Developmental influence of mGluR1 on cerebellar ataxia

Metabotropic glutamate receptor 1 (mGluR1) deficient mice expose severe cerebellar ataxia. This phenotype is associated with a developmental defect in the maturation of neural connectivity and an inability to establish long- term depression (LTD) in adult mice, both affecting cerebellar Purkinje cells. Synapse elimination is believed to be the final step in neural circuit formation by producing the necessary refinement of redundant connectivity formed during development. Activity-dependent synapse selection occurs during a restricted period of time called the critical period, which varies in both duration and timing at different synapses. In the cerebellar cortex, the precise critical period for elimination of climbing fiber synapses has not yet been determined and in particular when and how mGluR1 participate to this event. In addition, a clear relationship between synaptic signalling from mGluR1 activation, Purkinje cell multiple climbing fiber innervation, impaired LTD and ataxia remains to be established. The primary aim of this project is to establish the relative importance of impaired connectivity and LTD for the development of cerebellar ataxia. For this purpose we want to establish a genetically modified mouse model, in which we replace the grm1 gene by a construct that allows us to selectively express mGluR1 in cerebellar Purkinje cells at any time during development. We intent to investigate four groups of mice 1) mGluR1 permanently induced; 2) mGluR1 suppressed for the first three weeks, but not thereafter; 3) mice depleted of mGluR1 only in adulthood; 4) permanently suppressed mice. With this setup we will be able to dissect the relative importance of impaired synapse elimination and LTD, as group 2 animals should have normal LTD, but supernumerous synapses, while group 3 animals will have normal synaptic connectivity, but impaired LTD. The proposed mice will express mGluR1 almost exclusively in cerebellar Purkinje cells, which is sufficient to rescue the ataxic phenotyp. This gives us the opportunity to study other functions of mGluR1 likewise in epilepsy or movement control, without the limitations of motor disabled animals.

The original aim of this project was to establish the relative importance of impaired connectivity and synaptic long- term depression for the development of cerebellar ataxia. For this purpose we wanted to establish a genetically modified mouse model, in which we replace the metabotropic glutamate receptor 1 (grm1) gene by a construct that allows us to selectively express mGluR1 in cerebellar Purkinje cells at any time during development. The proposed mice will express mGluR1 almost exclusively in cerebellar Purkinje cells, which is sufficient to rescue the ataxic phenotyp. This gives us the opportunity to study other functions of mGluR1 likewise in epilepsy or movement control, without the limitations of motor disabled animals. Unfortunately the generation of these mice by Ozgene (a company we had very good experience before) was massively delayed. Heterozygous mice were available only by the end of this project, so we could not do the proposed investigations. Therefore the PhD student Eduard Schunk investigated our prodynorphin deficient mice in respect of epilepsy and emotional control in more detail. The original plan was that he would participate in this project only to learn the necessary techniques. However, this alternative project was very successful. We actually could provide the proof of principle for the potential use of kappa-opioid-receptor (KOP) agonists in the treatment of temporal lobe epilepsy. Basically we were able to block progressing neurodegeneration in a mouse model of temporal lobe epilepsy. Most importantly, broad protective effects were observed also when this treatment was initiated several days after the initial insult. Thus, it is possible that a antiepileptic therapy based on KOP activation may delay or even stop the progression of epilepsy. On the other hand, we demonstrated that activation of KOP may influence emotional control. Thus, dynKO mice were markedly less anxious under conditions of increased stress than wild-type mice. It is also known that KOP agonists can induce dysphoria at higher concentrations. Therefore, we plan further experiments to optimize the treatment with KOP agonists in a way to maintain their neuroprotective effects, but to reduce aversive side effects as much as possible. In this respect it is important to point out, that epilepsy by itself may induce dysphoria, depression and mental deficits. Thus, the main readout for our study will not be the effects of KOP agonists themselves, but the net-effect from therapy and disease. From our data it is highly likely that KOP agonists depress self sustained epileptic seizures in progressing stages of epileptgenesis and may therefore be suitable to suppress progressive neurodegeneration and disturbances in emotional control and cognitive functions.