Day length and hormone levels
Why antlers actually shed. The physiological cue is the male hormone testosterone. The way this works is complicated, but changing day lengths are sensed by the eyes, which send this message, via the optic nerve, to the pineal gland. The pineal gland - a pea-sized organ at the base of the brain - produces many different hormones. One hormone produced is luteinizing hormone, which controls the amount of testosterone produced in the testes.
The antler cycle lags behind the changes in day length because the hormonal changes take time. During fall, decreasing day lengths cause melatonin production to increase, resulting in decreased production of both luteinizing hormone and testosterone. Decreasing testosterone levels then cause the antlers to shed.
Taken from
http://www.whitetailfanatic.com/html/rc_wds_antler.shtml :