S.J. Weigel

Critical periods for the role of ornithine decarboxylase and the polyamines in growth and development of the rat: Effects of exposure to α-difluoromethylornithine during discrete prenatal or postnatal intervals.

The roles of ornithine decarboxylase (ODC) and the polyamines in fetal and neonatal development were examined through the use of α‐difluoromethylornithine (DFMO), a specific irreversible inhibitor of ODC. Administration to pregnant rats of 500 mg/kg of DFMO every 12 h for a 4‐day period (8 DFMO injections) resulted in fetal and neonatal death; DFMO early in gestation produced fetal resorption whereas late gestational exposure did not compromise fetal viability but instead resulted in a delayed toxic effect, with high mortality in the first postnatal week. Generalized toxicity of DFMO was not apparent in later developmental periods, as 4 days of DFMO treatment begun postnatally did not produce any neonatal death. Shortening the course of gestational DFMO treatment to 2.5 days (5 DFMO injections) also did not adversely affect fetal or neonatal viability and thus permitted identification of critical periods in which various tissues are sensitive to DFMO. Examination of growth patterns of brain, heart and kidney and of neurochemical development of central and peripheral catecholaminergic neurons indicated that different critical periods exist for effects of DFMO on each tissue or even on the various cell types within a tissue. The separable sensitivities were apparent even though the effects of DFMO on ODC and the polyamines for any given treatment period were fairly uniform in all tissues studied. These results indicate that the ODC/polyamine system plays multiple roles in fetal survival and in tissue growth during discrete periods of development; because the time course of cellular maturation differs for each tissue or cell population, DFMO administered during any one brief period can produce organ‐specific developmental deficits.

Role of ornithine decarboxylase and the polyamines in nervous system development: Short-term postnatal administration of α-difluoromethylornithine, an irreversible inhibitor of ornithine decarboxylase.

The role of ornithine decarboxylase (ODC) and the polyamines in development of central and peripheral catecholaminergic neurons was examined through the use of α‐difluoromethylornithine (DFMO), a specific irreversible inhibitor of ODC. Short‐term postnatal administration of DFMO (500 mg/kg daily on days 1–6) to neonatal rats resulted in effective inhibition of ODC and depletion of both putrescine and spermidine in brain, heart and kidney; after cessation of DFMO administration, polyamine levels returned to normal by 10–13 days of age. There were no signs of generalized toxicity of short‐term DFMO treatment, as body weight gains were largely unaffected over the course of study (through weaning). However, development of peripheral sympathetic neurons was severely retarded by DFMO, with persistent and profound deficits of both cardiac and renal norepinephrine; the catecholamine deficiencies were unrelated to effects on end‐organ growth, as cardiac weights were essentially normal whereas kidney weights were adversely affected by DFMO. Development of the adrenal medulla, a peripheral catecholaminergic tissue which displays approximately the same developmental profile as do sympathetic neurons but which does not develop axonal projections, was not slowed by DFMO treatment; similarly, central noradrenergic and dopaminergic neurons, which undergo the majority of axonal outgrowth and synaptogenesis during the second to third postnatal week (just after the period in which polyamines returned to control levels), developed normally as assessed by measurements of transmitter levels, tyrosine hydroxylase activity and synaptosomal uptake of [3H]norepinephrine or [3H]dopamine. Extension of the period of DFMO treatment and consequent depletion of polyamines into the period in which central synaptogenesis occurs does, however, produce slowing of development of brain catecholamine neuronal projections. Thus, the ODC/polyamine system appears to play a critical postnatal role in catecholamine systems specifically undergoing active synaptogenesis.

Organ specificity of neonatal methyl mercury hydroxide poisoning in the rat: Effects on ornithine decarboxylase activity in developing tissues

To determine the organ specificity of neonatal mercury hydroxide (CH3HgOH) exposure on biochemical development of its potential target tissues, effects on rat brain, liver, heart and kidney were compared utilizing the ontogenetic pattern of ornithine decarboxylase (ODC) activity, an early index of perturbation of cellular maturation. CH3HgOH was given daily beginning at birth for up to 21 days, using three dose levels (1, 2.5 or 5 mg/kg s.c.). In the brain, CH3HgOH treatment resulted in an initial reduction in ODC followed by a subsequent elevation of activity, a maturational pattern known to be associated with delayed cellular development. In contrast to the effects of CH3HgOH on brain, the pattern obtained in the liver, an initial elevation followed by a subsequent decline, is usually associated with compression of the time couse of cellular development. In the heart and kidney, CH3HgOH produced sustained elevations of ODC representing prolongation of the developmental period of rapid tissue growth and development; these patterns were associated with tissue hypertrophy which was sustained through the preweaning stage for both tissues and well into the postweaning period for the kidney. The results obtained with ODC clearly demonstrate that neonatal CH3HgOH poisoning causes organ-specific biochemical lesions which can play a role in subsequent effects on overall tissue development.

