|
|
||||||||
Report
Department of Cognitive Neurology, Hertie-Institute for Clinical Brain Research, University of Tübingen, 72076 Tübingen, Germany
Submitted 21 February 2003; accepted in final form 2 October 2003
| ABSTRACT |
|---|
|
|
|---|
postnatal day (p) 13, no difference was found in gsyn between the two groups. At p14, gsyn in Lc/+ showed an increase, while those in WT stayed on the level found in younger animals. A peak-scaled nonstationary fluctuation analysis suggests that an increase in the average number of channels open at peak is the basis for the change in gsyn. The changes in gsyn, suitable to increase the efficacy of GABAergic transmission, occur in close temporal relationship to PC death and, thus, may reflect a functional adaptation to the loss of the DCN's main GABAergic afferents. | INTRODUCTION |
|---|
|
|
|---|
| METHODS |
|---|
|
|
|---|
Heterozygous B6CBACa-Aw-J/A-Lc (Jackson Laboratories, Bar Harbor, ME) were mated with B6CBA (Charles River, Sulzfeld, Germany) mice. B6CBA mice were also used as WT controls. Animals were kept and used in experiments according to the institutional and national animal care guidelines (also conforming with National Institutes of Health guidelines on animal care). Animals from the Lurcher progeny not showing the characteristic Lc/+ phenotype (ataxic gait and tendency to fall) were genotyped. The genomic DNA was isolated (DNeasy Tissue Kit, Qiagen, Hilden, Germany). Primers (MWG-Biotech, Ebersberg, Germany) 5'-TAAAAGCATATTGATGTTGTTG-3' or 5'-GCACTGAATGTGTATGACTTCAG-3' and 5'-CAGCATTTGTCAGGTTTGGTGAC-3' (Zuo et al. 1997
) were used in PCR to amplify the region of interest, including a guanine to adenine transition at nucleotide position 1960 in exon B of the mouse
2 glutamate receptor gene. The PCR product was purified (CONCERT Rapid PCR Purification System, GIBCO BRL, Rockville, MD) and sequenced employing the big dye terminator chemistry (PE Biosystems, Weiterstadt, Germany). The sequences were analyzed with an automated 3100 Genetic Analyzer (Applied Biosystems, Foster City, CA) using the fluorescent dideoxynucleotide technology and evaluated using Lasergene software (Dnastar, Madison, WI).
Electrophysiology and data analysis
The mice were deeply anesthetized by application of ketamine (150 mg/kg ip). In some cases, the mice were intracardially perfused with ice-cold modified artificial cerebrospinal fluid (ACSF) for 2 min before further preparation. The modified ACSF contained (in mM) 126 sucrose, 2.5 KCl, 1.3 NaH2PO4, 3 MgCl2, 26 NaHCO3, 0.1 CaCl2, and 20 D-glucose and was bubbled with 95% O25% CO2. The brains were removed and put in the ice-cold modified ACSF. The cerebellum was isolated and cut into two parasagittal halves. Parasagittal slices of 250 to 275 µm thickness were obtained using a vibratome (VT1000s, Leica, Nussloch, Germany) and transferred into modified ACSF at room temperature. The modified ACSF was replaced within the next 90 min with ACSF containing (in mM) 125 NaCl, 2.5 KCl, 1.3 NaH2PO4, 2 MgCl2, 26 NaHCO3, 2 CaCl2, and 20 D-glucose, oxygenated with 95% O25% CO2 at room temperature. Experiments were performed with this ACSF to which the selective non-N-methyl-D-aspartate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione [DNQX, 2025 µM, for recordings of spontaneous IPSCs (sIPSCs)], and in some recordings also TTX [1 µM, to detect miniature IPSCs (mIPSCs)] and/or the GABAA antagonist bicuculline (10 µM, Tocris, Bristol, UK), were added.
Whole-cell voltage-clamp recordings of the DCN were conducted with patch pipettes that had a resistance of 2.06.0 M
when filled with a solution containing (in mM) 124 CsCl, 10 K+HEPES, 5 EGTA, 4.6 MgCl2, 4 K+ATP, 0.4 Na+GTP, 0.1 CaCl2, and 5 QX-314 (Tocris), adjusted to pH 7.3 with CsOH, thus obtaining chloride-symmetrical experimental conditions. The patch procedure was visualized using a motorized (Luigs & Neumann, Ratingen, Germany) microscope (Axioscope, Zeiss, Göttingen, Germany) with water immersion objective (x40, Zeiss numerical aperture 0.75), infrared illumination, Normaski optics and infrared sensitive CCD camera (Newvicon C240007-C, Hamamatsu, Japan). The recordings were performed with an EPC-7 or EPC-8 amplifier (Heka, Lambrecht, Germany). The data were sampled at a rate of 20 kHz and subsequently low-pass filtered (cutoff frequency 1 kHz) (Spike2, CED, Cambridge, UK). Membrane resistances for the recordings used for quantitative analysis were 494.8 ± 379.4 M
(mean ± SD, throughout the paper) and the mean ratio serial resistance/membrane resistance was 0.078. The membrane and serial resistances did not differ significantly between genotypes. Neurons were voltage clamped at -70.4 ± 4.4 mV. Data were not quantitatively analyzed if under these conditions the serial resistance increased more than 25% of the initial value or if the membrane resistance of the recorded neuron was below 150 M
.
