Intermediate Medial Hyperstriatum Ventrale Research Articles

English

  Hatched from natural geomagnetic conditions, 1-day-old chicks were stimulated with Methyl Anthranilate (MeA) and labeled as the control group (CG). We found that the line density of dendritic spines (LDds) from the neurons in the memory-related nuclei [Intermediate Medial Hyperstriatum Ventrale (IMHV) and lobus parolfactorius (LPO)] was increased by 38.7%, and the total dendritic lengths (TLds) of individual neurons in IMHV were increased by 37.1 to 45.4%. In addition, the average LDds of individual neurons in imprinted chicks (ICs) were increased by 10.8%, and the neurons that had long dendrites were increased by 50%. In contrast, in the experimental group (EG), where chicks were hatched from hypomagnetic field, the average LDds of each neuron in IMHV and LPO of the native chick (NC) were similar to the NC of CG. After a 12 h exposure to MeA, the LDds from IMHV and LPO of the no-imprinted chick (n-IC) and IC were similar to NC but decreased by 17.4% to the n-IC and IC of CG. Furthermore, the average TLd of single neurons from the memory nuclei in NC, n-IC and IC were significantly decreased by 30.9%. However, the LPOs were not different in n-IC. These results indicate that if the natural geomagnetic environment is disrupted, the development of the dendritic spines from the neurons in the related memory nuclei is unchanged, but the hyperplasia of the dendritic spines in the neurons involved in long-term memory was ablated by MeA. Further, the development and growth of dendrites were significantly reduced.   Key words: Hypomagnetic field space, day-old chicks, memory-related nuclei (Medial Hyperstriatum Ventrale and lobus parolfactorius), neuron morphology.

Sex differences in pregnenolone sulphate in the chick brain after training

Pregnenolone and pregnenolone sulphate are potent memory enhancers when administered to rodents prior to various learning and memory paradigms. Here, we show that training on a passive avoidance task results in the increased concentration of pregnenolone sulphate in the medial striatum, formerly known as lobus parolfactorius of female but not male chicks. In addition, we demonstrated potential for neuronal synthesis of pregnenolone in the day-old chick brain, including in the intermediate medial mesopallium, formerly known as intermediate medial hyperstriatum ventrale and the medial striatum.

Contrasting roles for β 1, β 2 and β 3-adrenoceptors in memory formation in the chick

Noradrenaline plays distinct roles in the modulation and consolidation of memory for one-trial, discriminated, avoidance learning in the chick. We have previously shown that activation of β 2-, β 3- and α 1-adrenoceptors (ARs) by injection into the multimodal forebrain association region (intermediate medial hyperstriatum ventrale [IMHV] or intermediate medial mesopallium [IMM]) is involved in the consolidation of memory 30 min after training and that activation of α 2-ARs in the caudate putamen plays a role in the reinforcement of memory leading to consolidation in the IMM (IMHV). In this paper we provide evidence that noradrenaline acts at β 1-ARs in the basal ganglia (lobus parolfactorius or medial striatum) in short-term memory processing immediately post-training and demonstrate inhibition of memory by selective AR antagonists at particular times in the sequential memory processing sequence after training. These results support separate roles for β 2- and β 3-ARs in memory consolidation. Our studies suggest that, as a consequence of the learning experience, noradrenaline acts in different brain regions and at different times in memory processing, to enhance memory through distinct populations of ARs.

Post-synaptic GABA B receptors—possible controllers of coincidence detection?

Post-synaptic GABA B responses (slow, late hyperpolarisations which can be eliminated by perfusion with phaclofen) can be recorded in vitro from many, but not all, neurones in the intermediate medial hyperstriatum ventrale (IMHV). The IMHV is an area of the chick forebrain which is remarkable for its plasticity, and for its essential role in two specific types of early learning—imprinting, and a form of one-trial passive-avoidance learning. Post-synaptic GABA B responses are strongly statistically associated with other properties (such as high membrane resistance) which are, themselves, dependent on a bird’s past history. There is also evidence that their incidence changes with prior training in vivo and with age. GABA B hyperpolarisations are always offset to a varying extent by excitatory NMDA components. These two components follow a very similar time-course, so that the duration and (to a lesser extent), the magnitude of a response is controlled by the balance between the two systems. The evidence suggests that this balance fluctuates, and that its fluctuations determine the extent to which any neurone can function as a coincidence detector.

