Long-term memory is formed immediately without the need for protein synthesis-dependent consolidation in Drosophila.

It is believed that long-term memory (LTM) cannot be formed immediately because it must go through a protein synthesis-dependent consolidation process. However, the current study uses Drosophila aversive olfactory conditioning to show that such processes are dispensable for context-dependent LTM (cLTM). Single-trial conditioning yields cLTM that is formed immediately in a protein-synthesis independent manner and is sustained over 14 days without decay. Unlike retrieval of traditional LTM, which requires only the conditioned odour and is mediated by mushroom-body neurons, cLTM recall requires both the conditioned odour and reinstatement of the training-environmental context. It is mediated through lateral-horn neurons that connect to multiple sensory brain regions. The cLTM cannot be retrieved if synaptic transmission from any one of these centres is blocked, with effects similar to those of altered encoding context during retrieval. The present study provides strong evidence that long-term memory can be formed easily without the need for consolidation.

Genetic dissection of active forgetting in labile and consolidated memories in Drosophila.

Different memory components are forgotten through distinct molecular mechanisms. In Drosophila, the activation of 2 Rho GTPases (Rac1 and Cdc42), respectively, underlies the forgetting of an early labile memory (anesthesia-sensitive memory, ASM) and a form of consolidated memory (anesthesia-resistant memory, ARM). Here, we dissected the molecular mechanisms that tie Rac1 and Cdc42 to the different types of memory forgetting. We found that 2 WASP family proteins, SCAR/WAVE and WASp, act downstream of Rac1 and Cdc42 separately to regulate ASM and ARM forgetting in mushroom body neurons. Arp2/3 complex, which organizes branched actin polymerization, is a canonical downstream effector of WASP family proteins. However, we found that Arp2/3 complex is required in Cdc42/WASp-mediated ARM forgetting but not in Rac1/SCAR-mediated ASM forgetting. Instead, we identified that Rac1/SCAR may function with formin Diaphanous (Dia), a nucleator that facilitates linear actin polymerization, in ASM forgetting. The present study, complementing the previously identified Rac1/cofilin pathway that regulates actin depolymerization, suggests that Rho GTPases regulate forgetting by recruiting both actin polymerization and depolymerization pathways. Moreover, Rac1 and Cdc42 may regulate different types of memory forgetting by tapping into different actin polymerization mechanisms.

Social Isolation Induces Rac1-Dependent Forgetting of Social Memory.

Social isolation (SI) has detrimental effects on human and animal cognitive functions. In particular, acute isolation in adult mice impairs social recognition memory (SRM). Previous accounts of this impairment have focused primarily on memory consolidation. However, the current study suggests that impaired SRM results from enhanced forgetting. SI accelerates SRM decay without affecting memory formation. The impairment is caused by elevated Rac1 activity in the hippocampus. Using adeno-associated-virus-based genetic manipulation, we found that inhibition of Rac1 activity blocked forgetting of SRM in isolated adult mice, whereas activation of Rac1 accelerated forgetting in group-housed mice. Moreover, resocialization reversed the accelerated forgetting following isolation in correlation with suppression of Rac1 activity. In addition, accelerated long-term potentiation (LTP) decay in isolated mice brain slices was rescued by inhibition of Rac1 activity. Taken together, the findings lead us to conclude that social memory deficits in isolated mice are mediated by enhanced Rac1-dependent forgetting.

Fan-Shaped Body Neurons in the Drosophila Brain Regulate Both Innate and Conditioned Nociceptive Avoidance.

Multiple brain regions respond to harmful nociceptive stimuli. However, it remains unclear as to whether behavioral avoidance of such stimuli can be modulated within the same or distinct brain networks. Here, we found subgroups of neurons localized within a well-defined brain region capable of mediating both innate and conditioned nociceptive avoidance in Drosophila. Neurons in the ventral, but not the dorsal, of the multiple-layer organized fan-shaped body (FB) are responsive to electric shock (ES). Silencing ES-responsive neurons, but not non-responsive neurons, leads to reduced avoidance of harmful stimuli, including ES and heat shock. Activating these neurons consistently triggers avoidance and can serve as an unconditional stimulus in an aversive classical conditioning task. Among the three groups of responsive neurons identified, two also have reduced activity in ES-conditioned odor avoidance. These results demonstrate that both innate and conditioned nociceptive avoidance might be represented within neurons confined to a single brain region.

