Blockade Of Norepinephrine Synthesis Research Articles

The Selective Dopamine β-Hydroxylase Inhibitor Nepicastat Attenuates Multiple Aspects of Cocaine-Seeking Behavior

Although norepinephrine (NE) does not typically modulate cocaine self-administration under traditional schedules of reinforcement, it is required for different inducers of the reinstatement of cocaine-seeking behavior via activation of multiple adrenergic receptor subtypes. We predicted that blockade of NE synthesis would attenuate all known modalities of reinstatement and showed previously that the selective dopamine β-hydroxylase inhibitor, nepicastat, had no effect on either maintenance of operant cocaine self-administration maintained on a fixed-ratio 1 schedule or reinstatement of food seeking but did abolish cocaine-primed reinstatement. In the present series of studies, we first evaluated the dose-dependent effect of nepicastat (5, 50, or 100 mg/kg) on novelty-induced locomotor activity and found that it blunted exploration only at the highest dose. Next, we assessed the ability of nepicastat (50 mg/kg) to reduce breakpoint responding for cocaine on a progressive ratio schedule and reinstatement induced by drug-associated cues and stress. We found that nepicastat significantly lowered the breakpoint for cocaine, but not for regular chow or sucrose, and attenuated cue-, footshock-, and yohimbine-induced reinstatement. Combined, these results indicate that nepicastat can reduce the reinforcing properties of cocaine under a stringent schedule and can attenuate relapse-like behavior produced by cocaine, formerly cocaine-paired cues, and physiological and pharmacological stressors. Thus, nepicastat is one of those rare compounds that can reduce reinforced cocaine seeking as well as all three reinstatement modalities, while sparing exploratory behavior and natural reward seeking, making it a promising pharmacotherapy for cocaine addiction.

Induction of reversible growth retardation and growth hormone deficiency by blockade of norepinephrine synthesis in the rat

Norepinephrine is a major regulator of the release of growth hormone. Diethyldithiocarbamate, a dopamine-beta-hydroxylase inhibitor, reduces norepinephrine synthesis and acutely inhibits growth hormone (GH) secretion. To investigate the long-term effects of dopamine-beta-hydroxylase blockade on growth, we administered diethyldithiocarbamate (0, 40, 100 or 400 mg/kg sc b.i.d.) to 21-day-old female rats for 10 days. Food intake, body weight, and tail length were measured twice a week. Plasma GH levels and hypothalamic dopamine and norepinephrine content were measured; messenger ribonucleic acids (mRNAs) for GH-releasing hormone and somatostatin were determined by quantitative in situ hybridization. Diethyldithiocarbamate administration decreased GH levels (p < 0.05) and retarded growth in a dose-dependent manner (p < 0.05), without altering food intake. Co-administration of GH partially reversed the growth retardation in diethyldithiocarbamate-treated animals (p < 0.05). Diethyldithiocarbamate treatment also increased the hypothalamic dopamine/norepinephrine ratio (1.13 vs 0.41 control, p < 0.05). Local levels of GH-releasing hormone and somatostatin mRNA were not altered by treatment. After discontinuation of diethyldithiocarbamate, growth rates returned to normal or transiently even to supranormal values. Norepinephrine synthesis blockade with diethyldithiocarbamate provides a model for reversible growth retardation, in which GH levels are decreased in the absence of decreased GH-releasing hormone mRNA. These results support a role for norepinephrine in the regulation of normal growth.

Reduced norepinephrine turnover in mice with monosodium glutamate-induced obesity

The disappearance of norepinephrine from the heart, interscapular brown adipose tissue (BAT), and pancreas has been examined in mice with monosodium glutamate (MSG)-induced obesity and in untreated controls. MSG-treated mice became obese in the absence of increased food intake and their core temperature was significantly lower compared to control mice. The rate of norepinephrine turnover following blockade of norepinephrine synthesis with α-methylpara-tyrosine was significantly slower in heart and interscapular BAT of these mice than in untreated controls, but MSG had no effect on the pancreas. It is suggested that reduced norepinephrine turnover may be a major factor in the development of obesity after neonatal administration of MSG.

Catecholamine turnover in rats with ventromedial hypothalamic lesions.

The disappearance of norepinephrine (NE) from brown adipose tissue, heart, and pancreas in vivo has been examined in rats with ventromedial hypothalamic (VMH) lesions and in appropriate control rats. Two weeks after the introduction of these lesions, the rate of NE turnover following blockade of NE synthesis with alpha-methyl-p-tyrosine was significantly greater in brown adipose tissue and heart of rats with VMH lesions than in sham-operated controls. This significant increase occurred whether the animals were fed ad libitum or pair-gained to the sham-operated controls. In the pancreas, on the other hand, no effects of hypothalamic lesions could be detected. When rats were fasted, NE turnover slowed in both sham-operated and VMH-lesioned rats, but the turnover slowed more in sham-operated than in VMH-lesioned rats. When exposed to the cold for 6 h during measurements of NE turnover, VMH-lesioned rats increased their rate of NE turnover as did sham-operated rats. Fasted animals exposed to the cold had significantly higher rates of NE turnover whether or not they also had VMH lesions. Animals with electrolytic or knife-cut VMH lesions showed comparable rates of NE turnover in the fasted state. Eight weeks after hypothalamic lesions, similar studies were conducted. By this time NE turnover was not significantly different between the VMH-lesioned and sham-operated rats fed ad libitum. However, fasting, which slowed NE turnover in sham-operated rats, had no effect in VMH-lesioned animals. These data are discussed in relation to the autonomic hypothesis.

