Effects of chronic systemic administration of opioid peptides, naloxone and N-acetyl beta-endorphin antiserum on gonadotropins and testicular functions in the rat.

Systemic administration of opioid peptides, methionine-enkephalin and beta-endorphin, chronically, lowered gonadotropin levels in plasma and had an inhibitory effect mainly on the testicular enzymes hyaluronidase, acid phosphatase and on incorporation of 3[H] thymidine in the tissue. When rats were similarly treated with opioid peptide antagonist naloxone and N-acetyl beta-endorphin antiserum, induced an opposite effect. This is either the direct effect of opioid peptides/antagonist on the gonads or it may be via the circulating levels of gonadotropin.

Studies on the effect of intratesticular administration of opioid peptides, naloxone or N-acetyl beta-endorphin antiserum on some testicular parameters in rats.

Opioid peptides have been localized in a variety of peripheral tissues like placenta, thyroid, pancreas, gastrointestinal tract, in the reproductive tract of male and female and in the testes of rats. Immunoassayable material was detected in extracts of gonads, reproductive tract and accessory reproductive organs. Studies with naloxone have suggested that beta-endorphin may have an important role in steroidogenesis and may have a role in regulating transport of luminal material. In our studies met-enkephalin, beta-endorphin, naloxone or N-acetyl beta-endorphin antiserum were microinjected intra testicularly once on alternate days for one week and autopsied 24 h after the last injection. Intratesticular administration of 25, 50 and 100 micrograms doses of naloxone induced significant decrease in in vitro secretion of testosterone per se, which was significantly greater with 50 micrograms dose than with those of the other two doses. A 25 micrograms dose had no effect on hyaluronidase or acid phosphatase activity while 50 micrograms dose significantly decreased the enzyme activity. One hundred micrograms dose also significantly decreased hyaluronidase activity. Intratesticular injection of 10 micrograms met-enkephalin or 1 microgram beta-endorphin significantly decreased hyaluronidase activity whereas 20 microliters N-acetyl beta-endorphin antiserum increased the specific activity of hyaluronidase. There was no change in the weight of the testes on treatment with the above agents.

Testicular lactate dehydrogenase and sorbitol dehydrogenase activity after intratesticular injection of dynorphin and morphine in male rats.

Testicular lactate dehydrogenase (LDH) and sorbitol dehydrogenase (SDH) activity were measured at 1 and 4 hr following intratesticular injection of morphine and dynorphin. Twenty five and 50 micrograms doses of morphine sulfate significantly reduced LDH activity at 1 hr after injection. Five and 25 micrograms doses of dynorphin reduced LDH activity both at 1 and 4 hr after treatment. Testicular SDH activity was increased by morphine at 1 hr followed by a decrease at 4 hr. Both doses of dynorphin significantly reduced SDH activity at 1 and 4 hr after treatment. These results indicate paracrine regulatory role for opioids in testicular metabolism.

Prolactin suppression during pre and post-implantation periods on rat uterine glucosamine synthase activity.

Administration of bromocriptine (Bc), an ergot derivative having dopamine receptor agonist activity, to rats on day 1-5 of pregnancy prevented implantation of blastocysts and significantly suppressed uterine glucosamine 6-phosphate synthase activity. There was no effect on implantation or the enzyme activity when Bc was injected on day 7 or later of pregnancy. Injection of prolactin following Bc partially restored the enzyme activity and increased number of implantation sites. These results indicate that suppression of prolactin on day 1 to 5 of pregnancy causes failure of implantation. Bc on day 9 or later had no effect possibly due to the availability of placental LH/hCG to support the luteal cells.

Anti-implantation role for substance P and neurotensin in rat.

Substance P (SP) and neurotensin (NT), two structurally related peptides with contrasting biological actions, have been shown to have some role in peripheral reproductive processes. Intrauterine microinjection of SP or NT on day 4 or 5 of pregnancy in the rat significantly reduced the number of viable fetuses, weight and glycogen content of the uterus. The number of viable fetuses, uterine weight or glycogen content were not modified when SP/NT was microinjected on day 8, 9, 10 or on day 14, 15 and 16. The results indicate that the peptides possibly exert a direct local alteration in uterine vascular permeability causing failure in implantation.

Cholecystokinin antagonist, proglumide, stimulates growth hormone release in the rat.

Previous studies have revealed a stimulatory action of cholecystokinin on growth hormone release in the rat. To evaluate the physiologic significance of these effects we employed the cholecystokinin antagonist, proglumide and injected it intravenously and intraventricularly (third cerebral ventricle, 3V) to determine its actions on growth hormone. The experiments were performed in conscious, freely moving rats with indwelling cannulae in the 3V and/or external jugular vein. Intraventricular injection of 2 or 10 μg of proglumide significantly elevated plasma growth hormone concentrations in intact and castrated male rats and in ovariectomized females. Intravenous injections of 10 or 100 μg of proglumide were also effective in elevating growth hormone in a dose-related manner. Surprisingly, the response to the lower dose given intraventricularly was somewhat greater than that of the higher dose. We speculate that these stimulatory effects of proglumide given intraventricularly are due to the agonist action of proglumide at these doses since action of cholecystokinin itself is to increase plasma growth hormone following its intraventricular injection. The studies therefore do not establish a physiologically significant growth hormone-releasing action of brain cholecystokinin but provide more evidence that activation of cholecystokinin receptors in the brain can induce a stimulation of growth hormone release either by activation of the release of growth hormone-releasing hormone or by inhibition of the release of somatostatin or by a combination of these two actions.

