Search

Elimination of excess climbing fiber (CF)Purkinje cell synapses during cerebellar development involves a signaling pathway that includes type 1 metabotropic glutamate receptor, G[Alpha]q, and the [Gamma] isoform of protein kinase C. To identify phospholipase C (PLC) isoforms involved in this process, we generated mice deficient in PLC[Beta]4, one of two major isoforms expressed in Purkinje cells. PLC[Beta]4 mutant mice are viable but exhibit locomotor ataxia. Their cerebellar histology, parallel fiber synapse formation, and basic electrophysiology appear normal. However, developmental elimination of multiple CF innervation clearly is impaired in the rostral portion of the cerebellar vermis, in which PLC[Beta]4 mRNA is predominantly expressed. By contrast, CF synapse elimination is normal in the caudal cerebellum, in which low levels of PLC[Beta]4 mRNA but reciprocally high levels of PLC[Beta]3 mRNA are found. These results indicate that PLC[Beta]4 transduces signals that are required for CF synapse elimination in the rostral cerebellum.

Golgi cells expressing the human interleukin-2 receptor alpha subunit in transgenic mice underwent a novel cell ablation technology called immunotoxin-mediated cell targeting (IMCT) since the role played by Golgi cells in cerebellar function and motor coordination was unclear. The conditional disruption of the Golgi cells was through the administration of anti-TAC(Fv)-PE38 recombinant protein intrathecally. It was revealed that Golgi cell elimination resulted to severe motor disorders and weakening of the functional neurotransmitters needed in compound motor coordination.

In cerebeilar Purkinje neurons, [Gamma]-aminobutyric acid (GABA)-mediated inhibitory synaptic transmission undergoes a long-lasting 'rebound potentiation' after the activation of excitatory climbing fiber inputs. Rebound potentiation is triggered by the climbing-fiber-induced transient elevation of intracellular [Ca.sup.2+] concentration and is expressed as a long-lasting increase of postsynaptic [GABA.sub.A] receptor sensitivity. Herein we show that inhibitors of the [Ca.sup.2+]/calmodulin-dependent protein kinase II (CaM-KII) signal transduction pathway effectively block the induction of rebound potentiation. These inhibitors have no effect on the once established rebound potentiation, on voltage-gated [Ca.sup.2+] channel currents, or on the basal inhibitory transmission itself. Futhermore, a protein phosphatase inhibitor and the intracellularly applied CaM-KII markedly enhanced GABA-mediated currents in Purkinje neurons. Our results demonstrate that CaM-KII activation and the following phosphorylation are key steps for rebound potentiation.

The cellular basis of motor learning in the cerebellum has been attributed mostly to long-term depression (LTD) at excitatory parallel fiber (PF)-Purkinje cell (PC) synapses. LTD is induced when PFs are activated in conjunction with a climbing fiber (CF), the other excitatory input to PCs. Recently, by using whole-cell patch-clamp recording from PCs in cerebellar slices, a new form of synaptic plasticity was discovered. Stimulation of excitatory CFs induced a long-lasting (usually longer than 30 min) 'rebound potentiation (RP)' of [Gamma]-amino-butyric acid A (GABAA)-receptor mediated inhibitory postsynaptic currents (IPSCs). As in LTD, induction of RP requires transient elevation of intracellular calcium concentration ([[[Ca.sup.2+]].sub.i]) due to activation of voltage-gated [Ca.sup.2+] channels. Activity of inhibitory synapses seems also to be necessary for RP to occur. RP is mainly due to up-regulation of postsynaptic GABAA receptor function, since PC response to bath-applied exogenous GABA is also potentiated with a time course similar to RP. The difference in the time scale between the [Ca.sup.2+] transients (10-30 sec) and the durations of RP (>30 min) strongly suggests that some intracellular biochemical machinery is involved. Pharmacological evidence suggests that protein kinases are involved in RP of inhibitory synapses and LTD of excitatory PF synapses. Besides the well-described LTD, RP could be a cellular mechanism that plays an important role in motor learning. Key words: [Ca.sup.2+]; cerebellum; GABAA receptor; inhibitory synapse; long-lasting potentiation; protein kinase; Purkinje cell

Mutant mice deficient in protein kinase C-gamma are unable to coordinate motor activity but learn discrete motion elements such as eyeblinking conditioning. Cerebellar long-term depression is unaffected in the mutants.The motor coordination defects are probably due to the presence of excess multiple climbing fibers in the Purkinje cells of the mutants.

Protein kinase C-gamma (PKC-gamma) may function as a downstream element in eliminating excess climbing fiber (CF) synapses. Adult mutant mice lacking PKC-gamma show multiple CFs in 41% of the Purkinje cells (PCs) unlike the wild-type mice whose PCs have single CF innervation. The mutants have a normal anatomy and normal transmission of excitatory stimuli in CF-PC and the parallel fiber-PC synapses. The mutants show normal regression of multiple CF innervation until two weeks after birth followed by a reduction in regression.

mGLuR1-absent mice exhibit ataxic cerebellar symptoms and defective long term depression (LTD) and conditioned eyeblink response. They exhibit normal cerebellar anatomy, functional excitatory synaptic transmission from parallel and climbing fibers to the Purkinje cells and normal voltage-gated Ca+ channels of Purkinje cells. Observations on the mutant mice suggest that LTD activation requires mGLuR1 and that LTD deficiency leads to defective eyeblinking and ataxic symptoms.