VMN growth hormone-releasing hormone receptor regulation of counterregulatory transmission

Abstract

Ventromedial hypothalamic nucleus (VMN) growth hormone-releasing hormone (Ghrh) neurotransmission governs counterregulatory hormone release. Recent studies document Ghrh control of hypoglycemia-sensitive counterregulatory neurotransmitter expression in dorsomedial VMN (VMNdm) Ghrh/steroidogenic factor-1 (SF-1) neurons. In this study, Ghrh receptor (Ghrh-R) gene silencing was implemented in vivo to determine if VMN Ghrh-R shapes counterregulation. Intra-VMN Ghrh-R siRNA augmented corticosterone secretion in vehicle or insulin-injected male rats, but this hormone was correspondingly refractory or inhibited in eu- versus hypoglycemic females. In each sex, gene knockdown up- or down-regulated baseline glucagon and growth hormone (GH) release, but hypoglycemia reversed the direction of Ghrh-R control of each hormone. Single-cell laser catapult-microdissected VMNdm Ghrh/SF-1 neuron multiplex qPCR analysis revealed contrary VMN Ghrh-R gene-silencing effects on eu- versus hypoglycemic SF-1 mRNA levels. In both sexes, Ghrh-R siRNA up-regulated mRNAs encoding counterregulation-repressive (γ-aminobutyric acid) or -enhancing (nitric oxide) transmitter protein markers, unrelated to plasma glycemic profiles. Ghrh-R regulation of Ghrh gene transcription was absent (euglycemic) or stimulatory (hypoglycemic) in females, and receptor control of glutaminase mRNA, a marker for the counterregulatory-augmenting neurochemical glutamate, was lost in hypoglycemic males. Ghrh-R gene silencing caused uniform up-regulation of 5′-AMP-activated protein kinase alpha-2 (AMPKα2) mRNA in each sex, independent of glucose status, but caused dissimilar changes in AMPKα1 transcription in eu- versus hypoglycemic females. The outcomes provide novel evidence that VMN Ghrh-R signaling imposes glucose-dependent control of counterregulatory hormone secretion and distinctive VMNdm neuron counterregulatory transmitter marker gene profiles. Data infer that this metabolic control may involve SF-1 (both sexes)- and AMPKα1 (female)-dependent mechanisms.