Andrés E. Chávez

Endocannabinoid Signaling and Synaptic Function

Endocannabinoids are key modulators of synaptic function. By activating cannabinoid receptors expressed in the central nervous system, these lipid messengers can regulate several neural functions and behaviors. As experimental tools advance, the repertoire of known endocannabinoid-mediated effects at the synapse, and their underlying mechanism, continues to expand. Retrograde signaling is the principal mode by which endocannabinoids mediate short- and long-term forms of plasticity at both excitatory and inhibitory synapses. However, growing evidence suggests that endocannabinoids can also signal in a nonretrograde manner. In addition to mediating synaptic plasticity, the endocannabinoid system is itself subject to plastic changes. Multiple points of interaction with other neuromodulatory and signaling systems have now been identified. In this Review, we focus on new advances in synaptic endocannabinoid signaling in the mammalian brain. The emerging picture not only reinforces endocannabinoids as potent regulators of synaptic function but also reveals that endocannabinoid signaling is mechanistically more complex and diverse than originally thought.

TRPV1 activation by endogenous anandamide triggers postsynaptic long-term depression in dentate gyrus

The transient receptor potential TRPV1 is a nonselective cation channel that mediates pain sensations and is commonly activated by a wide variety of exogenous and endogenous, physical and chemical stimuli. Although TRPV1 receptors are mainly found in nociceptive neurons of the peripheral nervous system, these receptors have also been found in the brain, where their role is far less understood. Activation of TRPV1 reportedly regulates neurotransmitter release at several central synapses. However, we found that TRPV1 suppressed excitatory transmission in rat and mouse dentate gyrus by regulating postsynaptic function in an input-specific manner. This suppression was a result of Ca2+-calcineurin and clathrin-dependent internalization of AMPA receptors. Moreover, synaptic activation of TRPV1 triggered a form of long-term depression (TRPV1-LTD) mediated by the endocannabinoid anandamide in a type 1 cannabinoid receptor–independent manner. Thus, our findings reveal a previously unknown form of endocannabinoid- and TRPV1-mediated regulation of synaptic strength at central synapses.

REST-dependent epigenetic remodeling promotes the developmental switch in synaptic NMDA receptors

NMDA receptors (NMDARs) are critical to synaptogenesis, neural circuitry and higher cognitive functions. A hallmark feature of NMDARs is an early postnatal developmental switch from those containing primarily GluN2B to primarily GluN2A subunits. Although the switch in phenotype has been an area of intense interest for two decades, the mechanisms that trigger it and the link between experience and the switch are unclear. Here we show a new role for the transcriptional repressor REST in the developmental switch of synaptic NMDARs. REST is activated at a critical window of time and acts via epigenetic remodeling to repress Grin2b expression and alter NMDAR properties at rat hippocampal synapses. Knockdown of REST in vivo prevented the decline in GluN2B and developmental switch in NMDARs. Maternal deprivation impaired REST activation and acquisition of the mature NMDAR phenotype. Thus, REST is essential for experience-dependent fine-tuning of genes involved in synaptic plasticity.

Mechanisms Underlying Lateral GABAergic Feedback onto Rod Bipolar Cells in Rat Retina

GABAergic feedback inhibition from amacrine cells shapes visual signaling in the inner retina. Rod bipolar cells (RBCs), ON-sensitive cells that depolarize in response to light increments, receive reciprocal GABAergic feedback from A17 amacrine cells and additional GABAergic inputs from other amacrine cells located laterally in the inner plexiform layer. The circuitry and synaptic mechanisms underlying lateral GABAergic inhibition of RBCs are poorly understood. A-type and ρ-subunit-containing (C-type) GABA receptors (GABAARs and GABACRs) mediate both forms of inhibition, but their relative activation during synaptic transmission is unclear, and potential interactions between adjacent reciprocal and lateral synapses have not been explored. Here, we recorded from RBCs in acute slices of rat retina and isolated lateral GABAergic inhibition by pharmacologically ablating A17 amacrine cells. We found that amacrine cells providing lateral GABAergic inhibition to RBCs receive excitatory synaptic input mostly from ON bipolar cells via activation of both Ca2+-impermeable and Ca2+-permeable AMPA receptors (CP-AMPARs) but not NMDA receptors (NMDARs). Voltage-gated Ca2+ (Cav) channels mediate the majority of Ca2+ influx that triggers GABA release, although CP-AMPARs contribute a small component. The intracellular Ca2+ signal contributing to transmitter release is amplified by Ca2+-induced Ca2+ release from intracellular stores via activation of ryanodine receptors. Furthermore, lateral nonreciprocal feedback is mediated primarily by GABACRs that are activated independently from receptors mediating reciprocal feedback inhibition. These results illustrate numerous physiological differences that distinguish GABA release at reciprocal and lateral synapses, indicating complex, pathway-specific modulation of RBC signaling.

ELKS2α/CAST Deletion Selectively Increases Neurotransmitter Release at Inhibitory Synapses

The presynaptic active zone is composed of a protein network that contains ELKS2alpha (a.k.a. CAST) as a central component. Here we demonstrate that in mice, deletion of ELKS2alpha caused a large increase in inhibitory, but not excitatory, neurotransmitter release, and potentiated the size, but not the properties, of the readily-releasable pool of vesicles at inhibitory synapses. Quantitative electron microscopy revealed that the ELKS2alpha deletion did not change the number of docked vesicles or other ultrastructural parameters of synapses, except for a small decrease in synaptic vesicle numbers. The ELKS2alpha deletion did, however, alter the excitatory/inhibitory balance and exploratory behaviors, possibly as a result of the increased synaptic inhibition. Thus, as opposed to previous studies indicating that ELKS2alpha is essential for mediating neurotransmitter release, our results suggest that ELKS2alpha normally restricts release and limits the size of the readily-releasable pool of synaptic vesicles at the active zone of inhibitory synapses.

