Hemispheric asymmetry of synapses in chick medial hyperstriatum ventrale following passive avoidance training: a stereological investigation

Ouabain does not impair reconsolidation following a reminder of passive avoidance learning in the day-old chick

This study investigated the effect of ouabain, an inhibitor of NA(+) and K(+) ATPase, on consolidation and reconsolidation of a passive avoidance learning task in the day-old chick. In the consolidating trace, ouabain is thought to inhibit an intermediate-term memory phase, some aspects of which acts as a "trigger" for long-term stabilisation of the trace by new protein synthesis. It was hypothesised that a similar process may initiate the protein synthesis observed following reminder-activated reconsolidation in the young chick. Chicks were trained on a single trial passive avoidance task. A dose of 0.2 ug/kg ouabain was administered intracranially either 5 min post-training (consolidation processes) or 5 min post-reminder (reconsolidation processes). Consistent with previous research, ouabain administration induced a memory deficit following the initial learning trial. However, contrary to expectation, ouabain did not disrupt memory processing post-reactivation. This finding provides further evidence for the notion that consolidation and reconsolidation are associated with similar, but distinct, stages of processing.

Prenatal hypoxia impairs memory function but does not result in overt structural alterations in the postnatal chick brain

We showed previously that hypoxia in ovo impairs memory consolidation in the chick tested 2 days after hatching. Our present aim was to investigate whether we could detect any morphological effects of the same prenatal hypoxia. Hypoxia was induced by half-wrapping the egg with an impermeable membrane from either days 10-18 (W10-18 chicks) or days 14-18 (W14-18 chicks) of incubation (hatching approximately 21 days). Measurement of blood gases showed that reducing the surface area of the egg for gas exchange resulted in reduced pO2 and increased pCO2 2 days after wrapping. Although this hypoxia was sufficient to impair cognitive processing in the postnatal chick, our data suggest that it did not produce overt structural alterations or changes in the number of neurons, glutamine synthetase-immunoreactive cells or immunoreactivity to synaptophysin in the presynaptic vesicles in the multimodal integration (cortical) area compared to controls. Hence, we found no differences in the astrocyte to neuron ratio, synaptic density and/or vesicle number. Analysis of the ontogeny of astrocytes during the prenatal period of hypoxia showed them to be present at embryonic day 12, but not at the earlier ages examined. Although we found cognitive deficits in chicks from embryos made hypoxic during incubation, our regimen of prenatal hypoxia did not alter any of the parameters measured in the brains. This does not preclude the possibility that changes have occurred at the cellular or molecular levels or in specific neurotransmitter systems.

Quantitative Autoradiographic Demonstration of Changes in Binding to NMDA-sensitive [3H]Glutamate and [3H]MK801, but not [3H]AMPA Receptors in Chick Forebrain 30 min After Passive Avoidance Training

Day-old domestic chicks (Gallus domesticus) were trained on a one-trial passive avoidance task in which the aversive stimulus was an unpleasant tasting substance, methyl anthranilate. Thirty minutes later, localization of N-methyl-d-aspartic acid (NMDA)-sensitive [3H]glutamate receptor binding sites, [3H]MK801 and [3H]AMPA binding sites in 17 regions of the forebrain of methylanthranilate-trained and control (water-trained) chicks was determined using quantitative receptor autoradiography. Significant differences in binding to both MK801- and NMDA-sensitive glutamate receptors, but not alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, were found in three regions of the forebrain of trained compared to control chicks; two of these regions have been implicated from previous lesion, biochemical and morphological studies as playing a key role in the process of memory formation and storage following passive avoidance training. For NMDA-sensitive [3H]glutamate receptors, significant elevations in binding were observed in two regions, the left intermediate and medial hyperstriatum ventrale (IMHV) (39%) and the lobus parolfactorius (LPO) (34%), at 30 min post-training, but a decrease (44%) occurred in binding to the lateral neostriatum. Significant increases in binding to MK801 receptors were observed in the left IMHV (19%) and right IMHV (13%), and left LPO (22%) at 30 min post-training, though there was a decrease in the right LPO (15%). These findings, coupled with those described in a previous paper from our group (Burchuladze and Rose, Eur. J. Neurosci., 4, 533 - 538, 1992), demonstrate that a glutamate receptor subtype is involved in learning and memory formation in the chick.

Cerebral Glycoprotein Synthesis and Long-term Memory Formation in the Chick (Gallus domesticus) Following Passive Avoidance Training Depends on the Nature of the Aversive Stimulus

Chicks that peck a small bright bead coated in a distateful substance can learn in a single trial to subsequently avoid a similar bead. The taste aversant commonly used is methyl anthranilate, which also has a strong pervasive odour. We have compared the efficacy of methyl anthranilate and the apparently odourless quinine as aversants. Methyl anthranilate-trained chicks learnt the task and the memory apparently persisted undiminished for at least 24 h. Quinine-trained chicks exhibited a memory for the task similar to that of methyl anthranilate-trained chicks 45 min after training, this thereafter declined until, at 24 h after training, they showed no recall. We investigated the incorporation of a radio-labelled synaptic membrane glycoprotein precursor, [3H]fucose, into three regions of the chick forebrain; two of these regions have previously been implicated in learning using methyl anthranilate as the aversant. There was a significant increase in [3H]fucose incorporation into the left lateral cerebral area and numerically similar, but non-significant, increases in the intermediate part of the medial hyperstriatum ventrale and lobus parolfactorius. There were no such increases in the right hemisphere of methyl anthranilate-trained chicks or any region of either hemisphere of quinine-trained chicks. Thus, the memory for methyl anthranilate is longer-lasting than that for quinine and is associated with increased fucosylation in the left cerebral hemisphere and although in the short-term, chicks can retain a memory of the one-trial passive avoidance task with quinine as the aversant, this does not result in a localized increase in cerebral [3H]fucose incorporation.

Sequential training in separate paradigms impairs second task consolidation and learning-associated modulations of hippocampal NCAM polysialylation

As transient and time-dependent modulations of neural cell adhesion molecule polysialylation (NCAM PSA) are associated with morphofunctional change and required for the consolidation of spatial and nonspatial forms of learning, we determined the demands imposed on this system by sequential training in the Morris water maze followed by the passive avoidance paradigm. Animals trained in this manner had recall of the water maze but not the passive avoidance response as judged by their escape and avoidance latencies, respectively. Activation of NCAM PSA on dentate neurons at the 12-h post-training time suggested information processing; however, this was significantly less than that predicted for coincident acquisition of both tasks. When sequential training was separated by an interparadigm period of 2 h, an enduring NCAM PSA activation was observed which was indistinguishable from the sum of the expected activations for each individual task. These observations suggest that the NCAM PSA response may become saturated when alternate tasks are presented without an intervening period.

Visual imprinting and the neural mechanisms of recognition memory

To understand the neural bases of memory it is necessary to localize the regions storing information. Part of the hyperstriatum ventrale (IMHV) serves such a function for the learning process of imprinting in domestic chicks. Chicks exposed to an object learn its characteristics, and in doing so, the responsiveness of IMHV neurones to that object is selectively enhanced. Imprinting is associated with both pre- and postsynaptic changes in the region. Postsynaptic changes involve increases in the length of the postsynaptic density on dendritic spines and in the numbers of NMDA receptors; presynaptically, converging evidence points to an early and persistent enhancement of neurotransmitter release. Increases in the amounts of certain neural cell adhesion molecules a day after training might serve to stabilize the synaptic changes associated with a particular memory by strengthening pre- to postsynaptic adhesion, and by more strongly interconnecting the cytoskeletal frameworks of the dendritic spine and the synaptic terminal. Learning-related increases in the number of neurones staining positive for the transcription factor Fos in the IMHV give promise of identifying the neurones engaged in memory functions and of analysing their connections.

