Biomedical subjects

L Vollrath

Publications and source records attributed to L Vollrath.

At least 19 recordsLinked to original sources

PubMed2002

Chronic exposure to a GSM-like signal (mobile phone) does not stimulate the development of DMBA-induced mammary tumors in rats: results of three consecutive studies.

Certain epidemiological and experimental studies raised concerns about the safety of radiofrequency (RF) electromagnetic fields because of a possible increased risk of leukemia and lymphoma. In this study, an RF field used in mobile telecommunication was tested using 7,12-dimethylbenz[a]anthracene (DMBA)-induced mammary tumors in female Sprague-Dawley rats as a model for human breast cancer. Three experiments were carried out under strictly standardized conditions and were started on the same day of three consecutive years. The field consisted of a GSM-like signal (900 MHz pulsed at 217 Hz, pulse width 577 micros) of relatively low power flux density (100 microW/cm(2) +/- 3 dB) and was applied continuously throughout each experiment to freely moving animals. The specific absorption rates averaged over the whole body were 17.5-70 mW/kg. The highest values in young animals were at or around the exposure limit permissible for the general public (i.e. 80 mW/kg). The animals were palpated weekly for the presence of mammary tumors and were killed humanely when tumors reached a diameter of 1-2 cm to allow a reliable histopathological classification and a distinction between malignant and benign subtypes. The overall results of the three studies are that there was no statistically significant effect of RF-field exposure on tumor latency and that the cumulative tumor incidence at the end of the experiment was unaffected as well. The risk ratios were 1.08 (95% CI: 0.91-1.29) and 0.96 (95% CI: 0.85-1.07) for benign and malignant tumors, respectively. These observations are in agreement with other published findings. In the first experiment, however, the median latency for the development of the first malignant tumor in each animal was statistically significantly extended for RF-field-exposed animals compared to controls (278 days compared to 145 days, P = 0.009). No such differences were detected in the two subsequent experiments. These results show that low-level RF radiation does not appear to possess carcinogenic or cancer-promoting effects on DMBA-induced mammary tumors. To explain the mechanisms underlying the different results obtained in the three experiments, a hypothesis is presented which is based upon the neuroendocrine control mechanisms involved in the promotion of DMBA-induced mammary tumors. Despite the apparent absence of stimulatory effects of low-level RF-field exposure on the development and growth of solid tumors, it will be necessary to verify these results for leukemias and lymphomas, which may have completely different biological control mechanisms.

9,10-Dimethyl-1,2-benzanthracene↗

PubMed2001

No effects of pulsed radio frequency electromagnetic fields on melatonin, cortisol, and selected markers of the immune system in man.

There is growing public concern that radio frequency electromagnetic fields may have adverse biological effects. In the present study eight healthy male students were tested to see whether or not radio frequency electromagnetic fields as used in modern digital wireless telecommunication (GSM standard) have noticeable effects on salivary melatonin, cortisol, neopterin, and immunoglobulin A (sIgA) levels during and several hours after exposure. In a specifically designed, shielded experimental chamber, the circularly polarized electromagnetic field applied was transmitted by an antenna positioned 10 cm behind the head of upright sitting test persons. The carrier frequency of 900 MHz was pulsed with 217 Hz (average power flux density 1 W/m2). In double blind trials, each test person underwent a total of 20 randomly allotted 4 hour periods of exposure and sham exposure, equally distributed at day and night. The results obtained show that the salivary concentrations of melatonin, cortisol, neopterin and sIgA did not differ significantly between exposure and sham exposure.

Adult↗

PubMed2001

Sulfhydryl G proteins and phospholipase A(2)-associated G proteins are involved in adrenergic signal transduction in the rat pineal gland.

