BackgroundPain constitutes the major non motor syndrome in Parkinson's disease (PD) and includes neuropathic pain; however current drug therapies used to alleviate it have only limited efficacy. This is probably due to poor understanding of the mechanisms underlying it.AimsWe investigated a major class of trigeminal neuropathic pain, dynamic mechanical allodynia (DMA), in a rat model of PD and in which a bilateral 6-hydroxy dopamine (6-OHDA) injection was administered to produce a lesion of the nigrostriatal dopaminergic pathway.Results and discussionLesioned animals presented significant DMA in the orofacial area that occurred from 4 days to 5 weeks post-injury. To investigate a segmental implication in the neuropathic pain induced by dopamine depletion, the expression of the isoform gamma of the protein kinase C (PKCg) and phosphorylated extracellular signal-regulated kinases 1/2 (pERK1/2) was explored in the medullary dorsal horn (MDH). There was a high increase in PKCg expression in the III and IIi laminae of the MDH of lesioned-animals compared to shams. pERK1/2 expression was also significantly high in the ipsilateral MDH of lesioned rats in response to non-noxious tactile stimulus of the orofacial region. Since pERK1/2 is expressed only in response to nociceptive stimuli in the dorsal spinal horn, the current study demonstrates that non-noxious stimuli evoke allodynic response. Intraperitoneal and intracisternal administrations of bromocriptine, a dopamine 2 receptor (D2R) agonist, significantly decreased DMA compared to control rats injected with saline. These data demonstrate for the first time that nigrostriatal dopaminergic depletion produces trigeminal neuropathic pain that at least involves a segmental mechanism. In addition, bromocriptine was shown to have a remarkable analgesic effect on this neuropathic pain symptom.

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This study provides evidences strongly suggesting that PD-like lesion and L-dopa therapy were concomitant factors involved in striatal remodeling underlying the pramipexole-induced place preference. Molecular and pharmacological data suggest a key involvement of the glutamatergic pathway in this behavioral outcome.

Our data show that unilateral and bilateral dopamine depletion promoted trigeminal SMA comparable to that obtained after CCI-IoN. This allodynia can be alleviated by D2R activation, making D2R agonist a potential analgesic for orofacial neuropathic pain.

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BackgroundThis study investigated mesencephalic dopamine depletion effects on static mechanical allodynia (SMA) elicited by chronic constriction of the infraorbitary nerve (CCI-IoN).MethodsDopamine depletion (6-OHDA administration into the medial forebrain bundle) effects on CCI-IoN-induced SMA were explored using behavioral (nocifensive behavior score upon non-noxious stimuli using von Frey filament), pharmacological (bromocriptine injections) and immunohistochemical (PKCγ and pERK1/2) techniques.ResultsThe central dopamine depletion increased significantly the SMA score. Intraperitoneal and intracisternal injections of bromocriptine alleviated the allodynic behavior observed in both CCI-IoN and CCI-IoN + 6-OHDA animal groups. At the cellular level, dopamine depletion induced a significant increase in PKCγ expression in the medullary dorsal horn (MDH) in rat with CCI-IoN + 6-OHDA when compared to sham animals (CCI-IoN only). Similarly, after static non-noxious stimuli, the expression of pain marker proteins pERK1/2 within the MDH revealed significantly a higher number of positive cells in CCI-IoN + 6-OHDA rats when compared to the CCI-IoN group.ConclusionThis study demonstrates that nigrostriatal dopamine depletion exacerbates the neuropathic pain resulting from CCI-IoN. This effect is probably due to an action through descending pain inhibitory systems which increased pain sensitization at the MDH level. It demonstrates also an analgesic effect elicited by D2R activation at the segmental level.Electronic supplementary materialThe online version of this article (doi:10.1186/s10194-016-0607-z) contains supplementary material, which is available to authorized users.

Dopamine dysregulation syndrome (DDS) has been attributed to both dopamine replacement therapies (DRT) and the mesencephalic dopaminergic lesion. The DRT reinforcement effect is due to its action on the reward system, particularly on the nucleus accumbens (NAc). This nucleus receives two major projections, a glutamatergic from the prefrontal cortex and a dopaminergic from the posterior ventral tegmental area (pVTA). The latter modulate the former within the NAc. pVTA has been demonstrated to be implicated in the motivational effect of bromocriptine (dopamine 2 receptor (D2R) agonist) in bilateral pVTA-lesioned animals. Therefore the potential implication of the metabotropic glutamate receptor 5 (mGluR5) antagonist (MTEP: 3-((2-Methyl-1,3-thiazol-4-yl)ethynyl)pyridine) on bromocriptine-induced conditioned place preference (CPP) was explored. Results showed that the administration of the MTEP blocked completely the bromocriptine-induced CPP in bilateral pVTA-lesioned rats. Both the CPP acquisition and expression were abolished. These effects are due, at least to an increase of the glutamate concentration and that of mGlu5 receptor expression in the NAc shell of the pVTA-lesioned animals. Altogether these data demonstrated the importance of the mGlu5 receptor in the bromocriptine induced-reinforcement and that DDS is probably due to DRT effect on this glutamate receptor.