Postnatal developmental changes in AMPA and NMDA receptors in the rat vestibular nuclei

A developmental critical period for ocular dominance plasticity of binocular neurons in mouse superior colliculus

Detecting visual features in the environment is crucial for animals' survival. The superior colliculus (SC) is implicated in motion detection and processing, whereas how the SC integrates visual inputs from the two eyes remains unclear. Using in vivo electrophysiology, we show that mouse SC contains many binocular neurons that display robust ocular dominance (OD) plasticity in a critical period during early development, which is similar to, but not dependent on, the primary visual cortex. NR2A- and NR2B-containing N-methyl-D-aspartate (NMDA) receptors play an essential role in the regulation of SC plasticity. Blocking NMDA receptors can largely prevent the impairment of predatory hunting caused by monocular deprivation, indicating that maintaining the binocularity of SC neurons is required for efficient hunting behavior. Together, our studies reveal the existence and function of OD plasticity in SC, which broadens our understanding of the development of subcortical visual circuitry relating to motion detection and predatory hunting.

N-Methyl-d-Aspartate receptor and inflammation in dorsolateral prefrontal cortex in schizophrenia

Lower N-methyl-d-aspartate receptor (NMDAR) GluN1 subunit levels and heightened neuroinflammation are found in the cortex in schizophrenia. Since neuroinflammation can lead to changes in NMDAR function, it is possible that these observations are linked in schizophrenia. We aimed to extend our previous studies by measuring molecular indices of NMDARs that define key functional properties of this receptor - particularly the ratio of GluN2A and GluN2B subunits - in dorsolateral prefrontal cortex (DLPFC) from schizophrenia and control cases (37/37). We sought to test whether changes in these measures are specific to the subset of schizophrenia cases with high levels of inflammation-related mRNAs, defined as a high inflammatory subgroup. Quantitative autoradiography was used to detect 'functional' NMDARs ([³H]MK-801), GluN1-coupled-GluN2A subunits ([³H]CGP-39653), and GluN1-coupled-GluN2B subunits ([³H]Ifenprodil). Quantitative RT-PCR was used to measure NMDAR subunit transcripts (GRIN1, GRIN2A and GRIN2B). The ratios of GluN2A:GluN2B binding and GRIN2A:GRIN2B mRNAs were calculated as an index of putative NMDAR composition. We found: 1) GluN2A binding, and 2) the ratios of GluN2A:GluN2B binding and GRIN2A:GRIN2B mRNAs were lower in schizophrenia cases versus controls (p < 0.05), and 3) lower GluN2A:GluN2B binding and GRIN2A:GRIN2B mRNA ratios were exaggerated in the high inflammation/schizophrenia subgroup compared to the low inflammation/control subgroup (p < 0.05). No other NMDAR-related indices were significantly changed in the high inflammation/schizophrenia subgroup. This suggests that neuroinflammation may alter NMDAR stoichiometry rather than targeting total NMDAR levels overall, and future studies could aim to determine if anti-inflammatory treatment can alleviate this aspect of NMDAR-related pathology.

Pigeon as a model to study peripheral projections from the horizontal semicircular canal vestibular apparatus to a brainstem target immunoreactive for AMPA

Purpose:

To evaluate whether the pigeon (Columba livia) is a good model for evaluating the vestibular system involved with postural maintenance during movement.

Methods:

This study maps the brainstem targets of the horizontal ampullary inputs from the vestibular periphery of the pigeon. We used biotin dextran amine (BDA) injection in horizontal semicircular canal (HSCC), immunohistochemistry for GluR2/3 and GluR4 AMPA and computerized histomorphology reconstruction.

Results:

Our results show the same distribution pattern with ipsilateral projections to vestibular nuclear complex (VNC) from the HSCC, with the majority of labeled fibers being, long, thin, with few varicosities and many ramifications. Horizontal semicircular canal projections achieve neurons belonging to all nuclei of the VNC with exception of dorsal portion of lateral vestibular nucleus and this area express GluR2/3 and GluR4 AMPA receptors reinforcing the idea of glutamate participation in these connections.

Conclusions:

Pigeon is an appropriated experimental model to study of projections of HSCC and reinforcing the information that the vestibular system has strong relation with the fast responses necessary for postural control. Moreover, its phylogenetic organization apparently conservation, also seems to be a fundamental characteristic for vertebrates.

NMDA receptors control development of somatosensory callosal axonal projections

Callosal projections from primary somatosensory cortex (S1) are key for processing somatosensory inputs and integrating sensory-motor information. How the callosal innervation pattern in S1 is formed during early postnatal development is not clear. We found that the normal termination pattern of these callosal projections is disrupted in cortex specific NMDAR mutants. Rather than projecting selectively to the primary/secondary somatosensory cortex (S1/S2) border, axons were uniformly distributed throughout S1. In addition, the density of this projection increased over postnatal life until the mice died by P30. By combining genetic and antibody-mediated loss of function, we demonstrated that it is GluN2B-containing NMDA receptors in target S1 that mediate this guidance phenotype, thus playing a central role in interhemispheric connectivity. Furthermore, we found that this function of NMDA receptors in callosal circuit formation is independent of ion channel function and works with the EPHRIN-B/EPHB system. Thus, NMDAR in target S1 cortex regulates the formation callosal circuits perhaps by modulating EPH-dependent repulsion.

Postsynaptic GluN2B-containing NMDA receptors contribute to long-term depression induction in medial vestibular nucleus neurons of juvenile rats

Medial vestibular nucleus (MVN) neurons are involved in the regulation of eye movements to endure the stability of the image during head movement, and play a critical role in plasticity of the vestibulo-ocular reflex (VOR) during the juvenile period. We have previously shown that the long-term depression (LTD) of synaptic transmission was induced by high frequency stimulation (HFS) and blocked by N-methyl-D-aspartate (NMDA) receptor antagonist D-APV at the vestibular afferent synapses of type-B MVN neurons. In the present study, we used whole-cell patch-clamp recordings in vitro to investigate the subunit composition of these NMDA receptors in the induction of LTD in MVN slices from postnatal 13-16 day rats. We found that LTD induced in type-B neurons of the rat MVN with HFS was blocked by Ro 25-6981, a specific antagonist for GluN2B-containing NMDA receptors. Moreover, the other selective GluN2B-containing NMDA receptor antagonist (ifenprodil) also prevented the induction of LTD. However, bath application of the GluN2A-containing NMDA receptor antagonists (Zn²⁺ and TCN 201) had no influence on the induction of LTD. Similar results were obtained by exogenously applied two GluN2C/GluN2D-preferring NMDA receptor antagonists (PPDA and UBP 141). Furthermore, presynaptic NMDA receptor subunits are not necessary for vestibular LTD. These results suggest that the induction of LTD by HFS in vestibular afferent synapses of type-B MVN neurons requires postsynaptic GluN2B-containing NMDA receptors, but not GluN2A-containing NMDA receptors or GluN2C/GluN2D-containing NMDA receptors.

