442 Braht Research, 112 (1976) 442 446 ~f) Elsevier Scientific Publishing Company, Amsterdam - Prmted in The Netherlands Heavy metal content in subdivisions of the rat hippocampus (zinc, lead and copper) G. DANSCHER, E. J. FJERDINGSTAD, E. FJERD1NGSTAD and K. FREDENS Institute Of Anatomy, University of Aarhus, Aarhus and (E.F.) Institute of Hygiene, University of Copenhagen, Copenhagen (Denmark) (Accepted May 3rd, 1976) In recent years the Timm sulphide silver method lz for the histochemical dem- onstration of heavy metals has served as the starting point for a series of investiga- tions into the nature and function of metals in the hippocampal region. It was suggest- ed by studies with this method, and confirmed by flameless atomic absorption spectro- photometry, that the hippocampus and amygdala accumulate zinc and lead far in excess of the average contents of the whole brain 1,3,5,6. The giant boutons of the hippocampal mossy fibre system are especially strongly stained by the Timm method 4, s.9, and this has been postulated to be due to their content of zinc. It has further been shown that application of chelating drugs, which abolish Timm stainability, has pro- nounced behavioural effects 1,2,7,n. The present study was undertaken in an attempt to further delineate the localization and nature of the metals in question. Twenty male and female Wistar rats, weighing about 160 g, were used in this study. After decapitation, the brains were removed and transferred to 4 % analytical grade formalin (Merck) in de-ionized water. As noted earlier 5,6, this formaldehyde solution contains negligible amounts of heavy metal and does not extract metal from Fig. 1. Drawing of the hippocampal region of the rat showing the position of the mossy fibre system and demonstrating the cutting of the slices further described in Fig. 2. The dotted lines delimit the dorsal (A) and ventral 03) parts of the hippocampus which were not dissected.