Neuronal, astroglial and microglial cytokine expression after an excitotoxic lesion in the immature rat brain

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Abstract
Cytokines are important intercellular messengers involved in neuron–glia interactions and in the microglial‐astroglial crosstalk, modulating the glial response to brain injury and the lesion outcome. In this study, excitotoxic lesions were induced by the injection of N‐methyl‐d‐aspartate in postnatal day 9 rats, and the cytokines interleukin‐1 beta (IL‐1β), interleukin‐6 (IL‐6), tumour necrosis factor alpha (TNFα) and transforming growth factor beta 1 (TGF‐β1) analysed by ELISA and/or immunohistochemistry. Moreover, cytokine‐expressing glial cells were identified by means of double labelling with glial fibrillary acidic protein or tomato lectin binding. Our results show that both neurons and glia were capable of cytokine expression following different patterns in the excitotoxically damaged area vs. the nondegenerating surrounding grey matter (SGM). Excitotoxically damaged neurons showed upregulation of IL‐6 and downregulation of TNFα and TGF‐β1 before they degenerated. Moreover, in the SGM, an increased expression of neuronal IL‐6, TNFα and TGF‐β1 was observed. A subpopulation of microglial cells, located in the SGM and showing IL‐1β and TNFα expression, were the earliest glial cells producing cytokines, at 2–10 h postinjection. Later on, cytokine‐positive glial cells were found within the excitotoxically damaged area and the adjacent white matter: some reactive astrocytes expressed TNFα and IL‐6, and microglia/macrophages showed mild IL‐1β and TGF‐β1. Finally, the expression of all cytokines was observed in the glial scar. As discussed, this pattern of cytokine production suggests their implication in the evolution of excitotoxic neuronal damage and the associated glial response.