The rat pineal gland with its circadian noradrenaline-regulated melatonin rhythm is an excellent model for studying adrenergic signal transduction with respect to cAMP and cGMP formation. The stimulatory G(s) proteins play a well-established role in this process. In contrast, the potential roles of the inhibitory G(i) proteins, the functionally unclear other G(o) proteins, and a number of G protein subtypes are not known. The present study examines the effects on beta(1)- and beta(1)-plus-alpha(1)-stimulated cAMP and cGMP formation of a number of G protein modulators in rat pinealocyte suspension cultures. The effects of the nitric oxide donor sodium nitroprusside on cGMP were also examined. The results showed that drugs that activate G proteins of the G(i)/G(o) family, i.e., pertussis toxin, mastoparan, and compound 48/80, had no effect on unstimulated, isoproterenol (beta(1))-stimulated, or combined isoproterenol/phenylephrine (beta(1)-plus()-alpha(1))-stimulated cAMP and cGMP accumulation. However, in this experimental paradigm, the inhibitors of sulfhydryl G proteins (N-ethylmaleimide) and those of phospholipase A2-related G proteins (isotetrandrine) exerted a clear inhibitory effect. Sodium-nitroprusside-stimulated cGMP accumulation was also inhibited. These results confirm a previous report that members of the G(i)/G(o) family, which are present in the rat pineal gland, do not play a major role in adrenergic signal transduction. The new finding that sulfhydryl G proteins and phospholipase A2-associated G proteins exert a clear stimulatory effect on adrenergic signal transduction suggests that they are subtypes of G(s) proteins.

Adrenergic alpha-Agonists↗

PubMed2001

Synaptic vesicle alterations in rod photoreceptors of synaptophysin-deficient mice.

The abundance of the integral membrane protein synaptophysin in synaptic vesicles and its multiple possible functional contributions to transmitter exocytosis and synaptic vesicle formation stand in sharp contrast to the observed lack of defects in synaptophysin knockout mice. Assuming that deficiencies are compensated by the often coexpressed synaptophysin isoform synaptoporin, we now show that retinal rod photoreceptors, which do not synthesize synaptoporin either in wild-type or in knockout mice, are affected by the loss of synaptophysin. Multiple pale-appearing photoreceptors, as seen by electron microscopy, possess reduced cytoplasmic electron density, swollen mitochondria, an enlarged cell surface area, and, most importantly, a significantly reduced number of synaptic vesicles with an unusually bright interior. Quantification of the number of synaptic vesicles per unit area, not only in these, but also in all other rod terminals of knockout animals, reveals a considerable reduction in vesicles that is even more pronounced during the dark period, i.e., at times of highest synaptic activity. Moreover, activity-dependent reduction in synaptic vesicle diameter, typically occurring in wild-type mice, is not detected in knockout animals. The large number of clathrin-coated pits and vesicles in dark-adapted synaptophysin knockout mice is taken as an indication of compensatory usage of synaptophysin-independent pathway(s), and, conversely, in view of the overall reduction in the number of synaptic vesicles, as an indication for the presence of another synaptophysin-dependent synaptic vesicle recycling pathway. Our results provide in vivo evidence for the importance of the integral membrane protein synaptophysin for synaptic vesicle recycling and formation.

Action Potentials↗

PubMed2001

Serine/threonine phosphatase inhibitors decrease adrenergic arylalkylamine n-acetyltransferase induction in the rat pineal gland.

Adrenergic regulation of the pineal enzyme serotonin N-acetyltransferase [arylalkylamine N-acetyltransferase (AA-NAT); EC 2.3.1.87] accounts for the circadian rhythm in melatonin formation. In the present study, the role of protein phosphatases in the adrenergic regulation of rat pineal AA-NAT was investigated using specific inhibitors. In cultured pineals, the serine/threonine phosphatase type 1 and type 2A inhibitors okadaic acid and calyculin A significantly decreased adrenergically or cAMP-induced AA-NAT activity, whereas the serine/threonine phosphatase type 2B inhibitor cypermethrin and tyrosine phosphatase inhibitor dephostatin were ineffective. Reverse transcriptase-polymerase chain reaction (RT-PCR) data indicate that okadaic acid exerts its effect on cAMP-dependent AA-NAT induction by downregulating the amount of AA-NAT transcript. The 'third' messengers, inducible cAMP early repressor (ICER) and Fos-related antigene-2 (Fra-2), are believed to play a negative role in pineal AA-NAT transcription. Okadaic acid increased the cAMP responsiveness of neither ICER mRNA nor Fra-2 mRNA. Therefore, the regulatory role of pineal serine/threonine phosphatases in adrenergically stimulated AA-NAT expression probably does not depend on ICER or Fra-2.

