Basic electrotonic properties of primate pallidal neurons as inferred from a detailed analysis of their morphology: a modeling study.

We used a biophysical model to gain insight into the electrotonic properties of primate pallidal neurons. The model was built from the quantitative morphological analysis of internal (GPi) and external (GPe) pallidal neurons, labeled with biocytin filling or Golgi staining. Simulations showed a strong location dependence of the efficacy of single fast excitatory synaptic inputs, synaptic efficacy being the peak amplitude of somatic postsynaptic potentials (PPSs) evoked from dendritic sites. It markedly decreased with distance from the soma. The efficacy was also dependent on the dendritic branch order. At identical distances from the soma, there were no differences between the mean efficacy of GPi and GPe dendritic synaptic sites. However, the attenuation of the propagated PPSs was higher in GPi dendrites. This difference resulted from the smaller mean diameters of GPi dendrites. Diameters were also shown to often maximize the distal sites efficacy. A clear distance dependence was also found when simulating GABA(A) inhibitory activations. It was weaker than the one observed with fast excitatory PPSs. It is proposed that this difference could constitute a simple basis to create opposing center-surround patterns of activity in the basal ganglia output. In conclusion, this study suggests that GPi and GPe neurons have very similar electrotonic properties and that in such passive conditions all parts of their long dendrites significantly influence the somatic membrane potential. It suggests also that in spite of this similarity, neurons of the two pallidal segments differ in the way they integrate their synaptic inputs.