Stimulus-dependence of NO-mediated neuronal activation: a Fos expression study in the hippocampus and cortex of lactating rats

We investigated how simultaneous manipulation of suckling and i.c.v. inject ion of nitric oxide (NO)-related pharmacological agents affect Fos activati on in the hippocampus and cortex of lactating rats. Dams received drugs or vehicle injection just prior to reunion with their pups following a 12 h se paration and Fos protein activation was examined after 2 h of suckling. In conscious dams, sodium nitroprusside (SNP, an NO donor) significantly activ ated Fos expression in the hippocampus and cortex compared with vehicle or an NO synthase inhibitor. Urethane anaesthesia abolished Fos expression in the cortex, but not in the hippocampus following simultaneous SNP injection and resumption of suckling. Fos activation in the hippocampus and cortex w as significantly attenuated when SNP was injected alone (without dam-pups r eunion after separation). Coincident injection of SNP with a NO synthase in hibitor abolished all Fos activation in the hippocampus and cortex. In chro nically lactating rats (without any interruption of suckling) SNP injection failed to activate any Fos expression. These observations underscore the i mportance of stimulus modality in revealing the dynamics of NO-mediated neu ronal function within a discrete tissue entity such as the hippocampus or t he cortex, which may be relevant to other regions of the brain. NeuroReport 12:2859-2864 (C) 2001 Lippincott Williams & Wilkins.