Unimpaired prolactin response to teat stimulation during diminished response to continuous infusion to TRH

Repeated i.m. or i.v. injections of the synthetic Thyrotropin Releasing Hormone (TRH) are reported to have a progressively decreasing prolactin releasing effect in cows and in ewes. Continuous infusion of TRH was performed in two dairy cows and three heifers. The cows were treated with 125 μg TRH/h for four hours. The heifers received 3000 μg TRH/h for five, eight or nine hours. In all the animals infusion of TRH caused an initial maximal prolactin response (10 to 30 fold increase) within the first two to three hours of infusion. Thereafter prolactin plasma levels progressively declined. Stimulation of teats during or after infusion of TRH provoked a significant increase in the level of plasma prolactin, although prolactin response to TRH was already much diminished. Teat stimulation provoked a new prolactin surge with a subsequent decline whilst infusion was continued. At the end of the 9th hour, when infusion was stopped, teat stimulation again significantly raised the prolactin level. It is apparent that the diminishing prolactin response during continuous TRH infusion is not a result of progressive hypophyseal depletion of prolactin but rather progressive unresponsiveness of prolactin release to the specific prolactin stimulus.