Biomedical subjects

M I Vacas

Publications and source records attributed to M I Vacas.

At least 19 recordsLinked to original sources

PubMed1992

Binding sites for [3H]-melatonin in human platelets.

A number of in vitro effects of melatonin on human platelets were revealed in previous studies. In order to examine whether high affinity binding sites for [3H]-melatonin were present in membrane preparations of human platelets, a rapid filtration procedure through Whatman GFB paper was employed. Maximal melatonin binding was attained within 3 hr at 0 degree C. Scatchard analysis indicated a single population of binding sites with a dissociation constant (Kd) = 4.1 +/- 0.5 nM and maximal number of binding sites (Bmax) = 24.2 +/- 1.9 fmol/mg protein (mean +/- SEM of five experiments). When various indole analogs were tested for their ability to inhibit [3H]-melatonin binding, the following Ki (nM) were obtained: 6-chloromelatonin (11.4), 2-iodomelatonin (22.0), melatonin (24.7), 5-methoxytryptophol (49.9), N-acetylserotonin (68.9), 6-hydroxymelatonin (78.2), 5-methoxytryptamine (184). Serotonin was a potent inhibitor of [3H]-melatonin binding with a Ki = 20.6 nM. Except for 2-methylserotonin and alpha-methylserotonin, a number of serotonin agonists and antagonists tested did not affect melatonin binding to platelet membranes. Binding experiments carried out at either 0800 or 2000 did not reveal time-dependent differences in Kd or Bmax. The results suggest that high affinity melatonin acceptors are present in human platelets.

Adult↗

PubMed1990

Increase in adrenocorticotropin release during wallerian degeneration of peripheral sympathetic neurons after superior cervical ganglionectomy of rats.

The changes in adrenocorticotropin (ACTH) release before, during and after sympathetic nerve degeneration following superior cervical ganglionectomy (SCGx) were examined in male rats. A 12-fold increase of circulating ACTH was found in both SCGx and sham-operated rats 6 h after surgery. In sham-operated rats, plasma ACTH decreased by about half 16-22 h after surgery, whereas in SCGx rats it remained at a high concentration from 16 to 54 h after surgery, attaining basal values by 120 h post-SCGx. In SCGx rats, MBH corticotropin-releasing hormone (CRH) content decreased significantly from 16 to 54 h after surgery, while in controls it remained unmodified. Significantly smaller arginine vasopressin (AVP) contents were found in MBH of SCGx rats as compared to sham-operated controls, 16-54 h after surgery. In rats exposed to ether or immobilization stress 22 h after SCGx, plasma ACTH levels were significantly higher than in controls; however, since unstressed ACTH levels were about twice as high in SCGx rats, the percent increase of ACTH was smaller in the SCGx group. A decreased response of plasma ACTH to ether or immobilization stress was found in rats 7 days after SCGx. In rats subjected to a simultaneous adrenalectomy (Adx) and SCGx or sham-SCGx, plasma ACTH levels increased to a similar extent in both groups. ACTH increase after Adx was accompanied by decreases in MBH CRH, and absence of significant changes in MBH AVP contents. Rats subjected to pinealectomy (Px) or sham-Px 1 week earlier and killed 22 h earlier exhibited similar responses in plasma ACTH and MBH CRH to SCGx regardless of pineal intactness.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenocorticotropic Hormone↗

PubMed1990

Inhibition of human platelet aggregation and thromboxane-B2 production by melatonin: evidence for a diurnal variation.

The effects of melatonin on platelet aggregation and thromboxane-B2 (TxB2) production induced by 1-4 x 10(-6) M adenosine diphosphate (ADP) or 0.6 x 10(-3) M arachidonic acid (AA) were assessed in platelet-rich plasma (PRP). Micromolar concentrations of melatonin inhibited in a dose-dependent way ADP-induced platelet aggregation with individual inhibitions 40% or more at 10(-6)-10(-5) M. A significant depression of AA-induced platelet aggregation was observed only at 10(-5)-10(-4) M melatonin. Morning (0830 h)-evening (1800 h) studies of ADP-induced platelet aggregation in seven normal men showed a higher sensitivity at 1800 h when analyzed as a global inhibitory effect of melatonin (P less than 0.01). Moreover, only during the evening hours did melatonin induce reversible aggregation, an index of inhibition of the platelet secretory process elicited by ADP exposure. No diurnal variability in melatonin inhibition of AA-induced aggregation was detected. TxB2 production elicited by AA in the evening was inhibited significantly in a concentration-related manner by a 2-min preincubation with 10(-9)-10(-5) M melatonin, while during the morning hours the inhibition was significant only at 10(-6) M or higher melatonin concentrations. In the case of ADP, the inhibition of TxB2 release attained significance at 10(-5)-M (0830 h) or 10(-6)-M concentrations (1800 h). In the presence of either stimulatory agent, melatonin depression of TxB2 generation was about 2-fold greater at 1800 h than at 0830 h. The diurnal changes in melatonin effect on TxB2 production were also observed in thrombin-stimulated washed platelets. The present data indicate the existence of circadian variations in platelet responsiveness to melatonin in humans.

