determinant in relatively large proportions suggests a tissue-specific function for these glycans.

Spontaneous recovery after extinction of the conditioned proboscis extension response in the honeybee

In honeybees, the proboscis extension response (PER) can be conditioned by associating an odor stimulus (CS) to a sucrose reward (US). Conditioned responses to the CS, which are acquired by most bees after a single CS-US pairing, disappear after repeated unrewarded presentations of the CS, a process called extinction. Extinction is usually thought to be based either on (1) the disruption of the stored CS-US association, or (2) the formation of an inhibitory "CS-no US" association that is better retrieved than the initial CS-US association. The observation of spontaneous recovery, i.e., the reappearance of responses to the CS after time passes following extinction, is traditionally interpreted as a proof for the formation of a transient inhibitory association. To provide a better understanding of extinction in honeybees, we examined whether time intervals during training and extinction or the number of conditioning and extinction trials have an effect on the occurrence of spontaneous recovery. We found that spontaneous recovery mostly occurs when conditioning and testing took place in a massed fashion (1-min intertrial intervals). Moreover, spontaneous recovery depended on the time elapsed since extinction, 1 h being an optimum. Increasing the number of conditioning trials improved the spontaneous recovery level, whereas increasing the number of extinction trials reduced it. Lastly, we show that after single-trial conditioning, spontaneous recovery appears only once after extinction. These elements suggest that in honeybees extinction of the PER actually reflects the impairment of the CS-US association, but that depending on training parameters different memory substrates are affected.

Early socio-emotional experience induces expression of the immediate-early gene Arc/arg3.1 (activity-regulated cytoskeleton-associated protein/activity-regulated gene) in learning-relevant brain regions of the newborn chick

We have cloned a full-length complementary DNA from the chicken (Gallus gallus domesticus), which encodes a polypeptide that exhibits approximately 75% identity to the product of the mammalian gene Arc (activity-regulated cytoskeleton-associated protein), also known as arg3.1 (activity-regulated gene). Since this gene is an immediate-early gene that has been suggested to play a role in synaptic plasticity and learning and memory processes, its expression has been analyzed in a juvenile form of learning, namely, filial imprinting. Our results demonstrate that Arc/arg3.1 mRNA is detectable in the newborn chick brain, and that at this early age the level of this transcript can be altered by brief sensory/emotional experience. After postnatal exposure to a novel 30-min auditory imprinting stimulus, Arc/arg3.1 mRNA was found to be significantly increased in two higher associative areas, the mesopallium intermediomediale (P = 0.002) and the nidopallium dorso-caudale (P = 0.031), compared with naïve controls. The transcript level was also significantly elevated after imprinting in Area L pallii (P=0.045), which is analogous to the mammalian auditory cortex. In addition, increases were seen in the medio-rostral nidopallium/mesopallium (P = 0.054), which is presumed to be the analog of the mammalian prefrontal cortex, and the hyperpallium intercalatum (P = 0.054), but these did not quite reach significance. We discuss these data in the light of those obtained in an earlier study, in the same paradigm, for the avian immediate-early gene, zenk (an acronym for zif-268, egr-1, ngfi-a and krox-24, which are different names for the orthologous mammalian gene). We conclude that, although both the Arc/arg3.1 and zenk genes are induced by auditory imprinting, they are significantly up-regulated in different learning-relevant brain regions. It is, therefore, evident that they must be activated by different mechanisms.

Learning and memory

Learning and memory processes are thought to underlie a variety of human psychiatric disorders, including generalised anxiety disorder and post-traumatic stress disorder. Basic research performed in laboratory animals may help to elucidate the aetiology of the respective diseases. This chapter gives a short introduction into theoretical and practical aspects of animal experiments aimed at investigating acquisition, consolidation and extinction of aversive memories. It describes the behavioural paradigms most commonly used as well as neuroanatomical, cellular and molecular correlates of aversive memories. Finally, it discusses clinical implications of the results obtained in animal experiments in respect to the development of novel pharmacotherapeutic strategies for the treatment of human patients.

