THE EFFECT OF INJURY ON MONOAMINE CONCENTRATIONS IN THE RAT HYPOTHALAMUS

Abstract— The monoamine concentrations have been measured in four regions of the brain (hypothalamus, cortex, cerebellum and brain stem) in rats injured by either hind-limb ischaemia or scald. Both injuries produced a rapid fall in the noradrenaline concentration of the hypothalamus which recovered slowly if the injury was not fatal. This effect of injury was seen after pretreatment with a-methyl-p-tyrosine to inhibit noradrenaline synthesis, indicating an increased rate of utilization of noradrenaline after injury. These injuries did not affect the 5-hydroxytryptamine concentration in the hypothalamus, but changes were found in the concentration of this monoamine and in that of its metabolite, 5-hydroxyindole acetic acid, in the brain stem. It is concluded that these forms of injury had specific effects on the brain monoamines. The hypothalamic changes were not secondary to changes in core temperature or to hypotension or hypovolaemia and they are discussed in relation to the impairment of temperature regulation seen in the injured rat.     

An in vivo cholinergic influence on the retention of [3H]noradrenaline in regional brain synaptosomes

Rats were intraventricularly (icv) injected with [³H]noradrenaline and the retention of the amine was determined in synaptosomes obtained from cerebral cortex, hypothalamus and brain stem. Previous icv administration of hemicholinium-3, effective enough to markedly decrease brain acetylcholine levels, increased the retention of synaptosomal [³H]noradrenaline in hypothalamus and cerebral cortex; this increased retention did not occur in the brain stem. The increased retention of [³H]noradrenaline, produced by hemicholinium-3, was reversed by a concomitant icv dose of choline, which in turn reversed the decrease of acetylcholine caused by hemicholinium-3. These results are interpreted as brain cholinergic activity having an influence on the turnover of noradrenaline in some brain regions.     

Catecholamine biosynthesis in specific brain areas of the rat as determined by liquid chromatography and amperometric detection

Catecholamine turnover is examined using both steady and non-steady state methods. The steady state technique involved the injection of 5 μCi of ³H-tyrosine into the lateral cerebral ventricle in chronically cannulated rats. Catecholamines were separated by high performance reverse phase liquid chromatography and content analyzed with amperometric detection. Catecholamine synthesis was examined in the anterior hypothalamus, medial basal hypothalamus, brain stem and caudate-putamen. Dopamine synthesis was greater than norepinephrine in all areas but the brain stem. The decline in catecholamine content was demonstrated with a synthesis inhibitor, αMpT, and comparisons were made between the two methods. This study indicated that tyrosine hydroxylase inhibition alters catecholaminergic neuronal activity in the brain areas that we examined.     

Influence on Turnover and Level of Hypothalamic Noradrenaline by a New Antihypertensive Agent (GYKI 11679)

Abstract— In an attempt to delineate the possible importance of the concentration of noradrenaline at hypothalamic noradrenergic receptor sites in a hypotensive response to a drug, the action of a new antihypertensive agent, 1-(6-morpholino-3-pyridazynyi)-2-(1-[tert-butoxycarbonyi]-2-propylidene)-diazane (GYKI 11679), on the turnover rate and the endogenous level of noradrenaline (NA) in rat hypothalamus was examined. An effective, antihypertensive i.p. dose of the compound (10 mg/kg) produced a significant but relatively short-lasting reduction in the hypothalamic noradrenaline content, whereas no change was observed in the cardiac catecholamine level. The NA turnover determinations, carried out in GYKI 11679-pretreated rats by measuring the disappearance of labeled NA at 1, 2, 3, and 5 h after the injection of the radioactive amine, showed that a 10 mg/kg i.p. dose of the compound, given 1 h prior to the i.c.v. administration of the labeled NA, increased the turnover rate of noradrenaline to a great extent. The estimated half-lives of NA in the hypothalamus of the treated and of the non-treated animals were calculated as 1.72 and 3.62 h, respectively. In vitro studies showed that the spontaneous outflow of noradrenaline from hypothalamic slices was accelerated by GYKI 11679 in a dose-dependent manner in a concentration range of 10^?5 to 10^?7m . In a 10-fold higher range, GYKI 11679 produced inhibition of both the hypothalamic and the adrenal tyrosine hydroxylase activity but did not alter DOPA-decarboxylase, dopamine-β-hydroxylase, or monoamine oxidase activities. Direct in vivo measurements of catecholamine synthesis by determining the ³H-catecholamines (CA) formed from [³H]tyrosine in the hypothalamus after an i.c.v. administration of the labeled precursor showed a moderate increase in [³H]CA formation following a 10 mg/kg dose of the compound. When GYKI 11679 was administered in a 75 mg/kg i.p. dose to rats, the transformation was reduced by –50%. Adenylate cyclase activity measurements did not show stimulatory or inhibitory actions of the drug on the NA-stimulated adenylate cyclase of the rat hypothalamus, in accordance with previous results. This suggests that the increased NA turnover (utilization) caused by an effective, antihypertensive dose of GYKI 11679 is the direct consequence of an increased outflow, which occurs primarily in the hypothalamus. The increased activity of the noradrenergic neurons in this brain region might lead to a reduced sympathetic activity in the periphery and thus to a significant decrease in blood pressure.     