On the safety of repeated ketamine infusions for the treatment of depression: Effects of sex and developmental periods

In this review, we will discuss the safety of repeated treatments with ketamine for patients with treatment-resistant depression (TRD), a condition in which patients with major depression do not show any clinical improvements following treatments with at least two antidepressant drugs. We will discuss the effects of these treatments in both sexes at different developmental periods. Numerous small clinical studies have shown that a single, low-dose ketamine infusion can rapidly alleviate depressive symptoms and thoughts of suicidality in patients with TRD, and these effects can last for about one week. Interestingly, the antidepressant effects of ketamine can be prolonged with intermittent, repeated infusion regimens and produce more robust therapeutic effects when compared to a single infusion. The safety of such repeated treatments with ketamine has not been thoroughly investigated. Although more studies are needed, some clinical and preclinical reports indicated that repeated infusions of low doses of ketamine may have addictive properties, and suggested that adolescent and adult female subjects may be more sensitive to ketamine's addictive effects. Additionally, during ketamine infusions, many TRD patients report hallucinations and feelings of dissociation and depersonalization, and therefore the effects of repeated treatments of ketamine on cognition must be further examined. Some clinical reports indicated that, compared to women, men are more sensitive to the psychomimetic effects of ketamine. Preclinical studies extended these findings to both adolescent and adult male rodents and showed that male rodents at both developmental periods are more sensitive to ketamine's cognitive-altering effects. Accordingly, in this review we shall focus our discussion on the potential addictive and cognitive-impairing effects of repeated ketamine infusions in both sexes at two important developmental periods: adolescence and adulthood. Although more work about the safety of ketamine is warranted, we hope this review will bring some answers about the safety of treating TRD with repeated ketamine infusions.

Increasing the GluN2A/GluN2B Ratio in Neurons of the Mouse Basal and Lateral Amygdala Inhibits the Modification of an Existing Fear Memory Trace

Reconsolidation updating is a form of memory modification in which an existing memory can become destabilized upon retrieval and subsequently be modified via protein-synthesis-dependent reconsolidation. However, not all memories appear to destabilize upon retrieval and thus are not modifiable via reconsolidation updating approaches and the neurobiological basis for this remains poorly understood. Here, we report that auditory fear memories created with 10 tone-shock pairings are resistant to retrieval-dependent memory destabilization and are associated with an increase in the synaptic GluN2A/GluN2B ratio in neurons of the basal and lateral amygdala (BLA) compared with weaker fear memories created via one or three tone-shock pairings. To increase the GluN2A/GluN2B ratio after learning, we generated a line of mice that expresses an inducible and doxycycline-dependent GFP-GluN2A transgene specifically in α-CaMKII-positive neurons. Our findings indicate that increasing the GluN2A/GluN2B ratio in BLA α-CaMKII-positive neurons after a weak fear memory has consolidated inhibits retrieval-dependent memory destabilization and modification of the fear memory trace. This was associated with a reduction in retrieval-dependent AMPA receptor trafficking, as evidenced by a reduction in retrieval-dependent phosphorylation of GluR1 at serine-845. In addition, we determined that increasing the GluN2A/GluN2B ratio before fear learning significantly impaired long term memory consolidation, whereas short-term memory remained unaltered. An increase in the GluN2A/GluN2B ratio after fear learning had no influence on fear extinction or expression. Our results underscore the importance of NMDAR subunit composition for memory destabilization and suggest a mechanism for why some memories are resistant to modification.

SIGNIFICANCE STATEMENT

Memory modification using reconsolidation updating is being examined as one of the potential treatment approaches for attenuating maladaptive memories associated with emotional disorders. However, studies have shown that, whereas weak memories can be modified using reconsolidation updating, strong memories can be resistant to this approach. Therefore, treatments targeting the reconsolidation process are unlikely to be clinically effective unless methods are devised to enhance retrieval-dependent memory destabilization. Currently, little is known about the cellular and molecular events that influence the induction of reconsolidation updating. Here, we determined that an increase in the GluN2A/GluN2B ratio interferes with retrieval-dependent memory destabilization and inhibits the initiation of reconsolidation updating.

Efferent Vestibular Neurons Show Homogenous Discharge Output But Heterogeneous Synaptic Input Profile In Vitro

Despite the importance of our sense of balance we still know remarkably little about the central control of the peripheral balance system. While previous work has shown that activation of the efferent vestibular system results in modulation of afferent vestibular neuron discharge, the intrinsic and synaptic properties of efferent neurons themselves are largely unknown. Here we substantiate the location of the efferent vestibular nucleus (EVN) in the mouse, before characterizing the input and output properties of EVN neurons in vitro. We made transverse serial sections through the brainstem of 4-week-old mice, and performed immunohistochemistry for calcitonin gene-related peptide (CGRP) and choline acetyltransferase (ChAT), both expressed in the EVN of other species. We also injected fluorogold into the posterior canal and retrogradely labelled neurons in the EVN of ChAT:: tdTomato mice expressing tdTomato in all cholinergic neurons. As expected the EVN lies dorsolateral to the genu of the facial nerve (CNVII). We then made whole-cell current-, and voltage-clamp recordings from visually identified EVN neurons. In current-clamp, EVN neurons display a homogeneous discharge pattern. This is characterized by a high frequency burst of action potentials at the onset of a depolarizing stimulus and the offset of a hyperpolarizing stimulus that is mediated by T-type calcium channels. In voltage-clamp, EVN neurons receive either exclusively excitatory or inhibitory inputs, or a combination of both. Despite this heterogeneous mixture of inputs, we show that synaptic inputs onto EVN neurons are predominantly excitatory. Together these findings suggest that the inputs onto EVN neurons, and more specifically the origin of these inputs may underlie EVN neuron function.

The production of viral vectors designed to express large and difficult to express transgenes within neurons

BACKGROUND

Viral vectors are frequently used to deliver and direct expression of transgenes in a spatially and temporally restricted manner within the nervous system of numerous model organisms. Despite the common use of viral vectors to direct ectopic expression of transgenes within the nervous system, creating high titer viral vectors that are capable of expressing very large transgenes or difficult to express transgenes imposes unique challenges. Here we describe the development of adeno-associated viruses (AAV) and lentiviruses designed to express the large and difficult to express GluN2A or GluN2B subunits of the N-methyl-D-aspartate receptor (NMDA) receptor, specifically within neurons.

RESULTS

We created a number of custom designed AAV and lentiviral vectors that were optimized for large transgenes, by minimizing DNA sequences that were not essential, utilizing short promoter sequences of 8 widely used promoters (RSV, EFS, TRE3G, 0.4αCaMKII, 1.3αCaMKII, 0.5Synapsin, 1.1Synapsin and CMV) and utilizing a very short (~75 bps) 3' untranslated sequence. Not surprisingly these promoters differed in their ability to express the GluN2 subunits, however surprisingly we found that the neuron specific synapsin and αCaMKII, promoters were incapable of conferring detectable expression of full length GluN2 subunits and detectable expression could only be achieved from these promoters if the transgene included an intron or if the GluN2 subunit transgenes were truncated to only include the coding regions of the GluN2 transmembrane domains.

CONCLUSIONS

We determined that viral packaging limit, transgene promoter and the presence of an intron within the transgene were all important factors that contributed to being able to successfully develop viral vectors designed to deliver and express GluN2 transgenes in a neuron specific manner. Because these vectors have been optimized to accommodate large open reading frames and in some cases contain an intron to facilitate expression of difficult to express transgenes, these viral vectors likely could be useful for delivering and expressing many large or difficult to express transgenes in a neuron specific manner.