Active Protection: Learning-Activated Raf/MAPK Activity Protects Labile Memory from Rac1-Independent Forgetting.

Active forgetting explains the intrinsic instability of a labile memory lasting for hours. However, how such memory maintains stability against unwanted disruption is not completely understood. Here, we report a learning-activated active protection mechanism that enables labile memory to resist disruptive sensory experiences in Drosophila. Aversive olfactory conditioning activates mitogen-activated protein kinase (MAPK) transiently in the mushroom-body γ lobe, where labile-aversive memory is stored. This increased MAPK activity significantly prolongs labile memory retention and enhances its resistance to disruption induced by heat shock, electric shock, or odor reactivation. Such experience-induced forgetting cannot be prevented by inhibition of Rac1 activity. Instead, protection of Rac1-independent forgetting correlates with non-muscle myosin II activity and persistence of learning-induced presynaptic structural changes. Increased Raf/MAPK activity, together with suppressed Rac1 activity, completely blocks labile memory decay. Thus, learning not only leads to memory formation, but also activates active protection and active forgetting to regulate the formed memory.

Altered resting-state regional homogeneity after 13 weeks of paliperidone injection treatment in schizophrenia patients.

This study aimed to explore the effects of the long-acting antipsychotic drug palmitate paliperidone in resting-state brain activity of schizophrenia patients. Seventeen schizophrenia outpatients were included and received palmitate paliperidone injection (PAL) treatment for 13 weeks. These patients were compared to seventeen matched healthy controls. All subjects underwent two scan sessions of resting-state magnetic resonance imaging (baseline and the 13th week) and regional homogeneity (ReHo) at resting-state where compared. After 13 weeks of treatment, PAL increased ReHo of the prefrontal cortex, anterior cingulate gyrus and orbital frontal gyrus, while PAL decreased ReHo of the thalamus, parahippocampal gyrus and superior temporal gyrus. Furthermore, improvement of psychiatric symptoms correlated with changing amplitude of ReHo: positively correlated with postcentral gyrus and negatively correlated with the occipital cortex. Baseline ReHo values of the middle occipital gyrus were positively correlated with the rate of reduction of psychiatric symptoms and improvement of social function. These results suggested that PAL might achieve its clinical effect in schizophrenia by influencing the resting-state function of the occipital cortex, lateral prefrontal cortex and temporal lobe. Baseline function of the inferior occipital gyrus might potentially predict the short-term effect of PAL in schizophrenia.

Hippocampal Activation of Rac1 Regulates the Forgetting of Object Recognition Memory.

Forgetting is a universal feature for most types of memories. The best-defined and extensively characterized behaviors that depict forgetting are natural memory decay and interference-based forgetting [1, 2]. Molecular mechanisms underlying the active forgetting remain to be determined for memories in vertebrates. Recent progress has begun to unravel such mechanisms underlying the active forgetting [3-11] that is induced through the behavior-dependent activation of intracellular signaling pathways. In Drosophila, training-induced activation of the small G protein Rac1 mediates natural memory decay and interference-based forgetting of aversive conditioning memory [3]. In mice, the activation of photoactivable-Rac1 in recently potentiated spines in a motor learning task erases the motor memory [12]. These lines of evidence prompted us to investigate a role for Rac1 in time-based natural memory decay and interference-based forgetting in mice. The inhibition of Rac1 activity in hippocampal neurons through targeted expression of a dominant-negative Rac1 form extended object recognition memory from less than 72 hr to over 72 hr, whereas Rac1 activation accelerated memory decay within 24 hr. Interference-induced forgetting of this memory was correlated with Rac1 activation and was completely blocked by inhibition of Rac1 activity. Electrophysiological recordings of long-term potentiation provided independent evidence that further supported a role for Rac1 activation in forgetting. Thus, Rac1-dependent forgetting is evolutionarily conserved from invertebrates to vertebrates.

Cdc42-Dependent Forgetting Regulates Repetition Effect in Prolonging Memory Retention.

