Dopamine synapse is a neuroligin-2–mediated contact between dopaminergic presynaptic and GABAergic postsynaptic structures

Abstract

Midbrain dopamine neurons project densely to the striatum and form so-called dopamine synapses on medium spiny neurons (MSNs), principal neurons in the striatum. Because dopamine receptors are widely expressed away from dopamine synapses, it remains unclear how dopamine synapses are involved in dopaminergic transmission. Here we demonstrate that dopamine synapses are contacts formed between dopaminergic presynaptic and GABAergic postsynaptic structures. The presynaptic structure expressed tyrosine hydroxylase, vesicular monoamine transporter-2, and plasmalemmal dopamine transporter, which are essential for dopamine synthesis, vesicular filling, and recycling, but was below the detection threshold for molecules involving GABA synthesis and vesicular filling or for GABA itself. In contrast, the postsynaptic structure of dopamine synapses expressed GABAergic molecules, including postsynaptic adhesion molecule neuroligin-2, postsynaptic scaffolding molecule gephyrin, and GABA A receptor α1, without any specific clustering of dopamine receptors. Of these, neuroligin-2 promoted presynaptic differentiation in axons of midbrain dopamine neurons and striatal GABAergic neurons in culture. After neuroligin-2 knockdown in the striatum, a significant decrease of dopamine synapses coupled with a reciprocal increase of GABAergic synapses was observed on MSN dendrites. This finding suggests that neuroligin-2 controls striatal synapse formation by giving competitive advantage to heterologous dopamine synapses over conventional GABAergic synapses. Considering that MSN dendrites are preferential targets of dopamine synapses and express high levels of dopamine receptors, dopamine synapse formation may serve to increase the specificity and potency of dopaminergic modulation of striatal outputs by anchoring dopamine release sites to dopamine-sensing targets.dopamine synapse | neuroligin-2 | medium spiny neuron | striatum C hemical synapses comprise presynaptic machinery for transmitter release and postsynaptic machinery for receptor-mediated signal transduction in a neurochemically matched manner. They are classified into Gray type-I and type-II synapses by asymmetric or symmetric membrane density, respectively, of the preand postsynaptic structures (1). Most, if not all, asymmetric and symmetric synapses are excitatory and inhibitory, respectively, as they selectively express ionotropic glutamate or GABA/glycine receptors together with their specific scaffolding proteins (2). Neurochemical matching of chemical synapses is controlled by activity-dependent mechanisms (3, 4), and mediated by transmembrane adhesion proteins and secreted molecules (5). The neuroligin (NL) family comprises postsynaptic adhesion molecules that form transsynaptic contacts with presynaptic neurexins (Nrxn) (6). Of note, NL1 and NL2 are selectively expressed at glutamatergic and GABAergic synapses, respectively (7,8), and are required for activity-dependent specification of the corresponding synapses (9).Midbrain dopamine neurons project densely to the striat...

Dopamine Secretion Is Mediated by Sparse Active Zone-like Release Sites

“…Remarkably, dopamine may also mediate relatively fast postsynaptic currents (Gantz et al, 2013; Marcott et al, 2014). Furthermore, synaptic contacts of dopamine neurons are composed of dopaminergic presynaptic and GABAergic postsynaptic structures (Uchigashima et al, 2016), consistent with the observation that dopamine neurons synaptically co-release dopamine and GABA (Tritsch et al, 2012). These observations, together with our findings that only ~30% of dopamine varicosities contain active zone-like release sites, raise the possibility that dopamine is exclusively released at synaptic sites in the striatum.…”

“…These observations, together with our findings that only ~30% of dopamine varicosities contain active zone-like release sites, raise the possibility that dopamine is exclusively released at synaptic sites in the striatum. However, because dopamine receptors appear to be present mostly outside of synaptic structures (Cailld et al, 1996; Uchigashima et al, 2016; Yung et al, 1995), dopamine would still need to diffuse after release to extrasynaptic sites for signaling. To determine how dopamine codes precisely timed events, future studies will need to assess localization of dopamine receptors relative to its secretory sites.…”

“…Axons from these dopamine neurons project to the striatum, where dopamine regulates circuits for movement, motivation and reward (Howe and Dombeck, 2016; Schultz, 1998; Surmeier et al, 2014). In the striatum, dopamine receptors are mostly localized outside of synapses (Cailld et al, 1996; Uchigashima et al, 2016; Yung et al, 1995), many axonal dopamine varicosities are not associated with postsynaptic densities (Descarries et al, 1996), and dopamine receptors are G-protein coupled, whose signaling is more than two orders of magnitude slower than that of ionotropic receptors (Missale et al, 1998). These properties have led to the conclusion that dopamine is a volume transmitter that signals slowly and inaccurately, and suggests that an active zone that warrants speed and accuracy may be unnecessary for dopamine release.…”

A novel synaptic junction preparation for the identification and characterization of cleft proteins

“…Presynaptic neurexins are known to bind to postsynaptic neuroligins to form trans-synaptic complexes [19], [20]. Immuno-electron microscopy in intact neurons shows that label for neurexins and neuroligins localizes selectively to the synaptic junction to pre- and post-synaptic sides respectively of the synaptic cleft (Fig 6), [21], [22], [23], thus, in agreement with the results of biochemical analyses. Similarly, SJ fractions were enriched in N-cadherin, a protein that forms homophilic trans-synaptic bridges, in agreement with immuno-EM studies showing its selective localization at the synaptic junctional area [24], [25].…”

Synaptic Neurexin Complexes: A Molecular Code for the Logic of Neural Circuits

“…Neuroligins are differentially targeted to synapses. Nlgn1 is localized to excitatory synapses (Song et al, 1999) and Nlgn2 to inhibitory, dopaminergic and cholinergic synapses (Graf et al, 2004; Varoqueaux et al, 2004; Uchigashima et al, 2016; Takacs et al, 2013), possibly because dopaminergic and cholinergic synapses use GABA as a co-transmitter. Nlgn3 is found in both excitatory and inhibitory synapses (Budreck and Scheiffele, 2007), and Nlgn4 in glycinergic synapses (Hoon et al, 2011).…”