Stimulation of ornithine decarboxylase by histamine or norepinephrine in brain regions of the developing rat: evidence for biogenic amines as trophic agents in neonatal brain development

Norepinephrine, neurodevelopment and behavior

Neurotransmitters play critical roles in the developing nervous system. Among the neurotransmitters, norepinephrine (NE) is in particular postulated to be an important regulator of brain development. NE is expressed during early stages of development and is known to regulate both the development of noradrenergic neurons and the development of target areas. NE participates in the shaping and the wiring of the nervous system during the critical periods of development, and perturbations in this process can alter the brain's developmental trajectory, which in turn can cause long-lasting and even permanent changes in the brain function and behavior later in life. Here we will briefly review evidence for the role of noradrenergic system in neurodevelopmental processes and will discuss about the potential disruptors of noradrenergic system during development and their behavioral consequences.

Developmental exposure to terbutaline alters cell signaling in mature rat brain regions and augments the effects of subsequent neonatal exposure to the organophosphorus insecticide chlorpyrifos

Exposure to apparently unrelated neurotoxicants can nevertheless converge on common neurodevelopmental events. We examined the long-term effects of developmental exposure of rats to terbutaline, a beta-adrenoceptor agonist used to arrest preterm labor, and the organophosphorus insecticide chlorpyrifos (CPF) separately and together. Treatments mimicked the appropriate neurodevelopmental stages for human exposures: terbutaline on postnatal days (PN) 2-5 and CPF on PN11-14, with assessments conducted on PN45. Although neither treatment affected growth or viability, each elicited alterations in CNS cell signaling mediated by adenylyl cyclase (AC), a transduction pathway shared by numerous neuronal and hormonal signals. Terbutaline altered signaling in the brainstem and cerebellum, with gender differences particularly notable in the cerebellum (enhanced AC in males, suppressed in females). By itself, CPF exposure elicited deficits in AC signaling in the midbrain, brainstem, and striatum. However, sequential exposure to terbutaline followed by CPF produced larger alterations and involved a wider spectrum of brain regions than were obtained with either agent alone. In the cerebral cortex, adverse effects of the combined treatment intensified between PN45 and PN60, suggesting that exposures alter the long-term program for development of synaptic communication, leading to alterations in AC signaling that emerge even after adolescence. These findings indicate that terbutaline, like CPF, is a developmental neurotoxicant, and reinforce the idea that its use in preterm labor may create a subpopulation that is sensitized to long-term CNS effects of organophosphorus insecticides.

Altering the course of neurodevelopment: a framework for understanding the enduring effects of psychotropic drugs

Childhood is a time filled with wondrous changes, as brain plasticity permits experiences to shape the immature brain to meet the demands of the environment. Change occurs at various levels--from neuroanatomy, including within a given region and its connectivity to other regions, to the function of neurotransmitter systems and their reactivity to pharmacological agents in the short- and long-term. The nature and degree to which drug exposure influences the final adult topography is influenced greatly by the maturational phase of these critical factors. Moreover, evidence is slowly emerging that suggests that the long-term effects of drug exposure are delayed and expressed once the vulnerable system reaches maturation (i.e., typically during adulthood). This phenomenon is known as neuronal imprinting and occurs when the effects of drug exposure outlast the drug itself. Thus, understanding the persistent effects critically depends on the window of observation. Embracing this concept should influence how we conduct preclinical assessments of developmental drug exposure, and ultimately how we conduct clinical assessments of drug efficacy, effectiveness, and safety for the treatment of childhood psychiatric disorders. In this article, we present a model to provide a heuristic framework for making predictions about imprinted effects of childhood drug exposure. We then review epidemiological data on attention deficit hyperactivity disorder (ADHD) and childhood depression, prescription practices, and what is known regarding the long-term consequences of drug exposure in these populations. We conclude with a discussion of the current status of preclinical studies on juvenile stimulant exposure.

