Alterations in Hypothalamic Mechanisms Associated with Reduced Suckling-Induced Prolactin Secretion in the Lactation-Deficient OFA hr/hr Rat.

Introduction: OFA hr/hr rats have deficient lactation with impaired suckling-induced PRL release. Unlike their background strain, Sprague-Dawley (SD) rats, OFA rats display abnormal mediobasal hypothalamus (MBH) dopaminergic tone during late pregnancy and lactation. We explored if the expression of MBH components, including various receptors (R) and proteins that regulate the dopaminergic system, is altered in mid-lactating OFA compared to SD rats, which may be associated with the abnormality.

Methods: Four groups of mid-lactating rats were used: continuous lactation; pups separated overnight; 30-min suckling (S); and 2 h or 4 h S after separation. Mothers were sacrificed to obtain serum for PRL RIA and MBHs to determine tyrosine hydroxylase (TH), PRL-R, PRL signaling molecules (activator: STAT5b; inhibitors: SOCS1, SOCS3, CIS), opioids (PENK, PDYN), and µ- and κ-opioid R (MOR, KOR) mRNA expression by qPCR and phospho-TH (p-TH) and TH proteins by Western blot.

Results: Suckling-induced PRL was lower in OFA and p-TH expression diminished in both strains. Separation increased TH mRNA and protein in SD, which decreased after 4 h S, but OFA protein levels remained unchanged. Separation of pups also resulted in decreased PRL-R and CIS expression in SD but increased PRL-R and SOCS3 in OFA. Despite the lower PRL-R, STAT5b, SOCS1, and SOCS3 levels in OFA compared to SD, suckling diminished them further. We observed subtle changes in SD opioids and their R, but in OFA, suckling decreased PENK, KOR, and MOR.

Conclusion: The different patterns of TH, opioids, their R, and PRL signaling inhibitor expression with conserved TH activation by suckling may disturb the balance between stimulation and inhibition of PRL release resulting in impaired suckling-induced PRL secretion in OFA rats.