CHRONIC TREATMENT WITH LONG-ACTING BROMOCRIPTINE DOES NOT AFFECT DURATION OF THE BREEDING-SEASON, VOLUNTARY FOOD-INTAKE, BODY-WEIGHT, OR WOOL GROWTH IN THE SCOTTISH BLACKFACE EWE

The aim of this study was to determine whether suppression of the seasonal increase in prolactin concentrations by chronic treatment with the dopamine agonist bromocriptine would affect onset of anoestrus, voluntary food intake, body weight, and wool growth in a seasonal breed of sheep. Groups of eight Scottish Blackface ewes were injected i.m. each week with either the vehicle (Group A) or 2.0 mg (Group B), 6.0 mg (Group C), or 18.0 mg (Group D) of bromocriptine in a long-acting formulation, commencing on 18 January and terminating on 25 July (midwinter to midsummer in the northern hemisphere). Immediately before the bromocriptine injection, blood samples were taken for progesterone and prolactin determination. Voluntary food intakes were measured daily, and body weights were recorded every fortnight. Estimates of wool growth were made by weighing wool clipped from a measured area of skin once a month. Treatment had no effect on onset of anoestrus, voluntary food intake, body weight, or wool growth. Plasma prolactin concentrations increased significantly in all groups during the treatment period. From January to April, all doses of bromocriptine significantly reduced prolactin concentrations but later in the study (May and June) prolactin was significantly suppressed in Group D only, although even in this group prolactin concentrations increased between March and June. Pituitary prolactin content, measured at the end of the study in July, was also suppressed by bromocriptine. The gradual increase in prolactin concentrations in ewes receiving chronic bromocriptine was further investigated by treating a fifth group of ewes (Group E) with 18.0 mg of long-acting bromocriptine each week, commencing on 20 June. Prolactin concentrations declined to be similar to those in Group D within a week. When Groups A, D and E were treated with a further 10 mg of bromocriptine in an acute challenge, prolactin concentrations were suppressed in all groups, indicating that prolactin secretion in ewes receiving long-acting bromocriptine is sensitive to suppression with a dopamine agonist. This study demonstrates that the large summer increase in plasma prolactin concentrations in the ewe is not required for the onset of anoestrus or for the seasonal increase in voluntary food intake and wool growth.