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

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

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

PKA-activated ApAF-ApC/EBP heterodimer is a key downstream effector of ApCREB and is necessary and sufficient for the consolidation of long-term facilitation

Long-term memory requires transcriptional regulation by a combination of positive and negative transcription factors. Aplysia activating factor (ApAF) is known to be a positive transcription factor that forms heterodimers with ApC/EBP and ApCREB2. How these heterodimers are regulated and how they participate in the consolidation of long-term facilitation (LTF) has not, however, been characterized. We found that the functional activation of ApAF required phosphorylation of ApAF by PKA on Ser-266. In addition, ApAF lowered the threshold of LTF by forming a heterodimer with ApCREB2. Moreover, once activated by PKA, the ApAF-ApC/EBP heterodimer transactivates enhancer response element-containing genes and can induce LTF in the absence of CRE- and CREB-mediated gene expression. Collectively, these results suggest that PKA-activated ApAF-ApC/EBP heterodimer is a core downstream effector of ApCREB in the consolidation of LTF.

Consolidation of fear extinction requires protein synthesis in the medial prefrontal cortex

Extinction of conditioned fear is thought to form a long-term memory of safety, but the neural mechanisms are poorly understood. Consolidation of extinction learning in other paradigms requires protein synthesis, but the involvement of protein synthesis in extinction of conditioned fear remains unclear. Here, we show that rats infused intraventricularly with the protein synthesis inhibitor anisomycin extinguished normally within a session but were unable to recall extinction the following day. Anisomycin-treated rats showed no savings in the rate of re-learning of extinction, consistent with amnesia for extinction training. The identical effect was observed when anisomycin was microinfused into the medial prefrontal cortex (mPFC) but not the insular cortex. Furthermore, we observed that extinction training increased c-Fos levels in the mPFC but not in the insular cortex, consistent with extinction-induced gene expression in the mPFC. These findings extend previous lesion and unit-recording data by demonstrating that the mPFC is a critical storage site for extinction memory, rather than simply a pathway for expression of extinction. Understanding consolidation of fear extinction could lead to new treatments for anxiety disorders in which fear extinction is thought to be compromised.

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 microl 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.

Reminder effects - reconsolidation or retrieval deficit? Pharmacological dissection with protein synthesis inhibitors following reminder for a passive-avoidance task in young chicks

It is generally accepted that memory formation involves an irreversible passage via labile phases to the stable form of 'long-term memory' impervious to amnestic agents such as protein synthesis inhibitors. However, recent experiments demonstrate that reactivation of memory by way of a reminder renders it labile to such inhibitors, suggesting that such retrieval is followed by a so-called reconsolidation process similar or identical in its cellular and molecular correlates to that occurring during the initial consolidation. We compared the effects of the protein synthesis inhibitor anisomycin and the glycoprotein synthesis inhibitor 2-deoxygalactose on the temporal dynamics and pharmacological sensitivity of initial consolidation and memory expression following a reminder in a one-trial passive-avoidance task in day-old chicks. This comparison revealed three differences between the action of the inhibitors on newly formed compared with reactivated memory. First, the recall deficit after the reminder was temporary, whilst the amnesia following inhibitor treatment during training was stable. Second, the sensitive period for the effect of anisomycin was shorter in the reminder than in the training situation. Third, the effective dose for either inhibitor for reminder-associated amnesia was several times lower than for amnesia developing after training. Thus though like initial consolidation, memory expression at delayed periods following reminder depends on protein and glycoprotein synthesis, the differences between the temporal and pharmacological dynamics in the two situations point to the distinct character of the molecular processes involved in postreminder effects.

