NMDA receptor-dependent synaptic activation of TRPC channels in olfactory bulb granule cells

Canonical transient receptor potential (TRPC) channels are widely expressed throughout the nervous system including the olfactory bulb where their function is largely unknown. Here, we describe their contribution to central synaptic processing at the reciprocal mitral and tufted cell-granule cell microcircuit, the most abundant synapse of the mammalian olfactory bulb. Suprathreshold activation of the synapse causes sodium action potentials in mouse granule cells and a subsequent long-lasting depolarization (LLD) linked to a global dendritic postsynaptic calcium signal recorded with two-photon laser-scanning microscopy. These signals are not observed after action potentials evoked by current injection in the same cells. The LLD persists in the presence of group I metabotropic glutamate receptor antagonists but is entirely absent from granule cells deficient for the NMDA receptor subunit NR1. Moreover, both depolarization and Ca2? rise are sensitive to the blockade of NMDA receptors. The LLD and the accompanying Ca2? rise are also absent in granule cells from mice deficient for both TRPC channel subtypes 1 and 4, whereas the deletion of either TRPC1 or TRPC4 results in only a partial reduction of the LLD. Recordings from mitral cells in the absence of both subunits reveal a reduction of asynchronous neurotransmitter release from the granule cells during recurrent inhibition. We conclude that TRPC1 and TRPC4 can be activated downstream of NMDA receptor activation and contribute to slow synaptic transmission in the olfactory bulb, including the calcium dynamics required for asynchronous release from the granule cell spine.     

Mechanisms underlying NMDA receptor synaptic/extrasynaptic distribution and function

Research over the last few decades has shaped our understanding of the crucial involvement of the N-methyl-D-aspartate receptor (NMDAR) in mediating excitatory synaptic neurotransmission, neuronal development and learning and memory. The complexity of NMDAR modulation has escalated with the knowledge that receptors can traffic between synaptic and extrasynaptic sites, and that location on the plasma membrane profoundly affects the physiological function of NMDARs. Moreover, mechanisms that regulate NMDAR subcellular localization and function, such as protein-protein interactions, phosphorylation, palmitoylation, ubiquitination and receptor proteolytic cleavage, may differ for synaptic and extrasynaptic NMDARs. Recent studies suggest that NMDAR mislocalization is a dominant contributing factor to glutamatergic dysfunction and pathogenesis in neurological disorders such as Huntington's disease, Alzheimer's disease and ischemia. Therapeutic approaches that specifically rectify receptor mislocalization or target resulting downstream apoptotic signaling could be beneficial for preventing disease onset or progression across many disorders that are commonly caused by NMDAR dysfunction. This review will summarize the molecular mechanisms that regulate synaptic and extrasynaptic NMDAR localization in both physiologic and pathogenic states.     

Calcium, network activity, and the role of NMDA channels in synaptic plasticity in vitro.

Functionally effective neuronal circuits are constructed through a competitive process that requires patterned neuronal activity elicited by structured input from the environment. To explore the mechanisms of this activity-dependent synaptic restructuring, we have developed an in vitro preparation of mouse spinal cord neurons maintained in a 3-chambered cell-culture system. Sensory afferents that received chronic electrical stimulation for 3-5 d developed stronger synaptic connections than unstimulated afferents converging onto the same postsynaptic spinal cord neuron. Exposure to 100 microM DL-2-amino-5-phosphonovaleric acid (APV), an antagonist of the NMDA channel, during the stimulation period prevented the competitive advantage associated with electric stimulation. However, when APV was applied with a higher concentration of calcium (3 mM), activity-dependent synaptic plasticity was no longer inhibited by the NMDA receptor antagonist. This reversal of APV block of the plasticity was not impaired by reducing transmitter release with 3 mM magnesium (in addition to 3 mM calcium and APV). A suppressant effect of APV on spontaneous activity was observed, which was attributed to loss of the NMDA component of the EPSP. Activity-dependent plasticity was also blocked if spontaneous activity was suppressed with dilute tetrodotoxin (TTX; 5-10 nM), a dosage that reduces excitability of neurons but is insufficient to block sodium-dependent action potentials. These experiments bring into question how NMDA channel activation is involved in the processes of synaptic remodeling during development. The data suggest that postsynaptic activity is required for synaptic remodeling, but this activity need not involve NMDA receptor activation specifically for activity-evoked synaptic plasticity. Instead, the mechanism for plasticity appears to operate through calcium-dependent processes in general.     

