The Associations between COMT and MAO-B Genetic Variants with Negative Symptoms in Patients with Schizophrenia

Negative symptoms of schizophrenia, including anhedonia, represent a heavy burden on patients and their relatives. These symptoms are associated with cortical hypodopamynergia and impaired striatal dopamine release in response to reward stimuli. Catechol-O-methyltransferase (COMT) and monoamine oxidase type B (MAO-B) degrade dopamine and affect its neurotransmission. The study determined the association between COMT rs4680 and rs4818, MAO-B rs1799836 and rs6651806 polymorphisms, the severity of negative symptoms, and physical and social anhedonia in schizophrenia. Sex-dependent associations were detected in a research sample of 302 patients with schizophrenia. In female patients with schizophrenia, the presence of the G allele or GG genotype of COMT rs4680 and rs4818, as well as GG haplotype rs4818-rs4680, which were all related to higher COMT activity, was associated with an increase in several dimensions of negative symptoms and anhedonia. In male patients with schizophrenia, carriers of the MAO-B rs1799836 A allele, presumably associated with higher MAO-B activity, had a higher severity of alogia, while carriers of the A allele of the MAO-B rs6651806 had a higher severity of negative symptoms. These findings suggest that higher dopamine degradation, associated with COMT and MAO-B genetic variants, is associated with a sex-specific increase in the severity of negative symptoms in schizophrenia patients.

Cell-intrinsic effects of TorsinA(ΔE) disrupt dopamine release in a mouse model of DYT1-TOR1A dystonia

ABSTRACTDYT1-TOR1A dystonia is an inherited dystonia caused by a three base-pair deletion in the TOR1A gene (TOR1AΔE). Although the mechanisms underlying the dystonic movements are largely unknown, abnormalities in striatal dopamine and acetylcholine neurotransmission are consistently implicated whereby dopamine release is reduced while cholinergic tone is increased. Because striatal cholinergic neurotransmission mediates dopamine release, it is not known if the dopamine release deficit is mediated indirectly by abnormal acetylcholine neurotransmission or if Tor1a(ΔE) acts directly within dopaminergic neurons to attenuate release. To dissect the microcircuit that governs the deficit in dopamine release, we conditionally expressed Tor1a(ΔE) in either dopamine neurons or cholinergic interneurons in mice and assessed striatal dopamine release using ex vivo fast scan cyclic voltammetry or dopamine efflux using in vivo microdialysis. Conditional expression of Tor1a(ΔE) in cholinergic neurons did not affect striatal dopamine release. In contrast, conditional expression of Tor1a(ΔE) in dopamine neurons reduced dopamine release to 50% of normal, which is comparable to the deficit in Tor1a+/ΔE knockin mice that express the mutation ubiquitously. Despite the deficit in dopamine release, we found that the Tor1a(ΔE) mutation does not cause obvious nerve terminal dysfunction as other presynaptic mechanisms, including electrical excitability, vesicle recycling/refilling, Ca2+ signaling, D2 dopamine autoreceptor function and GABAB receptor function, are intact. Although the mechanistic link between Tor1a(ΔE) and dopamine release is unclear, these results clearly demonstrate that the defect in dopamine release is caused by the action of the Tor1a(ΔE) mutation within dopamine neurons.

BLOCKADE OF M4 MUSCARINIC RECEPTORS ON STRIATAL CHOLINERGIC INTERNEURONS NORMALIZES STRIATAL DOPAMINE RELEASE IN A MOUSE MODEL OF DYT1-TOR1A DYSTONIA

ABSTRACTTrihexyphenidyl (THP), a non-selective muscarinic receptor (mAChR) antagonist, is the preferred oral pharmaceutical for the treatment of DYT1-TOR1A dystonia. A better understanding of the mechanism of action of THP is a critical step in the development of better therapeutics with fewer side effects. Using a mouse model of DYT1-TOR1A dystonia (Tor1a+/ΔE KI mice), we recently found that THP normalized striatal DA release, revealing a plausible mechanism of action for this compound. However, the exact mAChR subtypes that mediate this effect remain unclear. In this study we used a combination of a newly developed M4 subtype-selective mAChR antagonist and cell-type specific mAChR KO mice to determine which mAChR subtypes mediate the DA enhancing effects of THP. We determined that THP and the M4 subtype-selective mAChR antagonist enhance striatal DA release by blocking M4 mAChR on striatal cholinergic interneurons in Tor1a+/ΔE KI mice. However, in Tor1a+/+ mice THP increases striatal DA release through a combination of M1 and M4 mAChR, which reveals an alteration in M1 mAChR function in Tor1a+/ΔE KI mice. Taken together these data implicate a principal role for M4 mAChR located on striatal cholinergic interneurons in the mechanism of action of THP and suggest that M4-subtype selective mAChR antagonists may be more efficacious therapeutics for DYT1-TOR1A dystonia.SIGNIFICANCE STATEMENTTrihexyphenidyl, a non-selective muscarinic receptor antagonist, is the preferred oral therapeutic for DYT1-TOR1A dystonia, but it is poorly tolerated due to significant side effects. A better understanding of the mechanism of action of trihexyphenidyl is needed for the development of improved therapeutics. We recently found that trihexyphenidyl rescues the deficit in both striatal dopamine release and steady-state extracellular striatal dopamine concentrations in a mouse model of DYT1-TOR1A dystonia. However, the precise muscarinic receptor subtype(s) that mediate these effects are unknown. We used a newly developed M4 muscarinic receptor subtype-selective antagonist along with M1 and M4 muscarinic receptor knockout mice to determine the precise muscarinic receptor subtypes that mediate the dopamine-enhancing effects of trihexyphenidyl.

Molecular and functional architecture of striatal dopamine release sites

Dopamine controls striatal circuit function, but its transmission mechanisms are not well understood. We recently showed that dopamine secretion requires RIM, suggesting that it occurs at active zone-like sites similar to conventional synapses. Here, we establish using a systematic conditional gene knockout approach that Munc13 and Liprin-α, active zone proteins for vesicle priming and release site organization, are important for dopamine secretion. Correspondingly, RIM zinc finger and C2B domains, which bind to Munc13 and Liprin-α, respectively, are needed to restore dopamine release in RIM knockout mice. In contrast, and different from conventional synapses, the active zone scaffolds RIM-BP and ELKS, and the RIM domains that bind to them, are expendable. Hence, dopamine release necessitates priming and release site scaffolding by RIM, Munc13, and Liprin-α, but other active zone proteins are dispensable. Our work establishes that molecularly simple but efficient release site architecture mediates fast dopamine exocytosis.

Axonal mechanisms mediating γ-aminobutyric acid receptor type A (GABA-A) inhibition of striatal dopamine release

Axons of dopaminergic neurons innervate the striatum where they contribute to movement and reinforcement learning. Past work has shown that striatal GABA tonically inhibits dopamine release, but whether GABA-A receptors directly modulate transmission or act indirectly through circuit elements is unresolved. Here, we use whole-cell and perforated-patch recordings to test for GABA-A receptors on the main dopaminergic neuron axons and branching processes within the striatum of adult mice. Application of GABA depolarized axons, but also decreased the amplitude of axonal spikes, limited propagation and reduced striatal dopamine release. The mechanism of inhibition involved sodium channel inactivation and shunting. Lastly, we show the positive allosteric modulator diazepam enhanced GABA-A currents on dopaminergic axons and directly inhibited release, but also likely acts by reducing excitation from cholinergic interneurons. Thus, we reveal the mechanisms of GABA-A receptor modulation of dopamine release and provide new insights into the actions of benzodiazepines within the striatum.