Plasma and Hypothalamic Peptide‐Hormone Levels Regulating Somatotroph Function and Energy Balance in Fed and Fasted States: A Comparative Study in Four Strains of Rats

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Abstract
Both growth hormone (GH)/insulin growth factor (IGF)‐1 axis and energy balance have been implicated in longevity independently. The aim of the present study was to characterize the effect of a 72‐h fasting period at 3 months of age in four different rat strains: (i) Wistar and (ii) Fischer 344 rats, which develop obesity with age, and (iii) Brown Norway and (iv) Lou C rats, which do not. Wistar rats ate more, were significantly bigger, and presented with higher plasma leptin and lower ghrelin levels and hypothalamic growth hormone‐releasing hormone (GHRH) content than rats from the three other strains. Plasma insulin and IGF‐1 levels were lower in Brown Norway and Lou C rats, and somatostatin content was lower in Brown Norway rats only. Glycaemia was lower in Lou C rats that displayed a lower relative food intake compared to Fischer and Wistar rats. Brown Norway rats showed a greater caloric efficiency than the three other strains. Concerning major hypothalamic neuropeptides implicated in feeding, similar amounts were detected in the four strains for neuropeptide Y, agouti‐related peptide, galanin, melanin‐concentrating hormone, α‐melanocortin‐stimulating hormone (α‐MSH) and corticotropin‐releasing hormone. Orexin A appeared to be slightly elevated in Fischer rats and cocaine amphetamine‐regulated transcript (CART)55−102 diminished in Brown Norway. At the mRNA level, orexin A, GHSR1, α‐MSH and CART expression were higher in Wistar and Lou C rats. Principal component analysis confirmed the presence of two main factors in the ad libitum rat population; the first being associated with growth‐related parameters and the second being associated with food intake regulation. Hypothalamic GHRH and somatostatin content were positively correlated with feeding‐related neuropeptides such as α‐MSH for GHRH, and orexin A and CART for both peptides. Plasma ghrelin levels were negatively correlated with leptin and IGF‐1 levels. Finally, a 72‐h fasting period affected minimally body weight, plasma IGF‐1 and leptin levels in Lou C rats compared to the three other strains, and plasma insulin levels were less affected in Brown Norway rats. In conclusion, Wistar shorter life span is consistent with its already fatter phenotype at 3 months of age. In terms of IGF‐1, glycaemia and leptin responses to fasting, the Lou strain, which presents with a low food intake/body weight and caloric efficiency, is the least affected. The link between food intake regulation, GH axis and ageing is further demonstrated by principal component analysis, where GHRH and somatostatin were found to be strongly associated with energy homeostasis parameters.