Levodopa-induced plasticity: a double-edged sword in Parkinson's disease?

Abstract

The long-term replacement therapy with the dopamine (DA) precursor 3,4-dihydroxy-l-phenylalanine (L-DOPA) is a milestone in the treatment of Parkinson's disease (PD). Although this drug precursor can be metabolized into the active neurotransmitter DA throughout the brain, its therapeutic benefit is due to restoring extracellular DA levels within the dorsal striatum, which lacks endogenous DA as a consequence of the neurodegenerative process induced by the disease. In the early phases of PD, L-DOPA treatment is able to restore both long-term depression (LTD) and long-term potentiation (LTP), two major forms of corticostriatal synaptic plasticity that are altered by dopaminergic denervation. However, unlike physiological DA transmission, this therapeutic approach in the advanced phase of the disease leads to abnormal peaks of DA, non-synaptically released, which are supposed to trigger behavioural sensitization, namely L-DOPA-induced dyskinesia. This condition is characterized by a loss of synaptic depotentiation, an inability to reverse previously induced LTP. In the advanced stages of PD, L-DOPA can also induce non-motor fluctuations with cognitive dysfunction and neuropsychiatric symptoms such as compulsive behaviours and impulse control disorders. Although the mechanisms underlying the role of L-DOPA in both motor and behavioural symptoms are still incompletely understood, recent data from electrophysiological and imaging studies have increased our understanding of the function of the brain areas involved and of the mechanisms implicated in both therapeutic and adverse actions of L-DOPA in PD patients.

Keywords: Levodopa treatment; Parkinson's disease; animal models; synaptic plasticity.

Figure 1.

Schematic representation of the possible mechanisms of action, the motor and behavioural effects induced by a long-term treatment with L-DOPA in PD patients.

Figure 2.

Scheme of the experimental procedure to investigate L-DOPA-dependent striatal synaptic plasticity in a rat model of PD induced by unilateral DA denervation by 6-OHDA. The unilateral injection of 6-OHDA in the medial forebrain bundle (MFB) causes a full DA denervation in the ipsilateral substantia nigra pars compacta (SNc) as detected by the staining for tyrosine hydroxylase (TH). This full denervation induces severe motor impairment and loss of both LTD and LTP induced by high-frequency stimulation (HFS) protocols. Short-term treatment with L-DOPA (3 days) allows the improvement of motor function as well as the recovery of both LTP and LTD. Conversely, a long-term treatment with L-DOPA (21 days) induces dyskinesia in the majority of the rats. Diskinetic animals show a loss of both LTD after HFS and depotentiation (reversal of LTP) after a low-frequency stimulation (LFS) protocol. For further experimental details, see Picconi et al. [16,17].