Excitatory synaptic potentials dependent on metabotropic glutamate receptor activation in guinea-pig hippocampal pyramidal cells

Persistent receptor activity underlies group I mGluR-mediated cellular plasticity in CA3 neuron

Plastic changes in cortical activities induced by group I metabotropic glutamate receptor (mGluR) stimulation include epileptogenesis, expressed in vitro as the conversion of normal neuronal activity to persistent, prolonged synchronized (ictal) discharges. At present, the mechanism that maintains group I mGluR-induced plasticity is not known. We examined this issue using hippocampal slices from guinea pigs and mice. Agonist [(S)-3,5-dihydroxyphenylglycine (DHPG), 30-50 μm)] stimulation of group I mGluRs induces persistent prolonged synchronized (ictal-like) discharges in CA3 that are associated with three identified excitatory cellular responses-suppression of spike afterhyperpolarizations, activation of a voltage-dependent cationic current, and increase in neuronal input resistance. Persistent prolonged synchronized discharges and the underlying excitatory cellular responses maintained following induction were reversibly blocked by mGluR1 antagonists [(S)-+-α-amino-4-carboxy-2-methylbenzeneacetic acid (LY 367385), 50, 100 μm; CPCCOEt (hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester, 100 μm], and to a lesser extent by the mGluR5 antagonist MPEP [2-methyl-6-(phenylethynyl)pyridine hydrochloride, 50 μm]. Activation of persistent cellular responses to DHPG were unaffected by tetrodotoxin (0.5-1 μm) or perfusion with low Ca(2+)(0.2 mm)-Mn(2+)(0.5 mm) media-conditions that suppress endogenous glutamate release. The pharmacological profile of the blocking action of the group I mGluR antagonist MCPG [(RS)-α-methyl-4-carboxyphenylglycine, 50-500 μm] on persistent cellular responses was different from that on cellular responses directly activated by DHPG. These data indicate that transient stimulation of group I mGluRs alters receptor properties, rendering them persistently active in the absence of applied agonist or endogenous glutamate activation. Persistent receptor activities, primarily involving mGluR1, maintain excitatory cellular responses and emergent prolonged synchronized discharges.

Cellular plasticity for group I mGluR-mediated epileptogenesis

Stimulation of group I metabotropic glutamate receptors (mGluRs) by the agonist (S)-dihydroxyphenylglycine in the hippocampus transforms normal neuronal activity into prolonged epileptiform discharges. The conversion is long lasting in that epileptiform discharges persist after washout of the inducing agonist and serves as a model of epileptogenesis. The group I mGluR model of epileptogenesis took on special significance because epilepsy associated with fragile X syndrome (FXS) may be caused by excessive group I mGluR signaling. At present, the plasticity mechanism underlying the group I mGluR-mediated epileptogenesis is unknown. I(mGluR(V)), a voltage-gated cationic current activated by group I mGluR agonists in CA3 pyramidal cells in the hippocampus, is a possible candidate. I(mGluR(V)) activation is associated with group I mGluR agonist-elicited epileptiform discharges. For I(mGluR(V)) to play a role in epileptogenesis, long-term activation of the current must occur after group I mGluR agonist exposure or synaptic stimulation. We observed that I(mGluR(V)), once induced by group I mGluR agonist stimulation in CA3 pyramidal cells, remained undiminished for hours after agonist washout. In slices prepared from FXS model mice, repeated stimulation of recurrent CA3 pyramidal cell synapses, effective in eliciting mGluR-mediated epileptiform discharges, also induced long-lasting I(mGluR(V)) in CA3 pyramidal cells. Similar to group I mGluR-mediated prolonged epileptiform discharges, persistent I(mGluR(V)) was no longer observed in preparations pretreated with inhibitors of tyrosine kinase, of extracellular signal-regulated kinase 1/2, or of mRNA protein synthesis. The results indicate that I(mGluR(V)) is an intrinsic plasticity mechanism associated with group I mGluR-mediated epileptogenesis.

Signaling mechanisms underlying group I mGluR-induced persistent AHP suppression in CA3 hippocampal neurons

Activation of group I metabotropic glutamate receptors (mGluRs) leads to a concerted modulation of spike afterpotentials in guinea pig hippocampal neurons including a suppression of both medium and slow afterhyperpolarizations (AHPs). Suppression of AHPs may be long-lasting, in that it persists after washout of the agonist. Here, we show that persistent AHP suppression differs from short-term, transient suppression in that distinct and additional signaling processes are required to render the suppression persistent. Persistent AHP suppression followed DHPG application for 30 min, but not DHPG application for 5 min. Persistent AHP suppression was temperature dependent, occurring at 30-31 degrees C, but not at 25-26 degrees C. Preincubation of slices in inhibitors of protein synthesis (cycloheximide or anisomycin) prevented the persistent suppression of AHPs by DHPG. Similarly, preincubation of slices in an inhibitor of p38 MAP kinase (SB 203580) prevented persistent AHP suppression. In contrast, a blocker of p42/44 MAP kinase activation (PD 98059) had no effect on persistent AHP suppression. Additionally, we show that the mGluR5 antagonist MPEP, but not the mGluR1 antagonist LY 367385, prevented DHPG-induced persistent AHP suppression. Thus persistent AHP suppression by DHPG in hippocampal neurons requires activation of mGluR5. In addition, activation of p38 MAP kinase signaling and protein synthesis are required to impart persistence to the DHPG-activated AHP suppression.

Prolonged epileptiform discharges induced by altered group I metabotropic glutamate receptor-mediated synaptic responses in hippocampal slices of a fragile X mouse model

Mutations in FMR1, which encodes the fragile X mental retardation protein (FMRP), are the cause of fragile X syndrome (FXS), an X-linked mental retardation disorder. Inactivation of the mouse gene Fmr1 confers a number of FXS-like phenotypes including an enhanced susceptibility to epileptogenesis during development. We find that in a FXS mouse model, in which the function of FMRP is suppressed, synaptically released glutamate induced prolonged epileptiform discharges resulting from enhanced group I metabotropic glutamate receptor (mGluR)-mediated responses in hippocampal slices. The induction of the group I mGluR-mediated, prolonged epileptiform discharges was inhibited in preparations that were pretreated with inhibitors of ERK1/2 (extracellular signal-regulated kinase 1/2) phosphorylation or of mRNA translation, and their maintenance was suppressed by group I mGluR antagonists. The results suggest that FMRP plays a key role in the control of signaling at the recurrent glutamatergic synapses in the hippocampus. The absence of this control causes the synaptically activated group I mGluRs to elicit translation-dependent epileptogenic activities.

Dendritic BC1 RNA: modulation by kindling-induced afterdischarges

Local protein synthesis in dendrites is thought to provide a mechanism for long-lasting modifications of synapses in response to physiological activity and behavioral experience. New synthesis of dendritic proteins may be triggered by various paradigms, including induction of epileptiform activity. Prerequisite for such modulated synthesis is a mechanism that limits translation of synaptodendritic mRNAs to times of demand. Recently identified as a translational repressor that is localized to dendrites, small untranslated BC1 RNA has been implicated in the regulation of postsynaptic protein synthesis. Here we show that translational repressor BC1 RNA is itself undergoing modulation as a result of neuronal stimulation. Induction of hippocampal epileptiform activity resulted in a significant decrease of BC1 RNA in the CA3 region over several hours after excitation. The observed decrease was cell-wide, thus indicating reduced expression rather than intracellular redistribution. We suggest that a downregulation of the translational repressor BC1 RNA serves to modulate postsynaptic protein complements in response to the induction of epileptiform activity. Such increased protein synthesis in dendrites may be required for the consolidation of enduring epileptogenic mechanisms.

