Effect of Thyrotropin Releasing Hormone and Pergolide on Plasma β-Endorphin Concentrations in Horses and Ponies

Abstract: Background: β-endorphin is a peptide hormone produced by the pituitary pars intermedia, and plasma concentrations may be increased with pituitary pars intermedia dysfunction (PPID). The potential value of measuring β-endorphin for the diagnosis and/or monitoring of PPID has yet to be determined.

Objectives: To evaluate the response of plasma β-endorphin concentrations to the thyrotropin releasing hormone (TRH) stimulation test and pergolide treatment, in horses and ponies with and without PPID.

Animals: Sixteen horses and ponies; eight with PPID and eight without PPID, matched by age and breed.

Methods: Animals underwent a standard TRH stimulation test (1 mg IV if ≥250 kg or 0.5 mg IV if < 250 kg), before receiving pergolide (2 µg/kg PO SID) or no drug for 4 weeks in a randomized crossover design. Plasma β-endorphin and ACTH concentrations were measured by radioimmunoassay and chemiluminescent immunoassay, respectively. Treatment effects were evaluated using Wilcoxon signed-rank tests and Spearman rank correlation, with significance accepted at P< 0.05.

Results: Basal plasma β-endorphin concentrations were significantly higher in PPID animals compared to non-PPID animals (median [range], 1503 [572-4907] pg/ml vs 508 [207-689] pg/ml; P=0.003). Both groups demonstrated an increase in plasma β-endorphin concentrations 10 minutes after TRH administration, which correlated with an increase in plasma ACTH concentrations (rs=0.91; P< 0.001). Plasma β-endorphin concentrations decreased following pergolide treatment in PPID animals (P=0.008) but were unchanged in non-PPID animals (P=0.74).

Conclusions and Clinical Importance: Measurement of plasma β-endorphin concentration may be a useful adjunct marker in the diagnosis and monitoring of PPID, and warrants further investigation.