PubMed Health⌕ Search

Biomedical subjects

J Vanecek

Publications and source records attributed to J Vanecek.

33 records · Page 2Linked to original sources
PubMed1995

Cellular mechanism of melatonin action in neonatal rat pituitary.

Melatonin inhibits gonadotropin-releasing hormone (GnRH)-induced luteinizing hormone (LH) release from neonatal rat anterior pituitary. Melatonin has been shown to decrease the concentration of several second messengers in neonatal pituitary, but it is not known which of them transduces the melatonin effect on LH release. In order to determine the mechanism of melatonin action, we tested the effect of melatonin on GnRH-induced LH release in the presence of specific drugs affecting second messengers. The calcium channel antagonist nifedipine inhibited LH release from cultured pituitary to a similar degree as melatonin and prevented the inhibitory effect of melatonin on LH release. The calcium channel agonist Bay K potentiated the GnRH-stimulated LH release and reduced the inhibitory effect of melatonin on LH release. These data strongly suggest that melatonin inhibits LH release via inhibition of calcium influx through voltage-sensitive channels. The cyclic AMP (cAMP) derivative 8-bromo-cAMP potentiated GnRH stimulation of LH release but did not prevent the melatonin-induced inhibition of LH release. However, when used in combination with Bay K, which reduced only partially the melatonin effect by itself, 8-bromo-cAMP completely blocked the melatonin effect. This observation suggests that decreased cAMP accumulation may also be involved in transduction of the melatonin effect on LH release.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1995

Vasoactive intestinal peptide elevates pinealocyte intracellular calcium concentrations by enhancing influx: evidence for involvement of a cyclic GMP-dependent mechanism.

Vasoactive intestinal peptide (VIP) receptor density is high in the pineal gland, which receives VIP innervation and responds to VIP with a relatively small increase in cAMP and cGMP levels. In the present study, we show that VIP (5-200 nM) treatment increased the intracellular calcium concentration ([Ca2+]i) in 64% of isolated individual pinealocytes; in comparison, norepinephrine (NE) elevated [Ca2+]i in 93% of the cells and produced more robust responses. Analysis of the role of second messengers indicated that [Ca2+]i was strongly elevated by cGMP analogs, but not by cAMP analogs. The nitric oxide-releasing agent S-nitro-N-acetylpenicillamine and 2,2-diethyl-1-nitroxyhydraxine also elevated [Ca2+]i. Investigation of the mechanisms revealed that responses to VIP or 8-bromo-cGMP involved Ca2+ influx, as did the plateau component of the response to NE; the large rapid component of the response to NE, however, appeared to reflect release from intracellular stores. Pharmacological studies indicated that the VIP-induced Ca2+ influx was mediated by a retinal rod-type cyclic nucleotidegated cation channel, expression of which was confirmed by reverse transcription-polymerase chain reaction analysis. These observations indicate that fundamentally different mechanisms generate the responses to NE and VIP. The dominant effect of VIP causing transient elevation of [Ca2+]i appears to be through cGMP gating aI-cis-diltiazem-sensitive rod-type cyclic nucleotide-gated cation channel. In contrast, the dominant effect of NE on [Ca2+]i is due to enhanced Ca2+ release from intracellular stores; the plateau component is due to influx through aI-cis-diltiazem-insensitive channel.

Animals↗
PubMed1994

Photoneural regulation of rat pineal nitric oxide synthase.

We report here a photoneural regulation of nitric oxide synthase (NOS) activity in the rat pineal gland. In the absence of the adrenergic stimulation following constant light exposure (LL) or denervation, pineal NOS activity is markedly reduced. A maximal drop is measured after 8 days in LL. When rats are housed back in normal light:dark (LD) conditions (12:12), pineal NOS activity returns to normal after 4 days. A partial decrease in pineal NOS activity is also observed when rats are placed for 8 days in LD 18:6 or shorter dark phases, indicating that pineal NOS activity reflects the length of the dark phase. Because it is known that norepinephrine (NE) is released at night from the nerve endings in the pineal gland and this release is blocked by exposure to light, our data suggest that NOS is controlled by adrenergic mechanisms. Our observation may also explain the lack of cyclic GMP response to NE observed in animals housed in constant light.