Beta-adrenergic receptors in the premammillary nucleus mediate the inhibition of LH release evoked by locus ceruleus stimulation.

The neurotransmitter and andrenoceptors involved in the inhibition of luteinizing hormone (LH) release induced by stimulating the locus ceruleus nucleus (LC) have been characterized in ovariectomized, estrogen-primed rats in which the release of LH was triggered by electrical stimulation of the medial preoptic area. Blockade of norepinephrine synthesis by the injection of the dopamine beta-hydroxylase inhibitors, diethyldithiocarbamate or fusaric acid, prevented the inhibition of LH release, whereas blockade of epinephrine synthesis by injecting the phenylethanolamine N-methyltransferase inhibitor, 2,3-dichloro-methylbenzylamine, had no effect. In addition, the inhibition of LH release was prevented by systemic injection of propranolol but not of phenoxybenzamine. This inhibition was also suppressed in rats in which the ventral premammillary (PM) nucleus ipsilateral to the stimulated LC was lesioned, or in animals with a transverse cut placed just in front of the PM. A similar response was seen in rats in which propranolol but not phenoxybenzamine was applied to the PM ipsilateral to the stimulated LC. The present results indicate that norepinephrine, acting through beta-adrenoceptors mediates the transmission of impulses which, originating in the LC, inhibit the release of LH. The LC adrenergic fibers synapse in the PM before the neural information they convey is transfered to the gonadotropin-releasing hormone containing neurons.

The effects of cysteamine on thyrotropin and immunoreactive beta-endorphin secretion in the rat.

We examined the effects of the thiol agent cysteamine (CSH), which is known to deplete the hypothalamus of immunoreactive somatostatin, on physiological TSH and beta- endorphin secretion in the adult male rat. CSH at doses of 90 and 300 mg/kg CSH produced a rapid decline in plasma TSH, whereas a dose of 30 mg/kg did not alter plasma TSH levels. After the higher doses of CSH, TSH levels in the blood remained lower than control values on day 2, but returned to normal by 1 week. This decrease in TSH within the plasma was not associated with a reduction in hypothalamic TRH concentrations. The TSH response to 500 ng/kg TRH was normal in CSH-treated animals. Blockade of norepinephrine synthesis with diethyldithiocarbamate (500 mg/kg) or fusaric acid (100 mg/kg) inhibited TSH secretion in a manner similar to that of CSH. beta-Endorphin-like immunoreactivity (bet-End-LI) was elevated in the plasma immediately after CSH (300 mg/kg) administration. This was associated with a 58% reduction in anterior pituitary beta-End-LI and no change in hypothalmic beta-End-LI. Plasma beta-End-LI returned to normal on day 2. The increase in plasma beta-End-LI induced by immobilization stress was not compromised by CSH treatment. The observed effects of CSH on both TSH and beta-End-LI are consistent with a reduction in central norepinephrine neurotransmission through the known actin of CSH to inhibit dopamine-beta-hydroxylase. Acute stress may play a role as well in the observed changes in TSH and beta-End-LI secretion.

Cysteamine effects on growth hormone secretion in the male rat.

The thiol agent cysteamine (CSH) is reported to deplete immunoreactive somatostatin (SS) in the central nervous system and the gastrointestinal tract of rats. We investigated whether CSH, given sc, alters physiological GH secretion in chronically cannulated male rats. A dose of 300 mg/kg totally disrupted episodic GH secretion, causing an elevation of GH baseline levels and an absence of GH secretory bursts. CSH at a dose of 90 m/gg elevated GH trough levels and slightly reduced GH peak amplitudes, whereas a dose of 30 mg/kg did not affect episodic GH secretion. GH patterns remained disrupted in most animals on day 2 after drug administration, but returned to normal by 1 week. The administration of anti-SS serum resulted in a further elevation of GH levels in CSH-treated animals. Both clonidine and morphine induced a GH rise in treated animals. The GH response to stress was attenuated in CSH-treated animals. CSH at the highest dose given (300 mg/kg) markedly reduced norepinephrine (NE) concentrations (56% of the control value) in the hypothalamus and median eminence 4 h after administration. In the same animals, dopamine was elevated (200%) in the hypothalamus, but was unaffected in the median eminence. The changes in catecholamine levels are consistent with the hypothesis that CSH inhibits the enzyme dopamine-β-hydroxylase. Furthermore, blockade of NE synthesis with the dopamine-β-hydroxylase inhibitors diethyldithiocarbamate (500 mg/kg) or fusaric acid (100 mg/kg) completely suppressed episodic GH secretion. These findings indicate that CSH has an important effect on physiological GH secretion and that its effect is dose dependent and reversible. Lower doses appear to have a specific effect on SS-mediated inhibition of GH secretion. On the other hand, high doses of CSH totally disrupt GH secretion by affecting both the SS inhibitory and the GRF stimulatory components of episodic GH secretion. The latter action of CSH is, perhaps, mediated by both an inhibition of the synthesis of NE in the hypothalamus and an acute stress response.