Animals↗

PubMed2001

Synaptic ribbon dynamics in photoreceptors of mice.

The present study deals with structural plasticity of a special type of chemical synapse, the ribbon synapse. Near the presynaptic membrane ribbon synapses contain conspicuous electron-dense synaptic bodies which appear mainly as rod-like profiles under the transmission electron microscope. In addition, club-shaped and spherical profiles may be present, the function of which is unclear. To gain some insight into the significance of the latter structures we studied their presence in rod-type photoreceptor cells of BALB/c mice under different lighting conditions. Quantification revealed that the club-shaped and the spherical profiles showed a clear light/dark dependence. They were virtually absent at night and increased strikingly in number when the animals were exposed to light. When darkness was extended into the morning, the profiles remained low in number. As the rod cells diminish their neurotransmitter release during the light phase, the present findings are interpreted as signs of synapse inactivation.

Animals↗

PubMed2000

A differential role of CREB phosphorylation in cAMP-inducible gene expression in the rat pineal.

In the rat pineal gland cAMP mediates nocturnal induction of the enzyme arylalkylamine N-acetyltransferase (AA-NAT) as well as of transcription factors such as inducible cAMP early repressor (ICER), Fos-related antigen-2 (Fra-2) and JunB. Cyclic AMP stimulates the phosphorylation of the DNA binding protein cAMP response element binding protein (CREB). While cAMP-induced CREB phosphorylation appears to be a prerequisite for AA-NAT and ICER gene expression, it is not known whether CREB phosphorylation accounts for the full cAMP response of the two genes. Furthermore, the significance of CREB phosphorylation in cAMP-activated Fra-2 and JunB transcription is unknown. In the present in vitro study we used the serine/threonine protein phosphatase inhibitor okadaic acid (OA) to phosphorylate CREB without altering intrapineal cAMP concentration. It was observed that OA (10(-7) M) was less effective than dibutyryl cAMP (dbcAMP; 10(-3) M) in inducing AA-NAT mRNA and ICER mRNA, respectively. On the basis of this finding, it is concluded that CREB phosphorylation alone is apparently not sufficient for the full cAMP response of the two genes. By contrast, OA and dbcAMP equally stimulated the accumulation of the mRNAs of Fra-2 and JunB. Therefore cAMP may induce Fra-2 and JunB transcripts via CREB phosphorylation. Our observations suggest that CREB phosphorylation plays a critical role in diversification of cAMP-dependent gene induction in the rat pineal.

Animals↗

PubMed2000

Single-cell recordings from chick pineal glands in vitro reveal ultradian and circadian oscillations.

Evidence is clear that each melatonin-producing cell in the chick pineal gland contains a circadian oscillator that continues to function in vitro, resulting in a prominent day/night rhythm of melatonin secretion. The aim of the present investigation was to examine whether the circadian organization of the gland has an electrophysiological correlate. To this end, single-cell recordings were made from isolated chick pineal glands kept in vitro under a light/dark cycle of 12:12 h, identical to that of the donors, or under continuous light or darkness. In all the glands investigated, a very small percentage of cells exhibited sodium-dependent spontaneous spike activity with a mean frequency below 10 Hz. The cells revealed rhythms with periods in the 15- to 60-min range and, additionally, exhibited ultradian and circadian rhythms in firing, with periods of 10.75+/-1.06 h and 26.25+/-1.26 h (mean +/- standard deviation), respectively. Most of the cells exhibited circadian rhythms with higher activity during daytime than at night, showing that the electrical activity and melatonin rhythm were out of phase. Under constant light or darkness, the circadian rhythm persisted. When the light/dark cycle of the donors was phase-advanced by 5 h, the cells revealed complete entrainment. We discuss whether the cells, albeit small in number, could function as a secondary ultradian/circadian oscillator contributing to the ultradian/circadian organization of the gland.

