George M. Ling

Modification by Acetylcholine of the Response of Rat Mesenteric Arteries to Sympathetic Stimulation

The aim of this study was to determine the effect of varying concentrations of acetylcholine on the vasoconstriction of the perfused mesenteric arteries of the rat caused by stimulation of their sympathetic postganglionic nerves. Perfusion was carried out with a peristaltic pump, and the constriction caused by stimulation was measured by the rise of pressure in the cannula tied in the artery. Two effects were found. At rates of stimulation between 2 and 8/sec, acetylcholine added to the perfusion fluid in very low concentrations (50 pg/ml) caused an increase in the vasoconstriction. An increase was also caused by adding a concentration of 2 ng/ml for 15 seconds. If, however, this or a greater concentration was added for a longer time, it had the opposite effect and the vasoconstriction was greatly reduced or blocked. This reduction or block was not due in the main to a reduction or block of the vasoconstrictor action of norepinephrine. Block of response to stimulation was also produced by guanethidine. The block produced by acetylcholine resembled the block produced by guanethidine in two respects. The block was removed by raising the calcium concentration, and it was also removed by adding d-amphetamine to the perfusion fluid. The block produced by a given concentration of acetylcholine was abolished by a much higher concentration of atropine or hyoscine.

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.

Some neurochemical alterations during acute DDT poisoning

Administration of a lethal dose (600 mg/kg) of 1,1,1-trichloro-2,2-bis( p -chlorophenyl)ethane ( p,p′ -DDT) to rats produced typical neurotoxic signs (e.g., hyperexcitability, tremors, convulsions and hyperthermia) and resulted in marked changes in brain acetylcholine (ACh), norepinephrine (NE) and 5-hydroxyindoleacetic acid (5-HIAA). Significant decreases in the amounts of cortical and striatal ACh as well as of brainstem NE were noted 5 hr after the administration of the insecticide. No significant alterations were found in striatal dopamine (DA) and brainstem 5-hydroxytryptamine (5-HT). However, brainstem 5-HIAA was significantly enhanced after treatment with p,p′ -DDT. In pargyline-pretreated rats, p,p′ -DDT produced increases in 5-HT which were greater than those observed in animals given the monoamine oxidase (MAO) inhibitor alone. Treatment with 6-fluorotryptophan (6-FT) or α -methyl- p -tyrosine ( α -MT), inhibitors of 5-HT or NE synthesis, respectively, prevented the DDT-induced hyperthermia, but not the tremors and convulsions. In contrast, administration of p -chlorophenylalanine ( p CPA) blocked all of the neurotoxic signs of p,p′ -DDT. In addition, cycloheximide, but not actinomycin D, prevented the insecticide-induced changes in striatal ACh and brainstem 5-HIAA, as well as the occurrence of tremors and hyperthermia. These results suggest that changes in the metabolism of brain 5-HT and NE may be responsible for the DDT-induced hyperthermia, while the tremors and convulsions seen after an acute lethal dose of this insecticide may be related to changes in brain ACh.

Role of brain acetylcholine and dopamine in acute neurotoxic effects of DDT

Abstract Neurotoxic effects (hyperexcitability, tremor) and hyperpyrexia seen after the acute administration of p , p ′-DDT (600 mg/kg) were associated with a significant decrease in ACh concentration without any concomitant changes in the levels of dopamine in the striatum of male and female rats. In contrast, the same dose of o , p ′-DDT failed to produce any of these symptoms and did not alter the concentration of the two neurohormones in rats of either sex. The results suggest that cholinergic mechanisms in the striatum may be involved in neurotoxic effects observed after the administration of p , p ′-DDT.

Effects of REMS deprivation on striatal dopamine and acetylcholine in rats

Changes in the concentration of striatal dopamine (DA) and acetylcholine (ACh) in rats deprived of REM sleep for 10 days were compared with those obtained after a 4 day deprivation procedure. Animals placed on small (7 cm dia.) islands surrounded by water were completely deprived of REM sleep but able to obtain some slow-wave sleep. Concentration of striatal DA was significantly increased after 4 days and 10 days of REM sleep deprivation by 73 and 133%, respectively when compared to controls. Levels of ACh in the striatum were significantly enhanced (by 28%) after 10 day, but failed to show significant change after 4 day REM sleep deprivation procedure. The short term locomotor activity was significantly higher in REM sleep-deprived animals. Our data indicate that REM sleep deprivation results in marked alterations of both cholinergic and dopaminergic mechanisms in the rat striatum.

