Lutz Vollrath

Effects of an artificial magnetic field on serotonin N-acetyltransferase activity and melatonin content of the rat pineal gland

SummaryIn the present study the effects of artificial magnetic fields on pineal serotonin-N-acetyltransferase (NAT) activity and melatonin content in male Sprague-Dawley rats were investigated to study the secretory activity of the pineal gland. Experimental inversion of the horizontal component of the natural magnetic field, performed at night-time, led to a significant decrease of both parameters investigated. During day-time, this effect was less conspicuous. During night-time, inversion of the horizontal component is followed by a reduced pineal secretory activity for about 2 h. After 24 h exposure to the inverted horizontal component, return to the natural condition was followed by a renewed clear depression of pineal NAT activity and melatonin content, indicating that the main stimulus is not the inverted magnetic field itself but rather its change. Changing the inclination of the local magnetic field from 63 ° to 58 °, 68 ° or 78 °, respectively also decreased the secretory activity of the rat pineal gland.

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.

Evidence for the involvement of the visual system in mediating magnetic field effects on pineal melatonin synthesis in the rat

In order to elucidate whether magnetic field effects on mammalian pineal function are direct, or instead indirect via retinal disturbances, acutely blinded and intact male rats were subjected to a single nocturnal magnetic stimulus. Then pineal N-acetyltransferase activity and melatonin content were assayed. Only in intact animals did the magnetic stimulus significantly reduce pineal activity, i.e. no effects were detected in blinded animals. These data point to a retinal magnetosensitivity which may serve to modulate pineal gland function.

Melatonin inhibits cyclic AMP and cyclic GMP accumulation in the rat pituitary

Subnanomolar concentrations of melatonin inhibit cyclic AMP and cyclic GMP accumulation in neonatal rat anterior pituitary stimulated in vitro with luteinizing-hormone releasing-hormone. Melatonin also inhibited forskolin-stimulated cyclic AMP accumulation in pars tuberalis. Inhibition of cyclic AMP accumulation is specific for melatonin, since its analogs N-acetylserotonin and 5-methoxytryptamine are 1000 times less potent. Cyclic nucleotides may thus serve as second messengers transducing the effect of melatonin on cellular level.

Plasticity of retinal ribbon synapses

Ribbon synapses differ from conventional chemical synapses in that they contain, within the cloud of synaptic vesicles (SVs), a specialized synaptic body, most often termed synaptic ribbon (SR). This body assumes various forms. Reconstructions reveal that what appear as rod‐ or ribbon‐like profiles in sections are in fact rectangular or horseshoe‐shaped plates. Moreover, spherical, T‐shaped, table‐shaped, and highly pleomorphic bodies may be present. In mammals, ribbon synapses are present in afferent synapses of photoreceptors, bipolar nerve cells, and hair cells of both the organ of Corti and the vestibular organ. Synaptic ribbons (SRs) are also found in the intrinsic cells of the third eye, the pineal gland, and in the lateral line system. The precise function of SRs is enigmatic. The prevailing concept is that SRs function as conveyor belts to channel SVs to the presynaptic membrane for neurotransmitter release by means of exocytosis. The present article reviews the evidence that speaks for a plasticity of these organelles in the retina and the third eye, as reflected in changes in number, size, shape, location, and grouping pattern. SR plasticity is especially pronounced in the mammalian and submammalian pineal gland and in cones and bipolar cells of teleost fishes. Here, SR number and size wax and wane according to the environmental lighting conditions. In the pineal SR numbers increase at night and decrease during the day. In teleost cones, SRs are in their prime during daytime and decrease or disappear at night, when transmitter release is enhanced. In addition to numerical changes, SRs may also exhibit changes in size, shape, grouping pattern, and location. In the mammalian retina of adults, in contrast to the developing retina, the reported signs of SR plasticity are subtle and not always consistent. They may reflect changes in function or may represent signs of degradation. To distinguish between the two, more detailed studies under selected experimental conditions are required. Probably the strongest evidence for SR plasticity in the mammalian retina is that in hibernating squirrels SRs leave the synaptic site and accumulate in areas as far as 5 μm from the synapse. Changes in shape include the occurrence of club‐shaped SRs and round SRs or synaptic spheres (SSs). SSs may represent a special type of synaptic body, yet belonging to the family of SRs, or may be related to the catabolism of SRs. SR number is regulated by Ca2+ in teleost cones, whereas in the mammalian pineal gland cGMP is involved. An interesting biochemical feature of ribbon synapses is that they lack synapsins. The presently reviewed results suggest to us that SRs do not primarily function as conveyor belts, but are devices to immobilize SVs in inactive ribbon synapses.

