Excitatory Amino Acid-Induced Degeneration of Dendrites of Catecholamine Neurons in Rat Substantia Nigra

Abstract

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-μm-thick SN slices cut in the horizontal plane on a vibratome. Slices were incubated in saline buffer at 35°C for 15 min to 6 h in the presence or absence or absence of kainic acid (KA) orN-methyl-d-aspartate (NMDA) in concentrations ranging from 10 to 500 μM. The slices were then fixed and resectioned into 40-μm 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 μM) and MK-801 (20 μM), 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 μM) of KA and within 15 min if the concentration was increased to 500 μM. 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.