Distribution of preproenkephalin, preprotachykinin A, and preprodynorphin mRNAs in the rat nucleus accumbens: effect of repeated administration of nicotine.

The effects of a repeated treatment with nicotine on the expression of mRNAs encoding preproenkephalin (PPE), preprotachykinin-A (PPT-A), and preprodynorphin (PPDYN) were examined by in situ hybridization histochemistry in various subregions of the nucleus accumbens (Acb). In saline-treated rats, optical density measurements on autoradiographic films showed marked anteroposterior decreasing gradients for PPE and PPT-A mRNAs in the rostral pole and the core, in the cone, and in the ventral shell of the Acb, whereas a lower anteroposterior gradient was observed for PPDYN mRNA signals. The intensity of the three mRNA signals also varied according to Acb subregion. However, analysis of percentages of prepropeptide mRNA-containing neurons as compared to total neurons showed, in the rostral pole, the core, and the cone, a similar percentage of PPE mRNA (around 45%)- and PPT-A mRNA (around 40%)-expressing neurons. The ventral shell can be distinguished from the other subregions by a lower percentage of PPE mRNA (35.8%)- and PPT-A mRNA (30.6%)-expressing neurons. The percentage of PPDYN mRNA-containing neurons, by contrast, was similar (around 37%) in the core, the cone, and the ventral shell. Repeated nicotine administration increases the PPE mRNA level in the rostral pole and the anterior third of the core without any change in PPT-A and PPDYN mRNA levels in the various Acb subregions examined. The PPE mRNA increase does not support an effect mediated through an interaction of nicotine with DA neurons. The effect could be linked to a nicotine activation of other afferents to the anterior Acb and/or to a direct nicotine stimulation of PPE mRNA neurons.