The effects of opiate agonists on growth hormone and prolactin release in rats

Various opioid receptor agonists, including Met5-enkephalin amide, Leu5-enkephalin amide, [D-Ala]2-Met5-enkephalin amide, [D-Ala]2-Leu5-enkephalin amide, morphine sulfate, d-methadone hydrochloride, and l-methadone hydrochloride were administered to adult male rats by subcutaneous injection. All opioid receptor agonists except Leu5-enkephalin amide significantly stimulated growth hormone and prolactin release. Naloxone and naltrexone blocked the hormone stimulatory effects of the opioids and both naloxone and naltrexone, when administered alone, significantly reduced serum growth hormone and prolactin concentrations. The dopaminergic agonist apomorphine, but not the alpha-adrenergic agonist clonidine, blocked opiate stimulation of prolactin. Morphine sulfate caused growth hormone release in rats pretreated with alpha-methyl-p-tryosine, a catecholamine synthesis inhibitor. Cholinergic agonists, physostigmine and pilocarpine, antagonized the growth hormone and prolactin release induced by morphine sulfate. The data suggest that the opiates stimulate prolactin via an interaction with catecholaminergic neurons controlling prolactin release and stimulate growth hormone via a mechanism independent of alpha-adrenergic or general catecholaminergic influence. The mechanism through which cholinergic agonists act to inhibit opiate agonist stimulation of growth hormone is presently unknown.     

Effects of naloxone on the secretion of prolactin and corticosterone induced by 5-hydroxytryptophan and a serotonergic agonist, mCPP

The effects of naloxone, an opiate “pure” receptor antagonist, on the release of prolactin and corticosterone in the rat were studied following the administration of the serotonin precursor 5-hydroxytryptophan or the serotonin receptor agonist (?) -$\text{m}$-chloropnehylpiperazine. Naloxone clearly antagonizes the release of prolactin induced by 5-hydroxytryptophan administered alone at a dosage of 50 mg/Kg/b.wt. or at dosage of 30 mg/Kg/b.wt. preceded 60 minutes before injection by the administration of the serotonin uptake blocker fluoxetine. The opiate antagonist does not modify the increase in blood level of prolactin induced by (?) ?$\text{m}$-chlorohenylpiperazine. Naloxone itself does not reduce the increase in plasma level of corticosterone induced by 5-hydroxytryptophan, 5-hydroxytryptophan +fluoxetine or (?)?$\text{m}$-chlorophenylpiperazine.The results suggest that endogenous opioids may be involved in the increase in serum level of prolactin induced by 5-hydroxytryptophan and also indicate the existence of different serotonergic neurotransmitter circuits capable of modulating the release of prolactin and corticosterone. A mutual interplay between serotonergic and opiate neurons may be involved in controlling the release of prolactin, but such an interplay does not seem to occur in the secretion of corticotrophin-releasing hormone.     

Naloxone inhibits prolactin and growth hormone release induced by intracellular glucopenia in the rats

Intraventricular administration of 2-deoxy-D-glucose (2DG), which causes intracellular glucopenia in the central nervous system, increased plasma prolactin and growth hormone levels in the urethane anesthetized male rats. Naloxone, an opiate antagonist, inhibited the 2DG-induced prolactin and growth hormone release. Apomorphine, a dopaminergic agonist, also inhibited the release of these hormones induced by 2DG. These results suggest that endorphins play a role in hypoglycemia-induced prolactin and growth hormone release and that the dopaminergic mechanism may be involved in this phenomenon.     

Suppression of serum prolactin by naloxone but not anti-β-endorphin antiserum in stressed and unstressed rats

The role of endogenous opioids in the regulation of tonic and stress-induced prolactin secretion was studied in male rats. Animals with chronically indwelling intra-atrial catheters were used and served as their own controls. Intravenous injection of a potent rabbit anti-β-endorphin antiserum produced no change in either baseline serum prolactin or in the rise induced by swimming for 15 minutes at 20 C. Naloxone, 0.5 mg/kg intravenously, produced a small but statistically significant lowering of baseline serum prolactin levels, as well as a mild blunting of the stress-induced serum prolactin rise. The failure of anti- - endorphin antiserum to affect serum prolactin may be explained either by failure of the antiserum to gain access to hypothalamic prolactin regulating loci, or to lack of involvement of β-endorphin in the control of serum prolactin. The suppression of prolactin secretion seen with naloxone indicates that endogeneous opioids are involved in prolactin regulation, though the relatively small magnitude of the changes observed suggests that their role under physiological conditions may be a minor one.     