Characterization of modulatory effects of postsynaptic metabotropic glutamate receptors on calcium currents in rat nucleus tractus solitarius

It is well known that metabotropic glutamate receptors (mGluRs) have multiple actions on neuronal excitability mediated by G-protein-coupled receptors, although the exact mechanisms by which these actions occur are not understood. This study examines the effects of mGluRs agonists on voltage-dependent Ca2+ channels (VDCCs) currents (ICa) in the nucleus tractus solitarius (NTS) of rats using patch-clamp recording methods. An application of (RS)-3,5-dihydroxyphenylglycine (DHPG, Group I mGluR agonist) caused both facilitation and inhibition of L-type and N/P/Q-types ICa, respectively. Neither (2S, 2'R, 3'R)-2-(2', 3'-dicarboxycyclopropyl)glycine (DCG, Group II mGluRs agonist) nor L-(+)-2-amino-4-phosphonobutyric acid (AP-4, Group III mGluRs agonist) nor (RS)-2-chloro-5-hydroxyphenylglycine (CHPG, mGluR5 agonist) modulated ICa. Intracellular dialysis of the Gq/11-protein antibody and Gi-protein antibody attenuated the DHPG-induced facilitation and inhibition, respectively. The phospholipase C (PLC) inhibitor, as well as inhibition of either the protein kinase C (PKC) or inositol-1,4,5-trisphosphate (IP3) attenuated the DHPG-induced facilitation of ICa but not a DHPG-induced inhibition. Application of a strong depolarizing voltage prepulse attenuated the DHPG-induced inhibition of ICa. These results indicate that mGluR1 facilitates L-type VDCCs via Gq/11-protein involving PKC including IP3 formation. On the other hand, mGluR1 inhibits N- and P/Q-types VDCCs via Gi-protein betagamma subunits.

Modulation of afterpotentials and firing pattern in guinea pig CA3 neurones by group I metabotropic glutamate receptors

Activation of group I metabotropic glutamate receptors (mGluRs) alters the firing patterns of individual CA3 pyramidal cells in guinea pig hippocampal slices. Following addition of the selective group I agonist (S)-3,5-dihydroxyphenylglycine (DHPG) to the bathing solution, pyramidal cells initially firing regular, single action potentials switched to firing in brief bursts. This change in firing pattern resulted from modulation by mGluRs of three afterpotentials. The medium and slow afterhyperpolarizations (m and sAHPs) were blocked by mGluR activation. In addition, a voltage-dependent after depolarization (ADP) was induced. Recordings from mutant mice lacking phospholipase C(beta1) (PLC(beta1)) showed that mGluR block of the mAHP, as well as induction of the ADP, depended on the phosphoinositide hydrolysis pathway. Block of the sAHP, however, was partly spared in the absence of PLC(beta1). Optical recordings of post spike intracellular Ca(2+) rises showed that mGluR block of the AHP was not mediated by alterations of action potential-associated Ca(2+) increases (Ca(2+) transients). The mGluR induction of an ADP was also independent of any changes in the Ca(2+) transient. The mGluR-induced change in the firing pattern of hippocampal pyramidal cells is thus the result of multiple mechanisms, including suppression of both m and sAHPs and activation of an ADP, that act together to produce a specific excitatory effect, namely an increased likelihood that a single action potential will lead immediately to one or more following action potentials.

Role of synaptic metabotropic glutamate receptors in epileptiform discharges in hippocampal slices

Application of group I metabotropic glutamate receptor (mGluR) agonists elicits seizure discharges in vivo and prolonged ictal-like activity in in vitro brain slices. In this study we examined 1) if group I mGluRs are activated by synaptically released glutamate during epileptiform discharges induced by convulsants in hippocampal slices and, if so, 2) whether the synaptically activated mGluRs contribute to the pattern of the epileptiform discharges. The GABA(A) receptor antagonist bicuculline (50 microM) was applied to induce short synchronized bursts of approximately 250 ms in mouse hippocampal slices. Addition of 4-aminopyridine (4-AP; 100 microM) prolonged these bursts to 0.7-2 s. The mGluR1 antagonist (S)-(+)-alpha-amino-4-carboxy-2-methylbenzeneacetic acid (LY 367385; 25-100 microM) and the mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP; 10-50 microM), applied separately, significantly reduced the duration of the synchronized discharges. The effects of these antagonists were additive when applied together, suggesting that mGluR1 and mGluR5 exert independent actions on the epileptiform bursts. In phospholipase C beta1 (PLCbeta1) knockout mice, bicuculline and 4-AP elicited prolonged synchronized discharges of comparable duration as those observed in slices from wild-type littermates. Furthermore, mGluR1 and mGluR5 antagonists reduced the duration of the epileptiform discharges to the same extent as they did in the wild-type preparations. The results suggest that mGluR1 and mGluR5 are activated synaptically during prolonged epileptiform discharges induced by bicuculline and 4-AP. Synaptic activation of these receptors extended the duration of synchronized discharges. In addition, the data indicate that the synaptic effects of the group I mGluRs on the duration of epileptiform discharges were mediated by a PLCbeta1-independent mechanism.

The requirement of presynaptic metabotropic glutamate receptors for the maintenance of locomotion

Spinal circuits known as central pattern generators maintain vertebrate locomotion. In the lamprey, the contralaterally alternating ventral root activity that defines this behavior is driven by ipsilateral glutamatergic excitation (Buchanan and Grillner, 1987) coupled with crossed glycinergic inhibition (Buchanan, 1982; Alford and Williams, 1989). These mechanisms are distributed throughout the spinal cord. Glutamatergic excitatory synapses activate AMPA and NMDA receptors known to be necessary for the maintenance of the locomotor rhythm. Less is known of the role and location of metabotropic glutamate receptors (mGluRs), although group I mGluRs enhance transmitter release at giant synapses in the lamprey spinal cord, whereas group II/III receptors may inhibit release. In this study we show that group I mGluR antagonists block fictive locomotion, a neural correlate of locomotion, by acting at the presynaptic terminal. Under physiological conditions, synaptically released glutamate activates presynaptic group I mGluRs (autoreceptors) during the repetitive activation of glutamatergic terminals. The resulting rise in [Ca2+]i caused by the release from presynaptic intracellular stores is coincident with an enhancement of synaptic transmission. Thus, blocking mGluRs reduces glutamate release during the repetitive activity that is characteristic of locomotion, leading to the arrest of locomotor activity. We found the effects of group I mGluRs on locomotion to be inconsistent with a postsynaptic effect on the central pattern generator. Consequently, the activation of metabotropic glutamate autoreceptors is necessary to maintain rhythmic motor output. Our results demonstrate the role of presynaptic mGluRs in the physiological control of movement for the first time.

Activation of group I mGluRs elicits different responses in murine CA1 and CA3 pyramidal cells

The group I metabotropic glutamate receptor agonist DHPG has been shown to produce two major effects on CA3 pyramidal cells at rest: a reduction in the background conductance and an activation of a voltage-gated inward current (I(mGluR(V))). Both effects contribute to depolarising CA3 pyramidal cells and the latter has been implicated in eliciting prolonged epileptiform population bursts. We observed that DHPG-induced depolarisation was smaller in CA1 pyramidal cells than in CA3 cells. Voltage clamp studies revealed that while DHPG elicited I(mGluR(V)) in CA3 pyramidal cells, such a response was absent in CA1 pyramidal cells. Both mGluR1 and mGluR5 have been localised in CA3 pyramidal cells, whereas only mGluR5 has been detected in CA1 pyramidal cells. Using mGluR1 knockout mice, we evaluated whether the absence of an I(mGluR(V)) response can be correlated with the absence of mGluR1. In these experiments, DHPG failed to elicit I(mGluR(V)) in CA3 pyramidal cells. This suggests that the smaller depolarising effects of DHPG on wild-type CA1 pyramidal cells is caused, at least in part, by the absence of I(mGluR(V)) in these cells and that the difference in the responses of CA1 and CA3 cells may be attributable to the lack of mGluR1 in CA1 pyramidal cells.

Glutamate receptor stimulation induces a persistent rhythmicity of the GABAergic inputs to rat midbrain dopaminergic neurons

The substantia nigra pars compacta and the ventral tegmental area are part of a complex network in the basal ganglia involved in behaviours as diverse as motor planning, generation of pleasure and drug addiction. Here we report that in the dopaminergic neurons of the rat ventral midbrain a brief coactivation of group I metabotropic and NMDA glutamate receptors may transform a temporally dispersed synaptic GABAergic input into a rhythmic pattern (range 4.5-22.5 Hz), probably through a mechanism involving electrotonic couplings. The plastic and long-lasting modification in the temporal code of the inhibitory synaptic activity induced by glutamate may be a key element in determining the function of midbrain dopaminergic neurons in both normal and pathological behaviour.

