Neurotoxic effects of kainic acid on substantia nigra neurons in rat brain slices

Trkb-IP3 Pathway Mediating Neuroprotection in Rat Hippocampal Neuronal Cell Culture Following Induction of Kainic Acid

BACKGROUND

Following brain injury, development of hippocampal sclerosis often led to the temporal lobe epilepsy which is sometimes resistant to common anti-epileptic drugs. Cellular and molecular changes underlying epileptogenesis in animal models were studied, however, the underlying mechanisms of kainic acid (KA) mediated neuronal damage in rat hippocampal neuron cell culture alone has not been elucidated yet.

METHODS

Embryonic day 18 (E-18) rat hippocampus neurons were cultured with poly-L-lysine coated glass coverslips. Following optimisation, KA (0.5 μM), a chemoconvulsant agent, was administered at three different time-points (30, 60 and 90 min) to induce seizure in rat hippocampal neuronal cell culture. We examined cell viability, neurite outgrowth density and immunoreactivity of the hippocampus neuron culture by measuring brain derived neurotrophic factor (BDNF), γ-amino butyric acid A (GABAA) subunit α-1 (GABRA1), tyrosine receptor kinase B (TrkB), and inositol trisphosphate receptor (IP3R/IP3) levels.

RESULTS

The results revealed significantly decreased and increased immunoreactivity changes in TrkB (a BDNF receptor) and IP3R, respectively, at 60 min time point.

CONCLUSION

The current findings suggest that TrkB and IP3 could have a neuroprotective role which could be a potential pharmacological target for anti-epilepsy drugs.

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.

Involvement of adrenal medulla grafts in the open field behavior

Immunohistochemical and behavioral techniques were used to study the effects of adrenal medulla grafts, implanted in striatum after bilateral kainic acid (KA) lesions of this structure, on the open field behavior of mice. KA-induced behavioral changes in leaning, grooming and locomotor activity of the open field test were significantly improved after grafting of the adrenal medulla, and in some respects, fully restored. Immunohistochemical identification showed that grafts contained neuron-like cells with a tyrosine hydroxylase (TH), phenylethanolamine N-methyltransferase, gamma-aminobutyric acid (GABA), choline acetyltransferase (ChAT), and enkephalin-like immunostainings. A likely interpretation of this complex pattern of results is that adrenal medullary grafts may restore the deficits of GABAergic neurons which in turn reverse the abnormalities in emotionality and locomotion. Neurobiologically, these behavioral improvements probably involve GABAergic and catecholaminergic factors of adrenal medulla grafts, although other neuroactive substances, such as acetylcholine and enkephalins, cannot be excluded.

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

An early indicator of damage to substantia nigra dopamine neurons in vitro is loss of dendrites that precedes loss of the cell body. To investigate dendritic damage in vivo, rats were treated for 1 day or 1 week with kainic acid (KA; 5 or 10 mg/kg i.p.), the brain fixed and substantia nigra (SN) dopamine neurons and their dendrites labeled using an antibody to tyrosine hydroxylase (TH). KA (10 mg/kg) produced seizures initially and resulted in significant loss of TH immunoreactivity in dendrites of dopamine neurons 1 week, but not 1 day, after a single injection. Daily injections of 5 mg/kg KA, which did not produce seizures, resulted in more extensive dendritic damage. The findings indicate that loss of dendritic staining is a sensitive index of damage to SN dopamine neurons in vivo.

Dendrite loss is a characteristic early indicator of toxin-induced neurodegeneration in rat midbrain slices

In rat brain substantia nigra catecholamine neurons in vitro, a sensitive indicator of excitatory amino-acid-induced damage is dendritic degeneration that precedes the loss of the cell body. The present study has shown that dendritic loss is not specific for excitatory amino acids and is an early indicator of neurodegeneration produced by numerous agents that initiate damage by different primary cellular actions. Rats were anesthetised by fluothane inhalation and killed, and the brain was rapidly removed. Three-hundred-micrometer-thick slices containing substantia nigra were incubated for 2 h at 35 degrees C in the presence or absence of kainic acid (50 microM), 1-methyl-4-phenylpyridinium ion (10 or 50 microM), ouabain (10 or 30 microM), 6-hydroxydopamine (10 or 100 microM), potassium cyanide (100 microM or 1 mM), or elevated extracellular potassium chloride (25, 50, or 100 mM). The slices were fixed and recut into thin sections (30 micrometer) and substantia nigra dopamine neurons were immunolabeled for tyrosine hydroxylase coupled to diaminobenzidine. Both the cell body and the extensive dendritic projections were immunolabeled. Each agent caused a similar pattern of toxicity including loss of tyrosine-hydroxylase-immunolabeled dendrites at lower concentrations and damage to, or disintegration of, the cell bodies at higher concentrations. For example, 100 microM potassium cyanide reduced the proportion of substantia nigra neurons which exhibited dendrites from 66 +/- 4% (SEM) in controls to 54 +/- 7%, without obvious changes in cell bodies. After 1 mM potassium cyanide, only 13 +/- 2% of substantia nigra neurons retained dendrites and cell bodies were shrunken or disintegrated. Loss of dendrites was also evident in substantia nigra neurons stained with cresyl violet or immunolabeled for microtubule-associated protein 2. The findings suggest that disruption of the dendritic arbor is an early indicator of neurodegeneration, irrespective of how this is initiated. The approach that we have developed may therefore prove valuable in investigating the mechanisms of degeneration of catecholamine neurons.

