The Drosophila radish gene encodes a protein required for anesthesia-resistant memory

Long-term memory in Drosophila is separable into two components: consolidated, anesthesia-resistant memory and long-lasting, protein-synthesis-dependent memory. The Drosophila memory mutant radish is specifically deficient in anesthesia-resistant memory and so represents the only molecular avenue to understanding this memory component. Here, we have identified the radish gene by positional cloning and comparative sequencing, finding a mutant stop codon in gene CG15720 from the Drosophila Genome Project. Induction of a wild-type CG15720 transgene in adult flies acutely rescues the mutant's memory defect. The phospholipase A2 gene, previously identified as radish [Chiang et al. (2004) Curr. Biol. 14:263-272], maps 95 kb outside the behaviorally determined deletion interval and is unlikely to be radish. The Radish protein is highly expressed in the mushroom bodies, centers of olfactory memory. It encodes a protein with 23 predicted cyclic-AMP-dependent protein kinase (PKA) phosphorylation sequences. The Radish protein has recently been reported to bind to Rac1 [Formstecher et al. (2005) Genome Res. 15:376-384], a small GTPase that regulates cytoskeletal rearrangement and influences neuronal and synaptic morphology.

The inverted "u-shaped" dose-effect relationships in learning and memory: modulation of arousal and consolidation

In the ample field of biological non-linear relationships there is also the inverted U-shaped dose-effect. In relation to cognitive functions, this phenomenon has been widely reported for many active compounds, in several learning paradigms, in several animal species and does not depend on either administration route (systemic or endocerebral) or administration time (before or after training). This review summarizes its most interesting aspects. The hypothesized mechanisms supporting it are reported and discussed, with particular emphasis on the participation of emotional arousal levels in the modulation of memory processes. Findings on the well documented relationship between stress, emotional arousal, peripheral epinephrine levels, cerebral norepinephrine levels and memory consolidation are reported. These are discussed and the need for further research is underlined.

Time-dependent reorganization of the brain components underlying memory retention in trace eyeblink conditioning

Many studies have confirmed the time-limited involvement of the hippocampus in mnemonic processes and suggested that there is reorganization of the responsible brain circuitry during memory consolidation. To clarify such reorganization, we chose trace classical eyeblink conditioning, in which hippocampal ablation produces temporally graded retrograde amnesia. Here, we extended the temporal characterization of retrograde amnesia to other regions that are involved in acquisition during this task: the medial prefrontal cortex (mPFC) and the cerebellum. At a various time interval after establishing the trace conditioned response (CR), rats received an aspiration of one of the three regions. After recovery, the animals were tested for their CR retention. When ablated 1 d after the learning, both the hippocampal lesion and the cerebellar lesion group of rats exhibited a severe impairment in retention of the CR, whereas the mPFC lesion group showed only a slight decline. With an increase in interval between the lesion and the learning, the effect of the hippocampal lesion diminished and that of the mPFC lesion increased. When ablated 4 weeks after the learning, the hippocampal lesion group exhibited as robust CRs as its corresponding control group. In contrast, the mPFC lesion and the cerebellar lesion groups failed to retain the CRs. These results indicate that the hippocampus and the cerebellum, but only marginally the mPFC, constitute a brain circuitry that mediates recently acquired memory. As time elapses, the circuitry is reorganized to use mainly the mPFC and the cerebellum, but not the hippocampus, for remotely acquired memory.