Group I metabotropic glutamate receptors elicit epileptiform discharges in the hippocampus through PLCbeta1 signaling

Activation of metabotropic glutamate receptors (mGluRs) produces multiple effects in cortical neurons, resulting in the emergence of network activities including epileptiform discharges. The cellular mechanisms underlying such network responses are largely unknown. We examined the properties of group I mGluR-mediated cellular responses in CA3 neurons and attempted to determine their role in the generation of the network activities. Group I mGluR stimulation causes depolarization of hippocampal neurons. This depolarization is primarily mediated by two sets of conductance change: the opening of a voltage-dependent cationic conductance (mediating I(mGluR(V))) and the closing of a voltage-independent (background) K(+) conductance. I(mGluR(V)) was no longer elicited by group I mGluR agonists in the presence of U73122, a phospholipase C (PLC) blocker. Also, the current could not be activated in hippocampal CA3 neurons from PLCbeta1 knock-out mice. In contrast, suppression of PLC signaling did not affect the group I mGluR-mediated suppression of background K(+) conductance. Thus, the suppression of the background K(+) conductance occurred upstream to PLC activation, whereas the generation of I(mGluR(V)) occurred downstream to PLC activation. Group I mGluR agonists normally elicited rhythmic single cell and population burst responses in the CA3 neurons. In the absence of an I(mGluR(V)) response, CA3 neurons in slices prepared from PLCbeta1-/- mutant mice could no longer generate these responses. The results suggest that I(mGluR(V)) expression in CA3 hippocampal neuron is PLCbeta1-dependent and that I(mGluR(V)) plays a necessary role in the generation of rhythmic single cell bursts and synchronized epileptiform discharges in the CA3 region of the hippocampus.

Group I metabotropic glutamate receptors elicit epileptiform discharges in the hippocampus through PLCbeta1 signaling

Activation of metabotropic glutamate receptors (mGluRs) produces multiple effects in cortical neurons, resulting in the emergence of network activities including epileptiform discharges. The cellular mechanisms underlying such network responses are largely unknown. We examined the properties of group I mGluR-mediated cellular responses in CA3 neurons and attempted to determine their role in the generation of the network activities. Group I mGluR stimulation causes depolarization of hippocampal neurons. This depolarization is primarily mediated by two sets of conductance change: the opening of a voltage-dependent cationic conductance (mediating I(mGluR(V))) and the closing of a voltage-independent (background) K(+) conductance. I(mGluR(V)) was no longer elicited by group I mGluR agonists in the presence of U73122, a phospholipase C (PLC) blocker. Also, the current could not be activated in hippocampal CA3 neurons from PLCbeta1 knock-out mice. In contrast, suppression of PLC signaling did not affect the group I mGluR-mediated suppression of background K(+) conductance. Thus, the suppression of the background K(+) conductance occurred upstream to PLC activation, whereas the generation of I(mGluR(V)) occurred downstream to PLC activation. Group I mGluR agonists normally elicited rhythmic single cell and population burst responses in the CA3 neurons. In the absence of an I(mGluR(V)) response, CA3 neurons in slices prepared from PLCbeta1-/- mutant mice could no longer generate these responses. The results suggest that I(mGluR(V)) expression in CA3 hippocampal neuron is PLCbeta1-dependent and that I(mGluR(V)) plays a necessary role in the generation of rhythmic single cell bursts and synchronized epileptiform discharges in the CA3 region of the hippocampus.

Tetanic stimulation and metabotropic glutamate receptor agonists modify synaptic responses and protein kinase activity in rat auditory cortex

We investigated whether tetanic-stimulation and activation of metabotropic glutamate receptors (mGluRs) can modify field-synaptic-potentials and protein kinase activity in rat auditory cortex, specifically protein kinase A (PKA) and protein kinase C (PKC). Tetanic stimulation (50 Hz, 1 s) increases PKA and PKC activity only if the CNQX-sensitive field-EPSP (f-EPSP) is also potentiated. If the f-EPSP is unchanged, then PKA and PKC activity remains unchanged. Tetanic stimulation decreases a bicuculline-sensitive field-IPSP (f-IPSP), and this occurs whether the f-EPSP is potentiated or not. Potentiation of the f-EPSP is blocked by antagonists of mGluRs (MCPG) and PKC (calphostin-C, tamoxifen), suggesting that the potentiation of the f-EPSP is dependent on mGluRs and PKC. PKC antagonists block the rise in PKC and PKA activity, which suggests that these may be coupled. In contrast, ACPD (agonist at mGluRs) decreases both the f-EPSP and the f-IPSP, but increases PKC and PKA activity. Quisqualate (group I mGluR agonist), decreases the f-IPSP, and increases PKA activity, suggesting that the increase in PKA activity is a result of activation of group I mGluRs. Additionally, the increase in PKC and PKA activity appears to be independent of the decrease of the f-EPSP and f-IPSP, because PKC antagonists block the increase in PKC and PKA activity levels but do not block ACPD's effect on the f-EPSP or f-IPSP. These data suggest that group I mGluRs are involved in potentiating the f-EPSP by a PKC and possibly PKA dependent mechanism which is separate from the mechanism that decreases the f-EPSP and f-IPSP.

Identified serotonergic neurons in the Tritonia swim CPG activate both ionotropic and metabotropic receptors

Although G-protein-coupled (metabotropic) receptors are known to modulate the production of motor patterns, evidence from the escape swim central pattern generator (CPG) of the nudibranch mollusk, Tritonia diomedea, suggests that they might also participate in the generation of the motor pattern itself. The dorsal swim interneurons (DSIs), identified serotonergic neurons intrinsic to the Tritonia swim CPG, evoke dual component synaptic potentials onto other CPG neurons and premotor interneurons. Both the fast and slow components were previously shown to be due to serotonin (5-HT) acting at distinct postsynaptic receptors. We find that blocking or facilitating metabotropic receptors in a postsynaptic premotor interneuron differentially affects the fast and slow synaptic responses to DSI stimulation. Blocking G-protein activation by iontophoretically injecting the GDP-analogue guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S) did not significantly affect the DSI-evoked fast excitatory postsynaptic potential (EPSP) but decreased the amplitude of the slow component more than 50%. Injection of the GTP analogues guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) and 5'-guanylyl-imidodiphosphate, to prolong G-protein activation, had mixed effects on the fast component but increased the amplitude and duration of the slow component of the DSI-evoked response and, with repeated DSI stimulation, led to a persistent depolarization. These results indicate that the fast component of the biphasic synaptic potential evoked by a serotonergic CPG neuron onto premotor interneurons is mediated by ionotropic receptors (5-HT-gated ion channels), whereas the slow component is mediated by G-protein-coupled receptors. A similar synaptic activation of metabotropic receptors might also be found within the CPG itself, where it could exert a direct influence onto motor pattern generation.

Modulation of the spike activity of neocortex neurons during a conditioned reflex

Experiments were conducted on cats to study the effects of iontophoretic application of glutamate and a number of modulators on the spike activity of neurons in the sensorimotor cortex during a conditioned reflex. These studies showed that glutamate, as well as exerting a direct influence on neuron spike activity, also had a delayed facilitatory action lasting 10-20 min after iontophoresis was finished. Adrenomimetics were found to have a double modulatory effect on intracortical glutamate connections: inhibitory and facilitatory effects were mediated by beta1 and beta2 adrenoceptors respectively. Although dopamine, like glutamate, facilitated neuron spike activity during the period of application, the simultaneous facilitatory actions of glutamate and L-DOPA were accompanied by occlusion of spike activity, and simultaneous application of glutamate and haloperidol suppressed spike activity associated with the conditioned reflex response. Facilitation thus appears to show a significant level of dependence on metabotropic glutamate receptors which, like dopamine receptors, are linked to the intracellular medium via Gi proteins.