Excitotoxic lesions of the medial striatum delay extinction of a reinforcement color discrimination operant task in domestic chicks; a functional role of reward anticipation

To reveal the functional roles of the striatum, we examined the effects of excitotoxic lesions to the bilateral medial striatum (mSt) and nucleus accumbens (Ac) in a food reinforcement color discrimination operant task. With a food reward as reinforcement, 1-week-old domestic chicks were trained to peck selectively at red and yellow beads (S+) and not to peck at a blue bead (S-). Those chicks then received either lesions or sham operations and were tested in extinction training sessions, during which yellow turned out to be nonrewarding (S-), whereas red and blue remained unchanged. To further examine the effects on postoperant noninstrumental aspects of behavior, we also measured the "waiting time", during which chicks stayed at the empty feeder after pecking at yellow. Although the lesioned chicks showed significantly higher error rates in the nonrewarding yellow trials, their postoperant waiting time gradually decreased similarly to the sham controls. Furthermore, the lesioned chicks waited significantly longer than the controls, even from the first extinction block. In the blue trials, both lesioned and sham chicks consistently refrained from pecking, indicating that the delayed extinction was not due to a general disinhibition of pecking. Similarly, no effects were found in the novel training sessions, suggesting that the lesions had selective effects on the extinction of a learned operant. These results suggest that a neural representation of memory-based reward anticipation in the mSt/Ac could contribute to the anticipation error required for extinction.

Origin of the anterior forebrain pathway

The brain nuclei and pathways comprising the song system of oscine songbirds bear many similarities with circuits in other bird species and in mammals. This suggests that the song system evolved as a specialization of pre-existing circuits and may retain fundamental properties in common with those of other taxa. Here we review evidence for these similarities, including electrophysiological, morphological, and neurochemical data for identifying specific cell types. In addition, we discuss connectional data, addressing similarities in axonal projections among nuclei across taxa. We focus primarily on the anterior forebrain pathway, a circuit essential for song learning and vocal plasticity, because the evidence is strongest that this circuit is homologous to mammalian circuits. These fundamental similarities highlight the importance of comparative approaches; for example, understanding the role the anterior forebrain pathway plays in song plasticity may shed light on general principles of basal ganglia function. In addition, understanding specializations of such circuits in songbirds may illuminate specific innovations critical for vocal learning.

The brain angiotensin system and extracellular matrix molecules in neural plasticity, learning, and memory

The brain renin-angiotensin system (RAS) has long been known to regulate several classic physiologies including blood pressure, sodium and water balance, cyclicity of reproductive hormones and sexual behaviors, and pituitary gland hormones. These physiologies are thought to be under the control of the angiotensin II (AngII)/AT1 receptor subtype system. The AT2 receptor subtype is expressed during fetal development and is less abundant in the adult. This receptor appears to oppose growth responses facilitated by the AT1 receptor, as well as growth factor receptors. Recent evidence points to an important contribution by the brain RAS to non-classic physiologies mediated by the newly discovered angiotensin IV (AngIV)/AT4 receptor subtype system. These physiologies include the regulation of blood flow, modulation of exploratory behavior, and a facilitory role in learning and memory acquisition. This system appears to interact with brain matrix metalloproteinases in order to modify extracellular matrix molecules thus permitting the synaptic remodeling critical to the neural plasticity presumed to underlie memory consolidation, reconsolidation, and retrieval. There is support for an inhibitory influence by AngII activation of the AT1 subtype, and a facilitory role by AngIV activation of the AT4 subtype, on neuronal firing rate, long-term potentiation, associative and spatial learning. The discovery of the AT4 receptor subtype, and its facilitory influence upon learning and memory, suggest an important role for the brain RAS in normal cognitive processing and perhaps in the treatment of dysfunctional memory disease states.

Cortical synaptogenesis and motor map reorganization occur during late, but not early, phase of motor skill learning

Extensive motor skill training induces reorganization of movement representations and synaptogenesis within adult motor cortex. Motor skill does not, however, develop uniformly across training sessions. It is characterized by an initial fast phase, followed by a later slow phase of learning. How cortical plasticity emerges during these phases is unknown. Here, we examine motor map topography and synapse number within rat motor cortex during the early and late phases of motor learning. Adult rats were placed in either a skilled or unskilled reaching condition (SRC and URC, respectively) for 3, 7, or 10 d. Intracortical microstimulation of layer V was used to determine the topography of forelimb movement representations within caudal forelimb area of motor cortex contralateral to the trained paw. Quantitative electron microscopy was used to measure the number of synapses per neuron within layer V. SRC animals showed significant increases in reaching accuracy after 3, 7, and 10 d of training. In comparison with URC animals, SRC animals had significantly larger distal forelimb representations after 10 d of training only. Furthermore, SRC animals had significantly more synapses per neuron than URC animals after 7 and 10 d of training. These results show that both motor map reorganization and synapse formation occur during the late phase of skill learning. Furthermore, synaptogenesis precedes map reorganization. We propose that motor map reorganization and synapse formation do not contribute to the initial acquisition of motor skills but represent the consolidation of motor skill that occurs during late stages of training.