Post-synaptic GABAB receptors?possible controllers of coincidence detection?

Post-synaptic GABA B responses (slow, late hyperpolarisations which can be eliminated by perfusion with phaclofen) can be recorded in vitro from many, but not all, neurones in the intermediate medial hyperstriatum ventrale (IMHV). The IMHV is an area of the chick forebrain which is remarkable for its plasticity, and for its essential role in two specific types of early learning—imprinting, and a form of one-trial passive-avoidance learning. Post-synaptic GABA B responses are strongly statistically associated with other properties (such as high membrane resistance) which are, themselves, dependent on a bird’s past history. There is also evidence that their incidence changes with prior training in vivo and with age. GABA B hyperpolarisations are always offset to a varying extent by excitatory NMDA components. These two components follow a very similar time-course, so that the duration and (to a lesser extent), the magnitude of a response is controlled by the balance between the two systems. The evidence suggests that this balance fluctuates, and that its fluctuations determine the extent to which any neurone can function as a coincidence detector.

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.

α 2-adrenoceptors in the basal ganglia have a role in memory consolidation and reinforcement

This study demonstrates a role for α 2-adrenoceptors in the basal ganglia in the consolidation of memory using weakly and strongly reinforced models of discriminated avoidance learning in the chick. The memory enhancing action of noradrenaline injected into the basal ganglia (lobus parolfactorius—LPO) was reduced in the presence of the α 2-adrenoceptor antagonist yohimbine, but when noradrenaline was injected into the multi-modal association area (intermediate medial hyperstriatum ventrale—IMHV), yohimbine failed to prevent memory enhancement. Yohimbine injected into the LPO prevented, whereas the α 2-adrenoceptor agonists oxymetazoline and clonidine enhanced, consolidation of memory. The timing of the inhibitory effect of yohimbine in the LPO suggested that α 2-adrenoceptor involvement occurs 10–15 min after training, and that stimulation of α 2-ARs in LPO is necessary for subsequent consolidation of memory. Oxymetazoline, being hydrophilic, was ineffective injected into IMHV, whereas the action of the lipophilic α 2-adrenoceptor agonist clonidine in the IMHV was interpreted as an action at a site more distal in the brain, probably the LPO. The results suggest that noradrenaline release in the basal ganglia in the chick stimulates α 2-adrenoceptors, which modulate and consolidate memory formation mediated by β 2- or β 3-ARs in the association area. The LPO may be responsible for the reinforcement of memory in the IMHV.

Discriminative taste aversion learning: A learning task for older chickens

The study of learning and memory using the chicken model has relied on three learning paradigms, passive avoidance learning, imprinting and the pebble floor task. Passive avoidance learning and imprinting have been used predominantly in very young chickens and cannot be used to access learning and memory in older chickens. We have established a new behavioural learning paradigm, Discriminative Taste Aversion Learning (DTAL), that can be used with both young and older animals. The task requires chickens to discriminate between food crumbs dyed either red or yellow with one colour being associated with the aversive tasting substance, methylanthranilate. Learning can be tested at various times after the training session by presenting chickens with the coloured food crumbs without an aversive taste. Both chickens tested at 5 and 15 days post-hatch learned to avoid the aversive crumbs. Furthermore, the protein synthesis inhibitor anisomycin (30 mM; 10 μl per hemisphere) injected into the intermediate medial hyperstriatum ventrale 15 min pre-training or 45 min post-training blocked long-term memory for the DTAL task when tested 24 h later. Memory for the task was unaffected by anisomycin injection 120 min post-training or in control animals injected with saline at similar times. The timing of the cellular processes of protein synthesis needed for consolidation of the DTAL appears to be similar to those described for the other behavioural paradigms in young chickens.

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 γ-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 γ-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 γ-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 γ-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 γ-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.

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.

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 α-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 α-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.

