Expression of c-fos, NGFI-A and secretogranin II mRNA in brain regions during initiation of cocaine self-administration in mice.

Intravenous cocaine self-administration in mice was studied to find correlates of the acquisition of cocaine-oriented operant behaviour in the expression of nerve growth factor-induced clone A (NGFI-A), c-fos and secretogranin II mRNAs. Yoked control animals, receiving cocaine passively, served as controls for the neurochemical effect of non-contingent cocaine infusion. Animals controlling their cocaine infusions did more nose-pokes during a 30-min trial than yoked controls and animals receiving only saline, indicating a reinforcing effect of cocaine. Compared with saline, an increase in c-fos mRNA in lateral and basolateral amygdala was found in active cocaine-receiving animals, and a decrease in yoked controls receiving cocaine. There is previous evidence for an involvement of the amygdala, particularly its basolateral part, in cocaine's effects on motivation. In caudate putamen, both contingent and non-contingent cocaine increased c-fos mRNA. Non-contingent cocaine infusions increased NGFI-A mRNA expression in the core of nucleus accumbens, medial caudate putamen and frontal cortex, whereas self-administration eliminated these effects. In the core of the nucleus accumbens and piriform cortex there was increased, and in medial amygdala decreased secretogranin II mRNA in yoked controls compared with saline controls. In contrast, in basomedial and central nuclei of amygdala, increased secretogranin II mRNA was found in self-administering mice. Previous studies measuring gene expression after cocaine administration obviously did not give the complete picture of changes in gene expression in the drug-taking organism. As differences in c-fos and secretogranin II mRNA between active mice and yoked controls were robust, measuring these mRNAs may identify neurons selectively involved in acquisition of cocaine-taking behaviour.