Cdc42 activation is necessary for heterosynaptic cooperation and competition.

Synapses change their weights in response to neuronal activity and in turn, neuronal networks alter their response properties and ultimately allow the brain to store information as memories. As for memories, not all events are maintained over time. Maintenance of synaptic plasticity depends on the interplay between functional changes at synapses and the synthesis of plasticity-related proteins that are involved in stabilizing the initial functional changes. Different forms of synaptic plasticity coexist in time and across the neuronal dendritic area. Thus, homosynaptic plasticity refers to activity-dependent synaptic modifications that are input-specific, whereas heterosynaptic plasticity relates to changes in non-activated synapses. Heterosynaptic forms of plasticity, such as synaptic cooperation and competition allow neurons to integrate events that occur separated by relatively large time windows, up to one hour. Here, we show that activation of Cdc42, a Rho GTPase that regulates actin cytoskeleton dynamics, is necessary for the maintenance of long-term potentiation (LTP) in a time-dependent manner. Inhibiting Cdc42 activation does not alter the time-course of LTP induction and its initial expression but blocks its late maintenance. We show that Cdc42 activation is involved in the phosphorylation of cofilin, a protein involved in modulating actin filaments and that weak and strong synaptic activation leads to similar levels on cofilin phosphorylation, despite different levels of LTP expression. We show that Cdc42 activation is required for synapses to interact by cooperation or competition, supporting the hypothesis that modulation of the actin cytoskeleton provides an activity-dependent and time-restricted permissive state of synapses allowing synaptic plasticity to occur. We found that under competition, the sequence in which synapses are activated determines the degree of LTP destabilization, demonstrating that competition is an active destabilization process. Taken together, we show that modulation of actin cytoskeleton by Cdc42 activation is necessary for the expression of homosynaptic and heterosynaptic forms of plasticity. Determining the temporal and spatial rules that determine whether synapses cooperate or compete will allow us to understand how memories are associated.

Carotid Body Resection Prevents Short-Term Spatial Memory Decline in Prediabetic Rats Without Changing Insulin Signaling in the Hippocampus and Prefrontal Cortex.

Individuals who develop type 2 diabetes (T2D) at an early age are at higher risk of developing neurodegenerative disorders such as Alzheimer's and Parkinson's disease. A shared dysfunctional characteristic between T2D and these neurodegenerative disorders is insulin resistance. Recently, it was shown that prediabetes animals and patients exhibited increased carotid body (CB) activity. Moreover, these organs are deeply involved in the development of metabolic diseases, since upon abolishment of their activity via carotid sinus nerve (CSN) resection, several dysmetabolic features of T2D were reverted. Herein, we investigated if CSN resection may also prevent cognitive impairment associated with brain insulin resistance. We explored a diet-induced prediabetes animal model where Wistar rats are kept in a high fat-high sucrose (HFHSu) diet for 20 weeks. We evaluated CSN resection effects on behavioral parameters and on insulin signaling-related proteins levels, in the prefrontal cortex and the hippocampus. HFHSu animals exhibited impaired short-term memory evaluated by the y-maze test. Remarkably, CSN resection prevented the development of this phenotype. HFHSu diet or CSN resection did not promote significant alterations in insulin signaling-associated proteins levels. Our findings suggest that CBs modulation might have a role in preventing short-term spatial memory deficits associated with peripheral dysmetabolic states.

Resilience to fear: The role of individual factors in amygdala response to stressors.

