Loss of tyrosine hydroxylase immunoreactivity in dendrites is a sensitive index of kainic acid-induced damage in rat substantia nigra neurons in vivo

Low-dose 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine causes inflammatory activation of astrocytes in nuclear factor-κB reporter mice prior to loss of dopaminergic neurons

Neuroinflammation is implicated in the progression of numerous disease states of the CNS, but early inflammatory signaling events in glial cells that may predispose neurons to injury are not easily characterized in vivo. To address this question, we exposed transgenic mice expressing a nuclear factor-κB (NF-κB)-driven enhanced green fluorescent protein (EGFP) reporter construct to low doses of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and examined inflammatory activation of astrocytes in relation to neurobehavioral and neuropathological outcomes. The highest dose of MPTP (60 mg/kg total dose) caused a decrease in locomotor activity and a reduction in stride length. No significant loss of dopaminergic neurons in the substantia nigra was apparent at any dose. In contrast, expression of tyrosine hydroxylase in striatal fibers was reduced at 60 mg/kg MPTP, as were levels of dopamine and DOPAC. Colocalized expression of EGFP and inducible nitric oxide synthase (NOS2) occurred in astrocytes at 30 and 60 mg/kg MPTP and was associated with increased protein nitration in nigral dopaminergic neurons. Inhibition of NF-κB in primary astrocytes by expression of mutant IκBα suppressed expression of NOS2 and protected cocultured neurons from astrocyte-mediated apoptosis. These data indicate that inflammatory activation of astrocytes and enhanced nitrosative stress occurs at low doses of MPTP prior to loss of dopaminergic neurons. NF-κB-mediated expression of NOS2 appears to be a sensitive indicator of neuroinflammation that correlates with MPTP-induced neurochemical and neurobehavioral deficits prior to loss of dopaminergic neurons in the subtantia nigra.

Reduced dendrite growth and altered glutamic acid decarboxylase (GAD) 65- and 67-kDa isoform protein expression from mouse cortical GABAergic neurons following excitotoxic injury in vitro

The vulnerability of brain cells to neurologic insults varies greatly, depending on their neuronal subpopulation. However, cells surviving pathological insults such as ischemia or brain trauma may undergo structural changes, e.g., altered process growth, that could compromise brain function. In this study, we examined the effect of glutamate excitotoxicity on dendrite growth from surviving cortical GABAergic neurons in vitro. Glutamate exposure did not affect GABAergic neuron viability, however, it significantly reduced dendrite growth from GABAergic neurons. This effect was blocked by the AMPA receptor antagonists NBQX and CFM-2, and mimicked by AMPA, but not NMDA. Glutamate excitotoxicity also caused an NMDA receptor-mediated decrease in the GABA synthesizing enzyme glutamic acid decarboxylase (GAD65/67) immunoreactivity from GABAergic neurons, measured using immunocytochemical and Western blot techniques. GAD is necessary for GABA synthesis; however, reduction of GABA by 3-mercaptopropionic acid (3-MPA), which inhibits GABA synthesis, did not alter dendrite growth. These results suggest that GABAergic cortical neurons are relatively resistant to excitotoxic-induced cell death, but they can display morphological and biochemical alterations which may impair their function.

Analytical characterization of a sensitive radioassay for tyrosine hydroxylase activity in rodent striatum

Several buffer compositions with a wide range of pH values have been reported for radiometric assay of tyrosine hydroxylase (TH) in biological samples. Assay sensitivity becomes a prime concern while analyzing TH in minute samples like tissue biopsies or discrete regions of rodent brain wherein lower enzyme levels are anticipated due to smaller sample sizes. It was therefore rationalized to evaluate relative affinities of three commonly used assay buffers (sodium phosphate, sodium acetate, and Tris-acetate) with TH enzyme activity. The impact of buffer pH and cofactor concentration on the sensitivity of TH assay was also investigated. Striata from rats or mice were homogenized, respectively, with 1.0 or 0.5 ml of the assay buffer containing 0.5% Triton X-100. The supernatants (200 microl) were incubated (20 min, 37 degrees C) with 0.8 microCi [3H] L-tyrosine, 1.5 mM DL-6-methyl-5,6,7,8-tetrahydropterine (6-MPH4), 100 U catalase, and 1.0 microM dithiothreitol in a total volume of 300 microl. The reaction was terminated by 1-ml suspension of activated charcoal in 0.1 M HCl. After centrifugation, 200-microl aliquots were mixed with 5 ml of cocktail for quantitation of [3H] H2O in supernatant. The results showed significant impact of pH rather than the buffer composition on the sensitivity of TH assay. An optimal pH range was found to be 5.5-6.0, whereas TH activity was significantly inhibited at pH 5.0 and pH 6.8 (F = 55.09, P = 0.000). A significantly high TH activity was observed with 1.5 mM 6-MPH4, whereas higher concentrations (3.0-4.5 mM) inhibited TH activity (F = 7.47, P = 0.005). Analysis of serially diluted striatal homogenates showed a significant correlation between TH activity and sample amount. The assay reaction was linear for 20- and 30-min incubation for rat and mice striata, respectively.

