Membrane progesterone receptors localization in the mouse spinal cord

The recent molecular cloning of membrane receptors for progesterone (mPRs) has tremendous implications for understanding the multiple actions of the hormone in the nervous system. The three isoforms which have been cloned from several species, mPRα, mPRβ and mPRγ, have seven-transmembrane domains, are G protein-coupled and may thus account for the rapid modulation of many intracellular signaling cascades by progesterone. However, in order to elucidate the precise functions of mPRs within the nervous system it is first necessary to determine their expression patterns and also to develop new pharmacological and molecular tools. The aim of the present study was to profile mPR expression in the mouse spinal cord, where progesterone has been shown to exert pleiotropic actions on neurons and glial cells, and where the hormone can also be locally synthesized. Our results show a wide distribution of mPRα, which is expressed in most neurons, astrocytes, oligodendrocytes, and also in a large proportion of NG2+ progenitor cells. This mPR isoform is thus likely to play a major role in the neuroprotective and promyelinating effects of progesterone. On the contrary, mPRβ showed a more restricted distribution, and was mainly present in ventral horn motoneurons and in neurites, consistent with an important role in neuronal transmission and plasticity. Interestingly, mPRβ was not present in glial cells. These observations suggest that the two mPR isoforms mediate distinct and specific functions of progesterone in the spinal cord. A significant observation was their very stable expression, which was similar in both sexes and not influenced by the presence or absence of the classical progesterone receptors. Although mPRγ mRNA could be detected in spinal cord tissue by reverse transcriptase-polymerase chain reaction (RT-PCR), in situ hybridization analysis did not allow us to verify and to map its presence, probably due to its relatively low expression. The present study is the first precise map of the regional and cellular distribution of mPR expression in the nervous system, a prior requirement for in vivo molecular and pharmacological strategies aimed to elucidate their precise functions. It thus represents a first important step towards a new understanding of progesterone actions in the nervous system within a precise neuroanatomical context.