Jorge Bartolome

Role of ornithine decarboxylase and the polyamines in nervous system development: a review

Development of nervous tissue is controlled, in part, by the ornithine decarboxylase (ODC)/polyamine system. Each brain region possesses a unique ontogenetic pattern for ODC, with highest levels of the enzyme associated with periods of most rapid growth. For this reason, perturbation of the ODC profile has proven useful in examinations of teratologic mechanisms and detection of adverse environmental effects during development. More recently, the replication of neuronal cells in developing brain has been shown to require the maintenance of polyamine levels and consequently, depletion of polyamines by alpha-difluoromethylornithine (DFMO, an ODC inhibitor) arrests brain cell maturation. DFMO also interferes with neuronal migration, axonogenesis and synaptogenesis, leading to disruption of the cytoarchitectural organization of brain structures: these results imply a similarly important role for polyamines in post-replicative events. Indeed, [3H]DFMO-autoradiographic localization of ODC in developing cerebellar lamina indicates high levels of activity associated with neuropil, areas of axonal outgrowth, and post-mitotic granule cells. Axonal outgrowth during regeneration after nerve damage in the mature nervous system may display some of the same characteristics as in developing neurons, suggesting that the two processes share common polyamine-dependent mechanisms.

Impaired development of cerebellar cortex in rats treated postnatally with α-difluorometh ylornithine

alpha-Difluoromethylornithine specifically and irreversibly inhibits the enzyme ornithine decarboxylase. Ornithine decarboxylase catalyses the initial step in the synthesis of polyamines, which are thought to play an essential role in growth and development of mammalian tissues. The current study examined the effects of alpha-difluoromethylornithine on the ontogenic development of the rat cerebellar cortex. Animals injected daily with alpha-difluoromethylornithine on postnatal days 1-21 suffered a deficit in the number of granule cells and many of the remaining granule cells became trapped in the molecular layer during migration. Purkinje cells were also scattered throughout the molecular layer and their mean diameter was 38% smaller than in controls. In general, the cerebellar cortex of alpha-difluoromethylornithine-treated rats failed to progress much beyond the stage of development reached in control rats during the first postnatal week. These effects of alpha-difluoromethylornithine were already clearly visible at 10-15 days of age. The final size of the cerebellum as a whole and of individual folia was markedly subnormal. These data indicate that polyamines play an obligatory role in cerebellar neurogenesis and histogenesis.

Postnatal development of brain alpha1-adrenergic receptors: In vitro autoradiography with [125i]HEAT in normal rats and rats treated with alpha-difluoromethylornithine, a specific, irreversible inhibitor of ornithine decarboxylase

The postnatal development of brain alpha 1-adrenergic receptors was studied in the rat brain using in vitro autoradiography. In some regions, such as the globus pallidus, receptor-binding sites were present at birth and increased during the first week but then decreased to very low levels by adulthood. In contrast, other regions such as the olfactory bulb and cerebral cortex exhibited little binding at birth, with a subsequent increase in receptors during the second week of life that persisted into the mature stage. Several regions had an intermediate pattern with significant labelling at birth, an increase in the first few weeks and a smaller decrement in binding sites as adulthood was approached. The data suggested that there were two archetypal development patterns, one of which was potentially related to the arrival of noradrenergic nerve projections (olfactory bulb) and the other of which was determined intrinsically by differentiation (globus pallidus). The two patterns could be distinguished by their sensitivity to alpha-difluoromethylornithine, a drug that inhibits ornithine decarboxylase, leading to a slowing of cellular replication, differentiation and migration. Drug treatment dramatically delayed the developmental fall-off of binding in the globus pallidus such that receptor sites remained in high concentration well past the point at which they disappeared in control animals. In the olfactory bulb, however, alpha-difluoromethylornithine had little or no effect on the ontogeny of receptor binding. These studies provide evidence that alpha 1-adrenergic receptors in various brain regions develop at different rates and with at least two characteristic patterns. Autoradiographic techniques provide important insights into receptor development that cannot be garnered from biochemical methods using isolated membrane preparations.

[40] Ornithine decarboxylase: Marker of neuroendocrine and neurotransmitter actions

Publisher Summary Ornithine decarboxylase (ODC) catalyzes the formation of putrescine from ornithine, the initial step in the synthesis of the polyamines spermidine and spermine. The polyamines are ubiquitously distributed in the tissues, and they appear to be associated intimately with tissue growth and cellular replication and differentiation. The ODC molecule itself may participate in growth regulation, as it appears to be an initiation factor for RNA polymerase. In addition to stimulation during general growth, increases in ODC activity have been demonstrated to occur in target organs exposed to specific hormones or neurotransmitters. The rapid rise of ODC activity has enabled the enzyme to be useful as a marker for tissue stimulation. The chapter presents several mechanisms to explain regulation of ODC activity. These include transcriptional, posttranscriptional, translational and posttranslational processes.