Post-training intra-basolateral amygdala infusions of norepinephrine enhance consolidation of memory for contextual fear conditioning

Post-training infusions of drugs, including noradrenergic agonists and antagonists, into the basolateral amygdala (BLA) influence the consolidation of memory for training in several tasks, including inhibitory avoidance. There is, however, conflicting evidence concerning whether post-training intra-BLA drug infusions modulate the consolidation of contextual fear conditioning (CFC). In the present study, norepinephrine (NE) was infused bilaterally into the BLA of male Sprague Dawley rats immediately after training on two CFC tasks: a Y-maze and a straight alley. Post-training intra-BLA infusions enhanced memory of CFC training in the Y-maze, as assessed by percentage of time spent freezing and shock arm entrance latencies. Post-training intra-BLA infusions of NE enhanced 48 hr retention of CFC training in the straight alley, as assessed by shock compartment entrance latencies and the number of shocks required to learn to avoid entering the shock compartment. These findings indicate that the consolidation of memory for CFC, like that for inhibitory avoidance training, is influenced by post-training neuromodulatory influences within the BLA. Thus, the findings provide additional evidence consistent with the hypothesis that the BLA has a general role in modulating memory consolidation.

Blockade of NMDA receptors in prelimbic cortex induces an enduring amnesia for odor-reward associative learning

The competitive antagonist 2-amino-5-phosphonoeptanoic acid (APV) was injected intracerebroventricularly to determine the involvement of NMDA receptors in different stages of memory consolidation. Subsequent experiments used local injections to determine possible sites of drug action. Rats were trained in a rapidly learned olfactory task to find palatable food in a hole in a sponge impregnated with the target odor in the presence of two other sponges with nonrewarded odors. APV injections were made intracerebroventricularly 5 min or 2 hr after the end of the training, and a retention test was given 48 hr later. The results showed that blockade of NMDA receptors immediately after training induces a profound and enduring amnesia with no effect when the treatment is delayed at 2 hr after training. To address the question of the effective sites of action of the intracerebroventricular treatment, APV injections into the hippocampus and into the prelimblic region of the frontal cortex (PLC) were made. Blockade of NMDA receptors into the PLC but not into the hippocampus impaired memory formation of the odor-reward association. The amnesia is not transient, because the retention tests were made 48 hr after training. These results underlie the role of NMDA receptors in the early stage of consolidation of a simple odor-reward associative memory and confirm the role of the PLC in the consolidation of long-term memory.

Postlearning consolidation of birdsong: stabilizing effects of age and anterior forebrain lesions

Birdsong is a learned, sequenced motor skill. For the zebra finch, learned song normally remains unchanging beyond early adulthood. However, stable adult song will gradually deteriorate after deafening (Nordeen and Nordeen, 1992), indicating an ongoing influence of auditory feedback on learned song. This plasticity of adult song in response to deafening gradually declines with age (Lombardino and Nottebohm, 2000), suggesting that, after song learning, there continue to be changes in the brain that progressively stabilize the song motor program. A qualitatively similar stabilization of learned song can be precipitated artificially by lesions of a basal ganglia circuit in the songbird anterior forebrain (Brainard and Doupe, 2000), raising the question of whether and how these two forms of song stabilization are related. We investigated this issue by characterizing the deterioration of song that occurs after deafening in young adult birds and the degree to which that deterioration is reduced by age or by lesions of the anterior forebrain that were directed at the lateral portion of the magnocellular nucleus of the anterior neostriatum (LMAN). In most respects, LMAN lesions stabilized song to a significantly greater extent than did aging; whereas old-deafened birds eventually exhibited significant deterioration of song, lesioned-deafened birds generally did not differ from controls. The one exception was for song tempo, which was significantly stabilized by age, but not by LMAN lesions. The results indicate that LMAN lesions do not simply mimic a normal aging process, and likewise suggest that the anterior forebrain pathway continues to play a role even in the residual song plasticity that is observed after the age-dependent stabilization of song.