Repeated learning is used daily and is a powerful way to improve memory. A fundamental question is how multiple learning trials add up to improve memory. While the major studies so far of such a repetition effect have emphasized the strengthening of memory formation, the current study reveals a molecular mechanism through suppression of forgetting. We find that single-session training leads to formation of anesthesia-resistant memory (ARM) and then activation of the small G protein Cdc42 to cause decay or forgetting of ARM within 24 hr. Repetition suppresses the activation of Cdc42-dependent forgetting, instead of enhancing ARM formation, leading to prolonged ARM. Consistently, inhibition of Cdc42 activity through genetic manipulation mimicked the repetition effect, while repetition-induced ARM improvement was abolished by elevated Cdc42 activity. Thus, only the first session in repetitive training contributes to ARM formation, while the subsequent sessions are devoted not to acquiring information but to inhibiting forgetting.

Inability to activate Rac1-dependent forgetting contributes to behavioral inflexibility in mutants of multiple autism-risk genes.

The etiology of autism is so complicated because it involves the effects of variants of several hundred risk genes along with the contribution of environmental factors. Therefore, it has been challenging to identify the causal paths that lead to the core autistic symptoms such as social deficit, repetitive behaviors, and behavioral inflexibility. As an alternative approach, extensive efforts have been devoted to identifying the convergence of the targets and functions of the autism-risk genes to facilitate mapping out causal paths. In this study, we used a reversal-learning task to measure behavioral flexibility in Drosophila and determined the effects of loss-of-function mutations in multiple autism-risk gene homologs in flies. Mutations of five autism-risk genes with diversified molecular functions all led to a similar phenotype of behavioral inflexibility indicated by impaired reversal-learning. These reversal-learning defects resulted from the inability to forget or rather, specifically, to activate Rac1 (Ras-related C3 botulinum toxin substrate 1)-dependent forgetting. Thus, behavior-evoked activation of Rac1-dependent forgetting has a converging function for autism-risk genes.

Importin-7 mediates memory consolidation through regulation of nuclear translocation of training-activated MAPK in Drosophila.

Translocation of signaling molecules, MAPK in particular, from the cytosol to nucleus represents a universal key element in initiating the gene program that determines memory consolidation. Translocation mechanisms and their behavioral impact, however, remain to be determined. Here, we report that a highly conserved nuclear transporter, Drosophila importin-7 (DIM-7), regulates import of training-activated MAPK for consolidation of long-term memory (LTM). We show that silencing DIM-7 functions results in impaired LTM, whereas overexpression of DIM-7 enhances LTM. This DIM-7-dependent regulation of LTM is confined to a consolidation time window and in mushroom body neurons. Image data show that bidirectional alteration in DIM-7 expression results in proportional changes in the intensity of training-activated MAPK accumulated within the nuclei of mushroom body neurons during LTM consolidation. Such DIM-7-regulated nuclear accumulation of activated MAPK is observed only in the training specified for LTM induction and determines the amplitude, but not the time course, of memory consolidation.

Requirement of the combination of mushroom body γ lobe and α/ß lobes for the retrieval of both aversive and appetitive early memories in Drosophila.

Extensive studies of Drosophila mushroom body in formation and retrieval of olfactory memories allow us to delineate the functional logic for memory storage and retrieval. Currently, there is a questionable disassociation of circuits for memory storage and retrieval during Drosophila olfactory memory processing. Formation of the initial aversive olfactory memory involves mushroom body γ lobe, whereas α/ß lobes are reported to be necessary for the retrieval of such memory. In contrast, formation and retrieval of the short-term appetitive olfactory memory appears to involve γ lobe. With the help of newly identified Gal4 lines and of focusing on 3-h memory for both aversive and appetitive conditionings, our reexamination of the retrieval of aversive and appetitive olfactory memories suggests a new view. Blocking γ lobe output led to severe deficiency of aversive early memory retrieval and partial impairment of appetitive early memory retrieval. Interrupting α/ß lobe output impaired the retrieval of both aversive and appetitive early memories. The contribution of the γ lobe and α/ß lobes appeared to be additive for the retrieval of appetitive early memory. Thus, these results suggest that the retrieval of aversive and appetitive olfactory early memories requires the synaptic outputs from both γ lobe and α/ß lobe neurons. This discovery may help us to rethink how aversive and appetitive memories are processed from memory formation to memory retrieval.

The differential requirement of mushroom body α/ß subdivisions in long-term memory retrieval in Drosophila.