Effects of β-endorphin on ornithine decarboxylase in tissues of developing rats: a potential role for this endogenous neuropeptide in the modulation of tissue growth

Ornithine decarboxlyase (ODC) catalyzes the initial step in the bio-synthesis of the polyamines spermidine and spermine, which are key regulators of cell growth, proliferation and differentiation. Intracisternal administration of beta-endorphin (1 microgram) to 6 day-old rats markedly decreased brain, liver, heart and kidney ODC activity. Conversely, subcutaneous administration of beta-endorphin increased ODC activity in the heart and liver. Thus, ODC inhibition in peripheral organs in rat pups given beta-endorphin intracisternally appears to reflect central effects of this neuropeptide. Experiments were also carried out to test whether opioid receptors are involved in these tissue ODC responses. Naloxone prevented the decreases in brain ODC indicating the participation of opioid receptors in that process. In contrast, naloxone did not alter ODC responses in peripheral organs in rat pups given beta-endorphin intracisternally, indicating that these effects are independent of its classical opioid character. These results support the view that endogenous beta-endorphin may play an important role in organogenesis by modulating the growth-related enzyme ODC. The data also suggest that the regulation of peripheral organ development by beta-endorphin may be mediated through the release of growth regulatory substances from the CNS.

Corticotropin-Releasing Factor Concentrations Exhibit an Apparent Diurnal Rhythm in Hypothalamic and Extrahypothalamic Brain Regions: Differential Sensitivity to Corticosterone

Regional brain concentrations of corticotropin-releasing factor (CRF) exhibited marked differences in a number of regions depending upon the time of day rats were sacrificed. When compared with animals killed at 09.00 h, CRF concentrations at 15.30 h were elevated in the median eminence, hypothalamus minus median eminence, preoptic area/suprachiasmatic nucleus, bed nucleus of the stria terminalis, septum, frontal/parietal cortex, cerebellum, cingulate cortex, locus ceruleus and the nucleus of the solitary tract. Animals that received 7 days of continuous corticosterone supplementation via osmotic minipump either did not exhibit an afternoon rise in CRF concentrations (median eminence, cingulate cortex and locus ceruleus) or exhibited significantly attenuated afternoon increases in CRF concentrations (hypothalamus minus median eminence). In rats killed at 09.00 h, corticosterone reduced CRF concentrations in the median eminence. Chronic corticosterone exposure did not affect CRF concentrations in any of the extrahypothalamic brain regions studied in rats killed at 09.00 h. These results suggest that a number of CRF-containing neurons in both hypothalamic and extrahypothalamic brain regions presumably undergo diurnal changes in their activity as evidenced by changes in CRF concentrations. Furthermore, chronic corticosterone supplementation can alter these changes, particularly those in the endocrine hypothalamus.

Nutritional Influences on Adrenal Chromaffin Cell Development: Comparison with Central Neurons

ABSTRACT: Neurotransmitter systems in the developing brain are generally protected from growth retardation associated with nutritional deprivation. To investigate if such protective mechanisms extend to similar tissues in the peripheral sympathetic system, maturation of the chromaffin cells of the adrenal medulla and development of their centrally derived splanchnic innervation were evaluated in rats whose nutritional status had been altered during the neonatal period by increasing (16–17 pups/litter) or decreasing (five to six pups/litter) the litter size from the standard (11–12 pups/litter). Ontogeny of adrenal catecholamine stores and activities of catecholamine-biosynthetic enzymes tyrosine hydroxylase and phenylethanolamine N-methyltransferase were monitored, along with activity of choline acetyltransferase, a marker enzyme for the preganglionic neurons innervating the chromaffin cells. Neonatal nutritional deprivation slowed body weight gain and retarded development of the chromaffin cells, as evidenced by subnormal catecholamine stores, tyrosine hydroxylase and phenylethanolamine N-methyltransferase activities. The effects persisted despite the complete recovery of body weights postweaning. The developmental alterations were not caused by overcrowding stress, as plasma corticosterone levels were not elevated in the large litter group. Neonatal nutritional enrichment promoted body weight gain but failed to enhance development of adrenal catecholamines; tyrosine hydroxylase and phenylethanolamine N-methyltransferase activities were elevated only in the preweaning period. In contrast to effects on the chromaffin cells, altered neonatal nutritional status had only minor, transient effects on the development of the centrally derived cholinergic innervation of the adrenal and produced only small changes (<10%) in brain tyrosine hydroxylase activity. These results suggest that, unlike central transmitter systems, maturation of chromaffin cells is adversely affected by neonatal nutritional deprivation; ontogenetic gains may already be close to optimum at normal nutritional status.

