Metabolism of biogenic amines in neuroblastoma and glioma cells in culture

Regulation of the human corticotropin-releasing-hormone gene promoter activity by antidepressant drugs in Neuro-2A and AtT-20 cells

Major depression is frequently associated with hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis. Clinically effective therapy with antidepressant drugs normalizes the disturbed activity of HPA axis, in part, by decreasing corticotropin-releasing hormone (CRH) synthesis, but the mechanism of this action is poorly recognized. In order to find out whether antidepressants directly affect CRH gene promoter activity, we studied their effect on undifferentiated and differentiated Neuro-2A cells, and for comparison the effect of the selected antidepressants on AtT-20 cells was also determined. The cells were stably transfected with a human CRH promoter fragment (-663 to +124 bp) linked to the chloramphenicol acetyltransferase (CAT) reporter gene. The regulation of CRH gene promoter activity is similar in Neuro-2A cells, both intact and differentiated, and in AtT-20 cell line, and cAMP/PKA-dependent pathway plays an important role in the stimulation of CRH gene. It was found that imipramine, amitryptyline, desipramine, fluoxetine, and mianserin, present in the culture medium for 5 days, in a concentration-dependent manner inhibited basal hCRH gene promoter activity in undifferentiated Neuro-2A cells, while other drugs under study (citalopram, tianeptine, moclobemide, venlafaxine, reboxetine, mirtazapine, and milnacipram) were inactive. In the differentiated cells, all examined antidepressants, except moclobemide (no effect) and tianeptine (increase), inhibited hCRH gene transcription. Moreover, in differentiated cells, the drugs acted stronger and were effective at lower concentrations. Forskolin-induced CAT activity was attenuated by imipramine and fluoxetine and to a lesser degree by amitriptyline and desipramine in differentiated cells, whereas other drugs were inactive. Moreover, imipramine and fluoxetine, but not tianeptine, showed moderate inhibitory effect on CRH gene promoter activity also in AtT-20 cell line, commonly used in CRH gene regulation studies. These results indicate that neuron-like differentiated Neuro-2A cells are a better model than pituitary and intact neuroblastoma to investigate the mechanism of psychotropic drug action. Inhibition of CRH gene promoter activity by antidepressant drugs may be a molecular mechanism by which these drugs inhibit the activity of HPA axis.

Characterization of the rat catechol-O-methyltransferase gene proximal promoter: identification of a nuclear protein-DNA interaction that contributes to the tissue-specific regulation

The methylating enzyme catechol-O-methyltransferase (COMT) is an important inactivator of substrates containing catechol-structure, such as catechol neurotransmitters and hormones. In previous studies, the rat COMT gene has been cloned and characterized, and it has been shown that the two COMT polypeptides, S- and MB-COMT, are expressed from one gene by cooperation of two separate promoters. One promoter, P2, functions constitutively, whereas the other, the proximal P1 promoter, is regulated in a tissue-specific manner. In this report, a more detailed analysis of the rat P1 promoter is presented. By using reporter gene constructs, it is shown that upstream sequences of the P1 promoter contain several regions that modulate the expression either positively or negatively. These experiments also show that the region between the MB- and S-ATG translation initiation codons is indispensable for the activity of this promoter. Analysis of this region by DNase I footprinting and gel retardation assays identified the presence of several DNA elements with SP1 and NF1 recognition site homologies that bound both liver and brain nuclear proteins. However, one 11-nucleotide-long DNA region containing an overlapping consensus binding sequence for CREB and C/EBP-like factors reacted only with the liver nuclear lysate. Supershift experiments suggest that the transcription factor C/EBPalpha mediates the tissue-specific expression of the rat COMT P1 promoter.

Structure of the rat catechol-O-methyltransferase gene: separate promoters are used to produce mRNAs for soluble and membrane-bound forms of the enzyme

The enzyme catechol-O-methyltransferase (COMT) catalyzes the inactivation of catechol-containing molecules by methylation. The cDNAs for the rat and human COMT have recently been cloned and recombinant proteins expressed in prokaryotic and eukaryotic cells. We describe here the structure of the rat COMT gene and its 5'-flanking sequences. The gene spans at least 13 kb and is composed of 5 exons, the first one noncoding. The two ATG codons for the initiation of translation of the membrane-bound (MB-COMT) and soluble (S-COMT) forms of the enzyme reside in the second exon. The gene expresses two mRNA species of 1.6 kb and 1.9 kb that have different tissue distributions. The expression of the transcripts is regulated by at least two promoters, P1 and P2. The P1 promoter expresses the shorter transcript in a tissue-specific manner and is located between the ATG codons in the coding region of the longer transcript. The P2 promoter is constitutive and responsible for the expression of the longer transcript. The shorter 1.6-kb mRNA (S-mRNA) produces only the S-COMT polypeptide, whereas the longer 1.9-kb mRNA (MB-mRNA) is able to direct synthesis of both forms of the COMT enzyme.