Brain peptides with opiate antagonist action: their possible role in tolerance and dependence

(1) Extracts of brain taken from rats injected with increasing doses of morphine acted like antagonists on the mouse vas deferens preparation in vitro and on morphine analgesia in vivo. (2) After electrophoretic fractionation, the antagonist properties were found among the basic peptides. One of these, Arg-Tyr-Gly-Gly-Phe-Met, was synthcsized and reproduced the analgesia-inhibiting effect of the brain extract. However, when tested on mouse vas deferens, it did not show antagonist properties but acted like enkephalin. (3) Quantitative estimation of the agonist and antagonist concentration in the brain of morphine-injected rats showed increase in both types of peptides during the first 12 days of treatment followed by a particularly marked fall of the agonists, far below the normal level. (4) Preliminary experiments have shown that the endogenous antagonists and the synthetic peptide can precipitate opiate withdrawal symptoms and may, therefore, be involved in the mechanism of dependence. (5) The results reported suggest the possibility that a shift in the balance of endogenous agonists and antagonists may contribute to the development of tolerance and dependence.     

Long-term potentiation of high-frequency oscillation and synaptic transmission characterize in vitro NMDA receptor-dependent epileptogenesis in the hippocampus

The implication of high-frequency network oscillations (HFOs) in brain pathology resides in as yet unclear mechanisms. Employing field recordings from ventral hippocampal slices and two models of epileptogenesis (i.e. establishment of interictal-like persistent bursts), we found that HFOs associated with epileptiform bursts and excitatory synaptic transmission were co-modulated during epileptogenesis. NMDA receptor-dependent epileptogenesis in CA3 was consistently accompanied by long-lasting strengthening in synaptic transmission (by 94+/-17%, n=5) and HFOs (frequency, power and duration increased by 24+/-8%, 57+/-18% and 33+/-10%, respectively). Co-modulation of synaptic transmission and HFOs was also observed in NMDA receptor-independent epileptogenesis, although in individual experiments either enhancement or depression of both phenomena was observed. Pathological HFOs >200 Hz were unequivocally present in persistent bursts induced by NMDA receptor-dependent but not NMDA receptor-independent mechanisms. The duration of pathological HFOs associated with persistent bursts but not of HFOs associated with bursts before the establishment of epileptogenesis was linearly and strongly correlated with the duration of bursts (r=0.58, P<0.0001). We propose that interplay between spontaneous synchronous bursting and long-lasting synaptic potentiation accompanying certain forms of epileptogenesis may underlie long-lasting potentiation of HFOs, whose quantitative aspects may reliably signal the degree of network changes involved in epileptogenesis.     

Prenatal morphine exposure suppresses mineralocorticoid receptor-dependent basal synaptic transmission and synaptic plasticity in the lateral perforant path in adult male rats

The effects of prenatal morphine exposure (E11-18) on mineralocorticoid receptor (MR) modulation of synaptic plasticity were investigated in the lateral perforant path (LPP)-dentate gyrus granule cell synaptic system. Hippocampal slices were prepared from adult, prenatally saline- or morphine-exposed male rats. One hour prior to decapitation, some adult male rats were injected subcutaneously with saline or the MR antagonist, canrenoic acid (50 mg/kg). LPP was stimulated with high-frequency (2x100 Hz/0.5 s) and short-term plasticity (STP) and long-term potentiation (LTP) were evaluated at 5 and 30 min poststimulation, respectively. Prenatally saline-exposed male rats injected with saline 1 h prior to decapitation showed significantly higher levels of baseline, STP, and LTP than prenatally saline-exposed, canrenoic acid-treated males. In contrast, prenatally morphine-exposed male rats regardless of saline or canrenoic acid injection 1 h prior to decapitation were comparable in their baseline, STP, and LTP activities. Thus, the results demonstrate that canrenoic acid decreases the efficacy of the basal synaptic transmission in the LPP as well as suppresses synaptic plasticity in saline-exposed males. However, in adult morphine-exposed male rats, canrenoic acid has no other or further effects than a saline treatment suggesting that prenatal morphine exposure suppresses MR-dependent basal synaptic transmission as well as synaptic plasticity.     

