The Relative Contribution of NMDARs to Excitatory Postsynaptic Currents is Controlled by Ca(2+)-Induced Inactivation

Calcium- and calmodulin-dependent inhibition of NMDA receptor currents

Calcium ions (Ca2+) reduce NMDA receptor currents through several distinct mechanisms. Among these, calmodulin (CaM)-dependent inhibition (CDI) accomplishes rapid, reversible, and incomplete reduction of the NMDA receptor currents in response to elevations in intracellular Ca2+. Quantitative and mechanistic descriptions of CDI of NMDA receptor-mediated signals have been marred by variability originating, in part, from differences in the conditions and metrics used to evaluate this process across laboratories. Recent ratiometric approaches to measure the magnitude and kinetics of NMDA receptor CDI have facilitated rapid insights into this phenomenon. Notably, the kinetics and magnitude of NMDA receptor CDI depend on the degree of saturation of its CaM binding sites, which represent the bona fide calcium sensor for this type of inhibition, the kinetics and magnitude of the Ca2+ signal, which depends on the biophysical properties of the NMDA receptor or of adjacent Ca2+ sources, and on the relative distribution of Ca2+ sources and CaM molecules. Given that all these factors vary widely during development, across cell types, and with physiological and pathological states, it is important to understand how NMDA receptor CDI develops and how it contributes to signaling in the central nervous system. Here, we review briefly these recent advances and highlight remaining questions about the structural and kinetic mechanisms of NMDA receptor CDI. Given that pathologies can arise from several sources, including mutations in the NMDA receptor and in CaM, understanding how CaM responds to intracellular Ca2+ signals to initiate conformational changes in NMDA receptors, and mapping the structural domains responsible will help to envision novel therapeutic strategies to neuropsychiatric diseases, which presently have limited available treatments.

The medial orbitofrontal cortex governs reward-related circuits in an age-dependent manner

Nucleus accumbens (NAc) neurons integrate excitatory inputs from cortical and limbic structures, contributing to critical cognitive functions, including decision-making. As these afferents mature from adolescence through adulthood, incoming signals to the NAc may summate differently between age groups. Decision-making evaluates both reward and risk before action selection, suggesting an interplay between reward- and risk-related circuits. Medial orbitofrontal cortex (MO)-NAc circuits permit risk assessment behaviors and likely underlie risk information incorporation. As adolescents make reward-centric choices regardless of risk, we hypothesized the impact of MO activity alters reward-related NAc circuits in an age-dependent manner. To test this hypothesis, we used single-unit electrophysiology to measure MO train stimulation's effect on reward-related pathways, specifically the basolateral amygdala (BLA)-NAc circuit, in adult and adolescent rats. MO train stimulation altered the strength but not the timing of BLA-NAc interactions in a frequency-dependent manner. In adults, MO train stimulation produced a frequency-dependent, bidirectional effect on BLA-evoked NAc AP probability. Contrastingly, MO train stimulation uniformly attenuated BLA-NAc interactions in adolescents. While the mature MO can govern reward-related circuits in an activity-dependent manner, perhaps to adapt to positive or negative decision-making outcomes, the adolescent MO may be less able to bidirectionally impact reward-related pathways resulting in biased decision-making.

Overexpression of Calretinin Enhances Short-Term Synaptic Depression

Analysis of the effects of various proteins on short-term synaptic plasticity is a difficult task, which may require the use of knockout animals. Here, we propose an alternative experimental approach for studying the roles of desired proteins in synaptic plasticity. We packed the Ca²⁺-binding protein calretinin and the fluorescent protein Venus into AAV and injected the concentrated viral suspension into the neocortex of newborn rats. The infected layer 2/3 pyramidal cells were identified in rat cortical slices using Venus fluorescence. Analysis of short-term synaptic plasticity using paired patch clamp recordings between layer 2/3 pyramidal cells (presynaptic cell) and fast-spiking (FS) interneurons (post-synaptic cell) showed that calretinin expression in the pyramidal cells did not change the failure rate in this synapse but did decrease synaptic delay. Analysis of the parameters of short-term synaptic plasticity showed that the amplitude of the first EPSP in the train was not affected by calretinin, however, calretinin strongly enhanced short-term depression. In addition, we found that the effect of calretinin depended on the presynaptic firing frequency: an increase in frequency resulted in enhancement of synaptic depression.