Effect of nerve growth factor on C-1300 murine neuroblastoma tumor growth and catecholamine content in neonatally sympathectomized mice

The in situ C-1300 murine neuroblastoma (MNB) tumor model was used to investigate the influence of exogenously administered nerve growth factor (NGF) on tumor growth and tissue catecholamine concentration in mice sympathectomized with 6-hydroxy-dopamine (6-OHDA) on postnatal days 4-10. Mice were implanted with 1 x 10(6) disaggregated MNB cells 3 days after termination of 6-OHDA administration. NGF (12-15 micrograms/mouse/day) treatment was initiated at the time of MNB cell implantation and continued until sacrifice of the animal. The time interval between tumor cell implantation and detection of palpable tumor (tumor onset time), transverse tumor diameter, tumor weight, tumor weight to body weight ratio, and tumor catecholamine concentration were determined. Neonatal sympathectomy caused a decrease in myocardial norepinephrine concentration of 88% compared with vehicle-treated animals as well as a significant reduction in total body and organ weight. Average body, brain, heart, and spleen weights were decreased 31%, 16%, 25%, and 42%, respectively, below control values. The daily injection of NGF, from the time of MNB tumor implantation to sacrifice, did not prevent these effects of chemical sympathectomy from being expressed. Tumor onset time following implantation of MNB cells was significantly increased in neonatally sympathectomized mice and was not altered by treatment with NGF. In contrast, the decrease in MNB tumor growth rate observed in sympathectomized mice was reversed by administration of NGF. Mean tumor weight and mean tumor to body weight ratio were 89% and 115% of comparable control values, respectively, in sympathectomized mice receiving exogenous NGF.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurotoxicity of MPTP and MPP+ in vitro: characterization using specific cell lines

The effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and its putative toxic metabolite 1-methyl-4-phenylpyridinium ion MPP+ were studied with specific neuronal and glial cell lines in vitro. MPTP had no morphological effect on actively growing neuroblastoma N2AB-1 cells or C6 glioma cells nor did it affect cell numbers. However, a low dose of MPP+ (33.7 microM) was cytotoxic to mitotic N2AB-1 cells inducing vacuole formation, cell lysis, and inhibiting cell growth over a 3-day period. Protein synthesis was inhibited in a dose-dependent fashion in MPP+ treated N2AB-1 cells after 24 h exposure while 33.7 microM of this toxin induced a 50% decrease in protein synthesis as early as 5 h after treatment of these cells. Differentiated, neurite-bearing N2AB-1 cells exhibited a loss of neurites and a change in cell size and shape following exposure to 0.33, 3.37 and 33.7 microM MPP+ after 24 h and some cells appeared to be mitogenically stimulated indicating MPP+ may act as a teratogen. C6 glioma cells, however, were resistant to MPP+. While mitotic N2AB-1 cells incubated with MPTP produced only traces of MPP+, C6 glioma cells generated significant amounts of this metabolite (3.6 microM). Moreover, although the morphology and cell number of cocultures did not change in the presence of MPTP, glioma-neuroblastoma cocultures produced 2.90 microM MPP+ which decreased protein synthesis by 18%.(ABSTRACT TRUNCATED AT 250 WORDS)

Striatal membrane-bound and soluble catechol-O-methyl-transferase after selective neuronal lesions in the rat

Activities of the two forms of catechol-O-methyltransferase (COMT), viz. the soluble (S-COMT) and the membrane-bound (MB-COMT), have been studied in the rat striatum to characterize their localization in relation to the nigrostriatal dopaminergic neurons. Selective unilateral nigrostriatal dopaminergic lesions were produced by an intranigral injection of 6-hydroxydopamine (6-OHDA; 8 micrograms/site). 6-OHDA caused an extensive lesion of the dopaminergic neurons as revealed by non-detectable concentrations of dopamine in the striata of the lesioned sites. In spite of that neither S-COMT nor MB-COMT activities were altered in comparison with the intact control striata. The intrastriatal injection of kainic acid significantly increased S-COMT activity but to some extent decreased MB-COMT activity. Kainic acid did not alter the striatal concentration of dopamine. These results suggest that both S-COMT and MB-COMT reside postsynaptically the nigrostriatal dopaminergic neurons. S-COMT seems to be found mainly in striatal glial cells, whereas striatal MB-COMT might be located both in postsynaptic neuronal and extraneuronal cells.

