Prolactin and its receptors in the chronic mild stress rat model of depression.

Prolactin (PRL) exhibits many physiological functions with wide effects on the central nervous system including stress responses. Our study aimed to investigate the effect of chronic unpredictable mild stress (CMS) - which is a good animal model of depression - on PRL receptor (PRLR) expression in the rat brain. Rats were exposed to CMS for two weeks and subsequently to CMS in combination with imipramine (IMI) treatment for five consecutive weeks. Behavioral deficit measured in anhedonic animals is a reduced intake of sucrose solution. Two weeks of CMS procedure allowed the selection of animals reactive to stress and displaying anhedonia, and the group which is considered as stress-non-reactive as far as behavioral measures are concerned. In this group the elevated level of PRL in plasma was observed, decrease in dopamine release in the hypothalamus, increase in [(125)I]PRL binding to PRLR in the choroid plexus, increase of mRNA encoding the long form of PRLR in the arcuate nucleus and the decrease of mRNA encoding its short form, and decrease in the mRNA encoding dopamine D2 receptor. All these alterations indicate these parameters as involved in the phenomenon of stress-resilience. The prolongation of the CMS procedure for additional five weeks shows the form of habituation to the stressful conditions. The most interesting result, however, was the up-regulation of PRLR in the choroid plexus of rats subjected to full CMS procedure combined with treatment with IMI, which may speak in favor of the role of this receptor in the mechanisms of antidepressant action.