Cyclin S: a new member of the cyclin family plays a role in long-term memory

Memory is thought to be subserved by structural and functional alteration in synaptic connectivity. But although neuronal plasticity requires gene expression, the identity of the proteins involved is largely unknown. Using the chick 1-day-old passive avoidance learning paradigm and differential display RNA fingerprinting, we identified 13 candidate genes which are upregulated in the intermediate medial hyperstriatum ventrale (IMHV), an area that has been correlated with the initial processing of memory formation. One of the induced genes is a new member of the cyclin family, with high homology to cyclin L (ania-6a). Analysis of the expression pattern of this gene after training revealed two time waves of induction: the first correlated with learning and initial memory process in the IMHV; the second correlated with memory consolidation, first in the IMHV, and then in the lobus paraolefactoris. There is a correlation between methylanthranilate (MeA) concentrations (the malaise substrate in the passive avoidance training procedure), the duration of memory and the expression level of cyclin S. While training chicks on low concentrations of MeA causes short-term memory and low expression level of cyclin S, high concentration of MeA induces long-term memory and high expression level of cyclin S in the IMHV. The role of cyclins in the regulation of neuronal-plasticity-related gene expression was overlooked, and it might serve as a key step in long-term memory formation.

Effects of nitric oxide inhibition on avoidance learning in the chick are lateralized and localized

Bilateral administration of nitric oxide synthase inhibitors into the intermediate medial hyperstriatal (IMHV) region of the chick brain impairs memory formation for an avoidance task. The aim of the current study was to determine whether this effect was restricted to a particular location in the brain, and whether inhibition was equally effective in both hemispheres. White Leghorn x black Australorp chicks were administered 0.5 mM N(omega)-Nitro-L-arginine methyl ester bilaterally into the lobus parolfactorius (LPO), or unilaterally into the IMHV. Injections into the LPO between 5 min pre-training and 40 min post-training had no effect on retention. In contrast, unilateral injections into the IMHV impaired retention and memory loss occurred from 40 min post-training. The effective administration time was hemisphere-dependent, requiring left hemisphere administration around the time of training and right hemisphere administration between 15 and 25 min post-training. These data suggest that localized nitric oxide activity in each hemisphere of the chick brain is necessary for the consolidation of memory for this task.

Neural correlates of memorized associations and cued movements in archistriatum of the domestic chick

The archistriatum mediates a neural pathway from the medial part of intermediate hyperstriatum ventrale (in the dorsal pallium) to the lobus parolfactorius (in the medial striatum), thus is possibly involved in memory formation in the domestic chick. To elucidate the functional roles, we examined single neuron activities from archistriatum in unconstrained chicks during execution of a GO/NOGO task. In this task, a brief motor sound was given as initial cue, and immediately followed by presentation of a coloured bead. Chick was required to recall the memorized associations between the colour and reward, and pecked at the bead to gain food after a delay (GO trials) or stayed not pecking (NOGO trials). The ventral part of intermediate archistriatum proved to contain a group of neurons that selectively responded to the reward-associated colours before the reward was actually presented, possibly coding the memorized associations. Another group of neurons fired during the reward period, thus could code aspects of the food reward. Yet another group of neurons started to fire immediately on the cue sound and prior to the cued movements nonselectively in both GO and NOGO trials, thus could be involved in the sensori-motor link between the sound and the targeted body movements. It is concluded that even a subregion of archistriatum contains diverse neural codes for memorized associations and food rewards, and neural codes of movements cued by sounds, suggesting that archistriatum is a complex of different functional systems, possibly corresponding to striatum, limbic amygdala, and prefrontal cortex in mammals.

