Aims: 

We aimed to study the normalized distributions of dendritic synapses of different CNS neurons in terms of the attenuations and propagation delays of their PSPs to the soma in different soma-dendritic membrane models.

Methods: 

High-fidelity 3D reconstructions of hippocampal pyramidal and cerebellar Purkinje cells from mice and spinal motoneurons of frogs (n=3X5) were used to create passive segmental cable models of these neurons by the NEURON simulation environment. Distances of synapses (modelled by current injections) were computed from the soma in geometrical (in mm), electrotonic (in space constants) and in temporal space (in ms) to estimate and compare distributions of synapses assuming their constant density over the dendrites.

Results: 

Fractions of the total dendritic surface (~synapses) were neuron-type specific in 8 of the 10 equidistant regions of geometrical space (ANOVA, p<0.005). However, distributions of synapses in electrotonic space were highly conservative, with peaks at 40% of electrotonic size of dendrites, for all neuron types with homogeneous soma-dendritic membrane (ANOVA, p>0.05 for 9 of 10 regions). In inhomogeneous membrane models the synaptic distributions were neuron-type specific in electrotonic space and varied little with the size of background synaptic activity for a given neuron type. Distributions of synapses according to propagation delays of their PSPs to the soma differed (ANOVA, p<0.05) in 9 of the 10 temporal domains of CNS neuron types with homogeneous soma-dendritic membranes and these differences tended to disappear as the background synaptic activity was decreased (soma-dendritic inhomogeneity increased).

Conclusion: 

Despite the neuron-type specificity of distributions of dendritic synapses in geometrical space, normalized synaptic distributions may become similar in electrotonic and temporal space depending on the inhomogeneity of the soma-dendritic membrane imposed by inherent passive membrane properties or by the background synaptic activity over the dendrites.

Support: 

OTKA K67747 and ETT 025/2006