RT Journal Article
SR Electronic
T1 Calcium Dependence of Retrograde Inhibition by Endocannabinoids at Synapses onto Purkinje Cells
JF The Journal of Neuroscience
JO J. Neurosci.
FD Society for Neuroscience
SP 6373
OP 6384
DO 10.1523/JNEUROSCI.23-15-06373.2003
VO 23
IS 15
A1 Stephan D. Brenowitz
A1 Wade G. Regehr
YR 2003
UL http://www.jneurosci.org/content/23/15/6373.abstract
AB Many types of neurons release endocannabinoids from their dendrites in response to elevation of intracellular calcium levels. Endocannabinoids then activate presynaptic cannabinoid receptors, thereby inhibiting neurotransmitter release for tens of seconds. A crucial step in understanding the physiological role of this retrograde signaling is to determine its sensitivity to elevations of postsynaptic calcium. Here we determine and compare the calcium dependence of endocannabinoid-mediated retrograde inhibition at three types of synapses onto cerebellar Purkinje cells. Previous studies have shown that Purkinje cell depolarization results in endocannabinoid-mediated retrograde inhibition of synapses received from climbing fibers, granule cell parallel fibers, and inhibitory interneurons. Using several calcium indicators with a range of affinities, we performed a series of in situ and in vitro calibrations to quantify calcium levels in Purkinje cells. We found that postsynaptic calcium levels of ∼15 μM are required for half-maximal retrograde inhibition at all of these synapses. In contrast, previous studies had suggested that endocannabinoid release could occur with slight elevations of calcium above resting levels, which implies that inhibition should be widespread and continuously modulated by subtle changes in intracellular calcium levels. However, our results indicate that such small changes in intracellular calcium are not sufficient to evoke endocannabinoid release. Instead, because of its high requirement for calcium, retrograde inhibition mediated by calcium-dependent endocannabinoid release from Purkinje cells will occur under more restricted conditions and with greater spatial localization than previously appreciated.