Serum PRL levels increased 4-6 weeks earlier than controls in HAL (P<0.001) and HAL + MEL-treated mink (P<0.02).  In mink receiving HAL + MEL, serum PRL levels, peaked on 3/13 and then declined by 3/27 to levels that tended to be lower than control (P<0.05) s.  In mink treated with HAL only, serum PRL levels increased and were significantly higher than control mink on 2/27 (P<0.05), after which time the hormone levels were no different from controls.  In response to HAL or HAL + MEL treatment, two mink in each group entered anagen between 3/13 and 3/20, approximately 3-4 weeks earlier than controls (P<0.01), while the remaining 4 mink in each group entered anagen the same time as controls.  Thus, even though serum PRL levels were elevated in advance of control mink, this failed to induce early summer anagen and casts further doubt on the role of PRL in summer hair growth cycles of mink.

UNDER HAIR DENSITYUnder hair density analysis revealed that the number of under hairs in G-type bundles was lower than in either I or U-type bundles for all treatment groups (Figure 4: P<0.01).  Exogenous HAL reduced the number of under hair follicles in I (P<0.05) and U-type bundles (P<0.01), while HAL + MEL reduced the number of under hair follicles in U-type bundles only (P<0.05). Thus, elevated serum PRL levels, results in a reduced number of under hair follicles entering anagen.

Moreover, because serum PRL levels were greater in HAL-treated mink than in HAL + MEL- treated mink, suggests to us a dose-related effect of PRL on the inhibition of under hair follicle development.  MEL-treated mink were not included in this analysis, since none of these animals exhibited summer anagen. Although it would appear that PRL although not required for initiation of the summer fur growth cycle, does play an important role in determining fur density.

SKIN PRL-R CONCENTRATIONS
The control mink (N = 2) that were retained until 9/29, entered winter anagen around 9/4, as expected and had serum PRL levels that were low, but detectable (Fig 5).  All of the MEL-treated mink (N = 3) had PRL levels that were lower than controls (P<0.05), but interestingly, none of these mink entered winter anagen earlier than controls.  Indeed, one MEL-treated mink never exhibited hair growth as of 9/29, while the remainder entered anagen the same time as controls.  It would appear that other, as yet undetermined, signals are involved in the onset of winter fur growth.

Skin PRL-R levels on 9/29, were not different between Control, ADX + DOC, DOC, (all in anagen) or from MEL-treated mink skin in telogen (Fig 5). However, skin samples from MEL-treated mink in anagen were higher than telogen skin from MEL-treated mink, ADX’d, DOC and control animals.  Because serum PRL levels were almost non-detectable in response to MEL, suggests that long-term treatment with MEL stimulates production of the skin PRL-R, at least during anagen.

LONG TERM HAIR GROWTH RESPONSE TO ADX
In the ADX’d mink, we could at no time detect a period of zero hair growth which would indicate a telogen phase.  Following shearing on 5/22 and again on 7/24, we noted that the skin was black, indicative of melanogenesis and hair growth. Inspection 7 days after each shearing revealed longer guard hairs (Fig 6) also indicative of continued hair growth. Lastly, serum PRL levels, which we have shown to be unaffected by ADX, were in long-term ADX’d mink, lower than controls (P<0.05).

EXPERIMENT II:The time of onset of depilation-induced anagen in HAL (3/27), and MEL-treated mink (3/29) was earlier than controls (4/9); Fig; P<0.05). Serum PRL levels were non-detectable in MEL-treated mink, whereas in HAL-treated mink, PRL levels (7.8 + 1.1 ng/ml) were higher than in control mink (5.6 + 0.91 ng/ml).  That depilation, over rode the inhibitory effects of MEL during the spring (Rose et al., 1998), suggesting that PRL is not required for onset of anagen, at least in most hair follicles. Analysis of under hair follicle density shows that hyperprolactinemia (HAL) reduces the number of under hair follicles that enter anagen in all three bundle types (Fig., P<0.05), whereas hypoprolactinemia (MEL) increased hair follicle activation for all three bundle types (P<0.05).
 

Skin PRL-R concentrations were significantly lower in MEL-treated mink (4.4 + 0.17 fmoles/mg protein) when compared to controls (P<0.05; 9.20 + 0.54 fmoles/mg protein), and although lower, were not different from HAL-treated mink (6.9 + 0.75 fmoles/mg).  These findings are in contrast to those obtained for mink during the winter anagen (Hunt et al., 2002; this symposium), where PRL-R levels were increased following long term treatment with MEL.   Moreover, in the data presented in EXP 1, we observed an increase in PRL-R levels following long term treatment (Feb to Sept) with MEL (Fig - 4). Future research will be necessary to resolve this apparent discrepancy.