Component Of Excitatory Postsynaptic Currents Research Articles

Ca2+-independent but voltage dependent transmission (CiVDS) at a central synapse formed between dorsal root ganglion and dorsal horn neurons.

A central principle of synaptic transmission is that action potential-induced presynaptic neurotransmitter release occurs exclusively via Ca-dependent secretion (CDS). The discovery and mechanistic investigations of Ca-independent but voltage-dependent secretion (CiVDS) have demonstrated that the action potential per se is sufficient to trigger neurotransmission in the somata of primary sensory and sympathetic neurons in mammals. One key question remains, however, whether CiVDS contributes to central synaptic transmission. Here, we report, in the central transmission from presynaptic (dorsal root ganglion) to postsynaptic (spinal dorsal horn) neurons in vitro, (i) excitatory postsynaptic currents (EPSCs) are mediated by glutamate transmission through both CiVDS (up to 87%) and CDS; (ii) CiVDS-mediated EPSCs are independent of extracellular and intracellular Ca 2+; (iii) CiVDS is faster than CDS in vesicle recycling with much less short-term depression; (iv) the fusion machinery of CiVDS includes Cav2.2 (voltage sensor) and SNARE (fusion pore). Together, an essential component of activity-induced EPSCs is mediated by CiVDS in a central synapse.

Ca2+ -independent transmission at the central synapse formed between dorsal root ganglion and dorsal horn neurons.

A central principle of synaptic transmission is that action potential-induced presynaptic neurotransmitter release occurs exclusively via Ca2+ -dependent secretion (CDS). The discovery and mechanistic investigations of Ca2+ -independent but voltage-dependent secretion (CiVDS) have demonstrated that the action potential per se is sufficient to trigger neurotransmission in the somata of primary sensory and sympathetic neurons in mammals. One key question remains, however, whether CiVDS contributes to central synaptic transmission. Here, we report, in the central transmission from presynaptic (dorsal root ganglion) to postsynaptic (spinal dorsal horn) neurons in vitro, (i) excitatory postsynaptic currents (EPSCs) are mediated by glutamate transmission through both CiVDS (up to 87%) and CDS; (ii) CiVDS-mediated EPSCs are independent of extracellular and intracellular Ca2+ ; (iii) CiVDS is faster than CDS in vesicle recycling with much less short-term depression; (iv) the fusion machinery of CiVDS includes Cav2.2 (voltage sensor) and SNARE (fusion pore). Together, an essential component of activity-induced EPSCs is mediated by CiVDS in a central synapse.

Subsaturation of the N-methyl-D-aspartate receptor glycine site allows the regulation of bursting activity in juvenile rat nigral dopamine neurons.

The activation of N-methyl-D-aspartate receptors (NMDARs) in substantia nigra pars compacta (SNc) dopamine (DA) cells is central to generate the bursting activity, a phasic signal linked to DA-related behaviours via the change in postsynaptic DA release. NMDARs are recruited during excitatory synaptic transmission by glutamate release, but the glycine site level of occupancy of these receptors during basal action potential-dependent activity is not known for SNc DA neurons. We explored NMDAR-dependent signals during exogenous applications of co-agonists in midbrain slices from juvenile rats. We found that both glycine and D-serine strengthened the NMDAR-dependent component of excitatory postsynaptic currents (EPSCs) in a concentration-dependent manner. EPSCs were also increased by endogenous glycine via the blockade of the glycine transport. The glycine site of NMDARs contributing to synaptic transmission is therefore subsaturated. The behaviourally relevant burst firing was more sensitive to exogenous D-serine and endogenous glycine than to exogenous glycine. The mechanisms regulating the availability of the co-agonists exert consequently a critical influence on the excitability of DA neurons via NMDARs. The modulation of the phasic firing in DA neurons by ambient NMDAR co-agonists may be important for nigral information processing and downstream motor-related behaviour.

