Brain DNA Content Research Articles

N-alkyl-N-nitrosourea induced secondary structural changes in DNA from rat embryos and fetal brains in vivo.

N-methyl-N-nitrosourea (MNU) and N-ethyl-N-nitrosourea (ENU) are gestational stage dependent teratogens and transplacental carcinogens capable of inducing neurogenic tumours in rats. Intravenous treatment of gravid Wistar rats showed that MNU is teratogenic but ENU is a transplacental carcinogen and may be a teratogen when administered on day 12 of gestation. Twenty-four hours after single doses of 2, 5, or 10 mg MNU/kg on day 12, dose dependent decreases in embryonic wet weight and total embryonic DNA were observed. Rats similarly treated with 2 and 5 mg MNU/kg showed dose dependent decreases in fetal brain DNA synthesis, DNA content, and wet weight 9 days later. Administration of single ENU doses of 1.5, 3, 6, 12, 48, and 80 mg/kg to day 12 embryos resulted in a dose dependent reduction in [methyl-14C]-thymidine (14C-TdR) incorporation into DNA after 24 h although total DNA amounts and embryonic wet weights were unaffected. Benzoylated DEAE-cellulose (BD-cellulose) chromatography fractionates DNA on the basis of secondary structure by stepwise elution of double-stranded DNA with 1.0 M NaCl solution (SE-DNA) followed by elution of DNA containing single-stranded regions with caffeine solution (CE-DNA). Day 13 embryonic and day 21 fetal brain DNA was monitored by in vivo labelling with [methyl-3H]-thymidine on days 6 and 7 of gestation. Significant reduction in percentages of CE-DNA (%CE-DNA) 24 h after treatment of day 12 embryos with 2, 5, or 10 mg MNU/kg were attributed to the necrotic effect of MNU. Day 12 treatment with MNU produced no change in %CE-DNA values of day 21 fetal brains. A teratogenic dose of 80 mg ENU/kg to day 12 embryos resulted in significantly increased %CE-DNA values compared to controls but no changes were observed after 1.5 to 48 mg/kg. Analysis of the distribution of %CE-DNA values from the 80 mg ENU/kg treated litter showed that the increase in %CE-DNA was due to a second distinct population of embryos with higher %CE-DNA values than controls. Incorporation of 14C-TdR into embryonic and fetal brain DNA demonstrated the effects of treatment with these compounds on DNA synthesis in vivo. The relative %CE-DNA is expressed as the ratio of the percentage of caffeine-eluted 14C-labelled DNA to %CE-DNA (i.e., %CE-14C-DNA:%CE-3H-DNA). In the majority of control embryos the 14C-specific activity of CE-DNA was higher than the 14C-specific activity of SE-DNA.(ABSTRACT TRUNCATED AT 400 WORDS)

Regional patterns of brain growth during the first three weeks following early adrenalectomy

Day 11 adrenalectomized (ADX) and sham-operated control rats were compared with respect to overall and regional brain growth and DNA content at 1, 2, and 3 weeks postsurgery. Possible treatment effects on the rate of postnatal cell loss were also assessed by injecting the animals with 3H-thymidine on day 2 postnatal (9 days prior to surgery) and subsequently measuring the amount of radiolabelled DNA remaining in various brain regions at each time point. Adrenalectomy led to reliable increases in cerebral cortex and midbrain-diencephalon weights within 1 week postsurgery, whereas cerebellum, hippocampus, and overall brain weights were not significantly elevated until 1 or 2 weeks later. The effects of adrenalectomy on tissue DNA content were likewise regionally dependent. According to the 3H-thymidine results, there was no significant influence of adrenalectomy on the loss of previously labelled DNA. In conclusion, the timing of brain growth acceleration following adrenal removal is regionally specific, with the most rapid effects occurring in the cortex and midbrain-diencephalon. Furthermore, adrenalectomy-induced increases in brain cell number appear to depend primarily on increased cellular genesis (as previously demonstrated) rather than decreased cell death.

