Specificity of striatal dopamine D1 system in humans: implications for clinical use of D1 receptor-agonists in Parkinson's disease

Functional pathology of neuroleptic-induced dystonia based on the striatal striosome-matrix dopamine system in humans

Neuroleptic-induced dystonia is a source of great concern in clinical practice because of its iatrogenic nature which can potentially lead to life-threatening conditions. Since all neuroleptics (antipsychotics) share the ability to block the dopamine D2-type receptors (D2Rs) that are highly enriched in the striatum, this drug-induced dystonia is thought to be caused by decreased striatal D2R activity. However, how associations of striatal D2R inactivation with dystonia are formed remains elusive.A growing body of evidence suggests that imbalanced activities between D1R-expressing medium spiny neurons and D2R-expressing medium spiny neurons (D1-MSNs and D2-MSNs) in the striatal striosome-matrix system underlie the pathophysiology of various basal ganglia disorders including dystonia. Given the specificity of the striatal dopamine D1 system in 'humans', this article highlights the striatal striosome hypothesis in causing 'repetitive' and 'stereotyped' motor symptoms which are key clinical features of dystonia. It is suggested that exposure to neuroleptics may reduce striosomal D1-MSN activity and thereby cause dystonia symptoms. This may occur through an increase in the striatal cholinergic activity and the collateral inhibitory action of D2-MSNs onto neighbouring D1-MSNs within the striosome subfields. The article proposes a functional pathology of the striosome-matrix dopamine system for neuroleptic-induced acute dystonia or neuroleptic-withdrawal dystonia. A rationale for the effectiveness of dopaminergic or cholinergic pharmacotherapy is also provided for treating dystonias. This narrative review covers various aspects of the relevant field and provides a detailed discussion of the mechanisms of neuroleptic-induced dystonia.

Therapeutic Effects of Dual Dopaminergic Modulation With l-DOPA and Chlorpromazine in Patients With Idiopathic Cervical Dystonia

Objectives

Imbalanced activities between dopamine D1 and D2 signals in striatal striosome-matrix system have been proposed as a cause of dystonia symptoms. The aim of this study was to assess the therapeutic effects of dual dopaminergic modulation (DDM) with l-DOPA and chlorpromazine (CPZ) in patients with idiopathic cervical dystonia (CD).

Methods

We enrolled 21 patients with CD who responded poorly to botulinum toxin treatment. The severities of CD motor symptoms and CD-associated pain were determined using the Toronto Western Spasmodic Torticollis Rating Scale and the visual analog scale, respectively.

Results

In patients with CD (n = 7), oral administration of l-DOPA combined with CPZ significantly attenuated both CD motor symptoms and CD-associated pain in a dose-related manner. By contrast, there was no improvement of CD symptoms in patients (n = 7) who ingested l-DOPA alone nor in those (n = 7) who ingested CPZ alone.

Discussion

DDM with l-DOPA and CPZ may be an effective tool to treat dystonia symptoms in patients with botulinum toxin-resistant idiopathic CD. Our results may also indicate that CD dystonia symptoms could be attenuated through DDM inducing an increase in striosomal D1-signaling.

Classification of Evidence

This study provides Class III evidence that treatment of botulinum toxin-resistant idiopathic cervical dystonia with l-DOPA and chlorpromazine is superior to either one alone.