Persistent activation of histamine H1 receptors in the hippocampal CA1 region enhances NMDA receptor-mediated synaptic excitation and long-term potentiation in astrocyte- and D-serine-dependent manner

Behavioral studies using pharmacological tools have implicated histamine H1 receptors in cognitive function via their interactions with N-methyl-D-aspartate receptors (NMDARs) in the hippocampus. However, little is known about the neurophysiological mechanism that underlies the interaction between H1 receptors and NMDARs. To explore how H1 receptor activation affects hippocampal excitatory neurotransmission and synaptic plasticity, this study aimed to examine the effect of H1 receptor ligands on both NMDAR-mediated synaptic currents and long-term potentiation (LTP) at synapses between Schaffer collaterals and CA1 pyramidal neurons using acute mouse hippocampal slices. We found that the H1 receptor antagonist/inverse agonists, pyrilamine (0.1 μM) and cetirizine (10 μM), decreased the NMDAR-mediated component of stimulation-induced excitatory postsynaptic currents (EPSCs) recorded from CA1 pyramidal neurons without affecting the AMPA receptor-mediated component of EPSCs and its paired pulse ratio. Pretreatment of slices with either the glial metabolism inhibitor, fluoroacetate (5 mM), or D-serine (100 μM) diminished the pyrilamine- or cetirizine-induced attenuation of the NMDAR-mediated EPSCs. Furthermore, the LTP of field excitatory postsynaptic potentials induced following high frequency stimulation of Schaffer collaterals was attenuated with application of pyrilamine or cetirizine. Pretreatment with D-serine again attenuated the pyrilamine-induced suppression of LTP. Our data suggest that H1 receptors in the CA1 can undergo persistent activation induced by their constitutive receptor activity and/or tonic release of endogenous histamine, resulting in facilitation of the NMDAR activity in a manner dependent of astrocytes and the release of D-serine. This led to the enhancement of NMDA-component EPSC and LTP at the Schaffer collateral-CA1 pyramidal neuron synapses.

Kinesin Family of Proteins Kif11 and Kif21B Act as Inhibitory Constraints of Excitatory Synaptic Transmission Through Distinct Mechanisms

Despite our understanding of the functions of the kinesin family of motor proteins (Kifs) in neurons, their specific roles in neuronal communication are less understood. To address this, by carrying out RNAi-mediated loss of function studies, we assessed the necessity of 18 Kifs in excitatory synaptic transmission in mouse primary hippocampal neurons prepared from both sexes. Our measurements of excitatory post-synaptic currents (EPSCs) have identified 7 Kifs that were found to be not critical and 11 Kifs that are essential for synaptic transmission by impacting either frequency or amplitude or both components of EPSCs. Intriguingly we found that knockdown of mitotic Kif4A and Kif11 and post-mitotic Kif21B resulted in an increase in EPSCs suggesting that they function as inhibitory constraints on synaptic transmission. Furthermore, Kifs (11, 21B, 13B) with distinct effects on synaptic transmission are expressed in the same hippocampal neuron. Mechanistically, unlike Kif21B, Kif11 requires the activity of pre-synaptic NMDARs. In addition, we find that Kif11 knockdown enhanced dendritic arborization, synapse number, expression of synaptic vesicle proteins synaptophysin and active zone protein Piccolo. Moreover, expression of Piccolo constrained Kif11 function in synaptic transmission. Together these results suggest that neurons are able to utilize specific Kifs as tools for calibrating synaptic function. These studies bring novel insights into the biology of Kifs and functioning of neural circuits.

Different AMPA receptor subtypes mediate the distinct kinetic components of a biphasic EPSC in hippocampal interneurons.

CA1 hippocampal interneurons at the border between stratum radiatum (SR) and stratum lacunosum-moleculare (SLM) have AMPA receptor (AMPAR)-mediated excitatory postsynaptic currents (EPSCs) that consist of two distinct phases: a typical fast component (FC), and a highly unusual slow component (SC) that persists for hundreds of milliseconds. To determine whether these kinetically distinct components of the EPSC are mediated by distinct AMPAR subpopulations, we examined the relative contributions of GluA2-containing and—lacking AMPARs to the SC. GluA2-containing AMPARs mediated the majority of the FC whereas GluA2-lacking AMPARs preferentially generated the SC. When glutamate uptake through the glial glutamate transporter excitatory amino acid transporter (EAAT1) was inhibited, spill over-mediated AMPAR activation recruited an even slower third kinetic component that persisted for several seconds; however, this spillover-mediated current was mediated predominantly by GluA2-containing AMPARs and therefore was clearly distinct from the SC when uptake is intact. Thus, different AMPAR subpopulations that vary in GluA2 content mediate the distinct components of the AMPAR EPSC. The SC is developmentally downregulated in mice, declining after the second postnatal week. This downregulation affects both GluA2-containing and GluA2-lacking AMPARs mediating the SC, and is not accompanied by developmental changes in the GluA2 content of AMPARs underlying the FC. Thus, the downregulation of the SC appears to be independent of synaptic GluA2 expression, suggesting the involvement of another AMPAR subunit or an auxiliary protein. Our results therefore identify GluA2-dependent and GluA2-independent determinants of the SC: GluA2-lacking AMPARs preferentially contribute to the SC, while the developmental downregulation of the SC is independent of GluA2 content.