Distinct mechanisms for expression of Fos-like immunoreactivity and synaptic potentiation in telencephalic hyperstriatum of the quail chick

In the intermediate and medial hyperstriatum ventrale (IMHV), a telencephalic region essentially involved in the initial processes of early learning tasks in poultry chicks, induction of an immediate early gene c-fos correlates significantly with the degree of learning (K.V. Anokhin, R. Mileusnic, I.Y. Shamakina, S.P.R. Rose, Effects of early experience on c-fos gene expression in the chick forebrain, Brain Res. 544 (1991) 101-107; B.J. McCabe, G. Horn, Learning-related changes in Fos-like immunoreactivity in the chick forebrain after imprinting, Proc. Natl. Acad. Sci. USA 91 (1994) 11417-11421). In slices of IMHV in vitro, on the other hand, tetanic stimulation at a low frequency induces a potentiation of synaptic responses (P.M. Bradley, B.D. Burns, A.C. Webb, Potentiation of synaptic responses in slices from the chick forebrain, Proc. R. Soc. Lond. B. 243 (1991) 19-24; T. Matsushima, K. Aoki, Potentiation and depotentiation of DNQX-sensitive fast excitatory synaptic transmission in telencephalon of the quail chick, Neurosci. Lett. 185 (1995) 179-182). In this study, we have examined a possible causal link between these two forms of activity-dependent processes, c-fos expression and synaptic potentiation. C-fos was visualized immunohistochemically using antibody raised against the Fos-protein, and potentiation was evaluated on the basis of field potential responses to local electrical stimulation. Tetanic stimulation (5 Hz x 300 pulses) was required for potentiation, but not for c-fos expression. Conversely, a negative correlation appeared between them, and slices with relatively high density of Fos-like immunoreactive cells around the stimulation site failed to show potentiation. Furthermore, drugs similarly effective in blocking potentiation (such as AP5 (NMDA receptor antagonist) and bicuculline (GABA(A) receptor antagonist)) had different effects on the c-fos induction. While AP5 had minor, if any, effects on c-fos expression, bicuculline enhanced it selectively around the site of stimulation. Our results suggest that these two processes are basically distinct, and could represent different aspects in the formation of memory traces in IMHV.

Clathrin proteins and recognition memory

Strong converging evidence indicates that the intermediate and medial part of the hyperstriatum ventrale (IMHV) of the chick forebrain is a site of recognition memory for the learning process of imprinting. Clathrin proteins have been implicated in synaptic plasticity. In the present study we demonstrate for the first time that they are involved in vertebrate learning. Chicks were trained by exposure to a conspicuous object and their preference for it versus a novel object subsequently measured as a preference score (an index of learning). Trained chicks with low preference scores were classed as "poor learners" and those with high preference scores as "good learners". An additional group of chicks was untrained ("dark-reared"). Tissue was removed from the left and right IMHV, hyperstriatum accessorium and posterior neostriatum 9.5 h or 24 h after training. Clathrin heavy chain and clathrin light chains a and b were assayed using sodium dodecyl sulphate polyacrylamide gel electrophoresis and immunoblotting. In the IMHV, and only for clathrin heavy chain, was there a significant effect of training. The effect occurred 24 h but not 9.5 h after training, and was significant only in the left IMHV. In this region at 24 h, there was (i) significantly more clathrin heavy chain in good learners than in dark-reared chicks, and (ii) a significant positive correlation between the amount of clathrin heavy chain and preference score; the amount of protein present in the dark-reared chicks did not differ significantly from the amount predicted from the regression line for trained chicks performing at chance (preference score 50). These findings imply that for the left IMHV, visual experience per se, locomotor activity and other side effects of training did not affect the amount of clathrin heavy chain. Rather, the increase observed was a function of the amounts chick learned and, because it was delayed, is likely to be involved in long-term memory. The results for clathrin heavy chain taken together suggest that enhanced presynaptic events in the IMHV, possibly associated with an increase in synaptic vesicle release/uptake, are important in the recognition memory underlying imprinting.

Behavioral, structural and neurochemical asymmetries in the avian brain: a model system for studying visual development and processing

The emphasis of this review is on the visual systems and lateralized visually guided behavior in several avian species. Lateral asymmetry is known to be present in the tectofugal visual projections to the forebrain of the pigeon and in the thalamofugal visual projections to the forebrain of the chicken. These structural asymmetries are discussed in the context of the behavioral and neurochemical asymmetries. While recognizing the need to investigate the organization of both of the visual pathways within one avian species; this review reasons inductively that the lateralized organization of the two visual pathways leads to binocular input to the right hemisphere via the thalamofugal visual system and to the left hemisphere via the tectofugal visual system. For each system, input to the other hemisphere is primarily monocular. This specialization of the hemispheres for visual processing has predictable effects on behavior. The role of asymmetrical light stimulation of the eyes of the embryo in determining the lateralizations in the visual pathways and some behaviors is discussed, as are other lateralizations generated or altered by imprinting and passive avoidance learning.

The involvement of dopamine in the striatum in passive avoidance training in the chick

Quantitative receptor autoradiography was used to investigate the distribution of binding of [3H]SCH 23390 to dopamine (D1) and [3H]spiroperone to D2 receptors in regions of the forebrain of the one-day-old domestic chick (Gallus domesticus). High levels of specific binding of the D1 and D2 ligands were found in the striatal regions (paleostriatum augmentatum and lobus parolfactorius) of the one-day-old chick, as reported previously in the pigeon, turtle and rat, whilst binding levels were considerably lower in the pallidum (paleostriatum primitivum), hippocampus and hyperstriatum ventrale. The proportions of D1 and D2 receptor binding in the chick were relatively similar in the striatum and pallidum, apart from the paleostriatum augmentatum, where D2 receptors outnumber those of D1 by a factor of two. Binding of the D1 and D2 ligands to forebrain regions was also investigated 30 min after one-trial passive avoidance training of one-day-old chicks in which the aversive stimulus was a bead coated with a bitter tasting substance, methyl anthranilate. These experiments demonstrated a large and highly significant bilateral increase (compared to control birds) in binding to D1 (but not D2) receptors in the lobus parolfactorius. In this striatal region, equivalent to the caudate-putamen of mammals, previous studies have shown that synaptic and dendritic alterations occur following avoidance training. It is concluded that alterations in dopamine binding may be involved in processes that result in modification of the pecking response in chicks after avoidance training.

Age and the effects of 2-D,L-amino-5-phosphonovalerate in an area of the chick forebrain which is essential for early learning

The intermediate, medial part of the hyperstriatum ventrale (IMHV) is a region of the avian forebrain which is known to be essential for early learning in the domestic chick. The IMHV in an in vitro slice preparation displays two forms of synaptic plasticity. The incidence of both varies with age and is maximal between 3 and 5 days post-hatch. Since NMDA receptors are critical for at least one of these plasticities, we have investigated the relationship between age and the contribution of NMDA receptors to the field response evoked by local, low-frequency stimulation and have found that the magnitude of the NMDA-dependent component of the response varies with age peaking between 3 and 5 days post-hatch. Spontaneous neural activity, recorded intracellularly, can be completely and reversibly silenced by NMDA receptor blockade and the incidence of spontaneous activity also varies with age, peaking between 3 and 5 days. These results suggest that the IMHV contains NMDA receptors which can be activated near resting membrane potential. Either the efficiency or the numbers of these receptors is maximal at a specific point in development and their peak activity coincides with a peak in synaptic plasticity. These characteristics are similar to those reported for young mammals.