Effects of α-fluoromethylhistidine (FMH), an irreversible inhibitor of histidine decarboxylase, on development of brain histamine and catecholamine systems in the neonatal rat

Daily administration of FMH to neonatal rats produced long-lasting inhibition of histidine decarboxylase in hypothalamus and cerebral cortex and led to depletion of histamine in both brain regions. The onset of depletion was more rapid in cerebral cortex, a region in which non-neurotransmitter pools of histamine predominate in early postnatal life, appearing as early as postnatal day 3; depletion in the hypothalamus, a region rich in histaminergic neuronal projections, appeared later. No effects were seen on body or brain growth, nor was development of other biogenic amine systems affected. FMH thus provides a selective probe for examining the role of histamine in brain development.

Postnatal methyl mercury exposure: Effects on ontogeny of renal and hepatic ornithine decarboxylase responses to trophic stimuli

The effects of postnatal methyl mercury exposure on the ontogeny of renal and hepatic responsiveness to trophic stimuli were examined. Increased ornithine decarboxylase (ODC) activity was used as an index of tissue stimulation. In the rat, renal ODC responsiveness to growth hormone, angiotensin, vasopressin, isoproterenol, and serotonin was absent at birth and matured 3 to 4 weeks later. However, pups exposed to methyl mercury showed marked, ODC responses to these same agents as early as 10 to 19 days of postnatal age, accompanied by a significant renal hypertrophy. In contrast to the kidney, the liver of normally developing rats was responsive to trophic factors even in the neonate. In this tissue, there was no consistent effect of neonatal methyl mercury treatment on ODC responses at any developmental stage tested; although absolute liver weights were reduced, liver/body weight ratio was not affected. These results demonstrate that postnatal methyl mercury exposure causes a precocious onset of ODC responses to trophic agents specifically in the kidney. Altered responsiveness may mediate some of the effects of this organomercurial on overall renal development and function.

Maternal methadone administration: Deficit in development of alpha-noradrenergic responses in developing rat brain as assessed by norepinephrine stimulation of 33Pi incorporation into phospholipids in vivo

The effects of perinatal methadone exposure on the development of noradrenergic responses in the brain were examined by assessing the ability of intracisternally administered norepinephrine to stimulate 33Pi incorporation into phospholipids in vivo; the effect of norepinephrine is mediated by alpha1-receptors juxtaposed to noradrenergic nerve terminals. Although there was no difference in basal (unstimulated) incorporation of 33Pi, a deficit in norepinephrine-induced stimulation of incorporation was found throughout the preweaning period in offspring of dams treated daily with methadone beginning in midgestation. This effect was not seen when methadone was given during the postnatal period. Since perinatal methadone exposure also delays development of presynaptic catecholaminergic nerve terminals in the brain, these results support the view that perinatal exposure to methadone depresses overall central noradrenergic synaptic function; however, the effects on presynaptic development and on receptor-mediated responses appear to be separable in that they display differences in the critical age periods of sensitivity to perturbation by the drug.

Alterations in the development of catecholamine turnover induced by perinatal methadone: Differences in central vs. peripheral sympathetic nervous systems

Abstract Administration of methadone to pregnant and nursing rats slows synaptogenesis of central cathcholaminergic systems in the offspring but accelerates the onset of synaptic function in peripheral sympathetic pathways. Norepinephrine turnover, assessed by inhibiting catecholamine biosynthesis with alpha-methyl-p-tyrosine, was elevated in cardiac sympathetic nerve terminals in rats exposed perinatally to methadone. In contrast, turnover was unchanged in noradrenergic and dopaminergic systems in the brain. Similar results were obtained when methadone was given directly to the pups during postnatal life. These data suggest that opiate-induced alterations of impulse flow and transmitter turnover in a given neuron population may determine whether the effects of perinatal methadone exposure result in facilitation or inhibition of synaptic development.

Toxic effects of maternal methadone administration on cardiac development in the neonatal rat: Potential participation of altered polyamine levels in growth impairment

The pattern of development of polyamine levels in hearts of preweanling rats whose mothers received methadone indicated initial deficits in both spermidine and spermine followed by rebound elevations, a pattern consistent with delayed cellular development. Since polyamines are thought to play important roles in nucleic acid and protein synthesis during cellular maturation, these alterations may participate in the retardation of tissue growth seen in the perinatal opiate syndrome.