Effect of excess extracellular glutamate on dendrite growth from cerebral cortical neurons at 3 days in vitro: Involvement of NMDA receptors

Glutamate is an important regulator of dendrite development; however, during cerebral ischemia, massive glutamate release can lead to neurodegeneration and death. An early consequence of glutamate excitotoxicity is dendrite injury, which often precedes cell death. We examined the effect of glutamate on dendrite growth from embryonic day 18 (E18) mouse cortical neurons grown for 3 days in vitro (DIV) and immunolabeled with anti-microtubule-associated protein (MAP)2 and anti-neurofilament (NF)-H, to identify dendrites and axons, respectively. Cortical neurons exposed to excess extracellular glutamate (100 microM) displayed reduced dendrite growth, which occurred in the absence of cell death. This effect was mimicked by the ionotropic glutamate receptor agonist N-methyl-D-aspartate (NMDA) and blocked by the ionotropic glutamate receptor antagonist kynurenic acid and the NMDA receptor-specific antagonist MK-801. The non-NMDA receptor agonist AMPA, however, did not affect process growth. Neither NMDA nor AMPA influenced neuron survival. Immunolabeling and Western blot analysis of NMDA receptors using antibodies against the NR1 subunit, demonstrated that immature cortical neurons used in this study, express NMDA receptors. These results suggest that excess glutamate decreases dendrite growth through a mechanism resulting from NMDA receptor subclass activation. Furthermore, these data support the possibility that excess glutamate activation of NMDA receptors mediate both cell death in mature neurons and the inhibitory effect of excess glutamate on dendrite growth in immature neurons or in the absence of cell death.

Mitochondrial complex inhibitors preferentially damage substantia nigra dopamine neurons in rat brain slices

Using a rat brain slice preparation, we investigated the role of energy impairment on the selective loss of dopamine neurons in the substantia nigra (SN). Brain slices (400 microm) were incubated at 35 degrees C for 2 h in the presence or absence of mitochondrial complex inhibitors, rotenone, MPP+, 3-nitropropionic acid, and antimycin A. Slices were also incubated in rotenone with excitatory amino acid (EAA) receptor antagonists, MK-801 and CNQX, to determine whether rotenone-induced damage was mediated by EAAs. The slices were then fixed, recut into 30-microm sections, and immunolabeled for tyrosine hydroxylase (TH) to identify catecholamine neurons and to quantify loss of TH-labeled dendrites after treatment. Quantitative comparison was made between SN dopamine neurons, in which rotenone-induced dendrite loss was severe, and hypothalamic A11 dopamine neurons, which were spared. Adjacent sections that were immunolabeled for calbindin or stained with cresyl violet also revealed a striking dendritic degeneration of SN neurons in rotenone-exposed slices, whereas noncatecholamine neurons, such as those in the perifornical nucleus (PeF), were more resistant. Preferential damage to SN dopamine neurons was also evident with other mitochondrial complex inhibitors, MPP+ and antimycin A. EAA receptor antagonists provided partial protection to SN neurons in slices incubated with rotenone (3 microM). The particular vulnerability of SN dopamine neurons in the slice is consistent with the vulnerability of SN in Parkinson's disease. The selective effect of mitochondrial complex inhibition in SN dopamine neurons implies a fundamental deficit in the capacity of these neurons to defend against toxic insult.

Catecholamine neuron groups in rat brain slices differ in their susceptibility to excitatory amino acid induced dendritic degeneration

We investigated whether specific types of catecholamine neurons were differentially vulnerable to damage induced by excitatory amino acids (EAAs) in vitro in a rat brain slice preparation. Brain slices, 300 micro m thick, were cut horizontally, exposed to either N-methyl-D-aspartate (NMDA) or kainic acid (KA) for 2h, fixed and then cut into thin (30 micro m) sections in the same (horizontal) plane as the slice. The sections were immunolabelled for tyrosine hydroxylase to identify different groups of catecholamine neurons (substantia nigra (SN), paranigral (PN), interfascicular (IF) and hypothalamic A11, A13 and A14) which exhibited prominent dendritic projections in the horizontal plane. Loss of dendrites was used as a sensitive index of damage that precedes the loss of the cell body. Catecholamine neurons differed strikingly in their vulnerability of EAA-induced dendrite degeneration. The most vulnerable were those in the dorsal tier of the SN, whereas the most resistant were those in the hypothalamic A11 group. For example, in the dorsal tier of SN, NMDA (50 micro M) reduced the proportion of neurons with dendrites from 64% (+/- 8% SEM) in controls to 13% (+/- 7%) whereas the majority of A11 neurons (69 +/- 10%) retained their dendrites compared to controls (89% +/- 8%). The other groups of catecholamine neurons exhibited intermediate vulnerability. An essentially similar pattern of differential vulnerability was observed with KA. An understanding of the cellular mechanisms that underlie the particular vulnerability of SN neurons in the slice will aid the discovery of pharmacological therapies to prevent or slow the pathological process in neurodegenerative diseases which involve these neurons.