| The subthalamic nucleus occupies an important position in the indirect pathway of the basal ganglia. It is well known that the firing rate and pattern of the subthalamic neurons is directly related to the manifestation of Parkinson’s disease. Being a monoamine neurotransmitter,5-HT is involved in mediating many physiological functions in the central nervous system. Lots of evidence have confirmed that5-HT participates the pathophysiological processes in several movement disorders. Morphological studies have demonstrated that the subthalamic nucleus receives serotonergic innervation arising from the raphe nuclei, mainly the dorsal raphe nucleus. A high level of5-HT and different kinds of5-HT receptor subtypes were detected in the subthalamic nucleus.Object:Firstly, the aim was to study the effects of5-HT and selective5-HT1B,5-HT2C,5-HT4,5-HT1A receptor agonist on the firing rate of subthalamic neurons in normal rats and6-hydroxydopamine (6-OHDA) parkinsonian rats. Secondly, our aim was to compare the electrophysiological effects of5-HT on subthalamic nucleus neurons under the two states. Thirdly, to study the posturing regulation as well as the anticataleptic effects of5-HT in the subthalamic nucleus. Finally, in order to further elucidate the receptor mechanism of5-HT electrophysio logical effects, we observed the expression of5-HT receptor subtype5-HT1B,5-HT2C,5-HT4and5-HT1A in the subthalamic nucleus of normal rats.Methods:In vivo extracellular single unit recordings,6-OHDA-lesioned parkinsonian rats preparation, immunohistochemistry and behavioral test were used in the present study.Results:1. In normal rats, electrophysio logical experiments showed that, in recorded47neurons of subthalamic nucleus,5-HT increased the frequency of spontaneous discharges of32neurons by an average of32.02±4.31%. Meantimes, the other15neurons was inhibited by5-HT, whose frequency was deceased by26.08±4.58%. In the saline control group, we recorded8neurons in the subthalamic nucleus, whose firing rate increased by4.89±1.61%. Compared with the control group, both inceased group and deceased group were significant (P<0.001).Micro-pressure injection of5-HT1B receptor agonist, CP-93129,5-HT2C receptor agonist, RO-600175, and5-HT4receptor agonist, ML-10302, increased the spontaneous firing rate of subthalamic neurons in normal rats, respectively. These average increase was25.11±3.79%,22.82±4.02%and23.99±2.79%, which all were significantly different from that of normal saline control group (P<0.01). However, the application of5-HT1A receptor agonist,8-OH-DPAT, reduced the discharge frequency by18.75±2.70%. Meantime, we compared the excitatory effect of5-HT on the firing rate of neurons with CP-93129, RO-600175and ML-10302,respectively, there was no significant difference (P>0.05). The inhibitory effects of5-HT and8-OH-DPAT was compared, which had no statistical significance (P>0.05).In order to confirm the expression of several5-HT receptor subtypes on one single subthalamic neuron, both CP-93129and8-OH-DPAT were applied to the same one. The discharge frequency increased by23.92±7.03%and decreased by19.28±3.45%, respectively (n=7, P<0.05). In the same way, we injected RO-600175and ML-10302to the same subthalamic nucleus neuron. Both of them elicited excitatory effect significantly (RO-600175:20.04±4.40%; ML-10302:26.26±6.87%,n=11, P<0.01).2. In parkinsonian rats, electrophysio logical experiments indicated that, in recorded38subthalamic neurons in the lesioned side,5-HT increased the frequency of spontaneous discharges of25neurons by an average of31.12±4.27%; meantimes, the other13neurons was inhibited by5-HT, whose frequency was deceased by23.10±7.05%. Micro-pressure injection respectively of5-HT1B receptor agonist, CP-93129,5-HT2C receptor agonist, RO-600175, and5-HT4receptor agonist, ML-10302, increased the spontaneous firing rate of subthalamic neurons in the lesioned side, respectively. These average increase was28.76±4.43%,32.50±7.12%and26.16±6.61%. However, the application of5-HT1A receptor agonist,8-OH-DPAT, reduced the discharge frequency by18.72±3.81%.We recorded19subthalamic neurons in the unlesioned side of parkinsonian rats.5-HT increased the frequency of spontaneous discharges of10neurons by an average of26.54±6.41%. Meantimes, the other9neurons was inhibited by5-HT, whose frequency was deceased by21.94±5.95%. Micro-pressure injection CP-93129, RO-600175and ML-10302, increased the spontaneous firing rate of subthalamic neurons in the unlesioned side, respectively. These average increase was26.91±3.35%,32.75±8.72%and26.47±6.44%. However, application of8-OH-DPAT reduced the discharge frequency by23.27±5.17%.3. Compared with normal rats (the average frequency:7.80±0.54Hz), the basal firing rate of subthalamic neurons in the lesioned side of parkinsonian rats, was significantly higher (the average frequency:11.07±0.85Hz, P<0.001).We also observed that the overall distribution of discharge pattern changed (χ2=25.523, d.f.=2,P<0.001),in which the number of bursty firing neurons increased significantly (χ2=24.832,d.f.=1, P<0.001) and the number of regular firing decreased (x2=8.917, d.f.=l, P<0.01)We next compared the discharge frequency and pattern between the lesioned side and the unlesioned side of parkinsonian rats, the firing rate of subthalamic neurons in the lesioned side increased significantly (P<0.01), with which the number of bursty discharge increased (x2=8.316, d.f.=1, P<0.01), regular firing reduced (χ2=8.848, d.f.=1, P<0.01). To further confirm that the discharge pattern of subthalamic nucleus neurons changed in Parkinson’s disease state, we analyzed CV and FF of ISI in normal and parkinsonian rats. The data showed that CV of subthalamic neurons increased obviously in the lesioned side, compared with in normal rats and unlesioned side, respectively (normal:0.68±0.05; unlesioned side:0.66±0.08; lesioned side:1.00±0.09, P<0.01), and FF also increased remarkably (normal:0.21±0.04; unlesioned side:0.16±0.05; lesioned side:0.39±0.08, P<0.05).In addition, the electrophysiological effects of5-HT and4receptor agonists on subthalamic nucleus neurons in the lesioned side of of parkinsonian rats and in normal rats were compared, the results showed that no significant difference (P>0.05).4. In the bar test, haloperidol administration (0.25mg/kg, i.p.) produced a comparatively steady catalepsy state in rats. Bilateral micro injection of5-HT (0.1mM,0.3μl/side) into the subthalamic nucleus significantly alleviate the symptoms of rigidity caused by haloperidol (P<0.05compared with that of saline).5. By using immunohistochemistry, we further observed the expression of5-HT1B,5-HT2C,5-HT4and5-HT1A receptors in the subthalamic nucleus of normal rats. Positive staining of these4receptor subtype was observed on the subthalamic nucleus neurons in normal rats.Conclusion:Electrophysiological studies showed that exogenous5-HT can alter the excitability of subthalamic nucleus neurons in normal and parkinsonian rats, which showed an increase or a decrease in the frequency of discharge. The activation of5-HT1B,5-HT2C and5-HT4receptor increased the firing rate of subthalamic nucleus neurons, while when5-HT1A receptor was activated, the neuronal frequency was decreased. Compared with normal rats, in Parkinson’s disease state, although the discharge rate of subthalamic neurons was speeded and the number of burst discharge increased, the electrophysiological effects of5-HT and the corresponding receptor agonists on neurons did not significantly change. Morphological studies revealed that the subthalamic nucleus of normal rats expressed abundant5-HT1B,5-HT2C,5-HT4and5-HT1A receptors. |
PDF Full Text Request