Responses to Metabotropic Glutamate Receptor Activation in Cerebellar Purkinje Cells: Induction of an Inward Current

Metabotropic glutamate receptors are involved in the detection of IMP and L-amino acids by mouse taste sensory cells

G-protein-coupled receptors are thought to be involved in the detection of umami and L-amino acid taste. These include the heterodimer taste receptor type 1 member 1 (T1r1)+taste receptor type 1 member 3 (T1r3), taste and brain variants of mGluR4 and mGluR1, and calcium sensors. While several studies suggest T1r1+T1r3 is a broadly tuned lLamino acid receptor, little is known about the function of metabotropic glutamate receptors (mGluRs) in L-amino acid taste transduction. Calcium imaging of isolated taste sensory cells (TSCs) of T1r3-GFP and T1r3 knock-out (T1r3 KO) mice was performed using the ratiometric dye Fura 2 AM to investigate the role of different mGluRs in detecting various L-amino acids and inosine 5' monophosphate (IMP). Using agonists selective for various mGluRs such as (RS)-3,5-dihydroxyphenylglycine (DHPG) (an mGluR1 agonist) and L-(+)-2-amino-4-phosphonobutyric acid (l-AP4) (an mGluR4 agonist), we evaluated TSCs to determine if they might respond to these agonists, IMP, and three L-amino acids (monopotassium L-glutamate, L-serine and L-arginine). Additionally, we used selective antagonists against different mGluRs such as (RS)-L-aminoindan-1,5-dicarboxylic acid (AIDA) (an mGluR1 antagonist), and (RS)-α-methylserine-O-phosphate (MSOP) (an mGluR4 antagonist) to determine if they can block responses elicited by these L-amino acids and IMP. We found that L-amino acid- and IMP-responsive cells also responded to each agonist. Antagonists for mGluR4 and mGluR1 significantly blocked the responses elicited by IMP and each of the L-amino acids. Collectively, these data provide evidence for the involvement of taste and brain variants of mGluR1 and mGluR4 in L-amino acid and IMP taste responses in mice, and support the concept that multiple receptors contribute to IMP and L-amino acid taste.

Contributions of T-type voltage-gated calcium channels to postsynaptic calcium signaling within Purkinje neurons

Low threshold voltage-gated T-type calcium channels have long been implicated in the electrical excitability and calcium signaling of cerebellar Purkinje neurons although the molecular composition, localization, and modulation of T-type channels within Purkinje cells have only recently been addressed. The specific functional roles that T-type channels play in local synaptic integration within Purkinje spines are also currently being unraveled. Overall, Purkinje neurons represent a powerful model system to explore the potential roles of postsynaptic T-type channels throughout the nervous system. In this review, we present an overview of T-type calcium channel biophysical, pharmacological, and physiological characteristics that provides a foundation for understanding T-type channels within Purkinje neurons. We also describe the biophysical properties of T-type channels in context of other voltage-gated calcium channel currents found within Purkinje cells. The data thus far suggest that one specific T-type isoform, Ca_v3.1, is highly expressed within Purkinje spines and both physically and functionally couples to mGluR1 and other effectors within putative signaling microdomains. Finally, we discuss how the selective potentiation of Ca_v3.1 channels via activation of mGluR1 by parallel fiber inputs affects local synaptic integration and how this interaction may relate to the overall excitability of Purkinje neuron dendrites.

Pharmacological evidence of functional inhibitory metabotrophic glutamate receptors on mouse arousal-related cholinergic laterodorsal tegmental neurons

Cholinergic neurons of the pontine laterodorsal tegmentum (LDT) are importantly involved in neurobiological mechanisms governing states of arousal such as sleep and wakefulness as well as other appetitive behaviors, such as drug-seeking. Accordingly, mechanisms controlling their excitability are important to elucidate if we are to understand how these LDT neurons generate arousal states. Glutamate mediates the vast majority of excitatory synaptic transmission in the vertebrate CNS and while presence of glutamate input in the LDT has been shown and ionotropic responses to glutamate have been reported in the LDT, characterization of metabotropic responses is lacking. Therefore, electrophysiological responses and changes in levels of intracellular Ca(2+) in mouse cholinergic LDT neurons following application of specific mGluR agonists and antagonists were examined. Unexpectedly, both the mGluR(5)specific agonist, CHPG, and the group II mGluR (mGlu(2/3)) agonist, LY379268 (LY), induced a TTX-insensitive outward current/hyperpolarization. Both outward currents were significantly reduced by the mGluR antagonist MCPG and the CHPG-induced current was blocked by the specific mGluR(5) antagonist MTEP. Concurrent Ca(2+)imaging revealed that while CHPG actions did include release of Ca(2+) from CPA/thapsigargin-sensitive intracellular stores, actions of LY did not. Both CHPG- and LY-induced outward currents were mediated by a TEA-sensitive potassium conductance. The large-conductance, Ca(2+)-dependent potassium (BK) channel blocker, iberiotoxin, attenuated CHPG actions. Consistent with actions on the BK conductance, CHPG enhanced the amplitude of the fast component of the after hyperpolarizing potential, concurrent with a reduction in the firing rate. We conclude that stimulation of mGluR(5) and group II (mGluR(2/3)) elicits postsynaptically-mediated outward currents/hyperpolarizations in cholinergic LDT neurons. Effects of glutamatergic input would be, thus, expected not only to be excitation via stimulation of ionotropic glutamate receptors and mGluR(1), but also inhibition via actions at mGluR(5) and mGluR(2/3) on these neurons. As these two processes counteract each other, these surprising findings necessitate revision of predictions regarding the net level of excitation generated by glutamate input to cholinergic LDT cells and, by extension, the functional outcome of glutamate transmission on processes which these neurons regulate. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Disruption of metabotropic glutamate receptor signalling is a major defect at cerebellar parallel fibre-Purkinje cell synapses in staggerer mutant mice

