LEARNING-INDUCED MODULATION OF THE EFFECT OF ENDOCANNABINOIDS ON INHIBITORY SYNAPTIC TRANSMISSION

Endocannabinoids are key modulators that regulate central brain functions and behaviours, including learning and memory. At the cellular and molecular levels, endocannabinoids are potent modulators of excitatory and inhibitory synaptic function. Most effects of cannabinoids are thought to be mediated via G protein-coupled cannabinoids receptors. In particular, cannabinoids released from post-synaptic neurons are suggested to act as retrograde messengers, activating presynaptic Type-1 Cannabinoid receptors (CB1R), thereby inducing suppression of synaptic release. Another central mechanism of cannabinoids-induced action requires activation of astroglial CB1R. CB1R are also implicated in self-modulation of cortical neurons. Rats that are trained in a particularly difficult olfactory-discrimination task show a dramatic increased ability to acquire memories of new odors. The memory of the acquired high skill acquisition, termed 'rule learning' or 'learning set' lasts for many months. Using this behavioural paradigm, we show a novel function of action for CB1R; supporting long-term memory by maintaining persistent enhancement of inhibitory synaptic transmission. Long-lasting enhancement of inhibitory synaptic transmission is blocked by a CB1R inverse agonist. This effect is mediated by a novel purely post-synaptic mechanism, obtained by enhancing the single GABAA channel conductance that is PKA-dependent. The significant role that CB1R has in maintaining learning-induced long-term strengthening of synaptic inhibition suggests that endocannabinoids have a key role in maintaining long-term memory by enhancing synaptic inhibition.

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