Über die neurosekretorische Innervation der Adenohypophyse von Teleostiern, insbesondere von Hippocampus cuda und Tinca tinca

SummaryThe present study deals with the neurosecretory innervation of the adenohypophysis in teleost fishes.In Hippocampus cuda neurosecretory fibres (containing elementary granules of 1300 to 1500 Å, 1000 Å, and 500–600 Å diameter respectively) penetrate deeply into Pars intermedia and Pars distalis. All types of neurosecretory fibre make direct contacts with hormone producing cells. Two types of contact have been observed: 1. Direct contacts between neurosecretory fibres and endocrine cells which differ from synaptic contacts in that they show no thickening of the presynaptic membrane and no accumulation of synaptic vesicles. It is not known whether this relationship between neurosecretory fibres and endocrine cells is a functional one. 2. Synaptic contacts characterized by an accumulation of synaptic vesicles and a darkening of the presynaptic membrane. Synaptic contacts have been observed between neurosecretory fibres and 3 types of Pars distalis cell and 1 type of Pars intermedia cell. It is discussed whether these contacts represent transmitting synapses or whether they are specialized sites of hormone release. The close synaptic contact between neurosecretory fibres and Pars distalis cells strongly suggests that hypothalamic neurosecretion plays an important role in the regulation of Pars distalis function.In the pituitary of Tinca tinca neurosecretory fibres (containing elementary granules of 1900 Å, 1100 Å and 600–700 Å in diameter respectively) are surrounded and thus separated from the hormone producing cells by a single or a double basement membrane. It was rarely found that neurosecretory fibres penetrate the basement membrane and make direct contacts with endocrine cells. Synaptic contacts have not been observed. Since the nerve fibre tracts penetrate deeply into all lobes of the adenohypophysis and since they are relatively close together it appears that also this type of innervation provides good facilities for an interaction between neurosecretory fibres and endocrine cells.Furthermore it is pointed out that in the adenohypophysis of teleost fishes 4 types of neurosecretory innervation can be distinguished. This classification is based on how the transport of neurosecretory material from the nerve tracts to the endocrine cells is mediated.ZusammenfassungDie vorliegende Studie beschäftigt sich mit der neurosekretorischen Innervation der Adenohypophyse von Teleostiern.Bei Hippocampus cuda dringen neurosekretorische Nervenfasern (mit Elementargranula von 1300–1500 Å, 1000 Å und 500–600 Å Durchmesser) tief in die Pars intermedia und den Hypophysenvorderlappen ein. Alle Arten von neurosekretorischen Fasern treten in direkten Kontakt mit hormonproduzierenden Zellen. Zwei verschiedene Arten von Kontakten wurden beobachtet:1.Direkte Kontakte zwischen neurosekretorischen Nervenfasern und endokrinen Zellen, die sich von Synapsen dadurch unterscheiden, daß ihnen eine Verdickung der präsynaptischen Membran und Anhäufungen von synaptischen Bläschen fehlen. Es ist unklar, ob diesen Kontakten eine funktionelle Bedeutung zukommt.2.Synaptische Kontakte mit einer Anhäufung von synaptischen Bläschen und einer Verdichtung der präsynaptischen Membran. Synaptische Kontakte wurden zwischen neurosekretorischen Fasern und drei Zellarten der Pars distalis und einer der Pars intermedia beobachtet. Es wird diskutiert, ob diese Kontakte funktionell echte Synapsen darstellen oder ob sie spezialisierte Orte der Neurosekretabgabe über Membranen hinweg sind. Der enge synaptische Kontakt zwischen neurosekretorischen Nervenfasern und endokrinen Zellen der Pars distalis deutet darauf hin, daß hypothalamische neurosekretorische Pasern eine besondere Rolle bei der Funktionsregulierung des Hypophysenvorderlappens spielen.In der Hypophyse von Tinca tinca sind die neurosekretorischen Fasern (mit Elementargranula von 1900 Å, 1100 Å und 600–700 Å Durchmesser) von einer einfachen oder doppelten Basalmembran umgeben und somit von den hormonproduzierenden Zellen getrennt. Nur gelegentlich wurde beobachtet, daß neurosekretorische Fasern die Basalmembran durchbrechen und in direkten Kontakt mit endokrinen Zellen treten. Synapsen wurden nicht gefunden. Da die neurosekretorischen Nervenfasern in alle Teile der Adenohypophyse tief eindringen und relativ dicht beieinanderliegen, dürften die Bedingungen für eine neurosekretorische Beeinflussung der endokrinen Zellen nicht ungünstig sein.Es wird außerdem darauf hingewiesen, daß in der Adenohypophyse von Teleostiern vier verschiedene Innervationstypen vorkommen. Dieser Einteilung liegen die verschiedenen Wege zugrunde, auf denen das neurosekretorische Material von den Nervenfasertrakten zu den Erfolgszellen gelangt.