Sexual dimorphism in brain weight of meadow voles: role of gonadal hormones

In most adult mammals, brain weights of males exceed those of females. The role of androgens in the genesis of this sex difference was assessed in meadow voles by acute neonatal or chronic postweaning manipulation of testosterone titers. Female voles given a single injection of testosterone propionate (TP) on the second day of postnatal life had brain weights in adulthood that were indistinguishable from those of male voles and significantly heavier than those of control females. Whole brain DNA content, a measure of cell number, was not increased by neonatal TP treatment. Females treated with TP from day 19 to 70 had lower brain weights than control females and males gonadectomized at 19 days of age had greater brain weights than did intact male voles at day 70. The sex dimorphism in brain weight reflects organizational effects of testosterone during perinatal development. Beginning at weaning, and continuing through postpubertal development, testosterone decreases brain weight in both sexes. We suggest that testosterone affects brain weight by altering cell size or non-cellular components rather than cell number.

Long day lengths promote brain growth in meadow voles

Male meadow voles kept in a long photoperiod (LP) from birth to 70 days of age have heavier brains than those kept in a short photoperiod (SP). Brain weights of male voles kept in the LP first exceeded those of SP animals at 20 days of age; differences were greatest at 35 days (5.8%) and persisted through 140 days of age (2%), although the magnitude of the difference declined progressively. Accelerated compensatory increases in brain weight were observed in voles transferred from the SP to the LP at 70 days of age. Total brain DNA content, an index of cell number, was not significantly affected by initial or final photoperiod, although it increased 7.8% within 70 days after voles were transferred from the SP to the LP. Brain weights (but not DNA content) of males exceeded those of females, but this sex difference was present only in the LP. We suggest that short day lengths retard brain development by reducing rates of myelination and possibly reducing cell size as well; this is part of a general retardation of somatic growth associated with a delayed onset of puberty that can be reversed by a stimulatory LP but, ordinarily, occurs spontaneously as voles become refractory to short day lengths.

Enhanced Brain Cell Proliferation Following Early Adrenalectomy in Rats

We have previously demonstrated an increase in adult brain DNA content in rats adrenalectomized on postnatal day 11. The present studies examined cell proliferation in cerebral cortex, cerebellum, hippocampus, and midbrain-diencephalon following adrenalectomy at this age. Compared to sham-operated controls, adrenalectomized animals showed increased [3H]thymidine incorporation into DNA (measured at 1 h following a pulse injection) in all brain regions at 7 and 14 days postsurgery. In some areas, the effect was already present as early as 2 days following adrenalectomy. Chronic replacement with corticosterone prevented this increase in DNA labelling in a dose-dependent manner. When cell proliferation in the cerebral cortex and cerebellum was independently assessed by measuring changes in thymidine kinase activity, enzyme activity was significantly elevated in both areas at 7 and 14 days postsurgery. Finally, histological examination of the cerebellar cortex suggested a delayed disappearance of the external granular layer in several cerebellar lobules of adrenalectomized animals. Overall, these findings indicate that day-11 adrenalectomy leads to a prolonged stimulation of mitotic activity in areas where cell formation at this time is exclusively glial (i.e., cerebral cortex and mid-brain-diencephalon) as well as in areas where postnatal neurogenesis is also occurring (cerebellum and hippocampus). It is hypothesized that this stimulation results from the removal of a tonic inhibitory effect exerted by circulating glucocorticoids in the normal intact animal.

Chronic Caffeine Intake Alters the Composition of Various Parts of the Brain in Young Growing Rats

Time-pregnant rats were fed a regular laboratory diet until delivery. Litters delivered within an 8-hour period were combined and 8 randomly selected pups were assigned to each dam. Dams were then divided into two groups. Group I received a 20% protein diet. Group II was pair-fed to group I with a 20% protein diet containing caffeine (1 mg/100 g body weight) until day 22 postpartum. On weaning (day 22), only male pups were selected and fed the 20% protein or 20% protein + caffeine diet according to which diet their respective dams had been fed. On day 43, rats were killed and the brains were divided into 6 different areas. DNA, RNA, protein and cholesterol contents were measured. Caffeine supplementation resulted in a decrease in the total brain weight, and DNA and RNA content. DNA value of cortex-midbrain was smaller in the caffeine group whereas the values of medulla oblongata and striatum were greater in comparison with the controls. Caffeine's effects on RNA included an increase in the cerebellum, cortex-midbrain and hippocampus and a decrease in the medulla oblongata. Protein content of cerebellum was increased and of hippocampus decreased due to caffeine supplementation. Cholesterol content of the medulla oblongata and hippocampus was less in the caffeine group than in the control group. The present study shows that chronic caffeine intake during rapid periods of growth influences various parts of the brain in entirely different biochemical manners.

