Presence of pro-opiomelanocortin mRNA in the rat medial prefrontal cortex, nucleus accumbens and ventral tegmental area: Studies by RT-PCR and in situ hybridization techniques

Pro-opiomelanocortin (POMC) is a large proteic precursor which originates several biologically actives neuropeptides, such as β-lipotropin (β-LPH), β-endorphin (β-END), adenocorticotropic hormone (ACTH) and α-melanocyte-stimulating hormone (α-MSH). The arcuate nucleus of the hypothalamus is the main POMC producing cell group in brain and innervates several areas of the limbic system and brainstem. POMC-derived neuropeptides have been related to several motivated and rewarding behaviours, including sexual facilitation, feeding, and drug addiction. However, POMC mRNA has not been detected in regions of the dopaminergic mesocorticolimbic system, which represents the most important reward pathway. The aim of this work was to investigate if POMC mRNA is expressed in the medial prefrontal cortex (mPFC), the nucleus accumbens (NAcc) and the ventral tegmental area (VTA) of the rat. We used the reverse transcriptase reaction coupled to the polymerase chain reaction (RT-PCR). We also used the in situ hybridization technique to study the regional distribution of POMC mRNA in the same regions. We report that RT-PCR amplification of extracted RNA with two different pairs of primers generates the predicted 94 bp and 678 bp POMC–PCR products. Both the amplification of RNA obtained from the rat glial C-6 cell line (which does not express POMC mRNA) and the omission of reverse transcriptase from the RT reaction of rat brain samples showed no amplification products. We have shown for the first time that the rat medial prefrontal cortex, the nucleus accumbens and the ventral tegmental area contain POMC mRNA. This mRNA is in low concentration, ranging from 21% to 31% with respect to the hypothalamus. In situ hybridization experiments showed that POMC mRNA is homogeneously distributed in these areas. The presence of POMC mRNA in regions of the mesocorticolimbic system could have functional implications in motivated behaviours.