Group I mGluR activation turns on a voltage-gated inward current in hippocampal pyramidal cells

A unique property of the group I metabotropic glutamate receptor (mGluR)-induced depolarization in hippocampal cells is that the amplitude of the depolarization is larger when the response is elicited at more depolarized membrane potentials. Our understanding of the conductance mechanism underlying this voltage-dependent response is incomplete. Through the use of current-clamp and single-electrode voltage-clamp recordings in guinea pig hippocampal slices, we examined the group I mGluR-induced depolarization in CA3 pyramidal cells. The group I mGluR agonists (S)-3-hydroxyphenylglycine and (S)-3,5-dihydroxyphenylglycine turned on a voltage-gated inward current (I(mGluR(V))), which was pharmacologically distinct from the voltage-gated sodium and calcium currents intrinsic to the cells. I(mGluR(V)) was a slowly activating, noninactivating current with a threshold at about -75 mV. In addition to the activation of I(mGluR(V)), group I mGluR stimulation also produced a voltage-independent decrease in the K(+) conductance. Our results suggest that the depolarization induced by group I mGluR activation is generated by two ionic mechanisms-a heretofore unrecognized voltage-gated inward current (I(mGluR(V))) that is turned on by depolarization and a voltage-insensitive inward current that results from a turn-off of the K(+) conductance. The low-threshold and noninactivating properties of I(mGluR(V)) allow the current to play a significant role in setting the resting potential and firing pattern of CA3 pyramidal cells.

Muscarinic receptor activation induces depolarizing plateau potentials in bursting neurons of the rat subiculum

Acetylcholine functions as a neuromodulator in the mammalian brain by binding to specific receptors and thus bringing about profound changes in neuronal excitability. Activation of muscarinic receptors often results in an increased excitability of cortical cells. It is, however, unknown whether such an action is present in the subiculum, a limbic structure that may be involved in cognitive processes as well as in seizure propagation. Most rat subicular neurons are endowed of intrinsic membrane properties that make them fire action potential bursts. Using intracellular recordings from these bursting cells in a slice preparation, we report here that application of the cholinergic agonist carbachol (CCh, 30-100 microM) to medium containing ionotropic excitatory amino acid receptor antagonists reduces burst-afterhyperpolarizations (burst-AHPs) and discloses depolarizing plateau potentials that outlast the triggering current pulses by 140-2,800 ms. These plateau potentials appear with CCh concentrations >50 microM and are dependent on the resting membrane potential and on the intensity/duration of the triggering pulse; are recorded during application of tetrodotoxin (1 microM, n = 5 neurons); but are markedly reduced by replacing 82% of extracellular Na(+) with equimolar choline (n = 6). Plateau potentials also are abolished by Co(2+) (2 mM; n = 5) or Cd(2+) (1 mM; n = 2) application and by recording with electrodes containing the Ca(2+) chelator bis(2-aminophenoxy)ethane-N, N,N',N'-tetraacetic acid (0.2 M; n = 6). CCh-induced burst-AHP reduction and plateau potentials are reversed by the muscarinic antagonist atropine (0.5 microM, n = 7). In conclusion, our findings demonstrate a powerful muscarinic modulation of the intrinsic excitability of subicular bursting cells that is predominated by the appearance of plateau potentials. These changes in excitability may contribute to physiological processes such as learning or memory and play a role in the generation of epileptiform depolarizations. We propose that, as in other limbic structures, muscarinic plateau potentials in the subiculum are mainly due to a Ca(2+)-dependent nonselective cationic conductance.

Group I mGluR activation causes voltage-dependent and -independent Ca2+ rises in hippocampal pyramidal cells

Application of the metabotropic glutamate receptor (mGluR) agonist (1S, 3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) or the selective group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG) depolarized both CA3 and CA1 pyramidal cells in guinea pig hippocampal slices. Simultaneous recordings of voltage and intracellular Ca2+ levels revealed that the depolarization was accompanied by a biphasic elevation of intracellular Ca2+ concentration ([Ca2+]i): a transient calcium rise followed by a delayed, sustained elevation. The transient [Ca2+]i rise was independent of the membrane potential and was blocked when caffeine was added to the perfusing solution. The sustained [Ca2+]i rise appeared when membrane depolarization reached threshold for voltage-gated Ca2+ influx and was suppressed by membrane hyperpolarization. The depolarization was associated with an increased input resistance and persisted when either the transient or sustained [Ca2+]i responses was blocked. mGluR-mediated voltage and [Ca2+]i responses were blocked by (+)-alpha-methyl-4-carboxyphenylglycine (MCPG) or (S)-4-carboxy-3-hydroxyphenylglycine (4C3HPG). These data suggest that in both CA3 and CA1 hippocampal cells, activation of group I mGluRs produced a biphasic accumulation of [Ca2+]i via two paths: a transient release from intracellular stores, and subsequently, by influx through voltage-gated Ca2+ channels. The concurrent mGluR-induced membrane depolarization was not caused by the [Ca2+]i rise.

Differential effects of metabotropic glutamate receptor antagonists on bursting activity in the amygdala

Differential effects of metabotropic glutamate receptor antagonists on bursting activity in the amygdala. Metabotropic glutamate receptors (mGluRs) are implicated in both the activation and inhibition of epileptiform bursting activity in seizure models. We examined the role of mGluR agonists and antagonists on bursting in vitro with whole cell recordings from neurons in the basolateral amygdala (BLA) of amygdala-kindled rats. The broad-spectrum mGluR agonist 1S,3R-1-aminocyclopentane dicarboxylate (1S,3R-ACPD, 100 microM) and the group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG, 20 microM) evoked bursting in BLA neurons from amygdala-kindled rats but not in control neurons. Neither the group II agonist (2S,3S,4S)-alpha-(carboxycyclopropyl)-glycine (L-CCG-I, 10 microM) nor the group III agonist L-2-amino-4-phosphonobutyrate (L-AP4, 100 microM) evoked bursting. The agonist-induced bursting was inhibited by the mGluR1 antagonists (+)-alpha-methyl-4-carboxyphenylglycine [(+)-MCPG, 500 microM] and (S)-4-carboxy-3-hydroxyphenylglycine [(S)-4C3HPG, 300 microM]. Kindling enhanced synaptic strength from the lateral amygdala (LA) to the BLA, resulting in synaptically driven bursts at low stimulus intensity. Bursting was abolished by (S)-4C3HPG. Further increasing stimulus intensity in the presence of (S)-4C3HPG (300 microM) evoked action potential firing similar to control neurons but did not induce epileptiform bursting. In kindled rats, the same threshold stimulation that evoked epileptiform bursting in the absence of drugs elicited excitatory postsynaptic potentials in (S)-4C3HPG. In contrast (+)-MCPG had no effect on afferent-evoked bursting in kindled neurons. Because (+)-MCPG is a mGluR2 antagonist, whereas (S)-4C3HPG is a mGluR2 agonist, the different effects of these compounds suggest that mGluR2 activation decreases excitability. Together these data suggest that group I mGluRs may facilitate and group II mGluRs may attenuate epileptiform bursting observed in kindled rats. The mixed agonist-antagonist (S)-4C3HPG restored synaptic transmission to control levels at the LA-BLA synapse in kindled animals. The different actions of (S)-4C3HPG and (+)-MCPG on LA-evoked bursting suggests that the mGluR1 antagonist-mGluR2 agonist properties may be the distinctive pharmacology necessary for future anticonvulsant compounds.

Synchronized paroxysmal activity in the developing thalamocortical network mediated by corticothalamic projections and "silent" synapses

In mouse thalamocortical slices in vitro, the potassium channel blocker 4-AP and GABAA receptor antagonist bicuculline together induced spontaneous prolonged depolarizations in layer VI neurons from postnatal day 2 (P2), in ventroposterior nucleus neurons (VP) from P7, and in reticular nucleus neurons (RTN) from P8. Dual whole-cell recordings revealed that prolonged bursts were synchronized in layer VI, VP, and RTN. Bursts were present in cortex isolated from thalamus, but not in thalamus isolated from cortex, indicating that bursts originated in cortex and propagated to thalamus. Prolonged bursts were synchronized in layer VI when vertical cuts extended from pia mater through layers IV or V, but were no longer synchronized when cuts extended through layer VI and white matter. In voltage-clamp recordings before P10, burst conductance of all three neuronal populations was dominated by the NMDA receptor-mediated conductance, and therefore synapses were "silent". In cortex and RTN, after P10, bursts were associated with strong AMPA/kainate receptor-mediated conductances, and synapses had become "functional"; silent synapses persisted in a large proportion of VP cells after P10. Before P9, the NMDA receptor antagonist APV or the non-NMDA receptor antagonist CNQX blocked the prolonged bursts. After P9, CNQX continued to block the prolonged bursts, but APV merely shortened their duration. Thus, NMDA receptor-based silent synapses are essential for paroxysmal corticothalamic activity during early postnatal development, and connections between layer VI neurons are sufficient for horizontal cortical synchronization.