The repetition timing of high frequency afferent stimulation drives the bidirectional plasticity at central synapses in the rat medial vestibular nuclei

In this study we show that high frequency stimulation (HFS, 100Hz) of afferent fibers to the medial vestibular nucleus (MVN) can induce opposite long-term modifications of synaptic responses in the type B neurons depending upon the stimulation pattern. Long burst stimulation (LBS: 2s) and short burst stimulation (SBS: 0.55s) were applied with different burst number (BN) and inter-burst intervals (IBI). It results that both LBS and SBS can induce either N-methyl-d aspartate receptors (NMDARs)-mediated long-term potentiation (LTP) or long-term depression (LTD), depending on temporal organization of repetitive bursts. In particular, the IBI plays a relevant role in guiding the shift from LTP to LTD since by using both LBS and SBS LTP is induced by shorter IBI than LTD. By contrast, the sign of long-term effect does not depend on the mean impulse frequency evaluated within the entire stimulation period. Therefore, the patterns of repetitive vestibular activation with different ratios between periods of increased activity and periods of basal activity may lead to LTP or LTD probably causing different levels of postsynaptic Ca(2+). On the whole, this study demonstrates that glutamatergic vestibular synapse in the MVN can undergo NMDAR-dependent bidirectional plasticity and puts forward a new aspect for understanding the adaptive and compensatory plasticity of the oculomotor responses.

mGluR5 and NMDA receptors drive the experience- and activity-dependent NMDA receptor NR2B to NR2A subunit switch

In cerebral cortex there is a developmental switch from NR2B- to NR2A-containing NMDA receptors (NMDARs) driven by activity and sensory experience. This subunit switch alters NMDAR function, influences synaptic plasticity, and its dysregulation is associated with neurological disorders. However, the mechanisms driving the subunit switch are not known. Here, we show in hippocampal CA1 pyramidal neurons that the NR2B to NR2A switch driven acutely by activity requires activation of NMDARs and mGluR5, involves PLC, Ca(2+) release from IP(3)R-dependent stores, and PKC activity. In mGluR5 knockout mice the developmental NR2B-NR2A switch in CA1 is deficient. Moreover, in visual cortex of mGluR5 knockout mice, the NR2B-NR2A switch evoked in vivo by visual experience is absent. Thus, we establish that mGluR5 and NMDARs are required for the activity-dependent NR2B-NR2A switch and play a critical role in experience-dependent regulation of NMDAR subunit composition in vivo.

Long-term potentiation of synaptic response and intrinsic excitability in neurons of the rat medial vestibular nuclei

Using intracellular recordings, we investigated the effects of high frequency stimulation (HFS) of the primary vestibular afferents on the evoked excitatory postsynaptic potential (EPSP) and intrinsic excitability (IE) of type-A and type-B neurons of the medial vestibular nucleus (MVN), in male rat brainstem slices. HFS induces long-term potentiation (LTP) of both EPSP and IE, which may occur in combination or separately. Synaptic LTP is characterized by an increase in the amplitude, slope and decay time constant of EPSP and IE-LTP through enhancements of spontaneous and evoked neuron firing and of input resistance (Rin). Moreover, IE-LTP is associated with a decrease in action potential afterhyperpolarization (AHP) amplitude and an increase in interspike slope steepness (ISS). The more frequent effects of HFS are EPSP-LTP in type-B neurons and IE-LTP in type-A neurons. In addition, the development of EPSP-LTP is fast in type-B neurons but slow in type-A, whereas IE-LTP develops slowly in both types. We have demonstrated that activation of N-methyl-d aspartate receptors (NMDARs) is only required for EPSP-LTP induction, whereas metabotropic glutamate receptors type-1 (mGluR1) are necessary for IE-LTP induction as well as the full development and maintenance of EPSP-LTP. Taken together, these findings demonstrate that brief and intense activation of vestibular afferent input to the MVN neurons may provoke synaptic LTP and/or IE-LTP that, induced in combination or separately, may assure the different selectivity of the MVN neuron response enhancement to the afferent signals.

Membrane depolarization regulates AMPA receptor subunit expression in cerebellar granule cells in culture

The physiological responses of AMPA receptors can be modulated through the differential expression of their subunits and by modifying their number at the cell surface. Here we have studied the expression of AMPA receptor subunits (GluR1-4) mRNAs in cerebellar granule cells grown in depolarizing (25mMK(+)) medium, and we have evaluated the effect of decreasing the [K(+)] in the culture medium for 24 h on both GluR1-4 expression (both mRNA and protein) and their presence at the plasma membrane. The expression of the four AMPAR subunits increases as the [K(+)] decreases, although the increase in GluR2 and GluR3 was only observed in the cell soma but not in the dendrites. Calcium entry through L-type calcium channel and CaMKIV activation are responsible for the reduction in the expression of AMPA receptor subunits in cells cultured in depolarizing conditions. Indeed, prolonged reduction of extracellular [K(+)] or blockage of L-type calcium channels enhanced both the surface insertion of the four AMPAR subunits and the AMPA response measured through intracellular calcium increase. These findings reveal a balanced increase in functional AMPA receptors at the surface of cells that can trigger strong increases in calcium in response to the persistent reduction of calcium entry.

Synaptic plasticity in medial vestibular nucleus neurons: comparison with computational requirements of VOR adaptation

BACKGROUND

Vestibulo-ocular reflex (VOR) gain adaptation, a longstanding experimental model of cerebellar learning, utilizes sites of plasticity in both cerebellar cortex and brainstem. However, the mechanisms by which the activity of cortical Purkinje cells may guide synaptic plasticity in brainstem vestibular neurons are unclear. Theoretical analyses indicate that vestibular plasticity should depend upon the correlation between Purkinje cell and vestibular afferent inputs, so that, in gain-down learning for example, increased cortical activity should induce long-term depression (LTD) at vestibular synapses.

METHODOLOGY/PRINCIPAL FINDINGS

Here we expressed this correlational learning rule in its simplest form, as an anti-Hebbian, heterosynaptic spike-timing dependent plasticity interaction between excitatory (vestibular) and inhibitory (floccular) inputs converging on medial vestibular nucleus (MVN) neurons (input-spike-timing dependent plasticity, iSTDP). To test this rule, we stimulated vestibular afferents to evoke EPSCs in rat MVN neurons in vitro. Control EPSC recordings were followed by an induction protocol where membrane hyperpolarizing pulses, mimicking IPSPs evoked by flocculus inputs, were paired with single vestibular nerve stimuli. A robust LTD developed at vestibular synapses when the afferent EPSPs coincided with membrane hyperpolarization, while EPSPs occurring before or after the simulated IPSPs induced no lasting change. Furthermore, the iSTDP rule also successfully predicted the effects of a complex protocol using EPSP trains designed to mimic classical conditioning.

CONCLUSIONS

These results, in strong support of theoretical predictions, suggest that the cerebellum alters the strength of vestibular synapses on MVN neurons through hetero-synaptic, anti-Hebbian iSTDP. Since the iSTDP rule does not depend on post-synaptic firing, it suggests a possible mechanism for VOR adaptation without compromising gaze-holding and VOR performance in vivo.