Adult↗

PubMed1990

Diurnal variation in melatonin effect on adenosine triphosphate and serotonin release by human platelets.

The effect of the pineal hormone melatonin on adenosine diphosphate-induced human platelet aggregation and adenosine triphosphate release was assessed in platelet-rich plasma obtained from normal volunteers at 08.30 and 20.30 h. In 10(-7)-10(-5) mol/l concentrations melatonin inhibited ADP-induced platelet aggregation only in the evening (p less than 0.05). ADP-induced ATP release, an index of platelet secretory processes, showed a generally greater, dose-dependent inhibition after adding melatonin (10(-9)-10(-5)mol/l) at 20.30 h as compared with 08.30 h. The inhibitory activity of melatonin (10(-9)-10(-5) mol/l) on [3H]serotonin release elicited by thrombin in washed human platelets obtained from normal volunteers was dose-dependent; the effect was generally greater at 20.30 h. The activity of the potent platelet anti-aggregating agent prostacyclin did not exhibit diurnal differences with respect to impairing ADP-induced platelet-rich plasma aggregation. These results indicate the existence of a diurnal variation of sensitivity to melatonin in human platelets.

Adenosine Diphosphate↗

PubMed1989

Effect of adenosine on melatonin and norepinephrine release in rat pineal explants.

To study adenosine effect on melatonin production, rat pineal explants were incubated for 6 h with 10(-4) M adenosine or 2-chloroadenosine in the presence or absence of 5 x 10(-5) M norepinephrine (NE). Melatonin content in pineal gland and medium was measured by RIA. Both adenosine and 2-chloroadenosine increased melatonin production rate by 3-4-fold, and augmented NE stimulation by 30-40%. Addition of adenosine deaminase prior to NE reduced stimulated melatonin release by 40-46%. 2-Chloroadenosine counteracted the inhibition of NE response given by adenosine deaminase. Either adenosine or its A1 receptor agonist cyclohexyl adenosine (CHA) decreased by 20-22% 3H-transmitter release induced by a K+ depolarizing stimulus in rat pineal incubated with 3H-NE. These results suggest that adenosine affects both pre- and postsynaptic pineal mechanisms.

2-Chloroadenosine↗

PubMed1987

Melatonin, 5-methoxytryptamine and some of their analogs as cyclo-oxygenase inhibitors in rat medial basal hypothalamus.

Melatonin, and its analogs 6-chloro- and 6-fluoromelatonin inhibited in a dose-dependent way (10(-8)-10(-5) M) labeled prostaglandin (PG) E2, PGF2 alpha, thromboxane (Tx) B2 and 6-keto-PGF1 alpha production from [14C]arachidonate by rat medial basal hypothalamus (MBH). 5-Methoxytryptamine also depressed arachidonate metabolism; at 10(-8) M concentrations the effect of 5-methoxytryptamine on PGE2, PGF2 alpha and TxB2 synthesis (93-96% inhibition), and 6-keto-PGF1 alpha (75% inhibition) was greater than that observed for melatonin (51-56% and 44% inhibition, respectively). Neither 6-hydroxymelatonin nor serotonin affected MBH cyclo-oxygenase pathway in vitro.

5-Methoxytryptamine↗

PubMed1987

In vitro effect of neuropeptide Y on melatonin and norepinephrine release in rat pineal gland.

1. To study neuropeptide Y (NPY) effect on melatonin production, rat pineal explants were incubated for 6 hr with 10-1,000 nM NPY in the presence or absence of 10 microM norepinephrine (NE). Melatonin content in the pineal gland and media was measured by radioimmunoassay (RIA). 2. NPY (10-1,000 nM) increased melatonin production and, at 10 or 100 nM concentrations (but not 1,000 nM), enhanced NE stimulation of melatonin production. 3. NPY (1,000 nM) impaired 3H-labeled transmitter release induced by a K+ depolarizing stimulus in rat pineals incubated with 3H-NE. 4. These results suggest that NPY affects both pre- and postsynaptic pineal mechanisms.

Animals↗

PubMed1987

Cellular and molecular mechanisms controlling melatonin release by mammalian pineal glands.

