Radhey L. Singhal

CADMIUM ALTERS BEHAVIOUR AND THE BIOSYNTHETIC CAPACITY FOR CATECHOLAMINES AND SEROTONIN IN NEONATAL RAT BRAIN

Abstract— Daily exposure to cadmium (10 μg/100g) for 30 days since birth significantly increased spontaneous locomotor activity as well as striatal tyrosine hydroxylase and mid‐brain tryptophan hydroxylase. The endogenous levels of norepinephrine, dopamine and serotonin failed to change in various brain regions of cadmium‐treated rats. In contrast, the concentration of 5‐hydroxyindoleacetic acid tended to rise but was significantly different from controls only in the mid‐brain region. The data suggest that cadmium treatment in early life increased the synthesis and physiological utilization of these putative transmitters which in turn probably altered locomotor performance.

Effects of nortriptyline treatment on learned helplessness in the rat

Using an escape delay procedure previously shown to elicit behavioral deficits in mice exposed to uncontrollable shock, rats treated with inescapable but not escapable shock or no shock displayed comparable interference effects when tested in a two-way shuttle box 24 hr later. Treatment with 12.5 mg/kg nortriptyline for 4 or 6 days counteracted the escape deficits produced by inescapable shock while the 0 or 2 day administration regimens were without any appreciable effect. The finding that interference effects produced by inescapable shock were sensitive to sub-acute but not acute drug administration supports the utility of the learned helplessness model in evaluating potential antidepressant agents in experimental animals.

Brain biogenic amines and altered thyroid function.

Abstract Evidence has been presented that alterations in thyroidal status produce marked changes in the metabolism of several biogenic amines in developing brain. Neonatal hypothyroidism induced either by 131I or by an anti-thyroid agent, methimazole, markedly decreased the concentrations of norepinephrine, dopamine and 5-hydroxytryptamine and the activity of their rate-limiting enzymes, tyrosine hydroxylase and tryptophan hydroxylase. However, the levels of 5-hydroxyindoleacetic acid, the chief metabolite of 5-hydroxytryptamine were elevated in several regions of the brain. Whereas thyroid deficiency in early life produced no appreciable change in whole brain monoamine oxidase activity, it was increased in mid brain and decreased in the hypothalamus. Brain acetylcholine levels were significantly elevated and the activity of acetylcholinesterase remained unchanged in rats made hypothyroid at 1 day of age. Delaying thyroidectomy for 20 days after birth produced less appreciable changes in norepinephrine and 5-hydroxytryptamine metabolism. Thyroid deficiency suppressed the ontogenesis of behavioural arousal and spontaneous locomotor activity. The administration of L-triiodothyronine to hypothyroid animals in early life restored the metabolism of various neurohumors virtually to the normal limits. However, when the replacement therapy was postponed until adulthood, L-triiodothyronine failed to produce any restorative effects, suggesting that a critical period exists in early life during which thyroid hormone must be present to permit normal developmental pattern of central amines. Data also have been obtained demonstrating that neonatal hyperthyroidism induced by daily administration of L-triiodothyronine results in an increased turnover of norepinephrine and 5-hydroxytryptamine. These amine changes were accompanied by a marked rise in the spontaneous locomotor activity in hyperthyroid rats. Finally, chronic treatment with lithium, an antimanic drug, also known to suppress thyroid hormone production, significantly decreased not only the spontaneous locomotor activity, but also changes in the turnover of 5-hydroxytryptamine and norepinephrine in neonatally hyperthyroid rats.

Persistence of Cadmium-Induced Metabolic Changes in Liver and Kidney

Daily intraperitoneal injection of cadmium chloride (1 milligram per kilogram) for 45 days enhanced gluconeogenesis as evidenced by significant increases in the activities of liver and kidney cortex pyruvate carboxylase, phosphopyruvate carboxylase, hexosediphosphatase, and glucose-6-phosphatase, the quartet of key, rate-limiting enzymes involved in the biotransformation of noncarbohydrate precursors into glucose. Whereas cadmium treatment decreased the level of hepatic glycogen, the concentration of blood glucose and urea was significantly elevated by this heavy metal. Discontinuation of the heavy metal treatment for 28 days, in rats previously injected with cadmium for 45 days, failed to restore the observed biochemical alterations in hepatic and renal carbohydrate metabolism to control values. Evidence indicates that cadmium augments the glucose-synthesizing capacity of liver and kidney cortex and that various metabolic changes persist even after a 4-week period of withdrawal from exposure to the heavy metal.