Low-threshold N-methyl- d-aspartate receptor function correlates negatively with learning

The intermediate, medial hyperstriatum ventrale (IMHV) is an area of the forebrain of the domestic chick which exhibits great plasticity. Moreover, there is a strong link between plasticity in the IMHV and specific changes in behaviour. The IMHV in vitro is still plastic, and many of its physiological properties are age-dependent, peaking in slices taken from 3- or 4-day-old birds. This ‘window’ coincides with an important transitional period in a chick’s normal behavioural development. It has also been claimed that reversal training is at its most effective in 3- and 4-day-old birds - a proposition which was confirmed by the experiments reported here. A combination of in vivo training followed by in vitro electrophysiology also revealed that the function of low-threshold N-methyl- d-aspartate receptors (one of the age-related variables) is negatively related to the effectiveness of reversal training, when age is held constant.

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.

Adenosine–amino acid interactions in the chick brain: a role in passive avoidance learning

The present work describes interactions between adenosine and the amino acids glutamate and GABA in slices of intermediate medial hyperstriatum ventrale (IMHV), an area of the chick brain known to be involved in learning and memory events associated with a one-trial passive avoidance task. In slices derived from the IMHV of untrained chicks, the A 1 receptor agonist N 6-cyclohexyladenosine (CHA; 10 μM) specifically inhibited glutamate release. Conversely, cyclopentyltheophylline (CPT; 100 μM an A 1 antagonist) increased glutamate release from the slices and blocked the CHA-induced inhibition of glutamate. The A 2 receptor agonist 2- p-(2-carboxylethyl)-phenylamino-5′- N-ethylcarboxamido adenosine hydrochloride (CGS 21680) selectively increased glutamate release when applied at 5 μM while it selectively inhibited GABA release at a lower concentration (10 nM). The addition of NMDA to the medium, resulted in increased adenosine release equivalent to that found following stimulation with 50 mM KCl. Both the NMDA and the KCl-induced increases were eliminated by addition of d-2-amino-5 phosphopentanoic acid ( d-AP5), an NMDA-receptor antagonist. Slices prepared from the IMHV of chicks following successful training on the task showed enhanced adenosine release 30 min, 1, 3 and 6.5 h after training compared to chicks trained to peck a water-coated bead. The results show that changes in adenosine release from the IMHV accompany memory formation in the chick. We suggest that adenosine–amino acid transmitter interactions potentially via the activation of NMDA receptors, a necessary step in long-term memory formation for the task, may modulate the formation of memory for the one-trial passive avoidance task.

Lesions of the Intermediate Medial Hyperstriatum Ventrale Impair Sickness-Conditioned Learning in Day-Old Chicks

Lesion studies show that the intermediate medial hyperstriatum ventrale (IMHV), a forebrain visual association area in chicks, is involved in learning and memory for one-trial passive avoidance and imprinting. We examined the effects of IMHV lesions in a one-trial, nongustatory, sickness-conditioned learning task. This task is similar to passive avoidance and imprinting because all three tasks require the chick to remember visual cues in order to respond correctly. However, sickness-conditioned learning differs from imprinting and passive avoidance because it uses sickness as the aversive stimulus and there is a longer conditioned stimulus-unconditioned stimulus interval (30-min delay compared to seconds). Bilateral IMHV lesions given 24 h before training impaired the ability of the chicks to avoid a bead associated with sickness produced by lithium chloride injection, as did pretraining unilateral left or right IMHV lesions. Post-training IMHV lesions given 1 h after training did not impair avoidance of the test bead in the sickness-conditioned learning task. However, lesioned chicks showed generalized avoidance of all test beads. The pretraining lesion results are similar to those found in imprinting and passive avoidance learning; however, the effects of unilateral IMHV lesions differed. Post-training lesion effects are similar to those found in passive avoidance learning. We propose that both left and right IMHV are necessary for sickness-conditioned learning and that post-training IMHV lesions impair the ability of the chick to learn or remember the association between the color of the bead and the aversive consequences of LiCl injection.