EFFECT OF STRESS ON THE DISPOSITION OF CATECHOLAMINES LOCALIZED IN VARIOUS INTRANEURONAL STORAGE FORMS IN THE BRAIN STEM OF THE RAT

Abstract— The utilization of [³H]norepinephrine newly taken up or newly synthesized from [³H]tyrosine was studied in the brain stem of normal and stressed rats up to 5 h after the intracistemal injection of [³H]norepinephrine or [³H]tyrosine. The biphasic disappearance of the exogenous as well as of the endogenously synthesized [³HJnorepinephrine revealed that the amine is localized in at least two main compartments (A and B). The half-life of the amine newly taken up or newly synthesized, mainly localized in compartment A, is of short duration (between 15 and 30 min); the amine stored for a longer period of time and mainly distributed in compartment B is utilized more slowly (half-life, 180 to 260 min). A stress of short duration (15 min) induced by electric shocks applied to the feet increased the utilization of [³HJnorepinephrine newly taken up or newly synthesized from [³H]dopamine or [³H]tyrosine, but has no effect on the [³H]norepinephrine stored for a longer time period, indicating that the amine in compartment A is released in preference to that stored in compartment B. A stress of longer duration (180 min) increased the utilization of [³H] norepinephrine in both compartments and induced a sustained increased in norepinephrine synthesis as shown by the enhanced formation of [³H]norepinephrine from [³H]tyrosine in brain stem slices in vitro. The electrical stress was without effect on [³H]norepinephrine uptake. As for [³H]norepinephrine, the 15 min of stress enhanced the utilization of [³H] dopamine newly taken up or newly synthesized from [³H]tyrosine and had no effect on [³H]dopamine stored for a longer time period. These results suggest an increased release of both [³H]dopamine and [³H]norepinephrine from noradrenergic terminals of the rat brain stem. Finally, the 15 min of stress appeared to have no effect on the utilization of [³H] serotonin newly synthesized from [³H]tryptophan in serotonergic neurons of the brain stem, whereas the 180 min of stress increased the utilization of 5-HT in this structure.     

Effects of β-endorphin on brain serotonin metabolism

Human β-endorphin (15 μg) administered intracisternally increased concentrations of serotonin (5HT) and its metabolite, 5-hydroxyindoleacetic. acid (5-HIAA), in brain stem and hypothalamus and decreased 5-HIAA concentrations in hippocampus. These data are compatible with the hypothesis that β-endorphin increases 5HT turnover in brain stem and hypothalamus and decreases 5HT turnover in hippocampus. β-endorphin increased in brain stem and hypothalamus and decreased in hippocampus the rate of pargyline-induced decline of 5-HIAA. β-endorphin decreased the rate of pargyline-induced accumulation of 5HT in all these brain regions. The probenecid-induced accumulation of 5-HIAA in brain stem was decreased by β-endorphin. These data are compatible with the hypothesis that β-endorphin increases release of 5HT from neurons in brain stem and hypothalamus and decreases release of 5HT from neurons in hippocampus. The data require further a hypothesis that β-endorphin either decreases 5HT reuptake in these three brain regions or increases 5-HIAA egress from brain.     