EVIDENCE FOR THE ROLE OF ADRENOCORTICAL HORMONES IN THE REGULATION OF NORADRENALINE AND DOPAMINE METABOLISM IN CERTAIN BRAIN AREAS

1 Bilateral adrenalectomy suppressed body growth and increased the activity of tyrosine hydroxylase in rat striatum in a time‐dependent manner. Fifteen days after adrenalectomy, the concentrations of noradrenaline were decreased significantly in hypothalamus and striatum, as were those of dopamine in brain stem and striatum. 2 Catechol‐O‐methyltransferase failed to change in response to adrenalectomy, but the activity of monoamine oxidase in cortex was significantly increased 7 days after surgery. These changes in various neurochemical parameters were even more pronounced 15 days after adrenal ablation. 3 Administration of corticosterone (10 mg/kg i.p.) to adrenalectomized rats effectively reversed the observed effects on brain amine metabolism. Corticosterone treatment for 7 days beginning from the 8th day of adrenalectomy virtually restored the concentrations of noradrenaline and dopamine as well as the activities of striatal tyrosine hydroxylase and cerebrocortical monoamine oxidase to the values seen for sham‐operated controls. 4 Our data suggest that changes seen in brain noradrenaline and dopamine of adrenalectomized rats are specific to adrenocortical steroids and that these hormones play a role in the regulation of catecholamine formation.

Lead in blood and brain regions of rats chronically exposed to low doses of the metal

Abstract The concentration of lead in blood and cerebral cortex (Cx), striatum (St), hippocampus (Hc), and midbrain (Mb) was measured by flameless atomic absorption spectrophotometry in rats that had received (po) daily 0.005, 0.025, 0.1, or 1.0 mg lead/kg from 3 days until 4, 6, or 8 weeks of age. The blood lead levels and regional brain lead content of control rats increased with age. In addition to a dose-dependent increase in blood lead levels of rats exposed to 0.1 mg lead/kg (by 176%) and 1.0 mg lead/kg (by 396%), the concentration of the metal in blood of rats treated with the 0.1 mg/kg dose initially increased with the duration of exposure, reached a maximum value of 13.8 μg/dl at 6 weeks of age, and leveled off thereafter. Whereas exposure to small amounts of lead (0.025 mg/kg) resulted in a preferential accumulation of lead in the Hc (by 127%), a larger dose of the metal (0.1 mg/kg) administered for the same period of time produced significant increases in the lead content of the Cx (by 80%), St (by 106%), Hc (by 134%), and Mb (by 67%); no significant interregional differences in lead content were noted. The duration of treatment also influenced the distribution of lead. Exposure to 0.1 mg lead/kg for 4 weeks resulted in a significant increase in the lead levels of only the Cx (by 125%) and Hc (by 522%), while the same treatment for 8 weeks resulted in a significant and similar accumulation of lead in all of the regions examined. These data indicate that in the case of chronic low-level lead poisoning, brain regional lead distribution is influenced by both the dose of the metal administered and the duration of exposure. Significant amounts of lead persisted in brain tissue for as long as 2 and 4 weeks after the withdrawal of lead treatment.

DIABETOGENIC EFFECTS OF CHRONIC ORAL CADMIUM ADMINISTRATION TO NEONATAL RATS

1 Chronic exposure of neonatal rats to oral cadmium (Cd) (0.1 and 1.0 μg/g daily for 45 days) disturbed glucose homeostasis, as reflected by hyperglycaemia, reduced liver glycogen and enhanced gluconeogenic potential of hepatic tissue. 2 This Cd‐exposure regimen also increased hepatic cyclic adenosine 3′,5′‐monophosphate (cyclic AMP) which was accompanied by enhancement of basal, adrenaline and glucagon‐stimulated form(s) of adenylate cyclase. 3 In order to assess the responsiveness of pancreatic beta cells to glucose, islets isolated from control as well as Cd‐exposed animals were incubated in vitro and their rate of insulin secretion determined. In the presence of glucose 0.5 mg/ml, there was no significant difference in the rate of insulin release. However, at higher glucose concentrations (1.5 and 3.0 mg/ml), the islets from Cd‐exposed rats released significantly less insulin than those of control animals. 4 The results are discussed in relation to the possible mechanism of the diabetogenic effect of Cd.

Evidence for the role of brain biogenic amines in depressed motor activity seen in chemically thyroidectomized rats.

