The molecular composition of the nicotinic acetylcholine receptors (nAChRs) located on dopaminergic neurons and modulating their activity is unclear. Using the reverse transcriptase-polymerase chain reaction we have analyzed the mRNA for nAChR subunits expressed in the substantia nigra (SN) and ventral tegmental area (VTA) following unilateral 6-hydroxydopamine lesion of the dopaminergic system. In contrast to the unlesioned hemisphere, no signal was found in the lesioned hemisphere for alpha3, alpha5, alpha6 and beta4 subunits in the SN nor for alpha2, alpha3, alpha5, alpha6, alpha7 and beta4 subunits in the VTA, indicating the expression of these subunits in dopaminergic neurons. mRNA for alpha4, beta2 and beta3 subunits (and alpha7 in the SN) were still detected after lesion, suggesting that they are expressed in GABAergic neurons and interneurons of these brain areas. These results demonstrate the selective localisation of a number of nAChR subunit mRNA within dopaminergic neurons, strongly suggesting that a heterogenous population of nAChRs play a role in modulating dopaminergic neuronal activity.
Dopaminergic neurons in the substantia nigra pars compacta (SNpc) are characterized by the expression of genes required for dopamine synthesis, handling and reuptake and the expression of these genes is largely controlled by nuclear receptor related 1 (Nurr1). Nurr1 is also expressed in astrocytes and microglia where it functions to mitigate the release of proinflammatory cytokines and neurotoxic factors. Given that Parkinson’s disease (PD) pathogenesis has been linked to both loss of Nurr1 expression in the SNpc and inflammation, increasing levels of Nurr1 maybe a promising therapeutic strategy. In this study a novel Nurr1 agonist, SA00025, was tested for both its efficiency to induce the transcription of dopaminergic target genes in vivo and prevent dopaminergic neuron degeneration in an inflammation exacerbated 6-OHDA-lesion model of PD. SA00025 (30mg/kg p.o.) entered the brain and modulated the expression of the dopaminergic phenotype genes TH, VMAT, DAT, AADC and the GDNF receptor gene c-Ret in the SN of naive rats. Daily gavage treatment with SA00025 (30mg/kg) for 32 days also induced partial neuroprotection of dopaminergic neurons and fibers in rats administered a priming injection of polyinosinic-polycytidylic acid (poly(I:C) and subsequent injection of 6-OHDA. The neuroprotective effects of SA00025 in this dopamine neuron degeneration model were associated with changes in microglial morphology indicative of a resting state and a decrease in microglial specific IBA-1 staining intensity in the SNpc. Astrocyte specific GFAP staining intensity and IL-6 levels were also reduced. We conclude that Nurr1 agonist treatment causes neuroprotective and anti-inflammatory effects in an inflammation exacerbated 6-OHDA lesion model of PD.
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