The roles of thyroid hormones and prolactin in the control of fibre moult and associated changes in hair follicle activities in cashmere goats

Abstract

Five groups of 8 adult male castrated goats were used in an 8-month study. For a 6-month period commencing in November, goats of one group were dosed with propylthiouracil (PTU), a drug which reduces the rate of conversion of thyroxine (T₄) to triiodothyronine (T₃) in peripheral tissues. Animals of a second group were treated with bromocriptine (Br), a drug which reduces the circulating concentrations of prolactin. Animals of other groups were treated with PTU + Br, PTU + triiodothyronine (PTU + T₃), or vehicle only (control). The mean date of moult onset was later in animals of the PTU + T₃ (P < 0.05), PTU + Br (P < 0.05), and Br (P < 0.01) groups than in control animals. PTU + T₃ , PTU + Br, and Br animals also exhibited lower minimum levels of primary follicle activity than control animals (P < 0.01). On the basis of a periodic function applied to the data, there was a trend towards a later nadir of both primary and secondary follicle activity. There were no significant effects of treatment on profiles of thyroxine (T₄), triiodothyronine (T₃), growth hormone (GH), insulin-like growth factor-1, or insulin but a significant (P < 0.001) seasonal increase in mean concentration occurred between November and March–April in all except GH profiles. It is concluded that PTU and Br did not act additively or synergistically on follicle activity or moult. Nevertheless, it is further concluded that the mechanisms of action of the thyroid hormones and prolactin are not independent and that future studies of their effects on follicle function should address their interactions with other hormones and changes in thyroid hormone receptor activity within the follicles.