Papel del sistema nigro-estriado y del cerebelo en la actividad oscilatoria cortical y en la sincronización de fase. Un estudio mediante potenciales de estado estable evocado por un tono modulado por un chirp en la rata

Resumen

Role of the nigrostriatal system and cerebelum in the cortical oscillatory activity and in the phase synchronization. A study using steady-state potentials evoked by chirp modulated tone in rats. Marta Pérez Alcázar. Faculty of Science. University of Navarra. Pamplona. Navarra. 2008. Background The steady-state potentials evoked by a modulated tone are a method to evaluate the brain synchronization related to external stimuli, and therefore it is a way to study the cortical oscillatory activity. The role of the nigrostriatal system and the cerebellum in its generation is not known, even though the suspicion is that they might be involved given the anomalies in the oscillatory activity in patients with Parkinson¿s disease and in patients with myoclonia in cerebellar disorders. Aims of the study The aim of this study is to investigate the implication of these two systems in the generation of oscillatory response in the steady-state potentials evoked by auditory stimuli. The results will be important to understand better the mechanism of this response in humans, and to know the repercussion of these diseases in the cortical functioning. Method We studied five groups of rats (two controls without surgery, with small surgery in a silent area), a group with cerebellar lesion and a group with partial nigrostriatal lesion). We registered the steady-state potentials evoked by a modulated tone in variable frequencies, using epidural electrodes close the auditory area. The obtained responses were analyzed using the time-frequency transforms (Wavelet and Gabor) and phase-analysis techniques. They were applied on the single sweeps to determine if its origin is due to new responses evoked by the stimuli or to a change in the phase of pre-existent activity. Results The response obtained in rats by steady-state potential evoked by a modulated tone was analyzed for all modulation tones. We observed these responses better at low frequencies than high frequencies in awake and freely moving animals. This response was analyzed by time-frequency transforms, in order to study the amplitude and phase-locking. Under ketamine/xylazine condition, the response experimented significant changes comparing with awake animals. These differents were emphasized around 60 Hz, where energy decreased in amplitude and phase-locking, and over 150 Hz where it was observed a higher synchronization. Control animals presented the highest amplitude response around 40-50 Hz, this frequency changed in 6-OHDA lesioned animals. In this group the peak decreased in frequency and after administration of dopaminergic agonist (apomorphine) was recovered to control values. In high frequencies range (over 150 Hz) administration of apomorphine produced a decreased in phase-locking. Animals with cerebellum right hemisphere lesioned were the last studied group. They experimented an important decrease in amplitude along all frequencies range, as well as in synchronization values. After dopaminergic antagonist adminstration (haloperidol), amplitude and synchronization response were recovered until control group values in basal condition. Significative differences were observed in the basal oscillatory activity between control and cerebellum lesioned animals. Conclusions In conclusion, Basal Ganglia and cerebellum, play an important role in cortical oscillatory activity. When some of them are lesioned the oscillatory activity was modified. These changes were recovered after dopaminergic agonits administration, when the lesion occurs in basal ganglia, or with antagonist dopaminergics administration in cerebellum lesion.