Sex Differences in Acclimation of Hypothalamic Arcuate Glucokinase Gene and Protein Expression to Repeated Intermediate Insulin Administration

Abstract

Background: Repeated intermediate-acting insulin administration attenuates genomic reactivity of neurons in key autonomic metabolic structures in the male, but not female rat brain – results that support a central neural component of sex-specific response desensitization. The glucokinase (GK) enzyme functions as a glucose sensor in a body-wide system of metabolic monitoring structures, including the brain, and is expressed at high levels in the hypothalamic arcuate nucleus (ARH). Method: Quantitative real-time RT-PCR was used to investigate the hypothesis that habituation of ARH GK gene expression to neutral protamine Hagedorn insulin (NPH) injection differs among sexes. In lieu of evidence for region-based functional heterogeneity in this structure, effects of NPH on in situ GK protein-staining patterns were evaluated at different rostrocaudal levels of the ARH by immunocytochemistry. Results: Basal ARH GK mRNA levels were equivalent in sham-operated (SHAM) and orchidectomized (ORDX) male rats. SHAM males exhibited augmented GK gene profiles in response to acute NPH injection, as well as elevated numbers of GK-immunoreactive (-ir) neurons in the rostral ARH. ORDX abolished this stimulatory transcriptional response, but did not prevent increased GK labeling throughout this structure. Stimulatory effects of precedent insulin administration on baseline GK mRNA were reversed by ORDX. Serial dosing of SHAM males with NPH elicited no change in ARH GK transcription, but decreased GK-ir in the rostral ARH. Acute NPH injection had no impact on GK gene profiles in estradiol benzoate (EB)- or oil-implanted ovariectomized (OVX) female rats, but diminished GK-ir cell counts in the OVX + EB caudal ARH. Precedent NPH treatment did not modify baseline GK mRNA levels in either group of females, but resulted in decreased or elevated GK gene and protein expression during recurring injection in the presence or absence of EB, respectively. Conclusion: These results provide novel evidence for sex-specific patterns of acclimation of GK mRNA and protein expression within the rat ARH to serial NPH injection, and support the need to elucidate the physiological ramifications of these adaptations regarding behavioral and physiological responses to recurring intermediate insulin administration.