Comparative morphology of the vertebrate pineal complex.

Publisher Summary This chapter explores whether there is evidence for the assumption that the mammalian pineal organ is a complex rather than a single organ with a uniform function. It is pointed out that the pineal organ is structurally very complex, especially in rodents. The uneven development of the pineal organ in rodents and the topographical relationships of the pineal organs in other mammalian orders have prompted a classification which is based on the shape, the size, and localization of the pineal organ. A tentative classification of the pineal organs of thoroughly studied species shows that closely related species have similar types of pineals. Based on the different localizations of the pineal organs in relation to the third ventricle, the problem of a release of pineal hormones into the CSF is discussed. It is concluded that comparative morphological studies do not support the notion that secretion directly into the CSF is an important principle. The separation of pineal tissue into superficial and deep pineals in some rodents poses the question as to whether the two parts are structurally and functionally identical. The available data suggest that they are dissimilar. The chapter also discusses the possible subdivision of pineal tissue into cortical and medullary regions. Recent results obtained in rats illustrate the importance of carefully analyzing the pineal parenchyma in order not to miss a perhaps meaningful mosaic-like architecture of the organ. It is concluded that in some species the use of the term pineal complex is justified from a purely morphological point of view.

Mouse photoreceptor synaptic ribbons lose and regain material in response to illumination changes

Chemical synapses equipped with ribbons are tonically active, high‐output synapses. The ribbons may play a role in the trafficking of synaptic vesicles. Recent findings in retinal rod cells of BALB/c mice indicate that ribbons are large and smooth in the dark phase, and, due to the formation and release of protrusions, small during the light phase. As a consequence of these changes, ribbons may traffick fewer vesicles in the light than in the dark phases. The aim of the present study was to find out whether the above ribbon changes in this mouse strain are strictly illumination‐dependent and which signalling processes may be involved. Here, we show that ribbons form protrusions and release them into the cytoplasm within 30–60 min after lights on, the reverse occurring within 30 min after lights off. Under constant light or constant dark, no circadian rhythm of synaptic ribbon changes is observed. The illumination‐dependence of ribbon structure is supported by in vitro experiments showing that in dark‐adapted retinas, light induces the same morphological changes as in vivo. In vitro, the effect of light on the ribbons can be counteracted by cyclic guanosine monophosphate and melatonin. In dark‐adapted retinas, light effects can be produced by decreasing the calcium ion concentrations in the incubation media. These results suggest that in retinal rod cells, the well known phototransduction signalling mechanisms may be responsible for the ribbon changes presently and previously reported.

Different types of magnetically sensitive cells in the rat pineal gland.

Extracellular recordings from rat pinealocytes reveal different cell responses following experimental changes in the horizontal component of the ambient magnetic field. While two-thirds of the units recorded did not respond at all, one group is activated, the activation continuing after switching off the magnetic stimuli. These cells were not further activated by a second stimulus. Another group comprises cells characterized by a sustained inhibition or excitation, respectively, during magnetic stimulation. These cells could be influenced by a second stimulus. After guinea pigs and homing pigeons the rat is now the third species in which the pineal gland has been shown electrophysiologically to respond to changes of the ambient magnetic field.

Magnetic Field Effects on Pineal N-Acetyltransferase Activity and Melatonin Content in the Gerbil--Role of Pigmentation and Sex

The ambient geomagnetic field influences a variety of biological phenomena. Electrical and biochemical parameters of the rodent pineal gland are influenced by the alteration of weak magnetic fields (MF), the magnetic receptor probably residing in the retina. However, open questions concern the role of retinal pigmentation as well as species- and sex-specific differences in MF perception. We therefore exposed male and female naturally pigmented and albino Mongolian gerbils, as well as Sprague-Dawley (SD) rats to a 60 degrees rotation of the horizontal component of the ambient MF. Alteration of nocturnal pineal melatonin content and N-acetyltransferase (NAT) activity were utilized as a parameter for assessing magneto-sensitivity. In pigmented gerbils, MF exposure resulted in no significant changes in pineal melatonin synthesis. In contrast, albino gerbils and SD rats exhibited--regardless of sex--significant decreases in pineal NAT activity and melatonin content following MF exposure. These results suggest that in rodents hypopigmentation appears to favor magnetoperception. The available evidence indicates that the pigmentation of the retina could play a crucial role.