Action Potentials↗

PubMed2000

Transient synaptic ribbons in the mammalian retina at unusual sites.

In the vertebrate retina the presence of synaptic ribbons (SRs) is well documented in two sites only, viz., in photoreceptor axon terminals in the outer plexiform layer and in bipolar cell axons in the inner plexiform layer. The present paper reports the presence of non-photoreceptor SRs in the outer plexiform layer of cattle and mouse, where they were seen in small numbers in thin cell processes near cone pedicles of light-adapted animals. They were never seen near rod spherules. Quantitative data obtained in mice killed at different time-points revealed that the SRs under consideration increased in number during day time and were absent during the dark phase. Moreover, under high light intensity of 10000 lux they were more frequent in number compared to 100-lux-exposed animals. It is concluded that the cell processes revealing the temporary presence of SRs are processes of flat bipolar cells which may provide a feedback to cones during the light phase.

Adaptation, Ocular↗

PubMed1999

An intrinsic neuronal-like network in the rat pineal gland.

Recent studies have shown that in rat pineal glands kept in vitro action potential-producing cell clusters are demonstrable. To test whether the clusters interact, multiple-unit recordings were carried out simultaneously from different clusters, with or without electrical stimulation. Clusters with rhythmic burst activity exhibit highly synchronized firing and electrical stimulation of one cluster elicits an immediate response in another one, apparently involving synapses but not gap junctions. It is hypothesized that the interacting clusters form a network. As the firing is affected by norepinephrine, acetylcholine and Ca2+, the network may monitor the interstitial concentrations of these substances and spread this information in the gland, to modulate melatonin secretion.

Animals↗

PubMed1999

Circadian and seasonal changes of synaptic bodies in different parts of the rabbit pineal gland.

In the mammalian pineal gland, synaptic bodies (SBs) are poorly understood organelles. Previous studies in rabbits have shown that the organelles are rather heterogeneous in shape, are few in number during the day and increase in number at night. No studies are currently available on seasonal changes in this species and it is unknown whether the biological rhythms are identical in the proximal, intermediate and distal parts of the elongated pineal. To this end, a study was made of 84 rabbits kept under natural lighting conditions to examine numerical variations of the different types of SBs in the proximal, intermediate and distal regions of pineal glands procured at different timepoints of a 24-hour cycle and in each of the four annual seasons. In the present study, rod-like, sphere-like, ovoid, rectangular and triangular SB profiles were distinguished; the first two types being the most abundant. In addition to the well-known circadian changes, with low numbers of SB profiles during the day and high numbers at night, we found pronounced season-related differences as well as differences related to pineal regions. In autumn and winter, nighttime SR profile numbers were significantly higher than in spring and summer. With respect to regional differences it was found that the amplitude of the circadian rhythm increased in a proximo-distal direction in the gland. In autumn the strongly enhanced nocturnal increase was restricted to the distal region of the gland, whereas in winter it was seen in both the distal and the intermediate regions. The regional differences are probably related to the fact that the postganglionic sympathetic fibres, which regulate pineal function, enter the gland distally and proceed rostrally to the proximal region. Taken together, the results show that day- and nightlength are structurally coded in the pineal gland by means of SB numbers. Provided the SBs of the mammalian pineal gland are involved in synaptic processes, the results suggest that synaptic processes are enhanced at night as well as in autumn and winter.

Animals↗

PubMed1999

Ultrastructural changes of photoreceptor synaptic ribbons in relation to time of day and illumination.