Stress,synaptic plasticity,and psychopathology

It is now generally recognized that stressful events play a critical role in the genesis of psychopathology. The neurobiological mechanisms that mediate the contribution of stressful events to the manifestation of psychiatric disorders may include changes in synaptic efficacy in different brain areas. Numerous studies in animals have begun to identify some of these areas through experiments manipulating stressful components. This review focuses on alterations of synaptic efficacy in the hippocampus, the lateral septum, and the medial prefrontal cortex that mimic the pathophysiology of depression and post-traumatic stress disorder.     

Memory, synaptic plasticity and neurotoxins

Most neurotoxins induce serious impairments in cognitive or intellectual functioning; therefore, the ability of the individual to learn and remember forms a critical component of neurotoxin assessment. In depth investigations of neurotoxin-induced cognitive deficits have assisted in pinpointing the neural site of toxin activity. However, specific impairments in learning and memory can depend on the developmental stage at which the individual is exposed to the toxin, and certain neurotoxins show cognitive lifespan selectivity. There appears to be a basic sequence of events underlying neural development and information storage, and neurotoxins may disrupt this neuronal sequel critical for the storage or expression of "ancestral" or "environmental" memories. Certain primary rules govern the orderly development of the nervous system; these rules or mechanisms allow the expression of "ancestral memories" concerning neuronal differentiation and primitive behaviors. Following birth, these same mechanisms have been retained in a less robust form to allow learning and information storage, or the retention of "environmental memories." Neurotoxins which disrupt learning and memory capacities appear to interfere in this basic sequence of events, with the specific outcome depending on when during lifespan development the neurotoxin intervenes.     

The neurobiology of sound-specific auditory plasticity: A core neural circuit

Auditory learning or experience induces large-scale neural plasticity in not only the auditory cortex but also in the auditory thalamus and midbrain. Such plasticity is guided by acquired sound (sound-specific auditory plasticity). The mechanisms involved in this process have been studied from various approaches and support the presence of a core neural circuit consisting of a subcortico-cortico-subcortical tonotopic loop supplemented by neuromodulatory (e.g., cholinergic) inputs. This circuit has three key functions essential for establishing large-scale and sound-specific plasticity in the auditory cortex, auditory thalamus and auditory midbrain. They include the presence of sound information for guiding the plasticity, the communication between the cortex, thalamus and midbrain for coordinating the plastic changes and the adjustment of the circuit status for augmenting the plasticity. This review begins with an overview of sound-specific auditory plasticity in the central auditory system. It then introduces the core neural circuit which plays an essential role in inducing sound-specific auditory plasticity. Finally, the core neural circuit and its relationship to auditory learning and experience are discussed.     

The neurobiology of sound-specific auditory plasticity: A core neural circuit

Auditory learning or experience induces large-scale neural plasticity in not only the auditory cortex but also in the auditory thalamus and midbrain. Such plasticity is guided by acquired sound (sound-specific auditory plasticity). The mechanisms involved in this process have been studied from various approaches and support the presence of a core neural circuit consisting of a subcortico-cortico-subcortical tonotopic loop supplemented by neuromodulatory (e.g., cholinergic) inputs. This circuit has three key functions essential for establishing large-scale and sound-specific plasticity in the auditory cortex, auditory thalamus and auditory midbrain. They include the presence of sound information for guiding the plasticity, the communication between the cortex, thalamus and midbrain for coordinating the plastic changes and the adjustment of the circuit status for augmenting the plasticity. This review begins with an overview of sound-specific auditory plasticity in the central auditory system. It then introduces the core neural circuit which plays an essential role in inducing sound-specific auditory plasticity. Finally, the core neural circuit and its relationship to auditory learning and experience are discussed.     