Amino Acid Oxidoreductases↗
PubMed1993

A subpopulation of neonatal gonadotropin-releasing hormone-sensitive pituitary cells is responsive to melatonin.

Melatonin partially inhibits the GnRH-induced elevation of intracellular free Ca2+ ([Ca2+]i) and depolarization of the plasma membrane of neonatal rat GnRH-responsive pituitary cells. This effect is lost during development. In the present study, this line of investigation was extended using single cell analysis. This revealed that melatonin does not alter basal [Ca2+]i in GnRH-responsive cells, but it does inhibit the effect of GnRH on [Ca2+]i in approximately 40% of these cells. Complete inhibition is seen in only approximately 11% of the GnRH-sensitive cells. Analysis of membrane potential also indicated that melatonin hyperpolarizes only a subpopulation of neonatal GnRH-sensitive cells and reverses GnRH-induced depolarization. In the absence of extracellular Ca2+, this effect was greater and more frequently observed. Simultaneous analysis of membrane potential and [Ca2+]i in individual GnRH-treated cells indicated that melatonin altered both parameters in the same cell. This is consistent with the hypothesis that melatonin decreases [Ca2+]i by hyperpolarizing the cell, thereby inhibiting Ca2+ influx through voltage-sensitive channels. The finding that melatonin only acts on a subpopulation of GnRH-responsive cells probably explains why melatonin partially reverse the effects of GnRH in mixed population studies. The existence of a specific melatonin-sensitive population of cells raises the possibilities that the developmental loss of melatonin sensitivity might reflect their selective death or the decreased expression of melatonin receptors in these cells. In addition, it is possible that melatonin-sensitive GnRH-responsive cells might have other remarkable features, such as secretion of a biologically active substance not produced by melatonin-insensitive GnRH-responsive cells.

Animals↗
PubMed1992

Melatonin inhibits gonadotropin-releasing hormone-induced elevation of intracellular Ca2+ in neonatal rat pituitary cells.

GnRH stimulates LH release by increasing intracellular Ca2+ ([Ca2+]i). Melatonin is known to inhibit GnRH-stimulated LH release from neonatal rat pituitary cells. In the present report, the issue of whether melatonin acts through [Ca2+]i was addressed. [Ca2+]i was studied in cells in suspension, using Fluo-3 as a fluorescent indicator. In neonatal rat pituitary cells, melatonin inhibited the GnRH-induced [Ca2+]i increase in a dose-dependent manner; the GnRH-induced increase in [Ca2+]i was inhibited 40% by 100 nM melatonin. The relative potencies of several indoles as inhibitors of the GnRH stimulation of [Ca2+]i in neonatal pituitary cells (2-iodo-melatonin greater than melatonin greater than 6-hydroxymelatonin) correlate with their known potencies to inhibit LH release and with their binding affinity to high affinity melatonin receptors, which indicates that these receptors probably mediate the effects of melatonin. Further support for this interpretation comes from the observation that melatonin does not inhibit the GnRH effect on [Ca2+]i in cells obtained from adolescent rat pituitary glands, which lack melatonin receptors and are insensitive to melatonin as an inhibitor of GnRH-stimulated LH release. The possible involvement of an inhibitory G-protein was also investigated by studying the effects of pertussis toxin. Pretreatment with pertussis toxin antagonized the effects of melatonin on [Ca2+]i and LH release. This indicates that melatonin may inhibit the GnRH-induced increase in [Ca2+]i through a mechanism involving a pertussis toxin-sensitive G-protein. To examine the role of extracellular Ca2+ in this effect, the effects of melatonin were examined in a low Ca2+ medium. Under these conditions, the effect of melatonin was markedly reduced, which indicates that melatonin may act by inhibiting Ca2+ influx. These observations indicate that melatonin inhibits GnRH stimulation of [Ca2+]i in neonatal rat gonadotrophs, and this probably explains the inhibitory action of melatonin on GnRH stimulation of LH release.

Aging↗
PubMed1992

Sodium-dependent effects of melatonin on membrane potential of neonatal rat pituitary cells.