EFFECTS OF 6-HYDROXYDOPAMINE ON CATECHOLAMINE CONTAINING NEURONES IN THE RAT BRAIN

After the intraventricular injection of 6-hydroxydopamine (6-OHDA), there was a long lasting reduction in the brain concentrations of noradrenaline (NA) and dopamine (DA). The brain concentration of NA was affected by lower doses of 6-OHDA than were required to deplete DA. A high dose of 6-OHDA which depleted the brain of NA and DA by 81 per cent and 66 per cent respectively, had no significant effect on brain concentrations of 5-hydroxytryptamine (5-HT) or γ-aminobutyric acid (GABA). The fall in catecholamines was accompanied by a long lasting reduction in the activities of tyrosine hydroxylase and DOPA decarboxylase in the hypothalamus and striatum, areas in the brain which are rich in catecholamine containing nerve endings. There was, however, no consistent effect on catechol-O-methyl transferase or monamine oxidase activity in these brain regions. The initial accumulation of [³H]NA into slices of the hypothalamus and striatum was markedly reduced 22–30 days after 6-OHDA treatment. These results are consistent with the evidence in the peripheral sympathetic nervous system that 6-OHDA causes a selective destruction of adrenergic nerve endings and suggest that this compound may have a similar destructive effect on catecholamine neurones in the CNS.     

Regulation of synaptosomal tyrosine hydroxylation in different brain regions with noradrenergic innervation

Tyrosine hydroxylation rate was measured by a modified tritium release assay at the physiological pH of 7.4 in synaptosomes prepared from cerebellum, hippocampus and hypothalamus. Incubation in the presence of 2 mM 8 bromo cAMP increased tyrosine hydroxylation in all three regions. An almost identical activation was seen after membrane depolarization by 50 mM K⁺. Removal of Ca²⁺ from the incubation medium had no significant effect on the activation produced by either agent, however it did significantly increase the control tyrosine hydroxylation rate in the hypothalamus. The combined effect of 8 Br cAMP and high K⁺ was found to be additive in the cerebellum and hippocampus but not in the hypothalamus. A reduction in tyrosine hydroxylation was observed if incubation was carried out in the presence of 1 μM noradrenaline; the degree of inhibition was similar in the three regions. 2 mM 8 Br. cAMP added to the noradrenaline restored tyrosine hydroxylation to control levels in synaptosomes from the hypothalamus, but not the hippocampus and cerebellum. Tyrosine hydroxylase in the hypothalamus is associated with dopaminergic nerve terminals as well as noradrenergic nerve terminals derived from more than one cell group, the hippocampus and cerebellum however both receive their noradrenergic input entirely from the locus coeruleus. Differences between synaptosomes from the three brain regions may therefore reflect differences in the nature of the enzyme as well as local regulatory mechanisms.     

DT-n-PROPYLACETATE AND GABA DEGRADATION. PREFERENTIAL INHIBITION OF SUCCINIC SEMIALDEHYDE DEHYDROGENASE AND INDIRECT INHIBITION OF GABA-TRANSAMINASE

