Electrophysiological Effects Of Orexin On Substantia Nigra In Normal And Parkinsonian Rats

Substantia nigra occupies a very important position in the basal ganglia. Orexins represent a recently characterized family of hypothalamic neuropeptides. Orexinergic system directly participates in central motor control. Morphological studies have demonstrated that the substantia nigra receives a large number of intensive orexinergic fibers. Highest levels of orexin receptor expression were observed in the substantia nigra. The patients with late-stage Parkinson’s disease have a significant loss of the hypothalamic orexin-producing neurons, and the level of orexin was decreased in cerebrospinal fluid. Previous studies focused on the effects of orexin on sleep disturbance observed in parkinsonian patients. Object: To evaluate the electrophysiological effects of orexin-A and the orexin1(OX1) receptor antagonist, SB-334867, on the firing rate of substantia nigra neurons in normal and parkinsonian rats as well as the expression of OX1receptor in the substantia nigra neurons. Methods:In vivo extracellular single unit recordings,6-OHDA-lesioned parkinsonian rats, and immunohistochemical staining were performed in the present study. Results:1. In the substantia nigra pars compacta of normal rats,58dopaminergic neurons were recorded. Micropressure ejection of O.O1mM orexin-A increased the basal spontaneous firing rate in36out of the58(62.1%) dopaminergic neurons (Basal:2.3±0.2Hz, orexin-A:4.2±0.5Hz). The average increase was101.0±18.0%(P<0.001). In the substantia nigra pars reticulata of normal rats,54gamma-aminobutyric acid (GABA)-ergic neurons were recroded. Orexin-A increased the basal spontaneous firing rate from24.2±2.0Hz to31.4±2.7Hz in24out of the54(44.4%) GABAergic neurons. The average increase was31.3±5.3%(P<0.001).2. In19out of the30(63.3%) dopaminergic neurons of the substantia nigra pars compacta, micropressure ejection of0.1mM SB-334867decreased the basal spontaneous firing rate from3.1±0.5Hz to1.0±0.3Hz. The average decrease was66.3±6.7%(P<0.001). In10out of the21(47.6%) GABAergic neurons of the substantia nigra pars reticulata, SB-334867decreased the basal spontaneous firing rate from21.9±3.0Hz to16.2±3.4Hz. The average decrease was28.4±7.9%(P<0.01).3. On the lesioned side of6-OHDA parkinsonian rats, orexin-A increased the basal spontaneous firing rate in12out of the21(57.1%) dopaminergic neurons (Basal:4.1±0.6Hz, orexin-A:6.5±1.0Hz). The average increase was58.2±11.8%(P<0.01). Orexin-A also increased the basal spontaneous firing rate in7out of the12(58.3%) GABAergic neurons (Basal:19.6±2.7Hz, orexin-A:23.5±3.8Hz). The average increase was18.8±3.0%(P<0.05). In both normal and6-OHDA parkinsonian rats, orexin-A produced a stronger excitation on dopaminergic neurons than that on GABAergic neurons (normal rats: P<0.01, Parkinsonian rats:P<0.05).4. Positive immunolabeling for OXi receptors was observed in the substantia nigra pars compacta and reticulata of both normal and6-OHDA parkinsonian rats. Conclusion:The present in vivo electrophysiological studies provided evidence that orexin-A produced excitation on the subatantia nigra neurons. Endogenous OX1receptors are involved in the modulation of firing activity of both dopaminergic and GABAergic neurons. Orexin-A also produced excitation in6-OHDA parkinsonian rats. Finally, immunostaining showed positive expression of OXi receptors in the substantia nigra of both normal and parkinsonian rats. The present findings may provide a rationale for further investigations into the involvement of nigral orexin system in Parkinson’s disease.