DSCAM gene triplication causes excessive GABAergic synapses in the neocortex in Down syndrome mouse models.

Down syndrome (DS) is caused by the trisomy of human chromosome 21 (HSA21). A major challenge in DS research is to identify the HSA21 genes that cause specific symptoms. Down syndrome cell adhesion molecule (DSCAM) is encoded by a HSA21 gene. Previous studies have shown that the protein level of the Drosophila homolog of DSCAM determines the size of presynaptic terminals. However, whether the triplication of DSCAM contributes to presynaptic development in DS remains unknown. Here, we show that DSCAM levels regulate GABAergic synapses formed on neocortical pyramidal neurons (PyNs). In the Ts65Dn mouse model for DS, where DSCAM is overexpressed due to DSCAM triplication, GABAergic innervation of PyNs by basket and chandelier interneurons is increased. Genetic normalization of DSCAM expression rescues the excessive GABAergic innervations and the increased inhibition of PyNs. Conversely, loss of DSCAM impairs GABAergic synapse development and function. These findings demonstrate excessive GABAergic innervation and synaptic transmission in the neocortex of DS mouse models and identify DSCAM overexpression as the cause. They also implicate dysregulated DSCAM levels as a potential pathogenic driver in related neurological disorders.

Dopamine D4 receptor modulates inhibitory transmission in pallido-pallidal terminals and regulates motor behavior.

Two major groups of terminals release GABA within the Globus pallidus; one group is constituted by projections from striatal neurons, while endings of the intranuclear collaterals form the other one. Each neurons' population expresses different subtypes of dopamine D2-like receptors: D2 R subtype is expressed by encephalin-positive MSNs, while pallidal neurons express the D4 R subtype. The D2 R modulates the firing rate of striatal neurons and GABA release at their projection areas, while the D4 R regulates Globus pallidus neurons excitability and GABA release at their projection areas. However, it is unknown if these receptors control GABA release at pallido-pallidal collaterals and regulate motor behavior. Here, we present neurochemical evidence of protein content and binding of D4 R in pallidal synaptosomes, control of [3 H] GABA release in pallidal slices of rat, electrophysiological evidence of the presence of D4 R on pallidal recurrent collaterals in mouse slices, and turning behavior induced by D4 R antagonist microinjected in amphetamine challenged rats. As in projection areas of pallidal neurons, GABAergic transmission in pallido-pallidal recurrent synapses is under modulation of D4 R, while the D2 R subtype, as known, modulates striato-pallidal projections. Also, as in projection areas, D4 R contributes to control the motor activity differently than D2 R. This study could help to understand the organization of intra-pallidal circuitry.

Cannabinoid CB1 receptors activation and coactivation with D2 receptors modulate GABAergic neurotransmission in the globus pallidus and increase motor asymmetry.

The cannabinoid CB1 (CB1R) and dopaminergic D2 (D2R) receptors modify GABAergic transmission in the globus pallidus. Although dopaminergic denervation produces changes in the expression and supersensitization of these receptors, the consequences of these changes on GABAergic neurotransmission are unknown. The aim of this study was to show the effects of CB1R and D2R activation and coactivation on the uptake and release of [(3) H]GABA in the globus pallidus of hemiparkinsonian rats as well as their effects on motor behavior. The activation of CB1R blocked GABA uptake and decreased GABA release in the globus pallidus in the dopamine denervated side, whereas the co-activation of CB1R-D2R increased GABA release and had no effect on GABA uptake. A microinjection of the CB1R agonist ACEA into the globus pallidus ipsilaterally to a 6-OHDA lesion potentiated turning behavior that was induced by methamphetamine. However, a microinjection of the D2R agonist quinpirole did not modify this behavior, and a microinjection of a mixture of CB1R and D2R agonists significantly potentiated turning behavior. The behavioral effects produced after the activation of the CB1R and the co-activation of CB1R and D2R can be explained by increased GABAergic neurotransmission produced by a block of GABA uptake and an increase in the release of GABA in the globus pallidus, respectively.

Physical and functional convergence of the autism risk genes Scn2a and Ank2 in neocortical pyramidal cell dendrites.

Dysfunction in sodium channels and their ankyrin scaffolding partners have both been implicated in neurodevelopmental disorders, including autism spectrum disorder (ASD). In particular, the genes SCN2A, which encodes the sodium channel NaV1.2, and ANK2, which encodes ankyrin-B, have strong ASD association. Recent studies indicate that ASD-associated haploinsufficiency in Scn2a impairs dendritic excitability and synaptic function in neocortical pyramidal cells, but how NaV1.2 is anchored within dendritic regions is unknown. Here, we show that ankyrin-B is essential for scaffolding NaV1.2 to the dendritic membrane of mouse neocortical neurons and that haploinsufficiency of Ank2 phenocopies intrinsic dendritic excitability and synaptic deficits observed in Scn2a+/- conditions. These results establish a direct, convergent link between two major ASD risk genes and reinforce an emerging framework suggesting that neocortical pyramidal cell dendritic dysfunction can contribute to neurodevelopmental disorder pathophysiology.