BK channels modulate pre- and postsynaptic signaling at reciprocal synapses in retina

In the mammalian retina, A17 amacrine cells provide reciprocal inhibitory feedback to rod bipolar cells, thereby shaping the time course of visual signaling in vivo. Previous results have indicated that A17 feedback can be triggered by Ca2+ influx through Ca2+-permeable AMPA receptors and can occur independently of voltage-gated Ca2+ (Cav) channels, whose presence and functional role in A17 dendrites have not yet been explored. We combined electrophysiology, calcium imaging and immunohistochemistry and found that L-type Cav channels in rat A17 amacrine cells were located at the sites of reciprocal synaptic feedback and that their contribution to GABA release was diminished by large-conductance Ca2+-activated potassium (BK) channels, which suppress postsynaptic depolarization in A17s and limit Cav channel activation. We also found that BK channels, by limiting GABA release from A17s, regulate the flow of excitatory synaptic transmission through the rod pathway.

Diverse Mechanisms Underlie Glycinergic Feedback Transmission onto Rod Bipolar Cells in Rat Retina

Synaptic inhibition shapes visual signaling in the inner retina, but the physiology of most amacrine cells, the interneurons that mediate this inhibition, is poorly understood. Discerning the function of most individual amacrine cell types is a daunting task, because few molecular or morphological markers specifically distinguish between approximately two dozen different amacrine cell types. Here, we examine a functional subset of amacrine cells by pharmacologically isolating glycinergic inhibition and evoking feedback IPSCs in a single cell type, the rod bipolar cell (RBC), with brief glutamate applications in the inner plexiform layer. We find that glycinergic amacrine cells innervating RBCs receive excitatory inputs from ON and OFF bipolar cells primarily via NMDA receptors (NMDARs) and Ca2+-impermeable AMPA-type glutamate receptors. Glycine release from amacrine cells is triggered by Ca2+ influx through both voltage-gated Ca2+ (Cav) channels and NMDARs. These intracellular Ca2+signals are amplified by Ca2+-induced Ca2+ release via both ryanodine and IP3 receptors, which are activated independently by Ca2+ influx through Cav channels and NMDARs, respectively. Glycinergic feedback signaling depends strongly, although not completely, on voltage-gated Na+ channels, and the spatial extent of feedback inhibition is expanded by gap junction connections between glycinergic amacrine cells. These results indicate that a diversity of mechanisms underlie glycinergic feedback inhibition onto RBCs, yet they highlight several physiological themes that appear to distinguish amacrine cell function.

Actinin-4 Governs Dendritic Spine Dynamics and Promotes Their Remodeling by Metabotropic Glutamate Receptors.

Background: Group 1 mGluRs induce dendritic spine remodeling, but the underlying molecular mechanisms remain uncharacterized. Results: α-Actinin-4 regulates dendritic protrusion dynamics and morphogenesis and is required for the receptor-induced dynamic remodeling of dendritic protrusions. Conclusion: α-Actinin-4 is a novel molecular effector of mGluR-dependent spine remodeling. Significance: mGluR signaling via actinins could contribute to synaptic plasticity and spine dysmorphogenesis in neurodevelopmental disorders. Dendritic spines are dynamic, actin-rich protrusions in neurons that undergo remodeling during neuronal development and activity-dependent plasticity within the central nervous system. Although group 1 metabotropic glutamate receptors (mGluRs) are critical for spine remodeling under physiopathological conditions, the molecular components linking receptor activity to structural plasticity remain unknown. Here we identify a Ca2+-sensitive actin-binding protein, α-actinin-4, as a novel group 1 mGluR-interacting partner that orchestrates spine dynamics and morphogenesis in primary neurons. Functional silencing of α-actinin-4 abolished spine elongation and turnover stimulated by group 1 mGluRs despite intact surface receptor expression and downstream ERK1/2 signaling. This function of α-actinin-4 in spine dynamics was underscored by gain-of-function phenotypes in untreated neurons. Here α-actinin-4 induced spine head enlargement, a morphological change requiring the C-terminal domain of α-actinin-4 that binds to CaMKII, an interaction we showed to be regulated by group 1 mGluR activation. Our data provide mechanistic insights into spine remodeling by metabotropic signaling and identify α-actinin-4 as a critical effector of structural plasticity within neurons.

LTP at Hilar Mossy Cell-Dentate Granule Cell Synapses Modulates Dentate Gyrus Output by Increasing Excitation/Inhibition Balance

Excitatory hilar mossy cells (MCs) in the dentate gyrus receive inputs from dentate granule cells (GCs) and project back to GCs locally, contralaterally, and along the longitudinal axis of the hippocampus, thereby establishing an associative positive-feedback loop and connecting functionally diverse hippocampal areas. MCs also synapse with GABAergic interneurons that mediate feed-forward inhibition onto GCs. Surprisingly, although these circuits have been implicated in both memory formation (e.g., pattern separation) and temporal lobe epilepsy, little is known about activity-dependent plasticity of their synaptic connections. Here, we report that MC-GC synapses undergo a presynaptic, NMDA-receptor-independent form of long-term potentiation (LTP) that requires postsynaptic brain-derived neurotrophic factor (BDNF)/TrkB and presynaptic cyclic AMP (cAMP)/PKA signaling. This LTP is input specific and selectively expressed at MC-GC synapses, but not at the disynaptic inhibitory loop. By increasing the excitation/inhibition balance, MC-GC LTP enhances GC output at the associative MC-GC recurrent circuit and may contribute to dentate-dependent forms of learning and epilepsy.