Glutamate input to noradrenergic neurons plays an essential role in the development of morphine dependence and psychomotor sensitization

The brain's noradrenergic system is involved in the development of behaviours induced by drugs of abuse, e.g. dependence and withdrawal, and also reward or psychomotor effects. To investigate how noradrenergic system activity is controlled in the context associated with drug-induced behaviours, we generated a Cre/loxP mouse model in which the essential glutamate NMDA receptor subunit NR1 is ablated in cells expressing dopamine β-hydroxylase (Dbh). As a result, the noradrenergic cells in NR1DbhCre mice lack the NMDA receptor-dependent component of excitatory post-synaptic currents. The mutant mice displayed no obvious behavioural alterations, had unchanged noradrenaline content and mild increase in dopamine levels in the nucleus accumbens. Interestingly, NR1DbhCre animals did not develop morphine-induced psychomotor sensitization. However, when the morphine injections were preceded by treatment with RX821002, an antagonist of α2-adrenergic receptors, the development of sensitization was restored. Conversely, pretreatment with clonidine, an agonist of α2-adrenergic receptors, blocked development of sensitization in wild-type mice. We also found that while the development of tolerance to morphine was normal in mutant mice, withdrawal symptoms were attenuated. These data reveal that NMDA receptors on noradrenergic neurons regulate development of opiate dependence and psychomotor sensitization, by controlling drug-induced noradrenaline signalling.

Altered function of glutamatergic cortico-striatal synapses causes output pathway abnormalities in a chronic model of parkinsonism

Objective In Parkinson's disease, chronic striatal dopamine depletion results in over-activity and under-activity of the indirect and direct striatal output pathways respectively. In this study, we investigated changes in the function of glutamatergic cortico-striatal synapses that contribute to abnormalities in striatal efferents. Methods Whole-cell recordings were performed in striatal slices prepared from adult bacterial artificial chromosome mice, chronically lesioned with 6-hydroxydopamine (6-OHDA). Paired pulse facilitation, spontaneous synaptic activity, the ratio of AMPAR to NMDAR-mediated components of excitatory postsynaptic currents, AMPAR and NMDAR kinetics, current–voltage relationship and intrinsic membrane properties were assessed in indirect and direct pathway medium spiny neurons (MSNs), which were identified on the basis of expression of GFP, driven by the promoters of A2A or D1 receptor expression. The trajectory of striatal efferents, with respect to selective targeting of the globus pallidus and substantia nigra was also compared in sham-operated versus 6-OHDA-lesioned mice. Results Dopamine depletion did not affect the number of pathway specific output neurons or the trajectory of striatal outputs. In sham-operated animals, cortico-striatal synapses of both striatal efferent populations exhibited paired pulse facilitation and similar ratios of AMPAR to NMDAR-mediated components of excitatory postsynaptic currents. Following striatal dopamine depletion, indirect pathway neurons exhibited decreased levels of paired pulse facilitation, enhanced sensitivity to presynaptic stimulation and an increase in the relative contribution of NMDAR to the EPSC but no change in spontaneous synaptic activity. In sham-operated mice, neurons of the direct pathway exhibited lower firing frequency compared to the indirect pathway following current injection. However, in 6-OHDA-lesioned mice, in the direct pathway, firing threshold was reduced, spike frequency adaptation developed and the frequency of spontaneous activity was also reduced. In addition, changes in the properties of NMDAR kinetics suggest that these receptors were desensitised. Discussion Increased synchronicity between pre and postsynaptic neurons, as indicated by decreased paired pulse facilitation, and increased sensitivity to extracellular stimulation, combined with an increase in the contribution of NMDARs to the EPSC at cortico-striatal synapses, may contribute to the over-activity of indirect pathway neurons in the parkinsonian striatum. In contrast, a decrease in spontaneous activity, postsynaptic desensitisation to excitatory stimuli and spike frequency adaptation of cortico-striatal synapses may underlie under-activity of the direct pathway.