1. The pineal gland is regulated primarily by photoperiodic information attaining the organ through a multisynaptic pathway initiated in the retina and the retinohypothalamic tract. 2. Norepinephrine (NE) released from superior cervical ganglion (SCG) neurons that provide sympathetic innervation to the pineal acts through alpha1- and beta 1- adrenoceptors to stimulate melatonin synthesis and release. 3. The increase in cyclic AMP mediated by beta 1-adrenergic activation is potentiated by the increase in Ca2+ flux, inositol phospholipid turnover, and prostaglandin and leukotriene synthesis produced by alpha 1-adrenergic activation. 4. Central pinealopetal connections may also participate in pineal control mechanisms; transmitters and modulators in these pathways include several neuropeptides, amino acids such as gamma-aminobutyric acid (GABA) and glutamate, and biogenic amines such as serotonin, acetylcholine, and dopamine. 5. Secondary regulatory signals for pineal secretory activity are several hormones that act on receptors sites on pineal cells or at any stage of the neuronal pinealopetal pathway.

Adrenergic Fibers↗

PubMed1987

Neuroendocrine integrative mechanisms in mammalian pineal gland: effects of steroid and adenohypophysial hormones on melatonin synthesis in vitro.

The time course for the decrease in norepinephrine concentration of rat pineal explants in culture indicated a significant fall starting at the 4th hour and completed after 16-24 h of incubation. Significant decreases of serotonin and 5-hydroxyindoleacetic acid (HIAA) levels in tissue, an increase of HIAA/serotonin ratio, and an increase of melatonin production rate in vitro were also observed as a function of the incubation time. Estradiol (10(-7)-10(-5) M) increased rat pineal melatonin content, testosterone (10(-5) M) decreased it and progesterone was devoid of activity when incubated with explants for up to 6 h. The in vitro stimulatory effect of estradiol on rat pineal methoxyindole synthesis was blocked by propranolol but not by phentolamine; propranolol also blocked the increase of nuclear estradiol-receptor complex produced by estrogen exposure of pineal explants. TSH (1-100 ng/ml), growth hormone (10-100 ng/ml) and LH (10 ng/ml) augmented rat pineal melatonin content while 100 ng/ml of FSH decreased it significantly. Prolactin exerted a biphasic effect on rat pineal explants, the lowest concentration augmenting melatonin content while the high concentration depressed it. Deep, intermediate and superficial segments of guinea-pig pineal glands showed an increase in melatonin concentration after a 6-h incubation in the presence of 10(-7)-10(-5) M estradiol.

Animals↗

PubMed1987

In vitro effects of adenohypophysial hormones on rat pineal melatonin content and release.

The effect of adenohypophysial hormones on rat pineal melatonin content and release was examined in vitro. Medium concentration of radioimmunoassayable melatonin decreased after a 6 h exposure to 1-100 ng/ml FSH; pineal levels of melatonin were only decreased by 100 ng/ml FSH. LH (1-100 ng/ml) augmented significantly medium melatonin concentration, tissue levels being increased at 10 ng/ml LH. Parallel increases of explant and medium melatonin content were found after exposure to 1-100 ng/ml TSH. At the smallest concentration employed (1 ng/ml) prolactin increased melatonin content and release while at 100 ng/ml a significant depression of both parameters was found. Growth hormone (1-10 ng/ml) augmented melatonin levels in medium but failed to modify them at 100 ng/ml, although at this concentration tissue melatonin levels increased. ACTH did not modify pineal melatonin synthesis in vitro.

Adrenocorticotropic Hormone↗

PubMed1987

Involvement of 5-lipoxygenase pathway in norepinephrine stimulation of rat pineal melatonin synthesis.

The effect of lipoxygenase inhibition, leukotriene agonists and antagonists, and 5-hydroxy-6,8,11,14-eicosatetraenoic acid (5-HETE) was examined in the rat pineal gland in organ culture. To study melatonin secretion pineal explants were incubated for 6 h in tissue culture medium 199 with the different drugs. Melatonin concentration in the pineal gland and the medium was measured by RIA. Exposure of explants to norepinephrine (NE) brought about a 2- to 5-fold increase in both parameters, an effect that was reduced but not abolished, by the lipoxygenase inhibitor nordihydroguaiaretic acid (NDGA; 10(-5) M). Lilly 171883 (10(-5) M) or FPL 55712 (10(-5) M; both antagonists of leukotrienes) reduced NE-induced melatonin production. Neither NDGA nor Lilly 171883 affected melatonin production in the absence of NE. Leukotrienes C4 and D4 increased melatonin release to the media at all concentrations tested (1-1,000 nM) with a maximum effect at 1 nM (leukotriene C4) and 10 nM (leukotriene D4). Significantly higher tissue melatonin concentrations as compared to controls were observed after exposure of pineal explants to 1 and 100 nM of leukotriene C4, or 100 nM of leukotriene D4. Another 5-lipoxygenase metabolite, 5-HETE, increased pineal melatonin content at concentrations of 1, 10 and 100 nM whereas only 1,000 nM stimulated melatonin release. These results suggest that the 5-lipoxygenase pathway plays a significant role in NE-stimulated melatonin production by the rat pineal gland.