PURPOSE: Electron microscopic sections through rod and cone ribbon synapses reveal mainly rodlike synaptic ribbon profiles, but a few unusual spherical and club-shaped profiles also occur. To elucidate the meaning of the latter two forms, the authors have investigated these ribbon synapses at different times during the 24-hour cycle and under various lighting conditions. METHODS: The various types of ribbon profiles were counted, and their sizes were measured by means of transmission electron microscopy in retinas of male BALB/c mice exposed to 12 hours light (lights on at 6 AM) and 12 hours dark (LD 12:12), continuous light, or continuous darkness for 4 days. RESULTS: A 24-hour study of mice exposed to LD 12:12 showed that spherical and club-shaped profile numbers ranged from 0% to 29%, depending on the time of day. They reached a maximum at 3 hours after light onset, followed by a gradual decrease to approach zero at night and reappearing after light onset the next morning. After 4 days of continuous light, the spherical profiles were significantly decreased in number (examined at 9 AM). After continuous darkness, the spherical and club-shaped profiles were significantly reduced in number. Administration of 4 hours of light after 92 hours of continuous darkness restored the number of spherical and club-shaped profiles to normal values. The rodlike ribbon profiles were found to be longer in darkness than in light. In rod terminals containing spherical profiles, the rodlike ribbon profiles were shorter than in terminals without spherical profiles. CONCLUSIONS. The club-shaped and the spherical profiles were related to the turnover of the synaptic ribbons. Soon after light exposure in the morning, the synaptic ribbons formed distal swellings, giving rise to club-shaped profiles and a decrease in length. The swellings appeared to bud off, thus forming spherical synaptic bodies. This article discusses whether these changes are signs of degradation of spent ribbons, or whether they play a physiological role related to the inactivation of the ribbon synapses after light exposure.

Animals↗

PubMed1998

Modulatory role of acetylcholine in the rat pineal gland.

The function of acetylcholine (ACh) in the mammalian pineal gland is unknown. To test the hypothesis that ACh exerts a modulatory role in this organ, in the present study electrophysiogical multiunit recordings were carried out in ex-vivo rat pineal glands superfused with different drugs. It was found that ACh (10(-7) M) as well as the cholinergic agonists oxotremorine (10(-7) M) and nicotine (10(-6) M) increased the discharge rates of most of the spontaneously active units and led to burst activity in previously regularly firing cells. It is concluded that ACh may play a modulatory role in the pineal by influencing the firing of a special population of pineal cells with perhaps receptor function.

Acetylcholine↗

PubMed1998

Demonstration of action-potential-producing cells in the rat pineal gland in vitro and their regulation by norepinephrine and nitric oxide.

There is evidence that sympathetically innervated mammalian pineal glands contain cells that exhibit action potentials. It is unknown whether ex vivo pineal glands deprived of their nervous input are still capable of firing. In the present study, multiple-unit recordings from rat pineals revealed spontaneously active cell clusters with a mean firing frequency of 1.5 +/- 0.3 Hz which could be abolished by tedrodotoxin. Regularly firing clusters showed no inherent periodicity in the minute range, whereas rhythmical clusters with periodically repeated bursts had period lengths of 12.6 min (day) and 9.5 min (night). Superfusion of norepinephrine reduced the firing frequency of both cluster types, or had no effect, and the rhythmical clusters became regular. The effects of norepinephrine appear to be mediated via beta-adrenoceptors because isoproterenol, a beta-agonist, had the same effect as norepinephrine and as the alpha-agonist phenylephrine was without effect. Evidence was obtained that the inhibitory effect of norepinephrine may be mediated by nitric oxide, which appears to affect the electrical discharge of the cells directly or indirectly through cGMP. It is discussed whether the spontaneously active pineal cells represent an intrapineal oscillator or are driven by an intrinsic oscillator which has yet to be defined.

8-Bromo Cyclic Adenosine Monophosphate↗

PubMed1998

Nitric oxide is formed in a subpopulation of rat pineal cells and acts as an intercellular messenger.