The kinetic constants for 4-aminobutyrate: 2-oxoglutarate aminotransferase (GABA-trans-aminase) and succinate-semialdehyde: NAD⁺ oxidoreductase (SSA-DH) have been determined using rat brain homogenate. The Michaelis constants for GABA-T at saturated substrate concentrations were calculated to be K_gaba= 1.5 mM, K_2-OG= 0.25 mM, K_GLU= 620 μM, and K_SSA= 87 μm. The V_max for the reaction using GABA and 2-oxoglutarate (2-OG) as substrates (forward reaction) was found to be 35.2 μmol/ These results indicate that MOPEG is a measure for changes in central noradrenaline turnover and that drugs affect MOPEG in the brain and spinal cord similarly. Fractional rate constants of MOPEG in the rat brain and spinal cord were estimated with the exponential decline curves produced by treatment with pargyline. Turnover rates of 193 pmol/gh and 167 pmol/g These results indicate that MOPEG is a measure for changes in central noradrenaline turnover and that drugs affect MOPEG in the brain and spinal cord similarly. Fractional rate constants of MOPEG in the rat brain and spinal cord were estimated with the exponential decline curves produced by treatment with pargyline. Turnover rates of 193 pmol/g/h and 167 pmol/g/h in the brain and spinal cord respectively were calculated. The kinetics of GABA-T have been shown to be consistent with a Ping Pong Bi Bi mechanism. Substrate inhibition of the forward reaction, through formation of a dead-end complex, was found to occur with 2-OG (K_i 3.3 mM), whereas GABA was found to be a product inhibitor of the reverse reaction (K_i= 0.6 mM). Using the appropriate Haldane relationship, a K_eq of 0.04 for GGBA-T was found, indicating that the reaction was strongly biased towards GABA. For SSA-DH, the K_m of SSA was determined (9.1 μM) and the V_max was 27.5 μmol/ These results indicate that MOPEG is a measure for changes in central noradrenaline turnover and that drugs affect MOPEG in the brain and spinal cord similarly. Fractional rate constants of MOPEG in the rat brain and spinal cord were estimated with the exponential decline curves produced by treatment with pargyline. Turnover rates of 193 pmol/g/h and 167 pmol/g These results indicate that MOPEG is a measure for changes in central noradrenaline turnover and that drugs affect MOPEG in the brain and spinal cord similarly. Fractional rate constants of MOPEG in the rat brain and spinal cord were estimated with the exponential decline curves produced by treatment with pargyline. Turnover rates of 193 pmol/g/h and 167 pmol/g/h in the brain and spinal cord respectively were calculated. h. The effect of di-n-propylacetate (DPA) on both GABA-T and SSA-DH was measured. DPA inhibited SSA-DH competitively with respect to SSA, giving a K_i of 0.5 mM. GABA-T was only slightly inhibited. The K_i of DPA for the forward reaction was 23.2 mM with respect to GABA, which was 40-50 times higher than that for SSA-DH. For the reverse reaction the K_i of DPA was found to be nearly the same (15.2 mM with respect to Glu and 22.9 mM with respect to SSA). These results suggest that GABA accumulation in the brain, after administration of DPA in vivo, is caused by SSA-DH inhibition. Two mechanisms are indicated by the data. (1) The higher level of SSA, which results from inhibition of SSA-DH, initiates the reverse reaction of GABA-T, thus increasing the level of GABA via conversion of SSA. (2) The degradation of GABA is inhibited by SSA, since SSA has a strong inhibitory effect on the forward reaction, as calculated from the present data.     

NORADRENALINE NERVE TERMINALS IN THE CEREBRAL CORTEX: EFFECTS ON NORADRENALINE UPTAKE AND STORAGE FOLLOWING AXONAL LESION WITH 6-HYDROXYDOPAMINE

The ascending noradrenaline-containing neuronal system from the locus coeruleus to the cerebral cortex was unilaterally lesioned by an intracerebral injection of 8 μg 6-hydroxydopamine in the dorsomedial reticular formation in the caudal mesencephalon. The 6-hydroxydopamine caused injury to axons of the dorsal catecholamine bundle associated with its specific neurotoxic action, while very limited unspecific tissue necrosis was observed. Following this treatment the endogenous noradrenaline in the ipsilateral cerebral cortex (neocortex) increased acutely (up to 2 days), as observed both with noradrenaline assay and fluorescence histochemistry. The noradrenaline concentration then gradually decreased to 15 per cent of the contralateral side 15 days after the lesion. At this time interval and up to at least 90 days no fluorescent catecholamine nerve terminals could be detected. The acute noradrenaline increase could be blocked partially by tyrosine hydroxylase inhibition produced by α-methyl-p-tyrosine. The disappearance of endogenous noradrenaline following tyrosine hydroxylase inhibition was also reduced after the 6-hydroxydopamine lesion. Studies on the in vitro uptake of [³H]noradrenaline (0.1 μM for 5 min) in slices from the neocortex after the 6-hydroxydopamine lesion showed a gradual decline in uptake reaching maximal reduction (35-40 per cent of the contralateral side) after 15 days. No recovery of [³H]noradrenaline uptake was seen up to 90 days after the lesion. The formation of [³H]noradrenaline from [³H]dopamine in vitro was reduced to 15 per cent of the contralateral side after a chronic lesion. The present results indicate that the disappearance of noradrenaline uptake-storage mechanisms in the neocortex is due to an anterograde degeneration of axons and nerve terminals of the dorsal catecholamine bundle. The data on endogenous noradrenaline and noradrenaline synthesis suggest that approx. 15 per cent of the noradrenaline nerve terminals in the neocortex remain intact following the lesion, while the [³H]noradrenaline uptake data reflect uptake in other tissue structures in addition to noradrenaline nerve terminals, e.g. dopamine nerve terminals, pericytes and/or glial cells.     