Location and characterization of opiate receptors regulating pituitary secretion

The site at which opiate agonists and antagonists act to alter secretion of prolactin, growth hormone and luteinizing hormone as well as the pharmacological specificity of the opiate receptors mediating these effects were examined in rats. Injection of β-endorphin but not a 10 fold higher dose of the non opiate peptide β-endorphin, increased release of prolactin and growth hormone in male rats while inhibiting luteinizing hormone release in ovariectomized, estrogen primed female rats. Prior treatment with naltrexone i.p. blocked these responses. Injection of naltrexone into the hypothalamus lowered prolactin release. In rats with a surgically formed hypothalamic island systemic administration of morphine or naltrexone altered prolactin release in the same manner as was observed in intact animals. In contrast no effects of β-endorphin or naltrexone were observed on the spontaneous secretion of prolactin $\text{in}$$\text{vitro}$. In addition β-endorphin did not alter the inhibition of prolactin release produced by apomorphine $\text{in}$$\text{vitro}$. The ED₅₀ for stimulation of prolactin release following intraventricular administration of β-endorphin or the synthetic enkephalin analog FK 33-824 was the same, approximately 0.1 ng/rat. However FK 33-824 at 0.2 ng/rat was able to produce much greater analgesia and catatonia than β-endorphin. The metabolism and distribution of β-endorphin was examined but did not account for these differential effects. These results indicate that opiate agonists and antagonists can act at the hypothalamic but not the anterior pituitary level to alter release of prolactin, growth hormone and luteinizing hormone. In addition the data suggest that the opiate receptors mediating release of prolactin may have a different pharmacological specificity from those involved with analgesia and catatonia.     

Morphine stimulates prolactin release in normal but not in castrated male rats

Morphine (200 micrograms/rat) was injected intraventricularly (i.v.t.) into normal and into long-term castrated (4 weeks) adult male rats. Animals were killed 10, 20, 40 and 60 min after treatment. In normal animals, the treatment with morphine resulted in a significant increase of serum prolactin concentrations at all time intervals considered. However, the i.v.t. injection of 200 micrograms morphine/rat into castrated rats did not exert any significant effect on prolactin release at any time interval considered. When morphine (200 micrograms/rat) was administered i.v.t. together with the specific opioid receptor blocker naloxone (7.5 or 15 micrograms/rat) the stimulatory effect of morphine on prolactin release was diminished at 10 min, and totally blocked at 20 min. Naloxone given alone did not influence serum prolactin concentrations. The results suggest that the presence of endogenous androgens is essential to permit the stimulatory effect of morphine on prolactin release.     

The effects of partial opiate agonists on plasma prolactin and growth hormone levels in the rat.

Opiate agonists, partial agonists, and antagonists differed in their effects on release of prolactin and growth hormone. Agonists (morphine, methadone or meperidine) elevated plasma levels of both hormones. An antagonist (naloxone) lowered levels of prolactin but not growth hormone. All partial agonists studied raised growth hormone levels; among these, levallorphan, nalorphine, and ciramadol lowered prolactin levels while pentazocine and meptazinol did not. Naloxone blocked morphine-induced release of prolactin and growth hormone. The partial agonists suppressed morphine-induced prolactin release, and several suppressed the elevated growth hormone levels as well. Data from the opiate radioreceptor assay (displacement of ³H-naloxone) in the presence and absence of sodium agrees with the above placement of agents into three classes. These results suggest that classification of opioid compounds into agonists, partial agonists and antagonists may be made by their effects on prolactin and growth hormone release.     

Relation of endogenous opioid peptides and morphine to neuroendocrine functions.