Reversed effects of RU486 and anisomycin on memory retention of light exposure or corticosterone facilitation in the dark-incubated chicks

Memory formation for a weak passive avoidance task in the dark-incubated chicks is facilitated by light exposure or corticosterone administration at optimally pre-hatch time points. To explore the potential mechanisms underlying activation of brain memory function development by light or corticosterone exposure during late embryo, steroid receptor antagonist RU486, or protein synthetic inhibitor anisomycin, was administered intraembryonically to the embryos of either only 24-h light-exposure or complete dark-hatched on embryonic day 20 (E20). The results showed that RU486 and anisomycin significantly retarded the facilitated retention both by light and corticosterone exposure in the dark-incubated chicks. They also suggest that the act of corticosterone or light exposure on the development of brain memory function is mediated by the effect of steroid receptor, or afterward on related protein syntheses that is involved in memory formation of post-hatched performance of day-old chicks.

Chloramphenicol-induced amnesia for passive avoidance training in the day-old chick

The antibiotic chloramphenicol, an inhibitor of mitochondrial protein synthesis, was used to investigate the time-related changes in protein synthesis following passive avoidance training in the day-old chick (white leghorn-black Australorp). Retention of memory for this simple learning task is known to be prevented by an inhibitor of cytosolic protein synthesis, anisomycin, in a biphasic manner, with the first phase of sensitivity occurring up to 90 min post-training and the second phase between 4 and 5 h post-training (Freeman, Rose, & Scholey, 1995). Birds received bilateral intracranial injections of chloramphenicol (10 microl/hemisphere of a 7.4 mM solution) at various times relative to training and were tested 24 h later. This report shows that at the second phase of anisomycin susceptibility there was a chloramphenicol-sensitive period (5 h post-training) which had an onset time less than 1 h after injection. The effect of chloramphenicol appears not to be due to the mitochondria being energetically compromised since intracranial injections of an uncoupler of mitochondrial oxidative phosphorylation, 2,4-dinitrophenol (0.1 mM), did not disrupt memory formation when injected 5 h after training, even though it did cause amnesia when injected at the earlier time point of 20 min post-training. These results are discussed in the context of what is already known about memory formation in the day-old chick.

Microinjections of anisomycin into the intermediate cerebellum during learning affect the acquisition of classically conditioned responses in the rabbit

The purpose of this study was to examine the effects of protein synthesis inhibition in the intermediate cerebellum on the acquisition and expression of classically conditioned nictitating membrane responses in the rabbit. Animals were conditioned for three days in a standard delay paradigm. Before each training session, either a solution of anisomycin (a protein synthesis inhibitor) or vehicle was bilaterally injected into the interposed cerebellar nuclear. Following these three training sessions, rabbits were tested to determine whether the previous training under the influence of anisomycin or vehicle resulted in the acquisition of conditioned responses. In this test, animals that were injected previously with the protein synthesis inhibitor exhibited significantly less retention of conditioned responses than rabbits injected with vehicle. Additional experiments demonstrated that anisomycin does not block the expression of conditioned responses during conditioning or in well-trained animals. Microinjections of muscimol at the same sites of the previous drug infusions suppressed the expression of conditioned responses, indicating that the protein synthesis inhibitor was applied to the eyeblink-related parts of cerebellar circuits. The obtained data are the first to demonstrate that a manipulation of cerebellar circuits, which does not affect the performance of learned behavior, can affect the process of learning. These results suggest that the synthesis of new proteins in the intermediate cerebellum participates in the formation of plastic changes responsible for eyeblink conditioning.

Two critical periods of protein and glycoprotein synthesis in memory consolidation for visual categorization learning in chicks