Effect of suvorexant on morphine tolerance and dependence in mice: Role of NMDA,AMPA, ERK and CREB proteins

The major problems of morphine use in the clinic are its tolerance and dependence. This study aimed to investigate the effect of suvorexant, a dual orexin receptor antagonist, on morphine-induced dependence and tolerance in mice and evaluate the level of NMDA, AMPA, ERK, p-ERK, CREB and p-CREB proteins in the brain. Tolerance and dependence were induced by repeated injection of morphine in mice (three times a day for 3 days, 50, 50, and 75 mg/kg /day). To evaluate the effects of the drugs on morphine-induced tolerance and dependence, suvorexant (30, 60 and 90 mg/kg), clonidine (positive control, 0.1 mg/kg) and saline were injected intraperitoneally 30 min before each injection of morphine. Tolerance and locomotor activity were assessed by tail-flick and open-field tests, respectively. The effect of suvorexant on the naloxone (5 mg/kg, ip)-induced morphine withdrawal, was also evaluated. Finally, the expression of proteins in the brain of mice was measured by western blot. Administration of suvorexant with morphine significantly reduced morphine-induced tolerance. Also, suvorexant attenuated the naloxone-precipitated opioid withdrawal. Suvorexant decreased morphine-enhanced levels of CREB and p-ERK proteins but did not affect the expression of NMDA and AMPA proteins compared to the morphine group. Suvorexant reduced morphine-induced tolerance and dependence through the inhibition of orexin receptors as well as changes in CREB and p-ERK protein levels in the brain.     

Thalamocortical synaptic connections: efficacy, modulation, inhibition and plasticity

The thalamic input to the neocortex is communicated by glutamatergic synapses. The properties and organization of these synapses determine the primary level of cortical processing. Similar to intracortical synapses, both AMPA and NMDA receptors in young and mature animals mediate thalamocortical transmission. Kainate receptors participate in thalamocortical transmission during early development. The shape of thalamocortical synaptic potentials is similar to the shape of intracortical potentials. On the other hand, thalamocortical synapses have on average a higher release probability than intracortical synapses, and a much higher number of release sites per axon. As a result, the transmission of each thalamocortical axon is significantly more reliable and efficient than most intracortical axons. Thalamic axons specifically innervate a subset of inhibitory cells, to create a strong and secure feed-forward inhibitory pathway. Thalamocortical connections display many forms of synaptic plasticity in the first postnatal week, but not afterwards. The implications of the functional organization of thalamocortical synapses for neocortical processing are discussed.     

Inquiry into endorphinergic feedback mechanisms during the development of opiate tolerance/dependence

Met-enkephalin and β-endorphin levels were determined in the pituitary and brain of rats after treatment for several weeks with either agonists of high receptor affinity, such as levorphanol and etorphine, or with the narcotic antagonist naloxone. Long-term activation of opiate receptors failed to change the endorphin levels in restricted areas of brain and pituitary, although a high degree of tolerance/dependence is apparent in those animals. Chronic blockade of opiate receptors by naloxone also fails to affect endorphin levels in the pituitary, but selectively increases metenkephalin levels in the striatum. The present data do not support the notion of negative feedback mechanisms to regulate endorphinergic functions during the development of opiate tolerance/dependence.     

Facilitated NMDA receptor-mediated synaptic plasticity in the hippocampal CA1 area of dystrophin-deficient mice.