Analysis of the postsynaptic currents was performed with the MiniAnalysis program (Synaptosoft, Decatur, GA). For the analysis of the synaptic waveforms only synaptic events were evaluated that were not interrupted by other synaptic events. On average, 145 ± 190 events (n = 92 recordings) were evaluated. Average synaptic conductance (gsyn) values were calculated as gsyn = Imean /(Vh - E), where Imean is the mean peak amplitude of the IPSCs, Vh is the holding potential, and E is the measured equilibrium potential for the IPSCs of the respective recording. Decay time mentioned in this paper is the time it takes the current values to decay to 1/e of the peak values of the IPSC amplitudes. Peak-scaled nonstationary fluctuation analysis was performed using the respective module of MiniAnalysis, version 5.6.29 (adapted from the original algorithm by Traynelis et al. 1993
). Those IPSCs that were interrupted by other events and did not return to baseline within the chosen time window were discarded. The recording sections used for the nonstationary fluctuation analysis showed no statistically significant correlation between IPSC amplitude and time, decay time and time, IPSC amplitude and decay time, or IPSC amplitude and half-width. The unitary current (i) was estimated by fitting the equation
(t)2 = iI(t) - I(t)2/N +
2basal, where
2 represents the variance at given time t of the mIPSC, I the current at given time t, and
2basal the variance in baseline noise measured before the peak. The algorithm forces the probability of channel opening at the peak of the mIPSC to approach unity (1.0), consequently N represents the average number of channels open at peak (Nopen,peak). Fitting was done after subtracting the baseline variance and without including the offset. The unitary conductance values (
) were calculated as
= i/(Vh - E).
Statistically significant differences and correlations were tested with the Student's t-test and Pearson product moment correlation test for normally distributed data, otherwise with the MannWhitney test and Spearman rank order correlation test (italicized P values throughout the paper). Statistical significance was assumed for P < 0.05.
| RESULTS AND DISCUSSION |
|---|
|
|
|---|
In those experiments in which mutants were studied well after the onset of the PC degeneration, i.e., 14 or more postnatal days, the gsyn values in Lc/+ were found enlarged compared with those in WT [for sIPSCs: 1.38 ± 0.50 nS (n = 12, Lc/+) versus 0.66 ± 0.28 nS (n = 14, WT), P
0.001; for mIPSCs: 1.23 ± 0.51 nS (n = 10, Lc/+) versus 0.57 ± 0.21 nS (n = 19, WT), P
0.001)].
We next asked whether the conductance changes appear in parallel to PC axonal degeneration or follow a different time course. Figure 1 shows recordings of sIPSCs and mIPSCs from the DCN of Lc/+ and WT at different postnatal days. An increase of gsyn is obvious in the recordings obtained at p14 in the mutants, but not in the WT. The gsyn in animals 13 or fewer postnatal days did not show any statistically significant difference between Lc/+ in comparison to the healthy control group [for sIPSCs: 0.57 ± 0.23 nS (n = 16, Lc/+
p13) versus 0.62 ± 0.32 nS (n = 14, WT
p13), P = 0.95]. The respective values for recordings of mIPSCs were 0.50 ± 0.39 nS (n = 3, Lc/+
p13) versus 0.39 ± 0.16 nS (n = 4, WT
p13), P = 0.62. At p14, gsyn in Lc/+ showed an increase while those in WT stayed on the level found in younger animals. Hence the change in conductance occurs only after the onset of PC degeneration. Figure 2, A and B summarize the findings. The gsyn are plotted as a function of the postnatal age of the recorded neuron. An analysis of amplitude distribution revealed that this increase in conductance was due to a decrease in the number of small events accompanied by an increase in the number of large events. There was no obvious indication for the appearance of a new population of IPSCs in Lc/+ compared with WT 14 postnatal days or older (Fig. 2C).