Transiently overexpressed alpha2-adrenoceptors and their control of DNA synthesis in the developing brain

During brain development, neurotransmitters act as trophic factors controlling the patterns of cell replication and differentiation. Alpha2-adrenoceptors (alpha2ARs) are transiently overexpressed in zones with high mitotic activity and we evaluated whether these receptors are linked to DNA synthesis in the perinatal rat brain. Acute administration of clonidine (2 mg/kg), an alpha2AR agonist, elicited dramatic decreases in DNA synthesis in the forebrain, brainstem, and cerebellum whether given on gestational day (GD) 21, or on postnatal days (PN) 1 or 8. However, alpha2AR blockade elicited by yohimbine (2.5 mg/kg) also resulted in decreased DNA synthesis on GD21 and PN8, albeit to a smaller extent than with clonidine. Yohimbine was able to blunt the effects of clonidine, verifying that both drugs are acting through the same receptor population. Because betaARs are also known to regulate DNA synthesis, we used propranolol (10 mg/kg) blockade of betaARs to evaluate whether the alpha2AR effects were mediated by presynaptic autoreceptors that regulate the release of norepinephrine and consequent betaAR responses; the effects of yohimbine were still discernible in the presence of propranolol. Accordingly, transiently overexpressed alpha2ARs in the developing brain participate in the control of DNA synthesis in a biphasic manner, with promotional actions at low, endogenous levels of stimulation, but inhibitory effects when stimulation is high. Effects on alpha2ARs are likely to contribute to long-term consequences of adrenergic agents used in obstetrics or neurotoxicants that affect adrenergic activity.

Pre- and post-hatching developmental changes in beta-adrenoceptor subtypes in chick brain

This study used [3H]CGP 12177 as a radioligand to determine the beta1 and beta2-adrenoceptor changes from the pre-hatching E17 stage, where the beta2 subtype is first detected, to the post-hatching P30 stage. While beta1-adrenoceptors were found to be present from E18 and were limited to cerebellum and hyperstriatum in all stages studied, beta2-adrenoceptors showed a wider distribution throughout the brain. In most of the structures analysed both beta1- and beta2-adrenoceptor binding values reached a maximum in the P2 stage, followed by a decrease over the following days. A second increase in both subtypes was detected again in the P15 and P30 stages. These results support the notion of a specific role for beta-adrenoceptors in neural plasticity in the first week after hatching and suggest that the beta2 subtype is the main adrenoceptor in chick brain throughout its development.

The tuberomammillary nucleus projections in the control of learning, memory and reinforcement processes: evidence for an inhibitory role

The tuberomammillary nucleus (TM), a cluster of magnocellular cells in the posterior hypothalamus, is the main source of neuronal histamine in the brain. Although this nucleus is well described in terms of anatomy and neurochemistry, only little is known about its function. Our earlier work showed that the TM projection system may be involved in behavioral asymmetries and behavioral recovery after unilateral manipulations of the brain. Using horseradish peroxidase (HRP) labeling we found an increase in strength (structure and/or activity) in the crossed and uncrossed tuberomammillary-striatal projections in the course of recovery from behavioral asymmetries produced by unilateral removal of the rats' vibrissae, which were in the same direction as the asymmetries found in projections from the substantia nigra to the striatum. Experiments performed with unilateral lesions of the TM region provide evidence for an involvement of the TM system in reinforcement mechanisms. Unilateral destruction of the TM with direct current (DC) or ibotenic acid was found to increase the rate of lateral hypothalamic self-stimulation ipsilateral to the lesion site, suggesting that the TM (particularly the E2 subgroup in its rostral part) may function as a reinforcement inhibiting neural substrate. Experiments performed with bilateral DC or ibotenic acid lesions of the TM region suggest a role of the nucleus in learning and mnemonic processes. A bilateral electrolytic or neurotoxic lesion of the TM region was found to facilitate the performance of adult and behaviorally impaired aged rats in a variety of learning tasks, including a habituation paradigm, aversively motivated learning tasks and water mazes. Examination of the site of the neurotoxic lesion in the TM region with immunohistochemical techniques revealed a marked decline of histamine-staining neurons mainly in the rostral part of the TM nucleus, suggesting that the facilitatory effects on reinforcement and mnemonic processes might be related to the destruction of TM intrinsic histaminergic cells. In summary, the present results indicate that the TM nucleus is involved in neural plasticity and functional recovery following damage to the CNS and may function as an inhibitory neural substrate in the control of reinforcement and mnemonic processes.

