Recording of glutamate-activated currents in membrane patches was combined with RT-PCR-mediated AMPA receptor (AMPAR) subunit mRNA analysis in single identified cells of rat brain slices. Analysis of AMPARs in principal neurons and interneurons of hippocampus and neocortex and in auditory relay neurons and Bergmann glial cells indicates that the GluR-B subunit in its flip version determines formation of receptors with relatively slow gating, whereas the GluR-D subunit promotes assembly of more rapidly gated receptors. The relation between Ca2+ permeability of AMPAR channels and the relative GluR-B mRNA abundance is consistent with the dominance of this subunit in determining the Ca2+ permeability of native receptors. The results suggest that differential expression of GluR-B and GluR-D subunit genes, as well as splicing and editing of their mRNAs, account for the differences in gating and Ca2+ permeability of native AMPAR channels.

The arginine residue at position 586 of the GluR-B subunit renders heteromeric alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-sensitive glutamate receptor channels impermeable to calcium. The codon for this arginine is introduced at the precursor messenger RNA (pre-mRNA) stage by site-selective adenosine editing of a glutamine codon. Heterozygous mice engineered by gene targeting to harbor an editing-incompetent GluR-B allele synthesized unedited GluR-B subunits and, in principal neurons and interneurons, expressed AMPA receptors with increased calcium permeability. These mice developed seizures and died by 3 weeks of age, showing that GluR-B pre-mRNA editing is essential for brain function.

Ca2+-sensor synaptotagmin-1 is thought to trigger membrane fusion by binding to acidic membrane lipids and SNARE proteins. Previous work has shown that binding is mediated by electrostatic interactions that are sensitive to the ionic environment. However, the influence of divalent or polyvalent ions, at physiological concentrations, on synaptotagmin binding to membranes or SNAREs has not been explored. Here we show that binding of rat synaptotagmin-1 to membranes containing PIP2 is regulated by charge shielding caused by the presence of divalent cations. Surprisingly, polyvalent ions such as ATP and Mg2+ completely abrogate synaptotagmin-1 binding to SNAREs regardless of whether Ca2+ is present or not. Altogether, our data suggest that at physiological ion concentrations Ca2+-dependent synaptotagmin-1 binding is confined to PIP2-containing membrane patches in the plasma membrane, suggesting that membrane interaction of synaptotagmin-1 rather than SNARE binding triggers exocytosis of vesicles.

1. The influence of intracellular factors on current rectification of different subtypes of native a-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors (AMPARs) was studied in rat brain slices by combining fast application of glutamate with patch pipette perfusion. 2. The peak current-voltage (I-V) relation of the AMPARs expressed in Bergmann glial cells of cerebellum and dentate gyrus (DG) basket cells of hippocampus was weakly rectifying in outside-out patches and nystatin-perforated vesicles, but showed a doubly rectifying shape with a region of reduced slope between 0 and +40 mV in nucleated patches. The I-V relation of AMPARs expressed in hippocampal CA3 pyramidal neurones was linear in all recording configurations. 3. Intracellular application of 25 /SM spermine, a naturally occurring polyamine, blocked outward currents in outside-out patches from Bergmann glial cells and DG basket cells in a voltage-dependent manner, generating I-V relations with a doubly rectifying shape which were similar to those recorded in nucleated patches. AMPARs in CA3 pyramidal cell patches were unaffected by 25 ,/M spermine.4. The half-maximal blocking concentration of spermine at +40 mV was 0'3 /M in Bergmann glial cell patches and 1S5,UM in DG basket cell patches, whereas it was much higher (> 100 sM) for CA3 pyramidal cell patches. Spermidine also affected current rectification, but with lower affinity. The block of outward current by polyamines following voltage jumps developed within < 0'5 ms. 5. We conclude that current rectification, rather than being an intrinsic property of the Ca2+-permeable AMPAR channel, is generated by polyamine block. a-Amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors (AMPARs) expressed in different types of neurones of the mammalian central nervous system (CNS) are molecularly diverse. Molecular cloning identified four AMPAR subunits, designated as GluR-A, -B, -C, and -D (or GluR1, 2, 3, and 4), from which the native receptors presumably are assembled. Studies on recombinant AMPARs expressed in Xenopus oocytes and human embryonic kidney (HEK 293) cells indicate that the subunit composition controls both Ca2+ permeability and current rectification. Recombinant AMPARs containing the edited GluR-B subunit show low Ca2+ permeability and linear or outwardly rectifying current-voltage (I-V) relations, whereas AMPARs lacking GluR-B exhibit high Ca2+ permeability and doubly or inwardly rectifying I-V relations (Hollmann, Hartley & Heinemann, 1991;Burnashev, Monyer, Seeburg & Sakmann, 1992b). Both functional properties are determined by the amino acid residue at the Q/R site of the pore-forming region which is either an arginine in the edited GluR-B subunit or a glutamine in the other subunits (Hollmann et at. 1991; Burnashev et al. 1992 b).For native AMPARs, the relation between CaP' permeability and current rectification is less clear than for recombinant AMPARs. In principal neurones of hippocampus and neocortex, AMPARs exhibit low Ca2+ permeability and linear or outwardly rectifying I-V ...