|
|
|
To determine whether the potentiation of IPSC amplitudes in Lc/+ is due to a difference in the number of receptors or due to a modulation of the single receptor channel conductance, we applied a peak-scaled nonstationary fluctuation analysis as described by Traynelis et al. (1993
). In addition to an estimation of the mean unitary conductance (
), this method also estimates the average number of channels open at peak of the IPSC (Nopen,peak). Using mIPSC recordings from Lc/+ (n = 7) and WT (n = 5) p14 or older, we found a statistically significant increase of Nopen,peak from 22.65 ± 7.59 in the WT to 45.45 ± 16.11 in the Lc/+ (P = 0.015). In contrast,
showed no statistically significant difference between mutants and WT [42.60 ± 21.96 pS (Lc/+) versus 33.25 ± 19.19 pS (WT), P = 0.462, Fig. 4). The most parsimonious explanation of the different Nopen,peak is a change in the total number of postsynaptic receptors as has been shown by studies directly assessing the number of receptors (Nusser et al. 1998
; Kilman et al. 2002
). A more remote possibility, which, however, cannot be excluded on the basis of the present data, is that neurotransmitter vesicle content increases in Lc/+.
|
Electrophysiological in vitro recordings from the DCN are possible in a limited postnatal time window only. The high myelinization of the structure in older animals complicates the isolation of viable neurons and the formation of a stable patch configuration andin our handsrenders such recordings virtually impossible in older animals. Due to this limited postnatal time window it is difficult to issue a statement about the electrophysiological properties in older animals. It is therefore not possible at this point to decide if our present data reflect a transient or permanent mechanism of adaptation.
GABAergic synapses on DCN neurons have been shown to undergo long-term potentiation (LTP) in response to an electrical pulse train to their afferents in the white matter or after sole intracellular depolarization (Aizenman et al. 1998
; Ouardouz and Sastry 2000
). This form of LTP is accompanied by increases in mIPSC frequency without changes in their amplitude but stronger response to extracellularly applied GABA agonists, a finding that could be explained by the activation of previously "silent" synapses (Ouardouz and Sastry 2000
). While the potentiation of GABAergic IPSCs in Lc/+ as described here might be taken as a form of LTP, it must employ a mechanism different from the one described earlier (Ouardouz and Sastry 2000
) because amplitudes of mIPSC are increased. Similar types of LTP, based on changes in Nopen,peak without alterations in the
values, have been observed in other GABAergic synapses, e.g., after kindling of hippocampal inhibitory synapses (Otis et al. 1994
; Nusser et al. 1998
) or after depriving activity at neocortical synapses (Kilman et al. 2002
). An important implication of the present study is therefore that forms of LTP may not only be involved in learning and memory, but may also be important for adaptive processes following brain damage or degeneration (Mittmann and Eysel 2001
; Wilson et al. 1979
) and that they might even have an adaptive value, compensating for the loss of the extrinsic inhibition.
| ACKNOWLEDGMENTS |
|---|
|
|
|---|
hennig and B. Baumann for technical assistance and Drs. M. Möck and V. Gauck of our laboratory and Dr. B. Wissinger of the Molecular Genetics Laboratory, University Eye Hospital Tübingen for discussion, advice, and support. GRANTS
This work was supported by the German Science Foundation (SFB 430-C6), the Center for Interdisciplinary Clinical Research, Tübingen (IZKF IC1), and the Hermann and Lilly Schilling Foundation.
| FOOTNOTES |
|---|
* C. Linnemann and F. Sultan contributed equally to this study. ![]()
Address for reprint requests and other correspondence: C. Linnemann, Department of Cognitive Neurology, Hertie-Institute for Clinical Brain Research, University of Tübingen, Auf der Morgenstelle 15, 72076 Tübingen, Germany (E-mail: Christoph.Linnemann{at}uni-tuebingen.de).
| REFERENCES |
|---|
|
|
|---|
Caddy KW and Biscoe TJ. Structural and quantitative studies on the normal C3H and Lurcher mutant mouse. Philos Trans R Soc Lond B 287: 167-201, 1979.[ISI][Medline]
Caston J, Vasseur F, Stelz T, Chianale C, Delhaye-Bouchaud N, and Mariani J. Differential roles of cerebellar cortex and deep cerebellar nuclei in the learning of the equilibrium behavior: studies in intact and cerebellectomized lurcher mutant mice. Brain Res Dev Brain Res 86: 311-316, 1995.[Medline]
Dumesnil-Bousez N and Sotelo C. Early development of the Lurcher cerebellum: Purkinje cell alterations and impairment of synaptogenesis. J Neurocytol 21: 506-529, 1992.[CrossRef][ISI][Medline]
Dunning DD, Hoover CL, Soltesz I, Smith MA, and O'Dowd DK. GABAA receptor-mediated miniature postsynaptic currents and
-subunit expression in developing cortical neurons. J Neurophysiol 82: 3286-3297, 1999.
