1. Dopaminergic pathophysiology: hypothesis on RLS mechanisms Walter Paulus Göttingen Germany

2. Why does l-dopa work at all if the firing rate of dopaminergic neurons is not altered?

3. Increase of vesicular size by 350 % More dopamine release per spike

4. Overtreatment: iron deficiency deteriorates function of the dopamine transporter leading to even higher dopamine levels untreated RLS: impairment of tyrosine hydroxylase? Iron deficiency reduces D2 receptors Erikson et al J Nutr 2000. Iron deficiency: different implications in untreated and overtreated patients

5. How much L-DOPA do we need in RLS?

6. 10 mg L-Dopa are supposed to finally enter the brain 1 mg is available for dopamine synthesis which amounts to 3 x 10 18 molecules of dopamine being synthesised „empty vesicles“ can be refilled completely within 10 – 15 min after absorption (cf Segawa disease) What happens with 100 mg L-Dopa? de la Fuente-Fernandez, 2004 Σ 1 tablet L-Dopa can „double“ the dopamine content of the dopaminergic system

7. 100 mg L-DOPA very likely to be (very or too?) efficient in RLS

8. With increasing L-Dopa dosage symptoms reoccur: U shaped dose response curve

9. Why too much dopamine with 200 to 300 mg/day in RLS and not in Parkinson‘s disease? In contrast to Parkinson‘s disease in RLS there is probably no degeneration of dopaminergic neurones (exception A11 degeneration?), hence small doses of L-DOPA may cause an overflow of dopamine.

10. Why too much dopamine with 200 to 300 mg/day in RLS and not in Parkinson‘s disease?. “suggesting absence of short and long loop regulation of this system within the spinal cord” “undoubtedly affects their response to DA agonist treatment” Spinal cord under dopaminergic overdosage and iron deficiency: particular high potential for too much dopamine in the synaptic cleft

11. “suggesting absence of short and long loop regulation of this system within the spinal cord” “undoubtedly affects their response to DA agonist treatment” Spinal cord under dopaminergic overdosage and iron deficiency: particular high potential for too much dopamine in the synaptic cleft

12. How much L-Dopa is too much and causes augmentation ? Second key question: How can the same drug l-DOPA be beneficial in RLS in low concentrations and worsening in high concentrations?

13. Dopaminergic A11 neurones only source of spinal dopamine

15. D2 / D3 / D4 antinociceptiv D1 / D5 pronociceptiv

16. Augmentation: The D1 hypothesis Paulus and Trenkwalder, 2006 A relative overstimulation of the dopamine D1 receptors compared to D2 receptors (in the spinal cord) may lead to D1-related pain and generate periodic limb movements –More D1 receptors are located outsite the synaptic cleft –Increased dopmine in the synaptic cleft: (too much) dopamine leaves the synaptic cleft –private synapses convert to social synapses Hyperdopaminergic state: D2 receptors reduced in number, unlike D1 receptors that are recycled to the plasma membrane (Bartlett, 2005)

17. How does a higher dopamine concentration induce a shift towards D1 stimulation?

18. Dopaminergic excitation of spinal locomotor circuits appears to be facilitated mainly by D1 receptors in mice by –decreasing IA current –SKCa (small-conductance calcium-activated K channel) currents –increasing glutamatergic synaptic transmission Further support for a „D1 Hypothesis“

19. A11 lesion reduces D1, D2, D3 receptors Iron deficiendy shifts D1 : D2/D3 balance Combination induces largest shift in balance

21. Relative shift from D2/D3 to D1 receptors in a) RLS (in iron deficiency) b) Augmentation So far D1 and D2/D3 separated in space! Augmentation develops over time We need to take care for a separation in time as well!

22. Augmentation is comparable to medication overuse headache, short term improvement with drug therapy and long term worsening

23. SCH: D1 Blockade In 4 of the 14 rats treated with the 200-mg/kg dose, hyperalgesia reappeared between 70 to 90 rmin after the administration. No such late hyperalgesic phase was observed with the 100-mg/kg dose. amelioration increase pain

24. SKF: D1 Agonist increase amelioration pain

25. increase amelioration pain

26. Increase in receptor internalization may occur rapidly within 1 min in response to increased synaptic DA concentrations; Kim et al. (2004) Relatively long time-course of amphetamine-induced reduction in D2 tracer binding as measured by PET as compared with the shorter timescale of increased DA concentrations as assessed by microdialysis.

27. Sensitivity of dopaminergic receptors was augmented by L-DOPA, and a drug-free period was required to develop the receptor super-sensitivity. For L-DOPA induced hyperalgesia a 1 h drug-free period to produce hyperalgesia was required. Repeated intermittent L-DOPA treatment is usually required to produce sensitization Sulpiride induces hyperalgesia in L-DOPA-naive mice. It however failed to enhance L-DOPA-induced hyperalgesia, suggesting that the endogenous dopaminergic inhibitory system of pain sensation was impaired.

28. The D1 antagonist SCH23390 significantly depressed the enhancement of pain perception. D1 receptors modify the pain conduction only in L-DOPA-primed animals, not in the L-DOPA-naive animals. In the physiological condition, D1 receptors may slightly facilitate the conduction of pain sensation and this effect can be suppressed by an activation of D2 receptors After a withdrawal of LDOPA, effect of D1 receptors to facilitate pain conduction could become apparent. This suggests that D1 receptors only work in pathological conditions. The dopaminergic inhibitory system for pain conduction, in which D2 receptors are mainly involved, is impaired after L-DOPA priming, and D1 receptors take part in the pain conduction system in this pathological condition. Further statements from Shimizu et al, 2007 immediately related to augmentation

29. D1 receptors modify the pain conduction only in L-DOPA-primed animals, not in the L-DOPA-naive animals. This suggests that D1 receptors only work in pathological conditions. In the physiological condition, D1 receptors may slightly facilitate the The present results suggest that dopaminergic inhibitory system for pain conduction, in which D2 receptors are mainly involved, is impaired after L-DOPA priming, and D1 receptors take part in the pain conduction system in this pathological condition.

31. 4 pieces of evidence of long term plastic alterations by L- DOPA or dopamine agonists

33. L-Dopa treatment: Acute: alleviation of RLS sympt, not of mechanical hyperalgesia, Chronic: alleviation of both CW = cotton whisp QT = Q-tip BR = brush

34. Induction of augmentation by „inhibitory“ D2 receptor agonism over time

35. Kuo et al, 2008 Dopamine dramatically Prolongs tDCS (transcranial direct current Stimulation) and PAS (paired associated Stimulation) aftereffects

36. Some open questions Dopamine (production) deficit versus increased dopamine turnover? RLS untreated: shifted balance between dopamine D1 / D2 receptor stimulation versus D2 / D3 / D4 D1 / D2 receptor balance depending on absolute dopamine concentration? Augmentation: long term shift towards D1 / D5? If so, why can D2 agonists cause augmentation at all? short versus long term time course of D1 versus D2 receptor trafficking Do plastic changes like those in the ELL-DOPA study play a role in RLS? Role of Priming by L-DOPA