Staggerer mutant mice have functional loss of a transcription factor, retinoid-related orphan receptor α (RORα), which is abundantly expressed in Purkinje cells (PCs) of the cerebellum.Homozygous staggerer (sg/sg)mice show cerebellar hypoplasia and congenital ataxia. Sg/sg mice serve as an important extreme mouse model of the hereditary spinocerebellar ataxia type 1 (SCA1), since it has been shown that RORα dysfunction is strongly correlated with SCA1 pathogenesis. However, synaptic abnormalities, especially at parallel fibre (PF)-PC synapses, in SCA1-related sg/sg mice have not been examined in detail electrophysiologically. In this study, we report that PFs can still establish functional synapses onto PCs in sg/sg mice in spite of reduction in the number of PF-PC synapses. Compared with PF-evoked EPSCs in the wild-type or heterozygotes, the success rate of the EPSC recordings in sg/sg was quite low (∼40%) and the EPSCs showed faster kinetics and slightly decreased paired pulse facilitation at short intervals. The prominent synaptic dysfunction is that sg/sg mice lack metabotropic glutamate receptor (mGluR)-mediated slow EPSCs completely. Neither intense PF stimulation nor an exogenously applied mGluR agonist, DHPG, could elicit mGluR-mediated responses.Western blot analysis in the sg/sg cerebellum revealed low-level expression of mGluR1 and TRPC3, both of which underlie mGluR-mediated slow currents in PCs. Immunohistochemical data demonstrated marked mislocalization of mGluR1 on sg/sg PCs.We found that mGluR-mediated retrograde suppression of PF-PC EPSCs by endocannabinoid is also impaired completely in sg/sg mice. These results suggest that disruption of mGluR signalling at PF-PC synapses is one of the major synaptic defects in sg/sg mice and may manifest itself in SCA1 pathology.

Group I mGluR activation enhances Ca(2+)-dependent nonselective cation currents and rhythmic bursting in main olfactory bulb external tufted cells

In the main olfactory bulb, activation of group I metabotropic glutamate receptors (mGluRs) by olfactory nerve stimulation generates slow (2 Hz) oscillations near the basal respiratory frequency. These oscillations arise in the glomerular layer and may be generated, in part, by the intrinsic neurons, the juxtaglomerular neurons. We investigated the physiological effects of group I mGluR agonists on one population of juxtaglomerular neurons, external tufted (ET) cells, which rhythmically burst at respiratory frequencies and synchronize the intraglomerular network. Electrophysiological studies in rat main olfactory bulb slices demonstrated that the mGluR agonist 3,4-dihydroxyphenylglycine (DHPG) amplified the strength of ET cell spike bursts, principally by increasing the number of spikes per burst. Voltage-clamp and Ca(2+)-imaging studies showed that DHPG elicits a Ca(2+)-dependent nonselective cation current (I(CAN)) in the dendrites of ET cells triggered by Ca(2+) release from internal stores. The DHPG effects on bursting and membrane current were attenuated by flufenamic acid and SKF96365, agents known to antagonize I(CAN) in a variety of neurons. DHPG also elicited slow membrane current oscillations and spikelets in ET cells when synaptic transmission and intrinsic membrane channels were inoperative. These findings indicate that DHPG may passively (by increasing burst strength) or actively (by increasing conductance of gap junctions) enhance the strength of electrical synapses between ET cells. Together, these findings indicate that activation of group I mGluRs on the dendrites of ET cells play a key role in the generation of slow rhythmic oscillation in the glomerular network, which is in turn tuned to sniffing of the animal in vivo.

Functional coupling between mGluR1 and Cav3.1 T-type calcium channels contributes to parallel fiber-induced fast calcium signaling within Purkinje cell dendritic spines

T-type voltage-gated calcium channels are expressed in the dendrites of many neurons, although their functional interactions with postsynaptic receptors and contributions to synaptic signaling are not well understood. We combine electrophysiological and ultrafast two-photon calcium imaging to demonstrate that mGluR1 activation potentiates cerebellar Purkinje cell Ca(v)3.1 T-type currents via a G-protein- and tyrosine-phosphatase-dependent pathway. Immunohistochemical and electron microscopic investigations on wild-type and Ca(v)3.1 gene knock-out animals show that Ca(v)3.1 T-type channels are preferentially expressed in Purkinje cell dendritic spines and colocalize with mGluR1s. We further demonstrate that parallel fiber stimulation induces fast subthreshold calcium signaling in dendritic spines and that the synaptic Ca(v)3.1-mediated calcium transients are potentiated by mGluR1 selectively during bursts of excitatory parallel fiber inputs. Our data identify a new fast calcium signaling pathway in Purkinje cell dendritic spines triggered by short burst of parallel fiber inputs and mediated by T-type calcium channels and mGluR1s.

Role of the metabotropic glutamate receptor subtype 1 in the harmaline-induced tremor in rats

The present study shows that JNJ 16259685-a selective antagonist of glutamate metabotropic receptor subtype 1 (mGluR1) injected in doses of 0.16 and 0.32 mg/kg sc strongly enhances tremor of forelimbs, head and trunk, hypolocomotion, and ataxia induced by harmaline (7.5 mg/kg ip) in rats. JNJ 16259685 inhibited locomotor and exploratory activity per se. These results may suggest an inhibitory influence of mGluR1 on the harmaline-induced motor disturbances.

Metabotropic glutamate 1 receptor: current concepts and perspectives

Almost 25 years after the first report that glutamate can activate receptors coupled to heterotrimeric G-proteins, tremendous progress has been made in the field of metabotropic glutamate receptors. Now, eight members of this family of glutamate receptors, encoded by eight different genes that share distinctive structural features have been identified. The first cloned receptor, the metabotropic glutamate (mGlu) receptor mGlu1 has probably been the most extensively studied mGlu receptor, and in many respects it represents a prototypical subtype for this family of receptors. Its biochemical, anatomical, physiological, and pharmacological characteristics have been intensely investigated. Together with subtype 5, mGlu1 receptors constitute a subgroup of receptors that couple to phospholipase C and mobilize Ca(2+) from intracellular stores. Several alternatively spliced variants of mGlu1 receptors, which differ primarily in the length of their C-terminal domain and anatomical localization, have been reported. Use of a number of genetic approaches and the recent development of selective antagonists have provided a means for clarifying the role played by this receptor in a number of neuronal systems. In this article we discuss recent advancements in the pharmacology and concepts about the intracellular transduction and pathophysiological role of mGlu1 receptors and review earlier data in view of these novel findings. The impact that this new and better understanding of the specific role of these receptors may have on novel treatment strategies for a variety of neurological and psychiatric disorders is considered.