Developmental maturation of ionotropic glutamate receptor subunits in rat vestibular nuclear neurons responsive to vertical linear acceleration

We investigated the maturation profile of subunits of ionotropic glutamate receptors in vestibular nuclear neurons that were activated by sinusoidal linear acceleration along the vertical plane. The otolithic origin of Fos expression in these neurons was confirmed as a marker of functional activation when labyrinthectomized and/or stationary control rats contrasted by showing sporadically scattered Fos-labeled neurons in the vestibular nuclei. By double immunohistochemistry for Fos and one of the receptor subunits, otolith-related neurons that expressed either alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate or N-methyl-d-aspartate subunits were first identified in the medial vestibular nucleus, spinal vestibular nucleus and Group x by postnatal day (P)7, and in the lateral vestibular nucleus and Group y by P9. No double-labeled neurons were found in the superior vestibular nucleus. Within each vestibular subnucleus, these double-labeled neurons constituted approximately 90% of the total Fos-labeled neurons. The percentage of Fos-labeled neurons expressing the GluR1 or NR2A subunit showed developmental invariance in all subnuclei. For Fos-labeled neurons expressing the NR1 subunit, similar invariance was observed except that, in Group y, these neurons decreased from P14 onwards. For Fos-labeled neurons expressing the GluR2, GluR2/3, GluR4 or NR2B subunit, a significant decrease was found by the adult stage. In particular, those expressing the GluR4 subunit showed a two- to threefold decrease in the medial vestibular nucleus, spinal vestibular nucleus and Group y. Also, those expressing the NR2B subunit showed a twofold decrease in Group y. Taken together, the postsynaptic expression of ionotropic glutamate receptor subunits in different vestibular subnuclei suggests that glutamatergic transmission within subregions plays differential developmental roles in the coding of gravity-related vertical spatial information.

Dynamics of glutamatergic synapses in the medial vestibular nucleus of the mouse

During sinusoidal rotation or translation, primary vestibular afferents modulate their discharge rates at the frequency of motion, effectively transmitting frequency-modulated (FM) signals. This study indicates a possible role for excitatory synapses in the processing of FM signals by vestibular brainstem pathways. Inputs to medial vestibular neurons were activated with FM pulse trains, while inhibitory transmission was blocked. The relationship between the presynaptic pulse rate and the postsynaptic membrane potential was found to be linear within a range of pulse rates. Short-term plasticity was a factor contributing to sensitivity at higher modulating frequencies. The amount of low-pass filtering was correlated with excitatory postsynaptic potential (EPSP) shape, which affected temporal summation during the train. Although the NMDA component of glutamatergic transmission affected EPSP shape, it made only a minor contribution to the dynamics of synaptic transmission. Most responses showed low-pass filtering over the entire 1-16 Hz range. Overall, excitatory synapses in the medial vestibular nucleus contribute a low-pass filter to central vestibular processing and complement the high-pass filtering that is introduced both by peripheral vestibular dynamics and by the intrinsic dynamics of secondary vestibular neurons.

Postsynaptic mechanisms of excitotoxicity: Involvement of postsynaptic density proteins, radicals, and oxidant molecules

Traditional models of neuronal excitotoxicity focused on the overactivation of receptors such as the ionotropic N-methyl-D-aspartate (NMDA)-subtype glutamate receptor. Recent developments have shifted focus to downstream neurotoxic signaling molecules with exciting implications to specific strategies for treating excitotoxic disorders. This review outlines these developments and introduces newly emerging evidence implicating the involvement of the melastatin subfamily in anoxic neuronal death. Both of these converge on the production of reactive oxygen species (ROS), including superoxide, nitric oxide (NO) and the oxidant peroxynitrite.

Early auditory deprivation alters expression of NMDA receptor subunit NR1 mRNA in the rat auditory cortex

The expression of NMDA receptor NR1 subunit mRNA was studied in rat auditory cortex (AC) on different postnatal days using digoxigenin-labeled oligonucleotide probes. The results showed that NR1 expression increased from birth to postnatal day 35 (P35) and remained constant until P56. The most significant increases occurred between P7 and P14. Changes in NR1 mRNA expression in rats subjected to monaural hearing deprivation on P7, P21, P35, and P49 were examined on P56. Between P7 and P21, when the rat auditory system was still in a critical period of development, NR1 mRNA expression was lower in the contralateral AC, which received auditory signals from the plugged ear, than in the ipsilateral AC. However, no significant difference was observed between the rats deprived of hearing on P35 and those deprived of hearing on P42, the end of the critical period of auditory development. These results showed that monaural hearing deprivation during early postnatal development was associated with decreased NR1 mRNA expression in the contralateral AC and suggested the involvement of NR1 in auditory function during development. They also indicated that, during postnatal development, environmental factors changed the functional plasticity of neurons in the AC through NR1 receptor expression. Taken together, these findings provide a possible underlying mechanism for the development of postnatal auditory function.

Glutamate receptor dynamics in dendritic microdomains

Among diverse factors regulating excitatory synaptic transmission, the abundance of postsynaptic glutamate receptors figures prominently in molecular memory and learning-related synaptic plasticity. To allow for both long-term maintenance of synaptic transmission and acute changes in synaptic strength, the relative rates of glutamate receptor insertion and removal must be tightly regulated. Interactions with scaffolding proteins control the targeting and signaling properties of glutamate receptors within the postsynaptic membrane. In addition, extrasynaptic receptor populations control the equilibrium of receptor exchange at synapses and activate distinct signaling pathways involved in plasticity. Here, we review recent findings that have shaped our current understanding of receptor mobility between synaptic and extrasynaptic compartments at glutamatergic synapses, focusing on AMPA and NMDA receptors. We also examine the cooperative relationship between intracellular trafficking and surface diffusion of glutamate receptors that underlies the expression of learning-related synaptic plasticity.

Developmental expression of NMDA and AMPA receptor subunits in vestibular nuclear neurons that encode gravity-related horizontal orientations

We examined the expression profile of subunits of ionotropic glutamate receptors [N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-proprionate (AMPA)] during postnatal development of connectivity in the rat vestibular nucleus. Vestibular nuclear neurons were functionally activated by constant velocity off-vertical axis rotation, a strategy to stimulate otolith organs in the inner ear. These neurons indicated Fos expression as a result. By immunodetection for Fos, otolith-related neurons that expressed NMDA/AMPA receptor subunits were identified as early as P7, and these neurons were found to increase progressively up to adulthood. Although there was developmental invariance in the percentage of Fos-immunoreactive neurons expressing the NR1, NR2A, GluR1, or GluR2/3 subunits, those expressing the NR2B subunit decreased from P14 onward, and those expressing the GluR4 subunit decreased in adults. These double-immunohistochemical data were corroborated by combined immuno-/hybridization histochemical data obtained from Fos-immunoreactive neurons expressing NR2B mRNA or GluR4 mRNA. The staining of both NR2B and GluR4 in the cytoplasm of these neurons decreased upon maturation. The percentage of Fos-immunoreactive neurons expressing the other ionotropic glutamate receptor subunits (viz. NR1, NR2A, GluR1, and GluR2/3) remained relatively constant throughout postnatal maturation. Triple immunofluorescence further demonstrated coexpression of NR1 and NR2 subunits in Fos-immunoreactive neurons. Coexpression of NR1 subunit with each of the GluR subunits was also observed among the Fos-immunoreactive neurons. Taken together, the different expression profiles of ionotropic glutamate receptor subunits constitute the histological basis for glutamatergic neurotransmission in the maturation of central vestibular connectivity for the coding of gravity-related horizontal head movements.

