Brain Advance Access originally published online on November 27, 2008
Brain 2009 132(2):319-335; doi:10.1093/brain/awn305
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Impact of grafted serotonin and dopamine neurons on development of L-DOPA-induced dyskinesias in parkinsonian rats is determined by the extent of dopamine neuron degeneration
1 Wallenberg Neuroscience Center, Department Experimental Medical Science, Lund University, BMC D11, 221 84 Lund, Sweden 2 Brain Repair and Imaging in Neural Systems (B.R.A.I.N.S) Unit, Lund University, BMC D11, 221 84 Lund, Sweden 3 Department of Neurology, Neurocentre, University of Freiburg, 79106 Freiburg, Germany
Correspondence to: Anders Björklund, Wallenberg Neuroscience Center, BMC A11, 221 84, Lund, Sweden E-mail: Anders.Bjorklund{at}med.lu.se
Previous studies have shown that serotonin neurons play an important role in the induction and maintenance of L-DOPA-induced dyskinesia in animals with lesion of the nigrostriatal dopamine system. Patients with Parkinson's disease that receive transplants of foetal ventral mesencephalic tissue, the graft cell preparation is likely to contain, in addition to dopamine neurons, serotonin neurons that will vary in number depending on the landmarks used for dissection. Here, we have studied the impact of grafted serotonin neurons—alone or mixed with dopamine neurons—on the development of L-DOPA-induced dyskinesia in rats with a partial 6-hydroxydopamine lesion of the host nigrostriatal projection. In these rats, which showed only low-level dyskinesia at the time of transplantation, serotonin grafts induced a worsening in the severity of dyskinesia that developed during continued L-DOPA treatment, while the dopamine-rich graft had the opposite, dampening effect. The detrimental effect seen in animals with serotonin neuron grafts was dramatically increased when the residual dopamine innervation in the striatum was removed by a second 6-hydroxydopamine lesion. Interestingly, rats with grafts that contained a mixture of dopamine and serotonin neurons (in 
2:1) showed a marked reduction in L-DOPA-induced dyskinesia over time, and the appearance of severe dyskinesia induced by the removal of the residual dopamine innervation, seen in the animals with transplants of serotonin neurons alone, was blocked. FosB expression in the striatal projection neurons, which is associated with dyskinesias, was also normalized by the dopamine-rich grafts, but not by the serotonin neuron grafts. These data indicate that as long as a sufficient portion, some 10–20%, of the dopamine innervation still remains, the increased host serotonin innervation generated by the grafted serotonin neurons will have limited effect on the development or severity of L-DOPA-induced dyskinesias. At more advanced stages of the disease, when the dopamine innervation of the putamen is reduced below this critical threshold, grafted serotonin neurons are likely to aggravate L-DOPA-induced dyskinesia in those cases where the dopamine re-innervation derived from the grafted neurons is insufficient in magnitude or do not cover the critical dyskinesia-inducing sub-regions of the grafted putamen. We conclude that it is not the absolute number of serotonin neurons in the grafts, but the relative densities of dopamine and serotonin innervations in the grafted striatum that is the critical factor in determining the long-term effect of foetal tissue graft, beneficial or detrimental, on dyskinesia in grafted Parkinson's disease patients.
Key Words: 5-HT; cell transplantation; levodopa; ventral mesenchephalon; Parkinson's disease
Abbreviations: AIMs, abnormal involuntary movements; DA, dopamine; 5,7-DHT, 5,7-dihydroxytryptamine; MFB, medial forebrain bundle; 6-OHDA, 6-hydroxydopamine; SERT, serotonin transporter; TH, tyrosine hydroxylase; VM, ventral mesencephalic
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Received June 11, 2008. Revised October 21, 2008. Accepted October 22, 2008.
*Present address: Experimental Neurology Unit, Department of Neurology, Phillips University Marburg, Hans Meerwein Strasse, 350 43 Marburg, Germany.
These authors contributed equal to this work.
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