Alteration of tyrosine hydroxylase and dopamine-β-hydroxylase activity in the locus coeruleus after 5,6-dihydroxytryptamine

Inhibition of catecholamine synthesis with alpha-methyl-p-tyrosine apparently increases brain serotoninergic activity in the rat: no influence of previous chronic immobilization stress

The functional relationship between brain catecholamines and serotoninergic function was studied in stress-naive and chronically immobilized rats after blockade of catecholamine synthesis with alpha-methyl-p-tyrosine (alpha MpT). The levels of noradrenaline (NA), serotonin, and 5-hydroxyindole acetic acid (5-HIAA) in pons plus medulla, brainstem, hypothalamus, hippocampus, and frontal cortex, and those of 3-methoxy, 4-hydroxyphenile-tileneglicol sulphate (MHPG-SO4) in the hypothalamus were measured by HPLC. Chronic immobilization (IMO) resulted in higher NA levels in pons plus medulla and hypothalamus, the latter area (the only one in which the NA metabolite was determined) also showing slightly elevated MHPG-SO4 levels as compared to stress-naive rats. Chronic IMO did not alter either serotonin or 5-HIAA levels, but acute stress consistently increased 5-HIAA levels in all areas, independently of previous chronic stress. Administration of alpha-MpT drastically reduced NA and increased 5-HIAA levels in all brain regions excepting the frontal cortex. The effect of the drug on serotoninergic function was not altered by previous chronic exposure to IMO. These data suggest that the noradrenergic system appears to exert a tonic inhibitory effect on serotoninergic activity in the brain, with the intensity of the effect depending on the brain area studied. In addition, chronic stress does not appear to alter the functional relationship between noradrenergic and serotoninergic activities, although interactions might exist in more restricted brain areas; this deserves further study.

Long-term alteration in tyrosine hydroxylase mRNA levels in rat locus coeruleus after intraventricular injection of 5,6-dihydroxytryptamine

The time course variations in tyrosine hydroxylase (TH) activity and specific mRNA were measured in the rat locus coeruleus (LC) and substantia nigra after an intracerebroventricular (i.c.v.) injection of 5,6-dihydroxytryptamine (5,6-DHT), a neurotoxin known to selectively destroy serotoninergic neurons. In this study, the TH activity and TH mRNA were both analyzed from homogenates of single tissue samples (micropunches). TH mRNA was extracted and quantified by densitometry using a northern blot method and an artificial TH RNA as an external standard. 5,6-DHT injection led to a long-lasting increase in TH activity and TH mRNA in LC but not in substantia nigra. The elevation in LC was progressive and reached its maximum value (+75%) at day 4 and day 8 after 5,6-DHT. This effect on TH activity was accompanied by a parallel change in TH mRNA whose amplitude was +57%, +81% and +45% at day 2, 4, and 8 respectively after the neurotoxin injection. Return to normal values was observed at day 16. Variations in TH activity and TH mRNA in LC were of similar amplitude. These results suggest that serotonin could be a potent modulator of TH gene expression within noradrenergic LC neurons.

Somatotopic organization of tyrosine hydroxylase expression in the rat locus coeruleus: long term effect of RU24722

Tyrosine hydroxylase (TH) tissue concentration was determined by immunostaining of tissue sections directly transferred onto nitrocellulose membranes in the restricted region of the noradrenergic perikarya of the locus coeruleus (LC) along its postero-anterior axis. TH containing cells were systematically counted on adjacent post fixed sections stained by immunohistochemistry. The absolute quantity of TH was estimated in each section and was found to be linearly related to the number of TH immuno-positive cells found in the adjacent section. The ratio between these two parameters was thus used as an index of the cellular concentration of TH in noradrenergic cells. In the LC of control rats, the TH cellular concentration was lower (-39%) in the anterior than in the posterior half of the structure. Three days after an injection of 20 mg/kg of RU24722, an eburnamine derivative known to increase the quantity of TH in the LC, increases in quantities of TH were found in both portions of the LC. Moreover in the posterior LC the increase in the amount of TH resulted from a significant increase in the number of TH-immunopositive cells. In the anterior part, however, it was primarily the result of a significant increase in TH cellular concentration. Throughout the LC there was an increase in the cellular concentration of TH which was inversely proportional to the concentrations found in control animals. TH mRNA content was measured by a quantitative in situ hybridization in sections of both the posterior and anterior LC one day after a single injection of RU24722 at the same dose. The quantity of TH mRNA was significantly increased in both parts. The number of TH mRNA-expressing neurons also increased, especially in the anterior LC. Thus the effects at the level of TH protein and TH mRNA were strikingly parallel though increase in TH protein occurred later than the increase in the TH mRNA. These results suggest that in the rat LC: (1) there is a significant population of 'sleeping cells' in which TH expression is either inactivated or, at a low level of activation; (2) TH cellular concentration could exert a retrocontrol on its own expression in cells of the LC that contained TH and (3) TH expression appears to be regulated by different selective mechanisms in these two different subpopulations of noradrenergic cells within the LC.