The contribution of the cytoskeletal membrane-associated protein dystrophin in glutamatergic transmission and related plasticity was investigated in the hippocampal CA1 area of wild-type and dystrophin-deficient (mdx) mice, using extracellular recordings in the ex vivo slice preparation. Presynaptic fiber volleys and field excitatory postsynaptic potentials (fEPSPs) mediated through N-methyl-D-Aspartate receptors (NMDAr) or non-NMDAr were compared in both strains. Comparable synaptic responses were observed in wild-type and mdx mice, suggesting that basal glutamatergic transmission is not altered in the mutants. By contrast, the synaptic strengthening induced by a conditioning stimulation of either 10, 30, or 100 Hz was significantly greater in mdx mice during the first minutes posttetanus. Because the posttetanic potentiation induced in the presence of the NMDAr antagonist D-APV was not affected in the mutants, a critical role of NMDAr in this increase was suggested. The magnitude of the potentiation induced by a 30 Hz stimulation in mdx mice was normalized as compared to wild-type mice by increasing the extracellular magnesium concentration from 1.5 to 3 mM. Moreover, the transitory depression of fEPSPs induced by bath-applied NMDA (50 microM for 30s) was more sensitive to an increased extracellular magnesium concentration in wild-type than in mdx mice. Our results suggest that the absence of dystrophin may facilitate NMDAr activation in the CA1 hippocampal subfield of mdx mice, which may be partly due to a reduction of the voltage-dependent block of this receptor by magnesium.     

Resistance to ethanol sensitization is associated with a loss of synaptic plasticity in the hippocampus

Behavioral sensitization to repeated ethanol (EtOH) exposure induces an increase in locomotor activity in mice. However, not all animals express such sensitization. Although the literature indicated that the hippocampus may play a role in EtOH sensitization, it is not known whether behavioral sensitization to EtOH is associated with preferential changes in bidirectional synaptic plasticity, i.e., LTP and LTD, two markers of learning capabilities that have also been shown to be involved in addictive behavior. In the present study, we examined whether the vulnerability to develop and express behavioral sensitization to EtOH is associated with altered bidirectional synaptic plasticity in the CA1 area of the dorsal hippocampus. For this purpose, we analyzed both LTP and LTD in resistant and sensitized mice during the expression phase, i.e., 7 days after 10 days of repeated EtOH i.p. administration. We found that resistant mice showed a lack of LTD without changes in LTP. The lack of LTD was associated with an increase in GluN2A protein level and was not due to an altered level of neuronal activity, since no difference was observed between the number of c‐FOS positive neurons in sensitized and resistant mice. Given that both types of synaptic plasticity signals may have distinct roles in specific learning and behaviors, our results suggest that resistant mice could exhibit different phenotypes in terms of learning/memory and addictive behaviors compared to sensitized ones. Synapse 71:e21899, 2017 . © 2016 Wiley Periodicals, Inc.     

Regulation of NMDA receptor transport: a KIF17-cargo binding/releasing underlies synaptic plasticity and memory in vivo

Regulation of NMDA receptor trafficking is crucial to modulate neuronal communication. Ca(2+)/calmodulin-dependent protein kinase phosphorylates the tail domain of KIF17, a member of the kinesin superfamily, to control NMDA receptor subunit 2B (GluN2B) transport by changing the KIF17-cargo interaction in vitro. However, the mechanisms of regulation of GluN2B transport in vivo and its physiological significance are unknown. We generated transgenic mice carrying wild-type KIF17 (TgS), or KIF17 with S1029A (TgA) or S1029D (TgD) phosphomimic mutations in kif17(-/-) background. TgA/kif17(-/-) and TgD/kif17(-/-) mice exhibited reductions in synaptic NMDA receptors because of their inability to load/unload GluN2B onto/from KIF17, leading to impaired neuronal plasticity, CREB activation, and spatial memory. Expression of GFP-KIF17 in TgS/kif17(-/-) mouse neurons rescued the synaptic and behavioral defects of kif17(-/-) mice. These results suggest that phosphorylation-based regulation of NMDA receptor transport is critical for learning and memory in vivo.     

Benzodiazepines: patterns of use, tolerance and dependence

The authors review recent studies on benzodiazepine, the most largely used drug for insomnia and anxiety. In this paper are summarized: the development, patterns of use and abuse, mechanism of action, development of differential tolerance to its many effects, and the phenomena of withdrawal and dependence on the benzodiazepines.