Morphine and the endogenous opioid peptides (EOP) exert similar effects on the neuroendocrine system. When adminstered acutely, they stimulate growth hormone (GH), prolactin (PRL), and adrenocorticotropin (ACTH) release, and inhibit release of luteinizing hormone (LH), follicle stimulating hormone (FSH),and thyrotropin (TSH). Recent studies indicate that the EOP probably have a physiological role in regulating pituitary hormone secretion. Thus injection of naloxone (opiate antagonist) alone in rats resulted in a rapid fall in serum concentrations of GH and PRL, and a rise in serum LH and FSH, suggesting that the EOP help maintain basal secretion of these hormones. Prior administration of naloxone or naltrexon inhibited stress-induced PRL release, and elevated serum LH in castrated male rats to greater than normal castrate levels. Studies on the mechanisms of action of the EOP and morphine on hormone secretion indicate that they have no direct effect on the pituitary, but act via the hypothalamus. There is no evidence that the EOP or morphine alter the action of the hypothalamic hypophysiotropic hormones on pituitary hormone secretion; they probably act via hypothalamic neurotransmitters to influence release of the hypothalamic hormones into the pituitary portal vessels. Preliminary observations indicate that they may increase serotonin and decrease dopamine metabolism in the hypothalamus, which could account for practically all of their effects on pituitary hormone secretion.     

The effect of a serotonin uptake inhibitor (Lilly 110140) on the secretion of prolactin in the rat

Serotonin reuptake inhibitor 3-(p-trifluoromethylphenoxy)-N-methyl-3-phenylpropylamine (Lilly 110140) in a dose of 10 mg/kg i.p. had no effect on the resting levels of serum prolactin in normal male rats. However, pretreatment of the animals with this drug strongly potentiated the prolactin releasing effect of 5-hydroxytryptophan as well as the prolactin release induced by ether stress and blood withdrawal. These results indicate that a central serotoninergic mechanism is involved in the stress-induced prolactin release in the rat.     

Influence of endogenous opiates on anterior pituitary function

In general, the endogenous opioid peptides (EOP), morphine (MOR), and related drugs exert similar effects on acute release of pituitary hormones. Thus administration of opiates produces a rapid increase in release of prolactin (PRL), growth hormone (GH), adrenocorticotropin (ACTH), and antidiuretic hormone (ADH), and a decrease in release of gonadotropins and thyrotropin (TSH). Although not yet fully established, there is growing evidence that the EOP participate in the physiological regulation of pituitary hormone secretion. Thus naloxone (NAL), a specific opiate antagonist, has been shown to reduce basal serum levels of PRL and GH, and to elevate serum levels of LH and follicle stimulating hormone in male rats. Other reports have shown that NAL can inhibit the stress-induced rise in serum PRL, raise the castration-induced increase in serum LH to greater than normal castrate values, and counteract the inhibitory effects of estrogen and testosterone on LH secretion. Opiates appear to have no direct action on the pituitary, but there is evidence that they can alter activity of hypothalamic dopamine and serotonin in modulating secretion of pituitary hormones.     

Effect of serotonin depletion by p-chlorophenylalanine on serum prolactin levels in estrogen-treated ovariectomized rats: insights concerning the serotoninergic, dopaminergic and opioid systems.

The stimulatory effect of serotonin on prolactin secretion is well documented, and the administration of an inhibitor of serotonin synthesis (p-chlorophenylalanine - pCPA) has the expected inhibitory action on prolactin release in most experimental situations. However, there is evidence that in certain physiological or experimental conditions, activation of the serotoninergic system can also determine inhibition of prolactin secretion. The aim of the present study was to investigate the ability of estrogen to modify the effect of pCPA on prolactin secretion and to evaluate the participation of opioid and/or dopaminergic systems in regulating pCPA-induced prolactin secretion in estradiol-treated rats. We observed that pCPA administration (200 mg/kg/day, s.c., 2 days) to ovariectomized (OVX) female rats treated with estradiol benzoate (300 microg/week for 2 weeks, or 50 microg/week for 4 weeks, s.c.) causes a significant increase in serum prolactin, whereas no effect is observed in intact rats or in OVX rats without treatment. Bromocriptine administration completely reversed prolactin values previously increased by estradiol and by pCPA [OVX rats + estradiol = 86.50 ng/ml (68.90-175.02), OVX + estradiol + pCPA = 211.30 ng/ml (142.03-311.00), OVX + estradiol + pCPA + bromocriptine = 29.35 ng/ml (23.01 - 48.74), p<0.05. Naloxone administration partially reduced estrogen-induced high prolactin concentrations, but did not affect prolactin secretion stimulation determined by pCPA. Overall, the data from this report confirm the involvement of the dopaminergic system and, to a lesser degree, of endogenous opioids in prolactin secretion stimulation determined by estradiol. Furthermore, our results suggest that the stimulatory action of pCPA on prolactin secretion in estradiol-treated OVX rats is mediated by serotonin, which may also act indirectly on dopamine neurons.     