Lamotrigine inhibits postsynaptic AMPA receptor and glutamate release in the dentate gyrus

The dentate gyrus (DG) is a gateway that regulates seizure activity in the hippocampus. We investigated the site of action of lamotrigine (LTG), an effective anticonvulsant, in the regulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and N-methyl-D-aspartic acid (NMDA) receptor-mediated excitatory synaptic transmission on DG. Evoked AMPA and NMDA receptor-mediated excitatory postsynaptic currents (eEPSCampa and eEPSCnmda) were recorded by whole-cell patch-clamp recording from the granule cells of DG in brain slice preparation of young Wistar rats (60-120 g). Exogenously applied AMPA and NMDA-induced currents and AMPA receptor-mediated miniature EPSC (mEPSCampa) were recorded in the presence of specific antagonists. LTG inhibited both eEPSCampa and eEPSCnmda, and had no effect on exogenously applied NMDA-induced current indicating LTG inhibited glutamate release. Previous studies demonstrated that alteration in glutamate concentration in synaptic cleft causes parallel changes of eEPSCampa and eEPSCnmda. Our results showed that LTG inhibited eEPSCampa significantly more than eEPSCnmda (p < 0.05), suggesting that LTG may also have blocked the postsynaptic AMPA receptor. The hypothesis is further supported by the facts that; (1) LTG (30-100 microM) inhibited direct exogenously applied AMPA-induced currents (to 90%), (2) LTG significantly reduced the amplitude, but not the frequency of mEPSCampa and asynchronous (EPSC), and (3) LTG-induced reduction of eEPSCampa was not associated with a modification of the paired-pulse ratio. To sum up, LTG exerts a postsynaptic inhibitory mechanism on the AMPA receptor. Our results demonstrate that LTG suppresses postsynaptic AMPA receptors and reduces glutamate release in granule cells of DG. The postsynaptic effect can be one of the underlying mechanisms of LTG's anticonvulsant action.

Subthalamic stimulation evokes complex EPSCs in the rat substantia nigra pars reticulata in vitro

The subthalamic nucleus (STN) plays an important role in movement control by exerting its excitatory influence on the substantia nigra pars reticulata (SNR), a major output structure of the basal ganglia. Moreover, excessive burst firing of SNR neurons seen in Parkinson's disease has been attributed to excessive transmission in the subthalamonigral pathway. Using the 'blind' whole-cell patch clamp recording technique in rat brain slices, we found that focal electrical stimulation of the STN evoked complex, long-duration excitatory postsynaptic currents (EPSCs) in SNR neurons. Complex EPSCs lasted 200-500 ms and consisted of an initial monosynaptic EPSC followed by a series of late EPSCs superimposed on a slow inward shift in holding current. Focal stimulation of regions outside the STN failed to evoke complex EPSCs. The late component of complex EPSCs was markedly reduced by ionotropic glutamate receptor antagonists (2-amino-5-phosphonopentanoic acid and 6-cyano-7-nitro-quinoxalone) and by a GABAA receptor agonist (isoguvacine) when these agents were applied directly to the STN using a fast-flow microapplicator. Moreover, the complex EPSC was greatly enhanced by bath application of the GABAA receptor antagonists picrotoxin or bicuculline. These data suggest that recurrent glutamate synapses in the STN generate polysynaptic, complex EPSCs that are under tonic inhibition by GABA. Because complex EPSCs are expected to generate bursts of action potentials in SNR neurons, we suggest that complex EPSCs may contribute to the pathological burst firing that is associated with the symptoms of Parkinson's disease.

