TRH and TRH-like peptide levels covary with caloric restriction and oral metformin in rat heart and testis

Background

Obesity, type 2 diabetes, and major depression are common comorbidities in aging populations. Metformin (MF) mimics effects of caloric restriction (CR) which increases health and life spans. CR suppresses central TRH activity resulting in bradycardia and suppression of energy expenditure. TRH and TRH-like peptides with sequence pGlu-X-Pro-NH₂, (X-TRH) where ‘X’ can be any amino acid residue, have anorectic, antidepressant, anxiolytic, analeptic, anti-epileptic, neuroprotective and anti-aging effects. The present studies compare the effects of CR and oral MF on expression of TRH and TRH-like peptides. CR experiment: Sixteen male Sprague-Dawley rats divided into 4 groups: ad libitum fed (CR-CON), 24 h fasted (CR-AC), fed (ADF1) or fasted (ADF2) 24 h after 10 days of alternate day fasting. MF experiment: Sixteen rats divided into 4 groups: water (MF-CON), 6 mg MF/ml water 24 h (MF-AC), 10 days MF (CHR) or 8 days MF then 48 h water (WD).

Results

Comparison of chronic MF and ADF2 treatment effects on TRH and TRH-like peptide levels in testis revealed a consistent pattern for peptide responses observed in the two experiments with Glu-TRH > Val-TRH > TRH > Peak 2 > Trp-TRH > Phe-TRH > Tyr-TRH. TRH levels increased in heart during CR and MF treatment consistent with cardiac responses to congestive heart failure and infarction.

Conclusions

Because TRH reverses aging-induced testicular degeneration and mediates the pathophysiology of congestive heart failure these tripeptides may participate in the therapeutic effects of MF in ameliorating the cardiometabolic risk factors and fertility impairments associated with type 2 diabetes.