Mglur1/TRPC3-Mediated Synaptic Transmission and Calcium Signaling in Mammalian Central Neurons
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Downloaded from http://cshperspectives.cshlp.org/ on October 6, 2021 - Published by Cold Spring Harbor Laboratory Press mGluR1/TRPC3-mediated Synaptic Transmission and Calcium Signaling in Mammalian Central Neurons Jana Hartmann, Horst A. Henning, and Arthur Konnerth Institute of Neuroscience and Center for Integrated Protein Science, Technical University of Munich, Munich, Germany Correspondence: [email protected] Metabotropic glutamate receptors type 1 (mGluR1s) are required for a normal function of the mammalian brain. They are particularly important for synaptic signaling and plasticity in the cerebellum. Unlike ionotropic glutamate receptors that mediate rapid synaptic trans- mission, mGluR1s produce in cerebellar Purkinje cells a complex postsynaptic response consisting of two distinct signal components, namely a local dendritic calcium signal and a slow excitatory postsynaptic potential. The basic mechanisms underlying these synaptic responses were clarified in recent years. First, the work of several groups established that the dendritic calcium signal results from IP3 receptor-mediated calcium release from internal stores. Second, it was recently found that mGluR1-mediated slow excitatory postsynaptic potentials are mediated by the transient receptor potential channel TRPC3. This surprising finding established TRPC3 as a novel postsynaptic channel for glutamatergic synaptic transmission. lutamate is the predominant neurotrans- agonists: AMPA- (a-amino-3-hydroxy-5-meth- Gmitter used by excitatory synapses in the yl-4-isoxazolepropionic acid), NMDA receptors mammalian brain (Hayashi 1952; Curtis et al. (N-methyl D-aspartate receptors) and kainate 1959). At postsynaptic sites, glutamate binds receptors (Alexander et al. 2009). The other class to two different classes of receptors, namely of glutamate receptors, the mGluRs, consists of the ionotropic glutamate receptors (iGluRs) and receptors that are coupled to G proteins and act the metabotropic glutamate receptors (mGluRs) through distinct downstream signaling cascades. (Sladeczek et al. 1985; Nicoletti et al. 1986; They are structurally different from iGluRs and Sugiyama et al. 1987). The iGluRs represent li- characterized by the presence of seven trans- gand-gated nonselective cation channels that un- membrane domains (Houamed et al. 1991; derlie excitatory postsynaptic currents (EPSCs). Masu et al. 1991). The mGluRs exist as homo- Based on their subunit composition, gating, dimers that do not by themselves form an ion- and permeability properties, they are subdi- permeable pore in the membrane (Ozawa et al. vided into three groups named after specific 1998). To date, eight different genes (and more Editors: Martin D. Bootman, Michael J. Berridge, James W. Putney, and H. Llewelyn Roderick Additional Perspectives on Calcium Signaling available at www.cshperspectives.org Copyright # 2011 Cold Spring Harbor Laboratory Press; all rights reserved. Advanced Online Article. Cite this article as Cold Spring Harb Perspect Biol doi: 10.1101/cshperspect.a006726 1 Downloaded from http://cshperspectives.cshlp.org/ on October 6, 2021 - Published by Cold Spring Harbor Laboratory Press Hartmann et al. splice variants) encoding mGluRs have been of the general mGluR1-knockout mice is res- identified and form the mGluR1 through cued by the insertion of the gene encoding mGluR8 subtypes (Alexander et al. 2009). Based mGluR1 exclusively into cerebellar Purkinje on the amino acid sequence homology, down- cells (Ichise et al. 2000) and blockade of stream signal transduction pathways, and phar- mGluR1 expression only in Purkinje cells of macological properties, each of the subtypes adult mice leads to impaired motor coordina- was assigned to one of three groups. Group I tion (Nakao et al. 2007). These findings estab- receptors consist of mGluR1 and mGluR5 that lished mGluR1 in Purkinje cell as synaptic positively couple to the phospholipase C (PLC). receptors that are indispensable for a normal The receptors mGluR2 and mGluR3 constitute cerebellar function. group II, whereas the remaining mGluRs, name- Synaptic transmission involving mGluR1s ly mGluR4, mGluR6, mGluR7, and mGluR8, is found at both parallel fiber-Purkinje cell belong to group III. Both groups II and III synapses (Batchelor and Garthwaite 1993; inhibit the adenylyl cyclase and thereby reduce Batchelor et al. 1994) as well as at climbing fi- the concentration of cAMP in the cytosol. ber-Purkinje cell synapses (Dzubay and Otis Of all different subtypes, mGluR1 is the 2002). Most of our knowledge on the mGluR1 most abundantly expressed mGluR in the mam- was gained from the analysis of the parallel fi- malian central nervous system. In the brain, ber