| The globus pallidus is an important structure in the indirect pathway of the basal ganglia circuit. By innervating all the other basal ganglia nuclei, the globus pallidus plays a critical role in movement regulation. The abnormal structure and function of globus pallidus are closely related to several neural degenerative diseases such as Parkinson’s disease(PD). Orexins represent a recently characterized family of hypothalamic neuropeptides, which play important roles in central movement regulation. Morphological studies have demonstrated that the globus pallidus receives intensive orexinergic fibers and expresses high levels of orexin 1 receptor(OX1). It is reported that the orexinergic neurons degenerate significantly in the hypothalamus of advanced Parkinson’s disease patients. The orexin levels of cerebrospinal fluid decline significantly and negative correlated with the severity of Parkinson’s disease. Object: To evaluate the electrophysiological effects of orexin-A on the spontaneous firing rate of globus pallidus neurons in both normal and 6-OHDA parkinsonian rats as well as the expression of OX1 receptor in the globus pallidus neurons. We also observe the behavioral effects of orexin-A in haloperidol rigidity rats. Methods: In vivo extracellular single unit recordings, 6-OHDA-lesioned parkinsonian rats, haloperidol rigidity rats, cannula implantation, immunohistochemical staining and behavioral test were performed in the present study.Results:1. In 38 pallidal neurons of normal rats, micropressure ejection of 0.01 m M orexin-A increased the basal firing rate from 14.22±2.15 Hz to 18.48±2.42 Hz(P<0.001). The average increase was 24.95±6.20%, which was significantly different(P<0.001) from that of vehicle(normal saline) injection. Furthermore, more than 20% increase in firing rate was observed in 12 out of the 38 pallidal neurons receiving orexin-A administration(basal: 9.43±2.27 Hz, orexin-A: 13.74±2.96 Hz; average increase: 67.65±11.24%).2. In 10 globus pallidus neurons with orexin-A induced more than 20% increase in firing rate(57.38±8.49%), co-application of 0.01 m M orexin-A and 0.1m M SB-334867(the specific OX1 receptor antagonist) increased the firing rate from 11.37±3.63 Hz to 14.90±4.83 Hz. The average increase was 29.74±6.39%, which was decreased significantly compared to that of orexin-A alone(P<0.05).3. In 6-OHDA parkinsonian rats, micropressure ejection of 0.01 mM orexin-Aincreased the basal firing rate from 9.02±2.07 Hz to 12.43±2.01 Hz(P<0.001) in 34 pallidal neurons. The average increase was 59.96±12.74%, which was significantly different from that of the vehicle(normal saline) injection(P<0.001). More than 20% increase in firing rate was observed in 14 out of the 34 pallidal neurons receiving orexin-A administration(basal: 4.23±1.01 Hz, orexin-A: 7.85±1.74 Hz; average increase: 107.43±15.78%). The orexin-A induced increase in firing rate of pallidal neurons on the lesioned side of parkinsonian rats was stronger than that of normal rats(P<0.001) as well as unlesioned side of parkinsonian rats(P<0.001).4. The immunohistochemical staining showed the developmental change of OX1 receptor expression in different age groups. OX1 receptors first appeared at an age of two weeks after birth, and reach the extensive level at an age of three weeks. Then the levels of OX1 receptor expression persist continuously until two month age and decreased slightly at three month age. Further immunostaining showed that the number of OX1 receptor positive pallidal neurons decreased significantly on the lesioned side of 6-OHDA parkinsonian rats(P<0.01).5. In behaving tests, unilateral microinjection of orexin-A into the globus pallidus induced contralateral deflection in haloperidol rigidity rats, which is significantly different compared to that of control group(normal saline injection).Conclusion: The present study indicated that orexin-A increased the spontaneous firing rate of globus pallidus neurons in both normal and 6-OHDA lesioned parkinsonian rats. The orexin-A induced increase in firing rate of pallidal neurons mainly through OX1 receptors. Orexin-A exerted stronger excitatory effects in parkinsonian rats. Furthermore, immunohistochemical studies showed positive expression of OX1 receptors in the globus pallidus neurons of both normal and parkinsonian rats. The number of OX1 receptors positive neurons decreased in parkinsonian rats. Finally, orexin-A improved rigidity of haloperidol rats. The present electrophysiological, behavioral and immunohistochemical studies may provide further rational and experimental basis for investigations into the potential of orexin-A neurotransmission in the treatment of Parkinson’s disease. |
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