Alteration of neuronal nitric oxide synthase activity and expression in the cerebellum and the forebrain of microencephalic rats

D-serine regulates cerebellar LTD and motor coordination through the δ2 glutamate receptor

D-serine (D-Ser) is an endogenous co-agonist for NMDA receptors and regulates neurotransmission and synaptic plasticity in the forebrain. D-Ser is also found in the cerebellum during the early postnatal period. Although D-Ser binds to the δ2 glutamate receptor (GluD2, Grid2) in vitro, its physiological significance has remained unclear. Here we show that D-Ser serves as an endogenous ligand for GluD2 to regulate long-term depression (LTD) at synapses between parallel fibers and Purkinje cells in the immature cerebellum. D-Ser was released mainly from Bergmann glia after the burst stimulation of parallel fibers in immature, but not mature, cerebellum. D-Ser rapidly induced endocytosis of AMPA receptors and mutually occluded LTD in wild-type, but not Grid2-null, Purkinje cells. Moreover, mice expressing mutant GluD2 in which the binding site for D-Ser was disrupted showed impaired LTD and motor dyscoordination during development. These results indicate that glial D-Ser regulates synaptic plasticity and cerebellar functions by interacting with GluD2.

Partial neuroprotection by 17-β-estradiol in neonatal γ-irradiated rat cerebellum

Acute and long-term complications can occur in patients receiving radiation therapy. It has been suggested that cytoprotection might decrease the incidence and severity of therapy-related toxicity in these patients. Developing cerebellum is highly radiosensitive and for that reason it is a useful structure to test potential neuroprotective substances to prevent radiation induced abnormalities. Recent studies have shown that estrogen can rapidly modulate intracellular signalling pathways involved in cell survival. Thus, it has been demonstrated that estrogens mediate neuroprotection by promoting growth, cell survival and by preventing axonal pruning. The aim of this work was to evaluate the effect of the treatment with 17-β-estradiol on the motor, structural and biochemical changes induced by neonatal ionizing radiation exposure, and to investigate the participation of nitric oxide and protein kinase C, two important intracellular messengers involved in neuronal activity. Our results show that perinatal chronic 17-β-estradiol treatment partially protects against radiation-induced cerebellar disorganization and motor abnormalities. PKC and NOS activities could be implicated in its neuroprotective mechanisms. These data provide new evidence about the mechanisms underlying estrogen neuroprotection, which could have therapeutic relevance for patients treated with radiotherapy.

Choline acetyltransferase activity at different ages in brain of Ts65Dn mice, an animal model for Down's syndrome and related neurodegenerative diseases

Ts65Dn mice, trisomic for a portion of chromosome 16 segmentally homologous to human chromosome 21, are an animal model for Down's syndrome and related neurodegenerative diseases, such as dementia of the Alzheimer type. In these mice, cognitive deficits and alterations in number of basal forebrain cholinergic neurons have been described. We have measured in Ts65Dn mice the catalytic activity of the cholinergic marker, choline acetyltransferase (ChAT), as well as the activity of the acetylcholine-degrading enzyme acetylcholinesterase (AChE), in the hippocampus and in cortical targets of basal forebrain cholinergic neurons. In mice aged 10 months, ChAT activity was significantly higher in Ts65Dn mice, compared to 2N animals, in the hippocampus, olfactory bulb, olfactory cortex, pre-frontal cortex, but not in other neocortical regions. At 19 months of age, on the other hand, no differences in ChAT activity were found. Thus, alterations of ChAT activity in these forebrain areas seem to recapitulate those recently described in patients scored as cases of mild cognitive impairment or mild Alzheimer's disease. Other neurochemical markers putatively associated with the disease progression, such as those implicating astrocytic hyperactivity and overproduction of amyloid precursor protein family, were preferentially found altered in some brain regions at the oldest age examined (19 months).