A protein synthesis inhibitor, anisomycin (ANI), and an inhibitor of glycoprotein synthesis, 2-deoxygalactose (2-D-gal), were used to investigate memory consolidation following visual categorization training in 2-day-old chicks. ANI (0.6 micromole/chick) and 2-D-gal (40 micromoles/chick) were injected intracerebrally at different time intervals from 1 hr before to 23 hr after the training. Retention was tested 24 hr post-training. Both ANI and 2-D-gal injections revealed two periods of memory sensitivity to pharmacological intervention. ANI impaired retention when injected from 5 min before to 30 min after the training or from 4 hr to 5 hr post-training, thus demonstrating that consolidation of long-term memory in this task requires two periods of protein synthesis. 2-D-Gal first produced an amnesia when it was injected in the interval from 5 min before to 5 min after the training. Injections made between 5 min and 5 hr post-training were without effect on the retention. The second period of memory impairment by 2-D-gal started at 5 hr post-training and lasted until 21 hr after the training. Administration of 2-D-gal made 23 hr after the training did not influence retention in the test at either 24 hr or 26 hr. These results are consistent with the hypothesis that two waves of protein and glycoprotein synthesis are necessary for the formation of long-term memory. The prolonged duration of performance impairment by 2-D-gal in the present task might reflect an extended memory consolidation period for a categorization form of learning.

Long-term habituation (LTH) in the crab Chasmagnathus: a model for behavioral and mechanistic studies of memory

A decade of studies on long-term habituation (LTH) in the crab Chasmagnathus is reviewed. Upon sudden presentation of a passing object overhead, the crab reacts with an escape response that habituates promptly and for at least five days. LTH proved to be an instance of associative memory and showed context, stimulus frequency and circadian phase specificity. A strong training protocol (STP) (> or = 15 trials, intertrial interval (ITI) of 171 s) invariably yielded LTH, while a weak training protocol (WTP) (< or = 10 trials, ITI = 171 s) invariably failed. STP was used with a presumably amnestic agent and WTP with a presumably hypermnestic agent. Remarkably, systemic administration of low doses was effective, which is likely to be due to the lack of an endothelial blood-brain barrier. LTH was blocked by inhibitors of protein and RNA synthesis, enhanced by protein kinase A (PKA) activators and reduced by PKA inhibitors, facilitated by angiotensin II and IV and disrupted by saralasin. The presence of angiotensins and related compounds in the crab brain was demonstrated. Diverse results suggest that LTH includes two components: an initial memory produced by spaced training and mainly expressed at an initial phase of testing, and a retraining memory produced by massed training and expressed at a later phase of testing (retraining). The initial memory would be associative, context specific and sensitive to cycloheximide, while the retraining memory would be nonassociative, context independent and insensitive to cycloheximide.

Cycloheximide inhibits context memory and long-term habituation in the crab Chasmagnathus

A shadow moving over head elicits an escape response in the crab Chasmagnathus that habituates promptly and for a long period. The effect of the protein synthesis inhibitor cycloheximide (CY) on this long-term memory was analyzed. Two hours after injection, 10 micrograms CY inhibited [14C]-amino amino acid incorporation into cerebral plus thoracic ganglia by 88% and 20 micrograms by 92%, but no inhibition was found at 24 h. A single injection of 10-20 micrograms CY given 30 min before training, failed to affect the short-term habituation. Similar doses impaired both context memory (CM) and long-term habituation (LTH) when tested at 72 and 120 h but only CM at 24 h. Such a disparity was explained by an unspecific depressing effect upon the response, attributed to an interaction between CY and training. The hypothesis was confirmed, because CY injected immediately after training disclosed amnestic effect at 24 h on both CM and LTH. A similar effect was proven when animals were injected at 2 h but not at 6 h after training. Results from experiments with pretraining and pretesting injections put aside a state-dependence or retrieval deficit effects of the drug. Taken together, findings of this article argue strongly for de novo protein synthesis as a mechanism of LTH and for the close relation between CM and LTH.

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.

