H.T. Chen

Effects of L-tryptophan and restraint stress on hypothalmic and brain serotonin turnover, and pituitary TSH and prolactin release in rats

Abstract The dose response and time course effects of L-tryptophan and restraint stress on the metabolism of serotonin and release of thyroid stimulating hormone (TSH) and prolactin (PRL) were tested in male rats. Both treatments increased serotonin turnover in the hypothalamus (H) and remaining brain tissue minus the cerebellum (brain) as determined by enhanced accumulation of serotonin following monoamine oxidase (MAO) inhibition. L-tryptophan but not restraint stress elevated levels of tryptophan in the cerebellum. Both L-tryptophan and restraint stress inhibited TSH release and stimulated PRL release. These findings indicate that enhanced rates of serotonin turnover produced by L-tryptophan and physical restraint are associated with inhibition of TSH and stimulation of PRL release from the anterior pituitary.

Interaction between opiates and hypothalamic dopamine on prolactin release.

Abstract Opiate stimulation of prolactin (PRL) release appears to involve a hypothalamic mechanism(s). The present study utilized both central acting drugs and direct measurement of hypothalamic dopamine (DA) to investigate this problem. Administration of L-dopa, the precursor of DA; piribedil, a DA agonist; or amineptine, a DA reuptake inhibitor, each decreased serum PRL concentrations. Morphine sulfate (MS) and haloperidol (HAL) significantly increased serum PRL levels. L-dopa and piribedil reversed the stimulatory effect of MS on serum PRL concentrations by increasing dopamine activity. MS blocked the inhibitory effects of amineptine on serum PRL release, possibly by decreasing the concentration of DA available for reuptake. Injection of subeffective doses of HAL concurrently with a subeffective dose of MS increased serum PRL concentrations, by an additive inhibitory action on dopaminergic activity. β-endorphin, an endogenous opioid peptide, decreased the rate of DA turnover in the median eminence, and increased serum PRL levels approximately 10 - fold. These observations indicate that opiates stimulate PRL release by decreasing DA activity in the median eminence.

Effects of morphine and naloxone on serum levels of luteinizing hormone and prolactin in prepubertal male and female rats.

The effects of morphine (M) and naloxone (N) on serum levels of luteinizing hormone (LH) and prolactin (PRL) in prepubertal male and female rats were investigated. N raised serum LH concentrations in female rats at 10, 15, 20, 25 and 30 days of age, but increased serum LH levels in male prepubertal rats only at 30 days of age. M significantly depressed serum LH values in both sexes only at 15 days of age. M increased serum PRL levels in immature rats of both sexes in all age groups, except in 25-day-old males, whereas N decreased serum PRL only in 25-day-old male rats. These data show that there are differences in the pituitary LH and PRL responses to M or N of immature as compared to the responses previously reported in mature rats, and suggest that the endogenous opioid peptides may have a role in regulating LH and PRL secretion in immature rats.

Inhibition by prolactin of post-castration rise in LH.

Castration of male and female rats resulted in a marked rise in serum LH. The rise in serum LH was partially or completely prevented by injection of prolactin (Prl), by implantation of a small amount of Prl in the median eminence (ME), by grafting 2 anterior pituitaries (APs) underneath the kidney capsule, or by transplantation of a Prl-secreting pituitary tumor underneath the skin. The larger pituitary tumor transplants secreted more Prl and were more effective in reducing LH release than the smaller tumors which secreted less Prl. Suppression of LH release generally was greater during the earlier than in the later phases of the different treatments. The pituitary LH response to synthetic LH-RH was the same in ovariectomized rats with or without pituitary grafts, and the decrease in hypothalamic LH-RH after orchidectomy was prevented by pituitary grafts. These results indicate that Prl can depress LH release after castration and that these effects are mediated via the hypothalamus.

Positive feedback by estrogen and progesterone on LH release in old and young rats.

Summary Young cycling rats 60 to 70 days of age, and old irregular-cycling or con-stant-estrous rats 10 to 11 months of age were ovariectomized. At 25 or 52 days after ovariectomy, they were each given a single injection of 8 μg of estradiol benzoate/100 g of body weight, followed 72 hr later by a single injection of 0.8 mg of progesterone/ 100 g of body weight. Radioimmunoassay of serum LH from blood samples removed 4,6,8, and 10 hr after progesterone administration showed that LH rose significantly more in the young than in the old, former constant-estrous rats 25 days after ovariec-tomy, and more than in either group of old rats at 55 days after ovariectomy. These results indicate that the positive feedback of estrogen and progesterone on LH release is reduced in old as compared to young cycling female rats, and is believed to account at least in part for the irregular cycling, and failure of ovulation in the old constant-estrous rats.

Naloxone stimulation of luteinizing hormones release in prepubertal female rats; role of serotonergic system☆

Abstract The purpose of this study was to determine whether naloxone stimulated LH release via a serotonergic mechanism. Injection of naloxone hydrochloride (2 mg/kg B.W.) into 25-day old female prepubertal rats resulted in a significant elevation in serum LH 30 min later. Injection of this dose of naloxone together with morphine sulfate (2 or 5 mg/kg B.E.) resulted in inhibition of naloxone-induced LH release. When rats were first injected with 5-hydroxytryptophan (5-HTP) to increase hypothalamic serotonin content, naloxone failed to increase serum LH levels. On the other hand, when parachlorophenylalanine (PCPA) was given first to reduce hypothalamic serotonin content, naloxone-induced LH release was potentiated. Morphine failed to inhibit the naloxone-induced rise in serum LH when PCPA was first administered. Neither 5-HTP nor PCPA, when injected alone, altered serum LH values. These results suggest that naloxone promotes LH release by reducing hypothalamic serotonergic activity, and morphine inhibits LH release by increasing hypothalamic serotonergic activity. This does not exclude possible involvement of other neurotransmitters.