Passive avoidance training results in increased responsiveness of voltage- and ligand-gated calcium channels in chick brain synaptoneurosomes

A temporal cascade of events has been described from a number of biochemical investigations of passive avoidance training in day-old chicks. Among these, within minutes of training, there is a transient, enhanced release of glutamate and increased agonist and antagonist binding to N-methyl- d-aspartate-sensitive glutamate receptors in the intermediate medial hyperstriatum ventrale of the forebrain. Some 6.5 h later, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate binding to glutamate receptors is also increased in the same region. These processes might be predicted to affect the uptake of calcium via voltage-sensitive calcium channels or glutamate receptor-associated channels, thereby changing the intracellular calcium concentration. To test this possibility, we have measured the calcium concentration in synaptoneurosomes, containing both pre- and postsynaptic elements, prepared from left and right intermediate medial hyperstriatum ventrale at various times following training, using Fura 2-AM as the indicator of intracellular calcium concentration. Synaptoneurosomes, prepared immediately and 5 min after training, were stimulated with 70 mM potassium chloride in the presence of 2 mM calcium, resulting in a significantly enhanced increase in calcium concentration in synaptoneurosomes from the left hemisphere of trained chicks. This effect was absent in samples obtained at later times after training. N-Methyl- d-aspartate (0.5 mM) induced a significant enhancement in the increase in calcium concentration in intermediate medial hyperstriatum ventrale from both left and right hemispheres 10 min and 30 min after training. At 3 h and 6 h after training, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (0.5 mM) induced a significantly enhanced increase in calcium concentration in samples from either hemisphere. These results suggest that immediately after training there is an engagement of both pre- and postsynaptic voltage-sensitive calcium channels, followed by an increased reponse to N-methyl- d-aspartate receptor stimulation, and coinciding with the enhanced calcium-dependent glutamate release and an increase in N-methyl- d-aspartate-sensitive glutamate receptor binding that has been reported previously. The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-sensitive mechanisms are activated at a later stage of memory formation, when increased α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate binding to glutamate receptors has been reported. Thus, responsiveness of calcium channels to agonist stimulation is implicated in temporally diverse stages in the cascade of events involved in memory formation following passive avoidance training in the chick.

Expression of Fos and Jun Proteins Following Passive Avoidance Training in the Day-Old Chick

It has been shown previously that the immediate-early genes, c-fos and c-jun mRNA are induced in the 1-day-old chick forebrain after one-trial passive avoidance training in which chicks learn to avoid pecking at a bitter-tasting bead. Here, we have studied the expression of their proteins using antibodies to Fos and Jun. Western blotting disclosed two immunoreactive bands for the anti-Fos antibody (47 and 54 kD) and two immunoreactive bands for the anti-Jun antibody (39 and 54 kD). Two hours post-training there was an increase in the number of Fos-positive stained nuclei in right intermediate medial hyperstriatum ventrale (IMHV) (P < 0.01), left IMHV (P < 0.05), right lobus parolfactorius (LPO) (P < 0.025) and left LPO (P < 0.05) of birds trained on the bitter bead compared with controls that had pecked a water-coated bead. Staining for Jun protein was significantly greater in the right LPO of trained chicks (P < 0.01). Other forebrain regions showed no increase over quiet control levels. The findings are discussed in the context of the cascade of events involved in passive avoidance memory consolidation in the day-old chick.

Changes in phosphorylation of Ca2+/calmodulin-dependent protein kinase II (CaMKII) in processing of short-term and long-term memories after passive avoidance learning.

