Ivan Izquierdo

Retrieval of memory for fear-motivated training initiates extinction requiring protein synthesis in the rat hippocampus

Evidence that protein synthesis inhibitors induce amnesia in a variety of species and learning paradigms indicates that the consolidation of newly acquired information into stable memories requires the synthesis of new proteins. Because extinction of a response also requires acquisition of new information, extinction, like original learning, would be expected to require protein synthesis. The present experiments examined the involvement of protein synthesis in the hippocampus in the extinction of a learned fear-based response known to involve the hippocampus. Rats were trained in a one-trial inhibitory avoidance task in which they received footshock after stepping from a small platform to a grid floor. They were then given daily retention tests without footshock. The inhibitory response (e.g., remaining on the platform) gradually extinguished with repeated testing over several days. Footshock administered in a different context, instead of a retention test, prevented the extinction. Infusions of the protein synthesis inhibitor anisomycin (80 μg) into the CA1 region of the hippocampus (bilaterally) 10 min before inhibitory avoidance training impaired retention on all subsequent tests. Anisomycin infused into the hippocampus immediately after the 1st retention test blocked extinction of the response. Infusions administered before the 1st retention test induced a temporary (i.e., 1 day) reduction in retention performance and blocked subsequent extinction. These findings are consistent with other evidence that anisomycin blocks both the consolidation of original learning and extinction.

Two Time Periods of Hippocampal mRNA Synthesis Are Required for Memory Consolidation of Fear-Motivated Learning

Information storage in the brain is a temporally graded process involving different memory types or phases. It has been assumed for over a century that one or more short-term memory (STM) processes are involved in processing new information while long-term memory (LTM) is being formed. It has been repeatedly reported that LTM requiresde novo RNA synthesis around the time of training. Here we show that LTM formation of a one-trial inhibitory avoidance training in rats, a hippocampal-dependent form of contextual fear conditioning, depends on two consolidation periods requiring synthesis of new mRNAs. By injecting the RNA polymerase II inhibitors 5,6-dichloro-1-β-d-ribofuranosylbenzimidazole or α-amanitin into the CA1 region of the dorsal hippocampus at various times before and after training, we found that hippocampal gene expression is critical in two time windows: around the time of training and 3–6 hr after training. Interestingly, these two periods of sensitivity to transcriptional inhibitors are similar to those observed using the protein synthesis inhibitor anisomycin. These findings underscore the parallel dependence of LTM formation of contextual fear on mRNA and protein synthesis in the hippocampus and suggest that the two time periods of anisomycin-induced amnesia depend at least in part on new mRNA synthesis.

Hippocampal cGMP and cAMP are differentially involved in memory processing of inhibitory avoidance learning

Cyclic GMP (cGMP) and cyclic AMP (cAMP) have been proposed to participate in the early and late stages of long-term potentiation (LTP), respectively. Here we report on the effect of post-training intrahippocampal infusion of membrane-permeable analogues of these cyclic nucleotides on the consolidation of inhibitory avoidance learning in rats and on the effect of this task on hippocampal cGMP and cAMP levels. Bilateral intrahippocampal microinjection of 8 Br-cGMP (1.25 μg per side) enhanced retention test performance when given immediately (0 min), but not when given 180 min, after training. In marked contrast, intrahippocampal infusion of the same dose of 8 Br-cAMP facilitated memory consolidation when given 180 min, but not 0 min, after training. Rats submitted to an inhibitory avoidance task showed a significant increase in the amount of cGMP in the hippocampus at 0 and 30 min after training, and in the amount of cAMP 30 and 180 min after training. Taken together, these results indicate that cGMP-regulated processes in the hippocampus play an important role in the early stages of memory consolidation and that cAMP signalling pathways are involved in the late post-training memory processing of inhibitory avoidance learning.

Muscimol injections in the medial septum impair spatial learning

These experiments examined the role of GABAergic systems in modulating septohippocampal cholinergic influences on learning. Microinjections of the GABA(A) agonist muscimol (0.5, 1.0 or 5.0 nmol) or physiological saline were administered (0.5 microliters) into the medial septum of rats via chronically implanted cannulae just prior to daily training in the Morris water maze spatial learning task. The animals received 3 training trials on each of 4 days. The escape latencies of rats trained with a submerged escape platform at a fixed location were significantly shorter than those trained with a randomly located platform. Rate of learning of the fixed location was significantly impaired in rats given pretraining muscimol injections in the medial septum at doses (1.0 and 5.0 nmol) that significantly reduced hippocampal high-affinity choline uptake (HACU). Analyses of responses on a probe trial with no pretraining injections and no platform revealed that, in comparison with controls, animals that had received muscimol prior to each training session were likely to swim in the region where the platform had been located. The finding that muscimol-injected rats were subsequently able to learn the task when trained without muscimol injections indicates that the acquisition impairment was not due to a lasting effect of the drug injections. Our results are consistent with the view that the septal GABAergic modulation of the septohippocampal cholinergic pathway is involved in regulating the acquisition of spatial information.

Memory facilitation by naloxone is due to release of dopaminergic and beta-adrenergic systems from tonic inhibition

The post-training IP administration of naloxone (0.8 mg/kg) facilitates memory consolidation of the habituation of a rearing response to a tone in rats. Amphetamine (1.0–2.5 mg/kg or nicotine (0.2–0.5 mg/kg), and amphetamine (2.5 mg/kg) plus nicotine (0.5 mg/kg) have no effect. The higher doses of amphetamine or nicotine, however, when given together with a dose of naloxone which is ineffective alone (0.2 mg/kg), markedly enhance consolidation. Haloperidol (0.5 mg/kg), propranolol (0.5 mg/kg), and phenoxybenzamine (2.0 mg/kg) have no effect on their own; whereas tolazoline (2.0 mg/kg) impairs consolidation. The effect of naloxone (0.8 mg/kg) is antagonized by haloperidol and by propranolol, but not by phenoxybenzamine or tolazoline. The results suggest that naloxone causes memory facilitation through the release of central dopaminergic and beta-adrenergic mechanisms from a tonic inhibitory influence of endogenous opiate peptide systems.

