THE DIOXIN RECEPTOR AND ITS NUCLEAR TRANSLOCATOR (ARNT) IN THE RAT-BRAIN

DIOXINS are environmental pollutants, whose detrimental effects on hea lth are the cause of wide public concern due to their accumulation in the food chain and resistance to metabolism. The most well known dioxi n is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Dioxins exert their e ffects through a ligand activated transcription factor termed the diox in or aryl hydrocarbon receptor (Ahr), which acts in concert with anot her structurally related protein: the aryl hydrocarbon nuclear translo cator (Arnt). In the present study, we have employed in situ hybridiza tion to study the localization of the mRNAs for these two proteins in the rat brain. We found mRNAs for both Ahr and Arnt predominantly in t he same neuronal populations: in the olfactory bulb, the hippocampus, and the cerebral and cerebellar cortices. Arnt, however, had a more wi despread expression than Ahr in the brain. The present results demonst rate that dioxins may act directly in the brain and that the effects o f dioxin may occur in discrete neuronal populations. However, in some parts of the brain, e.g. the hypothalamus, that are thought to be targ ets of the toxic effects of dioxins, we did not observe detectable lev els of Ahr mRNA. Furthermore, it appears that Arnt may have additional functions in the brain, apart from being the heterodimerization partn er of Ahr, possibly through heterodimerizing with other transcription factors.