Effects of pargyline on hypothalamic biogenic amines and serum prolactin. LH and TSH in male rats.

Abstract The time course effects of pargyline on hypothalamic biogenic amines and serum prolactin (PRL), LH and TSH were studied in adult male rats. The rats were killed at intervals of 1–6 hrs after pargyline injection. Hypothalamic dopamine (DA) rose 79% by 1 hr and was 41% above “0” time by 6 hrs. Norepinephrine (NE) increased 31% by 1 hr and remained at about this level through 6 hrs, whereas serotonin (5HT) increased from 42% by 1 hr and to 95% by 6 hrs. Serum PRL LH and TSH fell significantly during the first 2 hrs, but all had returned to pretreatment values by 4 hrs. Serum PRL was about 4-fold above pretreatment values by 6 hrs, but LH and TSH remained at pretreatment levels. Stimulation by pargyline of PRL release was potentiated by Lilly compound 110140, a serotonin reuptake inhibitor, and blocked by parachlorophenylalanine, a serotonin synthesis inhibitor. These results suggest that the inhibitory effects of pargyline on PRL, LH, and TSH release during the first 2 hrs were associated mainly with a rapid increase in DA, and subsequent elevation of PRL release was related to the increase in 5HT. Return of serum LH and TSH to pretreatment levels at 4 and 6 hrs appeared to be associated mainly with the decrease in DA and perhaps to elevated NE levels. These results suggest that changes in relative concentrations of hypothalamic amines are related to differential release of PRL, LH and TSH.

Effects of Castration, Steroid Replacement, and Hypophysectomy on Hypothalamic LHRH and Serum LH

Summary The effect of castration (CAS), hypophysectomy (HYPOX), and gonadal steroids on hypothalamic luteinizing hormone-releasing hormone (LHRH) content was studied in male and female rats. Hypothalamic LHRH content was significantly reduced by 1 week after castration in male rats and was further reduced by 2 weeks. HYPOX decreased LHRH content in male rats to the same extent as in CAS rats alone, suggesting that loss of gonadal function was mainly responsible for the fall in hypothalamic LHRH in these rats. In castrated male rats testosterone propionate (TP) at a dose of 0.5 mg/300 g body weight raised hypothalamic LHRH content above that of intact rats and reduced serum LH below the intact level. However, in castrated-hypophysecto-mized rats, TP treatment only returned hypothalamic LHRH content to the intact level. Ovariectomy (OVX) for 3 weeks decreased hypothalamic LHRH content significantly, whereas estradiol benzoate (EB, 1 μg) replacement for 1 week returned LHRH content to intact levels and reduced serum LH significantly. Progesterone (PRG) (2.5 mg) alone had no effect on hypothalamic LHRH content, but decreased serum LH levels slightly in ovariec-tomized rats. Treatment with a combination of EB and PRG raised hypothalamic LHRH content to levels even higher than those in intact rats, but this difference was not significant. These observations indicate that gon-adal steroids can regulate LH secretion by acting on the hypothalamus to modulate LHRH secretion, and suggest that the effect of hypophysectomy in reducing LHRH content is due mainly to loss of testosterone secretion.

Effects of hypo- and hyperthyroidism on 5-hydroxytriptophan and chlorpromazine- induced prolactin release in the rat.

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.

Effect of adrenalectomy and 5-hydroxytryptophan on phasic release of luteinizing hormone☆

The effect of 5-hydroxytryptophan (5-HTP) on serum progesterone and the possible role of adrenal progesterone in mediating stimulation by 5-HTP of phasic release of luteinizing. hormone (LH) were investigated in estradiol benzoate (EB)-treated ovariectomized rats. LH surges were induced in long-term (at least two weeks) ovariectomized rats by two injections of EB (20 micrograms/rat, s.c.) with an interval of 72 hrs. Administration of 5-HTP (50 mg/kg, i.p.) at 1000 hr in EB-treated ovariectomized rats resulted in a four-fold increase in serum progesterone within 30 mins, and significantly stimulated the LH surge at 1600 hr. This facilitative effect of 5-HTP on serum LH, but not progesterone, was further potentiated in rats pretreated with P-chlorophenylalanine (PCPA) 72 hrs earlier. Adrenalectomy shortly before 5-HTP administration attenuated the LH surge in saline treated controls, and completely blocked the facilitative effect of 5-HTP on the afternoon surge of LH in rats pretreated with PCPA 72 hrs earlier. On the other hand, chronic adrenalectomy (for 6 days) followed by hydrocortisone (0.2 mg/rat/day) replacement not only had no effect on the LH surge in saline treated controls, but also failed to prevent 5-HTP from facilitating the LH surge in PCPA pretreated rats. On the first day of bleeding, the basal LH value at 1000 hr in sham operated controls was significantly suppressed by PCPA pretreatment 48 hrs earlier. The second dose of 5-HTP administered on the next day failed to potentiate LH surges in either sham operated or adrenalectomized rats.(ABSTRACT TRUNCATED AT 250 WORDS)