[Role and regulation of dopamine D1 receptors in the striatum: implications for the genesis of dyskinesia in Parkinson's disease]

Pain in Parkinson's disease: a neuroanatomy-based approach

Parkinson's disease is a progressive neurodegenerative disorder characterized by the deposition of misfolded alpha-synuclein in different regions of the central and peripheral nervous system. Motor impairment represents the signature clinical expression of Parkinson's disease. Nevertheless, non-motor symptoms are invariably present at different stages of the disease and constitute an important therapeutic challenge with a high impact for the patients' quality of life. Among non-motor symptoms, pain is frequently experienced by patients, being present in a range of 24-85% of Parkinson's disease population. Moreover, in more than 5% of patients, pain represents the first clinical manifestation, preceding by decades the exordium of motor symptoms. Pain implies a complex biopsychosocial experience with a downstream complex anatomical network involved in pain perception, modulation, and processing. Interestingly, all the anatomical areas involved in pain network can be affected by a-synuclein pathology, suggesting that pathophysiology of pain in Parkinson's disease encompasses a 'pain spectrum', involving different anatomical and neurochemical substrates. Here the various anatomical sites recruited in pain perception, modulation and processing are discussed, highlighting the consequences of their possible degeneration in course of Parkinson's disease. Starting from peripheral small fibres neuropathy and pathological alterations at the level of the posterior laminae of the spinal cord, we then describe the multifaceted role of noradrenaline and dopamine loss in driving dysregulated pain perception. Finally, we focus on the possible role of the intertwined circuits between amygdala, nucleus accumbens and habenula in determining the psycho-emotional, autonomic and cognitive experience of pain in Parkinson's disease. This narrative review provides the first anatomically driven comprehension of pain in Parkinson's disease, aiming at fostering new insights for personalized clinical diagnosis and therapeutic interventions.

Increased sensitivity in the interaction of the dopaminergic/adenosinergic system at the level of the adenylate cyclase activity in the striatum of the "weaver" mouse

The specific antagonistic interaction between dopamine D1 and adenosine A1 receptors (D1/A1), as well as between dopamine D2 and adenosine A2a receptors (D2/A2a) exist not only at the receptor/receptor level, but also at the level of the secondary messengers. In this study, we examined the possible changes in these interactions at the level of cAMP formation in membrane preparation from "weaver" mouse striatum (a genetic model of Parkinson disease), by using specific agonists of these receptors. We also examined in the striatum of the "weaver" mouse the interaction between D1 and D2 dopamine receptors. Our results showed that in the striatum of "weaver" mice: a) the cAMP synthesis induced by D1 receptor activation (SKF 38393), was significantly reduced compared to control mice, while A1 receptor activation (L-PIA) leaded to a more intense inhibition of the D1-induced cAMP-formation compared to the controls, b) the cAMP synthesis which was induced by A2a receptor activation (CGS 21680), was significantly increased compared to the control mice. The specific D2 receptor agonist Quinpirole, added in low concentrations, caused a significant reduction of the A2a-induced cAMP formation, which was not observed in the control mouse. Furthermore, the D1 receptor induced cAMP synthesis was significantly higher in control compared to "weaver" striatum, which was more efficiently downregulated by D2 receptor agonist Quinpirole. These results suggest that the sensitivity to D1 and A2a receptor agonists is altered and that the interaction between D1/A1 and D2/A2a receptors is enhanced in the striatum of the "weaver" mutation, while an uncoupling between D1 and D2 receptors was observed. Since the adenylate cyclase basal activity did not differ between "weaver" and control striatum, the above-mentioned changes seem to be due to alterations in the function of the adenosine/dopamine receptors and their coupling to the G-proteins.

Intermittent Dopaminergic stimulation causes behavioral sensitization in the addicted brain and parkinsonism

The gold standard therapy for Parkinson's disease (PD) consists in chronic administration of pulses of the dopamine (DA) precursor l-dihydroxyphenylalanine (l-DOPA). Although the main brain area which is DA-deficient is the dorsal striatum (more the putamen than the caudate nucleus), other DA-innervated brain regions (i.e., the ventral striatum and other limbic areas) are affected by systemic administration of l-DOPA. While such a therapy produces an increase in synaptic and nonsynaptic DA, which replace the neurotransmitter deficiency, peaks of extracellular DA in the course of disease progression produce abnormal involuntary movements related to behavioral sensitization. Methamphetamine (METH), a widely abused drug, is known to produce behavioral sensitization, related to DA release (more in the ventral than dorsal striatum as well as other limbic regions). The present review discusses the overlapping between these treatments, based on pulses of DA stimulation with an emphasis on the class of DA receptors; signal transduction pathways; rearranged expression of neurotransmitters, cotransmitters, and their receptors coupled with ultrastructural changes. In fact, all these levels of synaptic plasticity show a surprising homology following these treatments, posing the mechanisms of behavioral sensitization during DA-replacement therapy in PD very close to the neurobiological mechanisms operating during METH abuse. In line with this view is the growing evidence of addictive behaviors in PD patients during the course of DA-replacement therapy.