Dynamics Of Neurotransmitter Release From Dopaminergic Neurons In Brain Slices

Dopamine (DA) transmission plays multiple important roles in motivation, movementinitiation and execution, reward signaling, and tobacco addiction. The action potential (AP)firing in the dopaminergic neurons locally triggers DA release in the somata and conductsalong the axons to trigger DA release in their terminals in the striatum (ascending pathway).Beyond the AP firing in dopaminergic neurons, synchronized firing in striatal cholinergicinterneurons also triggers transmitter release in dopaminergic terminals (hijack pathway).Timing of synaptic transmission is critical in neuronal circuits for executing behavioralprocesses. However, the timing property in either somatic DA release or terminal DA releaseremains unclear. The nicotinic modulation of striatal DA release is also elusive.Here, we investigated the dynamics of both somatic and terminal DA release in the freshbrain slices with combined path-clamp and electrochemical recordings, optogeneticstimulation, electrical-field stimulation, and amperometry recording. We found:(1)depolarization evoked quantal DA release from the somata of dopaminergic neurons in SNcslices. The median of the latency of the evoked DA release was486ms, which was~45timeslonger than the latency of terminal DA release in the striatum.(2) By selective stimulatingcholinergic intermeurons in the ChAT-ChR2-EYFP transgenic mice, we determined thelatency of DA release via the hijack pathway (17.8ms) and via the ascending pathway (10.8ms); After the optogenetic stimulation, cholinergic interneurons fired APs in the first2.8ms,cholinergic transmission completed in the following7.0ms, and the downstream DA releasestarted in the next10.8ms. The total latency of hijack DA release was regulated by bothpresynaptic vesicular Ach release and postsynaptic nAChRs.(3) In hijack pathway, thedopaminergic vesicle pool immediately replenished with exponential time constantτDT=15 s, while the apparent vesicle pool in the complex synapse replenished with τCTDT=17s afterabsolute refractory time T_ARP=1s.(4) Smoking-level-nicotine selectively blocked DArelease via the hijack pathway but not the ascending pathway, and reduced the depletion ofDA vesicle pool to permit the facilitation of following sustained release under the stimulationon both pathways.In summary, we determined the temperol components of somatic DA release andterminal DA release via the two pathways, and the modulation mechanism of DA release bysmoking-level-nicotine, which helps us to further understand the functions and properties ofdopamine release under physiological and tobacco addiction conditions.