Structural and neurochemical effects in mouse cerebellum following neonatal methylazoxymethanol and 6-hydroxydopamine treatment

Neuron specific alpha-adrenergic receptor expression in human cerebellum: implications for emerging cerebellar roles in neurologic disease

Recent data suggest novel functional roles for cerebellar involvement in a number of neurologic diseases. Function of cerebellar neurons is known to be modulated by norepinephrine and adrenergic receptors. The distribution of adrenergic receptor subtypes has been described in experimental animals, but corroboration of such studies in the human cerebellum, necessary for drug treatment, is still lacking. In the present work we studied cell-specific localizations of alpha1 adrenergic receptor subtype mRNA (alpha 1a, alpha 1b, alpha 1d), and alpha2 adrenergic receptor subtype mRNA (alpha 2a, alpha 2b, alpha 2c) by in situ hybridization on cryostat sections of human cerebellum (cortical layers and dentate nucleus). We observed unique neuron-specific alpha1 adrenergic receptor and alpha2 adrenergic receptor subtype distribution in human cerebellum. The cerebellar cortex expresses mRNA encoding all six alpha adrenergic receptor subtypes, whereas dentate nucleus neurons express all subtype mRNAs, except alpha 2a adrenergic receptor mRNA. All Purkinje cells label strongly for alpha 2a and alpha 2b adrenergic receptor mRNA. Additionally, Purkinje cells of the anterior lobe vermis (lobules I to V) and uvula/tonsil (lobules IX/HIX) express alpha 1a and alpha 2c subtypes, and Purkinje cells in the ansiform lobule (lobule HVII) and uvula/tonsil express alpha 1b and alpha 2c adrenergic receptor subtypes. Basket cells show a strong signal for alpha 1a, moderate signal for alpha 2a and light label for alpha 2b adrenergic receptor mRNA. In stellate cells, besides a strong label of alpha 2a adrenergic receptor mRNA in all and moderate label of alpha 2b message in select stellate cells, the inner stellate cells are also moderately positive for alpha 1b adrenergic receptor mRNA. Granule and Golgi cells express high levels of alpha 2a and alpha 2b adrenergic receptor mRNAs. These data contribute new information regarding specific location of adrenergic receptor subtypes in human cerebellar neurons. We discuss our observations in terms of possible modulatory roles of adrenergic receptor subtypes in cerebellar neurons responding to sensory and autonomic input signals, and review species differences in cerebellar adrenergic receptor expression.

Caspase-3-like proteases and 6-hydroxydopamine induced neuronal cell death

Neurotoxicity induced by 6-hydroxydopamine (6-OHDA) is believed to be due, in part, to the production of reactive oxygen species (ROS) and/or an inhibition of mitochondrial function. However, little is known about the ensuing intracellular events which ultimately result in cell death. Here we show that exposure to relatively low concentrations of 6-OHDA induces apoptosis of cerebellar granule neurons (CGN). 6-OHDA-induced apoptosis of CGN is associated with activation of a caspase-3-like protease. Western blots of cytosolic extracts from 6-OHDA-treated CGN reveal a translocation of cytochrome c from mitochondria to the cytosol, which precedes activation of the protease detected by Ac-DEVD-pNA. DNA laddering can be blocked by caspase inhibitors zVAD-FMK and Ac-DEVD-CHO, however cell death can only be attenuated for a short time period in the presence of these inhibitors. Our data suggest that 6-OHDA-induced apoptosis of CGN involves activation of a caspase-3-like protease. In contrast to the neurotoxicity induced by MPP+, however, the peptide inhibitors zVAD-FMK and Ac-DEVD-CHO can only attenuate early neuronal death induced by 6-OHDA. At later time points, neuronal death lacking DNA laddering occurs even in the presence of the peptide inhibitor zVAD-FMK or Ac-DEVD-CHO.

Neuroanatomical and functional alterations resulting from early postnatal cerebellar insults in rodents

This review examines neuroanatomical and functional alterations in rodents resulting from postnatal insults during cerebellar development. Treatments such as irradiation and methylazoxymethanol (MAM) administration produced near birth (< postnatal day 8 for irradiation treatment and < postnatal day 4 for MAM administration) result in more severe cerebellar damage than do similar treatments administered several days after birth. Prominent among the more severe alterations are foliation abnormalities, misalignment of Purkinje cells and continued multiple innervation of climbing fibers; few or none of these occur as a result of later treatments (> postnatal day 8 for irradiation treatment and > postnatal day 4 for MAM treatment). The functional alterations also differ: insults produced near birth result in hypoactivity, ataxia, tremor and accompanying learning deficits, whereas those produced later result in hyperactivity and few learning deficits. This hyperactivity may have relevance to human disorders. Brief discussions of cerebellar and functional alterations (e.g., hyperactivity) resulting from neonatal infection with the Borna disease virus and induction of hypo- and hyperthyroidism during the preweaning period are also presented.

