Maternal behaviour is exhibited by female rats post partum. This includes, nest building, nursing, pup retrieval, licking and grooming. Lactating mothers also show maternal defense behaviour with aggression directed against conspecifics. This behaviour has been attributed, in part, to the dramatic changes in steroid and neuropeptide hormones that accompany pregnancy and parturition.Peripheral steroid release is required for normal parturition and lactation. The brain is also a target for steroid hormones and evidence suggests that central release of steroid and peptides can induce many maternal behaviour. Neurosteroids are known to modulate a wide range of neurotransmitter and receptor systems in the central nervous system. They affect primary excitatory and inhibitory systems (glutamate and gamma amino butyric acid (GABA), and so regulate a range of behaviours including learning, anxiety, cognition, sexual behaviour and aggression. 5a- reductase type 1 (5a -R) and 3a- hydroxysteroid dehydrogenase (3a -HSD) are the two enzymes involved in the synthesis of neurosteroids; allopregnenolone (Allop) and tetrahydrodeoxycorticosterone (THDOC) in the brain from progesterone and deoxycorticosterone, respectively. In this study, brain regions such as the supraoptic nucleus (SON), medial amygdala (mAmyg), medial preoptic nucleus (MPO), bed nucleus of stria terminalis (BNST), and paraventricular nucleus (PVN) were activated in lactating mothers following the exhibition of maternal aggressive behaviour. Using immunocytochemistry for the immediate early gene (Fos -IR) as a marker of neuronal activation, there were approximately two to three folds more Fos -immunoreactive (Fos -IR) cells, in each brain region analysed in the aggressive lactating dams compared with non -aggressive controls.In brain regions such as the SON, mAmyg, MPO, BNST and PVN the Fos -IR cells were also double labelled for 5a -R and 3a -HSD. Glutamic acid decarboxylase (GAD) labelled cells were also observed in these brain regions but there were no Fos -IR cells labelled for GAD in any of the brain regions analysed. Together these results implicate neurosteroids in the exhibition of maternal aggression. Activation of cells containing the enzymes 5a -R and 3a -HSD suggest that neurosteroid synthesis in specific brain regions may play a role in regulating aggressive behaviour perhaps through the GABAergic mechanism, but further work is warranted