Involvement of AMPA receptors in maintenance of memory for a passive avoidance task in day-old domestic chicks (Gallus domesticus)

Day-old chicks (Gallus domesticus) were trained on a one-trial passive avoidance task where the aversive stimulus was an unpleasant tasting substance, methyl anthranilate. Chicks were killed 6.5 h after training. The kinetic parameters of [3H] alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid ([3H]AMPA) binding were determined using quantitative receptor autoradiography and Scatchard analyses in 15 discrete forebrain regions of trained and control (water-trained) chicks, revealing two components of binding in each. KD values showed some regional variation, but were 22.2 +/- 1.1 nmol l-1 for the high-affinity component and 685 +/- 25 nmol l-1 for the low-affinity component of binding to whole forebrain sections from control chicks. Analyses also revealed that Hill coefficients were significantly less than 1 in all regions measured. A significant decrease in KD for the low-affinity component occurred bilaterally in the intermediate and medial hyperstriatum ventrale (IMHV; left, 34.8%; right, 33.3%), a region that has previously been shown to be implicated in the processes of memory formation, following passive avoidance training. A significant decrease in KD for the high-affinity component occurred in the right palaeostriatum augmentatum (19.5%). Significant decreases in Bmax accompanied the KD alterations in both cases. Additionally, bilateral intracerebral injections (administered 4.5-5.5 h after training) into the IMHV of 500 nmol l-1 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a selective antagonist of non-NMDA glutamate receptors (particularly AMPA receptors), resulted in amnesia for one-trial passive avoidance training in day-old chicks tested 6.5 h after training.(ABSTRACT TRUNCATED AT 250 WORDS)

Population trends in the fine spatial re-organization of synaptic elements in forebrain regions of chicks 0.5 and 24 hours after passive avoidance training

Two regions in the forebrain of domestic chicks (Gallus domesticus), the intermediate and medial hyperstriatum ventrale and the lobus parolfactorius, have previously been shown to be important centres of biochemical, pharmacological and physiological change following one-trial passive avoidance training. The purpose of the present study was to examine, at the electron microscopic level, the fine spatial re-arrangement of synaptic structures in the intermediate and medial hyperstriatum ventrale (at 30 min), and in the lobus parolfactorius (at 24 h), post-training using comprehensive biometrical designs, image analysis and stochastic approaches. In intermediate and medial hyperstriatum ventrale, no significant differences in the numerical density of synapses either between control and trained chicks, or between hemispheres, were revealed using the disector method. However, after training, a nested-ANOVA demonstrated an increase in the thickness of pre- and post-synaptic electron densities (estimated via image analysis) only in the left intermediate and medial hyperstriatum ventrale, whereas synaptic apposition zone profiles increased in length bilaterally. In presynaptic terminals from the intermediate and medial hyperstriatum ventrale, stochastic analysis revealed that training resulted in the re-distribution of synaptic vesicles between two spatial pools relative to synaptic apposition zones, in both hemispheres producing a large number of synaptic vesicles closer to synaptic apposition zones; a nearest neighbour analysis of synaptic apposition zone profiles indicated that the lateral shape of the synaptic apposition zone after training is more complex in both hemispheres. In the lobus parolfactorius at 24 h post-training the main changes in synaptic fine structure involved a shift of synaptic vesicles away from synaptic apposition zones in the right hemisphere with the distance between synaptic apposition zones decreasing; in the left lobus parolfactorius, synaptic apposition zones became more regular/round in shape with a greater distance between them after training. These data suggest that the initial acquisition of memory involves population changes in the fine spatial organization of synaptic vesicles and synaptic apposition zones in synapses in the intermediate and medial hyperstriatum ventrale, which indicate a possible tendency towards greater synaptic efficacies. These changes are as dynamics as the molecular changes which have hitherto been considered the preserve of short-term correlates of memory formation.

MK-801 blockade of Fos and Jun expression following passive avoidance training in the chick

Training chicks on a one-trial passive avoidance task results in transient up-regulation of the N-methyl-D-aspartate (NMDA) receptor in the left intermediate medial hyperstriatum ventrale (IMHV) of the forebrain 30 min post-training. Injection of the non-competitive NMDA receptor inhibitor, (+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)-cyclohepten 5,10-imine maleate (MK-801), around the time of training renders chicks amnesic for the task. Training also results in enhanced expression of the immediate early gene (IEG) c-fos in the IMHV. To determine the relationship between NMDA receptor up-regulation and IEG induction during memory formation we have examined the expression of Fos, Jun and their related proteins 2 h following training in the presence/absence of the putative amnestic agent MK-801. Western blotting of IMHV samples revealed two protein bands with immunoreactivity to the Fos antibody at 47 and 54 kDa. Using an antibody to Jun, two immunoreactive bands were revealed at 39 and 54 kDa. All bands were enhanced in the left IMHV following passive avoidance training. Post-training intraperitoneal injections of MK-801 (75 mM) produced amnesia in approximately 50% of the birds when tested 1 h after training. Injection of MK-801 significantly attenuated expression of these proteins in birds rendered amnesic, but not in those that recalled the task. We conclude that NMDA receptor activation precedes immediate early gene expression in the memory formation cascade.

Increases in NMDA receptor binding are specifically related to memory formation for a passive avoidance task in the chick: a quantitative autoradiographic study

One-day-old chicks (Gallus domesticus) were trained on a one-trial passive avoidance task in which the aversive stimulus was an unpleasant lasting substance, methylanthranilate (MeA). Control birds were presented with a water (W) coated bead. Five minutes after training a group of McA-trained chicks were given a brief sub-convulsive trans-cranial electric shock, which rendered half amnesic whilst the remainder were able to show recall for the aversive stimulus. Thirty minutes after training birds were killed and quantitative receptor autoradiography was used to determine NMDA sensitive [3H]L-glutamate binding in specific regions of the forebrain of: (i) MeA-trained chicks; (ii) water-control chicks; (iii) MeA-trained electroshocked chicks showing recall; and (iv) MeA-electroshocked chicks amnesic for the aversive stimulus. Increases (805 in the left lobus parolfactorius and 67% in the left intermediate medial hyperstriatum ventrale) in NMDA sensitive [3H]L-glutamate binding occurred in electro-shocked chicks which showed recall of the aversive experience but were absent in MeA-trained chicks rendered amnesic by electro-shock. The increased binding in electroshocked MeA-trained birds which showed recall was similar to that observed previously in MeA-trained birds (without electroshock), compared to water control birds, whereas binding levels in McA-trained electroshocked amnesic birds were not different from those of water control birds. These data argue strongly in MeA-trained electroshocked amnesic birds were not different from those of water control birds. These data argue strongly that alterations in binding to glutamate receptor sub-types are specific to memory formation for the passive avoidance task.

Potentiation and depotentiation of DNQX-sensitive fast excitatory synaptic transmission in telencephalone of the quail chick

Whole-cell perforated-patch recording in slices revealed synaptic organization of the intermediate medial hyperstriatum ventrale, a telencephalic region intimately involved in the early learning processes in chicks. Local electrical stimuli elicited excitatory post-synaptic current (EPSC) mediated by monosynaptic activation of glutamate receptors of the non-N-methyl-D-aspartate type, which was followed by a late GABAA-ergic inhibition. The initial EPSC was potentiated at least for 60 min, when a tetanic stimulation (5 Hz x 60 s) was combined with post-synaptic depolarization at or above -30 mV. In some of these neurons, the potentiated responses underwent a subsequent depression when a second tetanus was accompanied by hyperpolarization at -80 mV or below.