Altered nitric oxide synthase and PKC activities in cerebellum of gamma-irradiated neonatal rats

In this study, we show that one single dose of gamma-irradiation at birth induces an inhibition of the cerebellar calcium dependent nitric oxide synthase (NOS) activity, probably correlated to the motor abnormalities and the disarrangement in the cerebellar cytoarchitecture observed in adult rats. This decrease in calcium dependent NOS activity could be associated with an increased protein kinase C (PKC) activity. PKC inhibition partially restores calcium dependent NOS activity, indicating that PKC activity could be negatively modulating the catalytic activity of calcium dependent NOS. These findings suggest that a decrease in nitric oxide (NO) production and the related increase in PKC activity could be intracellular events that participate in the onset of motor and cerebellar abnormalities induced by postnatal gamma-irradiation at early stages of life.

Neuronal nitric oxide synthase expression in cerebellar mutant mice

Nitric oxide (NO) is a diffusible, multifunctional signaling molecule found in many areas of the brain. NO signaling is involved in a wide array of neurophysiological functions including synaptogenesis, modulation of neurotransmitter release, synaptic plasticity, central nervous system blood flow and cell death. NO synthase (NOS) activity regulates the production of NO and the cerebellum expresses high levels of nitric oxide synthase (NOS) in granule, stellate and basket cells. Cerebellar mutant mice provide excellent opportunities to study changes of NO/NOS concentrations and activities to gain a greater understanding of the roles of NO and NOS in cerebellar function. Here, we have reviewed the current understanding of the functional roles of NO and NOS in the cerebellum and present NO/NOS activities that have been described in various cerebellar mutant mice and NOS knockout mice. NO appears to exert neuroprotective effects at low to moderate concentrations, whereas NO becomes neurotoxic as the concentration increases. Excessive NO production can cause oxidative stress to neurons, ultimately impairing neuronal function and result in neuronal cell death. Based on their genetics and cerebellar histopathology, some of cerebellar mutant mice display similarities with human neurological conditions and may prove to be valuable models to study several human neurological disorders, such as autism and schizophrenia.

Altered neuronal nitric oxide synthase expression in the cerebellum of calcium channel mutant mice

Tottering, rolling Nagoya, and leaner mutant mice all exhibit cerebellar ataxia to varying degrees, from mild (tottering mice) to severe (leaner mice). Collectively, these mice are regarded as tottering locus mutants because each of these mutant mice expresses a different autosomal recessive mutation in the gene coding for the alpha(1A) calcium ion channel protein, which is the pore forming subunit for P/Q-type high voltage activated calcium ion channels. These mutant mice all exhibit varying degrees of cerebellar dysfunction and neuronal cell death. Nitric oxide (NO) is an important messenger molecule in the central nervous system, especially in the cerebellum, and it is produced via the enzyme, nitric oxide synthase (NOS). We investigated expression of neuronal-NOS (n-NOS) in the cerebella of all three mutant mice, as revealed by NADPH-diaphorase (NADPH-d) histochemical staining, quantitation of n-NOS protein using Western blotting and quantitation of n-NOS mRNA using in situ hybridization. The expression of n-NOS mRNA and protein as well as the NADPH-d histochemical reaction were elevated in tottering and rolling Nagoya cerebella. n-NOS mRNA and the NADPH-d histochemical reaction were decreased in the leaner cerebellum, but the leaner mouse n-NOS protein concentration was not significantly different compared to age- and gender-matched controls. These findings suggest that NO may act as an important mediator in the production of the neuropathology observed in these mutant mice.

Developmental expression of the cell cycle and apoptosis controlling gene, Lot1, in the rat cerebellum and in cultures of cerebellar granule cells

The Lot1 gene encodes a zinc finger protein that, in vitro, concurrently regulates apoptosis and cell cycle arrest and belongs to a recently identified family of proteins with oncogenic and tumor-supressor functions. The present study, based on the development of the first antibody reportedly produced against rat Lot1, examines protein expression during normal development of the rat cerebellum and following methylazoxymethanol (MAM) administration, which results in hypoplasia of the cerebellar granule cell population. Using light microscopic immunocytochemistry, specific immunostaining for the Lot1 protein was observed at postnatal days 2 to 7 in the superficial external granule layer composed primarily of proliferating neuronal precursor cells. Purkinje cells showed distinct nuclear labeling at P7. In the adult cerebellum, the overall low Lot1 level was essentially associated with Purkinje cells. Experimentally altered developmental conditions, such as those obtained through MAM-induced microencephaly, did not drastically affect the pattern of Lot1 expression. In particular, Purkinje cells continued to show normal levels of immunoreactivity notwithstanding the altered cerebellar architecture. Primary cultures of cerebellar granule cells showed a temporal pattern of Lot1 expression resembling that of in vivo development, with mRNA and protein levels progressively decreasing with differentiation. When cerebellar granule cells were exposed to different neurotoxic challenges, Lot1 appeared not affected by purely apoptotic cell death, while transitorily induced by mixed necrotic-apoptotic cell death.