Morphometric analysis of the AMPA-type neurons in the Deiter's vestibular complex of the chick brain

Chicken (Gallus gallus) brains were used to investigate the typology and the immunolabel pattern for the subunits composing the AMPA-type glutamate receptors (GluR) of hindbrain neurons of the dorsal (dND) and ventral nuclei (vND) of the Deiter's vestibular complex (CD), which is the avian correspondent of the lateral vestibular nucleus (LVN) of mammals. Our results revealed that neurons of both divisions were poor in GluR1. The vND, the GluR2/3+ and GluR4+ label presented no area or neuronal size preference, although most neurons were around 75%. The dND neurons expressing GluR2/3 are primarily around 85%, medium to large-sized 85%, and predominantly 60% located in the medial portion of the rostral pole and in the lateral portion of the caudal pole. The majority of dND neurons containing GluR4 are also around 75%, larger (70% are large and giant), exhibiting a distribution that seems to be complementary to that of GluR2/3+ neurons. This distinct arrangement indicates functional differences into and between the DC nuclei, also signaling that such variation could be attributed to the diverse nature of the subunit composition of the GluRs. Discussion addresses the morphological and functional correlation of the avian DC with the LVN of mammals in addition to the high morphological correspondence, To include these data into the modern comparative approach we propose to adopt a similar nomenclature for the avian divisions dND and vND that could be referred as dLVN and vLVN.

Developmental regulation of the NMDA receptor subunits, NR3A and NR1, in human prefrontal cortex

Subunit composition of N-methyl-D-aspartate-type glutamate receptors (NMDARs) dictates their function, yet the ontogenic profiles of human NMDAR subunits from gestation to adulthood have not been determined. We examined NMDAR mRNA and protein development in human dorsolateral prefrontal cortex (DLPFC), an area in which NMDARs are critical for higher cognitive processing and NMDAR hypofunction is hypothesized in schizophrenia. Using quantitative reverse transcriptase-polymerase chain reaction and western blotting, we found NR1 expression begins low prenatally, peaks in adolescence, yet remains high throughout life, suggesting lifelong importance of NMDAR function. In contrast, NR3A levels are low during gestation, surge soon after birth, and decline progressively through adolescence and into adulthood. Because NR3A subunits uniquely attenuate NMDAR-mediated currents, limit calcium influx, and suppress dendritic spine formation, high levels during early childhood may be important for regulating neuroprotection and activity-dependent sculpting of synapses. We also examined whether subunit changes underlie reduced NMDAR activity in schizophrenia. Our results reveal normal NR1 and NR3A protein levels in DLPFC from schizophrenic patients, indicating that NMDAR hypofunction is unlikely to be maintained by gross changes in NR3A-containing NMDARs or overall NMDAR numbers. These data provide insights into NMDAR functions in the developing CNS and will contribute to designing pharmacotherapies for neurological disorders.

A model for synaptic development regulated by NMDA receptor subunit expression

Activation of NMDA receptors (NMDARs) is highly involved in the potentiation and depression of synaptic transmission. NMDARs comprise NR1 and NR2B subunits in the neonatal forebrain, while the expression of NR2A subunit is increased over time, leading to shortening of NMDAR-mediated synaptic currents. It has been suggested that the developmental switch in the NMDAR subunit composition regulates synaptic plasticity, but its physiological role remains unclear. In this study, we examine the effects of the NMDAR subunit switch on the spike-timing-dependent plasticity and the synaptic weight dynamics and demonstrate that the subunit switch contributes to inducing two consecutive processes-the potentiation of weak synapses and the induction of the competition between them-at an adequately rapid rate. Regulation of NMDAR subunit expression can be considered as a mechanism that promotes rapid and stable growth of immature synapses.

BRAG1, a Sec7 domain-containing protein, is a component of the postsynaptic density of excitatory synapses

The postsynaptic density (PSD) at excitatory synapses is a dynamic complex of glutamatergic receptors and associated proteins that governs synaptic structure and coordinates signal transduction. In this study, we report that BRAG1, a putative guanine nucleotide exchange factor for the Arf family of GTP-binding proteins, is a major component of the PSD. BRAG1 was identified in a 190 kDa band in the PSD fraction with the use of mass spectrometry coupled to searching of a protein sequence database. BRAG1 expression is abundant in the adult rat forebrain, and it is strongly enriched in the PSD fraction compared to forebrain homogenate and synaptosomes. Immunocytochemical localization of BRAG1 in dissociated hippocampal neurons shows that it forms discrete clusters that colocalize with the postsynaptic marker PSD-95 at sites along dendrites. BRAG1 contains a Sec7 domain, a domain that catalyzes exchange of GDP for GTP on the Arf family of small GTP-binding proteins. In their GTP-bound active state, Arfs regulate trafficking of vesicles and cytoskeletal structure. We demonstrate that the Sec7 domain of BRAG1 promotes binding of GTP to Arf in vitro. These data suggest that BRAG1 may modulate the functions of Arfs at synaptic sites.

Changes in D-serine levels and localization during postnatal development of the rat vestibular nuclei

The patterns of development of the vestibular nuclei (VN) and their main connections involving glutamate neurotransmission offer a good model for studying the function of the glial-derived neuromodulator D-serine in synaptic plasticity. In this study we show that D-serine is present in the VN and we analyzed its distribution and the levels of expression of serine racemase and D-amino acid oxidase (D-AAO) at different stages of postnatal (P) development. From birth to P21, high levels of D-serine were detected in glial cells and processes in all parts of the VN. This period corresponded to high expression of serine racemase and low expression of D-AAO. On the other hand, in the mature VN D-serine displayed very low levels and was mainly localized in neuronal cell bodies and dendrites. This drop of D-serine in adult stages corresponded to an increasing expression of D-AAO at mature stages. High levels of glial D-serine during the first 3 weeks of postnatal development correspond to an intense period of plasticity and synaptogenesis and maturation of VN afferents, suggesting that D-serine could be involved in these phenomena. These results demonstrate for the first time that changes in D-serine levels and distribution occur during postnatal development in the central nervous system. The strong decrease of D-serine levels and the glial-to-neuronal switch suggests that D-serine may have distinct functional roles depending on the developmental stage of the vestibular network.

Spatial coding capacity of central otolith neurons

This review focuses on recent approaches to unravel the capacity of otolith-related brainstem neurons for coding head orientations. In the first section, the spatiotemporal features of central vestibular neurons in response to natural otolithic stimulation are reviewed. Experiments that reveal convergent inputs from bilateral vestibular end organs bear important implications on the processing of spatiotemporal signals and integration of head orientational signals within central otolith neurons. Another section covers the maturation profile of central otolith neurons in the recognition of spatial information. Postnatal changes in the distribution pattern of neuronal subpopulations that subserve the horizontal and vertical otolith systems are highlighted. Lastly, the expression pattern of glutamate receptor subunits and neurotrophin receptors in otolith-related neurons within the vestibular nuclear complex are reviewed in relation to the potential roles of these receptors in the development of vestibular function.