Basolateral amygdala noradrenergic influence enables enhancement of memory consolidation induced by hippocampal glucocorticoid receptor activation

Previously, we reported that bilateral excitotoxic lesions of the basolateral nucleus of the amygdala (BLA) block the enhancing effects of posttraining systemic or intrahippocampal glucocorticoid administration on memory for inhibitory avoidance training. The present study further examined the basis of this permissive influence of the BLA on hippocampal memory functioning. Immediate posttraining unilateral infusions of the specific glucocorticoid receptor agonist RU 28362 (11beta,17beta-dihydroxy-6, 21-dimethyl-17alpha-pregna-4,6-trien-20-yn-3-one; 3.0, 10.0, or 30.0 ng in 0.5 microliter) administered into the dorsal hippocampus of male Sprague-Dawley rats induced dose-dependent enhancement of 48-h inhibitory avoidance retention. Infusions of the beta-adrenoceptor antagonist atenolol (0.5 microgram in 0.2 microliter) into the ipsilateral, but not the contralateral, BLA 10 min prior to training blocked the hippocampal glucocorticoid effects on memory consolidation. Infusions of the muscarinic cholinergic antagonist atropine (0.5 microgram in 0.2 microliter) into either the ipsilateral or contralateral BLA before training did not block the hippocampal glucocorticoid effects. These findings provide further evidence that beta-adrenergic activity in the BLA is essential in enabling glucocorticoid-induced modulation of memory consolidation and are consistent with the hypothesis that the BLA regulates the strength of memory consolidation in other brain structures. The ipsilateral nature of the BLA-hippocampus interaction indicates that BLA influences on hippocampal memory processes are mediated through neural pathways rather than by influences by means of the activation of peripheral stress responses.

Reactivation of hippocampal cell assemblies: effects of behavioral state, experience, and EEG dynamics

During slow wave sleep (SWS), traces of neuronal activity patterns from preceding behavior can be observed in rat hippocampus and neocortex. The spontaneous reactivation of these patterns is manifested as the reinstatement of the distribution of pairwise firing-rate correlations within a population of simultaneously recorded neurons. The effects of behavioral state [quiet wakefulness, SWS, and rapid eye movement (REM)], interactions between two successive spatial experiences, and global modulation during 200 Hz electroencephalographic (EEG) "ripples" on pattern reinstatement were studied in CA1 pyramidal cell population recordings. Pairwise firing-rate correlations during often repeated experiences accounted for a significant proportion of the variance in these interactions in subsequent SWS or quiet wakefulness and, to a lesser degree, during SWS before the experience on a given day. The latter effect was absent for novel experiences, suggesting that a persistent memory trace develops with experience. Pattern reinstatement was strongest during sharp wave-ripple oscillations, suggesting that these events may reflect system convergence onto attractor states corresponding to previous experiences. When two different experiences occurred in succession, the statistically independent effects of both were evident in subsequent SWS. Thus, the patterns of neural activity reemerge spontaneously, and in an interleaved manner, and do not necessarily reflect persistence of an active memory (i.e., reverberation). Firing-rate correlations during REM sleep were not related to the preceding familiar experience, possibly as a consequence of trace decay during the intervening SWS. REM episodes also did not detectably influence the correlation structure in subsequent SWS, suggesting a lack of strengthening of memory traces during REM sleep, at least in the case of familiar experiences.

Effect of corticotropin and desglycinamide 9 -lysine vasopressin on suppression of memory by puromycin

The puromycin-induced blockade of expression of maze learning in mice can be prevented by subcutaneous administration of "Purified Cortrophin Gel" up to 3 days prior to training. A similar protective effect was not found when highly purified corticotropin was tested, but was observed after administration of desglycinamide(9)-lysine vasopressin. It appears that pressorpeptides are more potent protective agents than corticotropin when administered subcutaneously, and that vasopressin contamination of the commercial preparation was probably responsible for the protective effects previously reported. These experiments suggest that vasopressin and its congeners modify memory consolidation in such a way that the "expression" of memory becomes insensitive to puromycin.

Pituitary peptides and the suppression of memory by puromycin

It has previously been shown that the expression of memory of maze-learning in mice is blocked by intracerebrally-injected puromycin one or more days after the training experience, and that bilateral adrenalectomy before training protects memory against this effect of puromycin. Purified cortrophin gel, injected subcutaneously up to 3 days before training, has now been found to give results like those obtained with adrenalectomy and, additionally, to provide a high degree of protection when injected up to 16 hr after training. Several possible explanations for these effects have been considered and found to be inadequate.