Cerebellar granule cells as a model to study mechanisms of neuronal apoptosis or survival in vivo and in vitro

Granule cells of the cerebellum constitute the largest homogeneous neuronal population of mammalian brain. Due to their postnatal generation and the feasibility of well characterized primary in vitro cultures, cerebellar granule cells are a model of election for the study of cellular and molecular correlates of mechanisms of survival/apoptosis and neurodegeneration/neuroprotection. The present review mainly deals with recent data on mechanisms and factors promoting survival or apoptotic elimination of cerebellar granule neurons, with a particular focus on the molecular correlates at the level of gene expression and induction of cellular signal pathways. The in vivo development is first analysed with particular reference to the role played by several neurotrophic factors and by the NMDA subtype of glutamate receptor. Then, mechanisms of survival/apoptosis are examined in the model of primary in vitro cultures, where the role of neurotrophins acting on cerebellar granule cells is followed by the large deal of data coming from the paradigm of potassium/serum withdrawal. The role of some key genes of the Bcl family, of some kinase systems and of transcriptional factors is primarily highlighted. Furthermore, the involvement of mitochondria, free radicals and proteases of the caspase family is considered. Finally, the use of cerebellar granule neurons in primary culture to experimentally address the issue of neurodegeneration and pharmacological neuroprotection is considered, with some comments on models at the borderline between necrosis and apoptosis, such as the excitotoxic neuronal damage. The overlapping of cellular signal pathways activated in granule neurons by apparently unrelated stimuli, such as neurotrophins and neurotransmitters/neuromodulators is stressed to put into light the special 'trophic' role played by activity in neurons. Finally, the advantage of designing and performing conceptually equivalent experiments on cerebellar granule neurons during development in vivo and in vitro, is stressed. On the basis of the reviewed material, it is concluded that cerebellar granule neurons have acquired a special position in modern neuroscience as one of the most reliable models for the study of neural development, function and pathology.

Developmental effects of in vivo and in vitro inhibition of nitric oxide synthase in neurons

The diffusible chemical messenger nitric oxide (NO) is involved in neuronal plasticity and it is, therefore, supposed to play a role in brain development. A shortage of NO during the critical period of brain maturation may theoretically have long-lasting consequences on the organization of the adult brain. We have performed in neonatal rats a chronic inhibition of the enzyme responsible for NO production, nitric oxide synthase (NOS), from postnatal day 3 to postnatal day 23, through administration of the competitive antagonist N-nitro-L-arginine methylester (L-NAME). The calcium-dependent catalytic activity resulted almost completely inhibited throughout the period of treatment and it took more than 4 days after its suspension to get a full recovery. The expression of the neuronal isoform of the enzyme (nNOS), revealed by immunoblotting, was unchanged during the treatment and after it. The histochemical reaction for NADPH diaphorase was reduced at the end of the treatment and recovered in concomitance with the recovery of the catalytic NOS activity. No gross structural alterations were detected in brain morphology. The levels of three neurotransmitter-related and one astrocytic marker were unchanged in the cerebellum, hippocampus and cortex of 60-day-old rats which had been neonatally treated. A similar lack of significant effects on neurochemical brain maturation was also noticed in a parallel series of experiments, in which a short pulse of NOS inhibition was performed at a critical prenatal time of brain development, from gestational day 14 to gestational day 19. In vitro, chronic exposure of cerebellar granule cells to L-NAME (500 microM) resulted in slight decrease of surviving neurons after 8 days in culture and in better resistance to the challenge of stressful culture conditions. The present results suggest that the basic plan of brain organization can be achieved despite an almost complete NOS inhibition during the maturation period. In vitro, NOS inhibition may bring to more pronounced consequences on neuronal viability and function.