Effects of serotonin on tyrosine hydroxylase and tau protein in a human neuroblastoma cell line

The direct effects of the neurotransmitter serotonin on the catecholaminergic enzyme, tyrosine hydroxylase and the microtubule-associated tau protein were studied in a human neuroblastoma cell line. Undifferentiated LAN-5 cells, cultured in serum supplemented basal medium, or cells induced to differentiate by 6 day exposure to 10 uM retinoic acid were treated for 48 hr with 50 nM and 50 uM serotonin. In undifferentiated cells, serotonin treatment (50 uM) decreased both tyrosine hydroxylase activity and a 50 kD cytoplasmic fraction tau protein while 50 nM serotonin treatment caused this 50 kD protein to increase in the cytoplasmic fraction but decrease in the membrane fraction. While basal tyrosine hydroxylase activity increased in differentiated vs. undifferentiated cells, serotonin treatment had no effect on the enzyme or tau in differentiated LAN-5. This study shows serotonin to have direct effects on the biochemistry and cytoskeleton of undifferentiated cultured human neuroblastoma.

Direct transfer into nitrocellulose and quantitative radioautographic anatomical determination of brain tyrosine hydroxylase protein concentration

An improved quantitative immunochemical determination of brain tyrosine hydroxylase (TH) concentrations was designed using direct transfer into nitrocellulose from 20-microns thick brain sections, followed by immunodetection and quantitative radioautography in three reference brain structures (locus ceruleus, substantia nigra, and ventral tegmental area). Results obtained by this methodology were similar to those obtained after extraction and Western blotting of the TH protein in control and reserpine-treated animals. Moreover, this methodology allows the combination of high sensitivity and high anatomical resolution in the study of the distribution of pharmacological effects. The locus ceruleus exhibited a significant posteroanterior distribution of TH protein concentration in control and reserpine-treated animals.

Long-term effects of RU24722 on tyrosine hydroxylase of the rat brain

The effects of RU24722 (14,15-dihydro-20,21-dinoreburnamine-14-ol) on tyrosine hydroxylase in central catecholaminergic neurons were studied in rats treated with different quantities of the molecule, and a time course was done for the minimal dose that gave the maximal effect. RU24722 induced increases in tyrosine hydroxylase activities and specific protein content in noradrenergic cells of the locus ceruleus and decreased all these parameters in dopaminergic neurons of the substantia nigra and ventral tegmental area. The results pointed out that the specific activity of newly synthesized tyrosine hydroxylase in the loci cerulei was potentially greater but was not expressed "in vivo" except 7 days after injection. The phenotypic specificity and the time course pattern of the action could be considered as a consequence of an induction mechanism. The comparison of long-term change in tyrosine hydroxylase values after piperoxane, RU24722, clonidine, and combined RU24722-clonidine treatment demonstrated that an activation during a few hours did not induce tyrosine hydroxylase in central noradrenergic neurons. Clonidine antagonized the activating effect of RU24722 following its injection but did not affect its long-term induction properties.