Ontogenetic development of histamine receptor subtypes in rat brain demonstrated by quantitative autoradiography

The postnatal ontogenetic development of the histamine receptor subtypes was studied in rat brain by quantitative receptor autoradiography with highly sensitive imaging plates. H1 receptor binding sites labeled with [3H]pyrilamine were detected on postnatal day 2 (P2) and increased very slowly until P9, and then rapidly reaching the adult levels in the hypothalamus, hippocampus, and amygdala by P16. The densities of H1 receptor binding sites in the cortex, striatum, thalamus, and substantia nigra were relatively low during development. H3 receptor binding sites labeled with [3H](R) alpha-methylhistamine were not detectable until P9. On P9, their density was higher in the substantia nigra than in other regions. Subsequently, H3 receptor binding increased, reaching the adult levels in the substantia nigra on P16 and in the other regions on P23. The histamine concentration was initially very high, but decreased to the adult level by P16. On the contrary, the activity of L-histidine decarboxylase of whole brain tissue was low on P5, and increased markedly from P16 to P23, to the adult level on P30. Administration of (S) alpha-fluoromethylhistidine (FMH), a specific inhibitor of L-histidine decarboxylase (HDC), significantly decreased both the HDC activity and histamine concentration during postnatal development. FMH treatment did not change H1 receptor binding in any brain region, but significantly increased H3 receptors in the substantia nigra and striatum on P23. Unilateral injection of 6-hydroxydopamine into the striatum on P2 resulted in up-regulation of H3 receptor binding sites in the dorsomedial (11%) and dorsolateral (18%) regions of the striatum and substantia nigra (31%) on P23, but no change in the H3 receptor density in the nucleus accumbens or frontal cortex on P11 and P23. These results demonstrate that the developmental patterns of H1 and H3 receptors are heterogeneous and independent of each other. There are marked mismatches of presynaptic and postsynaptic markers of the histaminergic neuron system as in other aminergic systems.

Role of histamine receptors in intestinal repair after ischemia-reperfusion in rats

BACKGROUND/AIMS

Previously, we showed that an elevated production of histamine promotes the healing of injured intestinal mucosa after ischemia-reperfusion. The aim of the present study was to determine whether histamine-mediated repair of the intestinal mucosa after ischemia-reperfusion involves the engagement of H1 or H2 receptors.

METHODS

The superior mesenteric artery was occluded for 15 minutes followed by reperfusion, and H1- or H2-receptor antagonists were infused intraduodenally. After ischemia-reperfusion, ornithine decarboxylase activity in the jejunal mucosa and lipid transport to mesenteric lymph were examined.

RESULTS

In jejunal mucosa, ornithine decarboxylase activity markedly increased at 6 hours after reperfusion and remained elevated at 48 hours. The ischemia-reperfusion-induced increase in ornithine decarboxylase activity was attenuated (in a dose-dependent manner) by an H1-receptor antagonist (chlorpheniramine maleate) but not by an H2 antagonist (cimetidine). Intraperitoneal injection of an H3 antagonist (thioperamide) increased histamine output in mesenteric lymph and stimulated intestinal ornithine decarboxylase activity. Transport of dietary lipid into mesenteric lymph was depressed 24 hours after an ischemic insult, yet it returned to the normal level 48 hours after ischemia-reperfusion. The recovery of the lipid transport normally observed at 48 hours after ischemia-reperfusion was attenuated by the H1 antagonist.

CONCLUSIONS

The beneficial effects of histamine on the repair of intestinal mucosa after ischemia-reperfusion results from the engagement and activation of the H1 receptor.

Regional neuropathology in schizophrenia: where are we? Where are we going?

This paper presents a neurologic formulation for the clinical features of the schizophrenic syndrome, and tests it against a systematic, region by region review of available postmortem neuroanatomical and neuropharmacological data. Based on this review a model is proposed that postulates a developmental lesion affecting the midline neurotransmitter-specific ascending projection systems. Due to the facilitatory role these systems play in the development of the brain regions to which they project, such a lesion is one parsimonious, and testable, explanation for virtually all the clinical, laboratory, and pathological findings reported to date in schizophrenia research. A case is made for establishing a global antemortem-postmortem collaboration using a Latin square design; the alternative may be that, as has happened in the past, the best efforts of dilligent researchers around the world may lead to little improvement in our understanding of schizophrenia.

Unilateral lesion in the tuberomammillary nucleus region: behavioral asymmetries and effects of histamine precursor

The subnuclei of tuberomammillary nucleus are located in the posterior part of the hypothalamus adjacent to the basolateral surface of the mammillary bodies. The neurons of this nucleus innervate extensive parts of the brain with several transmitters, particularly with histamine. In fact, they represent the only source of histaminergic projections in the brain. The present study deals with the effects of a lesion in this region on behavior. Unilateral electrolytic direct current (DC) lesions in the tuberomammillary nucleus led to an asymmetry in thigmotactic scanning; i.e., at 11 days, but not 1 day postlesion, the rats scanned the walls of an open field more with the vibrissae contralateral to the lesion than with those of the ipsilateral side. Furthermore, they emitted more ipsiversive than contraversive wide angle turns. The behavioral asymmetries are, in general, opposite in direction to those induced by lesion of the neighboring lateral hypothalamus and substantia nigra, indicating that they are specific to the tuberomammillary region destroyed. Application of the histamine precursor histidine led to a compensation of these asymmetries, suggesting that the tuberomammillary's histaminergic efferents are functionally related to the lesion-induced behavioral effects.

