Adrenal-thyroid pineal interaction and effect of exogenous melatonin during summer in crossbred cattle

Abstract

The circadian levels of catecholamines (epinephrine, norepinephrine and dopamine) and melatonin were measured in plasma samples of crossbred (Tharparker × Holstein, 6), 12-18 month-old, weighing 200±20 kg. Blood samples were collected at 4 hr interval during summer when the maximum ambient temperature (Db) was 35.5 Degree C and relative humidity at 08.30 hr was 55%. During the day epinephrine, norepinephrine and dopamine levels ranged from 317 LO 1575 pg/ml, 1551 to 1852 pg/ml and 725 to 2700 pg/ml respectively. The concentration of epinephrine and dopamine was high during day and low during night. Plasma norepinephrine levels increased during night prior to a peak in plasma melatonin at 00.30 hr. The norepinephrine levels declined to a mininium at 16.30 hr. Plasma melatonin levels were high (124.33±819 pg/ml at 00.30 hr) during night and low (23.33±4.16 pg/ml at 12.30 hr) during day. To study thyroidal-adrenal hormones and melatonin inter-relationship, 3 heifers were administered melalatonin subcutaneously (2 mg/100 kg.body weight)and jugular blood samples were collected. Melatonin administration decreased plasma T4 and T3 levels and the T3: T4 ratio increased. The levels of plasma T4 remained low for 4 hr and thereafter increased. Plasma T3 levels also declined after melatonin injection, but it was not significant. Plasma cortisol levels declined after melatonin injection. Plasma levels of epinephrine incsensed by 6.4-times after melatonin injection and declined, thereafter, but plasma norepinephrine and dopamine levels increased due to melatonin injection. The study points out that pineal plays an important role in regulation of thyroidal and adrenal hormones during summer in cattle and exogenous melatonin administration exerts a depressant modulatory effect on thyroid by altering thyroid stimulating hormone. The effect of melatonin on adrenals were also depressants as reflected by changes in medullaly and cortical hormones in cattle.