Differential Regulation of AMPA Receptor Subunit Trafficking by Palmitoylation of Two Distinct Sites

Modification of AMPA receptor function is a major mechanism for the regulation of synaptic transmission and underlies several forms of synaptic plasticity. Post-translational palmitoylation is a reversible modification that regulates localization of many proteins. Here, we report that palmitoylation of the AMPA receptor regulates receptor trafficking. All AMPA receptor subunits are palmitoylated on two cysteine residues in their transmembrane domain (TMD) 2 and in their C-terminal region. Palmitoylation on TMD 2 is upregulated by the palmitoyl acyl transferase GODZ and leads to an accumulation of the receptor in the Golgi and a reduction of receptor surface expression. C-terminal palmitoylation decreases interaction of the AMPA receptor with the 4.1N protein and regulates AMPA- and NMDA-induced AMPA receptor internalization. Moreover, depalmitoylation of the receptor is regulated by activation of glutamate receptors. These data suggest that regulated palmitoylation of AMPA receptor subunits modulates receptor trafficking and may be important for synaptic plasticity.

Selective muscarinic regulation of functional glutamatergic Schaffer collateral synapses in rat CA1 pyramidal neurons.

Analysis of the cholinergic regulation of glutamatergic neurotransmission is an essential step in understanding the hippocampus because it can influence forms of synaptic plasticity that are thought to underlie learning and memory. We studied in vitro the cholinergic regulation of excitatory postsynaptic currents (EPSCs) evoked in rat CA1 pyramidal neurons by Schaffer collateral (SC) stimulation. Using "minimal" stimulation, which activates one or very few synapses, the cholinergic agonist carbamylcholine (CCh) increased the failure rate of functional more (36 %) than of silent synapses (7 %), without changes in the EPSC amplitude. These effects of CCh were insensitive to manipulations that increased the probability of release, such as paired pulse facilitation, increases in temperature and increases in the extracellular Ca(2+) : Mg(2+) ratio. Using "conventional" stimulation, which activates a large number of synapses, CCh inhibited more the pharmacologically isolated non-NMDA (86 %) than the NMDA (47 %) EPSC. The changes in failure rate, EPSC variance and the increased paired pulse facilitation that paralleled the inhibition imply that CCh decreased release probability. Muscarine had similar effects. The inhibition by both CCh and by muscarine was prevented by atropine. We conclude that CCh reduces the non-NMDA component of SC EPSCs by selectively inhibiting transmitter release at functional synapses via activation of muscarinic receptors. The results suggest that SCs have two types of terminals, one in functional synapses, selectively sensitive to regulation through activation of muscarinic receptors, and the other in silent synapses less sensitive to that regulation. The specific inhibition of functional synapses would favour activity-dependent plastic phenomena through NMDA receptors at silent synapses without the activation of non-NMDA receptors and functional synapses.

Postsynaptic induction and presynaptic expression of group 1 mGluR-dependent LTD in the hippocampal CA1 region.

Activation of metabotropic glutamate receptors (mGluRs) with the group I mGluR selective agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) induces a long-term depression (LTD) of excitatory synaptic transmission in the CA1 region of the hippocampus. Here we investigated the potential roles of pre- and postsynaptic processes in the DHPG-induced LTD at excitatory synapses onto hippocampal pyramidal cells in the mouse hippocampus. Activation of mGluRs with DHPG, but not ACPD, induced LTD at both Schaffer collateral/commissural fiber synapses onto CA1 pyramidal cells and at associational/commissural fiber synapses onto CA3 pyramidal cells. DHPG-induced LTD was blocked when the G-protein inhibitor guanosine-5'-O-(2-thiodiphosphate) was selectively delivered into postsynaptic CA1 pyramidal cells via an intracellular recording electrode, suggesting that DHPG depresses synaptic transmission through a postsynaptic, GTP-dependent signaling pathway. The effects of DHPG were also strongly modulated, however, by experimental manipulations that altered presynaptic calcium influx. In these experiments, we found that elevating extracellular Ca(2+) concentrations ([Ca(2+)](o)) to 6 mM almost completely blocked the effects of DHPG, whereas lowering [Ca(2+)](o) to 1 mM significantly enhanced the ability of DHPG to depress synaptic transmission. Enhancing Ca(2+) influx by prolonging action potential duration with bath applications of the K(+) channel blocker 4-aminopyridine (4-AP) also strongly reduced the effects of DHPG in the presence of normal [Ca(2+)](o) (2 mM). Although these findings indicate that alterations in Ca(2+)-dependent signaling processes strongly regulate the effects of DHPG on synaptic transmission, they do not distinguish between potential pre- versus postsynaptic sites of action. We found, however, that while inhibiting both pre- and postsynaptic K(+) channels with bath-applied 4-AP blocked the effects of DHPG; inhibition of postsynaptic K(+) channels alone with intracellular Cs(+) and TEA had no effect on the ability of DHPG to inhibit synaptic transmission. This suggests that presynaptic changes in transmitter release contribute to the depression of synaptic transmission by DHPG. Consistent with this, DHPG induced a persistent depression of both AMPA and N-methyl-D-aspartate receptor-mediated components of excitatory postsynaptic currents in voltage-clamped pyramidal cells. Together our results suggest that activation of postsynaptic mGluRs suppresses transmission at excitatory synapses onto CA1 pyramidal cells through presynaptic effects on transmitter release.