Neonatal methylmercury poisoning in the rat: Effects on development of central catecholamine neurotransmitter systems

Abstract The effects of neonatal methylmercury (CH 3 -Hg) exposure on the biochemical development of central nervous system catecholamine synapses were examined by assessing synaptosomal and synaptic vesicular uptakes of [ 3 H]catecholamines, activity of tyrosine hydroxylase, the rate-limiting enzyme in catecholamine biosynthesis, and levels and turnover rates of norepinephrine and dopamine. Methylmercury was given daily for up to 21 days beginning at birth, using three dose levels (1, 2.5, or 5 mg/kg, sc). Only the highest dose retarded body and brain weight gains. Exposure of neonates to CH 3 -Hg produced initial inhibition of [ 3 H]dopamine synaptosomal uptake, followed by marked elevations of uptake from 20 days onward. The alterations in synaptosomal uptake were preceded by increases in the turnover rate of dopamine; however, normal developmental patterns were seen for other presynaptic terminal parameters (vesicular uptake, tyrosine hydroxylase activity, and dopamine content). Synaptosomal uptake of [ 3 H]norepinephrine also displayed an eventual elevation in CH 3 -Hg-treated animals again preceded by increased transmitter turnover. The alterations in uptake and turnover for both neurotransmitters indicate that the synaptic dynamics of developing central catecholaminergic neurons are indeed affected by CH 3 -Hg treatment. In keeping with the behavioral teratology of this substance, abnormalities of neurotransmitter biochemistry can be detected at doses well below those which influence growth of the organism.

Role of polyamines in isoproterenol-induced cardiac hypertrophy: Effects of α-difluoromethylornithine, an irreversible inhibitor of ornithine decarboxylase

Abstract DFMO is a selective irreversible inhibitor of ornithine decarboxylase (ODC), the initial enzyme in the polyamine biosynthetic pathway. DFMO was utilized to determine the role of polyamines in isoproterenol (ISO)-induced cardiac hypertrophy. Daily subcutaneous administration of 200 mg/kg of DFMO reduced cardiac putrescine levels but did not significantly alter the basal levels of spermidine or spermine, nor was normal cardiac growth affected. ISO-induced cardiac hypertrophy was accompanied by increased putrescine and spermidine levels but spermine was not significantly altered. DFMO reversed the ISO-induced increases in putrescine and inhibited or attenuated both the increases in spermidine content and the cardiac hypertrophy. Although normal ODC activity appears not to be necessary for the maintenance of basal levels of polyamines or for normal cardiac growth, sustained inhibition of ODC interferes with ISO-induced elevations of putrescine, spermidine and heart weight.

Effects of central administration of beta-endorphin on brain and liver DNA synthesis in preweanling rats

We have previously shown that central administration of beta-endorphin results in a reduction of ornithine decarboxylase activity. Ornithine decarboxylase catalyses the rate-limiting step in the biosynthesis of the polyamines putrescine, spermidine and spermine, thought to modulate nucleic acid synthesis. The present study examines the effects of intracisternal injection of beta-endorphin on brain and liver DNA synthesis in preweanling rats. In six-day-old rats, beta-endorphin (0.75 micrograms/g brain wt) produced approximately a 70% inhibition in brain and liver DNA synthesis 1 h after injection, and values were still subnormal in both tissues 10 h later. Subcutaneous administration of beta-endorphin did not alter liver DNA synthesis. Thus, it is most likely that the suppressed liver DNA synthesis observed in animals given beta-endorphin intracisternally is mediated by central mechanisms. Co-administration of naloxone plus beta-endorphin intracisternally prevented the response, indicating an opioid receptor-mediated phenomenon. Naloxone alone caused small but significant increases in brain and liver DNA synthesis, suggesting a tonic influence on tissue DNA by endogenous opioids in the CNS. Acute inhibition of ornithine decarboxylase activity by alpha-difluoromethylornithine did not alter DNA synthesis, indicating that the decreases in DNA synthesis induced by beta-endorphin are unrelated to the ornithine decarboxylase/polyamine system. The effect appears to be restricted to early development as no significant changes in DNA synthesis were obtained in 20-day-old animals. The results from these studies indicate that CNS beta-endorphin has the ability to influence DNA synthesis in central as well as in peripheral tissues.(ABSTRACT TRUNCATED AT 250 WORDS)