Long day lengths increase brain weight and DNA content in the meadow vole, Microtus pennsylvanicus

Male voles reared in long (spring) day lengths had heavier brains with greater total DNA content than did males housed in short (fall) day lengths; these effects were not observed in female littermates kept in the two photoperiods. Male brains were heavier than female brains in long but not in short photoperiods. Day length affected brain mass during early postnatal development, but not when treatments were initiated in adulthood. Seasonal differences in brain development, including the numbers of neurons and glia, may reflect different metabolic and behavioral demands faced by males born in spring and fall, respectively.

Effect of protein malnutrition and maternal caffeine intake on the growth of fetal rat brain.

Pregnant dams were divided into two subgroups on day 10 of gestation. Half were fed a 20% protein diet and the other half an 8% protein diet. A second group also subdivided was pair-fed with rats of the first group. Their diet was supplemented with caffeine in amount calculated to provide daily doses of 2 mg/100 g body weight. On days 18, 20, and 22 randomly selected dams were injected with 3H-thymidine intraperitoneally and 1.5 h later their fetuses were delivered surgically in order to determine the rate of DNA synthesis along the gestation. The rest of the fetuses were delivered surgically on day 22. Pups' brains were rapidly removed and DNA, RNA, protein and 3H-thymidine uptake were studied. Average body weights of the fetuses in the caffeine-supplemented control group were smaller than those of the noncaffeine group. Effects of caffeine that were similar in both diet groups included a decrease in brain DNA content and concentration and an increase in brain protein content and concentration. However, the percent decrease and increase, respectively, was different depending on the nutritional status. DNA synthesis was not affected by malnutrition or caffeine supplementation on day 18 of gestation. Caffeine's effect on the rate of DNA synthesis was different on day 20 of gestation depending on nutritional status. Caffeine supplementation resulted in a decrease in DNA synthesis in both groups on day 22 of gestation. These data indicate that caffeine intake during pregnancy produces differential effects on fetal rat brain depending on dietary protein content.

Effects of fetal thyroid states on RNA, DNA, and tubulin content in neonatal rat brain.

3,5-Dimethyl-3'-isopropyl-L-thyronine (DIMIT)-induced fetal hyperthyroid rats showed a marked accumulation of intracellular tubulin content in the cerebral cortex, hypothalamus, and cerebellum at 1 day old. When the fetuses were transplacentally radio-thyroidectomized with the administration of [131I]-Na, the brain weight was not changed at 1 day old. The DNA content was not affected by the radio-thyroidectomy (Tx), but intracellular RNA concentration (per DNA) was increased in the hypothalamus and cerebellum at that age. The DIMIT-supplement to Tx-fetuses failed to restore these abnormal values to normal. Delayed effects of the fetal Tx were observed in 11-day-old infants. These neonates showed decreased weight gain in the body, brain, cerebral cortex, and cerebellum. The DNA content (per wet tissue) was higher in the hypothalamus and cerebellum. The intracellular concentrations of RNA and tubulin (per DNA) were significantly decreased in the hypothalamus. These values were restored to normal by the administration of L-thyroxine(T4) to rats between 1 and 10 days old. These results demonstrate that fetal nervous tissues are sensitive to changes in thyroid functions. The different manner of response to the hormonal states among the brain areas may reflect that each nervous tissue has a specific critical period for thyroid hormones during development.

THE EFFECTS OF CHRONIC ALCOHOL CONSUMPTION ON PREGNANT RATS AND THEIR OFFSPRING

The effect of chronic ethanol intake during gestation was studied in rats fed a liquid diet in which ethanol provided 36% of the total calories. The animals were chronically alcoholised before mating, and the body weight gain and nutritional status during pregnancy were noted. Blood ethanol levels were measured during pregnancy and parturition. Specifically, we have shown that chronic ethanol intake during pregnancy lengthens the gestation period, decreases foetal viability, increases the placental weight and diminishes foetus, liver and brain weights, as well as the protein and DNA content of foetal brain. The reduced body weight of rats prenatally exposed to alcohol continued for the first two months of the postnatal period and was most apparent during lactation.

