Cellular plasticity at the climbing fibre-Purkinje cell synapse as a model of plasticity in adulthood and ageing

Elevated substance P (NK-1) receptor immunoreactivity in the cerebellum of seizure prone gerbil

In the present study, we performed a comparative analysis of the distribution of substance P (SP) receptor (NK-1) immunoreactivity in order to determine the characteristics of the SP system in the cerebelli of rat and gerbils. In the rat cerebellar cortex, only a few Purkinje cells exhibited weak NK-1 receptor immunoreactivity. Similar to the case of rat, NK-1 receptor immunoreactivity in the cerebellar cortex of seizure resistant (SR) gerbils was rarely detected. In contrast, in the cerebellar cortex of seizure sensitive (SS) gerbils, dendrites and cell bodies of Purkinje cell showed strong NK-1 receptor immunoreactivity. Similar to the cerebellar cortex, little NK-1 receptor immunoreactivity in deep cerebellar nuclei was observed in the rat. In SR gerbils, however, deep cerebellar nuclei showed weak NK-1 receptor immunoreactivity. NK-1 receptor immunoreactivity in the deep cerebellar nuclei of SS gerbils was markedly increased, as compared with SR gerbils. Based on the present data, we suggest that the SP system of cerebellar circuit in gerbil are different from rat, and over-expression of NK-1 receptor immunoreactivity in Purkinje cells of SS gerbils may be relevant to Purkinje cell loss induced by seizure activity.

The decreases in calcium binding proteins and neurofilament immunoreactivities in the Purkinje cell of the seizure sensitive gerbils

In previous studies, it has been reported that Purkinje cell degeneration during seizure is evoked by excitotoxicity due to an increase in the intracellular Ca(2+) level, though calbindin D-28k (CB) and parvalbumin (PV), intracellular free calcium buffers, are abundantly colocalized in these cells. In the present study, we investigated the expressions of CB, PV, neurofilament (NF) 68, 150, 200, and polyphosphorylated epitope in NF (RT 97), in the cerebellum of gerbils to identify the mechanism of Purkinje cell damages induced by seizure. In seizure resistant gerbils, nearly all the Purkinje cells showed CB, PA, NF 150, NF 200 and RT 97 immunoreactivity. In SS gerbils, however, a clear decrease in the number of CB(+) and PV(+) Purkinje cells was observed. The NF and RT 97 immunoreactivities, in the Purkinje cells was also lower (except NF 68), but not absent. These results suggest several points. First, the decrease in the concentrations of CB and PV may render the Purkinje cells more susceptible to intermittent Ca(2+) fluctuations and more prone to accumulating intolerable quantities of Ca(2+). Second, during the Ca(2+)-PV interaction PV plays an important role in facilitating donations of Mg(2+), which is a potent enzyme activator in phosphorylation. Thus the decline in PV concentration also implicated the defects of phosphorylation in the NF. Third, increases in both the intracellular Ca(2+) level and dephosphorylation trigger the degradation of the NF, particularly NF 200. Finally, these degradations in the NF induce the functional defects in Purkinje cell, which then cause Purkinje cell degeneration.

Microglial signalling cascades in neurodegenerative disease

Activated microglia release a number of substances, the specific cocktail released depending on the stimulus. Many of the substances released by microglia also serve to activate them, suggesting the presence of a number of autocrine/paracrine loops. Because of the low density of microglia present in the normal brain, such autocrine/paracrine loops may not be significant but during the initiation and ongoing states of neurodegeneration, the increased concentrations of microglia may allow the activation and escalated stimulation of these feedback pathways. The activation of p38 MAPK by A beta and cytokines may be part of a microglial autocrine loop which results in the fueling of the microglial inflammatory response. A novel class of cytokine suppressive anti-inflammatory drugs (CSAIDs) inhibit the activation of p38 kinase (Bhat et al., 1998) suggesting this kinase plays a key role in transducing microglial responses to activation stimuli (Badger et al., 1996).

Reciprocal trophic interactions between climbing fibres and Purkinje cells in the rat cerebellum

In the adult cerebellum both the climbing fibre arbour and the Purkinje cell are very plastic and each element is able to exert a remarkable action on the other one. The adult phenotype of the Purkinje cell is strictly dependent on the presence of its climbing fibre arbour. When the climbing fibre is missing, the Purkinje cell undergoes a hyperspiny transformation and becomes hyperinnervated by the parallel fibres. However, this change is fully reversible. The climbing fibre-deprived Purkinje cell is able to elicit sprouting of nearby located intact climbing fibres and the new arbour is able to fully restore synaptic connections which appear normal both morphologically and functionally. Multiple climbing fibre innervation of a single Purkinje cell persists in the adult hypogranular cerebellum. The different fibres are distributed to separate dendritic regions, suggesting a local competition between the different arbours for their territory. It is postulated that in the intact rat, an activity dependent mechanism of the parallel fibre favours the predominance of one arbour with the elimination of its competitors. When the Purkinje cell is deleted, the climbing fibre arbour becomes heavily atrophic and reduced in size. The analysis of the pattern of this atrophy indicates that the climbing fibre arbour is made by two compartments: a proximal one, whose survival depends on the integrity of the inferior olive, and a distal one, which represents the true pre-synaptic site, which strictly depends on the target. The climbing fibre terminal arbour is able to extend its territory of innervation not only when adult intact climbing fibres are confronted with nearby denervated Purkinje cells, but also when an embryonic cerebellum is grafted onto the surface of an adult unlesioned cerebellum. In this case, collaterals of intact climbing fibre arbours elongate through the pial surface, enter the graft to innervate the Purkinje cells. This growth is likely under the influence of a tropic signal released by the embryonic Purkinje cells. This suggests that the sprouting observed in the adult rat following a subtotal inferior olive lesion is also triggered by a similar factor. The axonal elongation and the consequent synaptogenesis are likely guided by local cues. In this condition, the distribution of the new collateral reinnervation occurs within its projectional map. In addition, when the inferior cerebellar peduncle is sectioned at birth, the climbing fibres of the non-deafferented hemicerebellum emit collaterals which cross the midline and innervate cerebellar strips which are symmetrically positioned relative to the intact side. In the grafting experiments, both the migrated and non-migrated Purkinje cells show the typical electrophysiological properties of the mature cerebellum. These data show that the disappearance of neuronal elements is not a necessary prerequisite to allow new neurones to become fully morphologically and functionally integrated into an adult brain. The reciprocal trophic influence between the climbing fibres and the Purkinje cells shown in the present series of experiments are likely operative in the adult brain not only in pathological conditions and they could give a basic contribution to the synaptic plasticity underlying learned behaviour.