Re: Winter coat retention

Eleanor Kellon, VMD

--- In EquineCushings@..., "horsecorrect"
<horsecorrect@...> wrote:
Now that Spring is here, I am concerned that
she is not shedding. My other horse is losing her coat
dramatically! I
have read that a reduction in pergolide dose seems to help the coat
What we know about shedding so far is (this is normal horses):

- early spring is the only time the pituitary is sensitive to
melatonin, the hormone that is released during darkness.

- beginning sometime in February to April (some individual
differences), prolactin levels in the pituitary begin to rise

- decreasing melatonin levels in the spring may trigger a surge of
prolactin, or change the sensitivity of the target cells to
prolactin, which in turn causes both shedding and resumption of
reproductive activity.

- ACTH levels are elevated in September, may still be elevated in
January, are low in May

What we don't know is what, if anything, happens to ACTH during the
spring transition period. Like ACTH and the other POMC derived
pituitary hormones, Prolactin secretion is suppressed by dopamine.
Blocking dopamine (dopamine inhibition) also releases the block on
ovarian activity in the winter so, conversely, dopamine also
suppresses ovarian activity.

We know that Cushing's and the elevated ACTH and prolactin (? some, ?
all) that goes along with it is caused by loss of dopaminergic
hormones. So far so good. Where the connection gets shakey is that
you then would predict the horse would shed. I don't think we have a
very good handle on why they don't.

Some/most horses shed on pergolide, some don't. Some even
paradoxically grow a longer coat. That latter group may be horses
that really didn't need it, since increasing dopamine activity and
suppressing prolactin release would be expected to mimic the fall and
cause a longer hair coat to grow.

If this seems rambling, it is - because I'm not sure there's an
answer to the questions. My "best guess" would be that the ones that
are not shedding may have oversuppression of the seasonal prolactin
rise that normally triggers shedding. If that's the case, lowering
the pergolide dose (after making sure ACTH is indeed controlled)
might indeed trigger shedding.

That doesn't really explain why Cushing's horses have long coats in
the first place though. If prolactin rises along with the pituitary
hormones (and we know it does at least in some mares), why don't they
shed instead of growing a long coat? Maybe they become insensitive to
Prolactin by having constantly high levels.


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