Synaptotagmins 1 and 7 play complementary roles in somatodendritic dopamine release

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 Ca²⁺ concentration ([Ca²⁺]_o). Here we tested the hypothesis that the high-affinity Ca²⁺ sensor, synaptotagmin 7 (Syt7), is a key determinant of somatodendritic DA release and its Ca²⁺ 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 D2 DA autoreceptor-dependent inhibitory currents (D2ICs). Single-cell application of an antibody to Syt7 (Syt7 Ab) decreased pulse-train evoked D2ICs, revealing a functional role for Syt7. Assessment of the Ca²⁺-dependence of pulse-train evoked D2ICs confirmed robust DA release in submillimolar [Ca²⁺]_o in wild-type (WT) neurons, but loss of this sensitivity with intracellular Syt7 Ab or in Syt7 knockout (KO) mice. In millimolar [Ca²⁺]_o, pulse-train evoked D2ICs in Syt7 KOs showed a greater reduction in decreased [Ca²⁺]_o than seen in WT mice; the effect on single-pulse evoked DA release, however, did not differ between genotypes. Single-cell application of an antibody to Syt1 (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. SIGNIFICANCE STATEMENT The respective Ca²⁺ dependence of somatodendritic and axonal dopamine (DA) release differs, resulting in persistence of somatodendritic DA release in submillimolar Ca²⁺ concentrations too low to support axonal release. We demonstrate that synaptotagmin7 (Syt7), a high affinity Ca²⁺ sensor, underlies phasic somatodendritic DA release and its Ca²⁺ sensitivity in the substantia nigra pars compacta. In contrast, we found that synaptotagmin 1 (Syt1), the Ca²⁺ sensor underlying axonal DA release, plays a role in tonic, but not phasic somatodendritic DA release in wild-type mice. However, Syt1 can facilitate phasic DA release after Syt7 deletion. Thus, we show that both Syt1 and Syt7 act as Ca²⁺ sensors subserving different aspects of somatodendritic DA release processes.