Retrograde Inhibition of Presynaptic Calcium Influx by Endogenous Cannabinoids at Excitatory Synapses onto Purkinje Cells

Abstract

Brief depolarization of cerebellar Purkinje cells was found to inhibit parallel fiber and climbing fiber EPSCs for tens of seconds. This depolarization-induced suppression of excitation (DSE) is accompanied by altered paired-pulse plasticity, suggesting a presynaptic locus. Fluorometric imaging revealed that postsynaptic depolarization also reduces presynaptic calcium influx. The inhibition of both presynaptic calcium influx and EPSCs is eliminated by buffering postsynaptic calcium with BAPTA. The cannabinoid CB1 receptor antagonist AM251 prevents DSE, and the agonist WIN 55,212-2 occludes DSE. These findings suggest that Purkinje cells release endogenous cannabinoids in response to elevated calcium, thereby inhibiting presynaptic calcium entry and suppressing transmitter release. DSE may provide a way for cells to use their firing rate to dynamically regulate synaptic inputs. Together with previous studies, these findings suggest a widespread role for endogenous cannabinoids in retrograde synaptic inhibition.

Endocannabinoid‐mediated short‐term synaptic plasticity: depolarization‐induced suppression of inhibition (DSI) and depolarization‐induced suppression of excitation (DSE)

“…In slices, these values correspond to the inhibition obtained with saturating concentration of a cannabinoid exogenous agonist, showing that standard DSI induction protocols are able to saturate presynaptic CBIRs. The maximal inhibition for DSE is slightly smaller than DSI for the synapses between cerebellar parallel fibers and Purkinje cells , whereas it is markedly smaller for the DSE of climbing fibers to Purkinje cell synapses (Kreitzer & Regehr, 2001a; and for hippocampal DSE (Ohno-Shosaku et al, 2002b;Chen et al, 2003). These differences could arise from a reduced number of functional CB1Rs, from a distinct location of CB1Rs on presynaptic glutamatergic terminals with respect to the postsynaptic endcannabinoid release machinery, or from a different sensitivity (possibly associated to a different isoform, see later) of endocannabinoid receptors to endogenous agonists (Ohno-Shosaku et al, 2002b).…”

“…Nearly simultaneously with the Wilson and Nicoll paper, Kreitzer and Regehr (2001a) described DSE for the first time figure. On the right, the effect of blocking the CB1R is shown: mIPSC frequency is unchanged after Purkinje cell depolarization (given at t ¼ 0 in both time plots; from Diana et al, 2002).…”

“…Consequently, the main body of the literature in the field concerns the modulation of GABAergic transmission. By contrast, the first descriptions of DSE are very recent (Kreitzer & Regehr, 2001a;OhnoShosaku et al, 2002b) so that comparatively little is known on DSE. Due to the broad similarity between the two forms of synaptic plasticity, we will treat DSI and DSE together.…”

“…DSE was found at parallel and climbing fiber inputs of Purkinje cells (Kreitzer & Regehr, 2001a) and at glutamatergic inputs onto CA1 pyramidal cells (Ohno-Shosaku et al, 2002b). By definition, DSI (as well as DSE) is triggered by postsynaptic depolarization.…”

“…It was quickly recognized that the calcium concentration rise that follows postsynaptic depolarization serves as a signal for DSI/DSE. Three lines of evidence indicate that this increase in intracellular calcium is both a necessary and a sufficient condition for induction: (1) blockers of calcium entry block DSI (Llano et al, 1991;Lenz et al, 1998); (2) postsynaptic application of calcium chelating agents like EGTA and/or BAPTA prevent DSI/DSE (Pitler & Alger, 1992;Glitsch et al, 2000;Kreitzer & Regehr, 2001a;Ohno-Shosaku et al, 2002a); (3) postsynaptic calcium uncaging is able to trigger synaptic depression (Wang & Zucker, 2000;.…”