Synchronized paroxysmal activity in the developing thalamocortical network mediated by corticothalamic projections and "silent" synapses

In mouse thalamocortical slices in vitro, the potassium channel blocker 4-AP and GABAA receptor antagonist bicuculline together induced spontaneous prolonged depolarizations in layer VI neurons from postnatal day 2 (P2), in ventroposterior nucleus neurons (VP) from P7, and in reticular nucleus neurons (RTN) from P8. Dual whole-cell recordings revealed that prolonged bursts were synchronized in layer VI, VP, and RTN. Bursts were present in cortex isolated from thalamus, but not in thalamus isolated from cortex, indicating that bursts originated in cortex and propagated to thalamus. Prolonged bursts were synchronized in layer VI when vertical cuts extended from pia mater through layers IV or V, but were no longer synchronized when cuts extended through layer VI and white matter. In voltage-clamp recordings before P10, burst conductance of all three neuronal populations was dominated by the NMDA receptor-mediated conductance, and therefore synapses were "silent". In cortex and RTN, after P10, bursts were associated with strong AMPA/kainate receptor-mediated conductances, and synapses had become "functional"; silent synapses persisted in a large proportion of VP cells after P10. Before P9, the NMDA receptor antagonist APV or the non-NMDA receptor antagonist CNQX blocked the prolonged bursts. After P9, CNQX continued to block the prolonged bursts, but APV merely shortened their duration. Thus, NMDA receptor-based silent synapses are essential for paroxysmal corticothalamic activity during early postnatal development, and connections between layer VI neurons are sufficient for horizontal cortical synchronization.

Subthalamic nucleus neurons switch from single-spike activity to burst-firing mode

The modification of the discharge pattern of subthalamic nucleus (STN) neurons from single-spike activity to mixed burst-firing mode is one of the characteristics of parkinsonism in rat and primates. However, the mechanism of this process is not yet understood. Intrinsic firing patterns of STN neurons were examined in rat brain slices with intracellular and patch-clamp techniques. Almost half of the STN neurons that spontaneously discharged in the single-spike mode had the intrinsic property of switching to pure or mixed burst-firing mode when the membrane was hyperpolarized from -41.3 +/- 1.0 mV (range, -35 to -50 mV; n = 15) to -51.0 +/- 1.0 mV (range, -42 to -60 mV; n = 20). This switch was greatly facilitated by activation of metabotropic glutamate receptors with 1S,3R-ACPD. Recurrent membrane oscillations underlying burst-firing mode were endogenous and Ca2+-dependent because they were largely reduced by nifedipine (3 microM), Ni2+ (40 microM), and BAPTA-AM (10-50 microM) at any potential tested, whereas TTX (1 microM) had no effect. In contrast, simultaneous application of TEA (1 mM) and apamin (0.2 microM) prolonged burst duration. Moreover, in response to intracellular stimulation at hyperpolarized potentials, a plateau potential with a voltage and ionic basis similar to those of spontaneous bursts was recorded in 82% of the tested STN neurons, all of which displayed a low-threshold Ni2+-sensitive spike. We propose that recurrent membrane oscillations during bursts result from the sequential activation of T/R- and L-type Ca2+ currents, a Ca2+-activated inward current, and Ca2+-activated K+ currents.

Subthalamic nucleus neurons switch from single-spike activity to burst-firing mode

The modification of the discharge pattern of subthalamic nucleus (STN) neurons from single-spike activity to mixed burst-firing mode is one of the characteristics of parkinsonism in rat and primates. However, the mechanism of this process is not yet understood. Intrinsic firing patterns of STN neurons were examined in rat brain slices with intracellular and patch-clamp techniques. Almost half of the STN neurons that spontaneously discharged in the single-spike mode had the intrinsic property of switching to pure or mixed burst-firing mode when the membrane was hyperpolarized from -41.3 +/- 1.0 mV (range, -35 to -50 mV; n = 15) to -51.0 +/- 1.0 mV (range, -42 to -60 mV; n = 20). This switch was greatly facilitated by activation of metabotropic glutamate receptors with 1S,3R-ACPD. Recurrent membrane oscillations underlying burst-firing mode were endogenous and Ca2+-dependent because they were largely reduced by nifedipine (3 microM), Ni2+ (40 microM), and BAPTA-AM (10-50 microM) at any potential tested, whereas TTX (1 microM) had no effect. In contrast, simultaneous application of TEA (1 mM) and apamin (0.2 microM) prolonged burst duration. Moreover, in response to intracellular stimulation at hyperpolarized potentials, a plateau potential with a voltage and ionic basis similar to those of spontaneous bursts was recorded in 82% of the tested STN neurons, all of which displayed a low-threshold Ni2+-sensitive spike. We propose that recurrent membrane oscillations during bursts result from the sequential activation of T/R- and L-type Ca2+ currents, a Ca2+-activated inward current, and Ca2+-activated K+ currents.

Metabotropic glutamate receptors: electrophysiological properties and role in plasticity

Electrophysiological research on mGluRs is now very extensive, and it is clear that activation of mGluRs results in a large number of diverse cellular actions. Studies of mGluRs and on ionic channels has clearly demonstrated that mGluR activation has a widespread and potent inhibitory action on both voltage-gated Ca2+ channels and K+ channels. Inhibition of N-type Ca2+ channels, and inhibition of Ca(++)-dependent K+ current, IAHP, and IM being particularly prominent. Potentiation of activation of both Ca2+ and K+ channels has also been observed, although less prominently than inhibition, but mGluR-mediated activation of non-selective cationic channels is widespread. In a small number of studies, generation of an mGluR-mediated slow excitatory postsynaptic potential has been demonstrated as a consequence of the effect of mGluR activation on ion channels, such as activation of a non-selective cationic channels. Although certain mGluR-modulation of channels is a consequence of direct G-protein-linked action, for example, inhibition of Ca2+ channels, many other effects occur as a result of activation of intracellular messenger pathways, but at present, little progress has been made on the identification of the messengers. The field of study of the involvement of mGluRs in synaptic plasticity is very large. Evidence for the involvement of mGluRs in one form of LTD induction in the cerebellum and hippocampus is now particularly impressive. However, the role of mGluRs in LTP induction continues to be a source of dispute, and resolution of the question of the exact involvement of mGluRs in the induction of LTP will have to await the production of more selective ligands and of selective gene knockouts.

Activity-dependent pH shifts and periodic recurrence of spontaneous interictal spikes in a model of focal epileptogenesis

The mechanisms that control the periodicity of spontaneous epileptiform cortical potentials were investigated in the in vitro isolated guinea pig brain preparation. A brief intracortical application of bicuculline in the piriform cortex induced spontaneous interictal spikes (sISs) that recurred with high periodicity (8.5 +/- 3.1 sec, mean +/- SD). Intracellular recordings from principal neurons showed that the early phase of the inter-sIS period is caused by a GABAb receptor-mediated inhibitory potential. The late component of the interspike period correlated to a slowly decaying depolarization abolished at membrane potentials positive to -32.1 +/- 5.3 mV and was not associated with membrane conductance changes. Specific pharmacological tests excluded the contribution of synaptic and intrinsic conductances to the late inter-sIS interval. Recordings with ion-sensitive electrodes demonstrated that sISs determined both a rapid increase in extracellular K+ concentration (0.5-1 mM) and an extracellular alkalinization (0.05-0.08 pH units) that slowly decayed during the inter-sIS period and returned to control values just before a subsequent sIS was generated. These observations were not congruous with the presence of a silent period, because both extracellular increase in K+ and alkalinization are commonly associated with an increase in neuronal excitability. Extracellular alkalinization could be correlated to an sIS-induced intracellular acidification, a phenomenon that reduces cell coupling by impairing gap junction function. When intracellular acidification was transiently prevented by arterial perfusion with NH4Cl (10-20 mM), spontaneous ictal-like epileptiform discharges were induced. In addition, the gap junction blockers octanol (0.2-2 mM) and 18-alpha-glycyrrethinic acid (20 microM) applied either via the arterial system or locally in the cortex completely and reversibly abolished the sIS. The results reported here suggest that the massive cell discharge associated with an sIS induce a strong inhibition, possibly secondary to a pH-dependent uncoupling of gap junctions, that regulates sIS periodicity.

