Immature chemodifferentiation of Purkinje cell synapses revealed by 5'-nucleotidase ecto-enzyme activity in the cerebellum of the reeler mouse

Survival of interneurons and parallel fiber synapses in a cerebellar cortex deprived of Purkinje cells: Studies in the double mutant mouseGrid2Lc/+;Bax−/−

The Lurcher mutation in the Grid2 gene causes the cell autonomous death of virtually all cerebellar Purkinje cells and the target-related death of 90% of the granule cells and 60-75% of the olivary neurons. Inactivation of Bax, a pro-apoptotic gene of the Bcl-2 family, in heterozygous Lurcher mutants (Grid2Lc/+) rescues approximately 60% of the granule cells, but does not rescue Purkinje or olivary neurons. Given the larger size of the cerebellar molecular layer in Grid2Lc/+;Bax(-/-) double mutants compared to Grid2Lc/+ mutants, we analyzed the survival of the stellate and basket interneurons as well as the synaptic connectivity of parallel fibers originating from the surviving granule cells in the absence of their Purkinje cell targets in the Grid2Lc/+;Bax(-/-) cerebellum. Quantification showed a significantly higher density of interneurons ( approximately 60%) in the molecular layer of the Grid2Lc/+;Bax(-/-) mice compared to Grid2Lc/+, suggesting that interneurons are subject to a BAX-dependent target-related death in the Lurcher mutants. Furthermore, electron microscopy showed the normal ultrastructural aspect of a number of parallel fibers in the molecular layer of the Grid2Lc/+; Bax(-/-) double mutant mice and preserved their numerous synaptic contacts on interneurons, suggesting that interneurons could play a trophic role for axon terminals of surviving granule cells. Finally, parallel fibers varicosities in the double mutant established "pseudo-synapses" on glia as well as displayed autophagic profiles, suggesting that the connections established by the parallel fibers in the absence of their Purkinje cell targets were subject to a high turnover involving autophagy.

Modulation of Seizures and Synaptic Plasticity by Adenosinergic Receptors in an Experimental Model of Temporal Lobe Epilepsy Induced by Pilocarpine in Rats

PURPOSE

Adenosine is a major negative neuromodulator of synaptic activity in the central nervous system and can exert anticonvulsant and neuroprotective effects in many experimental models of epilepsy. Extracellular adenosine can be formed by a membrane-anchored enzyme ecto-5'-nucleotidase. The purposes of this study were to characterize the role of adenosine receptors in modulating status epilepticus (SE) induced by pilocarpine and evaluate its neuroprotective action. Ecto-5'-nucleotidase activity was studied during the different phases of pilocarpine-induced epilepsy in rats.

METHODS

Adult rats were pretreated with different adenosinergic agents to evaluate the latency and incidence of SE induced by pilocarpine in rats. The neuroprotective effect also was evaluated.

RESULTS

A proconvulsant effect was observed with DPCPX and DMPX that reduced the latency of SE in almost all rats. Pretreatment with the MRS 1220 did not alter the incidence of SE but reduced the latency to develop SE. An anticonvulsant and neuroprotective effect was detected with R-PIA. Rats pretreated with R-PIA had a decreased number of apoptotic cells in the hippocampus, whereas pretreatment with DPCPX did not modify the hippocampal damage. An intensification of neuronal death was observed in the dentate gyrus and CA3 when rats were pretreated with DMPX. MRS-1220 did not modify the number of apoptotic cells in the hippocampus. An increase in the ecto-5 -nucleotidase staining was detected in the hippocampus during silent and chronic phases.

CONCLUSIONS

The present data show that adenosine released during pilocarpine-induced SE via A1-receptor stimulation can exhibit neuroprotective and anticonvulsant roles. Similar effects could also be inferred with A2a and A3 adenosinergic agents, but further experiments are necessary to confirm their roles. Ecto-5 -nucleotidase activity during silent and chronic phases might have a role in blocking spontaneous seizures by production of inhibitory neuromodulator adenosine, besides taking part in the mechanism that controls sprouting.

Olivocerebellar climbing fibers in the granuloprival cerebellum: morphological study of individual axonal projections in the X-irradiated rat

Elimination of cerebellar granule cells early during postnatal development produces abnormal neural organization that retains immature characteristics in the adult, including innervation of each Purkinje cell by multiple climbing fibers from the inferior olive. To elucidate mechanisms underlying development of the olivocerebellar projection, we studied light-microscopic morphology of single olivocerebellar axons labeled with biotinylated dextran amine in adult rats rendered agranular by a single postnatal X-irradiation. Each reconstructed olivocerebellar axon gave off approximately 12 climbing fibers, approximately twice as many as in normal rats. Terminal arborizations of climbing fibers made irregular tufts in most areas, whereas they were arranged vertically in a few mildly affected areas. Each climbing fiber terminal arborization innervated only part of the dendritic arbor of a Purkinje cell, and multiple climbing fibers innervated a single Purkinje cell. These climbing fibers originated either from the same olivocerebellar axon (pseudomultiple innervation) or from distinct axons (true multiple innervation). Abundant non-climbing fiber thin collaterals projected to all cortical layers. Although the longitudinal pattern of the zonal olivocerebellar projection was generally observed, lateral branching, including bilateral projections, was relatively frequent. These results suggest that the granule cell-parallel fiber system induces several important features of olivocerebellar projection: (1) organization of the climbing fiber terminal arborization tightly surrounding Purkinje cell dendrites, (2) elimination of pseudo- and true multiple innervations establishing one-to-one innervation, (3) retraction of non-climbing fiber thin collaterals from the molecular layer, and (4) probable refinement of the longitudinal projection domains by removing aberrant transverse branches.