The mind through chick eyes: memory, cognition and anticipation

To understand the animal mind, we have to reconstruct how animals recognize the external world through their own eyes. For the reconstruction to be realistic, explanations must be made both in their proximate causes (brain mechanisms) as well as ultimate causes (evolutionary backgrounds). Here, we review recent advances in the behavioral, psychological, and system-neuroscience studies accomplished using the domestic chick as subjects. Diverse behavioral paradigms are compared (such as filial imprinting, sexual imprinting, one-trial passive avoidance learning, and reinforcement operant conditioning) in their behavioral characterizations (development, sensory and motor aspects of functions, fitness gains) and relevant brain mechanisms. We will stress that common brain regions are shared by these distinct paradigms, particularly those in the ventral telencephalic structures such as AIv (in the archistriatum) and LPO (in the medial striatum). Neuronal ensembles in these regions could code the chick's anticipation for forthcoming events, particularly the quality/quantity and the temporal proximity of rewards. Without the internal representation of the anticipated proximity in LPO, behavioral tolerance will be lost, and the chick makes impulsive choice for a less optimized option. Functional roles of these regions proved compatible with their anatomical counterparts in the mammalian brain, thus suggesting that the neural systems linking between the memorized past and the anticipated future have remained highly conservative through the evolution of the amniotic vertebrates during the last 300 million years. With the conservative nature in mind, research efforts should be oriented toward a unifying theory, which could explain behavioral deviations from optimized foraging, such as "naïve curiosity," "contra-freeloading," "Concorde fallacy," and "altruism."

Localized lesion of caudal part of lobus parolfactorius caused impulsive choice in the domestic chick: evolutionarily conserved function of ventral striatum

Effects of bilateral chemical lesions of the medial basal ganglia [lobus parolfactorius (LPO)] were examined in 7- to 14-d-old domestic chicks. Chicks were trained in a color discrimination task, in which the subject had to peck one of the two colored beads associated with rewards that differed in quantity (amount of food) and/or temporal proximity (delay of food delivery from peck). In experiment 1, food was given without delay, and chicks successfully learned to choose a colored bead that was associated with a larger reward than the other. In experiment 2, a colored bead (red) was associated with a large reward delivered after a delay (D = 1, 2, or 3 sec), whereas another (yellow) was associated with a small reward delivered immediately. In intact and sham-operated conditions, chicks with a longer D chose the red bead progressively fewer times. Selective lesions to the caudal LPO (but not the rostral LPO) caused impulsive choice, and the ablated chicks chose the yellow bead and gained a small-immediate reward regardless of D. However, when retrained in a null-delay condition (D = 0 sec), the lesioned chick chose the red bead again. Ability to associate novel colors with reward was also unimpaired. These results suggest that the LPO may be responsible for the anticipation of reward proximity and involved in a suppression of impulsiveness by which animals seek immediate gains. The present results also indicate a striking similarity in functional roles between the avian LPO and the nucleus accumbens/ventral striatum in mammals.

Lesions of the ventro-medial basal ganglia impair the reinforcement but not the recall of memorized color discrimination in domestic chicks

Effects of bilateral chemical lesions of the ventro-medial basal ganglia (lobus parolfactorius, LPO) were examined in 3-9-day-old domestic chicks. In experiment-1, chicks were trained to peck at a blue bead that was associated with drops of water as a reward. Addition of passive avoidance training using a bitter yellow bead resulted in highly selective pecking between blue and yellow. LPO lesion (given 3-5 h after training) did not impair the selectivity when chicks were tested 24 h afterwards, while the novel reinforcement using a red bead was severely impaired. In experiment-2, chicks were trained in a GO/NO-GO color discrimination task with food reward. Trained chicks received bilateral LPO lesions, and they were tested 48 h afterwards for the number of pecks and latency of the first peck in each trial. The LPO lesion did not impair the recall of memorized color discrimination in tests, while the chicks were severely deficient in post-operative novel training. These results confirm that: (1) bilateral LPO ablation does not interfere with selective pecking based on the memorized color cues; but (2) it impairs reinforcement in novel training. LPO is thus supposed to be involved in acquisition, rather than execution of memorized behaviors.

