Identification of neuronal input to the arcuate nucleus (ARH) activated during lactation: implications in the activation of neuropeptide Y neurons

Activation of the neuropeptide Y (NPY) neuronal system in the arcuate nucle us of the hypothalamus (ARH) during lactation in the rat is likely due to t he neural impulses arising from the suckling stimulus. However, the afferen t neuronal input to the ARH that is activated during lactation and is respo nsible for activation of NPY neurons is currently unknown. Previously, usin g cFos as a marker for neuronal activation, we identified several brain are as in the lactating animals that were activated by the suckling stimulus. T hus, the objective of the present study was to determine if these activated areas observed in the lactating animals project directly into the ARH. The retrograde tracer, fluorogold (FG), was injected into the ARH on day 4 pos tpartum. Chronically suckled rats were then deprived of their eight-pup lit ters on day 9 postpartum, and 48 h later, the pups were returned to the fem ales to reinitiate the suckling stimulus for 90 min to induce cFos expressi on. The animals were then perfused and the brains were subjected to double- label immunohistochemistry to visualize both FG- and cFos-positive cells. S ubstantial FG/cFos double-labeled cells were found in forebrain regions, in cluding the medial preoptic area, periventricular preoptic area, bed nucleu s of the stria terminalis, and the medial amygdala, and in brainstem region s including the lateral parabrachial nucleus, peripeduncular area and ventr olateral medulla. The results of the present study demonstrate that specifi c areas in the brain are activated during lactation and send direct project ions to the ARH. Thus, these areas are potentially important candidates for mediating the activation of the NPY neuronal system in the ARH during lact ation. (C) 1999 Elsevier Science B.V. All rights reserved.