The reinforcing properties of nicotine are associated with a specific patterning of c-fos expression in the rat brain.

Abstract

Rats were trained for nicotine intravenous infusions in a self-administration paradigm. The effect of nicotine self-administration on regional brain activity was studied by mapping changes of c-fos expression. Specific nicotine effects were determine by comparing the patterning of Fos-like immunoreactivity (Fos-Ll) in nicotine self-administering rats with that in three different control groups. Controls included rats exposed to the same manipulation as nicotine self-administering rats who received intravenous saline instead of nicotine. In addition, two groups of untrained sham-operated rats exposed daily to the same operant boxes were included: one group had the same food restriction used in the operant training, the other was fed ad libitum. Nicotine self-administration, exposure to saline and food restriction increased Fos-Ll in 43, 33 and three brain regions, respectively, when compared with the control group fed ad libitum. Computer-assisted image analysis of Fos-Ll profiles performed on 16 relevant limbic and sensory structures showed that in saline-treated rats a significant (P < 0.01) increase of Fos-Ll profiles was observed in medial prefrontal cortex, lateral septum, core and ventral shell of nucleus accumbens, claustrum, amygdaloid nuclei, paraventricular thalamic nucleus and lateral geniculate nucleus. A significant (P < 0.01) further increase produced by nicotine was found in medial prefrontal cortex and ventral shell of nucleus accumbens. Interestingly, cingulate and piriform cortex, superior colliculus and medial terminal nucleus of the accessory optic tract were specifically activated by nicotine but not saline. These results show that nicotine self-administration activates sensory structures, as well as limbic structures involved in natural rewarding pathways. The results suggest the involvement of restricted terminal regions of the mesocorticolimbic dopaminergic system in the maintenance of nicotine self-administration.