Projections from ventrolateral hypothalamic neurons containing progesterone receptors and somatostatin to the midbrain central gray in the female guinea pig.

The goal of this study was to determine, by combining retrograde fluorescent tract tracing with double immunocytochemistry, whether neurons immunoreactive (IR) for both progesterone receptors (PR) and somatostatin (SOM) in the guinea-pig ventrolateral nucleus (VL) send their axons directly to the midbrain. Unilateral microinjections of true blue (TB) fluorescent dye were made into the midbrain central gray (MCG) of ovariectomized animals primed with estradiol to induce PR and injected intracerebroventricularly with colchicine to visualize SOM-IR neurons. The highest number of PR/SOM-IR neurons in the VL that projected to the midbrain was found when the fluorescent retrograde tracer was confined to the lateral region of the MCG at the mid-rostrocaudal level. More specifically, 29% of the progesterone target neurons containing SOM detected in the VL projected directly to the midbrain, accounting for 7% of the PR-IR neurons and for 9% of the SOM-IR neurons. These triple-labeled cells were found throughout the extent of the VL. Double-labeled cells also sent axons to the MCG and were either PR-IR (14%) or SOM-IR (12%). These results provide morphological evidence that a subset of the PR-IR neurons expressing SOM in the VL have long projections to the midbrain and suggest that the SOM system may modulate neural circuits involved in the regulation of steroid-influenced behaviors and neuroendocrine functions.