Effect of thyroid hormones on neurosecretory cells of supraoptic and paraventricular nuclei of rat hypothalamusin vitro

Abstract

The direct effect of thyroid hormones on oxytocin-and vasopressinergic cells is studied in vitro on cultured slices of rat hypothalamus containing the paraventricular and/or supraoptic nuclei. It is shown that 30-and 60-min exposure to thyroxine leads to considerable shrinkage of the nucleoli of oxytocin-and vasopressin-secreting cells in the supraoptic and paraventricular nuclei, which reflects the decrease in their functional activity. The addition of triiodothyronine leads to stimulation of neurosecretory cells (30-min incubation) followed by a drop in their functional activity. Key Words: thyroid hormones; vasopressin; oxytocin; paraventricular nucleus; supraoptic nucleusThe direct effect of the hypothalamic hormones vasopressin (VP) and oxytocin (OT) on functional activity of the thyroid gland is well established [1,2]. However, the possibility of feed-back regulation of VP-and OT-cells of the hypothalamus by thyroid hormones (TH) remains unsolved. TH receptors were identified in many areas of the brain, including the hypothalamus [7]. Conceivable TH influence the VP cells since different thyroid statuses are accompanied by shifts in the blood VP level [5,6]. However, under conditions of long-term hormone imbalance it is very difficult to determine which factor plays a key role in the regulation of endocrine organs. It has been shown that TH participate in the regulation of the OT gene in vitro [3,4].In the present study we examined the effect of short-term direct exposure of cells of paraventricular and supraoptic nuclei to TH under conditions of in vitro incubation of hypothalamic sections in the presence of TH. MATERIALS AND METHODSSections of the hypothalamus were obtained from male Wistar rats (120-140 g). After decapitation, the brain was promptly removed and 400-g sections containing supraoptic and/or paraventricular nuclei were prepared from the hypothalamic area. The sections were preincubated in Eafle's medium saturated with carbogen (95% 02+5% CO2) at 37~ for 90 min. The medium was replaced every 30 min.Two experimental series were carried out. In both series binding of triiodothyronine (T3) and thyroxine (T4) was measured using radiolabeled hormones: 125I-T3 and I-%I-T4.After preincubation, the sections were transferred to a medium containing 2.7x 10 .9 M ~zSI-T3 (74x 10 js Bq/mol, series I) or 4.5x 10 .9 M ~25I-T 4 (140x 10 ~5 Bq/mol, series II). Control sections were incubated without TH. After 30-and 60-min incubation, the sections were fixed in picric acid and formalin (3:5) at 37~ for 7 days and processed by standard histological methods. Histological sections (6 ~) were stained with paraldehyde-fuchsin by the Gomori--Gabu method and poststained with azan by the Heidenhain method.