A new role for T-type channels in fast "low-threshold" exocytosis

Evidence is accumulating on a key role of T-type channels in neurotransmitter release. Recent works have brought undisputable proofs that T-type channels are capable of controlling hormone and neurotransmitters release in association with exocytosis of large dense-core and synaptic vesicles. T-type channel-secretion coupling is not as ubiquitous as that shown for N- and P/Q-type channels in central neurons. In this case, the high-density of Cav2 channel types and co-localization to the release sites ensure high rates of vesicle release and synchronous synaptic responses. Nevertheless, when sufficiently expressed in distal dendrites and neurosecretory cells, T-type channels are able to drive the fast fusion of vesicles ready for release during "low-threshold" Ca2+-entry. T-type channels appear effectively coupled to fast vesicle depletion and may possibly regulate other Ca2+-dependent processes like vesicle recycling and vesicle mobilization from a reserve pool that are important mechanisms controlling synaptic activity during sustained stimulation. Here, we will briefly review the main findings that assign a specific task to T-type channels in fast exocytosis discussing their possible involvement in the control of the Ca2+-dependent processes regulating synaptic activity and vesicular hormone release.