Inhibition of tyrosine hydroxylase activity by serotonin in explants of newborn rat locus ceruleus

The long-term regulation of tyrosine hydroxylase (TH) by serotonin has been studied with cultures of newborn rat locus ceruleus explants. The presence of serotonin in the culture medium for a 24-h period was followed by an inhibition of TH activity in the explants. This effect lasted several days, with a maximal effect 2 days after treatment. Moreover, the decrease was reversible and dependent on the concentration of serotonin used (from 1 microM to 1 mM). The mechanisms of this regulation have been studied using drugs such as those known to act specifically on serotoninergic receptors and those known to interfere with protein synthesis. Thus, the action of serotonin (10(-5) M) on TH activity was suppressed with equimolar concentrations of serotoninergic antagonists such as metergoline or methiothepin. It was reproduced by quipazine, a drug capable of acting as a serotoninergic agonist. Inhibitors of protein synthesis acting either at the transcriptional or the translational levels can reproduce the inhibition of TH activity by serotonin alone. Furthermore, the effects of one or the other of these compounds and that of serotonin were not additive. This study confirms the hypothesis of an inhibitory control by serotonin on TH activity in the noradrenergic neurons of the locus ceruleus. Serotonin could regulate the synthesis of the enzyme through specific serotoninergic receptors.

Visualization of tyrosine hydroxylase-immunoreactive neurons in the cat dorsal motor vagal cells after treatment with parachlorophenylalanine

We examined tyrosine hydroxylase-immunoreactive (TH-IR) neuronal structures in the cat dorsal motor nucleus of the vagus (DMV) and its adjacent regions. We identified only a few in the caudal part of the DMV and no TH-IR cells at all in its rostral portion. However, after treatment with parachlorophenylalanine (PCPA), numerous TH-IR perikarya were visualized in the DMV. Comments are made on the central catecholamine regulation and the possible influence of serotonin afferents on this nucleus.

Alterations in tyrosine hydroxylase activity elicited by raphe nuclei lesions in the rat locus coeruleus: evidence for the involvement of serotonin afferents

The time course of the variations in tyrosine hydroxylase (TH) activity was measured in the rat locus coeruleus (LC) after lesions of the nucleus raphe dorsalis (NRD), nucleus raphe centralis superior (NRCS) and nucleus raphe pontis (NRP). A certain number of lesions were performed in the raphe magnus (RM), the caudal and rostral NRP and the caudal and rostral NRCS, lateral to raphe nuclei and in adrenalectomized animals. The serotonin (5-HT) content in the LC was also determined after these lesions. Only raphe nuclei producing significant decreases in the 5-HT content in the LC are successful in provoking increases in the TH activity in the LC, thus these results suggest that the noradrenaline (NA) synthesis in the LC may be regulated by 5-HT afferents. Moreover, intraventricular injections of 5,6-dihydroxytryptamine (5,6-DHT) and administration of parachlorophenylalanine (PCPA) also produce significant increases in TH in the LC. After immunotitrations of TH in the LC it was shown that, with exception of a high dose of 5,6-DHT (75 micrograms), all these treatments provoke an increase in the concentration of the enzyme. It therefore seems that one of the functional roles of 5-HT in the LC could be the regulation of the concentration of TH.

Alteration of tyrosine hydroxylase activity in the locus coeruleus after administration of p-chlorophenylalanine

The time course of the variations in tyrosine hydroxylase activity (THA) and serotonin (5-HT) content were measured in the rat locus coeruleus after parachlorophenylalanine (PCPA) administration. Highly significant decreases in the 5-HT content in LC were found 24-48 h after PCPA treatment (300 mg/kg daily). An increase in THA (in the LC) was found to be significant 4 days after 2 successive injections of PCPA and after 4 successive injections of the drug THA gradually increased, reaching a maximum around 4 days after the last injection. This maximum increase in THA was greatly reduced when 5-HTP was simultaneously administered with PCPA. These results join others which suggest that a serotonin-mediated mechanism could be one of the processes controlling noradrenaline metabolism in the locus coeruleus.

Correlation between the increase in tyrosine hydroxylase activity and the decrease in serotonin content in the rat locus coeruleus after 5,6-dihydroxytryptamine

A correlation is demonstrated between the increase in tyrosine hydroxylase activity and the decrease in serotonin concentration in the rat locus coeruleus after various doses of 5,6-dihydroxytryptamine and gradual protection of serotonin neurons by pretreatment with two serotonin uptake inhibitors: fluoxetine or citalopram.