TRPC3 channels are required for synaptic transmission and motor coordination

In the mammalian central nervous system, slow synaptic excitation involves the activation of metabotropic glutamate receptors (mGluRs). It has been proposed that C1-type transient receptor potential (TRPC1) channels underlie this synaptic excitation, but our analysis of TRPC1-deficient mice does not support this hypothesis. Here, we show unambiguously that it is TRPC3 that is needed for mGluR-dependent synaptic signaling in mouse cerebellar Purkinje cells. TRPC3 is the most abundantly expressed TRPC subunit in Purkinje cells. In mutant mice lacking TRPC3, both slow synaptic potentials and mGluR-mediated inward currents are completely absent, while the synaptically mediated Ca2+ release signals from intracellular stores are unchanged. Importantly, TRPC3 knockout mice exhibit an impaired walking behavior. Taken together, our results establish TRPC3 as a new type of postsynaptic channel that mediates mGluR-dependent synaptic transmission in cerebellar Purkinje cells and is crucial for motor coordination.

mGluR1 agonists elicit a Ca 2+ signal and membrane hyperpolarization mediated by apamin-sensitive potassium channels in immature rat purkinje neurons

The type 1 metabotropic glutamate receptor (mGluR1) plays an import role in the synaptic physiology and development of cerebellar Purkinje neurons. mGluR1 expression occurs early in the developmental program of Purkinje neurons, at an age that precedes expression of the dendritic structure. Few studies have investigated the physiological response produced by mGluR1 activation in early-developing Purkinje neurons. To address this question, simultaneous recording of membrane potential and intracellular Ca(2+) was performed in immature cultured Purkinje neurons coupled with exogenous application of mGluR1 agonists. Membrane potential was measured using the perforated patch method of whole-cell recording, and intracellular Ca(2+) was measured using fura-2-based Ca(2+) imaging. Brief, 1-sec micropressure application of the group I mGluR-selective agonist (S)-3,5-dihydroxyphenylglycine (DHPG) evoked a prominent Ca(2+) signal and coincident fast hyperpolarization in the immature neurons. The mGluR1-selective antagonist 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester blocked the Ca(2+) signal and fast hyperpolarization, confirming the involvement of mGluR1s. Amplitude of the fast hyperpolarization varied as a function of membrane potential and intracellular Ca(2+) and was blocked by apamin, an antagonist of the small-conductance Ca(2+)-activated K(+) channel (SK), identifying this K(+) channel as an underlying mechanism. In similar experiments with mature cultured Purkinje neurons, DHPG elicited a Ca(2+) signal, but fast membrane hyperpolarization was not evident. These results suggest that mGluR1 activation and the resulting release of Ca(2+) from intracellular stores and activation of SK channels may be a mechanism through which mGluR1 can modulate neuronal excitability of Purkinje neurons during early development.

Climbing fiber-triggered metabotropic slow potentials enhance dendritic calcium transients and simple spike firing in cerebellar Purkinje cells

Cerebellar Purkinje cells (PCs) receive synaptic input from numerous parallel fibers (PFs) and from a single climbing fiber (CF). At both types of synapses, fast synaptic transmission is mediated by AMPA receptors, while at PF synapses burst activity can additionally recruit metabotropic glutamate receptors (mGluRs) that mediate a slow depolarizing potential. Here, we show that mGluR-activated slow potentials can be evoked throughout the dendrite by CF-evoked complex spike firing in the presence of an mGluR agonist. The CF-triggered mGluR potential was not only blocked by an mGluR antagonist but also when the CF-induced Ca(2+) transient was blocked by an AMPA receptor antagonist, suggesting the possibility that the slow potential can be activated by the simultaneous occurrence of agonist binding at mGluRs and a CF-evoked Ca(2+) transient. In turn, these CF-triggered slow mGluR potentials enhance the complex spike-associated calcium signals throughout the dendrite. Moreover, they provide a mechanism by which CFs can modulate the simple spike frequency of PCs.

Two new non-competitive mGlu1 receptor antagonists are potent tools to unravel functions of this mGlu receptor subtype

The validation of the selective, potent and systemically active non-competitive mGlu1 antagonists YM-298198 and JNJ16259685 in a physiological functional assay will facilitate elucidation of this receptor's role in brain function and as a potential drug target.

Involvement of Na+-Ca2+ exchanger on metabotropic glutamate receptor 1-mediated [Ca2+]i transients in rat cerebellar Purkinje neurons

Cerebellar Purkinje neurons have intracellular regulatory systems including Ca2+-binding proteins, intracellular Ca2+ stores, Ca2+-ATPase and Na+-Ca2+ exchanger (NCX) that keep intracellular Ca2+ concentration ([Ca2+]i) in physiological range. Among these, NCX interacts with AMPA receptors, activation of which induces cerebellar synaptic plasticity. And the activation of metabotropic glutamate receptor 1 (mGluR1) is also involved in the induction of cerebellar long-term depression. The interaction of NCX with mGluR1 is not known yet. Thus, in this study, the functional relationship between NCX and mGluR1 in modulating the [Ca2+]i in rat Purkinje neurons was investigated. The interaction between NCX and mGluR1 in Purkinje neurons was studied by measuring intracellular Ca2+ transients induced by an agonist of group I mGluRs, 3,5-dihydroxyphenylglycine (DHPG). The DHPG-induced Ca2+ transient was significantly reduced by treatments of NCX inhibitors, bepridil and KB-R7943. When cells were pretreated with antisense oligodeoxynucleotides of NCX, the DHPG-induced Ca2+ transient was also inhibited. These results suggest that NCX modulates the activity of mGluR1 in cerebellar Purkinje neurons. Therefore, NCX appears to play an important role in the physiological function of cerebellar Purkinje neurons such as synaptic plasticity.

Group I metabotropic glutamate receptors are differentially expressed by two populations of olfactory bulb granule cells

In the main olfactory bulb, several populations of granule cells (GCs) can be distinguished based on the soma location either superficially, interspersed with mitral cells within the mitral cell layer (MCL), or deeper, within the GC layer (GCL). Little is known about the physiological properties of superficial GCs (sGCs) versus deep GCs (dGCs). Here, we used patch-clamp recording methods to explore the role of Group I metabotropic glutamate receptors (mGluRs) in regulating the activity of GCs in slices from wildtype and mGluR-/- mutant mice. In wildtype mice, bath application of the selective Group I mGluR agonist DHPG depolarized and increased the firing rate of both GC subtypes. In the presence of blockers of fast synaptic transmission (APV, CNQX, gabazine), DHPG directly depolarized both GC subtypes, although the two GC subtypes responded differentially to DHPG in mGluR1-/- and mGluR5-/- mice. DHPG depolarized sGCs in slices from mGluR5-/- mice, although it had no effect on sGCs in slices from mGluR1-/- mice. By contrast, DHPG depolarized dGCs in slices from mGluR1-/- mice but had no effect on dGCs in slices from mGluR5-/- mice. Previous studies showed that mitral cells express mGluR1 but not mGluR5. The present results therefore suggest that sGCs are more similar to mitral cells than dGCs in terms of mGluR expression.

