@entity1063 and @entity935 regulation of XXXX secretion in sheep: role in @entity31008 homeostasis but not photoperiodism.

@entity1169 @entity3160 @entity2847 @entity935 @entity3216 @entity31008 @entity2647 @entity13397 @entity1063
The role of @entity1063 (NA) and @entity935 (DA) in the hypothalamic control of @entity31008 (PRL) secretion was investigated in hypothalamic intact (control) and hypothalamo-pituitary disconnected (HPD) Soay rams. The animals were exposed to alternating 16-weekly periods of short (8 L : 16D) and long days (16 L : 8D) to induce marked cyclical changes in PRL secretion in both groups (as demonstrated previously). Selective NA and DA receptor antagonists (dose: 1.2 micromol/kg) were administered under short days (low endogenous PRL secretion), and agonists (dose: 0.0012-0.12 micromol/kg) were administered under long days (high endogenous PRL secretion). The acute changes in blood PRL concentrations were measured over 4 h as the index of responsiveness. Under short days, treatment with @entity13397 (alpha-1 adenoceptor antagonist), and @entity3160 (alpha-2 antagonist), consistently increased PRL secretion in control, but not in HPD rams. The treatments produced similar acute, drug-specific behavioural effects in both groups. @entity1169 (beta antagonist) had no effect on PRL secretion, while @entity3216 (DA D-2 antagonist) induced a marked increase in blood PRL concentrations in control rams (> 4 h), and a transient effect in HPD rams (15 min). Under long days, when endogenous PRL secretion was increased, @entity2647 (alpha-1 agonist) produced no effects, while @entity2847 (DA D-2 agonist) robustly decreased PRL concentrations in both control and HPD rams, even at the lowest treatment dose. Overall, the positive responses to the antagonists in the control rams, support the view that DA (acting via D-2 receptors), and to a lesser extent NA (acting via alpha-1/alpha-2 receptors), negatively regulate PRL secretion. In contrast, the lack of responses to the antagonists in the HPD rams, support the view that neither DA, nor NA, mediate the photoperiodic control of PRL secretion.