synapses. The parallel fiber synapse is quite mGluR1 is highly expressed in the olfactory unique in the central nervous system regarding bulb, dentate gyrus, and cerebellum (Lein et al. its endowment with neurotransmitter recep- 2007). The highest expression level of mGluR1 tors. In contrast to most other glutamatergic in the brain is found in Purkinje cells, the prin- synapses in the mammalian brain, it lacks cipal neurons of the cerebellar cortex (Shige- functional NMDA receptors (Shin and Linden moto et al. 1992; Lein et al. 2007). Together 2005). The entire synaptic transmission at with the AMPA receptors, mGluR1s are part these synapses relies on AMPA receptors and of the excitatory synapses formed between on mGluR1 (Takechi et al. 1998). Although parallel fibers and Purkinje cells (Fig. 1A). AMPA receptors are effectively activated even Each Purkinje cell is innervated by 100,000– with single shock stimuli (Konnerth et al. 200,000 parallel fibers (Ito 2006) that are axons 1990; Llano et al. 1991b), activation of mGluRs of the cerebellar granule cells, the most abun- requires repetitive stimulation (Batchelor and dant type of neuron in the brain. A second Garthwaite 1993; Batchelor et al. 1994; Batche- type of excitatory input to Purkinje cells is rep- lor and Garthwaite 1997; Takechi et al. 1998). resented by the climbing fibers that originate in A possible explanation for the need of repetitive the inferior olive in the brain stem (Ito 2006). stimulation may relate to the observation that The two excitatory synaptic inputs to Purkinje mGluR1s are found mostly at the periphery of cells are important determinants for the main the subsynaptic region (Nusser et al. 1994). At functions of the cerebellum, including the real- these sites outside the synaptic cleft, glutamate time control of movement precision, error- levels that are sufficiently high for receptor ac- correction, and control of posture as well as tivation may be reached only with repetitive the procedural learning of complex movement stimulation. sequences and conditioned responses. At parallel fiber-Purkinje cell synapses, re- It is expected that mGluR1 is involved in petitive stimulation produces an initial AMPA many of these cerebellar functions. This view receptor postsynaptic signal component, fol- is supported by the observation that mGluR1- lowed by a more prolonged mGluR1 compo- deficient knockout mice show severe impair- nent (Fig. 1). Figure 1B shows a current clamp ments in motor coordination. In particular, the recording of this response consisting of an gait of these mice is strongly affected as well early burst of action potentials, followed by a as their ability for motor learning and general prolonged depolarization known as a “slow coordination (Aiba et al. 1994). The phenotype excitatory postsynaptic potential” (slow EPSP) 2 Advanced Online Article. Cite this article as Cold Spring Harb Perspect Biol doi: 10.1101/cshperspect.a006726 Downloaded from http://cshperspectives.cshlp.org/ on October 6, 2021 - Published by Cold Spring Harbor Laboratory Press Synaptic Transmission Mediated by mGluR1 and TRPC3 A Parallel fiber C Whole-cell recording Stim Purkinje cell Granule cell 20 μm BDmGluR1-dependent Ca2+ signal 50% 20 mV ΔF/F 250 ms CPCCOEt 5 μm Slow EPSP EPSC Slow 200 pA EPSC MCPG 1 s 0 400 ΔF/F Figure 1. Parallel fiber-evoked mGluR1-dependent signals. (A) Diagram showing the parallel fiber synaptic input to Purkinje cell dendrites. (B) Microelectrode recording of glutamatergic postsynaptic potentials from a Purkinje cell in an acute slice of adult rat cerebellum. Short trains of stimuli to the parallel fibers (5–6 at 50 Hz) caused summation of the early AMPA receptor-dependent EPSPs (leading to spike firing) and a slow, delayed, depolarizing potential (slow EPSP), which was reversibly inhibited by antagonist of mGluRs (þ)-MCPG (1mM). (C) Confocal image of a patch-clamped Purkinje cell in a cerebellar slice of an adult mouse. The patch-clamp pipette and the glass capillary used forelectrical stimulation of parallel fibers are depicted sche- matically. The site of stimulation is shown at higher magnification in D.(D) Left: Parallel fiber-evoked (five pulses at 200 Hz, in 10 mM CNQX) synaptic responses consisting of a dendritic mGluR1-dependent Ca2þ tran- sient (DF/F, top) and an early rapid and a slow excitatory postsynaptic current (EPSC, bottom). Block of the mGluR1-dependent components by the group I-specific mGluR-antagonist CPCCOEt (200 mM) is shown as indicated. Right: Pseudocolor image of the synaptic Ca2þ signal. (B, Reprinted with modifications, with permis- sion, from Batchelor and Gaithwaite 1997 [Nature Publishing Group].) (Batchelor and Garthwaite 1993; Batchelor et al. (Fig. 1D) (Llano et al. 1991a; Finch and Augus- 1994; Batchelor and Garthwaite 1997). Voltage- tine 1998; Takechi et al. 1998). clamp