1. Glutamate receptor (GluR) channels were studied in basket cells in the dentate gyrus of rat hippocampal slices. Basket cells were identified by their location, dendritic morphology and high frequency of action potentials generated during sustained current injection. 2. Dual-component currents were activated by fast application of glutamate to outside-out membrane patches isolated from basket cell somata (10 /SM glycine, no external Mg2+). The fast component was selectively blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), the slow component by D-2-amino-5-phosphonopentanoic acid (D-AP5). This suggests that the two components were mediated by a-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor (AMPAR)/kainate receptor and N-methyl-D-aspartate receptor (NMDAR) channels, respectively. The mean ratio of the peak current of the NMDAR component to that of the AMPAR/kainate receptor component was 0-22 (1 ms pulses of 10 mm glutamate). 3. The AMPAR/kainate receptor component, which was studied in isolation in the presence of D-AP5, was identified as AMPAR mediated on the basis of the preferential activation by AMPA as compared with kainate, the weak desensitization of kainate-activated currents, the cross-desensitization between AMPA and kainate, and the reduction of desensitization by cyclothiazide. 4. Deactivation of basket cell AMPARs following 1 ms pulses of glutamate occurred with a time constant (T) of 1-2 + 0 1 ms (mean + S.E.M.). During 100 ms glutamate pulses, AMPARs desensitized with a r of 3.7 + 0-2 ms. 5. The peak current-voltage (I-V) relation of AMPAR-mediated currents in Na+-rich extracellular solution showed a reversal potential of -4 0 + 2-6 mV and was characterized by a doubly rectifying shape. The conductance of single AMPAR channels was estimated as 22-6 + 1 6 pS using non-stationary fluctuation analysis. AMPARs expressed in hippocampal basket cells were highly Ca2+ permeable (Pca/PK = 1-79). 6. NMDARs in hippocampal basket cells were studied in isolation in the presence of CNQX.Deactivation of NMDARs activated by glutamate pulses occurred bi-exponentially with mean r values of 266 + 23 ms (76 %) and 2620 + 383 ms (24 %). 7. The peak I-V relation of the NMDAR-mediated component in Na+-rich extracellular solution showed a reversal potential of 1.5 + 0-6 mV and a region of negative slope at negative

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We induced Ca 2ϩ loads in mouse pancreatic ␤-cells by short membrane depolarizations while monitoring changes of the cytosolic free Ca 2ϩ . Agents were applied to block each of the potential clearance mechanisms selectively. Our results show that the SERCA pumps dominate the clearance after depolarization. RESEARCH DESIGN AND METHODSChemicals. Indo-1-AM, pluronic 147, and BCECF-AM were from Molecular Probes (Eugene, OR), and thapsigargin (TG) and cyclopiazonic acid (CPA) were from Calbiochem (La Jolla, CA). Culture medium, serum, and antibiotics were from Invitrogen (Carlsbad, CA), and all other chemicals from Sigma (St. Louis, MO). Cell preparation. Animal care followed the University of Washington Animal Medicine guidelines. The pancreas was removed from male Balb/c mice (4 -7 weeks old) killed with CO 2 (16), and islets of Langerhans were obtained by incubating small pancreatic pieces for 35 min in modified Hank's buffered solution, containing 5 mg/ml collagenase P (Boehringer, Germany), 1 mg/ml BSA, 20 mmol/l HEPES, and 10 mmol/l glucose. Single cells were dispersed by shaking islets in Ca 2ϩ -free Hank's buffered solution containing 1 mmol/l EGTA, 5 mmol/l glucose, and 10 mg/ml BSA. Isolated cells plated on coverslips precoated with poly-ornithine were kept in a 37°C, 5% CO 2 incubator for 2-5 days in RPMI-1640 culture medium containing 10 mmol/l glucose, 10% FBS, 100 g/ml streptomycin, and 100 IU/ml penicillin. Results were the same on culture days 2-5. Non-␤-cells were excluded by selecting the larger cells (17). Frequent tests showed that these cells respond to high glucose with Ca 2ϩ elevations and secretion (by amperometry). Solutions. The control bath solution (called Na7.4) contained NaCl 130 mmol/l, KCl 2.5 mmol/l, CaCl 2 2 mmol/l, MgCl 2 1 mmol/l, HEPES 10 mmol/l, glucose 15 mmol/l, and diazoxide 250 mol/l (pH 7.4 with NaOH). We included high glucose to mimic clearance under nutrient stimulus and diazoxide to minimize changes of the resting potential due to variations of cytoplasmic ATP. The experiments involved rapid changes (Ͻ500 ms) of solution by a fast local perfusion system controlled digitally. Except in Figs. 1