In the rat pineal, formation of the second messenger cyclic GMP (cGMP) is under adrenergic control. Two important sequential steps mediate adrenergic signal transduction by cGMP, receptor-stimulated nitric oxide (NO) formation by the enzyme NO synthase I (NOS I), and NO-induced cGMP formation by the cytosolic enzyme guanylyl cyclase. With regard to the first step in cGMP transduction (i.e. NO formation) we found, by means of NOS I immunostaining and NADPH-diaphorase staining, that the presence of NOS I was restricted to a subpopulation of pineal cells, generally surrounded by NOS I-negative cells. Considering the fact that NO is able to permeate the cell membrane, the question arises whether the second step in cGMP signalling (i.e. NO-induced cGMP formation) is restricted to the NO-forming cell itself or takes place in neighboring cells as well. It was found that oxyhemoglobin, which scavenges NO but does not enter cells, strongly decreased adrenergic cGMP response. This suggests that a considerable part of adrenergically induced NO stimulates cGMP formation in neighboring cells. Our findings indicate that in the rat pineal the components of adrenergic cGMP transduction are largely distributed among two types of cells: NOS I-containing pineal cells where adrenoceptor stimulation induces NO formation, and neighboring cells in which NO stimulates cGMP formation.

Animals↗

PubMed1997

Day-night rhythm of acetylcholine in the rat pineal gland.

Using high-performance-liquid-chromatography (HPLC) measurement of acetylcholine, choline acetyltransferase (ChAT) enzyme assay and anti-ChAT immunohistochemistry, we have investigated the expression of the cholinergic system in pineal glands of male rats. Glands procured during the day period (1200 h) contained significant amounts of acetylcholine (0.5 pmol/gland). A similar content was found in pineal glands after a 48 h culture period, i.e. when the intrapineal nerve fibres have degenerated. This strongly indicates that the pinealocytes are the cells which contain acetylcholine. To confirm this conclusion we demonstrate substantial ChAT-like immunoreactivity in pinealocytes. ChAT enzyme activity measured in homogenized glands (day period) was 7 +/- 3 nmol/mg per h. Acetylcholine content as well as ChAT enzyme activity increased about 10-fold in pineal glands during the night period (2400 h). The present study demonstrates for the first time the presence of a day-night rhythm of ChAT and acetylcholine in rat pinealocytes. The function of pineal acetylcholine is not clear, but there are indications that acetylcholine may depress noradrenaline release from intrapineal sympathetic fibres and hence melatonin synthesis.

Acetylcholine↗

PubMed1997

Nitric oxide inhibits electrically active units in the rat pineal gland.

Extracellular multiple unit recordings were performed in isolated rat pineal glands to determine a possible effect of nitric oxide (NO) on the spontaneous electrical activity of pinealocytes. Spontaneously active cells forming clusters of 3-5 cells fell into two categories: more or less regularly firing clusters (REG, 64%) and irregularly discharging clusters with periodically repeated bursts (RHY, 36%). The NO-donor sodium nitroprusside (SNP) reduced the discharge rate of the great majority of REG clusters and of all the RHY clusters examined. Moreover, the burst activity of RHY clusters was abolished. These results could be completely reproduced by using another NO-donor, S-nitroso-N-acetyl-penicillamine (SNAP). The NO synthase inhibitor NMLA had no effect on REG and RHY clusters. The results show that spontaneous electrical activity is an intrinsic function of the rat pineal gland. NO can modulate the electrical activity affecting discharge rate and discharge pattern.

Animals↗

PubMed1997

The shape of synaptic ribbons in the rat pineal gland.

Under the transmission electron microscope, synaptic ribbons (SRs) of the mammalian pineal gland appear as rod-like organelles. Their three-dimensional structure is not precisely known. In the present study, pineal SRs were investigated using serial sections obtained from rats killed at noon and midnight. The shape of the SRs was reconstructed based on SR profile length and the number of sections in which the profiles were contained. The results obtained show that SRs are basically flat plate-like structures with polymorphic lateral edges. Reconstructions of SRs revealed that they had average dimensions of 300x150x35 nm and were 19.3% larger at night than at day; the difference in SR size points to perhaps major differences in synaptic function between day and night.

Animals↗