Naloxone inhibits the plasma epinephrine response to ACTH but not to 2-deoxy-D-glucose in rats

Intravenous injection of (1-24) ACTH and 2-deoxy-d-glucose (2DG) stimulated the plasma epinephrine and norepinephrine levels in pentobarbital-anesthetized male rats. Naloxone, a specific opiate antagonist, inhibited the plasma epinephrine response to ACTH but not to 2DG. Norepinephrine release induced by ACTH or 2DG was not affected by naloxone. These results suggest that the opioid peptidergic synapse might be involved in the ACTH- but not in the 2DG-induced epinephrine release.     

家兔中枢内源性阿片样物质在紧张性高血糖反应中的作用

本工作观察家兔内源性阿片样物质在紧张性高血糖反应中的作用。通过向家兔侧脑室内注射阿片受体阻断剂纳洛酮或羧基肽酶 A 的抑制剂 D-苯丙氨酸以分别减弱或加强脑内内源性阿片样物质的作用。结果表明,纳洛酮能使由乙醚或2-脱氧葡萄糖所引起的高血糖反应减弱,而使由胰岛素所引起的低血糖反应加强并延搁其回复过程。D-苯丙氨酸表现为相反的效应。在已对吗啡形成耐受的家兔,2-脱氧葡萄糖所引起的高血糖反应也呈减弱。这些结果提示,脑内内源性阿片样物质与紧张性高血糖反应有关。     

Effects of enkephalin in analogues on prolactin release in the rat.

The effects of nineteen enkephalin analogues on the circulating levels of prolactin in the male rat following intraventricular injection of the peptides were determined and compared with that of Met- and Leu-enkephalin. Eleven of the 19 analogues stimulated prolactin secretion. It was found, in general, that the structure activity relationship for enkephalin stimulation of prolactin secretion was similar to that for opiate receptor activity. Analogues which contained a [DAla²] substitution were generally effective in stimulating prolonged prolactin release. Some, but not all analogues containing [DTrp²] or [DLeu⁵] were active. Analogues containing the [DTrp¹], [DPhe⁴] or [DMet⁵] substitutions were ineffective. The prolactin releasing effect of intravenous Tyr-DAla-Gly-Phe-DLeu was reversed by naloxone. Naloxone had no effect on the haloperidol- and alpha-methylparatyrosine induced increases in plasma prolactin levels. The results of these studies are discussed in the light of the suggestion that the enkephalins may function as neuroendocrine modulators.     

Effect of passive immunization against substance P in rats with hyperprolactinemia

Substance P, an undecapeptide isolated from gut and brain tissues, was reported to stimulate prolactin release. It was suggested that substance P may play a role in the control of prolactin secretion. In this investigation we studied the effects of the blockade of endogenous substance P by the administration of a specific anti-substance P serum on serum prolactin levels in rats in the evening of proestrus, in lactating rats after suckling, and in male rats with hyperprolactinemia induced by grafting 2 anterior pituitary glands under the kidney capsule. The injection of the anti-substance P serum was followed by a significant decrease of the prolactin surge induced by 30 min suckling in lactating rats, when the antiserum was administered 24 hr but not 5.30 hr earlier. Anti-substance P serum also induced a significant decrease in serum prolactin levels in pituitary grafted rats, but induced no change in the proestrous surge of prolactin and LH. These results show that substance P may be involved in the release of prolactin induced by suckling and that this peptide may have an intrapituitary role in the process of prolactin release. On the other hand, substance P does not seem to play a significant role in the proestrous peak of prolactin and LH.     

Effects of naloxone,morphine and methionine enkephalin on serum prolactin,luteinizing hormone,follicle stimulating hormone,thyroid stimulating hormone and growth hormone

The response of 5 anterior pituitary hormones to single injections of naloxone, morphine and metenkephalin administration was measured in male rats. Morphine and met-enkephalin significantly increased serum prolactin and GH concentrations, and significantly decreased serum LH and TSH concentrations. Naloxone reduced serum prolactin and GH concentrations, increased serum LH and FSH, but had little effect on serum TSH concentrations. Concurrent injections of naloxone with morphine or met-enkephalin reduced the response to each of the drugs given separtely. These results suggest that endogenous morphinomimetic substances may participate in regulating secretion of anterior pituitary hormones.     

