Ritardo di sviluppo di coorti di Xenopus laevis in prometamorfosi esposte a campi magnetici a bassa frequenza e confronto del contenuto di melatonina retinica tra esposti e controlli
Author(s)
Alilla, Roberta
Date Issued
April 4, 2008
Type
Doctoral Thesis
Abstract
Developmental delay of Xenopus laevis cohorts in prometamorphosis exposed to weak
low frequency magnetic fields and comparison of retina’s melatonin concentration
between exposed and control cohorts.
Objective of my doctor’s degree thesis was to confirm by experiments that exposure of
Xenopus laevis (Daudin) tadpoles to weak low frequency magnetic fields can slow down
animal’s ontogenetic development, to check particular amplitude-frequency combinations
of magnetic fields in promoting this phenomenon, and to attempt, still through
experiments, understanding if melatonin can be considered as a target molecule of
magnetic energy. I proposed this research starting from the results of a previous one, in
which my tutor observed a significant (P <0.001) metamorphosis delay in X. laevis
tadpoles exposed to the magnetic field of a TV set [Severini et al., 2003]. It is well known
that TV magnetic field has a ‘saw-tooth’ shape, and that it consists of different harmonic
components; then, it is not possible to ascertain which frequency-amplitude combination(s)
of TV magnetic field is responsible of the observed maturation retard. In order to check
the role of different amplitude-frequency magnetic field combinations in causing X. laevis
developmental retard, a tuneable solenoid in which aquariums with tadpoles can be lodged
for long (toxic) exposures was planned and constructed. With this solenoid as magnetic
field source were performed six experiments: three in the first year and the remaining three
in the second. In the third year, when the role of melatonin was taken into consideration,
tadpoles were exposed to a magnetic field emitted from a TV device, as in the former
Severini’s experiments.
In the three experiments of the first year, four cohorts of 35 tadpoles were reared in
aquariums in comparable conditions (of temperature, water availability, feeding and
photoperiod). Two of them were exposed inside the solenoid to two slightly different
combination of magnetic field frequency and intensity: (50 Hz, 71.1 μT rms) and (50 Hz,
73.7 μT rms), respectively. The remaining two cohorts, the control ones, were exposed to
static geomagnetic field (20,6 μT) and background variable field (0.05 μT) [Severini,
Alilla et al., in review]. The statistical analysis of results via two way ANOVA shows that
the strongest ELF magnetic field (73.7 μT rms) causes a highly significant metamorphosis
delay (4.05 days, P < 0.001) with respect to the controls, whereas the retard caused from
the weakest one (71.1 μT rms) is smaller (1.57 days) and not significant (P <.40). This
result not only confirms that magnetic exposure can slow down tadpoles development, but
it also suggests that a sharp intensity threshold between 71 μT rms and 74 μT rms
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(amplitude 100 μT and 104 μT) must exist in considering developmental retards associated
to exposures to 50 Hz electromagnetic fields.
In each of the three experiments of the second year, two cohorts were exposed to the same
frequency-intensity combinations of magnetic field in the solenoid: (50 Hz, 71.1 μT rms)
and (50 Hz, 73.7 μT rms) with the following difference as to the first year. Then, the
constant static (geomagnetic) field was (20,6 μT), while in the new series of experiments
the constant static magnetic field in the solenoid was augmented artificially up to 65 μT.
This was done to perform this series of experiments in condition of Ca2+ cyclotron
resonance, according to Liboff’s theory [Liboff, 1985] and to check if this theory could
explain (at least partially) the observed retards. The results show that the magnetic field
Ca2+ cyclotron resonance produced a significant developmental delay (0.86 days, P<.05),
though shorter than that of the first series. Comparing the results of the last three
experiments in the solenoid to the former three, we can exclude that calcium ions play an
important role in retarding metamorphosis via cyclotronic resonance. This is in agreement
with the main criticism of the Liboff’s theory, according to which thermal molecular
agitation in matter destroys the effects of cyclotronic resonance [Adair, 1991]. In
conclusion, cellular calcium ions cannot be considered as target molecules of
electromagnetic photons in biological matter.
In attempt to find a biological mediator between electromagnetic long wave radiation and
developmental delay, the third year of my research was dedicated to investigate the role of
melatonin. In Amphibians, metamorphosis is induced by a gradual rise in the thyroid
hormones (TH) that get to their maximum values during metamorphic climax. Melatonin
has an inhibitory effect on the thyroid hormones in all stages of the life cycle [Wright et
al., 2000] and it is considered a mediator of electromagnetic fields [Reiter, 1998]. In my
two experiments I exposed a cohort of X. laevis tadpoles to the magnetic field emitted by a
TV set, as in Severini’s experiment [2003].
Removal of Xenopus retinas started during animal’s prometamorphosis about at stage 55,
according to Nieuwkoop & Faber classification [1956], and melatonin concentrations was
measured by the ELISA (IBL-Hamburg) test. The results show that melatonin in Xenopus
retinas has a circadian rhythm of synthesis with high levels at night and low levels during
the day. Up to now, it was known that X. laevis retinas synthesize and release melatonin
rhythmically in early embryonic and larval stages (26 and 47) [Green et al,. 1999]; our
results show that this happens also in prometamorphosis. The exposure to the TV set
magnetic field caused a developmental slow down already significant three weeks after
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oviposition, however, the difference between melatonin concentrations of exposed and
unexposed cohorts does not result significant. It is known that melatonin concentrations
decrease as X. laevis tadpoles approach metamorphosis, and presumably, differences in
melatonin concentrations between exposed and not exposed tadpoles progressively
diminish. As consequence, the number of removed retinas must increase with respect to as
we did in order to show a difference of retinal melatonin concentration between exposed
and not exposed tadpoles.
Additional information
Dottorato di ricerca in Genetica e biologia cellulare
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