Dendritic spine density in the lobus parolfactorius of the domestic chick is increased 24 h after one-trial passive avoidance training

One to three day old chicks spontaneously peck at small objects. When presented with a chrome bead coated with the bitter tasting substance methyl anthranilate (MeA), chicks peck once, display a characteristic disgust response and subsequently avoid a similar bead. Chicks that are trained on a water coated bead continue to peck a similar bead on retrial. Twenty four hours after training on this one-trial passive avoidance paradigm, chicks were tested for retention. The brains of chicks displaying the correct behavioural response (> 90%) were removed and the lobus parolfactorius from each hemisphere was dissected from the brain and impregnated using a rapid Golgi technique. Analysis of large multipolar neurones by centrifugal dendritic branch order showed that there were significantly more spines on all orders examined in the left hemispheres of MeA-trained chicks compared to water-trained control chicks. Significantly higher spine densities were also found on 4th and 5th order branches of neurones in the right lobus parolfactorius of MeA-trained chicks compared to water-trained chicks. No significant difference in dendritic length was observed. These results suggest that substantial plasticity occurs in post-synaptic structures in the lobus parolfactorius following passive avoidance training. It is suggested that this plasticity is related to processes involved in long term information storage.

Transient cerebral ischemia disrupts performance on a one-trial passive avoidance task in the domestic chick and is associated with neuronal degeneration in the central nervous system

We have examined the effects of transient cerebral ischemia on performance of a one-trial passive avoidance task by chicks. Transient forebrain ischemia was induced by bilateral carotid artery occlusion for a period of 10 min. In one experimental group, ischemia was produced prior to training on the avoidance task whereas in the other group ischemic intervention was not made until 3 h after initial training. Sham-operated groups were matched to each of the experimental groups. All four groups were tested for retention of the avoidance response 24 h post-surgery. The sham-operated birds and those receiving post-training ischemia showed good retention of the avoidance response, whereas in birds which received ischemia prior to training there was significant amnesia. Neuronal damage, determined qualitatively using a silver impregnation method, was observed in several forebrain regions including the hippocampus, hyperstriatal regions, paleostriatum primitivum, ventral archistriatum, and lateral corticoid area. Damage was also observed in the Purkinje cells of the cerebellum. The behavioural and anatomical effects of transient forebrain ischemia have not been previously investigated in an avian species and the finding of significant amnesia for a learning task following ischemia is in good agreement with several behavioural studies in mammals.

The effects of archistriatal lesions on one-trial passive avoidance learning in the chick

The avian archistriatum, part of which may be homologous with the mammalian amygdala, has been implicated in fear and avoidance behaviour. There is also evidence to suggest its involvement in learning. One-trial passive avoidance learning (PAL) has been used extensively to study memory formation in the chick. Young chicks will peck spontaneously at a small, visually conspicuous bead. If the bead has been coated with an aversive substance, the chicks show a disgust response and learn in a single trial not to peck a similar bead on subsequent presentation. Successful acquisition of this one-trial PAL task involves the formation of a learned association between the bead and a noxious taste, followed by the expression of an avoidance response. In view of the possible involvement of the archistriatum in avoidance and learning behaviour, its role in one-trial PAL was investigated by ablation. Bilateral electrolytic lesions of the entire archistriatum prevented the acquisition of the one-trial PAL task. Neither bilateral lesions of the lateral cerebral area nor sham operation affected learning. The impairment of one-trial PAL caused by archistriatal lesions was not due to effects on the visual or motor components of pecking behaviour. The archistriatum could therefore be directly involved in memory formation. It could also be involved in the organization of avoidance behaviour associated with the task, or it could form part of a circuit linking two other forebrain regions previously implicated in one-trial PAL, the intermediate part of the medial hyperstriatum ventrale and the lobus parolfactorius.

Increases in neuronal bursting recorded from the chick lobus parolfactorius after training are both time-dependent and memory-specific

Day-old-chicks can be trained in one trial to avoid a methylanthranilate-coated bead (methyl-chicks). The lobus parolfactorius of the chick forebrain is an important structure for memory of this avoidance response. To examine training-induced electrophysiological changes in this structure, spontaneous neuronal bursting activity was measured from the lobus parolfactorius of anaesthetized, day-old methyl- and water-chicks (the latter chicks trained to peck at a water-coated bead) over the period 1-10 h post-test. Bursting was significantly higher in methyl-chicks over this period. This post-test increase was time-dependent: bursting in methyl-chicks was significantly higher only during the period 4-7 h post-test. In a second experiment, methyl-chicks were subjected to brief, subconvulsive electroshock 5 min post-training. When tested 1 h later about half of these chicks showed recall (avoided the bead) and half were amnesic (pecked the bead). These chicks were anaesthetized and bursting was recorded from the lobus parolfactorius. Chicks that showed recall exhibited a significantly higher level of bursting over the period 1-10 h post-test when compared to chicks that were amnesic. The time course of bursting was similar to that seen in non-electroshocked methyl-chicks. These results suggest that passive avoidance training induces a memory-specific, time-dependent increase in neuronal activity within the lobus parolfactorius of day-old chicks. This increase may be directly associated with long-term consolidation of memory for the task.

Training-induced increases in neuronal activity recorded from the forebrain of the day-old chick are time dependent

Spontaneous neuronal bursting occurs in many areas of chick forebrain. Day-old chicks trained using a one-trial task to avoid a methylanthranilate-coated bead (methyl-chicks) show a significant increase in bursting when compared to chicks trained to peck a water-coated bead (water-chicks). This increase occurs in two forebrain areas: the intermediate medial hyperstriatum ventrale and the lobus parolfactorius. Bursting was recorded from the intermediate medial hyperstriatum ventrale of anaesthetized methyl- and water-chicks at eight time-points over the period 1-9 h post-test. Data merged over this period showed that methyl-chicks displayed an overall increase in bursting in both left and right hemispheres when compared to water-chicks. When burst activity was compared against time, bursting in methyl-chicks was significantly elevated only during the period 3-7 h post-test. Maximal bursting in methyl-chicks was seen 6-7 h post-test. These results suggest that the training-induced increase in bursting seen in the intermediate medial hyperstriatum ventrale of methyl-chicks is not a simple, generalized increase with time but rather has a significant temporal aspect. These results may have particular relevance to previously proposed models of memory formation in the chick.

The molecular neurobiology of early learning, development, and sensitive periods, with emphasis on the avian brain

The subcellular processes that correlate with early learning and memory formation in the chick and sensitive periods for this learning are discussed. Imprinting and passive avoidance learning are followed by a number of cellular processes, each of which persists for a characteristic time in certain brain regions, and may culminate in synaptic structure modification. In the chick brain, the NMDA subtype of glutamate receptor appears to play an important role in both memory formation and sensitive periods during development, similar to its demonstrated role in neural plasticity in the mammalian brain. Two important findings have emerged from the studies using chickens. First, memory formation appears to occur at multiple sites in the forebrain and, most importantly, it appears to "flow" from one site to another, leaving neurochemical traces in each as it moves on. Second, the memory is laid down either in different sites or in different subcellular events in the left and right forebrain hemispheres. Hence, we are alerted to the possibility of similar asymmetrical processes occurring in memory consolidation in the mammalian brain. The similarities between early memory formation and experience-dependent plasticity of the brain during development are discussed.