Metabotropic glutamate receptors in the main olfactory bulb drive granule cell-mediated inhibition

Main olfactory bulb (MOB) granule cells (GCs) express high levels of the group I metabotropic glutamate receptor (mGluR), mGluR5. We investigated the role of mGluRs in regulating GC activity in rodent MOB slices using whole cell patch-clamp electrophysiology. The group I/II mGluR agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (ACPD) or the selective group I agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) depolarized ( approximately 20 mV) and increased the firing rate of GCs. In the presence of ionotropic glutamate and GABA receptor antagonists, DHPG evoked a more modest depolarization ( approximately 8 mV). In voltage clamp, DHPG, but not group II [(2S,2'R,3)-2-(2',3'-dicarboxycyclopropyl)glycine, DCG-IV] or group III [L(+)-2-amino-4-phosphonobutyric acid, L-AP4] mGluR agonists, induced an inward current. The inward current reversed polarity near the potassium equilibrium potential, suggesting mediation by closure of potassium channels. The DHPG-evoked inward current was unaffected by the mGluR1 antagonist (S)-(+)-alpha-amino-4-carboxy-2-methylbenzeneacetic acid (LY367385), was blocked by the group I/II mGluR antagonist (alphaS)-alpha-amino-alpha-[(1S,2S)-2-carboxycyclopropyl]-9H-xanthine-9-propanoic acid (LY341495), and was absent in GCs from mGluR5 knockout mice. LY341495 also attenuated mitral cell-evoked voltage-sensitive dye signals in the external plexiform layer and mitral cell-evoked spikes in GCs. These results suggest that activation of mGluR5 increases GC excitability, an effect that should increase GC-mediated GABAergic inhibition of mitral cells. In support of this: DHPG increased the frequency of spontaneous GABAergic inhibitory postsynaptic currents in mitral cells and LY341495 attenuated the feedback GABAergic postsynaptic potential elicited by intracellular depolarization of mitral cells. Our results suggest that activation of mGluR5 participates in feedforward and/or feedback inhibition at mitral cell to GC dendrodendritic synapses, possibly to modulate lateral inhibition and contrast in the MOB.

Olfactory nerve stimulation-evoked mGluR1 slow potentials, oscillations, and calcium signaling in mouse olfactory bulb mitral cells

Fast synaptic transmission between olfactory receptor neurons and mitral cells (MCs) is mediated through AMPA and NMDA ionotropic glutamate receptors. MCs also express high levels of metabotropic glutamate receptor 1 (mGluR1) whose functional significance is less understood. Here we characterized a slow mGluR1-mediated potential that was evoked by high-frequency (100-Hz) olfactory nerve (ON) stimulation in the presence of NBQX and D-APV, blockers of ionotropic glutamate receptors, and that was associated with a local Ca2+ transient in the MC dendritic tuft. High-frequency ON stimulation in the presence of NBQX and D-APV also evoked a slow, nearly 2-Hz oscillation of MC membrane potential that was abolished by the mGluR1 antagonist LY367385 (50 microM). Both mGluR slow potential and slow oscillation persisted in the presence of gabazine (10 microM), a GABA(A) receptor antagonist, and intracellular QX-314 (10 mM), a Na+ channel blocker. In contrast to a slow mGluR1 potential in cerebellar Purkinje neurons, the MC mGluR1 potential was not depressed by SKF96365 (< or =250 microM) and thus is likely not mediated by TRPC1 cation channels, nor was it potentiated by an elevation of intracellular Ca2+ level. Imaging with the Na+ indicator SBFI revealed a Na+ transient in the MC dendrite accompanying the mGluR1 slow potential. We conclude that the MC mGluR1 potential triggered by glutamate released from the ON supports oscillations and synchronizations of MCs associated within one glomerulus.

Group I Metabotropic Glutamate Receptors on Second-Order Baroreceptor Neurons Are Tonically Activated and Induce a Na+–Ca2+Exchange Current

The nucleus tractus solitarius (NTS) is essential for coordinating baroreflex control of blood pressure. The baroreceptor sensory fibers make glutamatergic synapses onto second-order NTS neurons. Glutamate spillover activates Group II and III presynaptic metabotropic glutamate receptors (mGluRs) on the baroreceptor central terminals to inhibit synaptic transmission, but the role of postsynaptic mGluRs is less understood. We used whole cell patch-clamping in anatomically identified second-order baroreceptor neurons in a brain stem slice to test whether Group I, II, and III mGluRs had postsynaptic effects at this first central synapse in the baroreceptor afferent pathway. The Group I agonist DHPG induced a depolarization and spiking that was mimicked by endogenous glutamate. Group I mGluR blockade prevented the depolarization and slightly hyperpolarized the neurons, suggesting a small tonic Group I mGluR activation. The DHPG-induced inward current consisted of voltage-dependent and -independent components; the former was blocked by TEA and the latter was blocked by replacing extracellular NaCl with LiCl or Tris-HCl. The DHPG current was potentiated in a Ca2+-free external solution and was diminished by intracellular dialysis with BAPTA and by perfusion with Na+–Ca2+exchanger blockers, KB-R7943 or 3′,4′-dichlorobenzamil. Intracellular dialysis with GDPβS or heparin and perfusion with the PLC inhibitor U-73122 or the Ca2+-calmodulin inhibitor W-7 significantly decreased the DHPG current. The data suggest that Group I mGluRs on baroreceptor neurons are functional; are activated by endogenous glutamate; and activate a Na+–Ca2+exchanger through G-protein, PLC, IP3, and Ca2+-calmodulin mechanisms to excite the cell, thus providing postsynaptic mechanisms to enhance or prolong baroreceptor signal transmission.