Activity-dependent pH shifts and periodic recurrence of spontaneous interictal spikes in a model of focal epileptogenesis

The mechanisms that control the periodicity of spontaneous epileptiform cortical potentials were investigated in the in vitro isolated guinea pig brain preparation. A brief intracortical application of bicuculline in the piriform cortex induced spontaneous interictal spikes (sISs) that recurred with high periodicity (8.5 +/- 3.1 sec, mean +/- SD). Intracellular recordings from principal neurons showed that the early phase of the inter-sIS period is caused by a GABAb receptor-mediated inhibitory potential. The late component of the interspike period correlated to a slowly decaying depolarization abolished at membrane potentials positive to -32.1 +/- 5.3 mV and was not associated with membrane conductance changes. Specific pharmacological tests excluded the contribution of synaptic and intrinsic conductances to the late inter-sIS interval. Recordings with ion-sensitive electrodes demonstrated that sISs determined both a rapid increase in extracellular K+ concentration (0.5-1 mM) and an extracellular alkalinization (0.05-0.08 pH units) that slowly decayed during the inter-sIS period and returned to control values just before a subsequent sIS was generated. These observations were not congruous with the presence of a silent period, because both extracellular increase in K+ and alkalinization are commonly associated with an increase in neuronal excitability. Extracellular alkalinization could be correlated to an sIS-induced intracellular acidification, a phenomenon that reduces cell coupling by impairing gap junction function. When intracellular acidification was transiently prevented by arterial perfusion with NH4Cl (10-20 mM), spontaneous ictal-like epileptiform discharges were induced. In addition, the gap junction blockers octanol (0.2-2 mM) and 18-alpha-glycyrrethinic acid (20 microM) applied either via the arterial system or locally in the cortex completely and reversibly abolished the sIS. The results reported here suggest that the massive cell discharge associated with an sIS induce a strong inhibition, possibly secondary to a pH-dependent uncoupling of gap junctions, that regulates sIS periodicity.

Progression of change in NMDA, non-NMDA, and metabotropic glutamate receptor function at the developing corticothalamic synapse

The development of receptor function at corticothalamic synapses during the first 20 days of postnatal development is described. Whole cell excitatory postsynaptic currents (EPSCs) were evoked in relay neurons of the ventral posterior nucleus (VP) by stimulation of corticothalamic fibers in in vitro slices of mouse brain from postnatal day 1 (P1). During P1-P12, excitatory postsynaptic conductances showed strong voltage dependence at peak current and at 100 ms after the stimulus and were almost completely antagonized by -2-amino-5-phosphonopentoic acid (APV), indicating that N-methyl--aspartate (NMDA) receptor-mediated currents dominate corticothalamic EPSCs at this time. After P12, in 42% of cells, excitatory postsynaptic conductances showed no voltage-dependence at peak current but still showed voltage-dependence 100-ms poststimulus. This voltage-dependent conductance was antagonized by APV. The nonvoltage-dependent component was APV resistant, showed fast decay, and was antagonized by the nonNMDA antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). In the remaining 58% of cells after P12, excitatory postsynaptic conductances showed moderate voltage dependence at peak conductance and strong voltage dependence 100 ms after the stimulus. Analysis of EPSCs before and after APV showed a significant increase in the relative contribution of the non-NMDA conductance after the second postnatal week. From P1 to P16, there was a significant decrease in the time constant of decay of the NMDA EPSC but no change in the voltage dependence of the NMDA response. After P8, slow EPSPs, 1.5-30 s in duration and mediated by metabotropic glutamate receptors (mGluRs), could be evoked by high-frequency stimulation of corticothalamic fibers in the presence of APV and CNQX. Similar slow depolarizations could be evoked by local application of the mGluR agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (t-ACPD) but from P0. Both conductances were blocked by the mGluR antagonist, (RS)-alpha-methyl-4-carboxyphenylglycine. Hence functional mGluR receptors are present on VP cells from birth, but their synaptic activation at corticothalamic synapses can only be detected after P8. In voltage clamp, the extrapolated reversal potential of the t-ACPD current, with potassium gluconate-based internal solution, was +12 +/- 10 (SE) mV, and the measured reversal potential with cesium gluconate-based internal solution was 1.5 +/- 9.9 mV, suggesting that the mGluR-mediated depolarization was mediated by a nonselective cation current. Replacement of NaCl in the external solution caused the reversal potential of the current to shift to -18 +/- 2 mV, indicating that Na+ is a charge carrier in the current. The current amplitude was not reduced by application of Cs+, Ba2+, and Cd2+, indicating that the t-ACPD current was distinct from the hyperpolarization-activated cation current (IH) and distinct from certain other previously characterized mGluR-activated, nonselective cation conductances.

Interaction between dopamine and glutamate in the sensorimotor cortex during conditioned placing reaction

Changes in impulse activity of sensorimotor cortex neurons associated with interaction of glutamate and dopamine during conditioned placing reaction were investigated in experiments on cats. Application of either glutamate or levodopa as a dopamine precursor increased background and evoked impulse activity in many of sensorimotor cortex neurons. It occurred occasionally that an increased impulse activity of cortical neurons produced by joint application of glutamate and levodopa could be much more intense than that produced by one of these substances. Amphetamine acted on cortical neurons in a similar way as levodopa. Haloperidol, a non-selective blocker of dopamine1 and dopamine2 receptors, increased or did not change background and evoked impulse activity in some cortical neurons. In contrast to application of glutamate alone, simultaneous application of glutamate and haloperidol to the neocortex depressed neuronal responses connected with conditioned movement. Thus, glutamate cannot exert its potentiating effect on evoked neuronal activity due to the depressing action of haloperidol. This means that glutamate potentiation is realized to a great extent through molecular mechanisms common for glutamate and dopamine, possibly through G-proteins which are common for glutamate metabotropic and dopamine receptors.

Metabotropic glutamate receptor activation in vivo induces intraneuronal amyloid immunoreactivity in guinea pig hippocampus

Stimulation of metabotropic glutamate receptors in vitro has been shown to accelerate the breakdown of amyloid precursor protein (APP) to form increased production of non-amyloidogenic secreted APP (sAPP). The mechanism whereby this occurs is not entirely clear but it is presumed to be linked to generation of diacylglycerol and activation of protein kinase C because other neurotransmitter receptors such as m1 and m3 muscarinic receptors, known to be coupled to this second messenger cascade, likewise increase sAPP production. Although it is presumed that a reciprocal relationship exists between the formation of amyloid beta protein (Abeta) and the production of sAPP, recent evidence suggests alternative processing can occur. Given the fact that much of the observations on APP metabolism have been made in vitro we sought to investigate the effect of metabotropic receptor activation on Abeta in vivo in a species known to contain the same amino acid sequence of Abeta as found in humans. Intrahippocampal injection of the mGluR agonist 1S,3R-ACPD in guinea pigs produced neurodegeneration of CA1 hippocampal pyramidal neurons at 12 h postinjection. Immunocytochemistry of sections from ACPD injected animals using selective antibodies to Abeta revealed the presence of punctate intraneuronal granules in pyramidal neurons of the hippocampus. These structures appeared to be localized within the nucleus and were particularly prominent in neurons within the region of neurodegeneration. Immunoreactivity was not observed in vehicle injected controls nor in sections from ACPD injected animals stained with preadsorbed antiserum. Abeta immunodetection was correlated with the onset of neurodegeneration since animals evaluated at 1 h and 4 h postinjection lacked both Abeta immunoreactivity as well as neurodegeneration. Evaluation of animals injected with NMDA revealed neurodegeneration but no Abeta immunoreactivity suggesting Abeta formation did not appear to be due to non-selective excitotoxicity. Staining of sections with antibodies directed to various regions of APP demonstrated increased C-terminal APP immunoreactivity in pyramidal neurons in the vicinity of degeneration. These data support recent in vitro studies illustrating that Abeta can be found intracellularly within neurons.