Resilience to stress is an adaptive process that varies individually. Resilience refers to the adaptation, or the ability to maintain or regain mental health, despite being subject to adverse situation. Resilience is a dynamic concept that reflects a combination of internal individual factors, including age and gender interacting with external factors such as social, cultural and environmental factors. In the last decade, we have witnessed an increase in the prevalence of anxiety disorders, including post-traumatic stress disorder. Given that stress in unavoidable, it is of great interest to understand the neurophysiological mechanisms of resilience, the individual factors that may contribute to susceptibility and promote efficacious approaches to improve resilience. Here, we address this complex question, attempting at defining clear and operational definitions that may allow us to improve our analysis of behavior incorporating individuality. We examine how individual perception of the stressor can alter the outcome of an adverse situation using as an example, the fear-conditioning paradigm and discuss how individual differences in the reward system can contribute to resilience. Given the central role of the endocannabinoid system in regulating fear responses and anxiety, we discuss the evidence that polymorphisms in several molecules of this signaling system contribute to different anxiety phenotypes. The endocannabinoid system is highly interconnected with the serotoninergic and dopaminergic modulatory systems, contributing to individual differences in stress perception and coping mechanisms. We review how the individual variability in these modulatory systems can be used towards a multivariable assessment of stress risk. Incorporating individuality in our research will allow us to define biomarkers of anxiety disorders as well as assess prognosis, towards a personalized clinical approach to mental health.

Temporal Gating of Synaptic Competition in the Amygdala by Cannabinoid Receptor Activation.

The acquisition of fear memories involves plasticity of the thalamic and cortical pathways to the lateral amygdala (LA). In turn, the maintenance of synaptic plasticity requires the interplay between input-specific synaptic tags and the allocation of plasticity-related proteins. Based on this interplay, weakly activated synapses can express long-lasting forms of synaptic plasticity by cooperating with strongly activated synapses. Increasing the number of activated synapses can shift cooperation to competition. Synaptic cooperation and competition can determine whether two events, separated in time, are associated or whether a particular event is selected for storage. The rules that determine whether synapses cooperate or compete are unknown. We found that synaptic cooperation and competition, in the LA, are determined by the temporal sequence of cortical and thalamic stimulation and that the strength of the synaptic tag is modulated by the endocannabinoid signaling. This modulation is particularly effective in thalamic synapses, supporting a critical role of endocannabinoids in restricting thalamic plasticity. Also, we found that the availability of synaptic proteins is activity-dependent, shifting competition to cooperation. Our data present the first evidence that presynaptic modulation of synaptic activation, by the cannabinoid signaling, functions as a temporal gating mechanism limiting synaptic cooperation and competition.

Actin remodeling, the synaptic tag and the maintenance of synaptic plasticity.

Activity-dependent plasticity of synaptic connections is a hallmark of the mammalian brain and represents a key mechanism for rewiring neural circuits during development, experience-dependent plasticity, and brain disorders. Cellular models of memory, such as long-term potentiation and long-term depression, share common principles to memory consolidation. As for memory, the maintenance of synaptic plasticity is dependent on the synthesis of de novo protein synthesis. The synaptic-tagging and capture hypothesis states that the maintenance of synaptic plasticity is dependent on the interplay between input-specific synaptic tags and the allocation or capture of plasticity-related proteins (PRPs) at activated synapses. The setting of the synaptic tag and the capture of PRPs are independent processes that can occur separated in time and different groups of activated synapses. How are these two processes orchestrated in time and space? Here, we discuss the synaptic-tagging and capture hypothesis in the light of neuronal compartmentalization models and address the role of actin as a putative synaptic tag. If different groups of synapses interact by synaptic-tagging and capture mechanisms, understanding the spatial rules of such interaction is key to define the relevant neuronal compartment. We also discuss how actin modulation can allow an input-specific capture of PRPs and try to conciliate the temporal dynamics of synaptic actin with the maintenance of plasticity. Understanding how multiple synapses interact in time and space is fundamental to predict how neurons integrate information and ultimately how memory is acquired.

Endocannabinoid signaling and memory dynamics: A synaptic perspective.