Abstract— The effects of exposure to an antithyroid drug, methimazole, on brain tyrosine hydroxylase and tryptophan hydroxylase activity, as well as the levels of norepinephrine, dopamine, 5‐hydroxytryptamine and 5‐hydroxyindoleacetic acid have been investigated in maturing brain. Daily treatment of neonatal rats with methimazole for 30 days induced chemical thyroidectomy as evidenced by significant impairment of body and brain growth. The activities or brain tyrosine hydroxylase and tryptophan hydroxylase and the levels of norepinephrine, dopamine and 5‐hydroxytryptamine were markedly altered in a dose‐ and time‐dependent manner in methimazole‐treated rats. Conversely, the concentration of brain 5‐hydroxyindoleacetic acid was elevated (46%) by methimazole administration. Treatment with the antithyroid drug failed to exert any significant effect on the endogenous levels of brain tryptophan, as well as on the activity of the deaminating enzyme, monoamine oxidase. Administration of triiodothyronine (25 or 100 μg/100 g) to hypothyroid rats for 30 days did not produce any appreciable effect upon the neurochemical parameters related to either norepinephrine or 5‐hydroxytryptamine mctabolism. However, increasing the dose of triiodothyronine to 250 μg/100 g significantly elevated the levels of norepinephrine and 5‐hydroxytryplamine as well as the activities of the two synthesizing enzymes, tyrosine hydroxylase and tryptophan hydroxylase. Brain 5‐hydroxyindoleacetic acid levels were restored to normal values in thyroid hormone‐deficient rats treated with this higher dose of triiodothyronine. Evidencc also was obtained to show that chemical thyroidectomy suppressed the spontancous locomotor activity in neonatal rats; the changes being apparent at 15 days of age. Our data support the view that thyroid hormone in neonatal life displays an important regulatory effect on the metabolism of norepinephrine, dopamine and 5‐hydroxytryptamine. Since certain amines have been known to be implicated as the neurochemical substrates for behavioural arousal, it is conceivable that the observed hypoactivity in methimazolc‐treated rats may, at least in part, be related to impaired maturation of norepinephrine and dopamine‐synthesizing systems in brains of cretinous rats.

DDT and Related Chlorinated Hydrocarbon Insecticides: Pharmacological Basis of Their Toxicity in Mammals

Publisher Summary This chapter discusses the pharmacological basis of DDT and related chlorinated hydrocarbon insecticides toxicity in mammals. In mammals, the administration of acute large doses of DDT and related chlorinated hydrocarbon insecticides produces a variety of toxic effects and invariably leads to death. Methoxychlor, that appears to be replacing DDT in certain countries, exhibits toxicity that is about 30 times less than that seen with DDT. The major routes of DDT metabolism in mammals are (1) oxidation to DDA, (2) dehydrochlorination to DDE, and (3) reductive dechlorination to DDD. In vertebrates, the major metabolite of DDT in feces and urine is DDA. Of DDT-derived, ether-soluble material in the rat bile, DDT constitutes about 3%, DDE 1%, and free DDA 25-35%; the remaining consists of complexes of DDA or a closely related material. DDE is the principal storage form of ingested DDT in man. However, while men stores about 60% of DDT-derived material in the form of DDE, rats store only about 22-29% of DDE, and monkeys convert little or no DDT to DDE.

Metabolic control mechanisms in mammalian systems. Regulation of key glycolytic enzymes in developing brain during experimental cretinism.

Abstract— The regulation by thyroid hormone of the activities of hexokinase (ATP: D‐hexose 6‐phosphotransferase; EC 2.7.1.1), phosphofructokinase (ATP: D‐fructose‐6‐ phosphate 1‐phosphotransferase; EC 2.7.1.11) and pyruvate kinase (ATP: pyruvate phosphotransferase; EC 2.7.1.40) has been investigated in the soluble fractions of the cerebral cortex and cerebellum of the rat. Ontogenetic studies on these key glycolytic enzymes demonstrated marked increases in the normal cerebral cortex between 1 day and 1 yr of age; less pronounced increases in enzyme activities were noted in the normal cerebellum. Neonatal thyroidectomy, induced by treatment of 1‐day‐old rats with 100 μCi of 131I, ied to an impairment of body and brain growth and inhibited the developmental increases in hexokinase, phosphofructokinase and pyruvate kinase in both the cerebral cortex and cerebellum. Whereas 50 μCi of 131I had little or no effect on these brain enzymes, 200 μCi of the radioisotope markedly inhibited (35–65 per cent) the developmental increases of the various enzyme activities investigated. When administration of the radioisotope was delayed for 20 days after birth, little or no inhibition of the development of brain glycolytic enzymes was observed. Whereas treatment of normal neonatal animals with L‐tri‐iodothyronine had no significant effect on the activities of cerebro‐cortical and cerebellar glycolytic enzymes, the hormone increased their activities in young cretinous rats. However, when the initiation of tri‐iodothyronine treatment was delayed until neonatally thyroidectomized rats had reached adulthood, this hormone failed to produce any appreciable change in enzyme activity. Our results indicate that thyroid hormone exerts an important regulatory influence on the activities of hexokinase, phosphofructokinase and pyruvate kinase in the developing cerebral cortex and cerebellum.