Endocannabinoid Signaling Dynamics Probed with Optical Tools

Abstract

Intercellular signaling dynamics critically influence the functional roles that the signals can play. Small lipids are synthesized and released from neurons, acting as intercellular signals in regulating neurotransmitter release, modulating ion channels on target cells, and modifying synaptic plasticity. The repertoire of biological effects of lipids such as endocannabinoids (eCBs) is rapidly expanding, yet lipid signaling dynamics have not been studied. The eCB system constitutes a powerful tool for bioassaying the dynamics of lipid signaling. The eCBs are synthesized in, and released from, postsynaptic somatodendritic domains that are readily accessible to whole-cell patch electrodes. The dramatic effects of these lipid signals are detected electrophysiologically as CB1-dependent alterations in conventional synaptic transmission, which therefore serve as a sensitive reporter of eCB actions. We used electrophysiological recording, photolytic release of caged glutamate and a newly developed caged AEA (anandamide), together with rapid [Ca 2ϩ ] i measurements, to investigate the dynamics of retrograde eCB signaling between CA1 pyramidal cells and GABAergic synapses in rat hippocampus in vitro. We show that, at 22°C, eCB synthesis and release must occur within 75-190 ms after the initiating stimulus, almost an order of magnitude faster than previously thought. At 37°C, the time could be Ͻ50 ms. Activation of CB1 and downstream processes constitute a significant fraction of the total delay and are identified as major rate-limiting steps in retrograde signaling. Our findings imply that lipid messenger dynamics are comparable with those of metabotropic neurotransmitters and can modulate neuronal interactions on a similarly fast time scale.

Two Coincidence Detectors for Spike Timing-Dependent Plasticity in Somatosensory Cortex

“…Like L4-L2/3 t-LTD, L5 t-LTD is independent of postsynaptic NMDARs but requires presumably presynaptic NMDARs as well as CB1 receptor activation. Sjostrom et al (2003) proposed that CB1 receptors encode the precise time of postsynaptic spikes (via spike-elicited eCB release), and presynaptic NMDARs encode the time of presynaptic spikes (by acting as glutamate autoreceptors), and that coincident activation of these receptors drives t-LTD. A potential difficulty for this model is that eCB synthesis, diffusion, and CB1 receptor signaling may be too slow to allow precise encoding of postsynaptic spike timing (Wilson and Nicoll, 2002;Heinbockel et al, 2005). In the postsynaptic coincidence detector model, eCB signaling represents the output of the coincidence detector, the signal to initiate synaptic weakening, which does not require high temporal precision.…”

Presynaptic Modulation by Endocannabinoids

“…For example, a compelling recent study used a "caged" form of AEA to demonstrate the action and time course of EC and CB1-mediated presynaptic inhibition at a GABAergic synapse in hippocampal slices (Heinbockel et al 2005). This tool allowed the investigators to photolytically release the AEA with precise timing using an argon-ion laser for UV chemical uncaging.…”

“…First, the receptors mediating such effects are coupled to Gi/o-type G-proteins that are known to engage intracellular signaling pathways, including inhibition of adenylyl cyclase, inhibition of voltage-gated calcium channels, and activation of potassium channels (Dunwiddie and Lovinger 1993;Miller 1998). Second, receptors and downstream signaling can be activated on a subsecond timescale, providing rapid feedback for intercellular alterations in secretion (Mintz and Bean 1993;Heinbockel et al 2005). Third, regulation of intracellular calcium, due mainly to inhibition of the aforementioned calcium channels, appears to play a prominent role in receptormediated decreases in neurotransmitter release (Wu and Saggau 1997).…”

Flashy Science: Controlling Neural Function with Light

“…When perfused into hippocampal slice preparations, the caged AEA serves as a latent eCB pool, and focal photolysis rapidly liberates highly hydrophobic AEA in situ to activate CB 1 receptors. They have used whole-cell voltageclamp recording and intracellular Ca 2ϩ measurement in combination with photorelease of caged AEA and Ncm-Glu (Cai et al, 2004) to probe the dynamics of eCB signaling in the hippocampal CA1 region (Heinbockel et al, 2005). Photorelease of AEA transiently suppressed spontaneous IPSCs with a time course comparable with that of DSI (Fig.…”