Passive avoidance training enhances cell proliferation in 1-day-old chicks

One-day-old domestic chicks were injected i.p. with bromodeoxyuridine (BrdU) before training on a one-trial passive avoidance task where the aversive experience was a bead coated with a bitter tasting substance, methyl anthranilate (MeA). Animals were tested 24 h later; those avoiding (if MeA-trained) or pecking if water (W)-trained (which they peck appetitively), along with a group of untrained naïve chicks, were used to determine cell proliferation either 24 h or 9 days post BrdU injection. In all three groups, BrdU positive cells were identified sparsely throughout the forebrain but labelling was pronounced around ventricular zone (VZ) surfaces at both 24 h and 9 days post-BrdU-injection. Double immunolabelling with neuronal specific antibodies, to either NeuN, or beta-tubulin III, confirmed that most BrdU labelled cells appeared to be neurons. Unbiased stereological analysis of labelled cells in selected forebrain areas 24 h post BrdU injection showed a significant MeA-training induced increase in labelled cells in both the dorsal VZ surface bordering the intermediate and medial hyperstriatum ventrale (IMHV) and the tuberculum olfactorium (TO). By 9 days post-BrdU-injection, there was a significantly greater number of BrdU labelled cells in MeA-trained birds within the IMHV, lobus parolfactorius (LPO) and TO. These results demonstrate that avoidance training in 1-day-old chicks has a marked effect on cell proliferation, in the LPO and IMHV, regions of the chick previously identified as a key loci of memory formation, and in a second region (TO), which has olfactory functions, but has not been previously investigated in relation to avoidance learning.

Fibronectin domains of extracellular matrix molecule tenascin-C modulate hippocampal learning and synaptic plasticity

The extracellular matrix molecule tenascin-C (TN-C) has been shown to be involved in hippocampal synaptic plasticity in vitro. Here, we describe a deficit in hippocampus-dependent contextual memory in TN-C-deficient mice using the step-down avoidance paradigm. We further show that a fragment of TN-C containing the fibronectin type-III repeats 6-8 (FN6-8), but not a fragment containing repeats 3-5, bound to pyramidal and granule cell somata in the hippocampal formation of C57BL/6J mice and repelled axons of pyramidal neurons when presented as a border in vitro. Injection of the FN6-8 fragment into the hippocampus inhibited retention of memory in the step-down paradigm and reduced levels of long-term potentiation in the CA1 region of the hippocampus. In summary, our data show that TN-C is involved in hippocampus-dependent contextual memory and synaptic plasticity and identify the FN6-8 domain as one of molecular determinants mediating these functions.

Role of adrenoceptor subtypes in memory consolidation

Noradrenaline release in areas within the forebrain occurs following activation of noradrenergic cells in the locus coeruleus (LoC). Release of noradrenaline by attentional/arousal/vigilance factors appears to be essential for learning and is responsible for the consolidation of memory. Noradrenaline can activate any of nine different adrenoceptor (AR) subtypes in the brain and selectivity of action may be achieved by the spatial location and relative density of the AR subtypes, by different affinities of the different subtypes and by temporal selectivity in terms of when the different ARs are activated in the memory formation process. This review examines the use of selective agonists and antagonists to determine the roles of the AR subtypes in the one-trial discriminated avoidance learning paradigm in the chick. A model is developed that integrates noradrenergic activity in basal ganglia (lobus parolfactorius (LPO)) and association cortex (intermediate medial hyperstriatum ventrale (IMHV)) leading to the consolidation of memory 30 min after training. There is evidence that beta(2)- and beta(3)-ARs are important in the association area but require input from alpha(2)-AR stimulated activity in the basal ganglia for consolidation. On the other hand, alpha(1)-AR activation in the IMHV is inhibitory and prevents consolidation. While there is no role for beta(1)-ARs in memory consolidation, they play a role in short-term memory (STM). The use of the precocial chick has clear advantages in having a temporally discrete learning task which allows for discrimination memory and whose development can be followed at discrete intervals after learning. These studies reveal clear roles for AR subtypes in the formation and consolidation of memory in the chick, which have allowed the development of a model that can now be tested in mammalian systems.

