| Parkinson's disease (PD) is a common neurodegeneration disease. Etiology studies have indicated that PD is related to genie mutations and environmental factors, but only 10% are familial PD cases, over 90% of idiopathic PD are induced by environmental factors. Studies have shown that pesticide exposure is associated with the pathogenesis of PD. Previous studies of our laboratory have also showed that the exposure of rotenone, paraquat (PQ) and maneb (MB) are linked to PD. Otherwise, pesticides were often sprayed overlappingly, which resulted that two or more kinds of pesticide would exist in the local environment, the joint neurotoxic effects of these pesticides still need to be proved using animal exposing.In the present study, the effects of chronic exposure to PQ and PQ co-administration with MB on rats'behavior were observed, and the changes of substantia nigra (SN) neurons were accordingly detected. It was aimed to establish the PQ/MB-induced PD model of rats. Previous studis have shown that the movement dysfunction of PD is derived from the degeneration of nigral dopaminergic (DA-ergic) neurons, and is closely related to physiological activity of basal ganglia. These studies were mostly focused on globus pallidus and subthalamus. However, compared to normal animals, the electrical activity of striatal neurons in animal of PD model and the changes of DA receptors are less involved. In this paper, the changes of spontaneous activity of striatal neurons were further explored in PD animal model induced by PQ/MB, and the sensitivity changes of D1 and D2 receptors were observed using the method of electrophysiology and micro-electrophoresis of DA receptor agonist.The main results are as follows:1. The exposure of PQ (10 mg/kg) alone and in combination with MB (30 mg/kg) for 6 weeks could induce the changes of movement and neuronal pathology of SN in rats. The results of behavioral test showed that the numbers of rats'sliding down the declined plane and the latencies of rat's moving on the vertical grid were significantly increased compared to both the control group and the same group before exposure. In the open-field test, the numbers of rats'head dipping of exposed rats was significantly reduced, the time of rearing in the combined exposure group was also significantly reduced, and the time of rats'inactive sitting and the numbers of line crossing were significantly increased compared to the control group and the same group before exposure. Compared with the PQ separate exposure group, the combined exposure of PQ and MB could induce more severe motor dysfunction in rats. Pathology results showed that the exposure of PQ alone and in combination with MB resulted in the progressive degeneration of neurons and the neural loss of 43% and 71% in SN respectively. Consistent with the behavior change of rats, the co-exposure of PQ and MB could induce more severe damage in SN neurons.2. The firing patterns of spontaneous discharge of striatal neurons were changed after the exposure of PQ alone and co-exposure of PQ and MB. Compared to the control group, the neurons with the regular single-pulse firing increased, but the irregular single-pulse firing reduced in the exposure group, moreover the compound bursting firing neurons were significantly increased in the combined exposure group. Both the exposure of PQ and the combined exposure of PQ and MB could induce the significant increase of firing rate of striatal neurons compare to the control group. It was shown that spontaneous firing neurons lower than 1 Hz were significantly decreased, but ones higher than 10 Hz were significantly increased. Furthermore, the percentage distribution of the medium spiny-like neurons (MSNs), large aspiny-like neurons (LANs) and fast-spiking (FS) interneurons in striatum were changed, and their firing activities were significantly enhanced after the co-exposure of PQ and MB.3. Following D1 receptor agonist, SKF38393 or D2 receptor agonist, LY171555 iontophoretically administrated respectively, effects of increase and decrease on the firing activity were both observed in striatal neurons. However, stronger inhibitory effects of activating D1 receptors and weaker excitatory effects of activating D2 receptors were found in the exposed rats as compared to controls. SKF38393 and LY171555 also induced the changes of waveforms of spontaneous discharge in the excitatory striatal neurons. The effects in the exposed and control group showed different characteristics.These results above indicated that:1. Chronic systemic exposure to PQ alone and its joint with MB could induce like-PD motor dysfunction including motrigidity, bradykinesia and hypokinesia, and result with the pathological change of the degeneration in SN neurons. In addition, both behavior and pathology detection showed that the toxicity effect of the combined exposure of PQ and MB was stronger than that of the PQ alone exposure, which suggested that PQ and MB had some collaborative neurotoxicity.2. The exposure of PQ alone and in combination with MB could bring on enhancement of spontaneous activity of striatal neurons including MSNs, LANs and FS interneurons in rats. It suggested that the electrophysiological activity of MSNs and FS interneurons could be associated with motor dysfunction of PD.3. After local activation of striatal D1 or D2 receptors, the different impact effects of electrical activities of striatal neurons were observed in control and PQ/MB exposed rats. Stronger inhibitory effects were induced by activating D1 receptors in PQ/MB-induced rats, while the activation of D2 receptors could induce weaker excitatory effects, which showed the different sensitivities of D1 and D2 receptors. It indicated the differential changes in the sensitivities of D1 and D2 receptors, and further supported that the functions of D1 and D2 receptors could be linked to the direct and indirect pathways of basal ganglia, and also indicated that the imbalance of the direct pathway and the indirect pathway associated with D1 and D2 receptors is the important cause of movement dysfunction of PD.From the perspective of behavior and pathology, the neurotoxic effects of exposure to PQ alone and its joint with MB in rats were explored in the present work. It confirmed that PQ had neurotoxic effect of inducing PD and that MB had a certain synergistic toxicity with PQ. Using in vivo single-cell extracellular recording combined with micro-electrophoresis technology, our results suggested that the changes of neuronal activities in striatum were linked to movement dysfunction of PD. The present study firstly discovered the differential changes of D1 and D2 receptor sensitivities in striatum in PD animal model. Our results would provide evidences for environmental etiology of PD, and provide physiological basis for illustrating the mechanism of movement regulation of basal ganglia associated with PD, and also provide a clue for the use of DA receptor agonist in the treatment of PD.
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