Abnormal responses of globus pallidus units to cortical stimulation in experimental parkinsonism

ZOLD, CAMILA; MURER, GUSTAVO; RIQUELME, LUIS

Mar del PLata, Buenos Aires, Argentina

Congreso; Congreso Conjunto de Sociedades Biomédicas; 2004

The normal operation of the basal ganglia (BG) requires the processing of cortical information through segregated parallel channels along with the integration of cortical-striatal and cortical-subthalamic inputs. Nigrostriatal lesions would alter essential aspects of information processing in the BG causing the clinical signs of parkinsonism. As the globus pallidus (GP) receives cortical inputs through the striatum (inhibitory) and subthalamus (excitatory), we decided to study the effects of nigrostriatal lesions on the responses of GP units to electrical stimulation of the motor cortex. In 6-OHDA-lesioned rats (a widespread model of parkinsonism) 26 of 27 (98%) GP units responded to motor cortex stimulation against 23% (4 of 17 units) in control animals (p<0.001, Fisher exact test). This increased responsiveness was due to a greater incidence of both excitations (96% vs 23%, p<0.001) and inhibitions (40% vs 12%, p<0.05).  We also found an abnormal integration of inhibitory (cortical-striatum-GP) and excitatory (cortical-subthalamus-GP) responses in GP units: none of the neurons of control rats showed excitation-inhibition-excitation responses against 6 of 27 in the 6-OHDA rats (p<0.05).  These results indicate that: i) the representation of a focus of cortical activity is distributed more extensively in the GP in experimental parkinsonism, suggesting a degradation of the capacity of parallel processing; ii) this alteration could be explained by an increased transfer of activity through both the cortical-striatum-GP and cortical-subthalamus-GP pathways; iii) the presence of greater number of triphasic responses in 6-OHDA rats suggests increased convergence cortical-striatum-GP and cortical-subthalamus-GP inputs in parkinsonian rats.