Organ growth and cellular development in ethanol-exposed rats

Effects of perinatal exposure to ethanol on growth and cellular development were investigated. Alcohol was administered in liquid diets designed to provide optimal nutrition during pregnancy. Pair-fed and ad lib control groups were included. The 3 groups of females were similar in body weight during gestation and lactation, and offspring weights were similar on gestation Day 21 and at birth. By Day 9 of lactation control pups weighed more than both alcohol and pair-fed pups which were similar in body weight. Weights of brain, heart, liver and kidney were reduced in alcohol pups compared to pair-feds and controls. Decreased liver weight reflected both decreased cell size and decreased protein content, but was primarily due to decreased caloric intake. Decreased heart weight appeared to result from a direct effect of ethanol on heart protein content. Even more marked were the adverse effects of ethanol on kidney protein content and kidney DNA (reflecting a decrease in cell number). In contrast, although both absolute brain weight and DNA content were decreased in ethanol-exposed offspring, relative brain weight was increased. Finally, maternal ethanol consumption significantly increased relative placenta weights as well as placental DNA and protein content.

Extracellular (36Cl) space, electrolyte, protein, and DNA content in brain of DBA and C57 mice: effects of age.

DBA/2J (DBA) mice are susceptible to audiogenic seizures (AGSs) in an age-dependent manner, susceptibility being maximal at 21 days and absent at 110 days of age. Previous studies have demonstrated that there is a decrease in anion transport and an increase in carbonic anhydrase (CA) activity in brain from DBA mice as compared with C57BL/6J (C57, non-AGS) mice at 21 days. Since these results suggest that there are alterations in cellular and electrolyte composition of brain from DBA mice, the present work was directed toward determining electrolyte content, extracellular space, and DNA content of brain from DBA and C57 mice at 21 and 110 days of age. There was a decrease in intracellular chloride and sodium content, and an increase in intracellular potassium content in cerebral cortex, cerebellum, and brainstem from DBA mice at both 21 and 110 days. Also, extracellular space was larger in these three brain tissues from DBA mice at both ages. DNA content, not different between the strains at 21 days, was significantly lower in cerebellum from DBA mice at 110 days. These findings give further evidence of alterations in the transport of ions in brain from DBA mice. In addition, they demonstrate that there are alterations in the intracellular and extracellular space values, an indication of changes in cellular composition of brain in these mice. Such changes may contribute to AGS susceptibility by disrupting the balance that normally exists in the neuronal microenvironment of the central nervous system.

Experimental maternal hyperpipecolatemia decreases DNA in the mouse brain

Pipecolic acid represents the major metabolite of lysine in the CNS. This study aimed to mimic the known pathological condition of hyperpipecolatemia in human neonates by maintaining high and steady concentrations in the mouse brain both pre- and postnatally. Although histological observations did not reveal any abnormal findings in the brain, liver and kidney, the offsprings of mothers which were injected during pregnancy with 250 mg/kg PA presented a significant decrease in both cortical and whole brain DNA content. Protein, RNA and CNPase activity were not altered. The changes in DNA content are of particular interest since they demonstrate that a high and steady level of pipecolic acid, which is comparable to the level seen in human hyperpipecolatemic brain, can induce a neurotoxic effect in the fetal mouse brain. Mice injected with the same dose from day 1 to day 29 after birth did not show any change in DNA content in the cortex. These results suggest that the critical period from pipecolic acid neurotoxic effect is prenatal, probably due to a selective vulnerability of developing neurons to high levels of pipecolic acid.The decrease in DNA concentration induced by maternal hyperpipecolatemia suggests a neurotoxic effect of pipecolic acid on cell proliferation during fetal development which could be related to the neuronal damage observed in the brain of hyperpipecolatemic neonates.

Effect of prenatal imipramine exposure on development of the postnatal rat heart and brain.

Imipramine (IMI) was administered s.c. at 0, 5, or 10 mg/kg/day to pregnant rats on gestation days 8-20 to assess possible alterations in postnatal heart and brain development. Maternal weight gain was significantly reduced in a dose-response manner, but litter size and pup weight on postnatal day (PND) 1 were unaffected. On PND 1, litters were culled to 10 pups for analysis on PNDs 4/5, 7/8, 14/15, and 21/22. Pup body weight was not affected at any age measured, but heart weight was significantly reduced at 10 mg/kg IMI on PNDs 4/5 and 7/8. Brain weight was increased in a dose-related pattern on PNDs 4/5 and 7/8 and was significantly higher at 5 mg/kg IMI on PND 14/15. No significant effect was observed in heart or brain protein and DNA content or in cardiac beta-adrenergic receptor concentration. Prenatal IMI exposure had no effect on basal cardiac ornithine decarboxylase (ODC), an enzyme associated with growth and development, but basal brain ODC was lower at 5 mg/kg IMI at all ages measured. Cardiac ODC stimulation by insulin was unaffected by prenatal exposure to IMI, but isoproterenol-stimulated ODC was increased on PND 21/22 at 5 mg/kg IMI. In conclusion, the IMI-related changes in several parameters suggest that when maternal IMI treatment is used, alterations in postnatal heart and brain development must be considered as possible outcomes.