THE ROLE OF BRAIN SEROTONIN IN DDT‐INDUCED HYPERPYREXTA

Abstract— Administration of a single dose of p,p′‐DDT to rats resulted in a marked increase in the concentration of 5‐hydroxyindoleacetic acid in the brain stem. No significant change was observed in the amount of 5‐hydroxytryptamine (serotonin) following treatment with this insecticide. However, when p,p′‐DDT was given to pargyline‐treated rats, a significantly greater increase in serotonin was observed when compared to that with the monoamine oxidase inhibitor alone. The concomitant administration of p,p′‐DDT and dl‐p‐chlorophenylalanine not only blocked the observed rise in 5‐hydroxyindoleacetic acid but also suppressed almost completely the neurotoxic symptoms (hyperpyrexia, tremors and convulsions) seen with the pesticide. In contrast, DL‐6‐fluorotryptophan (a more specific inhibitor of serotonin synthesis) prevented the DDT‐induced rise in body temperature as well as the increase in hydroxyindoleacetic acid, but did not affect the occurrence of tremor and convulsions. Our data suggest that an increase in the turnover rate of brain serotonin may underlie the hyperpyrexia induced by p,p′‐DDT.

Changes in brain cyclic AMP metabolism and acetylcholine and dopamine during narcotic dependence and withdrawal

Abstract Effects of morphine administration were studied on cyclic AMP metabolism in several regions of rat brain. In the cortex, cerebellum and thalamus-hypothalamus, morphine dependence did not alter the activity of either adenylate cyclase or phosphodiesterase. However, during withdrawal from the opiate treatment, adenylate cyclase activity declined in all three regions studied. In contrast, the striatal cyclic AMP metabolism was enhanced during morphine treatment as reflected by elevated endogenous cyclic AMP and increased adenylate cyclase. Furthermore, narcotic dependence produced significant increases in acetylcholinesterase activity of rat striatum. Whereas morphine withdrawal reversed the changes in striatal acetylcholine levels and acetylcholinesterase activity, the enhanced striatal dopamine remained unaltered. Although the activity of striatal adenylate cyclase was significantly reduced when compared to the morphine-dependent rats, the drop in cyclic AMP levels was not significant. Methadone replacement did not affect the changes in striatal dopamine seen in morphine-withdrawn rats. Whereas dopamine stimulated equally well the striatal adenylate cyclase from control or morphine-dependent animals, it failed to stimulate the striatal enzyme from rats undergoing withdrawal. The crude synaptosomal fraction of the whole brain from morphine-dependent rats exhibited an increase in cyclic AMP which was accompanied by elevated adenylate cyclase and protein kinase activity. Naloxone administration suppressed this rise in cyclic AMP and reversed the morphine-stimulated increases in the activities of adenylate cyclase and protein kinase. Following the withdrawal of morphine treatment, alterations in cyclic AMP metabolism were similar to those noted in morphine-naloxone group. Furthermore, substitution of morphine with methadone antagonized the observed alterations in cyclic nucleotide metabolism during withdrawal.

REGULATION OF PYRUVATE KINASE IN THE RAT CEREBRAL CORTEX

—The distribution of pyruvate kinase (ATP: pyruvate phosphotransferase; EC 2.7.1.40) in several areas of the central nervous system, the ontogenic changes in the cerebro‐cortical enzyme, and its modulation by certain metabolites were investigated in the rat. No significant differences in activity of pyruvate kinase in different regions of the nervous system were detected when activity was expressed per g of tissue. Studies on the ontogeny of pyruvate kinase in cerebral cortex revealed extremely low levels of enzymic activity at birth. A two‐fold increase occurred between 1 and 20 days of postnatal age, with a further two‐fold increase between 20 and 60 days of age.

Alterations in striatal acetylcholine, acetylcholine esterase and dopamine after methadone replacement in morphine-dependent rats

Abstract Chronic treatment of rats with morphine was found to produced a significant increase in endogenous ACh and DA as well as a decrease in AChE activity of rat striatum. Narcotic withdrawal for 48 hr resulted in a reversal of morphine-induced changes in striatal ACh levels and AChE activity whereas the enhanced striatal DA remained unaltered. Methadone replacement failed to affect the changes in striatal ACh and AChE activity but abolished the increases in striatal DA seen in morphine-withdrawn rats. Our results suggest that ‘morphine addiction’ in rats may involve alterations in the cholinergic and dopaminergic mechanisms of the striatum and that methadone replacement influences primarily the changes in the dopaminergic system produced by chronic morphine and its subsequent withdrawal.