[Abstract Title]. - Society for Neuroscience

Italian MIUR
San Paolo Foundation
Ministry of Health
Title: Activity-dependent involvement of GABAergic terminals in the heterologous competition
for the mature Purkinje cell innervation
Authors: *R. CESA 1 , L. MORANDO 2 , P. STRATA 1,3,4 ;
1 Dept Neurosci., Univ. Turin, Turin, Italy; 2 Dept Anat., Univ Turin, Italy; 3 Natl. Inst. of
Neurosci., Turin, Italy; 4 IRCCS Santa Lucia Fndn., Rome, Italy
Abstract: Competition among different axons to reach the somatodendritic region of the target
neuron is an important step during development in order to achieve the final architectural design
typical of the mature brain. Such a competition is activity-dependent and can occur between
homologous fibers or between two different neuronal populations (heterologous competition)
innervating contiguous target territories. Such a heterologous competition has been described in
the cerebellar cortex between the two main excitatory inputs which innervate the two dendritic
domains of the Purkinje cell (PC); the parallel fibers (PFs) and the climbing fiber (CF).
A prolonged block of electrical activity by tetrodotoxin (TTX) or of the ionotropic glutamate
receptors leads to the appearance in the proximal dendritic region of the PC of a high number of
new spines mainly innervated by PFs, an input normally restricted to the branchlets. In contrast,
CFs undergo a morphological modification consisting of a reduced size of their varicosities and a
loss of synaptic contacts with the PCs. This finding has led to the hypothesis that in the absence
of activity the PC is intrinsically endowed with cues to be innervated by the PFs. The CFs, in
order to achieve and maintain their dendritic territory, have to be active, thus displacing the cue
of the excitatory competitor afferents.
In addition to the two excitatory inputs, the PC receives also a strong inhibitory GABAergic
input from both basket and stellate neurons which is distributed along the entire extent of the PC
somatodendritic region, directly onto the dendritic shaft. Following the prolonged
intraparenchymal administration of TTX, a new type of synapse appears made by GABAergic
terminals onto the newly-formed PC spines in the proximal dendrites. The surprising observation
of the presence of GABAergic terminals on spines which normally receive an excitatory input
has prompted us to study the inhibitory input distribution both on the dendritic shaft and on the
spines. Here we show that the density of labelled GABAergic terminals per area of molecular
layer and per length of PC dendritic membrane is increased after the block of electrical activity
and that such an expansion takes place only in the PC proximal domain. Thus, the competition
for the innervation of the PC is not limited to the two excitatory inputs and activity of the CF has
a fundamental role in the maintenance of the proper synaptic excitatory and inhibitory
architectural wiring.
Disclosures: R. Cesa, None; L. Morando, None; P. Strata, None.