Effects of neonatal methylmercury exposure n development of nucleic acids and proteins in rat brain: Regional specificity

Exposure of neonatal rats to methylmercury (1 or 2.5 mg/kg SC daily) during the preweaning period caused regionally-specific alterations in DNA, RNA and protein content in brain. In midbrain + brainstem, where neuronal replication and differentiation conclude early, reduced DNA content was prominent at either dose and was apparent well before evidence of general body growth impairment; small deficits in protein content and brain region weight were seen. In contrast, cerebral cortex showed an elevation of DNA in the high dose group and a tendency toward supranormal RNA values at either dose. In cerebellum, where maturation occurs last, both DNA and RNA were markedly stimulated in pups receiving either 1 or 2.5 mg/kg and little effect was seen on proteins or region weight. These results indicate that the response of the developing central nervous system to cell loss caused by methylmercury is dependent upon the stage of development at which exposure occurs: whereas clear-cut reductions in cell number (DNA content) occur in fairly mature, post-replicative regions, those areas undergoing rapid replication and differentiation are likely to exhibit compensatory stimulation or replacement of cells.

Effects of orally administered caffeine on cellular response in protein energy-malnourished neonatal rat brain.

The purpose of this study was to determine the effects of caffeine on growing rats and how protein energy malnutrition can modify these potential effects. Caffeine (1,3,7-trimethylxanthine) is not only the most commonly consumed, neurally active stimulant in our daily lives, but it is widely used in the management of apnea in the premature neonate. One group of dams (20%) (n = 6) was begun on a 20% protein diet ad libitum. The second group (6%) (n = 4) was begun on a 6% protein diet. A third group (20% + C) (n = 4) was pair-fed to group 1 (20%) with the 20% protein diet, but beginning on day 3 the pups received 10 mg/kg body weight of caffeine via intragastric feeding needle every other day. The fourth group (6% + C) (n = 5) was pair-fed with group 2 (6%) with the 6% protein diet and the pups received 10 mg/kg of caffeine in the same manner as the 20% + C group from day 3. Although the 6% protein diet was associated with the expected reduced body and brain growth, there were no additional growth alterations associated with caffeine administration in either the 20% or 6% diet groups. This growth failure was accompanied by the expected reductions in total whole brain DNA, RNA, protein, and cholesterol content regardless of whether caffeine was received or not. Effects of caffeine which were similar in both diet groups included an increase in brain RNA content and concentration and an increase in the RNA/DNA ratio. However, there were differential effects of caffeine seen depending on diet group assignment.(ABSTRACT TRUNCATED AT 250 WORDS)

Fetal fuels. VII. Ketone bodies inhibit synthesis of purines in fetal rat brain.

Maternal starvation in late gestation results in ketonemia and a lowered DNA content in fetal rat brain. Because purines are needed for formation of new RNA and DNA, we examined the de novo pathway for purine biosynthesis in fetal rat brain slices by measuring the incorporation of [14C]formate into [14C]adenine. Maternal starvation days 18-20 resulted in a slight but nonsignificant fall in purine biosynthesis from 0.230 +/- 0.006 to 0.216 +/- 0.006 mumol X g-1 X 2 h-1. Graded concentrations of DL-3-hydroxybutyrate (3OHB) produced a progressive inhibition of formate incorporation in fetal brain slices, and rates of purine biosynthesis in the presence of 5.4 mM 3OHB were significantly lower (P less than 0.01) in fetal brain slices from both fed and starved mothers, i.e., 0.199 +/- 0.006 and 0.189 +/- 0.006 mumol X g-1 X 2 h-1, respectively. Acetoacetate caused an inhibition similar in magnitude to 3OHB. That 3OHB did not act via a protein biosynthetic step was evidenced by unimpaired biosynthesis of purines and sustained 3OHB inhibition in the presence of 1 mM cycloheximide. Unlike the de novo pathway, the salvage pathway measured by incorporation of [8-14C]adenine into labeled nucleotides was not significantly inhibited even by supraphysiological levels of 3OHB (21.6 mM). Serial measurements of the de novo and salvage pathways in neonatal brain slices showed a maintenance of salvage activity during the first 2 wk but a progressive fall in activity of the de novo pathway. Thus ketone bodies could act to restrain the synthesis of purine nucleotide building blocks for new cell formation in fetal but not in neonatal rat brain.

