INHIBITORY EFFECT OF INCREASED PHOTOPERIOD ON WOOL FOLLICLE GROWTH

The relationships between circulating prolactin (PRL), wool follicle g rowth and daylength were investigated in 24 New Zealand Wiltshire ewes housed indoors from September 1989 to May 1991. Twelve control (C) ew es were maintained under natural photoperiod. Two other groups were he ld in short days (SD; 8 h light:16 h darkness) commencing from the win ter solstice (22 June 1990) for either three (group SD3, n=7) or six ( group SD6, n=5) months before reversion to natural daylength. Skin was sampled at one- to four-week intervals for histological determination of percentages of growing primary and secondary follicles. Hourly blo od samples over 24 h were collected via jugular cannulae fr om C sheep in March and July and then monthly from all animals until December 19 90 for estimation of mean monthly PRL concentrations for each treatmen t group. Between autumn (March 1990) and winter (July) primary follicl e activity (PFA) and secondary follicle activity (SFA) declined in C e wes (PFA: 97 to 43%, SFA: 100 to 57%). Follicle regrowth during July a nd August in eight C ewes preceded the initial rise in plasma PRL from the winter minimum (1.6 ng/ml). Across the three groups, four instanc es of decreased follicle activity were observed, closely following or concurrent with increases in plasma PRL concentrations. The resumption of spring growth in four C sheep was temporarily checked by falls in follicle activities during September and October as PRL concentrations began to increase (3.4 to 8.9 ng/ml). Follicle activity also declined in November and December in eight C sheep, coincident with the rapid rise in PRL to a seasonal maximum in late November (165.4 ng/ml). The increase in SD3 follicle activity over spring was not delayed by short days but during October, after release from treatment, PRL concentrat ions rose (1.8 to 12.0 ng/ml) and follicle activity declined (PFA: 65 to 38%, SFA: 68 to 43%). In SD6 ewes, PRL concentrations were suppress ed (2 mu ng/ml) and relatively constant levels of follicle activity (P FA: 73%, SFA: 95%) were maintained throughout shea-day treatment. Rele ase of SD6 ewes into summer photoperiod in January 1991 temporarily in terrupted follicle growth (PFA: 68 to 17%, SFA: 96 to 19%) and caused out-of-season shedding in March and April. Contemporary C follicle act ivities were high (PFA: 95%, SFA: 98%). These data suggest that natura l and experimental increases in daylength have a shea-term inhibitory effect on growing wool follicles which could be mediated through risin g concentrations of plasma prolactin.