Effects of postnatal treatment with naloxone on plasma gonadotropin prolactin, testosterone and testicular functions in male rats.

Opioid peptides are implicated in the control of gonadotropin and prolactin secretion. The role of opioid antagonist naloxone and its effects on plasma gonadotropin, prolactin, testosterone levels and testicular hyaluronidase, acid phosphatase, [3H]uridine and thymidine incorporation, RNA, DNA and protein concentrations were evaluated in rats after administration of naloxone beginning day 1 through 21 and autopsied on 45, 60 and 90 days of age. Plasma gonadotropin and testosterone levels were significantly elevated after naloxone treatment. Testicular hyaluronidase and acid phosphatase activity increased till 60 days post treatment and declined thereafter. Concentrations of RNA and protein did not change significantly but the concentration of DNA declined at 45 and 60 days of age. These results suggest that endogenous opioid peptides exert regulatory influence on gonadotropin secretion which in turn control the testicular function in the male rat.

Role of neurotransmitters and neuropeptides in the control of gonadotropin release: A review.

Rapid progress has been recorded recently in the understanding of the role of neurotransmitters and neuropeptides in the control of reproduction and on their apparent potential in the regulation of fertility. Peptides, as well as monoamines, are important in the control of lutinizing hormone releasing hormone and gonadotropin release. The input from brainstem noradrenergic neurons as well as dopamine mediated stimulated release of lutinizing hormone. In addition considerable evidence exist for the occurrence of a specific follicle stimulating hormone—releasing factor. A large number of brain peptides affect the secretion of lutinizing hormone releasing hormone and the endogenous opioid peptides appear to have a physiologically important function in restraining the influence on lutinizing hormone releasing hormone release under most circumstances. Vasoactive intestinal peptide and substance Ρ stimulate whereas cholecystokinin, neurotensin, gastrin, secretin, somatostatin, α-melanosite stimulating hormone and vasotocin inhibit lutinizing hormone release. Of the inhibitory peptides, cholecystokinin and arg-vasotocin are the most potent. Inhibin injected into the ventricle selectively suppresses follicle stimulating hormone release by a hypothalamic action. Thus the control of gonadotropin release is complex and a number of aminergic and peptidergic transmitters are involved.

Plasma gonadotropin, prolactin levels and hypothalamic tyrosine hydroxylase activity in rats during estrous cycle, after overiectomy and after blockade of catecholamine biosynthesis.

Plasma gonadotropin, prolactin levels and hypothalamic tyrosine hydroxylase activity were evaluated at 0900, 1200 and 1700 h during diestrus, proestrus and estrus, ovariectomized and after systemic administration of reserpine or α-methyl p-tyrosine, which interfere with catecholamine biosynthesis, in rats. Gonadotropin and prolactin levels showed peak values during the afternoon of proestrus, while hypothalamic tyrosine hydroxylase activity was markedly lowered at 1200 on proestrus. Gonadotropin levels were slightly lowered whereas prolactin concentrations and hypothalamic tyrosine hydroxylase activity were significantly increased by reserpine. Depletion of hypothalamic dopamine by reserpine apparently resulted in significant elevation of prolactin levels which inturn induce tyrosine hydroxylase. Gonadotropin levels and hypothalamic tyrosine hydroxylase activity were significantly suppressed after the administration of α-methyl p-tyrosine. Prolactin levels, however, were elevated significantly. These results indicate that catecholamines are involved in the control of gonadotropin and prolactin release during estrous cycle and inhibition of catecholamines biosynthesis by α-methyl p-tyrosine could result in suppression of gonadotropin levels, whereas removal of tonic inhibition of hypothalamic dopamine by α- methyl-p-tyrosine elevate prolactin levels.

Dopaminergic mediation of Y -aminobutyric acid in the control of prolactin release: Plasma prolactin and brain tyrosine hydroxylase levels in ovariectomized conscious rats.

The present experiments were carried out to further elucidate the mechanism by which dopamine mediates the actions of Y-aminobutyric acid on prolactin release from anterior pituitary following its intraventricular injection in overiectomized conscious rats, YAminobutyric acid significantly suppressed the prolactin levels at 0.1 μmol concentration while at 4 μmol dose, the level was elevated. The activity of tyrosine hydroxylase was increased significantly in the anterior pituitary at the lower dose while the higher concentration of Yaminobutyric acid did not bring about any change in the activity both in the hypothalamus and the anterior pituitary. The results presented suggest that intracellular dopamine in the anterior pituitary may directly inhibit prolactin release; the plasma prolactin level is elevated by Yaminobutyric acid, by way of either inhibiting dopaminergic tone or possible stimulation of a physiological prolactin releasin g hormone.