Maternal care activates the ventral tegmental area but not dopaminergic cells in the rat

The ventral tegmental area (VTA), together with the preoptic area, is part of a neural circuit necessary for the expression of maternal behaviour (MB); destruction of either area disrupts MB in postpartum rats. Central to the proposal of VTA activation are dopaminergic cells, for which the cell bodies lie in the VTA and project to forebrain structures. This mesolimbic system is a motivational circuit involved in rewarding behaviours such as sex and MB. Despite their recognised importance, surprisingly, unlike the preoptic area, there are no anatomical descriptions of the pattern of VTA activation or of the dopaminergic cell activation, specifically in relation to MB in the rat. In the present study, we explore the possible activation (as indicated by Fos protein via immunohistochemistry) of the anterior and medial portions of the VTA and in the dopaminergic cells in these regions, as well as in the medial preoptic area, in lactating rats, at postpartum day 7 (after a 12-hour mother/pups separation), and in dioestrous females. After 12 hours, mothers were perfused at that moment or after a 90 minutes of interaction, or not, with their pups. We found a strong significant Fos induction in both the preoptic area and in the anterior portion of VTA in dams that interacted with their pups. The number of dopaminergic cells that coexpressed Fos did not differ across groups. Additionally, we determined Fos and GABA colocalisation in the anterior part of the VTA and found dense GABAergic processes, possibly varicosities, in the area of increased Fos expression. The results of the present study support a proposed GABAergic pathway from medial preoptic area to VTA cells, critical for the expression of MB. Future experiments are warranted to explore the neurochemical identity of the Fos and no-Fos expressing cells that are recipients of GABAergic processes in the VTA, aiming to better understand the neural circuitry of the VTA in relation to MB.