Ionizing radiation, by definition, "ionizes," that is, it pushes an electron out of its apogee about an diminutive nucleus, causing the accumulation of electrical accuse on atoms or molecules. If this electron comes from the DNA itself or from a adjoining atom and anon strikes and disrupts the DNA molecule, the aftereffect is alleged absolute action. This antecedent ionization takes abode actual quickly, in about 0.000000000000001 of a second. However, today it is estimated that about two-thirds of the accident acquired by x application is due to aberrant action. This occurs back the absolved electron does not anon bang the DNA, but instead strikes an accustomed baptize molecule. This ionizes the baptize molecule, eventually bearing what is accepted as a chargeless radical. A chargeless abolitionist reacts actual acerb with added molecules as it seeks to restore a abiding agreement of electrons. A chargeless abolitionist may alluvion about up to 10,000,000,000 times best than the time bare for the antecedent ionization (this is still a actual abbreviate time, about 0.00001 of a second), accretion the adventitious of it abolition the acute DNA molecule. This additionally increases the achievability that added substances could be alien that would abrogate chargeless radicals afore they do damage.

Neutrons act absolutely differently. A fast neutron will bypass orbiting electrons and occasionally blast anon into an diminutive nucleus, animadversion out ample particles such as alpha particles, protons, or beyond bits of the nucleus. The best accepted collisions are with carbon or oxygen nuclei. The particles created will themselves again set about ionizing adjacent electrons. A apathetic neutron will not accept the activity to beating out ample particles back it strikes a nucleus. Instead, the neutron and the basis will animation off anniversary other, like billiard balls. In so doing, the neutron will apathetic down, and the basis will accretion speed. The best accepted blow is with a hydrogen nucleus, a proton that can accelerate or ionize electrons in adjacent atoms.

Effect of cadmium administration (3 mg CdCl2/kg body weight i.p.) and/or whole-body

irradiation (3 Gy of gamma rays) on the rat brain was studied in the course of 21 d after

treatment. The effect was studied on the basis of quantitative analyses of DNA and RNA. It was

found that the changes of nucleic acids in the brain hemispheres were very slight and they lay in

a temporal decrease in DNA content on the 7th – 14th d after the irradiation alone and in

combination with cadmium (CdCl2 administered 30 min before irradiation). In the cerebellum,

the changes of nucleic acids were more profound than in the hemispheres. The decrease in DNA

content in the cerebellum was also only temporal and it occurred on the 1st d after the irradiation

alone. The most interesting finding seems to be the increase in DNA content in the cerebellum in

a later period after the irradiation alone and in combination with cadmium without concomitant

increase in RNA content. This finding suggests increasing number of cells with high

nucleoplasmic ratio (mainly glial elements) in the rat cerebellum in a later period after

irradiation. In general, effect of cadmium administration on the rat brain was less expressed than

effect of irradiation and combination of the two treatments did not induce any summation effect.

Key words: rat, brain, cadmium, gamma radiation, DNA, RNA.

Cadmium is a highly toxic heavy metal with half-life time of about 10 years in

human beings. It accumulates predominantly in the kidneys as well as in liver, which

are the critical target organs (2, 19). Biological role of cadmium in an animal organism

is unknown (9). It is designated as a carcinogen in humans and rodents (20, 23).

However, the concentrations of Cd required to induce overt DNA damage are usually

very high (4, 5, 16). The damage of nucleic acids is due to direct or indirect effects of

cadmium (7). Cadmium induces, for example, an oxidative DNA damage leading to

DNA strand-breaks, DNA-protein cross-linking and inhibition of DNA repair.

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The effects of ionizing radiation on DNA have been studied in detail for many

years (3, 6, 10, 15, 18, 11, 24). A lot of physical-chemical changes of DNA (e.g.

chemical changes of bases, breaks of phosphodiester bonds in one or both chains of

DNA) were found. Similarly to cadmium, these changes were caused by a direct or

indirect (mediated by reactive free radicals) effects of radiation. In the proliferatively

active tissues, such as haemopoietic and lymphoid tissues and regenerating liver, the

most striking effects of ionizing irradiation include inhibition of DNA synthesis and

mitosis and increase in the number of chromosome abnormalities. Due to the lack of

literature data concerning the influence of cadmium and ionizing radiation on nonproliferating

mammalian tissues, the cytotoxic effects of cadmium and gamma

radiation (alone or in combination) on the rat brain were studied in this model

experiment. The effects were evaluated on the basis of quantitative changes of DNA

and RNA because they can serve as an integral index of damage and recovery of cells

after the insults.

Material and Methods

The experiments were performed on 80 male rats of the SD strain (Anlab

Praha, Czech Republic). The animals were 6 weeks old at the beginning of the

experiments. They were kept under standard vivarium conditions (temperature 22 -

24°C, natural light rhythm), and had access to food and water ad libitum. The animals

were divided into four groups:

- non-treated control rats (C)

- rats treated with cadmium (Cd)

- rats irradiated with gamma rays (Ir)

- rats treated with cadmium 30 min before irradiation (Cd+Ir).