Acetophenones↗

PubMed1987

Prolactin inhibition by p-tyramine in the male rat: site of action.

In a previous report, a consistent hypoprolactinemic effect of p-tyramine was observed in male rats under several experimental conditions in vivo. In the present experiments the action of p-tyramine on PRL release in vitro, or after challenge with different hyperprolactinemic drugs (serotonin, morphine, and TRH) was tested. Furthermore the participation of octopamine, a metabolite of tyramine, was evaluated with regard to the hypoprolactinemic action of the amine. P-Tyramine inhibited PRL release from hemipituitaries incubated in vitro at doses of 10(-4) and 10(-6) M (inhibition to 31% and 59% of control values, respectively). When tested for its ability to displace [3H]spiperone binding in vitro to a crude fraction of anterior pituitary membranes it was found that it did not compete with the D2 receptor labeled by [3H]spiperone, even at the concentration of 10(-4) M. P-Tyramine (40 mg/kg) antagonized the elevation of serum PRL levels by morphine, serotonin, and TRH. On the other hand, octopamine, which is formed from tyramine, also inhibited high PRL values found after stress, though the effective dose was higher than that of tyramine. Pretreatment with diethyldithiocarbanic acid, which inhibits conversion of p-tyramine to octopamine, did not modify the effect of tyramine in stress. The present results indicate that tyramine can inhibit PRL release due to certain drugs, by acting directly at the pituitary level. It does not displace [3H]spiperone binding from anterior pituitary membranes, and octopamine which lowers PRL release itself, cannot account for the effect of tyramine.

Animals↗

PubMed1986

Changes in growth hormone and prolactin release after superior cervical ganglionectomy of rats.

Superior cervical ganglionectomy (SCG X) decreased significantly serum growth hormone (GH) levels in rats 14-96 h after surgery, during and immediately after anterograde degeneration of regional sympathetic terminals. At later times (up to 28 days after SCG X) an increase in serum GH was observed. SCG X augments prolactin (PRL) release, but only at the earliest time examined (14 h after surgery). Injection of the alpha-adrenoceptor blocker, phenoxybenzamine, but not of the beta-blocker, propranolol, negated the depression in serum GH found in SCG X rats 14 h after surgery, without affecting PRL release.

Animals↗

PubMed1986

Origins of the sympathetic projections to rat thyroid and parathyroid glands.

The present study aimed to characterize the localization and pathways of sympathetic neurons innervating the thyroid and parathyroid glands. In rats subjected to unilateral superior cervical ganglionectomy or unilateral decentralization of the superior cervical ganglion 7 days earlier, ipsilateral depression of thyroid norepinephrine (NE) and epinephrine content to 6-16% of the contralateral intact lobe was observed. In both groups of animals neuronal [3H]NE uptake by the ipsilateral thyroid lobe was suppressed. In unilaterally decentralized rats pineal catecholamine levels remained within normal values whereas in unilaterally ganglionectomized rats a 74% decrease of pineal NE and E content was found. Unilateral section of the external carotid nerve abolished, as did unilateral superior cervical ganglionectomy, neuronal [3H]NE uptake in the ipsilateral thyroid lobe. In contrast external carotid nerve section did not modify the neuronal uptake of [3H]NE in the pineal gland. Either unilateral superior cervical ganglionectomy or external carotid nerve section resulted in similar involution of ipsilateral thyroid lobes of hypophysectomized rats. These results indicate that postganglionic sympathetic perikarya innervating the thyroid-parathyroid territory are located in the middle and/or inferior cervical ganglia and send their axons through the SCG and the external carotid nerve to these glands.

Animals↗

PubMed1986

In vitro effects of estradiol, testosterone, and progesterone on 5-methoxyindole content, cyclic adenosine 3',5'-monophosphate synthesis, and norepinephrine release in different parts of the female guinea pig pineal complex.