Properties of Excitatory Synaptic Connections Mediated by the Corpus Callosum in the Developing Rat Neocortex

Despite the major role of excitatory cortico-cortical connections in mediating neocortical activities, little is known about these synapses at the cellular level. Here we have characterized the synaptic properties of long-range excitatory-to-excitatory contacts between visually identified layer V pyramidal neurons of agranular frontal cortex in callosally connected neocortical slices from postnatal day 13 to 21 (P13-21) rats. Midline stimulation of the corpus callosum with a minimal stimulation paradigm evoked inward excitatory postsynaptic currents (EPSCs) with an averaged peak amplitude of 56.5 +/- 5 pA under conditions of whole cell voltage clamp at -70 mV. EPSCs had fixed latencies from stimulus onset and could follow stimulus trains (1-20 Hz) without changes in kinetic properties. Bath application of 2,3-dihydro-6-nitro-7-sulfamoyl-benzo(F)quinoxaline (NBQX) abolished these responses completely, indicating that they were mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors (AMPARs). Evoked responses were isolated in picrotoxin to yield purely excitatory PSCs, and a low concentration of NBQX (0.1 microM) was used to partially block AMPARs and prevent epileptiform activity in the tissue. Depolarization of the recorded pyramidal neurons revealed a late, slowly decaying component that reversed at approximately 0 mV and was blocked by D-2-amino-5-phosphonovaleric acid. Thus AMPA and N-methyl-D-aspartate receptors (NMDARs) coexist at callosal synapses and are likely to be activated monosynaptically. The peak amplitudes and decay time constants for EPSCs evoked using minimal stimulation (+/-40 mV) were similar to spontaneously occurring sEPSCs. Typical conductances associated with AMPA and NMDAR-mediated components, deduced from their respective current-voltage (I-V) relationships, were 525 +/- 168 and 966 +/- 281 pS, respectively. AMPAR-mediated responses showed age-dependent changes in the rectification properties of their I-V relationships. While I-Vs from animals >P15 were linear, those in the younger (<P16) age group were inwardly rectifying. Although Ca2+ permeability in AMPARs can be correlated with inward rectification, outside-out somatic patches from younger animals were characterized by Ca2+-impermeable receptors, suggesting that somatic receptors might be functionally different from those located at synapses. While the biophysical properties of AMPAR components of callosally-evoked EPSCs were similar to those evoked by stimulation of local excitatory connections, the NMDA component displayed input-specific differences. NMDAR-mediated responses for local inputs were activated at more hyperpolarized holding potentials in contrast with those evoked by callosal stimulation. Paired stimuli used to assay presynaptic release properties showed paired-pulse depression (PPD) in animals <P16, which converted to facilitation (PPF) in older animals, suggesting a developmental transition from low probability of transmitter release to high P(r) at these synapses and/or alterations in the properties of the underlying postsynaptic receptors. Physiologic properties of neocortical e-e connections are thus input specific and subject to developmental changes in their postsynaptic receptors.

