Dendritic spines and long-term plasticity

Molecular composition of the endocannabinoid system at glutamatergic synapses

Endocannabinoids play central roles in retrograde signaling at a wide variety of synapses throughout the CNS. Although several molecular components of the endocannabinoid system have been identified recently, their precise location and contribution to retrograde synaptic signaling is essentially unknown. Here we show, by using two independent riboprobes, that principal cell populations of the hippocampus express high levels of diacylglycerol lipase alpha (DGL-alpha), the enzyme involved in generation of the endocannabinoid 2-arachidonoyl-glycerol (2-AG). Immunostaining with two independent antibodies against DGL-alpha revealed that this lipase was concentrated in heads of dendritic spines throughout the hippocampal formation. Furthermore, quantification of high-resolution immunoelectron microscopic data showed that this enzyme was highly compartmentalized into a wide perisynaptic annulus around the postsynaptic density of axospinous contacts but did not occur intrasynaptically. On the opposite side of the synapse, the axon terminals forming these excitatory contacts were found to be equipped with presynaptic CB1 cannabinoid receptors. This precise anatomical positioning suggests that 2-AG produced by DGL-alpha on spine heads may be involved in retrograde synaptic signaling at glutamatergic synapses, whereas CB1 receptors located on the afferent terminals are in an ideal position to bind 2-AG and thereby adjust presynaptic glutamate release as a function of postsynaptic activity. We propose that this molecular composition of the endocannabinoid system may be a general feature of most glutamatergic synapses throughout the brain and may contribute to homosynaptic plasticity of excitatory synapses and to heterosynaptic plasticity between excitatory and inhibitory contacts.

Common molecular mechanisms in explicit and implicit memory

Cellular and molecular studies of both implicit and explicit memory suggest that experience-dependent modulation of synaptic strength and structure is a fundamental mechanism by which these memories are encoded and stored within the brain. In this review, we focus on recent advances in our understanding of two types of memory storage: (i) sensitization in Aplysia, a simple form of implicit memory, and (ii) formation of explicit spatial memories in the mouse hippocampus. These two processes share common molecular mechanisms that have been highly conserved through evolution.

ARF6 and EFA6A regulate the development and maintenance of dendritic spines

The cellular and molecular mechanisms underlying the development and maintenance of dendritic spines are not fully understood. ADP-ribosylation factor 6 (ARF6) is a small GTPase known to regulate actin remodeling and membrane traffic. Here, we report involvement of ARF6 and exchange factor for ARF6 (EFA6A) in the regulation of spine development and maintenance. An active form of ARF6 promotes the formation of dendritic spines at the expense of filopodia. EFA6A promotes spine formation in an ARF6 activation-dependent manner. Knockdown of ARF6 and EFA6A by small interfering RNA decreases spine formation. Live imaging indicates that ARF6 knockdown decreases the conversion of filopodia to spines and the stability of early spines. The spine-promoting effect of ARF6 is partially blocked by Rac1. ARF6 and EFA6A protect mature spines from inactivity-induced destabilization. These results suggest that ARF6 and EFA6A may regulate the conversion of filopodia to spines and the stability of both early and mature spines.

Activity-dependent movements of postsynaptic scaffolds at inhibitory synapses

Dendritic spines show an activity-dependent cytoskeleton-based remodeling coupled with variations in receptor number and the functional properties of excitatory synapses. In this study, we analyzed the dynamics of gephyrin containing inhibitory postsynaptic scaffolds imaging a Venus::gephyrin (VeGe) chimera in dissociated spinal cord neurons. We provide evidence that the postsynaptic scaffolds at mature synapses display a submicrometric rapid lateral motion and are continuously moving on the dendritic shaft. This dynamic behavior is calcium dependent and is controlled by the cytoskeleton. Minute rearrangement within the gephyrin scaffold as well as the scaffold lateral displacements are F-actin dependent. The lateral movements are counteracted by microtubules. Moreover, the action of the potassium channel blocker 4-aminopyridine and receptor antagonists indicate that the dynamics of postsynaptic gephyrin scaffolds are controlled by synaptic activity.