Melatonin inhibits GnRH-stimulated release of LH from neonatal rat pituitary cells, probably by inhibiting GnRH-induced elevation of intracellular Ca2+. This effect of melatonin seems to involve inhibition of Ca2+ influx through voltage-sensitive channels. Accordingly, it is possible that melatonin could act by hyperpolarizing pituitary cells, which would close these channels. This issue was addressed here by determining if melatonin influences membrane potential. Membrane potential and intracellular Ca2+ were studied in neonatal rat pituitary cells in suspension, using bis-oxonol and Fluo-3 as fluorescent indicators, respectively. It was found that treatment with melatonin alone causes membrane hyperpolarization and that it has a repolarizing effect after GnRH-induced membrane depolarization. This effect on membrane potential appears to be mediated by high affinity melatonin receptors and a pertussis toxin-sensitive Na(+)-dependent mechanism; it is not dependent upon Ca2+, Cl-, or bicarbonate. This may be the molecular basis of action of melatonin in other tissues with high affinity melatonin receptors.

Aging↗
PubMed1990

Daily changes in melatonin binding sites and the effect of castration.

The effect of castration and/or neonatal administration of testosterone propionate (TP) on 125I-melatonin binding and its daily changes was studied in rat anterior pituitary (AP) and in pars tuberalis/median eminence (PT/ME). In animals kept on a light/dark cycle of 12:12 h there was a marked increase in binding site density (Bmax) in the evening as compared to the morning, while there were no differences in the affinity (Kd). On a light/dark cycle of 8:16 h the daily rhythm in Bmax was abolished and the values were intermediate. Neonatal TP administration which increases the sensitivity of the reproductive axis to melatonin and photoperiodic regulations had no effect on the binding parameters in AP and in PT/ME. Castration, however, increased binding site density in AP by 85% while it had no effect on the affinity of the binding site (Kd).

Animals↗
PubMed1990

Developmental changes and daily rhythm in melatonin-induced inhibition of 3',5'-cyclic AMP accumulation in the rat pituitary.

Melatonin's transduction mechanisms were investigated using in vitro cultured anterior hemipituitaries. Melatonin inhibited cAMP and 3',5'-cyclic GMP accumulation in neonatal rat anterior pituitary stimulated with LHRH. Maximal inhibitory effect was reached within 25 min and persisted for at least 20 min. Inhibition of cAMP accumulation is specific for melatonin because its analogs N-acetylserotonin and 5-methoxytryptamine are 1000 times less potent. Melatonin effect is age- and time-dependent. Marked inhibition was observed in 5-, 10-, and 14-day-old rats but not in 29-day-old ones. Melatonin was significantly more potent when applied at the end of the light period as compared with the first half of the day. Melatonin's effects on cAMP correlate with its effect on reproductive functions and on LH release. Cyclic nucleotides may thus serve as second messengers transducing the effect of melatonin on cellular level.

Aging↗
PubMed1989

Effects of melatonin on spontaneous electrical activity of neurons in rat suprachiasmatic nuclei: an in vitro iontophoretic study.

Circadian rhythms, endogenously generated in suprachiasmatic nuclei (SCN), seem to be under the direct influence of melatonin. Therefore, the effect of iontophoretically applied melatonin on electrical activity of SCN neurons was investigated in vitro. Usually, melatonin had an inhibitory effect. In the 3-h periods before (2.00-5.00 p.m.) or after (5.00-8.00 p.m.) the light-dark transition the percentage of SCN neurons sensitive to melatonin was very high (80% and 100%, respectively). However, efficacy of melatonin was low in the periods preceeding (20%) and following (33%) this 6-h time interval.

Action Potentials↗
PubMed1986

See-saw signal processing in pinealocytes involves reciprocal changes in the alpha 1-adrenergic component of the cyclic GMP response and the beta-adrenergic component of the cyclic AMP response.