Effects of chronic lithium treatment on neuronal excitability and GABAergic transmission in an Ank3 mutant mouse model.

Bipolar disorder (BD) is a common psychiatric disease that can lead to psychosocial disability, decreased quality of life, and high risk for suicide. Genome-wide association studies have shown that the ANK3 gene is a significant risk factor for BD, but the mechanisms involved in BD pathophysiology are not yet fully understood. Previous work has shown that ankyrin-G, the protein encoded by ANK3, stabilizes inhibitory synapses in vivo through its interaction with the GABAA receptor-associated protein (GABARAP). We generated a mouse model with a missense p.W1989R mutation in Ank3, that abolishes the interaction between ankyrin-G and GABARAP, which leads to reduced inhibitory signaling in the somatosensory cortex and increased pyramidal cell excitability. Humans with the same mutation exhibit BD symptoms, which can be attenuated with lithium therapy. In this study, we describe that chronic treatment of Ank3 p.W1989R mice with lithium normalizes neuronal excitability in cortical pyramidal neurons and increases inhibitory GABAergic postsynaptic currents. The same outcome in inhibitory transmission was observed when mice were treated with the GSK-3ß inhibitor Tideglusib. These results suggest that lithium treatment modulates the excitability of pyramidal neurons in the cerebral cortex by increasing GABAergic neurotransmission, likely via GSK-3 inhibition. In addition to the importance of these findings regarding ANK3 variants as a risk factor for BD development, this study may have significant implications for treating other psychiatric disorders associated with alterations in inhibitory signaling, such as schizophrenia, autism spectrum disorder, and major depressive disorder.

Synaptotagmin-7 facilitates acetylcholine release in splanchnic nerve-chromaffin cell synapses during nerve activity.

Disturbances that threaten homeostasis elicit activation of the sympathetic nervous system (SNS) and the adrenal medulla. The effectors discharge as a unit to drive global and immediate changes in whole-body physiology. Descending sympathetic information is conveyed to the adrenal medulla via preganglionic splanchnic fibers. These fibers pass into the gland and synapse onto chromaffin cells, which synthesize, store, and secrete catecholamines and vasoactive peptides. While the importance of the sympatho-adrenal branch of the autonomic nervous system has been appreciated for many decades, the mechanisms underlying transmission between presynaptic splanchnic neurons and postsynaptic chromaffin cells have remained obscure. In contrast to chromaffin cells, which have enjoyed sustained attention as a model system for exocytosis, even the Ca2+ sensors that are expressed within splanchnic terminals have not yet been identified. This study shows that a ubiquitous Ca2+-binding protein, synaptotagmin-7 (Syt7), is expressed within the fibers that innervate the adrenal medulla, and that its absence can alter synaptic transmission in the preganglionic terminals of chromaffin cells. The prevailing impact in synapses that lack Syt7 is a decrease in synaptic strength and neuronal short-term plasticity. Evoked excitatory postsynaptic currents (EPSCs) in Syt7 KO preganglionic terminals are smaller in amplitude than in wild-type synapses stimulated in an identical manner. Splanchnic inputs also display robust short-term presynaptic facilitation, which is compromised in the absence of Syt7. These data reveal, for the first time, a role for any synaptotagmin at the splanchnic-chromaffin cell synapse. They also suggest that Syt7 has actions at synaptic terminals that are conserved across central and peripheral branches of the nervous system.

Cannabinoid-induced depression of synaptic transmission is switched to stimulation when dopaminergic tone is increased in the globus pallidus of the rodent.

Because activation of D2 receptors reverses the neurochemical effects of cannabinoids, we examined whether increasing dopaminergic tone in the globus pallidus (GPe) switches cannabinoid induced depression of synaptic transmission. GABAergic synaptic currents evoked in pallidal neurons by stimulation of striatal projections (IPSCs) were depressed by perfusion with the CB1R agonist ACEA. Coactivation of D2Rs with quinpirole converted the depression into stimulation. Pretreatment with pertussis toxin (PTX) to limit Gi/o protein coupling also switched the CB1R-induced depression of IPSCs. The stimulation of IPSCs was blocked by the selective PKA blocker H89. Changes in the paired pulse ratio during both inhibitory and stimulatory responses indicate that the effects are due to changes in transmitter release. Postsynaptic depolarization induces endocannabinoid release that inhibits transmitter release (DSI). When D2Rs were activated with quinpirole, depolarization increased transmission instead of depressing it. This increase was blocked by AM251. We also examined the effects of CB1R/D2R coactivation on cAMP accumulation in the GPe to further verify that the AC/PKA cascade is involved. CB1R/D2R coactivation converted the inhibition of cAMP seen when each receptor is stimulated alone into a stimulation. We also determined the effects on turning behavior of unilateral injection of ACEA into the GPe of awake animals and its modification by dopamine antagonists. Blockade of D2 family receptors with sulpiride antagonized the motor effects of ACEA. We show, for the first time, that cannabinoid-inhibition of synaptic transmission in the GPe becomes a stimulation after D2Rs or PTX treatment and that the switch is probably relevant for the control of motor behavior.