Light microscopic identification and immunocytochemical characterization of glutamatergic synapses in brain sections

Presynaptic proteins are readily identified by light microscopic immunocytochemistry, but immunodetection of postsynaptic proteins in brain sections proves difficult. We performed immunofluorescent double labeling for the NR1 subunit of the N-methyl-D-aspartate receptor (NMDAR) and the vesicular glutamate transporter 1 (VGLUT1). In material fixed with 4% paraformaldehyde, NMDAR staining in somatosensory cortex was restricted to the section surface, whereas presynaptic staining extended deeper into the tissue. Staining for postsynaptic proteins was enhanced in weakly fixed material and in tissue treated with pepsin, as previously reported, but tissue quality was impaired. Staining was also markedly enhanced, and without impairment of tissue quality, by treatment during perfusion with a mixture of inhibitors of proteases and the ubiquitin/proteosome system. We performed quantitative analysis of confocal images to study how immunostaining varies with depth into the tissue. Virtually all puncta immunopositive for VGLUT1 colocalized with synaptophysin puncta; these presynaptic puncta were most numerous 1-2 microm beneath the section surface. In contrast, puncta immunopositive for the NR1 subunit were most numerous at the surface, as were puncta immunopositive for the NR2 subunit, SynGAP, and CaMKII. Punctate staining for all postsynaptic proteins, but not presynaptic markers, was substantially enhanced in material pretreated with antiproteolytic agents. The large majority of NR1-positive puncta at the surface associated with VGLUT1 in this material are likely to represent synaptic contacts. Approximately eighty-five percent of VGLUT1-positive puncta in layers II-III of SI are associated with NR1-positive puncta, and approximately 80% are associated with NR2, SynGAP, and CaMKII. This approach may permit systematic analysis of the chemistry of glutamatergic synapses with light microscopic immunocytochemistry.

Differential expression of NMDA and AMPA/KA receptor subunits in the inferior olive of postnatal rats

We have employed immunohistochemistry to determine the expression patterns of receptor subunits of N-methyl-d-aspartate (NMDA-NR1 and NR2A/B) and alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid/kainic acid (AMPA/KA-GluR1, GluR2, GluR2/3, GluR4, and GluR5/6/7) in the inferior olive of postnatal rats up to adulthood. Immunoreactivity for distinct receptor subunits was predominantly localized in the soma and dendrites of neurons. Semi-quantification showed that the overall immunoreactivity in the inferior olive of adults was intense for GluR1, moderate for NR1 and NR2A/B, and low for GluR2, GluR2/3, GluR4, and GluR5/6/7. At P7, GluR1 was restricted to the dorsomedial cell column, subnucleus beta, principal nucleus and ventrolateral protrusion while the other subunits were found in all subnuclei of the inferior olive. The immunoreactivities for all glutamate receptor subunits ranged from low to moderate. As the rats matured, the immunoreactivity of GluR4 decreased after the second postnatal week, while those of the other subunits showed a general trend of increase, reaching adult level during the third postnatal week. Double immunofluorescence revealed that all NR1-containing neurons exhibited NR2A/B immunoreactivity, indicating that native NMDA receptors comprise of hetero-oligomeric combinations of NR1 and NR2A/B. Furthermore, co-localization of NMDA and AMPA/KA receptor subunits was demonstrated in individual neurons of the inferior olive. All NR1-containing neurons exhibited GluR1 immunoreactivity, and all NR2A/B-containing neurons showed GluR5/6/7 immunoreactivity. Our data suggest that NMDA and AMPA/KA receptors are involved in glutamate-mediated neurotransmission, contributing to synaptic plasticity and reorganization of circuitry in the inferior olive during postnatal development.

Selective neuronal degeneration in Huntington's disease

Huntington's disease (HD) is a progressive neurodegenerative disorder that generally begins in middle age with abnormalities of movement, cognition, personality, and mood. Neuronal loss is most marked among the medium-sized projection neurons of the dorsal striatum. HD is an autosomal dominant genetic disorder caused by a CAG expansion in exon 1 of the HD gene, encoding an expanded polyglutamine (polyQ) tract near the N-terminus of the protein huntingtin. Despite identification of the gene mutation more than a decade ago, the normal function of this ubiquitously expressed protein is still under investigation and the mechanisms underlying selective neurodegeneration in HD remain poorly understood. Detailed postmortem analyses of brains of HD patients have provided important clues, and HD transgenic and knock-in mouse models have facilitated investigations into potential pathogenic mechanisms. Subcellular fractionation and immunolocalization studies suggest a role for huntingtin in organelle transport, protein trafficking, and regulation of energy metabolism. Consistent with this, evidence from vertebrate and invertebrate models of HD indicates that expression of the polyQ-expanded form of huntingtin results in early impairment of axonal transport and mitochondrial function. As well, alteration in activity of the N-methyl-d-aspartate (NMDA) type glutamate receptor, which has been implicated as a main mediator of excitotoxic neuronal death, especially in the striatum, is an early effect of mutant huntingtin. Proteolysis and nuclear localization of huntingtin also occur relatively early, while formation of ubiquitinated aggregates of huntingtin and transcriptional dysregulation occur as late effects of the gene mutation. Although each of these processes may contribute to neuronal loss in HD, here we review the data to support a strong role for NMDA receptor (NMDAR)-mediated excitotoxicity and mitochondrial dysfunction in conferring selective neuronal vulnerability in HD.

Rearranging receptors

The immature brain is highly susceptible to seizures. The heightened susceptibility to seizures appears to be due, at least in part, to developmental changes that skew the balance between excitatory and inhibitory neurotransmitter systems in the brain in favor of a state of excitation. Multiple factors, including changes in GABAergic and glutaminergic receptor composition, number, and distribution, all contribute to produce the characteristic limbic hyperexcitability seen during the early postnatal period. Infants and young children who experience prolonged or repetitive seizures have an increased risk of subsequently developing epilepsy. Evidence to date suggests that status epilepticus produces permanent changes in the molecular and cellular structure of limbic circuitry that, in turn, result in a long-lasting increase in hippocampal excitability and lower seizure thresholds in later life.

Role of group II metabotropic glutamate receptors 2/3 and group I metabotropic glutamate receptor 5 in developing rat medial vestibular nuclei

In brainstem slices from developing rats, metabotropic glutamate receptors mGluR2/3 and mGluR5 play different inhibitory roles in synaptic transmission and plasticity of the medial vestibular nuclei. The mGluR2/3 block (LY341495) reduces the occurrence of long-term depression after vestibular afferent high frequency stimulation at P8-P10, and increases that of long-term potentiation, while the mGluR5 block prevents high frequency stimulation long-term depression. Later on, the receptor block does not influence high frequency stimulation effects. In addition, while mGluR2/3 agonist (APDC) always provokes a transient reduction of synaptic responses, that of mGluR5 (CHPG) induces long-term depression per se at P8-P10. These results show a key role of mGluR5 in inducing high frequency stimulation long-term depression in developing medial vestibular nuclei, while mGluR2/3 modulate synaptic transmission, probably through presynaptic control of glutamate release.