Pineal cyclic AMP and cyclic GMP are regulated by norepinephrine (NE) acting through alpha 1- and beta-adrenoceptors. beta-Adrenergic stimulation appears to be an absolute requirement and alpha 1-adrenergic activation amplifies beta-adrenergic stimulation of the cyclic AMP response 10-fold and the cyclic GMP response 100-fold, respectively. Chronic deprivation of adrenergic stimulation, due to exposure to constant light (LL) or by surgical denervation, enhances the cyclic AMP response and diminishes the cyclic GMP response as compared to control animals in a 10:14 light/dark (LD) cycle. This phenomenon is termed see-saw signal processing. In the current study we find these changes do not reflect shifts in the time course or Ka of these responses. Dose-response studies indicate the beta-adrenergic component of cyclic AMP stimulation is enhanced and the alpha 1-adrenergic component of cyclic GMP stimulation is diminished in LL pinealocytes. Several observations indicate these changes may reflect alterations in Ca2+-sensitive postreceptor mechanisms.

Animals↗
PubMed1985

Atypical synergistic alpha 1- and beta-adrenergic regulation of adenosine 3',5'-monophosphate and guanosine 3',5'-monophosphate in rat pinealocytes.

The adrenergic control of cAMP and 3',5'-cyclic GMP (cGMP) in dispersed adult rat pinealocytes was investigated. Norepinephrine treatment increased cAMP and cGMP content 60- and 400-fold, respectively; both alpha- and beta-adrenoceptors had to be activated for these responses to occur. Beta-Adrenergic stimulation alone produced only about 6- and 2-fold increase in cAMP and cGMP content, respectively. Alpha-Adrenergic stimulation, which alone had no effect on either cyclic nucleotide concentration, markedly amplified the beta-adrenergic stimulation of both cAMP and cGMP. The relative potency of alpha-adrenergic agonists and antagonists indicates the alpha 1-subclass of adrenoceptors is involved. A role of alpha 1-adrenoceptors in the control of pineal cAMP is consistent with published evidence of the presence of alpha 1-adrenoceptors on pinealocytes and their role in the regulation of N-acetyltransferase activity and melatonin production.

Animals↗
PubMed1975

Sexual differences in the metabolism of salicylates.

During 6 h the plasma level of salicylates was followed after oral administration of aspirin. The plasma levels of salicylic acid were significantly lower in bull than in heifer calves during the whole observation period. The toxicity of the drug was higher in female animals.

Animals↗
PubMedDate not supplied

Activation of protein kinase C potentiates isoprenaline-induced cyclic AMP accumulation in rat pinealocytes.

The pineal gland has proven to be an excellent model for the study of adrenergic control systems. Noradrenaline, released from sympathetic nerve terminals in the pineal gland, regulates a large nocturnal increase in melatonin synthesis by stimulating the activity of arylalkylamine N-acetyltransferase (NAT, EC 2.3.1.87) 30-70-fold. An essential step in both the induction and maintenance of high NAT activity is an increase in intracellular cyclic AMP. Noradrenaline acts via beta-adrenoceptors to increase pineal cyclic AMP by activating adenylate cyclase, and the activation of pineal alpha 1-adrenoceptors potentiates beta-adrenergic stimulation not only of NAT but of both cyclic AMP and cyclic GMP. Here we describe investigations designed to test whether alpha 1-adrenergic potentiation of beta-adrenergic stimulation of pineal cyclic AMP involves protein kinase C. Our results suggest that kinase activation is involved and the data provide the first demonstration of a synergistic interaction between Ca2+-phospholipid-dependent protein kinase (protein kinase C) and neurotransmitter-dependent stimulation of cyclic AMP.

Animals↗
PubMedDate not supplied

Melatonin inhibits GnRH-induced Ca2+ mobilization and influx through voltage-regulated channels.

In neonatal rat gonadotrophs, melatonin acts through the high-affinity membrane-bound receptors to inhibit GnRH-induced [Ca2+]i increase. GnRH increases [Ca2+]i primarily by mobilization from the inositol trisphosphate-sensitive pool followed by Ca2+ influx through the voltage-sensitive channels. Melatonin inhibits the GnRH-induced [Ca2+]i increase. When added after the GnRH-induced spike, melatonin decreases [Ca2+]i in 52% of the gonadotrophs. The effect of melatonin is dependent on extracellular Ca2+ and may be mimicked by Ca2+-free medium or verapamil. When added before GnRH, melatonin inhibits the [Ca2+]i spike. This effect of melatonin is independent of extracellular Ca2+ as it persists in Ca2+-free medium. These findings indicate that melatonin blocks Ca2+ mobilization as well as Ca2+ influx in the gonadotrophs.

Animals↗