Estimation of numerical density and mean synaptic height in chick hippocampus 24 and 48 hours after passive avoidance training

The effects of passive avoidance learning on synaptic morphology and number in the dorsolateral hippocampus of chick were investigated at 24 and 48 h after training. Chicks of both sexes were used. The numerical density of synapses and mean synaptic height were determined using design-based quantitative electron microscopic techniques. Our results suggest that after training there is a significant increase in synaptic density in the dorsolateral hippocampus of chicks at both 24 and 48 h, and also that the mean synaptic height was significantly different between trained and control groups. The increase in synaptic density was due to shaft (type II) synapses. It is known that during synaptogenesis, shaft synapses are formed first and are then converted to spine synapses. The only hemispheric asymmetry was found in the 24 h water-trained (W-trained) males where the numerical density of spine synapses was significantly higher in the left hippocampus. No significant differences due to gender in either numerical synaptic density or synapse height were observed at either 24 and 48 h. Comparison of the 24 h with 48 h groups showed an increase in shaft synaptic density over time in the W-trained groups, and an increased density of both shaft and spine synapses with time in methylanthranilate-trained (MeA-trained) chicks. These results demonstrate that the dorsolateral hippocampus of the chick shows synaptic changes at both 24 and 48 h after training and implicates this region in the long-term memory process.

Rapid ocular dominance plasticity requires cortical but not geniculate protein synthesis

Synaptic plasticity is a multistep process in which rapid, early phases eventually give way to slower, more enduring stages. Diverse forms of synaptic change share a common requirement for protein synthesis in the late stages of plasticity, which are often associated with structural rearrangements. Ocular dominance plasticity in the primary visual cortex (V1) is a long-lasting form of activity-dependent plasticity comprised of well-defined physiological and anatomical stages. The molecular events underlying these stages remain poorly understood. Using the protein synthesis inhibitor cycloheximide, we investigated a role for protein synthesis in ocular dominance plasticity. Suppression of cortical, but not geniculate, protein synthesis impaired rapid ocular dominance plasticity, while leaving neuronal responsiveness intact. These findings suggest that structural changes underlying ocular dominance plasticity occur rapidly following monocular occlusion, and cortical changes guide subsequent alterations in thalamocortical afferents.

Dehydroepiandosterone and its sulphate enhance memory retention in day-old chicks

We report the presence of dehydroepiandosterone (DHEA) and DHEA sulphate (DHEA-S) in the day-old-chick brain, and their possible role in memory formation. DHEA and DHEA-S were present in the brain at higher concentrations than in the plasma. Radioimmunoassay examination of the intermediate medial hyperstriatum ventrale 5 or 30 min after training or the lobus parolfactorius 60 or 120 min after training on the passive avoidance task did not show learning-related differences in absolute levels of DHEA or DHEA-S. However, bilateral intracerebral injections of DHEA or DHEA-S before or after training on the weak passive avoidance task enhanced recall 24 h after training. Memory retention was enhanced by administration of DHEA and DHEA-S 15 min before training or 30 and 60 but not 180 min after training. Neurosteroids are present in high concentrations in regions of the chick brain known to be associated with learning and memory for an aversive one-trial task. Our study demonstrates that memory retention for this task is enhanced by administration of the neurosteroids DHEA-S and DHEA. These findings provide additional evidence that these neurosteroids have memory-enhancing properties and, thus, if common to other tasks and species, that DHEA-S and DHEA may constitute potential therapeutic tools for the treatment of cognitive deficits.

Amino acid release from the intermediate medial hyperstriatum ventrale (IMHV) of day-old chicks following a one-trial passive avoidance task

Indirect evidence has implicated glutamate and gamma-amino butyric acid in memory formation for one-trial passive avoidance learning. We have further examined this by following the time course of glutamate and gamma-amino butyric acid release from slices prepared from the intermediate medial hyperstriatum ventrale of day-old chicks (Ross 1 Chunky) trained to avoid a bead covered in the aversant methylanthranilate. At various times after training, slices of left and right intermediate medial hyperstriatum ventrale were incubated in medium containing 50 mM potassium chloride and amino acid release was determined. Thirty minutes after training there was a bilateral increase in calcium-dependent glutamate release in slices from methylanthranilate-trained chicks compared to those trained to peck water. This increase was sustained until 1 h in the left hyperstriatum when an increase in calcium-dependent gamma-amino butyric acid release was also apparent. Glutamate uptake was also enhanced in left hyperstriatum (30 and 60 min) and in the right at 30 min. In the right intermediate medial hyperstriatum ventrale of methylanthranilate birds glutamate release was increased from 3 to 6.5 h and gamma-amino butyric acid at 6.5 h: a time that corresponded to the mobilization of a late process required if long-term memory was to be formed. These results confirm that the amino acids glutamate and gamma-amino butyric acid are released from the intermediate hyperstriatum ventrale in a calcium-dependent, neurotransmitter-like manner. Furthermore, changes in the release of these two amino acids accompany memory formation for a one-trial learning task in the day-old chick.

