Effect of neurokinin 3 receptor agonist senktide on the activation of hypothalamic nNOS neurons in cycling female rats

Objective: To examine the effect of an neurokinin 3 receptor (NK3R) agonist, senktide, on neuronal nitric oxide synthase (nNOS) activation in the median eminence-arcuate nucleus (ME-ARC) and preoptic area (POA) regions of the hypothalamus across proestrus, diestrus, and ovariectomized states in female rats and its correlation with luteinizing hormone (LH) secretion. Methods: Adult female Sprague-Dawley rats were examined for proestrus and diestrus phases of the estrous cycle. Female rats were categorized into proestrus and diestrus groups, and each was further divided into four subgroups ( n =4). In both the diestrus and proestrus categories, Group 1 was the control group. Groups 2, 3, and 4 received senktide (100 μg/kg -1 ), NK3R antagonist SB222200 (10 mg/kg -1 ), and SB222200 followed by senktide, respectively. To evaluate the effect of sex steroids on NK3R agonist-induced nNOS activation, female rats underwent bilateral ovariectomy and were divided into four groups ( n =3). Group 1 served as the control. Group 2 received a subcutaneous injection of 17 β-estradiol 3-benzoate (E2, 3 μg/rat). Group 3 received E2 and progesterone (30 μg/rat). Group 4 was administered senktide (100 μg/kg). Female rats from each group were sacrificed, blood was collected for LH ELISA, and hypothalamic tissues were collected for Western blotting. Results: Senktide increased nNOS phosphorylation in the ME-ARC during both the proestrus and diestrus phases. In the POA, senktide increased nNOS phosphorylation only during the diestrus phase. In ovariectomized rats, senktide activated nNOS independent of sex steroid levels. Senktide also increased serum LH concentration in diestrus and ovariectomized female rats. Conclusions: Senktide, an NK3R agonist, activates nNOS in the POA and ME-ARC regions of the hypothalamus in a phase dependent manner. The activation of nNOS by senktide suggests a potential mechanism by which neurokinin B triggers nNOS activation in the ARC and POA regions and regulates GnRH/LH secretion.