J Physiol Volume 586, Number 2, 575-592, January 15, 2008 DOI: 10.1113/jphysiol.2007.146522
Sustained granule cell activity disinhibits juvenile mouse cerebellar stellate cells through presynaptic mechanisms
Simone Astori1 and
Georg Köhr1
1 Department of Molecular Neurobiology, Max-Planck-Institute for Medical Research, Jahnstrasse 29, D-69120 Heidelberg, Germany
GABA release from cerebellar molecular layer interneurons can be modulated by presynaptic glutamate and/or GABAB receptors upon perfusing the respective agonists. However, it is unclear how release and potential spillover of endogenous transmitter lead to activation of presynaptic receptors. High frequency firing of granule cells, as observed in vivo upon sensory stimulation, could lead to glutamate and/or GABA spillover. Here, we established sustained glutamatergic activity in the granule cell layer of acute mouse cerebellar slices and performed 190 paired recordings from connected stellate cells. Train stimulation at 50 Hz reduced by about 30% the peak amplitude of IPSCs evoked by brief depolarization of the presynaptic cell in 2-week-old mice. A presynaptic mechanism was indicated by changes in failure rate, paired-pulse ratio and coefficient of variation of evoked IPSCs. Furthermore, two-photon Ca2+ imaging in identified Ca2+ hot spots of stellate cell axons confirmed reduced presynaptic Ca2+ influx after train stimulation within the granular layer. Pharmacological experiments indicated that glutamate released from parallel fibres activated AMPARs in stellate cells, evoking GABA release from surrounding cells. Consequential GABA spillover activated presynaptic GABABRs, which reduced the amplitude of eIPSCs. Two-thirds of the total disinhibitory effect were mediated by GABABRs, one-third being attributable to presynaptic AMPARs. This estimation was confirmed by the observation that bath applied baclofen induced a more pronounced reduction of evoked IPSCs than kainate. Granule cell-mediated disinhibition persisted at near-physiological temperature but was strongly diminished in 3-week-old mice. At this age, GABA release probability was not reduced and presynaptic GABABRs were still detectable, but GABA uptake appeared to be advanced, attenuating GABA spillover. Thus, sustained granule cell activity modulates stellate cell-to-stellate cell synapses, involving transmitter spillover during a developmentally restricted period.
(Received 11 October 2007;
accepted after revision 14 November 2007;
first published online 22 November 2007)
Corresponding author G. Köhr: Department of Molecular Neurobiology, Max-Planck-Institute for Medical Research, Jahnstrasse 29, D-69120 Heidelberg, Germany. Email: kohr{at}mpimf-heidelberg.mpg.de
Copyright © 2008 The Physiological Society.
