Two independent forms of activity-dependent potentiation regulate electrical transmission at mixed synapses on the Mauthner cell

Mixed (electrical and chemical) synaptic contacts on the Mauthner cells, known as Club endings, constitute a valuable model for the study of vertebrate electrical transmission. While electrical synapses are still perceived by many as passive intercellular channels that lack modifiability, a wealth of experimental evidence shows that gap junctions at Club endings are subject to dynamic regulatory control by two independent activity-dependent mechanisms that lead to potentiation of electrical transmission. One of those mechanisms relies on activation of NMDA receptors and postsynaptic CaMKII. A second mechanism relies on mGluR activation and endocannabinoid production and is indirectly mediated via the release of dopamine from nearby varicosities, which in turn leads to potentiation of the synaptic response via a PKA-mediated postsynaptic mechanism. We review here these two forms of potentiation and their signaling mechanisms, which include the activation of two kinases with well-established roles as regulators of synaptic strength, as well as the functional implications of these two forms of potentiation. Special Issue entitled Electrical Synapses.