Reduced gonadotropin-releasing hormone gene expression with fasting in the male rat brain

Fasting causes a decrease in serum gonadotropin and testosterone (T) levels in the male rat. We hypothesized that this fasting-induced decrease in serum gonadotropins is due to reduced GnRH gene expression. PreproGnRH mRNA (GnRH mRNA) and serum gonadotropin and T levels by RIA were compared in 90 day old male Wistar rats fed ad lib or fasted for 60 hours (n = 8/group). GnRH mRNA was quantitated by in situ hybridization and computerized image analysis in 18 anatomically matched 20 microns coronal sections from the medial preoptic area (MPOA) and diagonal band of Broca (DBB), using a 35S-labeled 48 base oligodeoxynucleotide probe complementary to rat GnRH mRNA. LH and FSH levels were lower in fasted vs fed rats (LH: 0.17 +/- 0.03 vs 0.36 +/- 0.06 ng/ml, p < 0.01; FSH: 6.7 +/- 0.5 vs 8.7 +/- 0.6 ng/ml, p < 0.05). T levels were also decreased in fasted (1.34 +/- 0.36 ng/ml) vs fed (2.21 +/- 0.30 ng/ml) rats, although this was not statistically significant (p = 0.08). In both the MPOA and DBB, the number of neurons expressing GnRH was lower in fasted vs fed rats (10.6 +/- 0.4 vs 15.8 +/- 1.2 cells/section, p < 0.001), while cellular GnRH mRNA content was unchanged with fasting (83 +/- 1 vs 80 +/- 2 grains/cell). These data support the hypothesis that in the male Wistar rat, decreased gonadotropin secretion with fasting is due, at least in part, to a reduction in GnRH gene expression.