The mushroom body (MB), a bilateral brain structure possessing about 2000-2500 neurons per hemisphere, plays a central role in olfactory learning and memory in Drosophila melanogaster. Extensive studies have demonstrated that three major types of MB neurons (α/ß, α'/ß' and Γ) exhibit distinct functions in memory processing, including the critical role of approximately 1000 MB α/ß neurons in retrieving long-term memory. Inspired by recent findings that MB α/ß neurons can be further divided into three subdivisions (surface, posterior and core) and wherein the α/ß core neurons play an permissive role in long-term memory consolidation, we examined the functional differences of all the three morphological subdivisions of MB α/ß by temporally precise manipulation of their synaptic outputs during long-term memory retrieval. We found the normal neurotransmission from a combination of MB α/ß surface and posterior neurons is necessary for retrieving both aversive and appetitive long-term memory, whereas output from MB α/ß posterior or core subdivision alone is dispensable. These results imply a specific requirement of about 500 MB α/ß neurons in supporting long-term memory retrieval and a further functional partitioning for memory processing within the MB α/ß region.

Schizophrenia susceptibility gene dysbindin regulates glutamatergic and dopaminergic functions via distinctive mechanisms in Drosophila.

The dysfunction of multiple neurotransmitter systems is a striking pathophysiological feature of many mental disorders, schizophrenia in particular, but delineating the underlying mechanisms has been challenging. Here we show that manipulation of a single schizophrenia susceptibility gene, dysbindin, is capable of regulating both glutamatergic and dopaminergic functions through two independent mechanisms, consequently leading to two categories of clinically relevant behavioral phenotypes. Dysbindin has been reported to affect glutamatergic and dopaminergic functions as well as a range of clinically relevant behaviors in vertebrates and invertebrates but has been thought to have a mainly neuronal origin. We find that reduced expression of Drosophila dysbindin (Ddysb) in presynaptic neurons significantly suppresses glutamatergic synaptic transmission and that this glutamatergic defect is responsible for impaired memory. However, only the reduced expression of Ddysb in glial cells is the cause of hyperdopaminergic activities that lead to abnormal locomotion and altered mating orientation. This effect is attributable to the altered expression of a dopamine metabolic enzyme, Ebony, in glial cells. Thus, Ddysb regulates glutamatergic transmission through its neuronal function and regulates dopamine metabolism by regulating Ebony expression in glial cells.

Forgetting is regulated through Rac activity in Drosophila.

Initially acquired memory dissipates rapidly if not consolidated. Such memory decay is thought to result either from the inherently labile nature of newly acquired memories or from interference by subsequently attained information. Here we report that a small G protein Rac-dependent forgetting mechanism contributes to both passive memory decay and interference-induced forgetting in Drosophila. Inhibition of Rac activity leads to slower decay of early memory, extending it from a few hours to more than one day, and to blockade of interference-induced forgetting. Conversely, elevated Rac activity in mushroom body neurons accelerates memory decay. This forgetting mechanism does not affect memory acquisition and is independent of Rutabaga adenylyl cyclase-mediated memory formation mechanisms. Endogenous Rac activation is evoked on different time scales during gradual memory loss in passive decay and during acute memory removal in reversal learning. We suggest that Rac's role in actin cytoskeleton remodeling may contribute to memory erasure.

Suppressor of Hairless is required for long-term memory formation in Drosophila.

Suppressor of Hairless [Su(H)] is a DNA-binding protein of the Notch-signaling pathway, which is important for developmental processes and has been implicated in behavior plasticity. It acts as a transcriptional activator in the Notch pathway, but also as a repressor in the absence of Notch signaling. Our previous work has shown that Notch signaling contributes to long-term memory formation in the Drosophila adult brain. In the present report, we show that Su(H) null heterozygous mutants perform normally for learning, early memory, and anesthesia-resistant memory, whereas long-term memory is impaired. Interestingly, we find overexpressing wild- type Su(H) also causes long-term memory defect in Drosophila. Significantly, induction of a heat-shock inducible Su(H)(+) transgene before training can fully rescue the memory defect of Su(H) mutants, thereby demonstrating an acute role for Su(H) in behavioral plasticity. We show that Su(H) is widely expressed in the adult brain. Transgenic expression of wild-type Su(H) in the Mushroom Bodies is sufficient to rescue the memory defect of Su(H) mutants. Our data clearly demonstrate that transcriptional activity of Su(H) in Notch signaling in the mushroom bodies is critical for the formation of long-term memory.