Dopaminergic projections to the medial preoptic area of postpartum rats

Abstract

Dopamine receptor activity in the rodent medial preoptic area (mPOA) is crucial for the display of maternal behaviors, as well as numerous other physiological and behavioral functions. However, the origin of dopaminergic input to the mPOA has not been identified through neuroanatomical tracing. To accomplish this, the retrograde tracer Fluorogold was iontophoretically applied to the mPOA of postpartum laboratory rats, and dual-label immunocytochemistry for Fluorogold and tyrosine hydroxylase later performed to identify dopaminergic cells of the forebrain and midbrain projecting to the mPOA. Results indicate that the number of dopaminergic cells projecting to the mPOA is moderate ( approximately 90 cells to one hemisphere), and that these cells have an unexpectedly wide distribution. Even so, more than half of the dual-labeled cells were found in either what has been considered extensions of the A10 dopamine group (particularly the ventrocaudal posterior hypothalamus and adjacent medial supramammillary nucleus), or in the A10 group of the ventral tegmental area. The rostral hypothalamus and surrounding region also contained numerous dual-labeled cells, with the greatest number found within the mPOA itself (including in the anteroventral preoptic area and preoptic periventricular nucleus). Notably, dual-labeled cells were rare in the zona incerta (A13), a site previously suggested to provide dopaminergic input to the mPOA. This study is the first to use anatomical tracing to detail the dopaminergic projections to the mPOA in the laboratory rat, and indicates that much of this projection originates more caudally than previously suggested.

Figure 1

Schematic representations of the dense core of FG deposits in the mPOA (indicated with gray shading) of each of the subjects examined in detail. Subject numbers referenced in text are at top left of panels. Subjects arranged across from the top left to bottom right according to the rostrocaudal placement of FG deposit. Abbreviations as in list above.

Figure 2

Photomicrograph of the mPOA of Subject #57 after immunocytochemical processing for FG and TH. aco = anterior commissure. 3V = third ventricle. Bar = 200 µm.

Figure 3

Photomicrograph of TH-immunoreactive fibers in the mPOA of a lactating rat, contralateral to a deposit of FG. 3V = third ventricle. Bar = 200 µm.

Figure 4

Photomicrograph of FG-immunoreactive cells in the MeApd of a lactating rat after FG was iontophoretically applied to the mPOA one week before sacrifice. opt = optic tract. Bar = 200 µm.

Figure 5

TH-immunoreactive (blue labeled) and FG-immunoreactive (brown labeled) cells in the A) zona incerta and B) VTA of a lactating rat after FG was iontophoretically applied to the mPOA one week before sacrifice. 3V = third ventricle, mp = mammillary peduncle, fr = fasciculus retroflexus. Bars = 200 µm.

Figure 6

Diagrammatic representation of the distribution of dual-labeled cells throughout the brain of Subject #57. Each dot represents one cell retrogradely labeled after Fluorogold application into the mPOA, and also immunoreactive for TH. Shaded area on second panel indicates the FG deposit. Figures modified from Swanson (1998).

Figure 7

Photomicrographic representation of dual-labeled cells in the PHv of Subject #55. Top panel – Location of PHv, with gray-shaded box indicating area shown in photomicrograph immediately below. Figure modified from Swanson (1998). Lower panel - 200× magnification of area indicated in top panel. Brown-labeled cells contain FG immunoreactivity, while blue-labeled cells contain TH immunoreactivity. Bar = 50 µm. Boxes indicate representative cells shown further below at 600× magnification containing (A) FG immunoreactivity, (B) TH immunoreactivity, (C) both FG and TH immunoreactivity. Bars in 600× magnification photos = 10 µm.

Figure 8

Photomicrograph of TH-immunoreactive cells immediately adjacent to the FG deposit site of Subject #53. Arrows point to light blue-shaded cells whose cytoplasm contains TH-immunoreactivity, but not FG immunoreactivity, which is indicated in many nearby cells by dense brown cytoplasmic staining. 3V = third ventricle. Bar = 100 µm.

Figure 9

Schematic representation of a horizontal section through the rat brain, indicating sites providing TH-immunoreactive projections to the postpartum rat mPOA. Gray-shaded area indicates an FG deposit in the mPOA. Bold-faced text highlights the sites containing the greatest number of TH-immunoreactive cells projecting to the mPOA. Figure modified from Paxinos and Watson (2007). Abbreviations as in list above.