Dopaminergic neurons establish a distinctive axonal arbor with a majority of non-synaptic terminals

Abstract

30Chemical neurotransmission in the brain typically occurs through synapses, which are structurally 31 and functionally defined as sites of close apposition between an axon terminal and a postsynaptic 32 domain. Ultrastructural examinations of axon terminals established by monoamine neurons in 33 the brain often failed to identify a similar tight pre-and postsynaptic coupling, giving rise to the 34 concept of "diffuse" or "volume" transmission. Whether this results from intrinsic properties of 35 such modulatory neurons remains undefined. Using an efficient co-culture model, we find that 36 dopaminergic neurons establish an axonal arbor that is distinctive compared to glutamatergic or 37 GABAergic neurons in both size and propensity of terminals to avoid direct contact with target 38 neurons. Furthermore, while most dopaminergic varicosities express key proteins involved in 39 exocytosis such as synaptotagmin 1, only 20% of these are synaptic. The active zone protein 40 bassoon was found to be enriched in a subset of dopaminergic terminals that are in proximity to 41 a target cell. Irrespective of their structure, a majority of dopaminergic terminals were found to 42 be active. Finally, we found that the presynaptic protein Nrxn-1 SS4and the postsynaptic protein 43 NL-1 AB , two major components involved in excitatory synapse formation, play a critical role in the 44 formation of synapses by dopamine neurons. Taken together, our findings support the idea that 45 dopamine neurons in the brain are endowed with a distinctive developmental program that leads 46 them to adopt a fundamentally different mode of connectivity, compared to glutamatergic and 47 GABAergic neurons involved in fast point-to-point signaling. 48 49 51 SIGNIFICANCE STATEMENT 52Midbrain dopamine (DA) neurons regulate circuits controlling movement, motivation, and 53 learning. The axonal connectivity of DA neurons is intriguing due to its hyperdense nature, with a 54 particularly large number of release sites, most of which not adopting a classical synaptic 55 structure. In this study, we provide new evidence highlighting the unique ability of DA neurons to 56 establish a large and heterogeneous axonal arbor with terminals that, in striking contrast with 57 glutamate and GABA neurons, actively avoid contact with the target cells. The majority of synaptic 58 and non-synaptic terminals express proteins for exocytosis and are active. Finally, our finding 59 suggests that, NL-1 A+B and Nrxn-1 SS4-, play a critical role in the formation of synapses by DA 60 neurons. 61 62 postsynaptic coupling at most release sites, giving rise to the concept of "diffuse" or "volume" 85 transmission, whereby neurotransmitter release from non-synaptic axon terminals leads to 86 activation of metabotropic receptors on target cells located at a distance, within a sphere of a 87 few tens of microns (17)(18)(19)(20)(21)(22)(23)(24). 88The molecular mechanisms determining the ability of DA neurons to establish synaptic 89 and non-synaptic terminals are presently unknown. M...

Molecular and functional architecture of striatal dopamine release sites

“…What other mechanisms could contribute? One possibility is that Ca V s are organized through transmembrane proteins rather than active zone complexes, for example neurexins which organize Ca V s and may drive synapse formation in cultured dopamine neurons (Ducrot et al, 2020; Luo et al, 2020). Another possibility is that α2δ proteins or β subunits drive positioning of various Ca V s in dopamine neurons, which could explain why subtype-specific positioning mechanism are dispensable (Held et al, 2020; Hoppa et al, 2012).…”

Implication of synaptotagmins 4 and 7 in activity-dependent somatodendritic dopamine release

“…It is made The copyright holder for this preprint this version posted January 26, 2021. ; https://doi.org/10.1101/2021.01.25.427983 doi: bioRxiv preprint Finally, an important outstanding question is the identification of the molecular mechanisms of STD DA release. Prior work has demonstrated that many proteins involved in regulated exocytosis, such as the calcium sensor Syt1, are selectively targeted to the axonal domain of neurons and not in dendrites 3,19,[35][36][37] . Building on previous in vitro work suggesting possible roles of Syt4 and Syt7 19 , in the present study we tested the hypothesis that Syt4 and Syt7 play a key role in STD DA release in the intact brain by quantifying STD DA release in Syt4, Syt7 and Syt4/7 double knockout (KO) mice.…”

Novel insights in the pathophysiology of α-synuclein dysregulation on D2 receptor activity contributing to the vulnerability of dopamine neurons

“…α-synuclein overexpression reduces arborization of dopamine neurons and pretreatment with a D2 receptor agonist partially rescues the detrimental impact of α-synuclein. Dopaminergic neurons have extensive axonal arborizations and large terminal fields [85][86][87] , where one dopamine neuron is estimated to have ~245,000 release sites 88,89 . Studies in animal models of PD and postmortem data in human PD 90 show decreased axonal complexity and dendritic arborization, reduction of number of axon terminals and global neuronal size precede neuronal death 63,88,91 .…”