Influence of visual experience on developmental shift from long-term depression to long-term potentiation in the rat medial vestibular nuclei

The influence of visual experience deprivation on changes in synaptic plasticity during postnatal development was studied in the ventral part of the rat medial vestibular nuclei (vMVN). We analysed the differences in the occurrence, expressed as a percentage, of long-term depression (LTD) and long-term potentiation (LTP) induced by high frequency stimulation (HFS) of the primary vestibular afferents in rats reared in the light (LR) and those in the dark (DR). In LR rats, HFS only induced LTD in the early stages of development, but the occurrence of LTD progressively decreased to zero before their eyes opened, while that of LTP enhanced from zero to about 50%. Once the rats' eyes had opened, LTD was no longer inducible while LTP occurrence gradually reached the normal adult value (70%). In DR rats, a similar shift from LTD to LTP was observed before their eyes opened, showing only a slightly slower LTD decay and LTP growth, and the LTD annulment was delayed by 1 day. By contrast, the time courses of LTD and LTP development in DR and LR rats showed remarkable differences following eye opening. In fact, LTD occurrence increased to about 50% in a short period of time and remained high until the adult stage. In addition, the occurrence of LTP slowly decreased to less than 20%. The effect of light-deprivation was reversible, since the exposure of DR rats to light, 5 days after eye opening, caused a sudden disappearance of LTD and a partial recover of LTP occurrence. In addition, we observed that a week of light deprivation in LR adult rats did not affect the normal adult LTP occurrence. These results provide evidence that in a critical period of development visual input plays a crucial role in shaping synaptic plasticity of the vMVN, and suggest that the visual guided shift from LTD to LTP during development may be necessary to refine and consolidate vestibular circuitry.

Molecular biology and ontogeny of glutamate receptors in the mammalian central nervous system

Glutamate is the principal excitatory neurotransmitter in the mammalian central nervous system. After release from presynaptic terminals, glutamate binds to both ionotropic and metabotropic receptors to mediate fast, slow, and persistent effects on synaptic transmission and integrity. There are three types of ionotropic glutamate receptors. N-Methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA), and kainate receptors are principally activated by the agonist bearing its name and are permeable to cationic flux; hence, their activation results in membrane depolarization. All ionotropic glutamate receptors are believed to be composed of four distinct subunits, each of which is topologically arranged with three transmembrane-spanning and one pore-lining (hairpin loop) domain. In contrast, metabotropic glutamate receptors are G protein (guanine nucleotide-binding protein) -coupled receptors linked to second-messenger systems. Group I metabotropic glutamate receptors are linked to phospholipase C, which results in phosphoinositide hydrolysis and release of calcium from intracellular stores. Group II and group III metabotropic glutamate receptors are negatively linked to adenylate cyclase, which catalyzes the production of cyclic adenosine monophosphate. Each metabotropic glutamate receptor is composed of seven transmembrane-spanning domains, similar to other members of the superfamily of metabotropic receptors, which includes noradrenergic, muscarinic acetylcholinergic, dopaminergic, serotonergic (except type 3 receptors), and gamma-aminobutyric acid (GABA) type B receptors. This review summarizes the relevant molecular biology and ontogeny of glutamate receptors in the central nervous system and highlights some of the roles that they can play during brain development and in certain disease states.

Glutamate-induced differential mitochondrial response in young and adult rats

Excitatory amino acid glutamate is involved in neurotransmission in the nervous system but it becomes a potent neurotoxin under variety of conditions. However, the molecular mechanism of excitotoxicity is not known completely. We have studied the influence of glutamate on intracellular calcium and mitochondrial functions in cortical slices from young and adult rats. The slices from both the age groups exhibited comparable intracellular calcium changes upon glutamate stimulation. Glutamate treatment caused a decrease in adenosine 5'-diphosphate/adenosine 5'-triphosphate (ADP/ATP) and an increase in nicotinamide adenine dinucleotide/nicotinamide adenine dinucleotide reduced form (NAD/NADH) ratio in both the age groups but the magnitude and the nature of temporal change was different. Glutamate-induced decrease in ATP/ADP and increase in NAD/NADH ratio was significantly higher in slices from the adult as compared to the young rats. The slices from young rats elicited slightly higher mitochondrial depolarization than adult rats. However, the formation of reactive oxygen species (ROS) and lactate dehydrogenase (LDH) release were significantly higher in adult rats as compared to young rats. Glutamate-induced mitochondrial depolarization, ROS formation and LDH release were highly dependent on the presence of Ca(2+) in the extracellular medium. The treatment of slices with mitochondrial inhibitors rotenone and oligomycin inhibited ROS formation and LDH release substantially. Our results suggest that the glutamate-induced increase in intracellular calcium is not the only factor responsible for neuronal cell death but the mitochondrial functions could be crucial in excitotoxicity.

AMPA receptor subunit expression in chick vestibular nucleus neurons

The principal cells of the chick tangential nucleus are vestibular nucleus neurons whose responses on vestibular nerve stimulation are abolished by glutamate receptor antagonists. Using confocal microscopy, we quantified immunolabeling for AMPA receptor subunits GluR1, GluR2, GluR2/3, and GluR4 in principal cells that were identified by the neuronal marker, microtubule-associated protein 2 (MAP2). This work was focused primarily on 9 days after hatching (H9) when the principal cells have acquired some important mature electrophysiologic properties. At H9, the principal cell bodies stained strongly with GluR2/3 and GluR4, whereas GluR1 and GluR2 produced weak signals. Moreover, GluR2/3 and GluR4 receptor subunit clusters in principal cell bodies and dendrites were localized at sites contacted by biocytin-labeled vestibular nerve terminals and synaptotagmin-labeled terminals. Developmental expression of AMPA receptor immunolabeling was studied in the principal cell bodies at embryonic day 16 (E16) and hatching (H1). At E16, labeling for GluR4 was already strong, and continued to increase at H1 and H9. In contrast, GluR2/3 labeling was weak at E16, but increased significantly at H1, and more so by H9. GluR1 and GluR2 were present at low levels at E16 and H1. From E16 to H9, overall AMPA receptor subunit expression increased steadily, with H9 showing the strongest labeling. Ultrastructural observations at E16 and H3 confirmed the presence of immunogold labeling for AMPA receptor subunits at the vestibular nerve and non-vestibular nerve synapses on the principal cell bodies. In summary, these results indicate that GluR3 and GluR4 are the major AMPA receptor subunits involved in excitatory synaptic transmission in principal cells during the perinatal period.

