Synergetic effect of pimozide and thyrotropin releasing hormone on prolactin and thyrotropin release during the drying off of ewes.

The effect of pimozide and/or TRH was investigated on plasma prolactin, thyrotropin, T(4) and T(3) and udder distension in 38 ewes during drying off by feed restriction. The effect of daily injections of 2mg pimozide (s.c.), combined or not with TRH stimulation (200µg, i.v.) on three different days of the drying off period was examined. Blood samples were taken twice daily in each group for 9 days, while blood sampling on the days of TRH injection was also performed at 0, 15, 30min, and 1, 2 and 4h post-injection. Plasma was assayed for PRL, TSH, T(4) and T(3) levels. Udder distension and mastitis incidence were recorded at the end of the drying off period. TRH and pimozide both resulted in elevated plasma PRL levels and acted in a synergetic way. Udder distension and the incidence of mastitis was only influenced by pimozide. The TSH as well as the T(3) response to TRH was increased in ewes under a continuous influence of pimozide and T(3) peaks following TRH injection occurred earlier than T(4) peaks. The higher effect of pimozide upon TRH stimulated PRL and TSH release at day 8 compared to days 0 and 3 indicates a progressive involvement of dopamine on the inhibition of PRL and the sensitivity of the thyrotrophs to TRH during drying off.