Functionally different AMPA-type glutamate receptors in morphologically identified neurons in rat superficial superior colliculus

In the superficial superior colliculus, a center of sensory processing related to visual salience, glutamate is used as a major excitatory neurotransmitter. α-Amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type glutamate receptors include a Ca 2+-impermeable, outwardly rectifying type (type I) and a Ca 2+-permeable, inwardly rectifying type (type II). To study the contribution of these AMPA receptor subtypes to visual sensory processing in the superior colliculus, we investigated the expression of these two types of AMPA receptors in six morphologically identified subgroups of neurons in the superficial superior colliculus by whole-cell recording using slice preparations of young (17–23 days old) and adult (60–68 days old) rats. Both outwardly and inwardly rectifying current responses were observed to pressure applied 10 mM kainate, a non-desensitizing AMPA receptor agonist. These currents were completely abolished by the selective AMPA receptor antagonist 1-(4-aminophenyl)-3-methylcarbamyl-4-methyl-7,8-methylenedioxy-3,4-dihydro-5 H-2,3-benzodiazepine (100 μM). The type II receptor antagonist spermine (1 mM) suppressed inwardly rectifying responses. The degree of inward rectification was correlated with the ratio of suppression by spermine, and inversely correlated with estimated Ca 2+ permeability, indicating that the degree of rectification reflects the relative amount of co-expressed type I and type II receptors. An inwardly rectifying and spermine-sensitive AMPA component of excitatory postsynaptic currents was observed, suggesting involvement of type II receptors in synaptic transmission. Morphological analysis revealed that a substantial population of horizontal cells in both young and adult rats ( n=31/53 and 15/17, respectively) and all wide field multipolar cells in adult rats ( n=6) showed inwardly rectifying AMPA receptor responses. From these results we suggest that type I and type II AMPA receptors are co-expressed with varying ratios in individual neurons in the rat superficial superior colliculus, and that type II receptors are abundantly expressed in most horizontal cells and wide field multipolar cells. Since these neurons are putatively GABAergic inhibitory neurons and have wide dendritic trees, type II receptors may contribute to the regulation of remote inhibitory interaction in the visual field map in the the superficial superior colliculus.

Dopamine D1/D5 receptor modulation of excitatory synaptic inputs to layer V prefrontal cortex neurons

Dopamine acts mainly through the D1/D5 receptor in the prefrontal cortex (PFC) to modulate neural activity and behaviors associated with working memory. To understand the mechanism of this effect, we examined the modulation of excitatory synaptic inputs onto layer V PFC pyramidal neurons by D1/D5 receptor stimulation. D1/D5 agonists increased the size of N-methyl-d-aspartate (NMDA) component of excitatory postsynaptic currents (EPSCs) through a postsynaptic mechanism. In contrast, D1/D5 agonists caused a slight reduction in the size of the non-NMDA component of EPSCs through a small decrease in release probability. With 20 Hz synaptic trains, we found that the D1/D5 agonists increased depolarization of summating the NMDA component of excitatory postsynaptic potential (EPSP). By increasing the NMDA component of EPSCs, yet slightly reducing release, D1/D5 receptor activation selectively enhanced sustained synaptic inputs and equalized the sizes of EPSPs in a 20-Hz train.

Dopamine D1/D5 receptor modulation of excitatory synaptic inputs to layer V prefrontal cortex neurons.

Dopamine acts mainly through the D1/D5 receptor in the prefrontal cortex (PFC) to modulate neural activity and behaviors associated with working memory. To understand the mechanism of this effect, we examined the modulation of excitatory synaptic inputs onto layer V PFC pyramidal neurons by D1/D5 receptor stimulation. D1/D5 agonists increased the size of N-methyl-d-aspartate (NMDA) component of excitatory postsynaptic currents (EPSCs) through a postsynaptic mechanism. In contrast, D1/D5 agonists caused a slight reduction in the size of the non-NMDA component of EPSCs through a small decrease in release probability. With 20 Hz synaptic trains, we found that the D1/D5 agonists increased depolarization of summating the NMDA component of excitatory postsynaptic potential (EPSP). By increasing the NMDA component of EPSCs, yet slightly reducing release, D1/D5 receptor activation selectively enhanced sustained synaptic inputs and equalized the sizes of EPSPs in a 20-Hz train.