Modification of synapse formation of accessory olfactory bulb neurons by coculture with vomeronasal neurons

Previously, a coculture system of accessory olfactory bulb (AOB) neurons and vomeronasal (VN) neurons was established for studying the functional roles of AOB neurons in pheromonal signal processing. In this study, the effect of VN neurons on the development of AOB neurons was examined in a coculture system. Spine density was quantitatively measured for various culture periods of 21, 28, 36, and 42 days in vitro. The densities of dendritic spines were lower in the coculture than in single culture for all periods in vitro. Synapse formation on spines was analyzed immunocytochemically using an anti-synaptophysin antibody. The ratio of the density of synaptophysin-immunopositive spine/total spine density was larger in the coculture than in the single culture. The volume of spine head was larger in the coculture than in single culture. These changes were not observed in the coculture in which there was no physical contact between AOB neurons and VN neurons. These observations suggest that synapse formation on the spines of AOB neurons is modified by physical contact with VN neurons.

Molecular mechanisms of dendritic spine morphogenesis

Excitatory synapses are formed on dendritic spines, postsynaptic structures that change during development and in response to synaptic activity. Once mature, however, spines can remain stable for many months. The molecular mechanisms that control the formation and elimination, motility and stability, and size and shape of dendritic spines are being revealed. Multiple signaling pathways, particularly those involving Rho and Ras family small GTPases, converge on the actin cytoskeleton to regulate spine morphology and dynamics bidirectionally. Numerous cell surface receptors, scaffold proteins and actin binding proteins are concentrated in spines and engaged in spine morphogenesis.

Structural and functional modifications in glutamateric synapses following prolonged ethanol exposure

This article summarizes the proceedings of a symposium presented at the 2005 annual meeting of the Research Society on Alcoholism in Santa Barbara, California, USA. The organizer and chair was L. Judson Chandler. The presentations were (1) Chronic Ethanol Exposure, N-Methyl-D-Aspartate (NMDA) Receptor Dynamics, and Withdrawal Hyperexcitability, by Adam Hendricson, Regina Maldve, and Richard Morrisett; (2) Ethanol-Induced Synaptic Targeting of NMDA Receptors Is Associated With Enhanced Postsynaptic Density-95 Clustering and Spine Size, by Judson Chandler and Ezekiel Carpenter-Hyland; (3) Presynaptic and Postsynaptic Alterations in the Nucleus Accumbens Following Chronic Alcohol Exposure, by Feng Zhou, Youssef Sari, and Richard Bell; and (4) An Active Role for Accumbens Homer2 Expression in Alcohol-Induced Neural Plasticity, by Karen Szumlinski.

Axon initial segment ensheathed by extracellular matrix in perineuronal nets

Perineuronal nets of extracellular matrix are associated with distinct types of neurons in the cerebral cortex and many subcortical regions. Large complexes of aggregating proteoglycans form a chemically specified microenvironment around the somata, proximal dendrites and the axon initial segment, including the presynaptic boutons attached to these domains. The subcellular distribution and the temporal course of postnatal formation suggest that perineuronal nets may be involved in the regulation of synaptic plasticity. Here we investigate structural and cytochemical characteristics of the extracellular matrix around axon initial segments virtually devoid of synaptic contacts. Wisteria floribunda agglutinin staining, the immunocytochemical detection of aggrecan and tenascin-R, as well as affinity-labeling of hyaluronan were used to analyze perineuronal nets associated with large motoneurons in the mouse superior colliculus. The molecular composition of perineuronal nets was divergent between neurons but was identical around the different cellular domains of the individual neurons. The axon initial segments largely devoid of synapses were covered by a continuous matrix sheath infiltrating the adjacent neuropil. The periaxonal zone penetrated by matrix components often increased in diameter along the initial segment from the axon hillock toward the myelinated part of the axon. The axonal and somatodendritic domains of perineuronal nets were concomitantly formed during the first three weeks of postnatal development. The common molecular properties and major structural features of subcellular perineuronal net domains were retained in organotypic midbrain slice cultures. The results support the hypothesis that the aggrecan-related extracellular matrix of perineuronal nets provides a continuous micromilieu for different subcellular domains performing integration and generation of the electrical activity of neurons.