Time course of the metabolite patterns of intraventricularly injected [3H]noradrenaline in rat brain regions

In the hypothalamus, septum, pons with medulla, and hippocampus regions of rat brain, the level of radioactivity of [3H]noradrenaline and of five of its metabolites were determined up to 6 h after intraventricular injection of the tritiated amine. The following main results were found: In anterior hypothalamus and septum, the [3H]noradrenaline level declined in two phases. Similar turnover curves were obtained for the primary deaminated metabolites, with almost the same final half-lives as for [3H]noradrenaline. The level of the initial methylation product, normetanephrine, also showed a biphasic decline, which did not correspond to that of [3H]noradrenaline but rather was faster throughout the experiment. The final metabolites (i.e., the glycol sulfates) reached maximal levels in hypothalamus and septum earlier than in other regions. Thereafter, their levels declined with almost similar rates in all areas tested, but always faster than the [3H]noradrenaline level. The following conclusions were drawn: In areas rich in catecholaminergic nerve terminals, there seems to be a site, in addition to the vesicular storage pool, that accumulates exogenous noradrenaline and then releases it with relatively short half-lives. The contents of primary deaminated metabolites followed the turnover of [3H]noradrenaline at both sites. Exogenous [3H]noradrenaline seems to be methylated at two extraneuronal sites, which are distinguished by the rates of subsequent deamination. The size of the pool of slowly deaminated [3H]normetanephrine that is formed immediately after [3H]noradrenaline injection determined the apparent turnover of this product throughout the experiment and, thus, like the final metabolites, reflects for several hours the initial degradation of the unstored [3H]noradrenaline, rather than the metabolism of the stored amine.(ABSTRACT TRUNCATED AT 250 WORDS)

Further studies on the nature of postsynaptic dopamine uptake and metabolism in rat striatum: sodium dependency and investigation of a possible role for carrier-mediated uptake into serotonin neurons

The nature of postsynaptic sites involved in the uptake and metabolism of striatal 3,4-dihydroxyphenylethylamine (dopamine, DA) was investigated. The accumulation of [3H]DA (10(-7) M) into slices of rat striatum was found to be greatly dependent (greater than 99%) on the presence of sodium ion in the incubation medium. However, the formation of the [3H]dihydroxyphenylacetic acid (DOPAC) and [3H]homovanillic acid (HVA) was only partially reduced in the absence of sodium (DOPAC, 27% of control; HVA, 47% of control). Inhibition of carrier-mediated DA neuronal uptake with nomifensine (10(-5) M) significantly decreased DA accumulation (18% of control) and [3H]DOPAC formation (62% of control), but enhanced [3H]HVA production (143% of control). Inhibition of the 5-hydroxytryptamine (5-HT, serotonin) neuronal uptake system with fluoxetine (10(-6) M) or selective 5-HT neuronal lesions with 5,7-dihydroxytryptamine (5,7-DHT) had no effect on [3H]DOPAC or [3H]HVA formed from [3H]DA in the presence or absence of nomifensine. These results demonstrate that the uptake and subsequent metabolism of striatal DA to DOPAC and HVA is only partially dependent on carrier-mediated uptake mechanism(s) requiring sodium ion. These data support our previous findings suggesting a significant role for synaptic glial cell deamination and O-methylation of striatal DA. Further, experiments with fluoxetine or 5,7-DHT suggest that 5-HT neurons do not significantly contribute in the synaptic uptake and metabolism of striatal DA.

