Serum Steroid Concentrations at Different Phases of Production and Brain Monoamine Oxidase Type A Messenger Ribonucleic Acid in Buller Steers1

One field study was conducted to evaluate serum steroid hormone profiles of steers exhibiting characteristics attributed to buller steer syndrome and to assess the potential association between buller steer syndrome and the monoamine neurotransmitter enzyme monoamine oxidase type A (MAO-A) mRNA. Whole blood serum was collected at 3 time points from 600 crossbred steers: 1) before placement on intensively stocked early summer pastures (pregrass), 2) after arrival to a commercial feedlot (feedlot arrival; BW = 403 kg), and 3) when a steer exhibited classical buller characteristics (diagnosis of buller; n = 25). The pregrass progesterone period was suppressed (P < 0.05) only in sera obtained from bullers as compared with control animals (342 vs. 673 pg/mL). From feedlot arrival to the diagnosis of buller, a reduction (P < 0.05) in progesterone occurred in bullers (349 vs. 217 pg/mL). An increase (P < 0.05) in testosterone occurred in bullers from feedlot arrival to diagnosis of a buller (7.4 vs. 21.7 pg/mL). Estradiol-17β from bullers and controls increased (P < 0.01) from pregrass to feedlot arrival. Buller and control estradiol and testosterone levels were not different at any sampling time. From the same group of steers, 10 bullers and 12 controls, MAO-A mRNA levels were analyzed in postmortem samples removed from the frontal cortex region of the brain. The MAO-A mRNA levels measured by the real-time quantitative PCR method were 74.5% greater (P = 0.03) in bullers than controls. This study indicates bullers have suppressed serum concentrations of progesterone and a potential predisposition for heightened MAO-A activity.