Functional effects of methylazoxymethanol-induced cerebellar hypoplasia in rats

The behavioral effects of a series of methylazoxymethanol acetate (MAM) injections in neonatal rats were investigated. Pups were injected twice daily on days 5-8 after birth with 4 mg/kg MAM or saline. Similar treatment paradigms cause cerebellar hypoplasia, which is a result of a depletion of granule cells. MAM treatment reduced adult cerebellar weight to 92% that of control and was without effect on weight of other brain regions examined. Postweaning body weight gain in males was reduced. Nest odor preference and emergence (light/dark box) assessments indicated no significant effects. Complex maze assessments and performance in an operant test battery demonstrated no cognitive deficits. Indeed, MAM treated females performed better in a complex maze under lighted conditions than same-sex controls. Open field and running wheel activity levels in females were unaffected. Though not statistically significant, males exhibited a mild hyperactivity syndrome characterized by an increase in running wheel and open field activity, as well as a depressed startle response. Both sexes were hypersensitive to the locomotor-increasing effects of methamphetamine. These results suggest that the functional effects resulting from cerebellar hypoplasia produced by MAM treatment on PNDs 5-8 are milder than those resulting from MAM treatment beginning on the day of birth. The results are compared with other forms of cerebellar lesions and provide support for the hypothesis that early insult (day of birth or shortly after) produces hypoactivity whereas a later insult (day 4 or later after birth) produces a syndrome of hyperactivity.

Use of quantitative ultrastructural immunoperoxidase labeling for analysis of catecholamine neurotoxicity and plasticity

Levels of catecholamines and the synthesizing enzyme, tyrosine hydroxylase (TH) are markedly decreased in the dorsal striatum, caudate-putamen nuclei, following neurotoxic lesions with 6-hydroxy-dopamine (6-OHDA). We examined whether pre-embedding immunoperoxidase labeling of TH could be standardized for quantitatively examining the density and ultrastructure of spared dopaminergic terminals in the striatum of lesioned rats. The peroxidase-antiperoxidase (PAP) method was used to localize rabbit antiserum against TH in caudate-putamen nuclei of adult rats given unilateral nigral injections of either vehicle or 6-hydroxydopamine in the early postnatal period. Experimental differences in fixation and immunocytochemical labeling were minimized by limiting comparisons of immunoreactivity to co-processed sections from the same litters of animals. Imaging software and a Phillips CM-10 electron microscope were used to quantitatively examine immunoreactive profiles in a narrow zone of tissue in contact with the embedding resin. Under these conditions variables attributed to differences in penetration were minimized. There were no significant differences in numbers or mean-cross sectional diameter of immunoreactive terminals in striatum ipsilateral versus contralateral to vehicle injections. Ipsilateral to the 6-OHDA injections, the density (numbers/area) of striatal TH-immunoreactive terminals was reduced by 50-90% in the majority of animals. In the most extensively lesioned rats, the cross-sectional areas of the remaining immunoreactive axons were significantly larger than in the contralateral striatum of the same animal or either hemisphere of vehicle injected controls. These results confirm and extend earlier findings on the plasticity of residual dopaminergic terminals in adult animals after neurotoxic damage. They also establish a quantitative method for ultrastructural analysis of the density of immunoreactivity in thick sections of tissue labeled prior to plastic embedding. The method has broad applicability to quantitative studies of neurotoxicity and plasticity in brain.

Dopaminergic innervation and binding in the rat cerebellum

In the present study, we used an antiserum against dopamine (DA), and specific [3H]ligands in order to shed more light on the dopaminergic system of the rat cerebellum. The immunocytochemical approach showed that the entire rat cerebellum is innervated by DA fibers. All cerebellar layers were found to receive a considerable amount of DA afferents but the molecular layer was the most heavily innervated. The analysis of [3H]DA and [3H]spiperone binding showed that in the rat cerebellum there exists DAergic binding with kinetic parameters similar to those reported for the mouse cerebellum. The results of the present study support the existence of a DA system in the rat cerebellum.

Increased activity of tyrosine hydroxylase in the cerebellum of the X-irradiated dystonic rat

The exposure of the cephalic end of rats to repeated doses of X-irradiation (150 rad) immediately after birth induces a long-term increase in the noradrenaline (NA) content of cerebellum (CE) (+ 37.8%), and a decrease in cerebellar weight (65.2% of controls), which results in an increased NA concentration (+ 109%). This increase in the neurotransmitter level is accompanied by a dystonic syndrome and histological abnormalities: Purkinje cells (the target cells for NA afferents to CE) fail to arrange in a characteristic monolayer, and their primary dendritic tree appears randomly oriented. The injection of reserpine 0.9 and 1.2 mg/kg ip to adult rats for 18 h depletes cerebellar NA content in both controls (15.7 +/- 4 ng/CE and 2.8 +/- 1.5 ng/CE, respectively) and X-irradiated rats (17.1 +/- 1 ng/CE and 8.3 +/- 2 ng/CE, respectively). The activity of tyrosine hydroxylase (TH) in CE of adult rats, measured by an in vitro assay, is significantly increased in neonatally X-irradiated animals when compared to age-matched controls (16.4 +/- 1.4 vs 6.32 +/- 0.6 nmol CO2/h/mg prot., p less than 0.01). As observed for NA levels, a net increase in TH activity induced by the ionizing radiation is also measured: 308.9 +/- 23.8 vs 408.2 +/- 21.5 nmol CO2/h/CE, p less than 0.01 (controls and X-treated, respectively). These results suggest that X-irradiation at birth may induce an abnormal sprouting of noradrenergic afferents to CE. The possibility that these changes represent a response of the NA system to the dystonic syndrome is discussed.