Histamine and histidine decarboxylase are correlated with mucosal repair in rat small intestine after ischemia-reperfusion

The aim of this experiment was to demonstrate whether histamine and histidine decarboxylase (HDC) contribute to mucosal repair in small intestine subjected to ischemia-reperfusion (I/R). The superior mesenteric artery was occluded for 15 min followed by reperfusion. In jejunal mucosa, histamine content and HDC activity increased after I/R. Histamine output in mesenteric lymph was also elevated after I/R. These increases in HDC activity, and mucosal and lymph histamine levels were suppressed by pretreatment of alpha-fluoromethylhistidine (alpha-FMH), a suicide inhibitor of HDC. alpha-FMH also attenuated the increase of ornithine decarboxylase (ODC) activity normally observed after I/R. Transport of dietary lipid into lymph markedly decreased at 24 h after I/R, yet it was restored to normal at 48 h after I/R. alpha-FMH inhibitor led to a sustained deficit in lipid transport at 48 h after I/R. This sustained functional impairment in alpha-FMH treated animals was associated with blunted responses of HDC activity and histamine content to I/R. Our results suggest that histamine and HDC contribute to the restoration in mucosal function observed at 48 h after I/R. This response may be related, at least in part, to stimulation of ODC activity by histamine.

Autoradiography of adrenoceptors in rat and human brain: alpha-adrenoceptor and idazoxan binding sites

This chapter reviews the current classification of adrenoceptors, and notes the difficulties of combining the molecular biological and pharmacological classifications of adrenoceptors. Possibilities for mapping the distribution of the proposed subtypes of adrenoceptors using currently available ligands are discussed, and the autoradiographic visualisation of the broad subtypes of alpha 1-, alpha 2-, beta 1-, and beta 2-adrenoceptors in the rat, monkey and human brain described and illustrated. The non-selectivity of ligands currently being used to label alpha-adrenoceptors is shown; we compare the distribution of [3H]idazoxan binding sites with the distribution of alpha 2-adrenoceptors visualised using other ligands. Resolution limitations of current autoradiographic approaches are considered and we shown how in situ hybridisation can complement data from receptor labelling studies used to localise receptors to pre- or postsynaptic sites.

Beta 2-adrenergic receptors are colocalized and coregulated with "whisker barrels" in rat somatosensory cortex

Autoradiography has been used to visualize independently the subtypes of beta-adrenergic receptors in rat somatosensory cortex. Beta 2-Adrenergic receptors, but not beta 1-adrenergic receptors colocalize with "whisker barrels" in this tissue. Thus, each whisker sends a specific multisynaptic pathway to the somatosensory cortex that can be histochemically visualized and only one subtype of beta-adrenergic receptor is specifically associated with this cortical representation. Additionally, neonatal lesion of any or all of the whisker follicles results in loss of the corresponding barrel(s) as shown by histochemical markers. This loss is paralleled by a similar loss in the organization of beta 2-adrenergic receptors in the somatosensory cortex. Other results indicate that these beta 2-adrenergic receptors are not involved in moment-to-moment signal transmission in this pathway and, additionally, are not involved in a gross way in the development of whisker-barrel array.

Trophic control of the ornithine decarboxylase/polyamine system in neonatal rat brain regions: lesions caused by 6-hydroxydopamine produce effects selective for cerebellum

Norepinephrine has been hypothesized as a trophic factor influencing postnatal development of the cerebellum. In the current study, neonatal rats were given 6-hydroxydopamine (6-OHDA) to destroy noradrenergic projections and the effects on the ornithine decarboxylase (ODC)/polyamine system were evaluated; ODC initiates the synthesis of polyamines, which are known to control cellular development in the cerebellum, and neonatal ODC activity is regulated in part by beta 2-adrenergic receptors. Intracisternal administration of 6-OHDA resulted in complete and permanent depletion of cerebellar norepinephrine and a deficit in ODC, polyamine levels and cerebellar growth. Subcutaneous administration of 6-OHDA, which caused only a small initial reduction in cerebellar norepinephrine, did not affect ODC and had only minor effects on tissue growth. Indeed, levels of the polyamines tended to be elevated after subcutaneous 6-OHDA, associated with postweaning elevations in norepinephrine, results which are probably indicative of axonal regeneration. In contrast to the effects of 6-OHDA on cerebellar development, neither the intracisternal nor subcutaneous drug treatment had any effect on cerebral cortical polyamines or growth, although the intracisternal treatment did impair ODC activity early in development. These data suggest that postnatal noradrenergic input, acting through the ODC/polyamine pathway, plays a selective role in cerebellar development.

