The anabolic androgenic steroid, nandrolone decanoate, increases the density of Fos-like immunoreactive neurons in limbic regions of guinea-pig brain Pia Johansson-Steensland, 1 Fred Nyberg 1 and Loris Chahl 2 1 Department of Pharmaceutical Biosciences, Division of Biological Research on Drug Dependence, Uppsala University, Sweden 2 Experimental Pharmacology Unit, Faculty of Medicine and Health Sciences, University of Newcastle, Newcastle, NSW 2308, Australia Keywords: behaviour, central nervous system, dependence, drug abuse Abstract The increased abuse of anabolic androgenic steroids is a major concern because of physiological and psychological side-effects. In some individuals they induce dramatic behavioural changes such as increased aggression, anxiety and depression. The mechanisms behind these behavioural changes are still poorly understood. In order to obtain information on the brain regions affected by anabolic androgenic steroids, the distribution of neurons containing c-Fos, the protein product of the immediate early gene c-fos, and Fos-related antigens was studied following chronic treatment of guinea-pigs with a high dose of nandrolone decanoate (15 mg/kg i.m. daily for 14 days). The behaviour of the guinea-pigs was monitored for 1 h each day. Animals treated with nandrolone exhibited a signi®cantly greater incidence of biting behaviour during the 14 day treatment period than vehicle- treated animals. A signi®cantly greater density of c-Fos and Fos-related antigen-positive neurons was found in the central nucleus of the amygdala, and of Fos-related antigen-positive neurons in the frontal cortex, the shell of the nucleus accumbens and the supraoptic nucleus in nandrolone-treated animals than in vehicle controls. Therefore, nandrolone induced Fos in brain regions involved in stress, behavioural responses and reward. The increased Fos expression in these limbic brain regions is of particular interest in relation to the behavioural changes reported in humans who abuse anabolic androgenic steroids and the abuse potential of these drugs. Introduction Anabolic androgenic steroids (AAS) are used widely among body- builders and athletes to enhance performance and muscle mass (Lukas, 1993; Yesalis & Bahrke, 1995). However, during the 1990s a more widespread abuse of AAS has been reported among adolescents (Kindlundh et al., 1998). Behavioural changes observed in humans under the in¯uence of AAS include increased irritability, aggression and sensitivity to provocation (Bahrke et al., 1990; Svare, 1990; Pope & Katz, 1994; Thiblin et al., 1997). Depressive symptoms are common after discontinuation of AAS abuse (Pope & Katz, 1994). The steroid used in this study, nandrolone, has been linked clinically to mood changes (Bond et al., 1995), hostility and aggression (Hannan et al., 1991). AAS have also been suggested to activate brain reward pathways (Clark et al., 1996; Johansson et al., 1997; Johansson et al. 2000a), to produce dependence (Tennant et al., 1988; Hays et al., 1990; Copeland et al. 2000), and to act as a gateway to opioid dependence (Arvary & Pope, 2000). Although there are many reports of behavioural changes induced by AAS, the sites of action in the central nervous system (CNS) of these agents are poorly understood. Detection of c-Fos, the protein product of the immediate-early gene c-fos, provides information about the sites of action of various pharmacological, electrical or physiological stimuli in the brain with single cell resolution regardless of the mechanisms of action (Morgan & Curran, 1991; Hughes & Dragunow, 1995). The basal level of c- Fos expression is usually low or absent. Following a stimulus, the classical activation of c-fos is a rapid and transient induction (Morgan & Curran, 1991). However, more long-lasting expression of Fos- related antigens, for days or weeks, has also been observed (Morgan & Curran, 1995). Stable proteins of the Fos family, which are variants of Delta Fos B, have now been found following chronic perturbations of the brain, and these have been proposed to produce long-term neural plasticity and behavioural changes (Nye & Nestler, 1996; Chen et al., 1997). Chronic treatment with AAS has recently been shown to affect monoamines (Lindqvist et al., 2002; Thiblin et al., 1999), dopamine receptor density (Kindlundh et al., 2001), the expression of N-methyl- D-aspartate (NMDA) receptor subunits (Le Greve Ás et al., 1997), opioid peptides (Johansson et al., 1997; Johansson et al., 2000a, 2000b) and tachykinins (Hallberg et al., 2000) in various regions of the CNS. Acute AAS treatment has recently been shown not to induce Fos expression in the rat (Harlan et al., 2000). Nevertheless, the animals were killed 2 h after the AAS injection and as AAS are slow acting drugs, chronic treatment is more likely to induce Fos expression. The aim of the present study was to provide further insight into the mechanism by which AAS exert their effects on the CNS. In the present study behavioural changes in guinea-pigs treated chronically with the steroid nandrolone decanoate were observed, and Correspondence: Dr Loris Chahl, as above. E-mail: loris.chahl@newcastle.edu.au Received 1 October 2001, revised 4 December 2001, accepted 7 December 2001 European Journal of Neuroscience, Vol. 15, pp. 539±544, 2002 ã Federation of European Neuroscience Societies