Glutamate receptors in the mammalian central nervous system

Glutamate receptors (GluRs) mediate most of the excitatory neurotransmission in the mammalian central nervous system (CNS). In addition, they are involved in plastic changes in synaptic transmission as well as excitotoxic neuronal cell death that occurs in a variety of acute and chronic neurological disorders. The GluRs are divided into two distinct groups, ionotropic and metabotropic receptors. The ionotropic receptors (iGluRs) are further subdivided into three groups: alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), kainate and N-methyl-D-aspartate (NMDA) receptor channels. The metabotropic receptors (mGluRs) are coupled to GTP-binding proteins (G-proteins), and regulate the production of intracellular messengers. The application of molecular cloning technology has greatly advanced our understanding of the GluR system. To date, at least 14 cDNAs of subunit proteins constituting iGluRs and 8 cDNAs of proteins constituting mGluRs have been cloned in the mammalian CNS, and the molecular structure, distribution and developmental change in the CNS, functional and pharmacological properties of each receptor subunit have been elucidated. Furthermore, the obtained clones have provided valuable tools for conducting studies to clarify the physiological and pathophysiological significances of each subunit. For example, the generation of gene knockout mice has disclosed critical roles of some GluR subunits in brain functions. In this article, we review recent progress in the research for GluRs with special emphasis on the molecular diversity of the GluR system and its implications for physiology and pathology of the CNS.

A long-lasting calcium-activated nonselective cationic current is generated by synaptic stimulation or exogenous activation of group I metabotropic glutamate receptors in CA1 pyramidal neurons

We have shown previously that a selective metabotropic glutamate receptor (mGluR) agonist, 1S,3R-1-aminocyclo-pentane-1, 3-dicarboxylate (1S,3R-ACPD), evokes an inward current in CA1 pyramidal neurons of rat hippocampal slices in the presence of K+ channel blockers (). This current has been characterized as a Ca2+-activated nonselective cationic (CAN) current. Using whole-cell patch-clamp recordings and intracellular dialysis, we now have identified the mGluR subtype and the mechanisms underlying the CAN current (ICAN) and report for the first time the presence of a synaptic ICAN in the mammalian CNS. First, we have shown pharmacologically that activation of ICAN by 1S,3R-ACPD involves the group I mGluRs (and not the groups II and III) and a G-protein-dependent process. We also report that ICAN is modulated by the divalent cations (Mg2+, Cd2+, and Zn2+). Second, we have isolated a slow synaptic inward current evoked by a high-frequency stimulation in the presence of K+ channel blockers, ionotropic glutamate, and GABAA receptor antagonists. This current shows similar properties to the exogenously evoked ICAN: its reversal potential is close to the reversal potential of the 1S, 3R-ACPD-evoked ICAN, and it is G-protein- and Ca2+-dependent. Because the amplitude and duration of ICAN increased in the presence of a glutamate uptake blocker, we suggest that this synaptic current is generated via the activation of mGluRs. We propose that the synaptic ICAN, activated by a brief tetanic stimulation and leading to a long-lasting inward current, may be involved in neuronal plasticity and synchronized network-driven oscillations.

A long-lasting calcium-activated nonselective cationic current is generated by synaptic stimulation or exogenous activation of group I metabotropic glutamate receptors in CA1 pyramidal neurons

We have shown previously that a selective metabotropic glutamate receptor (mGluR) agonist, 1S,3R-1-aminocyclo-pentane-1, 3-dicarboxylate (1S,3R-ACPD), evokes an inward current in CA1 pyramidal neurons of rat hippocampal slices in the presence of K+ channel blockers (). This current has been characterized as a Ca2+-activated nonselective cationic (CAN) current. Using whole-cell patch-clamp recordings and intracellular dialysis, we now have identified the mGluR subtype and the mechanisms underlying the CAN current (ICAN) and report for the first time the presence of a synaptic ICAN in the mammalian CNS. First, we have shown pharmacologically that activation of ICAN by 1S,3R-ACPD involves the group I mGluRs (and not the groups II and III) and a G-protein-dependent process. We also report that ICAN is modulated by the divalent cations (Mg2+, Cd2+, and Zn2+). Second, we have isolated a slow synaptic inward current evoked by a high-frequency stimulation in the presence of K+ channel blockers, ionotropic glutamate, and GABAA receptor antagonists. This current shows similar properties to the exogenously evoked ICAN: its reversal potential is close to the reversal potential of the 1S, 3R-ACPD-evoked ICAN, and it is G-protein- and Ca2+-dependent. Because the amplitude and duration of ICAN increased in the presence of a glutamate uptake blocker, we suggest that this synaptic current is generated via the activation of mGluRs. We propose that the synaptic ICAN, activated by a brief tetanic stimulation and leading to a long-lasting inward current, may be involved in neuronal plasticity and synchronized network-driven oscillations.

Regulation of muscarinic acetylcholine receptor-mediated synaptic responses by adenosine receptors in the rat hippocampus

1. Intracellular current clamp recordings were made from CA1 pyramidal neurones in rat hippocampal slices. Experiments were performed in the presence of ionotropic glutamate receptor antagonists and gamma-aminobutyric acid (GABA) receptor antagonists to block all fast excitatory and inhibitory synaptic transmission. A single stimulus, delivered extracellularly in the stratum oriens, caused a reduction in spike frequency adaptation in response to a depolarizing current step delivered 2 s after the stimulus. A 2- to 10-fold increase in stimulus intensity evoked a slow excitatory postsynaptic potential (EPSP) which was associated with a small increase in input resistance. The peak amplitude of the EPSP occurred approximately 2.5 s after the stimulus and its magnitude (up to 30 mV) and duration (10-50 s) increased with increasing stimulus intensity. 2. The slow EPSP was unaffected by the metabotropic glutamate receptor antagonist (+)-alpha-methyl-4-carboxyphenylglycine ((+)-MCPG; 1000 microM) but was greatly enhanced by the acetylcholinesterase inhibitor physostigmine (1-5 microM). Both the slow EPSP and the stimulus-evoked reduction in spike frequency adaptation were inhibited by the muscarinic acetylcholine receptor (mAChR) antagonist atropine (1-5 microM). These results are consistent with these effects being mediated by mAChRs. 3. Both the mAChR-mediated EPSP (EPSPm) and the associated reduction in spike frequency adaptation were reversibly depressed (up to 97%) by either adenosine (100 microM) or its non-hydrolysable analogue 2-chloroadenosine (CADO; 0.1-5.0 microM). These effects were often accompanied by postsynaptic hyperpolarization (up to 8 mV) and a reduction in input resistance (up to 11%). The selective adenosine A1 receptor agonists 2-chloro-N6-cyclopentyladenosine (CCPA; 0.1-0.4 microM) and R(-)N6-(2-phenylisopropyl)-adenosine (R-PIA; 1 microM) both depressed the EPSPm. In contrast, the adenosine A2A receptor agonist 2-p-(2-carboxyethyl)-phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680; 0.5-1.0 microM) did not significantly affect the EPSPm. 4. The selective adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 0.2 microM) fully reversed the depressant effects of both adenosine (100 microM) and CADO (1 microM) on the EPSPm and the stimulus-evoked reductions in spike frequency adaptation. 5. DPCPX (0.2 microM) alone caused a small but variable mean increase in the EPSPm of 22 +/- 19% and enabled activation of an EPSPm by a previously subthreshold stimulus. In contrast, the selective adenosine kinase inhibitor 5-iodotubercidin (5-IT; 10 microM) inhibited the EPSPm by 74 +/- 10%, an effect that was reversed by DPCPX. 6. The concentration-response relationship for the depressant action of CADO on the EPSPm more closely paralleled that for its presynaptic depressant action on glutamate-mediated EPSPs than that for postsynaptic hyperpolarization. The respective mean IC50 and EC50 concentrations for these effects were 0.3, 0.8 and 3.0 microM. 7. CADO (1-5 microM) did not have a significant effect on the postsynaptic depolarization, increase in input resistance and reduction in spike frequency adaptation evoked by carbachol (0.5-3.0 microM). All these effects were abolished by atropine (1 microM). 8. These data provide good evidence for an adenosine A1 receptor-mediated inhibition of mAChR-mediated synaptic responses in hippocampal CA1 pyramidal neurones. This inhibition is mediated predominantly presynaptically, is active tonically and can be enhanced when extracellular levels of endogenous adenosine are raised.