Heckroth JA. Quantitative morphological analysis of the cerebellar nuclei in normal and lurcher mutant mice. I. Morphology and cell number. J Comp Neurol 343: 173-182, 1994.[CrossRef][ISI][Medline]
Hollrigel GS and Soltesz I. Slow kinetics of miniature IPSCs during early postnatal development in granule cells of the dentate gyrus. J Neurosci 17: 5119-5128, 1997.
Kilman V, Van Rossum MCW, and Turrigiano GG. Activity deprivation reduces miniature IPSC amplitude by decreasing the number of postsynaptic GABAA receptors clustered at neocortical synapses. J Neurosci 22: 1328-1337, 2002.
Lalonde R, Filali M, Bensoula AN, and Lestienne F. Sensorimotor learning in three cerebellar mutant mice. Neurobiol Learn Mem 65: 113-120, 1996.[CrossRef][ISI][Medline]
Mittmann T and Eysel UT. Increased synaptic plasticity in the surround of visual cortex lesions in rats. Neuroreport 12: 3341-3347, 2001.[CrossRef][ISI][Medline]
Nusser Z, Hájos N, Somogyi P, and Mody I. Increased number of synaptic GABAA receptors underlies potentiation at hippocampal inhibitory synapses. Nature 395: 172-177, 1998.[CrossRef][Medline]
Otis TS, De Koninck Y, and Mody I. Lasting potentiation of inhibition is associated with an increased number of gamma-aminobutyric acid type A receptors activated during miniature inhibitory postsynaptic currents. Proc Natl Acad Sci USA 91: 7698-7702, 1994.
Ouardouz M and Sastry BR. Mechanisms underlying LTP of inhibitory synaptic transmission in the deep cerebellar nuclei. J Neurophysiol 84: 1414-1421, 2000.
Phillips RJS. "Lurcher," new gene in the linkage group XI of the house mouse. J Genet 57: 35-42, 1960.
Sultan F, König T, Möck M, and Thier P. Quantitative organization of neurotransmitters in the deep cerebellar nuclei of the Lurcher mutant. J Comp Neurol 452: 311-323, 2002.[CrossRef][ISI][Medline]
Traynelis SF, Silver RA, and Cull-Candy SG. Estimated conductance of glutamate receptor channels activated during EPSCs at the cerebellar mossy fiber-granule cell synapse. Neuron 11: 279-289, 1993.[CrossRef][ISI][Medline]
Vicini S, Ferguson C, Prybylowski K, Kralic J, Morrow AL, and Homanics GE. GABAA receptor
1 subunit deletion prevents developmental changes of inhibitory synaptic currents in cerebellar neurons. J Neurosci 21: 3009-3016, 2001.
Wetts R and Herrup K. Interaction of granule, Purkinje and inferior olivary neurons in Lurcher chimaeric mice. I. Qualitative studies. J Embryol Exp Morphol 68: 87-98, 1982.[ISI][Medline]
Wilson RC, Levy WB, and Steward O. Functional effects of lesion-induced plasticity: long term potentiation in formal and lesion-induced temporodentate connections. Brain Res 176: 65-78, 1979.[CrossRef][ISI][Medline]
Zuo J, De Jager PL, Takahashi KA, Jiang W, Linden DJ, and Heintz N. Neurodegeneration in Lurcher mice caused by mutation in
2 glutamate receptor gene. Nature 388: 769-773, 1997.[CrossRef][Medline]
This article has been cited by other articles:
![]() |
E. C. Hurlock, A. McMahon, and R. H. Joho Purkinje-Cell-Restricted Restoration of Kv3.3 Function Restores Complex Spikes and Rescues Motor Coordination in Kcnc3 Mutants J. Neurosci., April 30, 2008; 28(18): 4640 - 4648. [Abstract] [Full Text] [PDF] |
||||
![]() |
S. I. Levin, Z. M. Khaliq, T. K. Aman, T. M. Grieco, J. A. Kearney, I. M. Raman, and M. H. Meisler Impaired Motor Function in Mice With Cell-Specific Knockout of Sodium Channel Scn8a (NaV1.6) in Cerebellar Purkinje Neurons and Granule Cells J Neurophysiol, August 1, 2006; 96(2): 785 - 793. [Abstract] [Full Text] [PDF] |
||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Visit Other APS Journals Online |