Regulation of main olfactory bulb mitral cell excitability by metabotropic glutamate receptor mGluR1

In the rodent main olfactory bulb (MOB), mitral cells (MCs) express high levels of the group I metabotropic glutamate receptor (mGluR) subtype, mGluR1. The significance of this receptor in modulating MC excitability is unknown. We investigated the physiological role of mGluR1 in regulating MC activity in rat and mouse MOB slices. The selective group I agonist (RS)-3,5-dihydroxyphenylglycine (DHPG), but not group II or III agonists, induced potent, dose-dependent, and reversible depolarization and increased firing of MCs. These effects persisted in the presence of blockers of fast synaptic transmission, indicating that they are due to direct activation of mGluRs on MCs. Voltage-clamp recordings showed that DHPG elicited a voltage-dependent inward current consisting of multiple components sensitive to potassium and calcium channel blockade and intracellular calcium chelation. MC excitatory responses to DHPG were absent in mGluR1 knockout mice but persisted in mGluR5 knockout mice. Broad-spectrum LY341495, MCPG, as well as preferential mGluR1 LY367385 antagonists blocked the excitatory effects of DHPG and also potently modulated MC spontaneous and olfactory nerve-evoked excitability. mGluR antagonists altered spontaneous membrane potential bistability, increasing the duration of the up and down states. mGluR antagonists also substantially attenuated MC responses to sensory input, decreasing the probability and increasing the latency of olfactory nerve-evoked spikes. These findings suggest that endogenous glutamate tonically modulates MC excitability and responsiveness to olfactory nerve input, and hence the operation of the MOB circuitry, via activation of mGluR1.

Group I metabotropic glutamate receptor actions in oriens/alveus interneurons of rat hippocampal CA1 region

Group I metabotropic glutamate receptors (mGluRs) are important for hippocampal interneuron function. We used whole-cell recording and confocal imaging to characterize group I mGluR actions in CA1 oriens/alveus interneurons in slices. In tetrodotoxin and ionotropic glutamate receptor antagonists, the group I mGluR specific agonist DHPG increased intradendritic Ca(2+) levels and depolarized interneurons, whereas the group II mGluR specific agonist DCG-IV and the group III mGluR specific agonist L-AP4 did not. DHPG-induced depolarizing and Ca(2+) responses were antagonized by the group I mGluR antagonist 4CPG, but only Ca(2+) responses were significantly inhibited by the mGluR1 antagonist CPCCOEt. DHPG-induced depolarizing responses were not blocked by the inositol-1,4,5-trisphosphate (IP(3)) receptor inhibitor heparin, the protein kinase C (PKC) antagonists GF-109203X, or the inhibitor of phospholipase C (PLC) U73122. Thus, these responses to DHPG may not be transduced by the PLC-->IP(3)/diacylglycerol (DAG) pathway classically linked to group I mGluRs. DHPG-induced depolarizations were not blocked by intracellular GDP beta S or bath-application of N-ethylmaleimide (NEM), suggesting the involvement of a G protein-independent pathway. Our findings indicate that group I mGluRs induce a depolarization of oriens/alveus interneurons via a G protein-independent mechanism different from their classic signalling pathway. Since depolarizations are associated with intracellular Ca(2+) rises, these actions may be important for their synaptic plasticity and vulnerability to excitotoxicity.

Involvement of transient receptor potential-like channels in responses to mGluR-I activation in midbrain dopamine neurons

We investigated the involvement of store-operated channels (SOCs) and transient receptor potential (TRP) channels in the response to activation of the group I metabotropic glutamate receptor subtype 1 (mGluR1) with the agonist (S)-3,5-dihydroxyphenylglycine (DHPG, puff application) in dopamine neurons in rat brain slices. The mGluR1-induced conductance reversed polarity close to 0 mV and at more positive potentials when extracellular potassium concentrations were increased, indicating the involvement of a cationic channel. DHPG currents but not intracellular calcium responses were reduced by low extracellular sodium concentrations but were not affected by sodium channel blockers, tetrodotoxin and saxitoxin or by inhibition of the h-current with cesium. Abolition of calcium responses with intracellular BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; 10 mm) did not affect current responses, indicating they were not calcium activated. Extracellular application of non-selective SOCs and TRP channel blockers 2-aminoethoxydiphenylborane (2-APB), SKF96365, ruthenium red and flufenamic acid (but not gadolinium) reduced DHPG current and calcium responses. Intracellular application of ruthenium red and 2-APB did not affect DHPG currents, indicating that IP3 and ryanodine receptors did not mediate their actions. Single-cell PCR revealed the presence of TRPC1 and 5 mRNA in most dopamine neurons and subtypes 3, 4 and 6 in some. Store depletion evoked calcium entry indicative of SOCs, providing the first functional observation of such channels in native central neurons. Store depletion with either cyclopiazonic acid or ryanodine abolished calcium but not current responses to DHPG. The electrophysiological and pharmacological properties of the mGluR1-induced inward current are consistent with the involvement of TRP channels whereas calcium responses are dependent on the function of SOCs in voltage clamp recordings.

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.

Climbing fiber activation of metabotropic glutamate receptors on cerebellar purkinje neurons

In the cerebellum, metabotropic glutamate receptors (mGluRs) are required for distinct forms of synaptic plasticity expressed at parallel fiber (PF) and climbing fiber (CF) synapses. At PF synapses, mGluR activation generates a slow synaptic current and triggers intracellular calcium release; at CF synapses, mGluR activation has not been observed. This has led some investigators to propose that mGluR-dependent changes in CF synaptic strength are induced heterosynaptically. Here we describe an mGluR-mediated response to CF stimulation consisting of two parallel signaling pathways: one leading to a slow synaptic conductance and the other leading to internal calcium release. This additional target for glutamate broadens the signaling capabilities of CF synapses and raises the possibility that changes in CF strength are homosynaptically triggered.