THE METABOLISM OF TRITIATED DOPAMINE IN REGIONS OF THE RAT BRAIN IN VIVO—II

Abstract— The levels of tritiated catecholamines and metabolites were measured in regions of the rat brain at intervals after the intraventricular injection of [³H]dopamine, [³H]nor-adrenaline and [³H]normetanephrine. The disappearance of catecholamines and appearance of metabolites with time and the regional turnover rates of these amines indicate that the major pathway of the metabolism of noradrenaline and dopamine actively released from physiological storage sites is to the neutral alcoholic metabolites. The acid metabolites, homovanillic acid and 3,4-dihydroxyphenylacetic acid appear to be only minor products of normal dopamine metabolism in rat brain regions including the striate, but are the main end products of the metabolism of excess exogenous dopamine.
The active metabolism of stored noradrenaline to alcohol metabolites is also indicated by the increase in neutral alcohol metabolites accompanying the increased noradrenaline turnover when rats were subjected to electroshock stress. Therefore in the rat brain, neutral alcohol metabolites of dopamine and noradrenaline have great significance in the study of physiological catecholamine turnover in any region.     

Influence of increased catecholamine levels in blood plasma during cold-adaptation and intramuscular infusion on thresholds of thermoregulatory reactions in guinea-pigs

Summary Catecholamines and some of their metabolites were determined in urine and blood plasma of guinea-pigs before, during and after acclimation to a cold or warm environment. During adaptation to 5°C the amounts of noradrenaline in plasma and 24-h urine samples continuously increased up to 600% compared with values obtained at an ambient temperature of 22°C. Higher levels of dihydroxyphenylglycol and 3-methoxy-4-hydroxyphenylglycol further indicated an increased turnover of noradrenaline during cold adaptation. Acclimation to an ambient temperature of 28°C reduced the peripheral release of noradrenaline in comparison to the release observed at 22°C. Cold-induced increases in metabolic rate and electrical muscle activity both occur at a considerably lower mean body temperature in cold-than in warm-adapted guinea-pigs. The shift of thermoregulatory cold defence reactions to a lower mean body temperature could also be observed in warm-adapted animals after intramuscular infusion of noradrenaline in amounts comparable to those released during cold adaptation.It is concluded that high peripheral sympathetic activity directly or indirectly inhibits noradrenergic neurons in the lower brain stem that modulate the thermoregulatory control system by means of their afferents to the hypothalamus. As a consequence of this peripheral influence the thermoregulatory set point is shifted to a lower mean body temperature.Abbreviations A adrenaline - CA cold adapted - CNS central nervous system - DHPG dihydroxyphenylglycol - EMA electrical muscle activity - HPLC high performance liquid chromatography - 5-HT 5-hydroxytryptamine (serotonine) - MHPG 3-methoxy-4-hydroxypheyylglycol - MR metabolic rate - NA noradrenaline - T _b mean body temperature - WA warm adapted     

Caffeine Ingestion by Rats Increases Noradrenaline Turnover and Results in Self-Biting

The effects of caffeine on the activity of central and peripheral catecholaminergic structures have been studied in rats ingesting high doses of caffeine. The activities of the enzymes tyrosine hydroxylase and dopamine-beta-hydroxylase were measured as well as 3,4-dihydroxyphenylethylamine (dopamine), adrenaline, and noradrenaline concentrations, in brain (striatum and hypothalamus), heart, and adrenal gland. At the peripheral level, we observed a significant increase in the dopamine and adrenaline plus noradrenaline content in the heart, but an increase in dopamine content only was found in the adrenal gland. Dopamine-beta-hydroxylase activity in serum was increased, but the only significant enzymic change in brain was an increase in the dopamine-beta-hydroxylase activity of the hypothalamus. However, an increase in catecholamine content was observed in both structures of the brain. These data suggest that the mechanisms involved in caffeine-induced self-biting in rats are not limited to the dopaminergic system, because we have also observed an increase in noradrenaline turnover.     