Monitoring the stimulated release of dopamine with in vivo voltammetry. II: Clearance of released dopamine from extracellular fluid

Microvoltammetric electrodes implanted in the caudate nucleus of the anesthetized rat have been used to monitor dopamine released following electrical stimulation of the medial forebrain bundle. These electrodes are fabricated from unmodified carbon fibers and have been used with normal pulse voltammetry. Dopamine appears in the vicinity of the electrode when the stimulation is initiated, and disappears almost immediately when the stimulation is terminated. The data suggest that the effective diffusion distance is less than 100 micron. Postmortem analysis using liquid chromatography with electrochemical detection shows that dopamine released in this manner is metabolized to 3,4-dihydroxyphenylacetic acid (DOPAC); however, neither substance is observed electrochemically in the extracellular fluid within seconds after the stimulation. In addition, inhibitors of neuronal uptake of dopamine, amphetamine (1.8 or 15 mg X kg-1) or benztropine (25 mg X kg-1), or of dopamine metabolism, pargyline (150 mg X kg-1) or tropolone (100 mg X kg-1), do not significantly affect the rate at which dopamine disappears from extracellular fluid, although they can affect the amount released. These results suggest that dopamine cannot freely diffuse in the extracellular fluid because an extraneuronal uptake mechanism exists that clears dopamine from extracellular fluid into an extraneuronal pool where metabolism to 3,4-dihydroxyphenylacetic acid occurs. Dopamine can be observed during electrical stimulation of the ascending fibers because neuronal and extraneuronal uptake systems are unable to remove dopamine on these short time scales.

Inhibition of murine neuroblastoma growth by dopamine antagonists

C-1300 murine neuroblastoma ( MNB ) contains the catecholamine biosynthetic pathway. This study investigated manipulation of this pathway for effects on cell growth and survival in tumor-bearing mice, and to correlate these findings with specific membrane-bound dopamine-binding activity. The dopamine antagonists domperidone, pimozide, and spiroperidol inhibited macromolecular synthesis in vitro as demonstrated by decreased [3H]TdR and [14C]leu incorporation in a dose-response fashion; 56, 49, and 43% inhibition was noted at 10(-6) M concentration of each drug, respectively, with no loss of cell viability. Dopamine agonists showed no significant inhibition. Scatchard analysis of dopamine binding was consistent with a single class of receptor sites with a mean concentration of 13.2 +/- 2.0 pmole/g wet weight of tissue and mean dissociation constant (Kd) = 0.69 +/- 0.38 nM, compared to a mean receptor concentration of 28.1 +/- 5.2 pmole/g wet weight of tissue and Kd = 0.38 +/- 0.09 nM in receptor-rich dog caudate nucleus, the normal control. A/J mice injected with 1 X 10(6) tumor cells and treated with daily pimozide or domperidone had a significant increase in disease-free survival when compared to controls (15 versus 8.5 days, P less than 0.001) as well as a significant increase in overall survival (35 versus 25 days, P less than 0.001). These data suggest that dopamine antagonists inhibit macromolecular synthesis in the C-1300 MNB . The inhibition of MNB tumor growth in vivo by dopamine antagonists suggests a specific chemotherapeutic approach to neuroblastoma, possibly mediated by dopamine receptors.

Characteristics of the electrical response to dopamine in neuroblastoma cells

1. The characteristics of the electrical response to dopamine in the mouse neuroblastoma cell line N1E-115 were studied. 2. Neuroblastoma cells responded to ionophoretically applied dopamine by generating a transient depolarization. Under voltage-clamp conditions, a transient inward current was recorded in response to dopamine application. 3. The receptor was more effectively activated by dopamine than by noradrenaline. Haloperidol blocked the dopamine-induced current with an apparent dissociation constant of 40 nM. Phentolamine was much less potent than haloperidol, and propranolol had no effect. 4. The dopamine-induced current was increased in amplitude by hyperpolarizing the membrane, decreased by depolarization, and reversed its polarity at + 14 mV. 5. When the external sodium concentration was decreased from 125 to 94 mM, the reversal potential was shifted in the direction of hyperpolarization by 10 mV. 6. Increasing the external potassium concentration from 0.2 to 20 mM caused a shift of the reversal potential by 13 mV in the direction of depolarization. 7. Replacement of external chloride with isethionate or glutamate caused little or no shift in the reversal potential, but increased the amplitude of the current. 8. Increase in external calcium concentration caused a block of the dopamine-induced current with an apparent dissociation constant of 1.3 mM, without altering its reversal potential. 9. It is concluded that the ionic channel activated by dopamine undergoes a conductance increase to both sodium and potassium but not to chloride or calcium.

Hypoxanthine-guanine phosphoribosyltransferase mutant glioma cells: diminished monamine oxidase activity

A defective capability of cultured rat glioma cells to reutilize purine bases (hypoxanthine-guanine phosphoribosyltransferase deficiency) was associated with a reduced capacity to oxidatively deaminate serotonin and tryptamine. The mutant glioma cells were also more sensitive to the cytotoxic effects of serotonin than were normal cells