Histamine increases ornithine decarboxylase activity in primary cultures of cerebellar granule cells

The effect of histamine on the activity of ornithine decarboxylase (ODC) of cerebellar granule neurons was studied using primary cultures grown both in serum-containing medium and in chemically defined medium. In comparison with granule neurons grown in chemically defined medium, the activity of ODC was about twice as great in the neurons grown in serum-containing medium. Treatment of cultured cerebellar neurons with histamine caused a dose-dependent increase in ODC activity. The maximum elevation was observed at 500 nM of histamine, when the increase in ODC activity was about 50% and 120% over controls in granule cells grown in serum-containing medium and in chemically defined medium, respectively. Histamine had no significant effect on the activity of lactate dehydrogenase in these cultures. The present findings provided direct evidence for the involvement of histamine in the regulation of ODC-related non-mitotic growth of granule neurons in the cerebellum.

Freund's complete adjuvant induces ornithine decarboxylase activity in the central nervous system of male rats and triggers the release of pituitary hormones

In male rats, inoculation of Freund's complete adjuvant (FCA, 0.5 mg/rat of Mycobacterium butyricum in paraffin oil) induced high levels of ornithine decarboxylase (ODC) in the hypothalamus and pituitary gland (285% and 245% of controls, respectively, within 12 h to 2 days). ODC activity also was altered in the cerebellum and left neocortex, but not in the right neocortex. This activity reflected a dynamic equilibrium which is influenced by ODC synthesis, degradation, activation, etc. The circadian rhythms of pituitary ODC activity and plasma prolactin level, 3-4 days after FCA, showed that enhancement of enzymatic activity during the dark phase correlated with a marked release of prolactin (Prl). During this early period after FCA, changes in plasma levels of other pituitary hormones were not significant or were less important. Pretreatment with bromocriptine microcapsules inhibited both basal and FCA-induced pituitary ODC activity, as well as Prl secretion. Further, significant increases in plasma luteinizing hormone and adrenocorticotropic hormone were noted from days 4 and 8, respectively, and onwards. Finally, a phase of reduced corticosterone secretion occurred during the latency period. This study shows that FCA influences central nervous system pathways and supports the idea that endogenous Prl is involved in some early events which lead to the development of adjuvant arthritis.

Effect of histamine on the development of astroglial cells in culture

The effect of histamine on different aspects of the growth of astrocytes was studied using primary cultures derived either from forebrain or from cerebellum of the rat. The influence on general growth and differentiation was monitored in terms of the activities of ornithine decarboxylase and glutamine synthetase enzymes, whereas [3H]thymidine incorporation into DNA was used as a specific index of cell proliferation. Treatment with 500 nM histamine of cells grown for 6 days in vitro, caused a time-dependent significant increase in ornithine decarboxylase activity of astrocytes from both sources. The maximum increase was observed at 4 h after histamine treatment, at that time the elevation in ornithine decarboxylase activity being about 80% and 300% over control values in the forebrain and the cerebellar astrocytes, respectively. Under similar experimental conditions, addition of histamine (500 nM) to medium resulted in a significant increase in [3H]thymidine incorporation into DNA in both types of cultures: in comparison with control, the elevation was about 45% at 48 h in forebrain astrocytes and at 24 h in cerebellar astrocytes. On the other hand, the specific activity of glutamine synthetase in cerebellar astrocytes was markedly enhanced (about 100%) by treatment with histamine (500 nM) for 4 days, but forebrain astrocytes were little affected. Addition of histamine to the culture medium produced no significant alteration in the activity of lactate dehydrogenase and protein content of either type of astroglial cells. The present findings, which support our earlier proposal that the biochemical properties of astrocytes differ between various brain regions, provide direct evidence for the involvement of histamine in the regulation of growth and development of astrocytes.

