Presynaptic NMDA receptors, cannabinoid release and
LTD induction in layer 5 cells
It is widely accepted that postsynaptic NMDA receptors (NMDARs) detect
coincident pre- and postsynaptic activity during the induction of
timing-dependent long-term potentiation (tLTP). Less is known, however, about
the mechanisms that underlie timing-dependent long-term depression (tLTD).
Although induction of tLTD is known to depend on activation of NMDARs, the
factors determining the temporal window for tLTD remain elusive. We set out to
investigate tLTD in connected pairs of visual cortical thick-tufted layer-5
(L5) neurons.
We find that L5 tLTD changes short-term depression and the coefficient
of variation consistent with presynaptic expression. The need for postsynaptic
activity and the presynaptic expression imply retrograde signalling. Synthetic
and endogenous agonists of the cannabinoid CB1 receptor produce depression that
mimics and occludes tLTD, suggesting that the retrograde signal is an
endocannabinoid. Just like tLTD, CB1 agonist-induced LTD requires presynaptic
activity and NMDAR activation, but (as opposed to tLTD) it does not rely on
postsynaptic Ca2+ influx.
LTD is produced only if the presynaptic neuron is active when
endocannabinoid is bound to CB1 receptors. This presynaptic activity dependence
relies on presynaptic NMDA autoreceptors that detect the release of glutamate.
These presynaptic NMDARs are NR2B subunit-containing at an age when
postsynaptic NMDARs are not. In agreement, tLTD, but not tLTP, is abolished by
the NR2B-specific blocker ifenprodil.
Since LTD requires the presence of endocannabinoid, reduced rates of
endocannbinoid breakdown may extend the tLTD temporal window. In agreement,
blockers of endocannabinoid hydrolysis significantly increase the tLTD timing
window.
In conclusion, tLTD requires simultaneous activation of presynaptic NMDA
and CB1 receptors. This novel form of presynaptic coincidence detection may
explain the temporal window of tLTD and may also impart synapse specificity to
cannabinoid retrograde signaling.