Hindbrain 5'-Monophosphate Activated Protein Kinase Mediates Inhibitory Effects of Short Term Food Deprivation in the Gonadotropin Releasing Hormone Luteinizing Hormone Axis: Role of Noradrenergic, Nitric Oxide and Opioid Neuron

Short term suspension of food intake planned or unplanned is an unavoidable and unpredictable metabolic stressor that can negatively impact the reproductive axis i.e. Hypothalamic-Pituitary-Gonadal Axis (HPG), particularly in females. But there is considerable knowledge gap regarding where and how the metabolic and gonadal cues converge in brain to curb Gonadotropin releasing hormone-luteinizing hormone (GnRH-LH) axis. This study investigated the hypothesis that food deprivation (FD) driven inhibitory inputs to reproductive neuroendocrine function involves adenosine monophosphate activated protein kinase (AMPK) sensor input from caudal hindbrain metabolic sensory noradrenergic neurons that integrates with upstream nitrergic and opioidergic neuron to regulate GnRH-LH axis. Food behavior study result showed that estrous cycle peak (mimicked by 300 mug/ml estradiol capsule implants), but not baseline (30 mug/ml) engage hindbrain AMPK signaling to intensify hyperphagia alongside elevated blood glucose and serum insulin in refed rats exposed to 18 hr. but not 12 hr. fast. This shows AMPK sensor connectivity to ingestion behavior is contingent on magnitude of metabolic stress and estradiol (E) strength. Hindbrain AMPK sensor activity is relayed to reproduction relevant forebrain structures through hindbrain norepinephrine (NE) inputs whose activity and directionality varied in site specific manner according to estradiol dose with peak and basal E respectively increasing and decreasing FD driven NE inputs. Hindbrain AMPK driven NE inputs suppress GnRH likely through nitric oxide signaling as result showed that during peak estrous cycle but not nadir level, 18 hr. food deprivation (FD-18) caused hindbrain AMPK mediated diminution of rostral preoptic (rPO) neuronal nitic oxide synthases (nNOS) and GnRH1 proteins alongside decreased pituitary LH secretion with gamma- aminobutyric acid (GABA) and anteroventricular periventricular (AVPV) Kisspeptin (KISS1) expression remaining refractory to FD irrespective of E dose. This parallel reduction of GnRH1 and nNOS was verified by lateral ventricular injection of nitric oxide (NO) donor SIN1 which at a dose of 100mug/ml reversed FD-18 induced suppression of GnRH and LH in high E rats by normalizing suppressed rPO Vesicular glutamate transporter-2 (Vglut2) & AVPV KISS1 and augmented dorsomedial hypothalamic RFamide related peptide-3 (RFRP-3) mRNA and/or protein profiles. FD-triggered noradrenergic input to the GnRH-I/LH axis also acts in part to enhance reproductive neuroendocrine sensitivity to mu receptor (mu-R) inhibition as shown by reversal of FD-18 induced mu-R expression with alpha1 inverse agonist Prazosin. Furthermore, result showed that releasing the opioid inhibitory brake with mu opioid antagonist CTOP prevented FD-18 suppression of estrogen positive feedback on GnRH LH axis in part by preventing down-regulation of key stimulatory neurochemicals, e.g. NO and kisspeptin.