Long-lasting effects of naltrexone, an opioid receptor antagonist, on cell proliferation in developing rat forebrain

Several studies have demonstrated in the past that endogenous opioid peptides and opioid receptors may be involved as mediators of brain tissue growth and function in the neonate. Applying histological and autoradiographic methods, we have examined the effect of the mu-receptor-specific antagonist, naltrexone, on the proliferation of the 4-12-week-old rat forebrain subependymal layer. We found that naltrexone, when given daily throughout the weaning period, evoked a long-lasting increase of the mitotic rate and the [3H]thymidine labelling index. This effect was most significant about 8-10 weeks after ending the naltrexone treatment. Although a direct influence of naltrexone on long-term subependymal cell proliferation cannot be excluded, we are discussing evidence of an indirect effect via suppression of noradrenergic activity in the forebrain.

Induction of ornithine decarboxylase in adult rat hippocampal slices

Several factors involved in the regulation of ornithine decarboxylase (ODC) activity in adult rat brain tissue have been identified by using the in vitro hippocampal slice preparation. The same amino acids that have previously been reported to induce ODC in tissue culture, i.e., asparagine and glutamine, were found to produce a concentration- and time-dependent increase in ODC activity that reached a 100 fold the control value after 6 h of incubation. The effect of asparagine was totally blocked by inhibition of either protein or RNA synthesis, suggesting that the inducing amino acids increase ODC activity by stimulating the transcription of genes directly or indirectly regulating ODC activity. The effect of the inducing amino acids was potentiated by a variety of factors which by themselves did not modify ODC activity. In particular, opioid peptides markedly potentiated the effect of asparagine. Although the opiate antagonists naloxone and naltrexone totally blocked the effects of the opioid peptides on ODC induction, they also produced an inhibition of the asparagine-mediated increase in ODC activity. Other factors like dibutyryl cyclic AMP and insulin also potentiated the effects of asparagine on ODC activity. These results provide the first description of ODC induction in an in vitro preparation of adult brain tissue and indicate that the hippocampal slice preparation could be used to study the molecular mechanisms which regulate the expression and activity of ODC in the adult central nervous system. Moreover the data suggest possible mechanisms which may be involved in the induction of ODC in hippocampus by seizure activity.

Adrenergic control of DNA synthesis in developing rat brain regions: effects of intracisternal administration of isoproterenol

Catecholamines are hypothesized to control cellular development in the central nervous system. In the current study, isoproterenol administered intracisternally to neonatal rats was found to inhibit DNA synthesis [( 3H]thymidine incorporation) in brain regions. The regional selectivity of effect corresponded to the sequence of cellular maturation, namely midbrain + brainstem greater than cerebral cortex greater than cerebellum, suggesting that the specific linkage of beta-adrenergic receptors to cessation of cell replication occurs during a specific maturational stage.

Histamine H2-receptor mediated activation of neonatal rat brain ornithine decarboxylase in vivo

The effect of histamine (HA) administered via intracerebroventricular injection on ornithine decarboxylase (ODC) activity was studied in neonatal rat brain. The HA effect was dose and time dependent. Maximal increase in ODC activity was achieved 2 hr after administration of 10 micrograms HA (38% over control levels). Impromidine (HA H2-agonist) mimicked the effect of HA on ODC and ranitidine (HA H2-antagonist) inhibited the response to HA. Neither 2-thiazolylethylamine (HA H1-agonist) nor mepyramine (HA H1-antagonist) modified control ODC activity. The HA-releasers, compound 48/80 and polymixin B sulfate, elicited an increase in brain ODC activity of 35% and 32%, respectively, over the control value.

Immunohistochemical localization of L-ornithine decarboxylase in developing rat brain

L-Ornithine decarboxylase, the rate limiting enzyme of polyamine biosynthesis and a marker enzyme of tissue proliferation and maturation, was localized immunocytochemically in the developing rat central nervous system. It can be noted that the distribution of the enzyme protein underlies temporal alterations. Conclusions are drawn from the localization of the enzyme and possible functional roles played by ornithine decarboxylase in discrete brain areas.

Do catecholamines contribute to the effects of neonatal hypoxia on development of brain and heart? Influence of concurrent alpha-adrenergic blockade on ornithine decarboxylase activity

Hypoxia in the neonate releases catecholamines from the adrenal medulla, a response which is necessary to survive. This study examines whether a similar dependence exists for the ability of brain and heart tissue to recover from hypoxia-induced damage, as assessed by measurements of ornithine decarboxylase (ODC) activity. Hypoxia at either 1 day or 8 days of age produced a subsequent elevation of brain ODC which persisted for 1 week, a pattern known to be associated with recovery from tissue damage and delayed cellular maturation. Pretreatment of the rats with phenoxybenzamine, an alpha-receptor blocking agent, resulted in attenuation of the long-term ODC response, but did not interfere with effects on the enzyme during the hypoxia itself. In the heart, hypoxia at 8 days of age displayed similar effects, with long-term ODC elevations which were attenuated by phenoxybenzamine. Hypoxia at 1 day of age also produced long-term heart ODC stimulation, but in this case the effect was exacerbated by phenoxybenzamine, an effect consistent with the greater dependence of cardiac tissue on alpha-receptor-mediated responses to hypoxia at that age. These results suggest that alpha-receptor stimulation by catecholamines released during neonatal hypoxia play a role in the metabolic adjustment of brain and heart tissue to damage and may aid in subsequent recovery.