Cyclosporin A, FK506 and rapamycin produce multiple, temporally distinct, effects on memory following single-trial, passive avoidance training in the chick

Few studies have used a pharmaco-behavioural methodology to directly investigate roles for the calcium-dependent protein phosphatase calcineurin (CaN) in memory formation, due partly to the absence of specific inhibitory agents. A number of drugs with different inhibitory profiles were used to examine this issue in groups of chicks trained on a single-trial, passive-avoidance task. Bilateral intracranial administration of the immunosuppressants FK506 and cyclosporin A (CyA) led to two temporally distinct effects, distinguished by the concentration of drug required and the effective time of administration relative to training. In addition to inhibiting CaN, CyA and FK506 inhibit distinct classes of peptidyl prolyl-cis/trans-isomerases (PPIases). Other agents known to inhibit these enzymes, including the Map kinase inhibitor Rapamycin, also induced memory deficits in a complex, dose- and time-of-administration-dependent, manner. The data fail to conclusively implicate CaN in memory formation, but are consistent with proposals that a phosphatase cascade may participate in an early stage of information storage. PPIases may be required at a later stage to catalyse the folding of new or translocated proteins, the synthesis of which is required for formation of long-term memory, although other possible explanations for the data remain to be investigated.

D1-receptor dependent synaptic potentiation in the basal ganglia of quail chicks

Properties of local synapses were analyzed in lobus parolfactorius (LPO; avian homologue of caudate-nucleus) of quail chicks by using slice preparations in vitro. Field-potential extracellular and whole-cell intracellular recordings revealed excitatory synaptic inputs converging from dorsal and ventral regions within LPO. With exogenous dopamine (100 microM) in the perfusate, synchronized conditioning stimulation induced biased changes in the dorsal and the ventral inputs; potentiation in the dorsal input and depression in the ventral input in average. On the other hand, de-synchronized conditioning failed to induce such biased changes, although the differences were not statistically significant. SCH-23390 (3 microM) blocked the dorsal potentiation, while AP-5 (100 microM) tended to block both of these changes. The plastic nature may underlie the memory formation in appetitive/aversive learning tasks.

Odor exposure causes central adaptation and morphological changes in selected olfactory glomeruli in Drosophila

In an attempt to correlate behavioral and neuronal changes, we examined the structural and functional effects of odor exposure in Drosophila. Young adult flies were exposed to a high concentration of the selected odor, usually benzaldehyde or isoamyl acetate, for 4 d and subsequently tested for their olfactory response to a variety of odorants and concentrations. The behavioral response showed specific adaptation to the exposed odor. By contrast, olfactory transduction, as measured in electroantennograms, remained normal. In vivo volume measurements were performed on olfactory glomeruli, the anatomical and functional units involved in odor processing. Pre-exposed flies exhibited volume reduction of certain glomeruli, in an odor-selective manner. Of a sample of eight glomeruli measured, dorsal medial (DM) 2 and ventral (V) were affected by benzaldehyde exposure, whereas DM6 was affected by isoamyl acetate. Estimation of the number of synapses indicates that volume reduction involves synapse loss that can reach 30% in the V glomerulus of flies adapted to benzaldehyde. Additional features of odorant-induced adaptation, including concentration dependence and perdurance, also show correlation, because both effects are elicited by high odor concentrations and are long-lasting (>1 week). Finally, the dunce mutant fails to develop behavioral adaptation as well as morphological changes in the olfactory glomeruli after exposure. These neural changes thus appear to require the cAMP signaling pathway.

Intrahippocampal administration of an antibody against the HNK-1 carbohydrate impairs memory consolidation in an inhibitory learning task in mice

Many cell adhesion molecules express the HNK-1 carbohydrate involved in formation and functioning of synapses. To assess its role in learning, we injected the monoclonal HNK-1 antibody or nonimmune IgG into the hippocampus of C57BL/6J mice 1 h after training in a step-down avoidance task. In animals treated with the HNK-1 antibody, latencies of step down in a recall session 48 h after injection did not change compared to training values and were significantly shorter versus IgG-treated controls, which acquired the task normally. Similar differences between the two treatments were also observed after a stronger training protocol in a step-down avoidance paradigm. The HNK-1 antibody was effective only when injected 1 h, but not 48 h after training, thus affecting memory consolidation but not memory recall itself. The HNK-1 antibody impaired memory also in tenascin-R knock-out mice, indicating that extracellular matrix molecule tenascin-R, one of the carriers of the HNK-1epitope in the hippocampus, does not mediate the function of the HNK-1 carbohydrate in this task. Our observations show that the HNK-1 carbohydrate is critically involved in memory consolidation in hippocampus-dependent learning in mammals.