In this experiment, all animals survived. In the previous experiments, by the

use of the same doses of cadmium and ionizing radiation, but realised on younger rats

and in another year season, mortality of 5-10 % of animals was recorded.

Cadmium administration. Cadmium (CdCl2, Lachema Brno, Czech

Republic) was administered i. p. in the dose of 3 mg/kg alone or 30 min before

irradiation (groups Cd and Cd+Ir, respectively).

Irradiation. Animals were whole-body irradiated with the dose of 3 Gy by

gamma rays from a 60Co source (Chisostat Chirana, Czech Republic) at a dose rate of

0.150 Gy.min-1 (groups Ir and Ir+Cd). Doses of cadmium and gamma radiation were

chosen from the wide range of doses on the basis of the results of preliminary

experiments. They appeared to be the highest tolerable doses (as to the survival of rats)

inducing clearly demonstrable biochemical changes in the rat brain.

Determination of RNA and DNA. The animals (6 in each group) were

sacrificed 1, 7, 14 and 21 d after treatment. The brain was immediately excised in toto

and weighed. It was divided into the hemispheres and cerebellum (without truncus

cerebri), because distinct findings were expected due to some differences in structure

and function of these main parts of the brain. RNA and DNA determinations were

carried out according to the method of Tsanev and Markov (22). Tissue samples (in

duplicates) were homogenized in 5% trichloracetic acid solution and then deproteinized

and purified by washing with methanol, chloroform-methanol, benzene and ether. The

separation of RNA and DNA was carried out by consecutive hydrolyses in alkaline

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(1 M KOH) and acid (0.2 M HClO4) solutions, respectively. The concentration of RNA

and DNA in the hydrolysates was determined by spectrophotometric measurements

(Hitachi, Tokio, Japan) at two wavelengths (RNA at 260 and 286 nm and DNA at 268

and 284 nm).

Statistical analysis. The experimental data were statistically evaluated by

Peritz´ F-test (12) at P≤ 0.05 and P ≤ 0.01 level of significance and they are given as

mean ± S.E.M .

Results

Hemispheres. No significant changes in RNA concentration and total content

were found in the hemispheres of rats exposed to cadmium and/or radiation comparing

to the control rats (Fig. 1). The decrease in DNA concentration in experimental rats

was not statistically significant (Fig. 2). The mean values of total DNA content

decreased after gamma irradiation (Ir) on the 7th d and after combined treatment with

cadmium and gamma irradiation (Cd+Ir) also on the 14th d of the treatment. Changes in

concentration and total content of DNA in experimental rats (Ir and Cd+Ir) were timedependent.

The application of cadmium alone did not cause any significant changes in

DNA in the rat hemispheres. As presented in Table 1, after cadmium administration or

irradiation alone (Cd, Ir) there was a tendency to some increase in the weight of rat

hemispheres. The effect of combined cadmium and radiation treatment (Cd+Ir) on the

weight of hemispheres was rather opposite.

Cerebellum. Cadmium and radiation alone or in the combination (Cd, Ir and

Cd+Ir) induced marked changes in the concentration of RNA in the cerebellum of rats.

As it follows from Fig. 3, statistically significant decrease in RNA concentration in

comparison to control rats was observed after 21 d of treatment. However, decrease in

RNA concentration was not accompanied by adequate decrease in total RNA content in

the organ. The concentration and content of DNA in the cerebellum (Fig. 4)

significantly decreased only 1 d after irradiation (Ir). Later, however, increase in DNA

concentration and especially in DNA content was found in the cerebellum of irradiated

rats of the groups Ir and Cd+Ir. After all treatments (Cd, Ir and Cd+Ir), marked

enhancement of the cerebellum weight was observed mainly in later stages of the

examination

Discussion

The results of the present study indicated that the quantitative changes of RNA

in the brain hemispheres and cerebellum of cadmium treated and/or gamma-irradiated

rats were less expressed than changes of DNA.

In the hemispheres, no quantitative changes of RNA occurred; changes of

DNA manifested themselves in a temporary decrease in total content of DNA in

irradiated rats of the groups Ir and Cd+Ir. We suppose that in both groups, the decrease

in DNA content on the 7th d was caused mainly by irradiation; additional cadmium

administration (Cd+Ir) resulted in extension of the decrease till the 14th d. The DNA

content is considered to be the best biochemical index of cellularity (1), hence, these

results indicate some loss of cells due to apoptosis. Apoptosis of cells is connected with

fragmentation of DNA and following loss of DNA. It seems, therefore, that effects of

irradiation alone and in combination with cadmium imitate process of cell senescence

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