Spatial re-arrangement of the vesicle apparatus in forebrain synapses of chicks 30 min after 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.

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.

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.

Transient, learning-induced ultrastructural change in spatially-clustered dentate granule cells of the adult rat hippocampus

In semithin, Toluidine Blue-stained plastic coronal sections, we have observed hyperchromatic granule cells in the dorsal crest of the adult rat dentate gyrus following passive avoidance learning. These exhibited a time-dependent, twenty- to thirty-fold increase in their frequency at the 5-7 h post-training time. The hyperchromatic cells formed a rostral-caudal ribbon, 250 microns in diameter and 60 microns in depth, in sections obtained from -2.6 to -4.5 mm with respect to bregma. This was not observed in passive animals or yoked controls. Ultrastructural analysis revealed their cytoplasm and dendrites to be enriched in ribosomes and microtubules, respectively. Dendrites associated with the hyperchromatic cells exhibited a two-fold increase in spine number as compared to those of normochromatic cells in the same region of the dorsal mid-molecular layer. These changes are suggested to be associated with modulation of L1 and neural cell adhesion molecule-mediated neuroplastic change within this discrete post-training period of memory consolidation.

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.

Long-term increases in synaptic density in chick CNS after passive avoidance training are blocked by an inhibitor of protein synthesis

Long-term increases in synaptic density (first recorded 24 h after training of chicks on a one-trial passive avoidance task, and still present 48 h post training), are found bilaterally in a part of the striatum, the lobus parolfactorius (LPO) [23,36], and are believed to reflect a trace of long-term memory formation. Such increases in synaptic density are most likely to occur by either de novo synthesis of new synaptic material, or via post-translational modification of pre-existing components. Several previous studies have shown that inhibitors of protein synthesis such as anisomycin injected just before, or after training, can prevent long-term memory formation in the chick. The present study therefore examined whether the long-term increases in synaptic density in the LPO that occur after passive avoidance training can be blocked by anisomycin. Our data show clearly that chicks injected with anisomycin 30 min pre-training were amnesic on testing 24 h later, and the bilateral increases in synaptic density (of spine and shaft synapses) seen in saline injected trained controls, were significantly reduced, demonstrating that protein synthesis de novo is involved in the post-training increase in synaptic density in the LPO.

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.

Morphological changes associated with stages of memory formation in the chick following passive avoidance training

Memory formation following learning is presumed to result from modification in the efficacy of neural circuitry, either through strengthening of pre-existing synapses, or formation of new contacts. An ideal paradigm to investigate memory formation is one-trial passive avoidance training of day-old chicks, in which the birds learn to avoid pecking a bead coated with an aversive substance, methylanthranilate. Following training, a sequence of biochemical, electro-physiological, pharmacological and morphological events takes place within two loci in the forebrain, the intermediate and medial hyperstriatum ventrale (IMHV), and part of the paleostriatal complex, the lobus parolfactorius (LPO). Our data reviewed here suggest that the initial acquisition of memory involves population changes in the fine spatial organization of synaptic vesicles and active zones in synapses in the IMHV whereas longer-term changes are more prominent in the LPO and involve, primarily, a bilateral increase in the density of synapses and dendritic spines. The short-term synaptic changes are as dynamic as the molecular changes which have hitherto been considered the preserve of short-term correlates of memory formation.