The effects of the acute administration of buprenorphine hydrochloride on the release of anterior pituitary hormones in the rat: evidence for the involvement of multiple opiate receptors

Multiple opiate receptor agonists and antagonists have been found to produce different patterns of anterior pituitary hormone release. The present studies examined the pattern of anterior pituitary hormone release produced by buprenorphine. The effects of the kappa agonist ethylketocyclazocine on thyroid stimulating hormone release were also examined. Following buprenorphine, serum levels of corticosterone and luteinizing hormone were not changed while growth hormone release was stimulated in a dose-dependent manner. Prolactin release was stimulated after the lowest dose of buprenorphine while the highest dose induced a fall in serum prolactin. Similar biphasic effects on thyroid stimulating hormone were seen after either buprenorphine or ethylketocyclazocine. The results provide support for the role of multiple opiate receptors in opiate-induced changes in anterior pituitary hormone release.     

beta-Endorphin controls vasopressin release during foot shock-induced stress in the rat

This study tested the possibility that beta-endorphin is involved in the regulation of vasopressin release during stress induced by inescapable electric foot shock. To this end, a specific anti-beta-endorphin antiserum or a control serum lacking the specific anti-beta-endorphin antibodies was administered to male rats. Plasma vasopressin concentrations, measured by radioimmunoassay, were not affected by brief foot shock stress in control rats, but were raised significantly by the stress in animals which had received an intracerebroventricular (i.c.v.) injection of the anti-beta-endorphin antiserum. In contrast, when the same volume of the anti-beta-endorphin antiserum was injected into a tail vein, foot shock stress produced only a slight effect on vasopressin release. I.c.v. injection of the antiserum changed neither basal nociceptive threshold nor stress-induced analgesia as revealed by the tail-flick latency. Vasopressin release induced by an osmotic stimulus was not influenced by the anti-beta-endorphin antiserum given i.c.v. The opiate antagonist naloxone or the glucocorticoid dexamethasone raised plasma vasopressin concentration in stressed rats which had received the control serum (i.c.v.); however, after i.c.v. injection of the anti-beta-endorphin antiserum neither naloxone nor dexamethasone elevated the plasma vasopressin concentration beyond the level reached by the anti-beta-endorphin antiserum (i.c.v.) alone. These results suggest that beta-endorphin inhibits the release of vasopressin during foot shock-induced stress in the rat.     

Stimulation of prolactin secretion in the rat by alpha-neo-endorphin, beta-neo-endorphin and dynorphin

Intraventricular injections of α-neo-endorphin, β-neo-endorphin and dynorphins (dynorphin[1–13], dynorphin[1–17], dynorphin[1–8]) resulted in an increase in plasma prolactin levels in urethane-anesthetized rats. Dynorphin [1–13] was the most potent to stimulate prolactin release among these opioid peptides. Plasma prolactin responses to these stimuli were blunted by naloxone, an opiate antagonist. In $\text{in}\text{vitro}$ studies, prolactin release from perfused pituitary cells was stimulated by α-neo-endorphin, and the effect was blunted by naloxone, whereas neither β-neo-endorphin nor dynorphin[1–13] affected prolactin release. These results suggest that newly identified “big” Leu-enkephalins in the brain stimulate prolactin secretion in the rat and that α-neo-endorphin has a possible direct action on the pituitary.     

II. Effect of aminooxyacetic acid and bicuculline on prolactin release in castrated male rats

To investigate the role of gamma aminobutyric acid (GABA) on prolactin secretion, castrated male rats were infused with aminooxyacetic acid (AOAA) or bicuculline, two drugs that affect GABA metabolism or its binding to the receptors, respectively. The infusion of AOAA or bicuculline for 2 hr did not significantly modify serum prolactin levels. A quick iv injection of sulpiride, a drug that induces hyperprolactinemia, brought about a significantly lower release of prolactin in rats infused with AOAA than in control rats, infused with saline. The response to sulpiride in rats infused with bicuculline was significantly greater, in terms of prolactin release, than in control rats. These results suggest that GABA may have an inhibitory role on the regulation of prolactin release.