Developmental shift from long-term depression to long-term potentiation in the rat medial vestibular nuclei: role of group I metabotropic glutamate receptors

The effects of high frequency stimulation (HFS) of the primary vestibular afferents on synaptic transmission in the ventral part of the medial vestibular nuclei (vMVN) were studied during postnatal development and compared with the changes in the expression of the group I metabotropic glutamate receptor (mGluR) subtypes, mGluR1 and mGluR5. During the first stages of development, HFS always induced a mGluR5- and GABAA-dependent long-term depression (LTD) which did not require NMDA receptor and mGluR1 activation. The probability of inducing LTD decreased progressively throughout the development and it was zero at about the end of the second postnatal week. Conversely, long-term potentiation (LTP) appeared at the beginning of the second week and its occurrence increased to reach the adult value at the end of the third week. Of interest, the sudden change in the LTP frequency occurred at the time of eye opening, about the end of the second postnatal week. LTP depended on NMDA receptor and mGluR1 activation. In parallel with the modifications in synaptic plasticity, we observed that the expression patterns and localizations of mGluR5 and mGluR1 in the medial vestibular nuclei (MVN) changed during postnatal development. At the earlier stages the mGluR1 expression was minimal, then increased progressively. In contrast, mGluR5 expression was initially high, then decreased. While mGluR1 was exclusively localized in neuronal compartments and concentrated at the postsynaptic sites at all stages observed, mGluR5 was found mainly in neuronal compartments at immature stages, then preferentially in glial compartments at mature stages. These results provide the first evidence for a progressive change from LTD to LTP accompanied by a distinct maturation expression of mGluR1 and mGluR5 during the development of the MVN.

Expression of Glutamate Transporters in the Medial and Lateral Vestibular Nuclei during Rat Postnatal Development

The postnatal developmental expression and the distribution of the glutamate transporters (GLAST, GLT-1 and EAAC1) were analyzed in rat vestibular nuclei (VN), at birth and during the following 4 weeks. Analyses were performed using reverse transcriptase-polymerase chain reaction and immunoblotting of GLAST, GLT-1 and EAAC1 mRNA and protein during the postnatal development of the VN neurons and their afferent connections. We also studied the distribution of each glutamate transporter in the medial and lateral VN by use of immunocytochemistry and confocal microscopy. GLAST, GLT-1 and EAAC1 mRNA and protein were present in the VN at each developmental stage. GLAST was highly expressed mainly in glia from birth to the adult stage, its distribution pattern was heterogeneous depending on the region of the medial and lateral VN. GLT-1 expression increased dramatically during the second and third postnatal weeks. At least during the first postnatal week, GLT-1 was expressed in the soma of neurons. EAAC1 was detected in neurons and decreased from the third week. These temporal and regional patterns of GLAST, GLT-1 and EAAC1 suggest that they play different roles in the maturation of glutamatergic synaptic transmission in the medial and lateral VN during postnatal development.

Characterization of spontaneous synaptic transmission in rat medial vestibular nucleus

Spontaneous synaptic currents of medial vestibular nucleus (MVN) neurons were studied using whole cell recording in slices prepared from 13- to 17-day-old rats. The spontaneous inhibitory postsynaptic currents (sIPSCs) were significantly reduced by the GABAA antagonist bicuculline (20 microM), but were not affected by the glycine antagonist strychnine (1 microM). The frequency, amplitude, and decay time constant of sIPSCs were 4.3 +/- 0.9 Hz, 18.1 +/- 2.0 pA and 8.9 +/- 0.4 ms, respectively. The specific AMPA receptor antagonist GYKI-52466 (50 microM) completely blocked the non-NMDA-mediated spontaneous excitatory postsynaptic currents (sEPSCs), indicating that they are mediated by an AMPA-preferring receptor. The AMPA-mediated sEPSCs were characterized by low frequency (1.5 +/- 0.4 Hz), small amplitude (13.9 +/- 1.9 pA) and rapid decay kinetics (2.8 +/- 0.2 ms). The majority (15/21) displayed linear I-V relationships, suggesting the presence of GluR2-containing AMPA receptors.

Distribution of alpha-amino-3-hydroxy-5-methyl-4 isoazolepropionic acid and N-methyl-D-aspartate receptor subunits in the vestibular and spiral ganglia of the mouse during early development

We investigated the distribution of the glutamate receptor subunits, alpha-amino-3-hydroxy-5-methyl-4 isoazolepropionic acid (AMPA) GluR2 and GluR2/R3, and N-methyl-D-aspartate (NMDA) NR1, and the timing of their appearance during early development of the mouse vestibular and spiral ganglia. NMDA NR1 was the first to be expressed, in the statoacoustic ganglion neurons on E11. GluR2/R3 immunoreactivity was detected in these neurons on E12. This signal probably corresponded exclusively to GluR3, as no signal was obtained for GluR2 alone at this stage. The appearance of these proteins began much earlier than previously reported. GluR2 staining was observed later, on E14 in the vestibular neurons and on E17 in the spiral neurons. The sequence in which these three glutamate receptors appeared suggested possible differences in their roles in the establishment of neuronal circuitry in the inner ear sensory epithelia. The production of NR1 and GluR2/R3 began during the early period of neuron growth and fasciculation. GluR2 appeared later and its expression paralleled synaptogenesis in the vestibular sensory epithelia and in the organ of Corti.

Kainate-induced excitotoxicity is dependent upon extracellular potassium concentrations that regulate the activity of AMPA/KA type glutamate receptors

In addition to well-known N-methyl-d-aspartate (NMDA) receptor-mediated excitotoxicity, recent studies suggest that non-NMDA type ionotropic glutamate receptors are also important mediators of excitotoxic neuronal death, and that their functional expression can be regulated by the cellular environment. In this study, we used cerebellar granule cells (CGCs) in culture to investigate kainate (KA)-induced excitotoxicity. Although previous reports indicated that KA induces apoptosis of CGCs in culture, no KA-induced excitotoxic cell death was observed in CGCs treated with KA when cells were maintained in high potassium media (24 mm K+). In contrast, when mature CGCs were shifted into low potassium media (3 mm K+), KA produced significant excitotoxicity. In electrophysiological studies, the KA-induced inward current density was significantly elevated in CGCs shifted into low K+ media compared with those maintained in high K+ media. Non-desensitizing aspects of KA currents observed in this study suggest that these responses were mediated by AMPA rather than KA receptors. In immunofluorescence studies, the surface expression of GluR1 subunits increased when mature CGCs were shifted into a low K+ environment. This study suggests that KA-induced excitotoxicity in mature CGCs is dependent upon the extracellular potassium concentration, which modulates functional expression and excitability of AMPA/KA receptors.

High-resolution iontophoresis for single-synapse stimulation

We present a technique for stimulating post-synaptic receptors with neurotransmitter locally at a single synapse and with a concentration profile that is comparable to endogenous stimulation. We modify the technique of iontophoresis to use a 0.1 microm electrode tip for local stimulation, and we combine it with fast capacitance compensation to achieve high-speed application from a high-resistance tip. Ejection of fluorescent dye from the electrode shows that transmitter can be limited to the width of a single synapse and to a time scale similar to an endogenous event. The speed and localization of transmitter is confirmed by iontophoretically stimulating single labeled synapses in cultured hippocampal neurons held under voltage clamp. The amount of transmitter ejected is linear and reproducible over a physiologically relevant range, making this technique useful for examining receptor kinetics and receptor insertion/removal. The system should be capable of delivering any charged neurotransmitter, and we show examples using glutamate and GABA. The technique is also combined with computer-controlled manipulation to study the strength and plasticity of multiple synapses in real-time.