Cholinergic neurotransmission and synaptic plasticity concerning memory processing

The brain is able to change the synaptic strength in response to stimuli that leave a memory trace. Long-term potentiation (LTP) and long-term depression (LTD) are forms of activity-dependent synaptic plasticity proposed to underlie memory. The induction of LTP appears mediated by glutamate acting on AMPA and then on NMDA receptors. Cholinergic muscarinic agonists facilitate learning and memory. Acetylcholine depolarizes pyramidal neurons, reduces inhibition, upregulates NMDA channels and activates the phosphoinositide cascade. Postsynaptic Ca2+ rises and stimulates Ca-dependent PK, promoting synaptic changes. Electroencephalographic desynchronization and hippocampal theta rhythm are related to learning and memory, are inducible by Cholinergic agonists and elicited by hippocampal Cholinergic terminals. Their loss results in memory deficits. Hence, Cholinergic pathways may act synergically with glutamatergic transmission, regulating and leading to synaptic plasticity. The stimulation that induces plasticity in vivo has not been established. The patterns for LTP/LTD induction in vitro may be due to the loss of ascending Cholinergic inputs. As a rat explores pyramidal cells fire bursts that could be relevant to plasticity.

Behavioral and genoprotective effects of Vaccinium berries intake in mice

Studies have shown that supplementation with berries rich in anthocyanins are effective in reducing oxidative stress associated with aging, and are beneficial in reversing age-related neuronal and behavioral changes. However, there are few reports on other biological activities of these polyphenols, such as genoprotective effects. The present experiments were performed to study the possible effects of 30-day administration of a lyophilized extract of Vaccinium ashei berries on cognitive performance using step-down inhibitory avoidance, open-field habituation and elevated plus-maze tasks, as well as on DNA damage in the hippocampus and cerebral cortex. The present study showed that the extract significantly enhanced long-term memory in the inhibitory avoidance task, induced an increase in the number of crossings during open-field habituation and had an anxiolytic effect in the elevated plus-maze task. Moreover, the extract reduced oxidative DNA damage in brain tissue in vitro. These results suggest that supplementation with V. ashei berries to mice improves performance on memory tasks and has a protective effect on DNA damage, possibly due to the antioxidant activity of polyphenols, including anthocyanins.

Beta-endorphin causes retrograde amnesia and is released from the rat brain by various forms of training and stimulation

The endogenous opiate peptide, beta-endorphin (0.4, 1.0, 2.0, and 10.0 μg/kg) was injected IP into rats immediately after training in a shuttle avoidance task, and its effect on memory retention was evaluated in test sessions carried out 24 h later. The drug was found to cause retrograde amnesia, the ED50 being 1.0 μg/kg. Beta-endorphin immunoreactivity was measured in the hypothalamus and rest of the brain of rats submitted to training, or test sessions of shuttle avoidance learning, pseudoconditioning in the shuttle-box, tones alone, or foot-shocks alone. After training in any of the four paradigms, there was a marked (46–60%) depletion of beta-endorphin immunoreactivity in the rest of the brain. No changes were detected in the hypothalamus or after test sessions. The loss of beta-endorphin immunoreactivity may be attributed to release of this substance caused by the stimuli used for training. From the present findings, as well as previous observations on the memory-facilitating influence of the opiate receptor antagonist, naloxone, it is concluded that there is a physiological amnesic mechanism mediated by beta-endorphin (and perhaps other opoid peptides as well), which is triggered by the non-associative factors present in the various forms of learning.

Role of hippocampal NO in the acquisition and consolidation of inhibitory avoidance learning

NITRIC oxide (NO), an unconventional neurotransmitter in the brain, has been postulated as a retrograde intercellular messenger necessary for the induction, but not the maintenance phase, of activity-dependent forms of synaptic plasticity in the hippocampus. Here we report on the effects of an inhibitory avoidance learning task on hippocampal NO synthase (NOS) activity and on the effects of intrahippocampal infusion of a NOS inhibitor in the acquisition and consolidation of this task in rats. NOS activity increases by 45% in the hippocampus immediately after training (0 min) but not at 60 min after training. No changes were observed in cerebellar NOS activity. The bilateral intrahippocampal microinjection of nitro-arginine (NO-arg), an NOS inhibitor, provoked retrograde amnesia for the inhibitory avoidance when given 10 min before or immediately after training, but not 60 min after training. These results suggest that NO-regulated processes in the hippocampus play an important role at the time of training or very shortly thereafter of an inhibitory avoidance learning.

Delayed wave of c-Fos expression in the dorsal hippocampus involved specifically in persistence of long-term memory storage

Memory formation is a temporally graded process during which transcription and translation steps are required in the first hours after acquisition. Although persistence is a key characteristic of memory storage, its mechanisms are scarcely characterized. Here, we show that long-lasting but not short-lived inhibitory avoidance long-term memory is associated with a delayed expression of c-Fos in the hippocampus. Importantly, this late wave of c-Fos is necessary for maintenance of inhibitory avoidance long-term storage. Moreover, inhibition of transcription in the dorsal hippocampus 24 h after training hinders persistence but not formation of long-term storage. These findings indicate that a delayed phase of transcription is essential for maintenance of a hippocampus-dependent memory trace. Our results support the hypothesis that recurrent rounds of consolidation-like events take place late after learning in the dorsal hippocampus to maintain memories.