Memory acquisition is a key brain feature in which our human nature relies on. Memories evolve over time. Initially after learning, memories are labile and sensitive to disruption by the interference of concurrent events. Later on, after consolidation, memories are resistant to disruption. However, reactivation of previously consolidated memories renders them again in an unstable state and therefore susceptible to perturbation. Additionally, and depending on the characteristics of the stimuli, a parallel process may be initiated which ultimately leads to the extinction of the previously acquired response. This dynamic aspect of memory maintenance opens the possibility for an updating of previously acquired memories but it also creates several conceptual challenges. What is the time window for memory updating? What determines whether reconsolidation or extinction is triggered? In this review, we tried to re-examine the relationship between consolidation, reconsolidation and extinction, aiming for a unifying view of memory dynamics. Since cellular models of memory share common principles, we present the evidence that similar rules apply to the maintenance of synaptic plasticity. Recently, a new function of the endocannabinoid (eCB) signaling system has been described for associative forms of synaptic plasticity in amygdala synapses. The eCB system has emerged as a key modulator of memory dynamics by adjusting the outcome to stimuli intensity. We propose a key function of eCB in discriminative forms of learning by restricting associative plasticity in amygdala synapses. Since many neuropsychiatric disorders are associated with a dysregulation in memory dynamics, understanding the rules underlying memory maintenance paves the path to better clinical interventions.

The interplay between neuronal activity and actin dynamics mimic the setting of an LTD synaptic tag.

Persistent forms of plasticity, such as long-term depression (LTD), are dependent on the interplay between activity-dependent synaptic tags and the capture of plasticity-related proteins. We propose that the synaptic tag represents a structural alteration that turns synapses permissive to change. We found that modulation of actin dynamics has different roles in the induction and maintenance of LTD. Inhibition of either actin depolymerisation or polymerization blocks LTD induction whereas only the inhibition of actin depolymerisation blocks LTD maintenance. Interestingly, we found that actin depolymerisation and CaMKII activation are involved in LTD synaptic-tagging and capture. Moreover, inhibition of actin polymerisation mimics the setting of a synaptic tag, in an activity-dependent manner, allowing the expression of LTD in non-stimulated synapses. Suspending synaptic activation also restricts the time window of synaptic capture, which can be restored by inhibiting actin polymerization. Our results support our hypothesis that modulation of the actin cytoskeleton provides an input-specific signal for synaptic protein capture.

The aging memory: Modulating epigenetic modifications to improve cognitive function.

Age-related cognitive decline is a major concern in society. Here, I discuss recent evidence that shows an age-related modulation of gene transcription by epigenetic modifications. Epigenetic modifications, such as histone acetylation, is unbalanced in aging, with an increase in histone deacetylation, that limits the expression of plasticity-related genes. By modifying the balance towards histone acetylation, histone deacetylase inhibitors present a new pharmacological approach to ameliorate age-related cognitive deficits.

Asymmetrical synaptic cooperation between cortical and thalamic inputs to the amygdale.

Fear conditioning, a form of associative learning is thought to involve the induction of an associative long-term potentiation of cortical and thalamic inputs to the lateral amygdala. Here, we show that stimulation of the thalamic input can reinforce a transient form of plasticity (E-LTP) induced by weak stimulation of the cortical inputs. This synaptic cooperation occurs within a time window of 30 min, suggesting that synaptic integration at amygdala synapses can occur within large time windows. Interestingly, we found that synaptic cooperation is not symmetrical. Reinforcement of a thalamic E-LTP by subsequent cortical stimulation is only observed within a shorter time window. We found that activation of endocannabinoid CB1 receptors is involved in the time restriction of thalamic and cortical synaptic cooperation in an activity-dependent manner. Our results support the hypothesis that synaptic cooperation can underlie associative learning and that synaptic tagging and capture is a general mechanism in synaptic plasticity.

Activity-dependent actin dynamics are required for the maintenance of long-term plasticity and for synaptic capture.