Calcium-Dependent Desensitization of NMDA Receptors

Glutamate receptors play the key role in excitatory synaptic transmission in the central nervous system (CNS). N-methyl-D-aspartate-activated glutamate receptors (NMDARs) are ion channels permeable to sodium, potassium, and calcium ions that localize to the pre- and postsynaptic membranes, as well as extrasynaptic neuronal membrane. Calcium entry into dendritic spines is essential for long-term potentiation (LTP) and long-term depression (LTD) of synaptic transmission. Both LTP and LTD represent morphological and functional changes occurring in the process of memory formation. NMDAR dysfunction is associated with epilepsy, schizophrenia, migraine, dementia, and neurodegenerative diseases. Prolonged activation of extrasynaptic NMDARs causes calcium overload and apoptosis of neurons. Here, we review recent findings on the molecular mechanisms of calcium-dependent NMDAR desensitization that ensures fast modulation of NMDAR conductance in the CNS and limits calcium entry into the cells under pathological conditions. We present the data on molecular determinants related to calcium-dependent NMDAR desensitization and functional interaction of NMDARs with other ion channels and transporters. We also describe association of NMDARs with lipid membrane microdomains.

Resident Calmodulin Primes NMDA Receptors for Ca2+-Dependent Inactivation

N-methyl-d-aspartate (NMDA) receptors are glutamate- and glycine-gated channels that flux Na+ and Ca2+ into postsynaptic neurons during synaptic transmission. The resulting intracellular Ca2+ transient is essential to physiological and pathological processes related to synaptic development, plasticity, and apoptosis. It also engages calmodulin (CaM) to reduce subsequent NMDA receptor activity in a process known as Ca2+-dependent inactivation (CDI). Here, we used whole-cell electrophysiology to measure CDI and computational modeling to dissect the sequence of events that underlies it. With these approaches, we estimate that CaM senses NMDA receptor Ca2+ influx at ∼9 nm from the channel pore. Further, when we controlled the frequency of Ca2+ influx through individual channels, we found that a kinetic model where apoCaM associates with channels before their activation best predicts the measured CDI. These results provide, to our knowledge, novel functional evidence for CaM preassociation to NMDA receptors in living cells. This particular mechanism for autoinhibitory feedback reveals strategies and challenges for Ca2+ regulation in neurons during physiological synaptic activity and disease.

Functional Analysis of Recombinant Channels in Host Cells Using a Fast Agonist Application System

A reduced recombinant system provides a unique opportunity to study the biophysical properties of NMDAR channels with known subunit compositions, by using a point mutation approach to analyze the structural determinants of receptor function (Wollmuth and Sobolevsky, Trends Neurosci 27:321-328, 2004). However, in addition to the well-developed repertoire of molecular biological techniques, these types of studies also require electrophysiological methods that allow a wide range of receptor activation protocols that can adequately assess desensitization, inactivation, ion permeability, and other properties of the channels. Currently, one of the most well-developed techniques suitable for addressing these issues is use of the fast agonist application system for rapid activation of ligand gated ion-channels (Colquhoun et al., J Physiol 458:261-287, 1992; Jonas and Sakmann, J Physiol 455:143-171, 1992).

Fast interaction between AMPA and NMDA receptors by intracellular calcium

Suppression of NMDA receptor (NMDAR)-mediated currents by intracellular Ca²⁺ has been described as a negative feedback loop in NMDAR modulation. In the time scale of tenths of milliseconds the depth of the suppression does not depend on the Ca²⁺ source. It may be caused by Ca²⁺ influx through voltage-gated calcium channels, NMDAR channels or release from intracellular stores. However, NMDARs are often co-expressed in synapses with Ca²⁺-permeable AMPA receptors (AMPARs). Due to significant differences in activation kinetics between these two types of glutamate receptors (GluRs), Ca²⁺ entry through AMPARs precedes full activation of NMDARs, and therefore, might have an impact on the amplitude of NMDAR-mediated currents. The study of Ca²⁺-mediated crosstalk between AMPAR and NMDAR in native synapses is challenging due to high NMDAR Ca²⁺ permeability. Therefore, recombinant Ca²⁺-permeable AMPAR and Ca²⁺-impermeable NMDAR mutant channels were co-expressed in HEK 293 cells to examine their interaction. An AMPAR-mediated increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) reversibly reduced the size of NMDAR-mediated whole-cell currents. The time course of the NMDAR channel inactivation and recovery from inactivation followed the time course of the [Ca²⁺]_i transient. When brief (1ms) pulses of glutamate were applied to outside-out patches, the degree of NMDAR inactivation increased with the increase in charge carried by the currents through co-activated AMPARs. However, AMPAR-mediated NMDAR inactivation was abolished in the presence of intracellular fast Ca²⁺ buffer BAPTA or in Ca²⁺-free extracellular solution. We conclude that Ca²⁺ entering through AMPARs inactivates co-localized NMDARs in the time range of excitatory postsynaptic currents.