A role for the neural cell adhesion molecule in a late, consolidating phase of glycoprotein synthesis six hours following passive avoidance training of the young chick

We have investigated the effect of intracranial injections of the amnestic anti-metabolite, 2-deoxygalactose, and antibodies to the neural cell adhesion molecule on retention of a one-trial passive avoidance task in chicks. Groups of chicks received bilateral intracranial injections of 10 mumol/hemisphere 2-deoxygalactose or 10 microliters/hemisphere anti-neural cell adhesion molecule and were tested 24 h following training. 2-Deoxygalactose injections were amnestic when administered at a previously established time (30 min pre-training). Here we show that the agent is also amnestic when injected within a second time window occurring specifically 6-8 h after training. Administration of 2-deoxygalactose between 2 and 6 h or after 8 h post-training was without effect on retention tested 24 h following training. Anti-neural cell adhesion molecule injections were amnestic only when performed at a time which coincided with the second phase of 2-deoxygalactose susceptibility. Further experiments demonstrated that the neural cell adhesion molecule is one of the molecules into which 2-deoxygalactose is incorporated. Additionally, we investigated the extent of diffusion of 2-deoxygalactose and anti-neural cell adhesion molecule following their injection, with respect to their residence in forebrain loci known to be involved in the memory for passive avoidance. We interpret these data as indicating that two waves of glycoprotein synthesis are necessary for the establishment of long-term memory for the experience of passive avoidance training. The evidence is discussed in the context of earlier results indicating that the two waves involve different glycoprotein species and, possibly, different forebrain regions. We speculate that the late phase of glycoprotein synthesis coincides with, and is required for, modulation of cell-cell adhesion processes, reflecting the selection and stabilization of synapses which maintain an enduring representation of long-term memory.

Nefiracetam (DM-9384) preserves hippocampal neural cell adhesion molecule-mediated memory consolidation processes during scopolamine disruption of passive avoidance training in the rat

Scopolamine (0.15 mg/kg), a muscarinic antagonist, when administered during training or at a discrete 6-h posttraining time point, is demonstrated to inhibit the recall of a step-down passive avoidance response when tested at 24 and 48 h after task acquisition. Nefiracetam (3 mg/kg), a piracetam-related nootropic, when given with scopolamine during training tended to improve task recall, and this effect was more pronounced when given at the 6-h posttraining time. Co-administration of nefiracetam with scopolamine was not necessary to achieve the antiamnesic action, as nefiracetam given during training significantly improved the memory deficits produced by scopolamine at the 6-h posttraining time. The paradigm-specific increase in hippocampal neural cell adhesion molecule sialylation, which is observed during consolidation of a passive avoidance response, was attenuated by the presence of scopolamine during training and at the 6-h posttraining time, and this effect was reversed by co-administration of nefiracetam, albeit in a paradigm-independent manner. These results suggest nefiracetam exerts a neurotrophic action that protects memory consolidation from drug interventive insults.

Quantitative autoradiographic demonstration of changes in binding to delta opioid, but not mu or kappa receptors, in chick forebrain 30 minutes after passive avoidance training

Day-old domestic chicks (Gallus domesticus) were trained on a one-trial passive avoidance task in which the aversive stimulus was a bitter tasting substance, methylanthranilate. Thirty minutes later, localization of binding of highly specific ligands (([D-Ala2, Gly-ol]-enkephalin ([3H]DAGO) for mu (mu) receptor sites, [D-Pen2,D-Pen5]-enkephalin ([3H]-DPDPE) for delta (delta) sites, and [3H]-U- 69593 for kappa (kappa 1) sites) to opioid receptors in various regions of the forebrain of methyl-anthranilate trained (M-) and control (water trained (W-)) chicks was determined using quantitative receptor autoradiography. Significant differences in binding to delta ([3H]-DPDPE), but not mu or kappa receptors, were found in several regions of the forebrain, of trained compared to control chicks. There were decreases in binding in the hyperstriatum dorsale of the left hemisphere (14%) and a decrease in binding in the lateral hyperstriatum ventrale of the right hemisphere (14%). However, significant increases were observed in delta binding in the paleostriatum augmentatum of the right hemisphere (16%) and the lobus parolfactorius of both hemispheres (left, 20%; right, 21%). In a control experiment designed to determine whether the taste of methylanthranilate contributed to the increase in 3H-DPDPE binding, there was no significant difference in the level of binding between blindfolded birds in which methylanthranilate was placed in the beak, and blindfolded birds in which water was placed on the bead and inserted into the beak. These findings demonstrate that changes occur in an opioid receptor sub-type in specific regions of forebrain of the chick following passive avoidance training which may be related to events concerned with the process of memory formation.

Long-term increases in the numerical density of synapses in the chick lobus parolfactorius after passive avoidance training

Passive avoidance training has been shown to cause an increase in synaptic density (Nvsyn) in the lobus parolfactorius (LPO) of the one-day old chick. The present study was conducted to investigate the time-course over which this plastic change takes place. Two groups of chicks were trained to peck at either a water coated bead (Control) or a methyl-anthranilate coated bead (M-trained). M-trained chicks showed avoidance responses when offered a similar but dry bead, 30 min later. Right and left hemisphere LPOs were obtained at intervals of 1, 6, 12, 24 and 48 h after training. Synaptic counts were made using a 3-dimensional stereological probe; the 'dissector'. A significantly larger mean Nvsyn (31%) was found in the left hemisphere of M-trained chicks 24 h after training, compared with Control chicks, and the difference fell to 10% at 48 h post-training. M-trained chicks also had a greater Nvsyn (17%) in the right hemisphere at 48 h post-training. The bilaterality of these findings is of particular interest, since unilateral lesions of the LPO fail to produce amnesia for the avoidance task. The importance of these results in the process of memory formation is discussed.

Cholinergic and dopaminergic agents which inhibit a passive avoidance response attenuate the paradigm-specific increases in NCAM sialylation state

The influence of cholinergic and dopaminergic agents on the acquisition of a passive avoidance response in the rat is demonstrated. Trifluoperazine (0.12 mg/kg), a dopamine antagonist, inhibited task acquisition when present during training or later, during consolidation, at the 10-12 h post-training period and at no other intervening time point. Induction of amnesia was dose-dependent and was not apparent when the dose exceeded 0.12 mg/kg. This effect appears to be due to an increase in dopamine release through presynaptic receptor antagonism as similar results could be obtained by the administration of apomorphine (0.5 mg/kg), a dopamine agonist, and this effect could be antagonized by the D1 receptor selective antagonist SCH-23390. Scopolamine (0.15 mg/kg), a muscarinic antagonist, impaired acquisition of the passive avoidance response when administered during training and, separately, at the 6 h post-training period. This could not be attributed to presynaptic antagonism as oxotremorine (0.2 mg/kg), a muscarinic agonist, had no amnesic action. Administration of apomorphine or scopolamine during training and at the appropriate post-training period prevented subsequent paradigm-specific increases of neural cell adhesion molecule sialylation state in hippocampal immunoprecipitates obtained at 24 h after task acquisition and 4 h following intraventricular infusion of the labelled sialic acid precursor - N-acetyl-D-mannosamine. Oxotremorine alone did not influence neural cell adhesion molecule sialylation state. These observations provide further evidence of a regulatory role for neural cell adhesion molecule sialylation state in information storage processes.

Visual input and lateralization of brain function in learning in the chick

Several lines of evidence (biochemical, neuroanatomical, electrophysiological, and behavioural) have indicated a critical role for the intermediate medial hyperstriatum ventrale of the chick forebrain in the acquisition of a passive avoidance response. Previous lesion studies indicated that bilateral or left, but not right, pretraining intermediate medial hyperstriatum ventrale lesions interfere with the acquisition of this task. We have further analysed this asymmetrical involvement of the intermediate medial hyperstriatum ventrale by use of a monocular learning protocol and intermediate medial hyperstriatum ventrale lesions (sham, bilateral, or unilateral). The results indicated that there is interocular transfer of information of passive avoidance learning between the two eye systems, with a tendency to be more successful from the right eye system to the left than in the opposite direction. As in binocular conditions, bilateral pretraining intermediate medial hyperstriatum ventrale lesions impair learning in monocularly trained animals. Unilateral lesions to either left or right monocularly trained experimental animals resulted in amnesia when they were made to the right intermediate medial hyperstriatum ventrale and the chicks were trained/tested with the left eye open. These results indicate that, although right intermediate medial hyperstriatum ventrale lesions do not result in amnesia in binocular animals, this region is capable of participating in memory acquisition processes. They also suggest a connection between lateralization of intermediate medial hyperstriatum ventrale function in passive avoidance learning and the behavioural and structural visual asymmetries known to occur in chicks.

