Neuronal Responses in the Brainstem and Hypothalamic Nuclei Following Insulin Treatment During the Late Follicular Phase in the Ewe

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The aim of this study was to determine the neuronal responses following insulin administration during the late follicular phase. Intact ewes were given either saline or insulin (5 IU/kg, i.v.) at 35 h after progesterone withdrawal and killed 3 h later. There was a marked increase in the number of Fos-positive noradrenergic neurones in the caudal brainstem of insulin-treated ewes. In the hypothalamic paraventricular nucleus, insulin treatment increased the presence of Fos-positive corticotrophin-releasing hormone neurones (from 2% to 98%) and Fos-positive arginine vasopressin parvocellular neurones (from 2% to 46%). Interestingly, after insulin treatment, despite a general increase in Fos-positive neurones in the arcuate nucleus (ARC), there was a marked reduction (from 47% to 1%) in Fos-positive β-endorphin neurones. Similarly, colocalized Fos and oestradiol receptor (ER) α-positive neurones decreased in the ARC after insulin (from 7% to 3%). Conversely, in the ventromedial nucleus, ERα-positive neurones with Fos increased (from 7% to 22%) alongside a general increase in Fos-positive neurones. Overall, a complex system of neurones in brainstem and hypothalamus is activated following insulin administration during the late follicular phase.