Activation of the NO-ergic system of the nucleus accumbens on presentation of contextual danger signals.

Studies in Sprague-Dawley rats using intracerebral microdialysis combined with HPLC showed that presentation of the animals with a chamber in which they had previously acquired a conditioned reflex fear reaction (combination of a tone and electric shocks) led to increases in extracellular citrulline (a co-product of NO synthesis) in the medial segment of the nucleus accumbens. This increase was prevented by local administration of the NO synthase inhibitor 7-nitroindazole (0.5 mM). The increase was significantly smaller in amplitude than the increase in the citrulline level induced by combined presentation of the tone and the chamber but was no different from changes in citrulline levels seen during this test in the lateral segment of the nucleus accumbens. These data provide evidence that contextual danger signals activate neuronal NO synthase in the medial and perhaps the lateral segments of the nucleus accumbens, leading to increases in extracellular citrulline and, probably, increased NO production in this part of the brain.