Excitatory amino acid-induced degeneration of dendrites of catecholamine neurons in rat substantia nigra

We have recently established a rat substantia nigra (SN) slice preparation in which a sensitive index of excitatory amino acid (EAA) toxicity was degeneration of the dendritic arbor of catecholamine neurons labelled by immunostaining for tyrosine hydroxylase (TH). The present study examined the pharmacological characteristics of EAA-induced neurotoxicity. Rats were anesthetised by halothane inhalation and killed, the brain was rapidly removed, and 400-microm-thick SN slices cut in the horizontal plane on a vibratome. Slices were incubated in saline buffer at 35 degreesC for 15 min to 6 h in the presence or absence or absence of kainic acid (KA) or N-methyl-D-aspartate (NMDA) in concentrations ranging from 10 to 500 microM. The slices were then fixed and resectioned into 40-microm sections that were coplanar with the parent slice. Dopaminergic SN neurons were labeled using antibody to tyrosine hydroxylase (TH) coupled to diaminobenzidine. A feature of the immunostaining was that it labeled not only the cell body but also the prolific dendritic arborization of SN neurons. Dendritic damage was quantified by counting the proportion of neurons with intact dendrites after treatment with EAA. KA and NMDA caused loss of dendrites that was prevented by CNQX (20 microM) and MK-801 (20 microM), respectively, indicating that activation of either NMDA or non-NMDA receptors produces neurotoxicity. EAA-induced dendritic damage was observed within 2 h of treatment with a low concentration (10 microM) of KA and within 15 min if the concentration was increased to 500 microM. Thus the loss of dendrites occurs rapidly and precedes disintegration of the cell bodies. Furthermore, brief (15 min) exposure to EAA initiated damage in the dendrites which progressed after the EAA was removed from its receptor. The observations are consistent with the postulated role of EAAs in neurodegenerative diseases. Labeling the dendritic arbor provides a sensitive approach to investigating the cellular mechanisms of neurodegeneration of catecholamine neurons.

Degeneration of the dendritic arbor as an index of neurotoxicity in identified catecholamine neurons in rat brain slices

Although catecholamine neurons are vulnerable targets for neurotoxins and degenerative disease, few in vitro studies have investigated the mechanisms of neurodegeneration in these cells. We therefore developed a brain slice preparation for this purpose. Rats were killed by cervical dislocation and 400-microm-thick horizontal slices containing midbrain catecholamine neurons were incubated for 2 h in the presence or absence of kainic acid (KA, 50 microM). After fixation, the slices were recut by a technique that provided thin (40 microm) sections in the same plane as the parent slice. Catecholamine neurons in these coplanar sections were labeled by immunostaining for tyrosine hydroxylase (TH) coupled with diaminobenzidine. The topographical organization of the horizontal plane of the brain was retained in the coplanar sections, enabling precise identification of catecholamine neurons in the thin sections, by reference to an atlas in the horizontal plane. In this study we examined neurons in the substantia nigra (SN). A key feature of the immunostaining was that it revealed both the cell body and also the extensive dendritic projections of SN neurons in the horizontal plane. After treatment with KA, cell bodies remained intact but the dendrites were truncated or fragmented. The loss of dendrites is a sensitive and readily quantifiable indicator of damage. KA caused significant reductions in the proportion of SN neurons with intact dendrites and in the total length of the dendrites, measured using a computer program. The sensitive index of damage and the facility to clearly distinguish catecholamine groups that are topographically close yet functionally distinct are the principal features of the experimental approach that we have developed. The preparation offers major advantages for investigating the selective vulnerability or resistance of particular types of catecholamine neurons to damage.