G-protein-mediated desensitization of metabotropic glutamatergic and muscarinic responses in CA3 cells in rat hippocampus

1. Desensitization of a metabotropic response was investigated in CA3 pyramidal neurons in hippocampal slice cultures using the patch-clamp technique. 2. 1S,3R-1-aminocyclopentane-1,3-dicarboxylate (1S,3R-ACPD), an agonist at metabotropic glutamate receptors (mGluRs), and metacholine (MCh), an agonist at muscarinic receptors, induced a cationic current that appears to be activated through a G-protein-independent transduction process, as previously shown. Prolonged or repetitive bath application of agonists led to rapid desensitization of the cationic current with a time constant of approximately 20 s. 3. Complete recovery from desensitization was observed within 6 min. 4. These responses mediated by mGluRs and muscarinic receptors cross-desensitized. 5. Preventing the activation of G-proteins by loading cells with GDP beta S strongly reduced or suppressed desensitization, and resulted in a sustained inward cationic current. When cells were filled with GTP gamma S to irreversibly activate G-proteins, the desensitization process was enhanced such that a first application of agonist caused a markedly reduced response. 6. These results show that a cationic current induced by metabotropic agonists in hippocampal pyramidal cells undergoes apparent desensitization and suggests that this process occurs through a G-protein-mediated inhibition of the underlying membrane conductance.

Simulation of gamma rhythms in networks of interneurons and pyramidal cells

Networks of hippocampal interneurons, with pyramidal neurons pharmacologically disconnected, can generate gamma-frequency (20 Hz and above) oscillations. Experiments and models have shown how the network frequency depends on excitation of the interneurons, and on the parameters of GABAA-mediated IPSCs between the interneurons (conductance and time course). Here we use network simulations to investigate how pyramidal cells, connected to the interneurons and to each other through AMPA-type and/or NMDA-type glutamate receptors, might modify the interneuron network oscillation. With or without AMPA-receptor mediated excitation of the interneurons, the pyramidal cells and interneurons fired in phase during the gamma oscillation. Synaptic excitation of the interneurons by pyramidal cells caused them to fire spike doublets or short bursts at gamma frequencies, thereby slowing the population rhythm. Rhythmic synchronized IPSPs allowed the pyramidal cells to encode their mean excitation by their phase of firing relative to the population waves. Recurrent excitation between the pyramidal cells could modify the phase of firing relative to the population waves. Our model suggests that pools of synaptically interconnected inhibitory cells are sufficient to produce gamma frequency rhythms, but the network behavior can be modified by participation of pyramidal cells.

Sequence-independent effects of phosphorothiolated oligonucleotides on synaptic transmission and excitability in the hippocampus in vivo

Antisense oligodeoxynucleotides (ODNs) have the potential to be a powerful tool for regulating gene expression and mRNA translation in spatially and temporally restricted domains. Prior to investigating the effects of antisense ODNs on hippocampal long-term potentiation, we investigated whether there are any non-specific effects of ODNs on perforant path synaptic transmission in the dentate gyrus of both pentobarbital-anaesthetized and awake, freely moving rats. Single injections of phosphorothioated antisense ODNs (4 nmol) to the immediate early gene zif/268 caused a rapid (within minutes) and long-lasting (>24 hr) profound depression of the perforant path evoked field potentials. This depressive effect was due to the phosphorothioate modification since a depression was not seen with unmodified antisense ODNs, relative to saline controls. Furthermore, the effect was not sequence-specific since modified sense ODNs caused the same degree of depression. The depression caused by the modified antisense ODNs was dose-dependent and specific to synaptic transmission, since antidromic population spikes elicited by mossy fibre stimulation were relatively unaffected compared to the orthodromic responses. A second unexpected side-effect of the modified ODNs was cellular hyperexcitability, such that bursts of epileptiform spikes in the EEG occurred both spontaneously and as a result of synaptic stimulation. While the mechanism of the synaptic depression remains unknown, these results indicate that phosphorothioate-modified ODNs exert profound non-specific effects on synaptic transmission in the hippocampus, that have the potential to seriously compromise any corresponding behavioural or electrophysiological studies.

Cholinergic-dependent plateau potential in hippocampal CA1 pyramidal neurons

Cholinergic stimulation of the hippocampal formation results in excitation and/or seizure. We report here, using whole-cell patch-clamp techniques in the hippocampal slice (34-35 degrees C), a cholinergic-dependent slow afterdepolarization (sADP) and long-lasting plateau potential (PP). In the presence of 20 microM carbachol, action potential firing evoked by weak intracellular current injection elicited an sADP that lasted several seconds. Increased spike firing evoked by stronger depolarizing stimuli resulted in long-duration PPs maintained close to -20 mV. Removal of either Na+ or Ca2+ from the external media, intracellular Ca2+ ([Ca2+]i) chelation with 10 mM bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid, or the addition of 100 microM Cd2+ to the perfusate abolished both the sADP and PP. The sADP was depressed and the PP was abolished by either 10 microM nimodipine or 1 microM omega-conotoxin, whereas 1.2 microM tetrodotoxin was ineffective. The involvement of a Na+/Ca2+ exchanger was minimal because both the sADP and PP persisted after equimolar substitution of 50 mM Li+ for Na+ in the external media or reduction of the bath temperature to 25 degrees C. Finally in the absence of carbachol the sADP and PP could not be evoked when K+ channels were suppressed, suggesting that depression of K+ conductances alone was not sufficient to unmask the conductance. Based on these data, we propose that a Ca2+-activated nonselective cation conductance was directly enhanced by muscarinic stimulation. The sADP, therefore, represents activation of this conductance by residual [Ca2+]i, whereas the PP represents a novel regenerative event involving the interplay between high-voltage-activated Ca2+ channels and the Ca2+-activated nonselective cation conductance. This latter mechanism may contribute significantly to ictal depolarizations observed during cholinergic-induced seizures.

Cholinergic-dependent plateau potential in hippocampal CA1 pyramidal neurons

Cholinergic stimulation of the hippocampal formation results in excitation and/or seizure. We report here, using whole-cell patch-clamp techniques in the hippocampal slice (34-35 degrees C), a cholinergic-dependent slow afterdepolarization (sADP) and long-lasting plateau potential (PP). In the presence of 20 microM carbachol, action potential firing evoked by weak intracellular current injection elicited an sADP that lasted several seconds. Increased spike firing evoked by stronger depolarizing stimuli resulted in long-duration PPs maintained close to -20 mV. Removal of either Na+ or Ca2+ from the external media, intracellular Ca2+ ([Ca2+]i) chelation with 10 mM bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid, or the addition of 100 microM Cd2+ to the perfusate abolished both the sADP and PP. The sADP was depressed and the PP was abolished by either 10 microM nimodipine or 1 microM omega-conotoxin, whereas 1.2 microM tetrodotoxin was ineffective. The involvement of a Na+/Ca2+ exchanger was minimal because both the sADP and PP persisted after equimolar substitution of 50 mM Li+ for Na+ in the external media or reduction of the bath temperature to 25 degrees C. Finally in the absence of carbachol the sADP and PP could not be evoked when K+ channels were suppressed, suggesting that depression of K+ conductances alone was not sufficient to unmask the conductance. Based on these data, we propose that a Ca2+-activated nonselective cation conductance was directly enhanced by muscarinic stimulation. The sADP, therefore, represents activation of this conductance by residual [Ca2+]i, whereas the PP represents a novel regenerative event involving the interplay between high-voltage-activated Ca2+ channels and the Ca2+-activated nonselective cation conductance. This latter mechanism may contribute significantly to ictal depolarizations observed during cholinergic-induced seizures.

Is activation of metabotropic glutamate receptors responsible for acute hyposic changes in hippocampal neurons?

In whole-cell recordings from CA1 neurons in slices from rats, the mGLUR agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (ACPD; 10 microM) had a depolarizing action on most cells, associated with an increase in input resistance and suppression of afterhyperpolarizations. Under voltage-clamp, there were corresponding changes in membrane current and conductance; in the presence of ACPD, the slow voltage-dependent outward current recorded at approximately -25 mV was smaller and was more clearly depressed by hypoxia. Neither ACPD nor mGLUR antagonists, L(+)-2-amino-3-phosphonoproprionic acid (L-AP3; 1 mM) and (+)-alpha-methyl-4-carboxyphenyl-glycine (MCPG; 0.5 mM), reduced the hyperpolarization or outward current (or the associated changes in input resistance or conductance) induced by 2 min of hypoxia. Early inward currents, corresponding to the early, transient depolarizing effect of hypoxia, wer also not significantly depressed by either MCPG or L-AP3. The hypoxic responses of CA1 neurons in slices are therefore unlikely to be caused mainly be glutamate release and activation of mGLURs.