To examine the effects of estradiol, testosterone, or progesterone on cyclic adenosine 3',5'-monophosphate (AMP) accumulation, 5-methoxyindole levels, and norepinephrine (NE) release by the female guinea pig pineal complex, samples of the deep, intermediate, or superficial portions of the complex were incubated in vitro with varied concentrations of either hormone. Exposure for 10 minutes to physiological amounts of estradiol (10 nM) or to 100 microM NE increased significantly cyclic AMP levels to the same extent in the three pineal regions. A maximal effect on cyclic AMP accumulation was observed at 100-nM concentrations of estradiol, with a tendency to return to basal levels at 1-10 microM of estradiol. Only high concentrations of testosterone or progesterone (i.e., 1-10 microM) increased cyclic AMP accumulation in incubated guinea pig pineal fragments. At a 100-nM concentration estradiol did not affect NE-stimulated cyclic AMP accumulation in guinea pig pineal fragments. In samples of either pineal region incubated for 6 hours in TC 199 medium with 10(-7) M or greater concentrations of estradiol and analyzed for melatonin, 5-methoxyindoleacetic acid, and 5-methoxytryptophol by high-pressure liquid chromatography with electrochemical detection, a significant increase of melatonin levels was found. Neither testosterone nor progesterone modified 5-methoxyindole levels of incubated explants. K+-stimulated transmitter release from guinea pig pineal fragments previously incubated with 3H-NE was not affected by hormone exposure. These results suggest that physiological concentrations of estradiol may exert a postsynaptic stimulation of cyclic AMP and melatonin synthesis to the same extent in all three regions of the female guinea pig pineal complex.

Animals↗

PubMed1985

Interaction between alpha- and beta-adrenoceptors in rat pineal adenosine cyclic 3',5'-monophosphate phosphodiesterase activation.

The adrenergic regulation of the low-Km pineal cAMP phosphodiesterase (PDE) activity was studied in adult female rats. PDE activity showed a transient enhancement (up to 42%) during the process of degeneration of pineal sympathetic nerve terminals that followed superior cervical ganglionectomy (SCGx), thus confirming the neural modulation of the enzyme. Treatment with isoproterenol (0.3-5.0 mg/Kg) increased significantly PDE activity within 2 hours. Phenylephrine induced a significant increase of pineal PDE only at a 10 mg/Kg dose, while at a lower dose (1 mg/Kg) it potentiated the stimulatory effect of isoproterenol. Treatment of pineal organ cultures with 100 microM propranolol inhibited norepinephrine (NE)-induced PDE activity while 100 microM phentolamine had no significant effect. Propranolol at doses unable to alter the in vitro NE-induced stimulation of pineal PDE activity (1 microM), antagonized such NE effect when used in combination with 1 microM phentolamine. At equimolecular concentrations (1 microM) the mixed alpha-beta-adrenergic agonist NE was more effective than the beta-adrenergic agonist isoproterenol to increase pineal PDE in vitro. These results suggest an alpha-beta-adrenergic interaction in the sympathetic modulation of low-Km PDE activity of rat pineal gland.

3',5'-Cyclic-AMP Phosphodiesterases↗

PubMed1984

Acute superior cervical ganglionectomy depresses the postcastration rise of gonadotropins in male rats.

In male rats, superior cervical ganglionectomy (SCGx) delayed the rise of serum FSH and LH induced by orchidectomy by 24 h. SCGx resulted in a decrease of median eminence norepinephrine (NE) content 16 h after surgery and in an increase of medio-basal hypothalamic cAMP synthesis and receptor occupancy. These data indicate that NE release from degenerating terminals originating in the superior cervical ganglion neurons modifies the regulatory mechanisms controlling the rise of gonadotropins after orchidectomy in rats.

Animals↗

PubMed1982

Testosterone decreases beta-adrenoceptor sites in rat pineal gland and brain.

Testosterone administration to orchidectomized rats brought about a significant, 55% decrease of beta-adrenoceptor sites in the pineal gland, assessed from the specific binding of radioactive dihydroalprenolol (DHA). The changes in density of binding sites were not accompanied by significant modifications of the Kd. FSH or LH treatment of acutely castrated animals did not affect pineal beta-adrenoceptor binding. The depressive effects of testosterone in beta-adrenergic receptors were also observed in crude membrane fractions of medial basal hypothalamus and cerebral cortex. Sympathetic denervation of the pineal gland by superior cervical ganglionectomy did not abolish the changes in pineal beta-adrenoceptor density caused by testosterone. Hormone effects did not depend on a direct effect of the hormone on beta-adrenoceptor sites because testosterone did not compete with [3H]-DHA for the binding sites, in vitro. These results suggest that testosterone depresses pineal beta-adrenergic sites by acting mainly on postsynaptic sites.

Animals↗