Intrinsic seasonal variation in basal growth hormone secretion from rainbow trout pituitaries.

Seasonal rhythms in growth hormone (GH) secretion are well-documented in salmonids, but whether these patterns reflect hypothalamic control or intrinsic pituitary programming remains unclear. Here I report that rainbow trout (Oncorhynchus mykiss) pituitaries exhibit robust seasonal variation in basal GH secretion under standardized in vitro perifusion conditions, independent of hypothalamic input. Analysis of 3,511 measurements from 66 individually perifused pituitaries across seven months revealed highly significant seasonal differences (one-way ANOVA: F6,3504 = 27.90, P < 0.001, η2 = 0.046; Kruskal-Wallis H = 417.75, P < 0.001). September represented the annual nadir (4.28 ± 0.26 ng/ml/5 min, 95% CI: 3.78-4.78), while October showed peak secretion (13.04 ± 0.55 ng/ml/5 min, 95% CI: 11.97-14.11), representing a 3.05-fold amplitude (Cohen's d = 0.74, medium-to-large effect). The persistence of these seasonal differences without hypothalamic stimulation or inhibition provides compelling evidence for intrinsic circannual timing mechanisms within somatotrophs. These findings advance our understanding of neuroendocrine seasonality in teleosts and have practical implications for photoperiod manipulation in salmonid aquaculture.