Make sure that your horse is diagnosed by a VET to be a non-sweater.  If you are unsure you can cause a host of problems by giving your horse One AC or other supplements for non-sweaters. 
DOPAMINE
Dopamine is required to cause the body to sweat.  If the horse is deficient in dopamine-he stops sweating.  If the horse is not deficient in dopamine and you give him supplements to increase it even further the horse can become extremely spooky, anxious and even depressed because when you increase dopamine you decrease serotonin. 
SEROTONIN
Ideally, serotonin and dopamine levels SHOULD BE BALANCED.  If you raise one too much the other one suffers.  Serotonin relaxes and creates a feeling of well being.  Dopamine enables focus, sweating and circulatory function.  Balancing can be affected by a horse's diet and the different proteins that the horse takes in.  Each horse is different and their moods should be closely monitored when trying to raise either serotonin or dopamine.