Many neuromodulators inhibit N-type Ca 2؉ currents via G protein-coupled pathways in acutely isolated superior cervical ganglion (SCG) neurons. Less is known about which neuromodulators affect release of norepinephrine (NE) at varicosities and terminals of these neurons. To address this question, we used carbon fiber amperometry to measure catecholamine secretion evoked by electrical stimulation at presumed sites of high terminal density in cultures of SCG neurons. The pharmacological properties of action potential-evoked NE release paralleled those of N-type Ca 2؉ channels: Release was completely blocked by Cd 2؉ or -conotoxin GVIA, reduced 50% by 10 M NE or 62% by 2 M UK-14,304, an ␣ 2 -adrenergic agonist, and reduced 63% by 10 M oxotremorine M (Oxo-M), a muscarinic agonist. Consistent with action at M 2 or M 4 receptor subtypes, Oxo-M could be antagonized by 10 M muscarinic antagonists methoctramine and tropicamide but not by pirenzepine. After overnight incubation with pertussis toxin, inhibition by UK-14,304 and Oxo-M was much reduced. Other neuromodulators known to inhibit Ca 2؉ channels in these cells, including adenosine, prostaglandin E 2 , somatostatin, and secretin, also depressed secretion by 34-44%. In cultures treated with -conotoxin GVIA, secretion dependent on L-type Ca 2؉ channels was evoked with long exposure to high K ؉ Ringer's solution. This secretion was not sensitive to UK-14,304 or Oxo-M. Evidently, many neuromodulators act on the secretory terminals of SCG neurons, and the depression of NE release at terminals closely parallels the membrane-delimited inhibition of N-type Ca 2؉ currents in the soma.Patch-clamp technique studies have shown that N-type Ca 2ϩ channels of the cell soma are modulated via many different G protein-coupled neurotransmitter receptors in superior cervical ganglion (SCG) neurons (1-6; for review, see ref. 7). Many neurotransmitter receptors, including ␣ 2 -adrenergic, somatostatin, prostaglandin E 2 (PGE 2 ), adenosine, M 4 muscarinic, pancreatic polypeptide, secretin, vasoactive intestinal peptide, and substance P, inhibit by a fast, membrane-delimited mechanism (7). Angiotensin II receptors and M 1 muscarinic receptors inhibit Ca 2ϩ channels via a slow, diffusible cytoplasmic messenger.Despite intensive work dissecting different modulatory pathways and investigating their underlying mechanisms (7-9), definite physiological roles for Ca 2ϩ channel inhibition in SCG cells have not yet been determined. Two have been proposed (7, 10): (i) In the soma, inhibition of Ca 2ϩ entry could alter cell excitability and action potential firing patterns because the somata possess Ca 2ϩ -activated K ϩ channels and Ca 2ϩ -sensitive M-type K ϩ channels (11-13), and (ii) at sympathetic nerve terminals and varicosities, inhibition of Ca 2ϩ influx could decrease norepinephrine (NE) secretion, as in Dunlap and Fischbach's (14) general concept of presynaptic inhibition. This hypothesis would require that N-type Ca 2ϩ channels be functionally coupled to many receptors at the dist...