Mechanisms underlying the inability to induce area CA1 LTP in the mouse after traumatic brain injury

Traumatic brain injury (TBI) is a significant health issue that often causes enduring cognitive deficits, in particular memory dysfunction. The hippocampus, a structure crucial in learning and memory, is frequently damaged during TBI. Since long-term potentiation (LTP) is the leading cellular model underlying learning and memory, this study was undertaken to examine how injury affects area CA1 LTP in mice using lateral fluid percussion injury (FPI). Brain slices derived from FPI animals demonstrated an inability to induce LTP in area CA1 7 days postinjury. However, area CA1 long-term depression could be induced in neurons 7 days postinjury, demonstrating that some forms of synaptic plasticity can still be elicited. Using a multi-disciplined approach, potential mechanisms underlying the inability to induce and maintain area CA1 LTP were investigated. This study demonstrates that injury leads to significantly smaller N-methyl-D-aspartate potentials and glutamate-induced excitatory currents, increased dendritic spine size, and decreased expression of alpha-calcium calmodulin kinase II. These findings may underlie the injury-induced lack of LTP and thus, contribute to cognitive impairments often associated with TBI. Furthermore, these results provide attractive sites for potential therapeutic intervention directed toward alleviating the devastating consequences of human TBI.

Modulation of dendritic spines in epilepsy: cellular mechanisms and functional implications

Epilepsy patients often suffer from significant neurological deficits, including memory impairment, behavioral problems, and psychiatric disorders. While the causes of neuropsychological dysfunction in epilepsy are multifactorial, accumulating evidence indicates that seizures themselves may directly cause brain injury. Although seizures sometimes result in neuronal death, they may also cause more subtle pathological changes in neuronal structure and function, including abnormalities in synaptic transmission. Dendritic spines receive a majority of the excitatory synaptic inputs to cortical neurons and are critically involved in synaptic plasticity and learning. Studies of human epilepsy and experimental animal models demonstrate that seizures may directly affect the morphological and functional properties of dendritic spines, suggesting that seizure-related changes in spines may represent a mechanistic basis for cognitive deficits in epilepsy. Novel therapeutic strategies directed at modulation of spine motility may prevent the detrimental effects of seizures on cognitive function in epilepsy.

Mechanisms of late-onset cognitive decline after early-life stress

Progressive cognitive deficits that emerge with aging are a result of complex interactions of genetic and environmental factors. Whereas much has been learned about the genetic underpinnings of these disorders, the nature of "acquired" contributing factors, and the mechanisms by which they promote progressive learning and memory dysfunction, remain largely unknown. Here, we demonstrate that a period of early-life "psychological" stress causes late-onset, selective deterioration of both complex behavior and synaptic plasticity: two forms of memory involving the hippocampus, were severely but selectively impaired in middle-aged, but not young adult, rats exposed to fragmented maternal care during the early postnatal period. At the cellular level, disturbances to hippocampal long-term potentiation paralleled the behavioral changes and were accompanied by dendritic atrophy and mossy fiber expansion. These findings constitute the first evidence that a short period of stress early in life can lead to delayed, progressive impairments of synaptic and behavioral measures of hippocampal function, with potential implications to the basis of age-related cognitive disorders in humans.

Changes in synaptic structure underlie the developmental speeding of AMPA receptor–mediated EPSCs

At many excitatory and inhibitory synapses throughout the nervous system, postsynaptic currents become faster as the synapse matures, primarily owing to changes in receptor subunit composition. The origin of the developmental acceleration of AMPA receptor (AMPAR)-mediated excitatory postsynaptic currents (EPSCs) remains elusive. We used patch-clamp recordings, electron microscopic immunogold localization of AMPARs, partial three-dimensional reconstruction of the neuropil and numerical simulations of glutamate diffusion and AMPAR activation to examine the factors underlying the developmental speeding of miniature EPSCs in mouse cerebellar granule cells. We found that the main developmental change that permits submillisecond transmission at mature synapses is an alteration in the glutamate concentration waveform as experienced by AMPARs. This can be accounted for by changes in the synaptic structure and surrounding neuropil, rather than by a change in AMPAR properties. Our findings raise the possibility that structural alterations could be a general mechanism underlying the change in the time course of AMPAR-mediated synaptic transmission.