Effects of neonatal hypoxia on brain development in the rat: immediate and long-term biochemical alterations in discrete regions

To evaluate the sensitivity of immature brain tissue to hypoxic insult, neonatal rats were exposed to 7% O2 for 2 h at critical stages of development (1, 8, 15, 23 days of postnatal age); the immediate and long-term impact of hypoxia was then assessed in cerebellum, cerebral cortex and midbrain through measurement of ornithine decarboxylase (ODC) activity, a biochemical determinant of cellular injury and subsequent maturation, and through measurements of protein synthesis, growth and synaptosomal uptake of norepinephrine (an index of noradrenergic synaptogenesis). In one-day-old rats, hypoxia caused stimulation of protein synthesis and short-term suppression of ODC activity which persisted for several hours after termination of low O2 exposure; over the ensuing days, there was a prolonged elevation of enzyme activity and a subsequent, regionally selective increase in synaptosomal uptake of norepinephrine without changes in brain growth. In contrast, hypoxia in 8-day-old rats produced signs of metabolic injury, with a short-term elevation of ODC throughout the brain and reduced protein synthetic rates, eventual shortfalls in brain regional growth and no net increase in synaptosomal uptake. The effects of hypoxia on brain regional growth in 8-day-old animals appeared to represent an age-specific effect, as low as O2 conditions in older animals did not affect growth (animals made hypoxic at 15 or 23 days), but did produce an eventual reduction in synaptosomal uptake (hypoxia at 15 days). Differences between one-day-old and 8-day-old rats were also apparent in cerebral responses simply to a 2-h separation from the dam under normoxic conditions. These results support the view that cellular development and synaptogenesis are compromised when neonatal brain tissue is exposed to hypoxic conditions, and that there are critical periods of sensitivity in which processes undergoing rapid maturational change are particularly vulnerable.

The in vivo inhibition of mouse brain protein kinase-C by retinoic acid

The intracisternal injection of either all-trans-retinoic acid or [alpha]-difluoromethylornithine (DFMO) into the brain of 9-day-old mice blocked (greater than 90%) phorbol ester-induced ornithine decarboxylase (ODC, EC 4.1.1.17) activity in a concentration-dependent fashion; this inhibition was not evident with the use of the biologically impotent furyl analog of retinoic acid. In a similar manner, retinoic acid reduced the soluble protein kinase-C (PK-C) activity by 60% as well as total EGTA-sensitive kinase activity (66%) associated with the plasma membrane. Sixty-six percent of the retinoic acid-induced loss of PK-C activity in the soluble fraction could be accounted for by the translocation of PK-C to the plasma membrane as measured by the specific binding of 12-O-[3H]tetradecanylphorbol-13-acetate (TPA). DFMO and furyl-retinoic acid were not effective in altering PK-C activity or TPA binding to PK-C. In the presence of retinoic acid, however, there was a 2.3-fold increase in specific [3H]TPA binding in the plasma membrane fraction, which was 3.4-fold greater than that lost from the cytosol. Because retinoids do not directly affect TPA binding to PK-C, the data suggest that (i) the presence of retinoic acid results in the exposure of heretofore cryptic TPA-binding sites in the membrane, where this binding is most likely related to the alteration of membrane structure and (ii) de novo ODC induction is not required for retinoid-dependent inhibition of PK-C, although the TPA induction of PK-C appears to be necessary with regard to ODC induction.