The making of a complex spike: ionic composition and plasticity

Climbing fiber (CF) activation evokes a large all-or-nothing electrical response in Purkinje cells (PCs), the complex spike. It has been suggested that the role of CFs (and thus complex spikes) is that of a "teacher" in simple learning paradigms such as associative eyeblink conditioning. An alternative hypothesis describes the olivocerebellar system as part of a timing device and denies a role of the CF input in learning. To date, neither of these hypotheses nor others can definitively be verified or discounted. Similarly, the complex spike evades a clear understanding when it comes to the cellular events underlying complex spike generation. What is known, however, is that complex spikes are associated with large dendritic calcium signals that are required for the induction of long-term depression (LTD) at the parallel fiber (PF)-PC synapse. PF-LTD is a form of long-term synaptic plasticity that has been suggested to underlie certain forms of cerebellar motor learning. In contrast to the PF input, the CF input has been considered invariant. Our recent discovery of LTD at the CF input shows that complex spikes are less static than previously assumed. In addition to depression of CF-evoked excitatory postsynaptic currents, long-lasting, selective reduction of slow complex spike components could be observed after brief CF tetanization. To understand the functional implications of CF-LTD, it is crucial to know the types of currents constituting the specific complex spike components. Here we review the "anatomy" of the complex spike as well as our observations of activity-dependent complex spike waveform modifications. In addition, we discuss which properties CF-LTD might add to the circuitry of the cerebellar cortex.

Metabotropic glutamate receptors: electrical and chemical signaling properties

Over the last two decades, glutamate has been established as the main excitatory neurotransmitter in the mammalian brain. Glutamate released from synapses activates ion channel-forming receptors at postsynaptic cells and consequently mediates fast postsynaptic potentials. These receptors are termed ionotropic glutamate receptors (iGluRs). The subsequent discovery of metabotropic glutamate receptors (mGluRs) revealed that glutamate can also mediate slow synaptic potentials, modulate ion channels, and directly couple to GTP binding proteins. In contrast to the iGluRs, the mGluRs possess seven transmembrane domains and a large intracellular C-terminus that involves interactions with a variety of other intracellular signaling systems. Eight functionally distinct mGluR subtypes are known to be localized to specific neuron types at presynaptic and/or postsynaptic membranes. Their physiological functions involve the generation of slow excitatory and inhibitory synaptic potentials, modulation of synaptic transmission, synaptic integration, and plasticity. The classical role of glutamate as a fast excitatory synaptic transmitter was largely extended by mGluRs acting as a neuromodulator and even as an activator of inhibitory mechanisms at certain synapses.

Parallel fiber plasticity

Cerebellar long-term depression (LTD) is classically observed when climbing fibers, originating from the inferior olive, and parallel fibers, axons of granule cells, are activated repetitively and synchronously. On the basis that the climbing fiber signals errors in motor performance, LTD provides a mechanism of learning whereby inappropriate motor signals, relayed to the cerebellar cortex by parallel fibers, are selectively weakened through their repeated, close temporal association with climbing fiber activity. LTD therefore provides a cellular substrate for error-driven motor learning in the cerebellar cortex. In recent years, it has become apparent that depression at this synapse can also occur without the need for concurrent climbing fiber activation provided the parallel fibers are activated in such a way as to mobilize calcium within the Purkinje cell. A form of long-term potentiation (LTP) has also been uncovered at this synapse, which similarly relies only upon parallel fiber activation. In brain slice preparations and contrary to expectation, each of these forms of parallel fiber induced plasticity, as well as classical LTD, does not remain confined to activated parallel fibers as previously thought, but both depression and potentiation have the capacity to spread to neighboring parallel fiber synapses several tens of microns away from the activated fibers. Here, the cellular mechanisms responsible for the induction and heterosynaptic spread of parallel fiber LTP and LTD are compared to those involved in classical LTD and the physiological implications that the heterosynaptic spread of plasticity may have on cerebellar signal processing are discussed.

Metabotropic glutamate receptors in the cerebellum with a focus on their function in Purkinje cells

Metabotropic glutamate receptors (mGluRs) are a family of proteins that have seven transmembrane segments and that couple to G proteins. They differ from ionotropic glutamate receptors in that they do not form ion channels but instead affect intracellular chemical messenger systems. Eight genes coding for different subtypes of mGluRs have been identified to date and numbered accordingly in the order in which the cDNAs were cloned. Based on their principal signal-transduction capabilities in recombinant expression systems and sequence similarities, the family of mGluR subtypes is subdivided into three groups. Group 1 mGluRs (consisting of mGluR1 and 5) functionally couple to phospholipase C and affect the IP3/Ca2+ signaling pathway. The subtypes of group 2 (mGluR2 and 3) and group 3 (mGluR4, 6 7 and 8) inhibit adenylate cyclase and, thereby, mediate a decrease in cAMP concentration. All mGluR subtypes are found in the cerebellar cortex with the exception of mGluR6 which is exclusively expressed in the retina. At the parallel fiber-Purkinje cell synapses mGluR1 is localized in the peri- and extra-synaptic membrane of Purkinje cells. The main focus of this review deals with the functions of this postsynaptically localized mGluR1. These functions include (i) mediation of an inward current and a slow excitatory postsynaptic potential, and (ii) a role in induction of parallel fiber-Purkinje cell long-term depression. We discuss the mechanism underlying the mGluR1-mediated postsynaptic current as well as current theories on the role of mGluR1 in parallel fiber-Purkinje cell long-term depression.

GABA(B) receptor activation enhances mGluR-mediated responses at cerebellar excitatory synapses

Metabotropic gamma-aminobutyric acid type B (GABAB) and glutamate receptors (mGluRs) are postsynaptically co-expressed at cerebellar parallel fiber (PF)-Purkinje cell (PC) excitatory synapses, but their functional interactions are unclear. We found that mGluR1 agonist-induced currents and [Ca2+]i increases in PCs were enhanced following co-activation of GABAB receptors. A GABAB antagonist and a G-protein uncoupler suppressed these effects. Low-concentration baclofen, a GABAB agonist, augmented mGluR1-mediated excitatory synaptic current produced by stimulating PFs. These results indicate that postsynaptic GABAB receptors functionally interact with mGluR1 and enhance mGluR1-mediated excitatory transmission at PF-PC synapses. The interaction between the two types of metabotropic receptors provides a likely mechanism for regulating cerebellar synaptic plasticity.