A presynaptic mechanism contributes to depression of autonomic signal transmission in NTS.

With increasing frequencies of autonomic afferent input to the nucleus tractus solitarii (NTS), postsynaptic responses are depressed. To test the hypothesis that a presynaptic mechanism contributes to this frequency-dependent depression, we used whole cell, voltage-clamp recordings in an NTS slice. First, we determined whether solitary tract stimulation (0.4-24 Hz) resulted in frequency-dependent depression of excitatory postsynaptic currents (EPSCs) in second-order neurons. Second, because decreases in presynaptic glutamate release result in a parallel depression of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and N-methyl-D-aspartic acid (NMDA) receptor-mediated components of EPSCs, we determined whether the magnitude, time course, and recovery from the depression were the same in both EPSC components. Third, to determine whether AMPA receptor desensitization contributed, we examined the depression during cyclothiazide. EPSCs decreased in a frequency-dependent manner by up to 76% in second- and 92% in higher-order neurons. AMPA and NMDA EPSC components were depressed with the same magnitude (by 83% and 83%) and time constant (113 and 103 ms). The time constant for the recovery was also not different (1.2 and 0.8 s). Cyclothiazide did not affect synaptic depression at >/=3 Hz. The data suggest that presynaptic mechanism(s) at the first NTS synapse mediate frequency-dependent synaptic depression.

The AMPA receptor interacts with and signals through the protein tyrosine kinase Lyn.

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system. The ionotropic glutamate receptors are classified into two groups, NMDA (N-methyl-D-aspartate) receptors and AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate) receptors. The AMPA receptor is a ligand-gated cation channel that mediates the fast component of excitatory postsynaptic currents in the central nervous system. Here we report that AMPA receptors function not only as ion channels but also as cell-surface signal transducers by means of their interaction with the Src-family non-receptor protein tyrosine kinase Lyn. In the cerebellum, Lyn is physically associated with the AMPA receptor and is rapidly activated following stimulation of the receptor. Activation of Lyn is independent of Ca2+ and Na+ influx through AMPA receptors. As a result of activation of Lyn, the mitogen-activated protein kinase (MAPK) signalling pathway is activated, and the expression of brain-derived neurotrophic factor (BDNF) messenger RNA is increased in a Lyn-kinase-dependent manner. Thus, AMPA receptors generate intracellular signals from the cell surface to the nucleus through the Lyn-MAPK pathway, which may contribute to synaptic plasticity by regulating the expression of BDNF.

Variability of AMPA and NMDA receptor mediated responses in CA1 pyramidal cells of young rats

The relative variability of excitatory postsynaptic currents (EPSCs) was studied using whole cell recording in CA1 pyramidal cells of hippocampal slices from 2 to 3-week-old rats. EPSCs were evoked by stimulating the Schaffer collateral–commissural pathway and recorded at holding potentials of −75, −30 or +40 mV. The recordings were either isolated α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or N-methyl- d-aspartate (NMDA) receptor mediated EPSCs, or composite ones. EPSC variability was quantified by coefficient of variation (CV). The inverse variability expressed as 1/CV 2 was employed in comparisons. Using early (5–15 ms) and late (40–100 ms) measurements to estimate the AMPA and NMDA components of composite EPSCs showed no difference in the variability of the two components. Comparing isolated AMPA EPSCs at −75 mV with NMDA EPSCs at −30 mV also failed to reveal a difference. However, in accord with previous studies by others, NMDA EPSCs recorded at +40 mV were less variable than AMPA EPSCs at −75 mV, the ratio of 1/CV 2 for NMDA vs. AMPA being around 1.7. A comparison between isolated AMPA EPSCs revealed a similar pattern of dependency of CV on membrane potential, the EPSCs at +40 mV being less variable than those at −30 or −75 mV (1/CV 2 ratios of 1.5–1.6). In conclusion, our results did not demonstrate any inherent difference in CV between AMPA and NMDA receptor mediated EPSCs and the observed differences in CV could be accounted for by a dependency on membrane potential, the mechanism of which remains to be resolved. The present results have implications for the interpretation of CV changes as observed, for instance, during synaptic plasticity.