Passive avoidance learning in the young chick results in time- and locus-specific elevations of alpha-tubulin immunoreactivity

A monoclonal antibody was used to examine changes in immunoreactivity of the cytoskeletal protein, alpha-tubulin, following passive avoidance learning in day-old chicks. Postmitochondrial fractions (16,000 g supernatants) were prepared from specific forebrain loci taken at several time points after training and assayed with the anti-alpha-tubulin antibody, YL1/2. Of the regions examined, elevations in the titre of YL1/2 were found in the left intermediate hyperstriatum ventrale 1 h, 6 h and 24 h following training, in the left lobus parolfactorius 1 h following training and in the right lobus parolfactorius 6 h and 24 h following training. No training-related changes were detected in a third forebrain region, the paleostriatum augmentatum. These results regarding the cellular dynamics of memory formation in the chick confirm and expand on earlier findings from our laboratory.

Structural asymmetry in the thalamofugal visual projections in 2-day-old chick is correlated with a hemispheric difference in synaptic density in the hyperstriatum accessorium

Differences in visual discrimination ability between the left and right eyes of chicks, which are most prominent in young males, may result from a structural asymmetry in the organization of the visual projections from the thalamus to the visual Wulst. This asymmetry in projections is no longer present by 21 days in males when the contralateral projections from the right thalamus to the left hyperstriatum have developed. Since the asymmetry of the thalamo-hyperstriatal system results in a differential input of fibres to regions of the hyperstriatum which in turn project to the hyperstriatum accessorium (HA), one of the major differences expected within this region would be an asymmetry in the numerical density of synapses (Nv.syn/microns3). When this was examined in the hyperstriatum accessorium of 2-day-old male chicks, the density of synapses in the right HA was found to be significantly higher (22%, P less than 0.05) than in the left HA. The consequences of this asymmetry in synaptic density in the HA could be widespread and influential within the chick visual system.

Hippocampal NCAM180 transiently increases sialylation during the acquisition and consolidation of a passive avoidance response in the adult rat

Synaptic connectivity change is a consistent anatomical feature of memory formation and the possibility that this is mediated by a replay of neurodevelopmental events has been investigated by measuring change in neural cell adhesion molecule sialylation state during the acquisition and consolidation of a passive avoidance response in the adult rat. The avoidance response was always generated after two to three trials and the animals remained on the platform for the criterion time of 5 min. In all cases training was complete within 5-8 min. Change in sialylation state was monitored following intraventricular infusion of the 3H-ManNAc precursor at 4 hr prior to the reference point. No task-specific change in general glycoconjugate sialylation was apparent in hippocampal P2 pellets at increasing times following training. Increased sialylation state was observed only in neural cell adhesion molecule (NCAM) immunoprecipitates of hippocampal membrane fractions at 12 and 24 hr after training. Change in hippocampal sialylation state could not be attributed to an increased accumulation of NCAM as detected by an immunoabsorbent assay. Immunoblotting of antibody precipitated NCAM demonstrated the 3H-ManNAc to be incorporated into the synapse-specific, 180 kDa isoform of NCAM and a novel 210 kDa isoform. Immunoprecipitation and immunoblotting procedures with an antibody specific for a2-8-polysialic acid showed the 180 and 210 kDa isoforms to be polysialylated. The role of NCAM180 sialylation as a mechanism for synapse selection in information storage is discussed.

The development of hemispheric asymmetries in neuronal activity in the domestic chick after visual experience

Previous experiments have shown that a restricted part of the forebrain, the intermediate and medial part of the hyperstriatum ventrale (IMHV), is involved in the learning process of imprinting. Through this process chicks selectively approach, and so recognise, a visually conspicuous object to which they have been exposed. Previous work has also demonstrated that the left and right IMHV regions play different roles in the recognition memory of imprinting, and that exposure to an object results in changes in synaptic organisation in the left, but not in the right IMHV. We have enquired whether training has a differential effect on spontaneous multiple-unit impulse activity in the two regions. Thirty-two chicks were reared in darkness. Sixteen were trained by exposing them to a flashing, rotating box; the remaining chicks serving as dark-reared controls. The trained chicks were anaesthetised either immediately after training (0-h trained chicks) or returned to the dark incubator and anaesthetised 6 h after the end of training (6-h trained chicks). A single microelectrode penetration was made through the left IMHV simultaneously with a single penetration through the right IMHV; a single penetration was also made through a visual projection area, the left hyperstriatum accessorium. Recordings were made from between 3 and 7 sites in each penetration, and the mean multiple-unit firing rate for these sites was calculated. Spontaneous activity in the right IMHV changed in respect of activity in the left IMHV with time after training; no such effects were found in the left hyperstriatum accessorium.(ABSTRACT TRUNCATED AT 250 WORDS)

Amnesia induced by 2-deoxygalactose in the day-old chick: lateralization of effects in two different one-trial learning tasks

2-Deoxy-D-galactose, an inhibitor of brain glycoprotein fucosylation, was injected intracranially (10 mumole dose in 10 microliters) into either the left or the right forebrain hemisphere of day-old chicks (Gallus domesticus). Bilateral injection of this dose of 2-deoxy-D-galactose is known to induce amnesia for several learning tasks including one-trial passive avoidance and sickness-induced learning. When a tritiated form of the drug was injected into one forebrain hemisphere only, a significantly large proportion of the dose remained in that hemisphere. Chicks were trained in two different one-trial learning tasks. The first was a passive avoidance task in which the chicks were allowed to peck at a green training stimulus (a small light-emitting diode, LED) coated in the bitter liquid, methylanthranilate, giving rise to a strong disgust response and consequent avoidance of the green stimulus. In the second paradigm the chicks were allowed to peck at a similarly colored dry stimulus but, 30 min later, were injected intraperitoneally with lithium chloride (0.1 ml of 1 M solution), causing a sickness-induced aversion for the green LED. 2-Deoxy-D-galactose caused amnesia for the passive avoidance task when injected before training into the right hemisphere but not the left. However, unilateral injection of the drug before training on the sickness-induced learning task did not cause amnesia. The results indicate that fucosylation of brain glycoproteins is required in the right hemisphere for learning the passive avoidance task but that memory for sickness-induced learning can be retained by either hemisphere.

Parallel evolution in mammalian and avian brains: comparative cytoarchitectonic and cytochemical analysis

Comparative morphology, which is based on the selection theory of evolution, analyses the impact of function upon structure and, therefore, emphasizes the adaptive events and biological advantage during the evolution of organs. A comparison based on analogies is described here as an adequate method. The hypothesis is proposed that the evolution of the brain follows the same trends in birds as in mammals. This hypothesis is proved by (1) allometric studies of brain weight and brain structure volume in relation to body weight in mammals and birds; (2) architectonic studies using image analysis on cell and fibre stains as well as on histochemical preparations and receptor autoradiography; and (3) hodological studies with injections of [3H]leucin, HRP and WGA-HRP. The results reveal a vast amount of structural and functional similarities in avian and mammalian brain organization, especially an expansion of structures that permit multimodal integration capacity in the telencephalon. Thus, a parallel evolution occurred in these two groups of vertebrates. It is argued that this may be a general phenomenon in evolution. A cladistic approach, which is based on the concept of homologies (plesio-, apomorphies), pushes aside the existence of analogies. For this reason, cladism does not seem to be a method to answer questions of evolutionary morphology adequately.