Retrograde modulation of transmitter release by postsynaptic subtype 1 metabotropic glutamate receptors in the rat cerebellum

1. The aim of the study was to elucidate the mechanisms underlying the depressant effect of the group I/II metabotropic glutamate receptor (mGluR) agonist 1S,3R-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) on parallel fibre (PF) to Purkinje cell (PC) synaptic transmission. Experiments were performed in rat cerebellar slices using the whole-cell patch-clamp technique and fluorometric measurements of presynaptic calcium variation 2. Analysis of short-term plasticity, fluctuation of EPSC amplitude and responses of PCs to exogenous glutamate showed that depression caused by 1S,3R-ACPD is presynaptic. 3. The effects of 1S,3R-ACPD were blocked and reproduced by group I mGluR antagonists and agonists, respectively. 4. These effects remained unchanged in mGluR5 knock-out mice and disappeared in mGluR1 knock-out mice. 5. 1S,3R-ACPD increased calcium concentration in PFs. This effect was abolished by AMPA/kainate (but not NMDA) receptor antagonists and mimicked by focally applied agonists of these receptors. Thus, it is not directly due to mGluRs but to presynaptic AMPA/kainate receptors indirectly activated by 1S,3R-ACPD. 6. Frequencies of spontaneous and evoked unitary EPSCs recorded in PCs were respectively increased and decreased by mGluR1 agonists. Similar results were obtained when mGluR1s were activated by tetanic stimulation of PFs. 7. Injecting 30 mM BAPTA into PCs blocked the effects of 1S,3R-ACPD on unitary EPSCs. 8. In conclusion, 1S,3R-ACPD reduces evoked release of glutamate from PFs. This effect is triggered by postsynaptic mGluR1s and thus implies that a retrograde messenger, probably glutamate, opens presynaptic AMPA/kainate receptors and consequently increases spontaneous release of glutamate from PF terminals and decreases evoked synaptic transmission.

Characterization of the mGluR(1)-mediated electrical and calcium signaling in Purkinje cells of mouse cerebellar slices

The metabotropic glutamate receptor 1 (mGluR(1)) plays a fundamental role in postnatal development and plasticity of ionotropic glutamate receptor-mediated synaptic excitation of cerebellar Purkinje cells. Synaptic activation of mGluR(1) by brief tetanic stimulation of parallel fibers evokes a slow excitatory postsynaptic current and an elevation of intracellular calcium concentration ([Ca2+](i)) in Purkinje cells. The mechanism underlying these responses has not been identified yet. Here we investigated the responses to synaptic and direct activation of mGluR(1) using whole cell patch-clamp recordings in combination with microfluorometric measurements of [Ca2+](i) in mouse Purkinje cells. Following pharmacological block of ionotropic glutamate receptors, two to six stimuli applied to parallel fibers at 100 Hz evoked a slow inward current that was associated with an elevation of [Ca2+](i). Both the inward current and the rise in [Ca2+](i) increased in size with increasing number of pulses albeit with no clear difference between the minimal number of pulses required to evoke these responses. Application of the mGluR(1) agonist (S)-3,5-dihydroxyphenylglycine (3,5-DHPG) by means of short-lasting (5-100 ms) pressure pulses delivered through an agonist-containing pipette positioned over the Purkinje cell dendrite, evoked responses resembling the synaptically induced inward current and elevation of [Ca2+](i). No increase in [Ca2+](i) was observed with inward currents of comparable amplitudes induced by the ionotropic glutamate receptor agonist AMPA. The 3,5-DHPG-induced inward current but not the associated increase in [Ca2+](i) was depressed when extracellular Na+ was replaced by choline, but, surprisingly, both responses were also depressed when bathing the tissue in a low calcium (0.125 mM) or calcium-free/EGTA solution. Thapsigargin (10 microM) and cyclopiazonic acid (30 microM), inhibitors of sarco-endoplasmic reticulum Ca2+-ATPase, had little effect on either the inward current or the elevation in [Ca2+](i) induced by 3,5-DHPG. Furthermore, the inward current induced by 3,5-DHPG was neither blocked by 1-[2-(4-methoxyphenyl)-2-[3-(4-methoxyphenyl)propoxy] ethyl-1H-imidazole, an inhibitor of store operated calcium influx, nor by nimodipine or omega-agatoxin, blockers of voltage-gated calcium channels. These electrophysiological and Ca2+-imaging experiments suggest that the mGluR(1)-mediated inward current, although mainly carried by Na+, involves influx of Ca2+ from the extracellular space.

Neuronal glutamate transporters control activation of postsynaptic metabotropic glutamate receptors and influence cerebellar long-term depression

Neuronal and glial isoforms of glutamate transporters show distinct distributions on membranes surrounding excitatory synapses, but specific roles for transporter subtypes remain unidentified. At parallel fiber (PF) synapses in cerebellum, neuronal glutamate transporters and metabotropic glutamate receptors (mGluRs) have overlapping postsynaptic distributions suggesting that postsynaptic transporters selectively regulate mGluR activation. We examined interactions between transporters and mGluRs by evoking mGluR-mediated excitatory postsynaptic currents (mGluR EPSCs) in slices of rat cerebellum. Selective inhibition of postsynaptic transporters enhanced mGluR EPSCs greater than 3-fold. Moreover, impairing glutamate uptake facilitated mGluR-dependent long-term depression at PF synapses. Our results demonstrate that uniquely positioned glutamate transporters strongly influence mGluR activation at cerebellar PF synapses. Postsynaptic glutamate uptake may serve as a general mechanism for regulating mGluR-initiated synaptic depression.

The G-protein-coupled receptor kinase GRK4 mediates homologous desensitization of metabotropic glutamate receptor 1

G-protein-coupled receptor kinases (GRKs) are involved in the regulation of many G-protein-coupled receptors. As opposed to the other GRKs, such as rhodopsin kinase (GRK1) or beta-adrenergic receptor kinase (beta ARK, GRK2), no receptor substrate for GRK4 has been so far identified. Here we show that GRK4 is expressed in cerebellar Purkinje cells, where it regulates mGlu(1) metabotropic glutamate receptors, as indicated by the following: 1) When coexpressed in heterologous cells (HEK293), mGlu(1) receptor signaling was desensitized by GRK4 in an agonist-dependent manner (homologous desensitization). 2) In transfected HEK293 and in cultured Purkinje cells, the exposure to glutamate agonists induced internalization of the receptor and redistribution of GRK4. There was a substantial colocalization of the receptor and kinase both under basal condition and after internalization. 3) Kinase activity was necessary for desensitizing mGlu(1a) receptor and agonist-dependent phosphorylation of this receptor was also documented. 4) Antisense treatment of cultured Purkinje cells, which significantly reduced the levels of GRK4 expression, induced a marked modification of the mGlu(1)-mediated functional response, consistent with an impaired receptor desensitization. The critical role for GRK4 in regulating mGlu(1) receptors implicates a major involvement of this kinase in the physiology of Purkinje cell and in motor learning.