The effect of maternal hyperthermia on the development of the brain in the guinea pig

The effect of maternal hyperthermia on the developmental state of newborn guinea pig brains has been studied. Mean litter size and mean body weights of young from control and experimental mothers did not differ. Mean brain weight of young from control mothers (2.72 ± 0.02 g) was larger than that from experimental mothers (2.3 ± 0.03 g). This reduced brain weight was expressed in terms of body weight and showed a mean deficit of 16.4 ± 1% (range 3.4 to 36.1%). A good correlation was observed between the degree of microencephaly and brain DNA content in experimental young, and the mean reduction in DNA level was 14.9% which compares well with the mean weight reduction. Data from dissected brains indicate that much of the reduction in brain weight and DNA content observed following maternal hyperthermia was owing to an effect on the cerebral hemisphere. Specific activities of synaptic and microsomal membrane marker enzymes were similar in preparations from control and microencephalic young. However, the activities of the enzymes, when expressed in terms of DNA content, were higher in cerebral preparations from microencephalic young as compared with controls. Microencephaly is associated with a reduced cell number (DNA content) but a compensatory elaboration of neuronal dendritic processes also seems to occur.

Chromosomal and biochemical studies on the effect of kat extract on laboratory rats.

Kat is being used extensively in many countries as a central nervous system stimulant. The effect of three doses of crude kat extract on chromosomal division and abnormalities in bone marrow, as well as on DNA, RNA, and total protein content in brain and liver was studied in laboratory rats in order to test the possible mutagenicity of the drug. Kat was given as a single subcutaneous injection at 0.05 (usage dose), 0.52 (intermediate dose), and 1.00 (sublethal dose) g/kg body weight. Animals were sacrificed at 6, 24, and 48 hr after treatment. Also, some animals were exposed subacutely for 5 consecutive days with sacrifice occurring 6 hr after the last injection. The mitotic index was reduced by all treatments, with the greatest effect occurring in the subacute treatment. Chromosomal abnormalities were induced by kat at all three doses, administered acutely or subacutely. The significant chromosomal aberrations were in the form of gaps, breaks, centromeric attenuations, and centric fusions. The concentration of DNA, RNA, and total protein in liver and brain decreased at all doses, with the greatest decrease occurring after subacute treatment. These findings suggest that kat has a profound effect on cell proliferation, on chromosomal abnormalities, and on DNA, RNA, and total protein synthesis.

Inhibitory influence of thiourea on brain of singi fish ( Heteropneustes fossilis bloch) and subsequent recovery by l-triiodothyronine

Immersion of Singi fish in thiourea-containing medium (1 mg/ml) for 45 days significantly decreased the cranio-somatic index, weight of different parts of brain, viz. cerebrum, cerebellum, midbrain and medulla oblongata, and also protein and RNA contents of these different regions. The DNA content of these substructures remained unchanged. In other sets of experiment, thiourea treatment for 33 days caused reduction in mitochondrial cytochrome-linked α-glycerophosphate dehydrogenase (EC 1.1.99.5) activity, and total protein and RNA contents of whole brain, while the amount of mitochondrial protein and total DNA content of whole brain did not undergo significant variation. A single injection of l-triiodothyronine (0.5 μg/g) enhanced this enzyme activity, mitochondrial protein amount and total protein and RNA contents of whole brain of thiourea-treated fish to almost such levels as obtained by l-triiodothyronine injection in normal (control) Singi fish within 3 days. The CSI in normal fish increased by triiodothyronine injection. In thiourea-treated fish, the reduction of CSI was restored to just control level by triiodothyronine. The results are, therefore, discussed as additional supportive evidence of the responsiveness of fish brain to thyroid hormone.