Electrophysiological Effects And Receptor Mechanisms Of Orexin-A On The Globus Pallidus Neurons In Both Normal And MPTP Parkinsonian Mice

Orexins represent a recently characterized family of hypothalamic neuropeptides,which play important roles in central movement regulation.Much evidence indicates that orexin levels decrease significantly in both Parkinson’s disease patients and animal models.The globus pallidus is an important relay nucleus in the indirect pathway of the basal ganglia,which plays significant roles in motor control under both health and pathological states.Morphological studies have demonstrated that the globus pallidus receives orexinergic fibers and expresses orexin-1(OX1R).Object: To elucidate the effects and receptor mechanisms of orexin-A on the firing activity of globus pallidus neurons in both normal and MPTP parkinsonian mice.Methods: In vivo extracellular single unit recordings,MPTP parkinsonian mice and immunohistochemical staining were performed in the present study.Results: 1.In normal mice,the spontaneous firing activity of 59 pallidal neurons were analyzed.The average frequency of spontaneous firing was 8.43±0.55 Hz.Three firing patterns of globus pallidus neurons in mice were recorded,with 29(49.15%)regular firing neurons,22(37.29%)irregular firing neurons and 8(13.56%)burst firing neurons.2.In normal mice,micro-pressure ejection of 0.01 mM orexin-A significantly increased the frequency of spontaneous firing from 7.77±0.94 Hz to 11.10±1.23 Hz in 21 out of the 38 pallidal neurons.The average increase was 51.26±6.36%(t=-7.20,P=0.000).Further analysis of firing pattern showed that orexin-A increased the CV(basal:0.50±0.09;orexin-A: 0.63±0.09;t=-2.48,P=0.02)but did not change the FF(basal:0.12±0.04;orexin-A: 0.11±0.04;t=0.47,P=0.65),suggesting that orexin-A changed the firing pattern with reducing the regularity of the spontaneous firing.3.In normal mice,micropressure ejection of the specific OX1 R antagonist,SB-334867(0.01 m M),decreased the basal firing rate from 8.71±0.95 Hz to 3.50±0.63 Hz in 23 out of the 33 neurons,and the average decrease was 61.77±5.42%(t=7.69,P=0.000).Further analysis of firing pattern showed that SB-334867 increased both CV(basal: 0.89±0.12;SB-334867:1.17±0.12;t=-3.32,P=0.003)and FF(basal: 0.21±0.07;SB-334867: 0.70±0.16;t=-3.52,P=0.002).4.In 7 globus pallidus neurons of normal mice,co-application of 0.01 m M SB-334867 and 0.01 mM orexin-A increased the spontaneous firing rate from4.66±1.28 Hz to 5.12±1.34 Hz.The average change of firing rate was 13.16±3.19%.However,after complete recovery of firing rate,micropressure ejection of 0.01 mM orexin-A alone increased the spontaneous firing rate from 5.15±1.16 Hz to 7.40±1.68 Hz.The average increase of firing rate was 46.43±7.22% in the same 7 neurons,which was significantly different(z=-2.49,P=0.011)from that of co-application of SB-334867 and orexin-A.5.In MPTP-treated parkinsonian mice,the basal firing rate was 7.64±0.96 Hz in 33 pallidal neurons recorded,which was not significantly different from that of normal mice(t=0.42,P=0.68).For the analysis of firing pattern,12 out of the 33(36.36%)pallidal neurons were identified as regular firing neurons,16(48.49%)were irregular firing neurons and 5(15.15%)were burst firing neurons.The fraction of irregular firing and burst firing neurons in MPTP parkinsonian mice tended to be more than that in normal mice although there was no statistical significance(χ2=1.42,d.f.=2,P>0.05).6.In MPTP parkinsonian mice,local administration of 0.01 mM orexin-A increased the firing rate from 6.14±1.17 Hz to 10.40±1.82 Hz in 14 out of the 23 pallidal neurons.The average increase was 83.78±16.82%(t=-5.20,P=0.000).Further firing pattern analysis showed that orexin-A did not change the CV(basal: 0.49±0.09;orexin-A: 0.60±0.11;t=-1.40,P=0.19)and FF(basal: 0.16±0.08;orexin-A: 0.13±0.07;t=0.22,P=0.83).In another 2 pallidal neurons,orexin-A decreased the spontaneous firing rate from 5.95±2.58 Hz to 3.12±1.02 Hz.The average decrease was 44.58±6.89%(t=1.81,P=0.32).7.In MPTP parkinsonian mice,micropressure ejection of SB-334867(0.01 m M)decreased the basal firing rate from 6.71±1.47 Hz to 2.49±1.09 Hz in 7 out of the 10 neurons,and the average decrease was 68.84±8.40%(t=5.91,P=0.001).Further analysis of firing pattern showed that SB-334867 increased both CV(basal: 0.99±0.12;SB-334867: 1.33±0.12;t=-6.09,P=0.001)and FF(basal: 0.18±0.04;SB-334867: 0.61±0.11;t=-5.60,P=0.001)significantly.8.The excitatory effects of orexin-A on the firing rate of globus pallidus neurons in MPTP parkinsonian mice were stronger(z=-2.02,P=0.04)than that in normal mice,while the SB-334867-induced inhibitory effects of firing rate were not significantly different between normal and parkinsonian mice(z=-0.19,P=0.85).9.Positive expression of OX1 receptor was observed in the globus pallidus of both normal and MPTP parkinsonian mice,and the expression in MPTP parkinsonian mice was similar to that in normal mice(P>0.05).Normal and MPTP parkinsonian mice showed similar levels of orexin-A expression in the lateral hypothalamus(P>0.05).Conclusion: In both normal and parkinsonian mice,micro-pressure ejection oforexin-A increased the spontaneous firing rate of pallidal neurons through OX1 receptor.Orexin-A-induced excitation of pallidal neurons in MPTP parkinsonian mice was stronger than that in normal mice.SB-334867 decreased the spontaneous firing rate of both normal and parkinsonian mice,suggesting the involvement of endogenous orexinergic system in the firing activity of globus pallidus through OX1 receptors.The present study provided a theoretical basis for further investigations of pallidal orexinergic system in the treatment of Parkinson’s disease.