Slow synaptic inhibition mediated by metabotropic glutamate receptor activation of GIRK channels

Glutamate is the predominant excitatory neurotransmitter in the vertebrate CNS. Ionotropic glutamate receptors mediate fast excitatory actions whereas metabotropic glutamate receptors (mGluRs) mediate a variety of slower effects. For example, mGluRs can mediate presynaptic inhibition, postsynaptic excitation, or, more rarely, postsynaptic inhibition. We previously described an unusually slow form of postsynaptic inhibition in one class of projection neuron in the song-control nucleus HVc of the songbird forebrain. These neurons, which participate in a circuit that is essential for vocal learning, exhibit an inhibitory postsynaptic potential (IPSP) that lasts several seconds. Only a portion of this slow IPSP is mediated by GABA(B) receptors. Since these cells are strongly hyperpolarized by agonists of mGluRs, we used intracellular recording from brain slices to investigate the mechanism of this hyperpolarization and to determine whether mGluRs contribute to the slow synaptic inhibition. We report that mGluRs hyperpolarize these HVc neurons by activating G protein-coupled, inwardly-rectifying potassium (GIRK) channels. MGluR antagonists blocked this response and the slow synaptic inhibition. Thus, glutamate can combine with GABA to mediate slow synaptic inhibition by activating GIRK channels in the CNS.

Quisqualate induces an inward current via mGluR activation in neocortical pyramidal neurons

Activation of metabotropic glutamate receptors (mGluRs) has multiple effects on the excitability of pyramidal neurons in rat frontal neocortex. Synaptic transmission and intrinsic excitability are both affected. During studies of the effects of quisqualate on synaptic activity, it was observed that quisqualate also induced a slow inward current. Whole-cell patch clamp recordings were obtained from layer II/III pyramidal neurons of neocortical slices in vitro. The bath solution contained APV, CNQX and bicuculline to block ionotropic glutamate and GABA(A) receptors. At a holding potential of -70 mV, quisqualate (2 microM) induced an inward current of about 60 pA. The response was reversible upon washing. This current was associated with an increase in membrane conductance and was still seen in the presence of TTX (0.5 microM). Bath application of the nonselective mGluR antagonist, (R, S)-alpha-methyl-4-carboxyphenyglycine (MCPG, 200-500 microM) reduced the current by 70%. Other mGluR agonists (ACPD, DHPG, L-CCG-1 and L-AP4) did not induce a significant inward current at the concentrations tested. The current-voltage relation of the quisqualate-induced current was linear with a reversal potential near 0 mV suggesting involvement of nonselective cation channels. The quisqualate-induced inward current was markedly reduced (72%) with 200 microM GDP-beta-S in the pipette solution, indicating that it is a postsynaptic phenomenon mediated by a G-protein dependent mechanism. These results suggest that mGluRs can directly increase the postsynaptic excitability of pyramidal cells.

Elevation of intradendritic sodium concentration mediated by synaptic activation of metabotropic glutamate receptors in cerebellar Purkinje cells

Cerebellar Purkinje cells express both ionotropic glutamate receptors and metabotropic glutamate receptors. Brief tetanic stimulation of parallel fibers in rat and mouse cerebellar slices evokes a slow excitatory postsynaptic current in Purkinje cells that is mediated by the mGluR1 subtype of metabotropic glutamate receptors. The effector system underlying this mGluR1 EPSC has not yet been identified. In the present study, we recorded the mGluR1 EPSC using the whole-cell patch-clamp technique in combination with microfluorometric recordings of the intracellular sodium concentration ([Na+]i) by means of the fluorescent sodium indicator SBFI. The mGluR1 EPSC was induced by local parallel fibre stimulation in the presence of the ionotropic glutamate receptor antagonists NBQX and D-APV and the GABAA receptor antagonists bicuculline or picrotoxin. The mGluR1 EPSC was associated with an increase in [Na+]i that was restricted to a specific portion of the dendritic tree. The mGluR1 EPSC as well as the increase in [Na+]i were inhibited by the mGluR antagonist S-MCPG. In the presence of NBQX, D-APV, pictrotoxin and TTX, bath application of the selective mGluR agonist 3,5-DHPG induced an elevation in [Na+]i which extended over the whole dendritic field of the Purkinje cell. This finding demonstrates that the mGluR1-mediated postsynaptic current leads to a significant influx of sodium into the dendritic cytoplasm of Purkinje cells and thereby provides a novel intracellular signalling mechanism that might be involved in mGluR1-dependent synaptic plasticity at this synapse.

Differential expression of group I metabotropic glutamate receptors in functionally distinct hippocampal interneurons

Metabotropic glutamate receptors (mGluRs) have been proposed to be involved in oscillatory rhythmic activity in the hippocampus. However, the subtypes of mGluRs involved and their precise distribution in different populations of interneurons is unclear. In this study, we combined functional analysis of mGluR-mediated inward currents in CA1 oriens-alveus interneurons with anatomical and immunocytochemical identification of these interneurons and expression analysis of group I mGluR using single-cell reverse transcription-PCR (RT-PCR). Four major interneuron subtypes could be distinguished based on the mGluR-mediated inward current induced by the application of 100 microm trans-(1S,3R)-1-aminocyclopentane-1, 3-dicarboxylic acid (ACPD) under voltage-clamp conditions and the action potential firing pattern under current-clamp conditions. Type I interneurons responded with a large inward current of approximately 224 pA, were positive for somatostatin, and the majority expressed both mGluR1 and mGluR5. Type II interneurons responded with an inward current of approximately 80 pA, contained calbindin, and expressed mainly mGluR1. Type III interneurons responded with an inward current of approximately 60 pA. These interneurons were fast-spiking, contained parvalbumin, and expressed mainly mGluR5. Type IV interneurons did not respond with an inward current upon application of ACPD, yet they expressed group I mGluRs. Activation of group I mGluRs under current-clamp conditions increased spike frequency and resulted in rhythmic firing activity in type I and II, but not in type III and IV, interneurons. RT-PCR results suggest that activation of mGluR1 in the subsets of GABAergic interneurons, classified here as type I and II, may play an important role in mediating synchronous activity.