Effects of Hypo- and Hyperthyroidism on 5-Hydroxytryptophan and Chlorpromazine-Induced Prolactin Release in the Rat 1

Abstract

Pituitary prolactin (PRL) content was reported to be decreased in hypothyroid animals (1, 2). Serum levels of PRL as measured by radioimmunoassay were observed to be unchanged by hypothyroidism in rats and human subjects (3, 4). Recently, the response of PRL secretion to thyrotrophin-releasing hormone (TRH) was shown to be higher in hypothyroid than in euthyroid animals, and the stimulating action of TRH on prolactin release was abolished by treatment with thyroid hormones (4, 5). Onishi et al. (6) reported that PRL release in response to TRH and to chlorpromazine (CPZ) was eliminated in hyperthyroid humans. The precursor of 5-hydroxytryptamine (5-HT), 5-hydroxytryptophan (5-HTP), and CPZ, an adrenergic receptor blocker, both have been observed to greatly increase PRL secretion (7-9), perhaps by decreasing prolactin release-inhibiting factor (PIF) or by increasing prolactin releasing factor (PRF) in the hypothalamus (9).

Materials and methods. Mature male Sprague-Dawley rats weighing 200-225 g were obtained from Spartan Research Animals (Haslett, Mich.). The rats were housed in a light-(14 hr light and 10 hr darkness daily) and temperature-controlled (26 ± 1°) room, and Received Wayne Lab Blox rat pellets (Allied Mills, Chicago, Ill.) and tap water ad libitum.

The rats were grouped as follows: (1) intact controls, (2) thyro-parathyroidectomized (THX), (3) THX and 2.5 μg thyroxine (T₄) per 100 g body wt, (4) THX and 10 pg T₄ per 100 b body wt. All surgically treated rats were given 0.2 ml Longicil (Fort Dodge Laboratories, Inc., Fort Dodge, Iowa), an antibiotic, by im injection to avoid infection. The sodium salt of L-T₄ (Nutritional Biochemical Co .) was dissolved in slightly alkaline saline (pH 8.0) and given sc to groups of three and four rats once daily. Rats in the intact control and THX groups were injected with alkaline saline (pH 8.0) only. After operation, the THX rats were given 1% calcium lactate solution instead of tap water to prevent tetany.