Brain serotonin turnover and plasma prolactin and growth hormone concentrations during changes in osmotic balance in the domestic fowl

Summary Young cockerels injected 24 h earlier with 0.9% saline,para-chorophenylalanine (pCPA, brain serotonin depletor) or alpha-methylpara-tyrosine (AMPT, brain catecholamine depletor) were deprived of access to water for 24 h. Plasma prolactin concentrations were markedly elevated by water deprivation and returned to normal on rehydration. pCPA, but not AMPT, significantly reduced the increase in prolactin. Concentrations of growth hormone were not affected by water deprivation. Brain serotonin concentrations were reduced by treatment with pCPA. Groups of cockerels were maintained under normal conditions or without access to drinking water for 12 h or 24h. Some were injected with the monoamine oxidase inhibitor pargyline, which increased the prolactin and decreased the growth hormone concentration in the plasma of the hydrated birds. The inhibitory effect of pargyline on growth hormone was augmented following water deprivation. Serotonin levels were not significantly affected by water deprivation but turnover (defined as accumulation of serotonin after pargyline treatment) was increased in the hypothalamus but not in remaining tissue. Injecting 30% saline solution intravenously markedly increased plasma prolactin whilst growth hormone concentrations were decreased. Serotonin turnover was increased in the hypothalamus but not in other brain regions. The results show that secretion of prolactin and growth hormone by the pituitary gland during osmotic imbalance in the fowl may be mediated by changes in hypothalamic scrotonin turnover.     

On the Recovery of [3H]Noradrenaline from Different Metabolic Compartments of Rat Brain with Respect to the Role of Catechol-O-Methyltransferase

Abstract: Rats were treated with reserpine, desmethylimipramine, or carrier, either alone or in combination with tropolone. Either 10 min (t₁) or 1 h (t₂) after intraventricular injection of [³H]noradrenaline, they were decapitated. The total ³H activity and the recovery of [³H]noradrenaline were determined in tissue extracts from various brain regions. Maximum total ³H activity was measured at t₁ in all tropolone-treated rats; the mean sum of these results served as an estimate of the initial tissue concentration of [³H]noradrenaline. At t₁, 40–50% of the sum of [³H]noradrenaline and its metabolites was recovered unchanged in normal rats; reserpine and DMI reduced the recovery to 18–27%. In all groups, the decline of [³H]noradrenaline was retarded after t₁. Inhibition of catechol-O-methyltransferase by tropolone caused consistently elevated [³H]noradrenaline levels, but did not affect the metabolic rate after t₁ when compared with similarly pretreated, but tropolone-free rats. Thus, if catechol-O-methyltransferase was inhibited during the injection of [³H]noradrenaline, a higher percentage of the amine had been taken up into spaces with a slow noradrenaline turnover. The maximum increase was seen when the neuronal uptake, was inhibited by desmethylimipramine. This supported the hypothesis that an additional extraneuronal space exists, in addition to the known intraneuronal and extraneuronal compartments, which has a slow noradrenaline turnover. The tropolone effect on the noradrenaline recovery possibly shows that there might be a saturable “methylating system,” similar to that described for the periphery, in which catechol-O-methyltransferase is linked to the extraneuronal uptake₂. By affecting the access of noradrenaline to non-neuronal cells it might influence the rate of noradrenaline elimination from the intercellular space.     

DEVELOPMENT OF THE BLOOD-BRAIN BARRIER FOR 6-HYDROXYDOPAMINE

The postnatal development of the blood-brain barrier for the neurotoxic action of 6-hydroxydopamine on central noradrenaline neurons has been investigated by recording the in vitro uptake of [³H]noradrenaline in slices from cerebral cortex, hypothalamus and spinal cord in rats treated with large doses of 6-hydroxydopamine at different ages. The [³H]noradranaline uptake was permanently and markedly reduced in all regions when the animals were treated at birth, certainly related to degeneration of noradrenaline neurons, caused by 6-OH-DA. In the cerebral cortex and hypothalamus an efficient protection against the effects of 6-OH-DA on [³H]noradrenaline uptake developed postnatally, while in the spinal cord this protection was never seen to become complete. The results obtained indicate a rapid formation of a blood-brain barrier for 6-OH-DA in the cerebral cortex between the 7th and 9th day after birth. In the hypothalamus the development of this barrier seemed to have a more gradual time-course, but appeared to be fully developed already at day 5 postnatally. Also in the spinal cord the barrier developed more gradually from birth to the adult age. It was observed, however, that both in the cerebral cortex and in the spinal cord, the blood-brain barrier developed, could not completely protect the central noradrenaline neurons from the neurotoxic actions of large doses of 6-OH-DA administered systemically to adult rats. Furthermore, the results obtained support the view that 6-OH-DA does not seem to apparently affect the outgrowth of remaining NA neurons which have not been destroyed by the 6-OH-DA treatment.     