Long-term memory formation in the chick requires mobilization of ryanodine-sensitive intracellular calcium stores

Training chicks (Gallus domesticus) on a one-trial passive avoidance task results in transient and time-dependent enhanced increases in N-methyl-d-aspartate- or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-stimulated intracellular calcium concentration in synaptoneurosomes isolated from a specific forebrain region, the intermediate medial hyperstriatum ventrale. This increase could result from either calcium entry from the extracellular medium or from mobilization of intracellular calcium stores. We have therefore examined the effects of dantrolene, an inhibitor of calcium release from the intracellular ryanodine-sensitive store, on these processes. Dantrolene, 50 nmol per hemisphere injected intracerebrally 30 min pre- or 30 min posttraining, blocked longer term memory for the passive avoidance task, whereas memory for the task was unaffected when dantrolene was injected at earlier or later times. Preincubation of synaptoneurosomes, isolated from the intermediate hyperstriatum ventrale 10 min after training, with 100 nM dantrolene abolished the enhanced training-induced increase in intracellular calcium concentration elicited by 0.5 mM N-methyl-d-aspartate. By contrast, the training-induced enhancement of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-stimulated increase in intracellular calcium concentration in synaptoneurosomes prepared 6 h posttraining was unaffected by preincubation with dantrolene, which was not amnestic at this time. Calcium release from ryanodine-sensitive intracellular stores may thus be a necessary stage in the early phase of the molecular cascade leading to the synaptic modulation required for long-term memory storage.

Participation of hippocampal metabotropic glutamate receptors, protein kinase A and mitogen-activated protein kinases in memory retrieval

The ability to recall past events is a major determinant of survival strategies in all species and is of paramount importance in determining our uniqueness as individuals. In contrast to memory formation, the information about the molecular mechanisms of memory retrieval is surprisingly scarce and fragmentary. Here we show that pretest inhibition of the specific upstream activator of mitogen-activated protein kinase kinase, or of protein kinase A in the hippocampus, blocked retrieval of long-term memory for an inhibitory avoidance task, a hippocampal-dependent learning task. An activator of protein kinase A enhanced retrieval. Mitogen-activated protein kinase activation increased in the hippocampus during retrieval, while protein kinase A activity remained unchanged. Pretest intrahippocampal blockade of metabotropic glutamate receptors or alpha-amino-3-hydroxy-5-methyl-4-isoxazolone propionic acid/kainate receptors, but not N-methyl-D-aspartate receptors or calcium/calmodulin dependent-protein kinase II, impaired retrieval. Thus, recall of inhibitory avoidance activates mitogen-activated protein kinase, which is necessary, along with metabotropic glutamate receptors, alpha-amino-3-hydroxy-5-methyl-4-isoxazolone propionic acid/kainate receptors, and protein kinase A, for long-term memory expression. Our results indicate that memory formation and retrieval may share some molecular mechanisms in the hippocampus.

God's organism? The chick as a model system for memory studies

The young chick is a powerful model system in which to study the biochemical and morphological processes underlying memory formation. Training chicks on a one trial passive avoidance task results in a molecular cascade in a specific brain region, the intermediate medial hyperstriatum ventrale. This cascade is initiated by glutamate release and engages a series of synaptic transients including increased calcium flux, up-regulation of NMDA-glutamate receptors, membrane protein phosphorylations, and the retrograde messenger NO. Expression of immediate early genes c-fos and c-jun precedes the synthesis, glycosylation, and redistribution, >4 hr downstream, of a number of synaptic membrane proteins, notably NCAM and L1. Other membrane proteins required in the early phase of memory formation include the amyloid precursor protein (APP) and apolipoprotein E. There are concomitant increases in dendritic spine number and changes in synaptic structure. Nonsynaptic factors, including corticosterone and BDNF, can modulate retention of the avoidance response, enhancing the salience of otherwise weakly retained memory. These results are discussed in relation to general concepts of memory formation and the spatio-temporal distribution of the putative memory trace.