Neural bases of recognition memory investigated through an analysis of imprinting

Through a learning process known as imprinting, the young of some animals, including the domestic chick, come to recognize an object by being exposed to it. Visually naive chicks vigorously approach a wide range of objects. After an adequate period of exposure to one object chicks selectively approach it in a recognition test. The nervous system of dark-reared chicks is not a tabula rasa, as chicks have predispositions to approach some stimuli rather than others. Nevertheless, visual imprinting leads to changes in a nervous system that may not have been 'marked' by previous visual experience, and so encourages the hope of discovering the neural bases of the learning process. The intermediate and medial part of the hyperstriatum ventrale, a sheet of cells within the cerebral hemispheres, plays a crucial role in visual imprinting, particularly in the memory process of recognition. The cellular and sub-cellular changes that take place in this part of the hyperstriatum ventrale after imprinting are described. The right and left hyperstriatum ventrale regions play different roles in the imprinting process, and evidence is given for the existence of multiple memory systems in the chick brain.

Visual memory lateralization in pigeons

Previous experiments employing simple visual discrimination tasks have revealed a cerebral lateralization in the visual system of pigeons with a dominance of the left hemisphere. Until now, visual memory lateralization in birds has not been investigated. To study possible asymmetries of visual memory functions, a simultaneous instrumental discrimination procedure was used. The animals were trained to discriminate 100 different visual patterns from a further 625 similar stimuli. Retention tests were conducted under binocular and monocular conditions. When the subjects looked monocularly, retention performance was significantly higher with the right eye (left hemisphere) than with the left eye (right hemisphere). The results suggest that the lateralization of the pigeon's visual system depends at least partly on an asymmetry in visual memory capacity.

Anisomycin and amnesia in the chick: state-dependent effects are not present with intracranial injections

It has recently been suggested that intraperitoneal (IP) injection of anisomycin (ANI) in the chick produces amnesia for a one-trial passive avoidance task in a state-dependent manner. We have examined the behavioral and biochemical effects of IP and intracranial (IC) injections of ANI in chicks trained on a one-trial passive avoidance task. IC injection of ANI produced 35% brain protein synthesis inhibition whereas IP injection produced only negligible amounts of protein synthesis inhibition in the brain. IC injection of ANI produced amnesia and was not state-dependent. Patterns of behavior consistent with state-dependent effects were produced by IP injection of ANI. These experiments indicate that there are differences in the pattern of results produced by IP and IC injection of ANI and support the hypothesis that the expression of long-term memory in chicks is associated with protein synthesis.

Quantitation of synaptic, neuronal and glial development in the intermediate and medial hyperstriatum ventrale (IMHV) of the chick Gallus domesticus, pre- and post-hatch

A quantitative analysis was made of the development of synapses, neurons and glia in both left and right intermediate and medial hyperstriatum ventrale (IMVH) of the forebrain of the domestic chick, Gallus domesticus from 16 days in ovo to 9 days post-hatch. There was a marked increase in total synapse numerical density (NVsyn), from 10 synapses per 100 microns3 at 16 days in ovo to 50 synapses per 100 microns3 at 9 days post-hatch; no significant left/right hemispheric differences were evident but there were differences between the development profiles for synapses with asymmetric as compared to symmetrical synaptic junctions. In contrast to synaptic development, the number of neurons per unit volume, NVneu, decreased by approximately 50% from the value at 16 days in ovo to that at day 0 (hatching). The neuronal population density remained unchanged to 3 days post-hatch and then, in 9-day-old birds, declined to almost a quarter of the original population size; no significant left/right hemisphere differences were evident. Glial cells declined in number from 16 days in ovo to 19 days in ovo, and then gradually increased to 9 days post-hatch. When the synapse to neuron ratio was examined, a trend was observed of a gradual increase with age, but no hemispheric differences were present. It is concluded that these changes are major events which must be considered in experiments aimed at determining the effects of behavioural, and/or environmental manipulations, on the morphology of the IMHV because they may mask other, more subtle, structural changes that occur as a result of behavioural experiences.

Amnesia of a passive avoidance task due to the beta 2-adrenoceptor antagonist ICI 118,551

The selective beta 2-adrenoceptor antagonist ICI 118,551 induces amnesia in the domestic chick when given systemically, 10 min after a one-trial PAL task. Young chicks will spontaneously peck at a small bright bead. If the bead has been coated with a distasteful substance, the chicks will learn in a single trial not to peck at a similar bead on subsequent presentation. Administration of ICI 118,551 prevented retention of this task. Vehicle-injected chicks which learnt the task, avoided a similar bead to the training bead in the retention test, but did not avoid a bead of a different colour. The effect of ICI 118,551 is unlikely to be a direct effect on performance since amnesic chicks pecked both beads freely and equally in the test.

Memory systems in the chick: dissociations and neuronal analysis

Young domestic chicks learn to recognize the visual characteristics of an object to which they are exposed. A restricted part of the forebrain, the intermediate and medial part of the hyperstriatum ventrale (IMHV) is implicated in this process. This form of exposure learning can be dissociated from (i) the ability to learn certain procedures or skills, and (ii) a predisposition to attend to particular types of naturalistic objects. The first of these dissociations is reminiscent of that found in certain human organic amnesias, whilst the second may have its counterpart in the processes involved in face recognition by infants. The right and left IMHV play different roles in the memory that underlies imprinting. The cellular and molecular processes involved in this form of memory are discussed.

Training induced dendritic spine density changes are specifically related to memory formation processes in the chick, Gallus domesticus

The density of dendritic spines on large, multipolar, projection neurons in the intermediate medial hyperstriatum ventrale (IMHV) of 1-day-old chicks was examined after training on a one-trial passive avoidance task. Chicks trained on the task were given a brief, subconvulsive transcranial electroshock 5 min after training, a procedure which rendered about half of them amnesic. The spine density was found to be significantly higher 24-26 h after training in the left IMHV in chicks that remembered the response compared to chicks rendered amnesic. These data, taken together with our earlier observations on spine density changes following training, argue strongly in favour of a specific role for dendritic spines in memory formation in the chick.

Changes in the number and structure of dendritic spines 25 hours after passive avoidance training in the domestic chick, Gallus domesticus

One-day-old chicks spontaneously peck at a shiny chrome bead. If the bead is coated with methylanthranilate, a bitter tasting substance (M-chicks), they peck once and avoid a similar bead subsequently. Control chicks peck, and continue pecking at a bead dipped in water (W-chicks). Twenty-five hours after this one-trial passive avoidance training the brains were fixed and Golgi-impregnated. A class of large, multipolar, projection neurons from the intermediate medial hyperstriatum ventrale (IMHV) of both hemispheres from M- and W-chicks were examined for changes in their spine density and spine shape. An estimate of the true spine number was obtained using the correction formula of Feldman and Peters. M-chicks showed highly significant increases (P less than 0.0001) in spine density of between 89-113% in the left hemisphere, and 37-69% in the right, compared with W-chicks. There was a significant hemispheric asymmetry in W-chicks: the right hemisphere had approximately 47% more spines per micron than the left, and this difference was abolished after training. Following passive avoidance training, significant increases in spine head diameter (by approximately 9%) and decreases in spine stem length (by approximately 17%), with no significant alterations in overall spine length, were observed in the left hemisphere. The mean dendrite lengths were not significantly changed after training, but an asymmetry of this measure in W-chicks (left greater than right) was present in dendrite orders 2 (P less than 0.01) and 3 (P less than 0.02). These results show that spine densities can increase rapidly (within 25 h) following a one-trial passive avoidance training task and that spine shape changes can be found on the same dendrites which also show changes in spine number. The data support the view that dendritic spines are involved in memory formation processes.