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)     

Influence of hypo- and hyperthyroidism on the turnover rate of noradrenaline, dopamine and serotonin in various rat cerebral structures]

The effect of chronic treatment with tyroxine (T4) or propylthiouracile (PTU) on the turnover of norepinephrine (NE), dopamine (DA) and 5-hydroxytryptamine (5-HT) has been studied in various areas of the rat brain (brain stem, hypothalamus, striatum and "rest of the brain"). The turnover of NE and DA was determined by the decay in endogenous levels after inhibition of tyrosine hydroxylase by alpha-methylparatyrosine and the turnover of 5-HT was evaluated by the initial accumulation of endogenous 5-HT after inhibition of monoamine oxydase by pargyline. T4 treatment accelerated the release of DA from the striatum but had no significant effects on NA release in the various cerebral areas : nevertheless the NE endogenous level was significantly reduced in the brain stem. PTU treatment delayed the release of DA and NA only from the "rest of the brain". Concerning 5-HT, the only significant variation was observed in the hypothalamus of PTU-treated rats and implied increased turnover. The possible relations between the changes in cerebral monoamines turnover and the behavioural alterations which are observed in thyroid disfunction are discussed.     

TRYPTOPHAN AND 5-HYDROXYTRYPTAMINE METABOLISM IN THE IMMATURE RAT BRAIN DURING RECOVERY FROM ASPHYXIA

Abstract— The turnover of cerebral 5-HT in the 5-day-old rat during recovery from asphyxia was assessed in the brain by determining 5-hydroxyindoleacetic acid concentration, and by following the accumulation of 5-HT after inhibition of MAO. Marked increases in turnover were found to persist for up to 3 h after resuscitation and this increase was accompanied by increases in brain tryptophan, plasma total tryptophan and the fraction of the plasma total tryptophan that was not bound to albumin. None of these parameters were different from those in control animals 24 h after resuscitation. Brain and plasma tyrosine levels remained unaltered by the treatment at all times investigated.     

6-AMINODOPAMINE-INDUCED DEGENERATION OF CATECHOLAMINE NEURONS

the effects of 6-aminodopamine on central and peripheral catecholamine neurons using fluorescence histochemical and isotope techniques have been investigated. Systematic administration of 6-aminodopamine (20 mg/kg intraveneously) produced a rapid (within 1 h) and long-lasting depletion of endogenous noradrenaline in adrenergic nerves of mouse atrium and iris with a concomitant loss of [³H]noradrenaline uptake. The effects were dosedependent. Accumulations of noradrenaline in non-terminal axons were observed histochemically, indicating that 6-aminodopamine induces neuronal damage. Desipramine completely blocked the 6-aminodopamine induced noradrenaline depletion and reduction in [³H]noradrenaline uptake, indicating that 6-aminodopamine has to be taken up by the axonal ‘membrane pump’ to produce its effects. Themonoamine oxidase inhibitor, nialamide, potentiated the effect of 6-aminodopamine on [³H]noradrenaline uptake. 6-Aminodopamine did not affect the cell bodies of the adrenergic neurons and there was a reappearance of adrenergic nerves and recovery of [³H]noradrenaline uptake. 6-Aminodopamine does not seem to pass the blood-brain barrier after systemic injection. Intraventricular injection of 6-aminodopamine in rats led to a considerable reduction in endogenous whole brain noradrenaline and [³H]noradrenaline uptake in slices from cerebral cortex and hypothalamus. Similar, but less pronounced effects were observed on dopamine neurons in the caudate nucleus. Histochemically, pronounced accumulations of transmitter were observed in the axons of the catecholamine neurons. The results obtained favour the view that 6-aminodopamine is able to produce an acute and selective degeneration of catecholamine neurons similar to that seen after the neurotoxicagent, 6-hydroxydopamine. Both compounds seemed to be approximately equally potent in their neurotoxicity, although 6-aminodopamine seemed to be more generally toxic.