The maintenance of long-lasting forms of plasticity, such as long-term potentiation (LTP) is dependent on the capture of plasticity-related proteins (PRPs) in an input-specific manner - synaptic capture. Here, it is shown that LTP, induced at Schaffer collaterals-CA1 synapses in acute rat hippocampal slice preparation, is not sensitive to protein synthesis inhibition if N-methyl-d-aspartate (NMDA) receptors are blocked, suggesting that synaptic activation is involved in the modulation of LTP maintenance. Similarly, it was found that synaptic activation also determines the sensitivity of LTP to manipulations of the actin cytoskeleton dynamics. Suspending synaptic activation or concomitant NMDA receptor inhibition is sufficient to rescue the impairment on LTP maintenance induced by actin polymerization blockade. Additionally, concomitant inhibition of protein degradation can partially prevent the LTP decay observed under actin polymerization blockade, suggesting that protein degradation is involved in the destabilization of LTP maintenance induced by actin polymerization blockade. Taken together, these observations suggest that LTP maintenance is determined by a balance of synthesis and degradation of PRPs modulated by synaptic activation and actin dynamics. Finally, it was uncovered that inhibition of actin depolymerization blocks synaptic capture, whereas inhibition of actin polymerization can extend the temporal window for synaptic capture. Additionally, inhibition of actin polymerization can rescue the impairment in synaptic capture induced by CaMKII inhibition, suggesting a link between CaMKII activation and modulation of actin dynamics during synaptic capture. These results show that an activity-dependent regulation of actin dynamics plays a critical role in LTP maintenance and synaptic capture.

LTD induction causes morphological changes of presynaptic boutons and reduces their contacts with spines.

Activity-dependent changes in the synaptic connections of the brain are thought to be important for learning and memory. Imaging techniques have enabled the examination of structural rearrangements during activity-dependent processes at the synapse. While many studies have examined structural changes of dendritic spines, little is known about structural plasticity of presynaptic boutons. We therefore examined how axonal boutons are affected during long-term depression (LTD). We used time lapse two-photon laser scanning microscopy and extracellular field recordings to monitor simultaneously synaptic morphology and activity for up to five hours in mouse organotypic hippocampal slice cultures. LTD induction dramatically increased the turnover of presynaptic boutons, while decreasing the number of putative synaptic contacts between Schaffer collateral boutons and spines of CA1 pyramidal cells. Our data indicate a substantial presynaptic contribution to activity-dependent morphological plasticity and provide opportunities for studying the molecular mechanisms of the structural remodeling of synaptic circuits.

A balance of protein synthesis and proteasome-dependent degradation determines the maintenance of LTP.

Long-lasting changes in synaptic strength are thought to play a pivotal role in activity-dependent plasticity and memory. There is ample evidence indicating that in hippocampal long-term potentiation (LTP) the synthesis of new proteins is crucial for enduring changes. However, whether protein degradation also plays a role in this process has only recently begun to receive attention. Here, we examine the effects of blocking protein degradation on LTP. We show that pharmacological inhibition of proteasome-dependent protein degradation, just like inhibition of protein synthesis, disrupts expression of late (L-)LTP. However, when protein degradation and protein synthesis are inhibited at the same time, LTP is restored to control levels, calling into question the commonly held hypothesis that synthesis of new proteins is indispensable for L-LTP. Instead, these findings point to a more facetted model, in which L-LTP is determined by the combined action of synthesis and degradation of plasticity proteins.

Neuronal activity determines the protein synthesis dependence of long-term potentiation.

Long-term potentiation (LTP) is generally divided into two phases, early (E-) and late (L-) LTP, of which only L-LTP is thought to depend on protein synthesis. Here we report that E-LTP can also be dependent on protein synthesis at higher levels of synaptic activation. Moreover, we show that the requirement for protein synthesis during L-LTP extends beyond the early induction phase and that it depends on synaptic stimulation. This suggests that the level of neuronal activity is a crucial determinant for the role of protein synthesis in E- and L-LTP.

Competing for memory: hippocampal LTP under regimes of reduced protein synthesis.

The persistence of synaptic potentiation in the hippocampus is known to depend on transcription and protein synthesis. We report here that, under regimes of reduced protein synthesis, competition between synapses for the relevant intracellular proteins can be demonstrated. Under such circumstances, the induction of additional protein synthesis-dependent long-term potentiation for a given set of postsynaptic neurons occurs at the expense of the maintenance of prior potentiation on an independent pathway. This new phenomenon, which we call "competitive maintenance," has important functional consequences, and it may be explained in terms of dynamic interactions between synapses and "plasticity factors" over extended periods of time.