Autoradiographic localization of ornithine decarboxylase in cerebellar cortex of the developing rat with [3H]alpha-difluoromethylornithine

Ornithine decarboxylase was autoradiographically localized in the developing rat cerebellar cortex after intracisternal injection of [3H]alpha-difluoromethylornithine, a specific, irreversible inhibitor of the enzyme. At nine days of age, when cerebellar ornithine decarboxylase activity is maximal, autoradiographic grains were distributed over all layers of the cerebellar cortex and throughout the brain stem. Within cerebellar folia, the highest grain density was associated with the molecular layer, whereas the internal and external granule cell layers were less densely labeled. Enhancement of ornithine decarboxylase activity by intracisternally-administered isoproterenol correspondingly increased the autoradiographic grain density over each layer. Thus much of the polyamine biosynthetic capability needed to support neuronal and/or glial differentiation appears to be associated with the developing cell processes. The combination of [3H]alpha-difluoromethylornithine autoradiography with localized injection techniques provides a potentially powerful tool for the study of the involvement of polyamine biosynthesis in brain development.

Effects of neonatal treatment with 6-aminonicotinamide on basal and isoproterenol-stimulated ornithine decarboxylase activity in cerebellum of the development rat

6-Aminonicotinamide (6-AN) is a nicotinic acid (vitamin B3) antagonist which, when administered to immature animals, has a profound influence on brain development. To explore the biochemical mechanisms which underlie these actions, we evaluated effects of 6-aminonicotinamide on ornithine decarboxylase, an enzyme involved in cellular replication and differentiation. The cerebellum of the neonatal rat was chosen for study because it represents a brain region which undergoes major maturational events postnatally. When given to neonatal rats, 6-aminonicotinamide (10 mg/kg, i.p., on days 1, 3, 5 and 7) caused a prompt and persistent inhibition of the enzyme well in advance of adverse effects on tissue weight or on general growth. In addition, the ability of the cerebellum to respond to trophic stimulation by a beta-adrenergic agonist, isoproterenol, was attenuated markedly. Assessment of cerebellar morphology indicated an early adverse effect of 6-AN on granule' cell division, resulting in eventual disruption of the characteristic laminar structure of this brain region. These data support the view that reduced ornithine decarboxylase activity and impairment of its reactivity to growth stimuli participate in the toxic effects of 6-aminonicotinamide on brain development.

Induction of mouse brain ornithine decarboxylase by 12-O-tetradecanoylphorbol-13-acetate is independent of TPA receptor concentration

Ornithine decarboxylase (ODC, E.C. 4.1.1.17) activity was measured in a 35,000 X g brain supernatant fraction, prepared 5 h after intracisternal injection of 12-O-tetradecanoylphorbol-13-acetate (TPA) into developing mouse brain. TPA-dependent induction of ODC activity was maximal on days 5 and 9 postnatally while on day 7, the developmental (endogenous) level of ODC in brain was high and, concurrently, the ability of TPA to induce ODC was reduced. Both TPA-dependent and developmental increases in mouse brain ODC activity were significantly reduced by intracisternal injection of retinoic acid (RA). The efficacy of TPA in elevating ODC activity at postnatal ages 1-220 days-old was independent of both soluble-and particulate-associated TPA receptor concentration. These observations suggest that although TPA receptor activation may be an obligatory event in ODC induction, TPA receptor activation and its concentration per se, are not sufficient determinants for ODC induction and tumorigenesis. Furthermore, the endogenous mechanism of ODC induction is distinct from that of the TPA-dependent increase in ODC enzyme activity.

Developmental effects of alpha-fluoromethylhistidine, an irreversible inhibitor of histidine decarboxylase, on growth and on levels and turnover of catecholamines

To examine the potential participation of histamine in cellular development, neonatal rats were given daily 50 mg/kg doses of alpha-fluoromethylhistidine (FMH), an irreversible inhibitor of histidine decarboxylase; previous studies have shown this regimen to deplete both neurotransmitter and nonneurotransmitter pools of histamine. No inhibition of growth was observed for either body weight, brain weight, heart weight or kidney weight; indeed, kidney weights tended to become supranormal toward weaning in the FMH-treated pups. Similarly, FMH failed to affect protein synthesis, confirming the lack of systemic toxicity of this amino acid as well as indicating that maintenance of histamine levels is not required for growth to proceed. In contrast, FMH did have a deleterious effect on development of the cardiac-sympathetic axis, with deficits in norepinephrine levels appearing during the third postnatal week. The deficits were not present in other catecholaminergic systems (brain noradrenergic or dopaminergic neurons and renal sympathetic neurons). The subnormal cardiac norepinephrine levels were preceded by a sharp increase in the turnover of norepinephrine at precisely the age at which central control of sympathetic tone first appears. The developmental effects of FMH indicate that, although it is unlikely that histamine participates in a major way in general control of cellular maturation, a more selective role for histamine as a trophic agent or neurotransmitter may exist during defined periods in nervous system development.

Effects of alpha-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.