Activation of neurons containing the enzyme nitric oxide synthase following exposure to an elevated plus maze

The elevated plus-maze (EPM) is one of the most used animal models of anxiety. Exposure to the EPM activates brain regions related to anxiety/fear. Systemic or intra-dorsolateral periaqueductal gray (dlPAG) inhibition of nitric oxide synthase (NOS) induces anxiolytic effect in animals submitted to an EPM. Additionally, exposure to an innate fear stimulus, such as a live predator, activates neurons containing NOS in regions related to defensive behavior. Considering these pieces of evidence, the present study investigated if neurons containing NOS localized in regions related to anxiety/fear are also activated after exposure to an EPM. Male Wistar rats were exposed to the EPM for 15 min and 2 h later their brains were removed and processed for c-Fos immunohistochemistry (a marker of neuronal functional activation) and NADPH-diaphorase histochemistry (NADPH-d; used to detect the presence of NOS neurons). Exposure to the EPM significantly increased double-stained cells (c-Fos + NADPHd positive neurons) in the parvocellular paraventricular (pPVN) and lateral (LH) hypothalamic nuclei, dlPAG and dorsal raphe nucleus (DRN), but not in the amygdaloid complex, bed nucleus of stria terminallis, dorsal premammillary nucleus of hypothalamus and inferior colicullus. These results suggest that exposure to an EPM activates NOS containing neurons in brain areas related to fear/anxiety.