Characteristic autophosphorylation of calcium/ calmodulin-dependent protein kinase II (CaMKII) and its consequences have made this kinase an interesting target in studying the molecular pathway for important neuronal functions including learning and memory formation. In this article, we use immunoprecipitation and immunoblotting methods to detect changes in phosphorylation of CaMKII during memory formation in 1-day-old chicks trained in a single trial passive avoidance task. A 60-kDa protein has been immunoprecipitated from the chick brain with an anti-rabbit CaMKII antibody. This protein shows strong and specific immunoactivities with antibodies against the calmodulin binding site of CaMKII, and the N and C terminals of beta-CaMKII. Commercially available anti-phosphoserine and anti-phosphothreonine antibodies are shown to sensitively detect phosphorylation of purified CaMKII. The basal phosphorylation of CaMKII from the intermediate medial hyperstriatum ventrale (IMHV) and lobus parolfactorius (LPO) regions of the chick brain is shown to be largely right hemisphere-lateralized. When chicks are subjected to a passive avoidance training experience, a specific increase in CaMKII phosphorylation is induced in the IMHV and LPO of the left hemisphere from those chicks whose memory for the training experience is successfully retrieved. While this specific increase in CaMKII phosphorylation is seen in both the left IMHV and left LPO in short-term memory, it is detectable only in the left LPO associated with long-term memory retrieval. The present results provide evidence that in vivo changes in phosphorylation of CaMKII are associated specifically with processing of distinct memory stages, which take place in specific brain regions.

Role of brain-derived neurotrophic factor and presynaptic proteins in passive avoidance learning in day-old domestic chicks

The consolidation of a one-trial passive avoidance learning task in the day-old chick involves a number of transient and longer-term biochemical processes, including increased release of glutamate. This study demonstrated that brain-derived neurotrophic factor, a proposed modulator of synaptic transmission and neurotransmitter release, is involved in the cascade associated with memory consolidation in the chick and that its actions were linked to modulation of expression of SNAP-25, syntaxin and synaptophysin, required for exocytosis. Intracerebral injections of 5 μl of antibodies to brain-derived neurotrophic factor into the left and right intermediate medial hyperstriatum ventrale resulted in a dose-dependent reduction in avoidance of an “aversive” bead by 3 h after training. Neurotrophin antibodies (0.5 μg/chick) administered between 1 h before, and up to 30 min after, training induced amnesia by 3 h which was sustained for at least 24 h. Injections of recombinant brain-derived neurotrophic factor (50 μg/ml; 0.5 μg/chick) just before training maintained avoidance in birds trained with a weaker aversant (10% methylanthranilate), such that chicks showed enhanced recall at times (24 h) beyond that when shorter-term forms of memory have decayed. In lysed synaptosomal membranes prepared from chicks injected with antibodies to brain-derived neurotrophic factor there was a decrease in expression of SNAP-25 and syntaxin in the left, but not the right, intermediate medial hyperstriatum ventrale, a region known to be associated with memory formation, which correlated with the decrease in neurotrophin concentration. Thus, these data indicate that brain-derived neurotrophic factor is involved in the formation of a long-term memory for an aversive stimulus and may function as a modulator of presynaptic proteins associated with exocytosis, enabling increases in neurotransmitter release.

Passive avoidance training and recall are associated with increased glutamate levels in the intermediate medial hyperstriatum ventrale of the day-old chick.

In the young chick, the intermediate medial hyperstriatum ventrale is involved in learning paradigms, including imprinting and passive avoidance learning. Biochemical changes in the intermediate medial hyperstriatum ventrale following learning include an up-regulation of amino-acid transmitter levels and receptor activity. To follow the changes of extracellular amino acid levels during passive avoidance training, we used an in vivo microdialysis technique. Probes were implanted in chicks before training the animals, either on a methyl- anthranylate-or water-coated bead. One hour later, recall was tested in both groups by presenting a similar bead. An increase of extra-cellular glutamate levels accompanied training and testing in both groups; during training, glutamate release was higher in methylanthranylate- trained than in water-trained chicks. When compared with the methylanthranylate-trained chicks during testing, the water-trained chicks showed enhanced extra-cellular glutamate levels. No other amino acid examined showed significant changes. After testing, the chicks were anesthetized and release- stimulated with an infusion of 50 mM potassium. Extra-cellular glutamate and taurine levels were significantly increased in both methylanthranylate-and water-trained chicks. The presentation of methylanthranylate as an. olfactory stimulus significantly enhanced glutamate levels, especially in methylanthranylate-trained chicks. The results suggest that such changes in extra-cellular glutamate levels in the intermediate medial hyperstriatum ventrale accompany pecking at either the water- or the methylanthranylate-bead. The taste of the aversant may be responsible for the greater increases found in methylanthranylate-trained birds.