Synaptotagmins 1 and 7 Play Complementary Roles in Somatodendritic Dopamine Release

Abstract

The molecular mechanisms underlying somatodendritic dopamine (DA) release remain unresolved, despite the passing of decades since its discovery. Our previous work showed robust release of somatodendritic DA in submillimolar extracellular Ca 21 concentration ([Ca 21 ] o ). Here we tested the hypothesis that the high-affinity Ca 21 sensor synaptotagmin 7 (Syt7), is a key determinant of somatodendritic DA release and its Ca 21 dependence. Somatodendritic DA release from SNc DA neurons was assessed using whole-cell recording in midbrain slices from male and female mice to monitor evoked DA-dependent D2 receptor-mediated inhibitory currents (D2ICs). Single-cell application of an antibody to Syt7 (Syt7 Ab) decreased pulse trainevoked D2ICs, revealing a functional role for Syt7. The assessment of the Ca 21 dependence of pulse train-evoked D2ICs confirmed robust DA release in submillimolar [Ca 21 ] o in wild-type (WT) neurons, but loss of this sensitivity with intracellular Syt7 Ab or in Syt7 knock-out (KO) mice. In millimolar [Ca 21 ] o , pulse train-evoked D2ICs in Syt7 KOs showed a greater reduction in decreased [Ca 21 ] o than seen in WT mice; the effect on single pulse-evoked DA release, however, did not differ between genotypes. Single-cell application of a Syt1 Ab had no effect on train-evoked D2ICs in WT SNc DA neurons, but did cause a decrease in D2IC amplitude in Syt7 KOs, indicating a functional substitution of Syt1 for Syt7. In addition, Syt1 Ab decreased single pulse-evoked D2ICs in WT cells, indicating the involvement of Syt1 in tonic DA release. Thus, Syt7 and Syt1 play complementary roles in somatodendritic DA release from SNc DA neurons.

Synaptotagmin-7 Counteracts Short-Term Depression during Phasic Dopamine Release

“…Dopamine is also released in the midbrain from somatodendritic compartments where it inhibits cell firing ( Beckstead et al, 2004 ). Recent reports have described a role of SYT7 in somatodendritic dopamine release similar to the above-described role in terminal regions ( Hikima et al, 2022 ). To confirm SYT7's role in modulating dopamine release from the somatodendritic compartment, we performed whole-cell voltage-clamp recordings of D2-IPSCs in the SNc from WT and Syt7 KO slices.…”

“…Variable results have also been observed for SYT7's role in the somatodendritic compartment. Intracellular application of a SYT7 antibody significantly reduced autaptic D2-IPSCs in response to stimulus trains ( Hikima et al, 2022 ). However, dopamine release triggered by stimulus trains measured using FSCV was not different in Syt7 KO mice compared with controls, though a double KO of Syt7 and Syt4 , a non–Ca 2+ -binding synaptotagmin isoform, displayed a marked reduction of release ( Delignat-Lavaud et al, 2022 ).…”

“…At most presynaptic terminals, including those of dopamine neurons, action potential-induced Ca 2+ influx triggers fast synchronous fusion by activating the low-affinity vesicular Ca 2+ sensor synaptotagmin-1 (SYT1; Banerjee et al, 2020 ; Hikima et al, 2022 ; Liu et al, 2022 ; Delignat-Lavaud et al, 2023 ; Lebowitz et al, 2023 ). After Ca 2+ influx, submicromolar “residual” Ca 2+ persists in the presynaptic cytoplasm for hundreds of milliseconds and activates the high-affinity Ca 2+ sensor synaptotagmin-7 (SYT7).…”

“…Syt7 RNA is expressed in many cell types in the brain but is enriched in SNc neurons ( Saunders et al, 2018 ). Studies using Syt7 KO animals or antibody-mediated inhibition suggest that SYT7 regulates dopamine release from the somatodendritic compartment ( Delignat-Lavaud et al, 2022 ; Hikima et al, 2022 ). Further, high-frequency stimulation restores somatodendritic dopamine release in SYT1 KO animals, and high-frequency stimulation and DAT blockade reveal a component of release from terminals in SYT1 KO animals that is postulated to be under the control of SYT7 ( Banerjee et al, 2020 ; Lebowitz et al, 2023 ).…”

Mechanisms of Synaptic Vesicle Exo- and Endocytosis

“…In contrast, synaptotagmin-1, underlying axonal dopamine release, plays a role in tonic dopamine release. However, synaptotagmi-1 can facilitate phasic dopamine release after synaptotagmin-7 deletion [ 88 ]. These results indicate that synaptotagmin Ca 2+ sensors subserve different aspects of the transmitter release processes.…”