Corticosterone enhances long-term retention in one-day-old chicks trained in a weak passive avoidance learning paradigm

Glucocorticoids are released during learning situations and can trigger neural actions through binding to receptors in different brain areas. The possible role of a glucocorticoid action in long-term memory formation was studied, in day-old chicks, by using a passive avoidance task which chicks otherwise only retain for a few hours (< 10) after training. Thus, we examined the effects of intracerebral corticosterone administration on retention 24 h posttraining. The results showed that chicks injected with corticosterone (1 microgram) at either 15 min pretraining or at 5, 30, 60 min (but not 120, 180, or 360 min) posttraining retained the passive avoidance response when tested 24 h posttraining. Studies with specific mineralocorticoid or glucocorticoid receptor antagonists (RU 28318 or RU 38486, respectively) indicated that this increase in retention by corticosterone might be mediated through glucocorticoid receptors. In order to assess whether the facilitatory effect of corticosterone was mediated through an effect on protein synthesis mechanisms, the protein synthesis inhibitor anisomycin was administered prior to corticosterone. However, this treatment only partially attenuated the effect of the steroid, suggesting that corticosterone may influence other cellular processes involved in the formation of long-term memory for the avoidance behaviour.

Inhibiting synthesis of the putative retrograde messenger nitric oxide results in amnesia in a passive avoidance task in the chick

It has been hypothesized that nitric oxide (NO) may act as a 'retrograde messenger' in the CNS, mediating intersynaptic communication in the context of neural plasticity during memory formation. To test this hypothesis the effects of the competitive NO synthase inhibitor N-nitro-L-arginine (NARG) on memory retention has been studied in a one-trial passive avoidance task in the day-old chick. Intracerebral injections before training into the intermediate medial hyperstriatum ventrale (IMHV), an area that is of crucial importance in learning in the chick, produced amnesia in this task when tested at various time points from 30 min to 24 h after training. Time- and dose dependencies of NARG when injected i.p. or i.c. were evaluated. Injection into one IMHV alone (left or right) proved to be sufficient to produce amnesia. Diffusion of NARG into the untreated hemisphere was ruled out by injecting it with L-arginine, which competes with NARG and prevents inhibition of NO synthase. Additional tests showed that the amnestic effect is not due to state-dependent learning, nor to interference of the drug with general motor ability or motivation.

Taste memory: the role of protein synthesis in gustatory cortex

Application of the protein synthesis inhibitor anisomycin to the rat gustatory cortex before and during training impairs conditioned taste aversion (CTA) to saccharin. No behavioral impairment is observed if the inhibitor is applied to an adjacent cortical area or to one cortical hemisphere only. The consumption of saccharin and of total fluid, as well as behavioral recognition of saccharin, is not affected. Preexposure of rats to saccharin several days before training markedly inhibits CTA to that taste. Injection of anisomycin to the gustatory cortex immediately prior to the preexposure period attenuates the latent inhibition. These results suggest that protein synthesis in the gustatory cortex is required for normal acquisition of the memory of taste.

Is anisomycin-induced amnesia for a passive avoidance task in chicks the result of state-dependent learning?

Many studies suggest that protein synthesis is required for formation of long-term memory. To test whether the protein synthesis inhibitor anisomycin (ANI) actually inhibits long-term memory formation or whether apparent amnesia could be attributed to state-dependency, chicks were both trained and tested under the influence of anisomycin (ANI). Two-day-old cockerels were trained in a 1-trial passive avoidance task. Intracerebral injections (10 microliters per hemisphere) of either saline (SAL) or 11.0 mM ANI were made into the medial hyperstriatum ventrale 5 min pretraining and 5 min pretest. The ANI inhibited cerebral protein synthesis by 70-80%, a level necessary to cause amnesia. Chicks that pecked a small bead dipped in methylanthranilate (MeA) and were injected with SAL both pretraining and pretest avoided pecking at test, showing memory for the bitter substance; chicks given ANI pretraining and SAL pretest pecked at the bead during test, which suggests amnesia. However, those given ANI both pretraining and pretest showed marked avoidance at test. Chicks trained to peck at a small bead dipped in water and given injections of either SAL or ANI pretraining and SAL pretest pecked readily at test. However, water-trained chicks given ANI pretest, regardless of pretraining injection, showed significantly higher avoidance at test. We conclude that peck aversion in the ANI-MeA-ANI group was not due to state-dependency but to generalized avoidance induced by pretest ANI.