Passive avoidance learning produces focal elevation of bursting activity in the chick brain: amnesia abolishes the increase

Presentation of a bright bead to day-old chicks (Gallus domesticus: Ross 1 Chunky Chicks) elicits spontaneous pecking. If the bead is coated with an aversive substance (e.g., methylanthranilate), they will avoid similar beads subsequently; if it is coated with water, they peck avidly on re-presentation. Formation of a memory for this one-trial passive avoidance task is unaffected by subconvulsive transcranial electroshock when applied 10 min after training in 60% of birds, whereas "immediate" post-training electroshock renders 63% of chicks amnesic. Memory formation and retention is associated with a large bilateral enhancement in trained over control chicks (320 and 350% in left and right hemispheres, respectively; p less than 0.001) of a particular spontaneous multi-unit activity firing pattern, that is, short-duration (15-40 ms) bursts of large-amplitude (greater than or equal to 200 microV, 450 microV max p-p), high-frequency (400-450 Hz) spiking in anesthetized chicks. This effect is observed in data lumped from 1-13 h after training and is restricted to the intermediate medial hyperstriatum ventrale. When chicks are rendered amnesic by electroshock immediately following training, there is a complete abolition of this increase in burst firing; in those chicks where this treatment fails to elicit amnesia, the increase in bursting is still observed. In birds in which the shock is delayed and memory formation occurs, the increase in bursting activity is maintained; however, if the delayed shock produces apparent amnesia, then the increase is once again abolished. The electroshock had no effect on bursting per se in untrained chicks. There was no significant difference in tonic spiking between the chicks. A marked increase in the occurrence of bursting epochs in the IMHV of anesthetized chicks following passive avoidance training is therefore closely associated with memory formation, but not with the nonspecific concomitants of the training procedure.

Asymmetric incorporation of [14C]cyanate and of fluorescein isothiocyanate in mamillary body of conditioned rats

A marked decrease in overall learning capacity has been observed in rats injected with cyanate. Therefore it was of interest to test whether learning influenced carbamylation of brain proteins. Incorporation of [14C]cyanate into proteins of the mamillary body was selectively modified following operant conditioning of the rat, so that trained rats showed an asymmetric image with higher levels of incorporation in the right side than in the left side, as compared to control rats. These results were confirmed using fluorescein isothiocyanate. The asymmetry persisted once the learning had been well established.

GABAergic structures in the chick telencephalon: GABA immunocytochemistry combined with light and electron microscope autoradiography, and Golgi impregnation

The distribution of gamma-aminobutyric acid (GABA)-ergic elements in 3 forebrain regions (medial mid-telencephalic hyperstriatum ventrale; paleostriatum augmentatum; lobus parolfactorius) of two-day-old domestic chicks was investigated using (1) light and electron microscope autoradiography following [3H]GABA uptake in vitro in combination with pre-embedding GABA immunocytochemistry and (2) Golgi impregnation and 'gold-toning' combined with postembedding GABA immunocytochemistry. In both the paleostriatal regions and the medial (mid-telencephalic) hyperstriatum ventrale, GABA immunolabelling was demonstrated with the pre-embedding technique. Radiolabelling with [3H]GABA was also shown in these regions, co-localised in many cases with the immunolabelling. In the paleostriatal regions, the majority of perikaryal labelling was found in ovoid, elongated or fusiform cell bodies of 6-7 micron diameter whereas in the medial (mid-telencephalic) hyperstriatum ventrale, larger (10-15 micron) multipolar and smaller (5-6 micron) bipolar neurons were found labelled. In the latter region, Golgi impregnated neurons of similar morphology were found to be immunopositive to GABA using the postembedding technique. The ultrastructure of [3H]GABA accumulating cells is characterised by pale or moderately granular nuclei with small invaginations, few mitochondria and a prominent Golgi apparatus. Astrocytes and ependymal cells are also labelled with [3H]GABA. GABA-labelled axon terminals represent 29-36% of the total in the 3 brain regions studied. They appear as electron-lucent boutons with few and often scattered synaptic vesicles and in most cases they form symmetrical axo-dendritic junctions.

The effects of protein synthesis inhibition on structural changes associated with learning in the chick

The effect of the protein synthesis inhibitor anisomycin on the structural changes associated with passive avoidance learning in the chick was investigated. Chicks were trained when they were 24 h old by allowing them to peck at a shiny bead coated with either water or the aversive-tasting substance methylanthranilate (MeA). Chicks which peck the MeA-coated bead will on subsequent testing avoid pecking a similar, but water-coated bead. Behavioural testing was carried out 12 h after training and immediately afterwards the chicks were killed and their brains prepared for electron microscopy. A specific region of the forebrain, the intermediate and medial part of the hyperstriatum ventrale (IMHV) was investigated. When the IMHV of the MeA trained chicks was compared with that of water-trained controls structural changes of the synapse were detected. These changes involved a significant increase in the mean length of the postsynaptic density (LPSD) of symmetrical synapses in the left IMHV. Chicks injected with 0.8 mg of anisomycin 30 min before training with a MeA-coated bead showed aversion for the shiny bead when tested 12 h later. Electron microscopic analysis of the IMHV from these amnestic chicks showed no evidence for the change in LPSD demonstrated in the water-injected controls. These results are discussed in relation to the nature of the memory trace induced by training on a passive avoidance task.

Alterations in synaptic structure in the paleostriatal complex of the domestic chick, Gallus domesticus, following passive avoidance training

A morphometric study was made of synapses in both left and right hemispheres of two regions of the chick paleostriatal complex, the paleostriatum augmentatum (PA) and the lobus parolfactorius (LPO), 24 h after passive avoidance training (methyl anthranilate, M-chicks), and in water-trained controls (W-chicks). The synaptic features examined were D, the mean length of the postsynaptic thickening; Nv.syn, the numerical density of synapses; Vv.syn, the volume density of the presynaptic bouton; V, the mean volume of the presynaptic bouton; Nv.ves, the numerical density of synaptic vesicles per bouton volume; ves.syn, the number of synaptic vesicles per presynaptic bouton; and K, the curvature of the synaptic contact zone. In the LPO there is a significant increase in the numerical density of synapses (Nv.syn) in both left and right hemispheres of M-compared with W-chicks (up to 59%, depending on the method of calculation used). A hemispheric asymmetry of postsynaptic thickening length (D) which is present in W-chicks (R greater than L by 10%) is reversed in M-chicks. There is no difference in the volume density of the presynaptic bouton (Vv.syn) or the mean bouton volume (V) either between W- and M-chicks, or between left and right hemispheres. Significant changes are found after avoidance training in both of the synaptic vesicle parameters measured. There is an increase of approximately 50%, both in the numerical density of synaptic vesicles (Nv.ves) and the number of vesicles per synaptic bouton (ves.syn), in the left hemisphere of M-chicks. No changes in the mean synaptic contact curvature (K) were observed after training, either of presynaptically concave, or presynaptically convex synapses, in either left or right hemispheres, nor did the percentage distribution of these different curvature classes vary greatly. In the PA there were no significant changes in D, Nv.syn or Vv.syn, either between M- and W-chicks, or between left and right hemispheres, 24 h after passive avoidance training. However, when the differences between the mean bouton volume (V) are examined, there is a significant increase in size of the boutons in the left hemisphere of M- compared to W-chicks. There is also a hemispheric asymmetry in both Nv.ves and ves.syn in W-chicks (R greater than L by approximately 15%) and this disappears on M-training. No changes in the mean synaptic contact curvature were observed after training, either of presynaptically concave, or presynaptically convex synapses, in either left or right hemispheres. However, the percentage distribution of these different curvature classes showed some variation.(ABSTRACT TRUNCATED AT 400 WORDS)