Influence of aging and tyrosine hydroxylase inhibition on tissue levels of norepinephrine during stress.

The effect of aging was evaluated on norepinephrine content of the heart (ventricles) and spleen of 3- and 24-month-old F344 and Sprague-Dawley rats utilizing a sensitive radioenzymatic assay. To assess sympathetic nervous system activity, the decline in organ norepinephrine content was compared in young and old F344 rats 6 hours after blockade of norepinephrine synthesis with alpha-methyl-p-tyrosine, a tyrosine hydroxylase inhibitor. Three conditions were studied: (a) normal, (b) brief starvation (54 hours), and (c) cold exposure (6 hours, 4 degrees C). There was no significant age-related difference in steady state organ norepinephrine concentration. Based on the response to alpha-methyl-p-tyrosine, aging did not affect the rate of heart and spleen norepinephrine synthesis and, therefore, sympathetic nervous system impulse activity during normal or cold stress conditions. Starvation, however, did not suppress sympathetic nervous system activity to the heart in old animals, as it did in the young rats.

Central noradrenergic activity and the formation of glycol sulfate metabolites of brain norepinephrine

The present study utilized intraventricular injection of Na 2 35SO 4 to detect drug induced changes in the in vivo formation of the two major metabolites of rat brain norepinephrine (NE) - the sulfate conjugates of 3-methoxy-4-hydroxyphenylglycol (MOPEG-SO 4) and 3,4-dihyd (DOPEG-SO 4). Assays involved the hypothalamus only. Rats pretreated with clonidine showed a reduced formation of both MOPEG- 35SO 4 and DOPEG- 35SO 4 after intraventricular Na 2 35SO 4 as well as reduced synthesis of 3H-NE from intraventricular 3H-tyrosine. Phenoxybenzamine (POB) produced increases in the synthesis of both 35S-labeled conjugates and 3H-NE. Neither drug altered the loss of exogenous 3H-MOPEG-SO 4 but clonidine increased both the accumulation of labeled sulfate and the sulfation of exogenous MOPEG in pheniprazine treated rats. These results show that the rates of formation of the labeled glycol sulfates are sensitive indicators of changes in brain NE turnover but can also be influenced by factors involved in sulfation that are unrelated to NE turnover. Blockade of NE synthesis with alpha methyltyrosine did not affect resting or POB-elevated levels of the labeled conjugates until stores of NE were reduced by 40%. The latter findings suggest that central noradrenergic neurons can release and metabolize NE at a normal rate despite synthesis blockade so long as adequate stores of NE are available.

Brown fat norepinephrine contents and turnover during cold acclimation and hibernation in the golden hamster ( mesocricetus auratus)

1. 1. Norepinephrine (NE) levels were measured fluorometrically in brown fat and certain other tissues of normal, cold acclimated, hibernating, and newly aroused hamsters. 2. 2. During cold acclimation the absolute quantity of NE in brown fat increased along with the tissue mass. 3. 3. Brown fat NE decreased significantly in hibernating hamsters. A primed system for accelerated NE turnover, which could be important for arousal thermogenesis, is suggested. 4. 4. NE turnover was estimated in brown fat, heart, and spleen by measuring the NE decline after blockade of NE synthesis with α-methyl- p-tyrosine. 5. 5. Brown fat NE turnover was greater in cold acclimated hamsters (4° C.) than in normal controls (23° C.). This could be important for efficient brown fat thermogenesis during cold exposure. 6. 6. Brown fat NE turnover was increased in both normal controls and cold acclimated hamsters exposed acutely to 4° C. Steady-state NE declined in normal brown fat but not in cold acclimated brown fat during acute cold exposure. This could indicate an increased capacity for NE synthesis as well as turnover in cold acclimated brown fat.

The relative role of storage and synthesis of brain norepinephrine in the psychomotor stimulation evoked by amphetamine or by desipramine and tetrabenazine.

The relative role of storage and synthesis of brain norepinephrine in the psychomotor stimulation evoked by amphetamine or by desipramine and tetrabenazine has been investigated. The blockade of norepinephrine synthesis by the tyrosine hydroxylase inhibitor α-MT abolishes the central stimulatory action of amphetamine but not that evoked by tetrabenazine in DMI pretreated rats. In contrast, depletion of norepinephrine by α-MMT prevents the central stimulation evoked by the DMI-tetrabenazine combination but not that elicited by amphetamine. The α-methylated tyrosine derivatives do not interfere with either the metabolism of amphetamine and DMI or their entry into the brain. The results of the present studies, thus, are consistent with the views that the central action of amphetamine requires an uninterrupted synthesis of brain norepinphrine whereas a rapid release of norepinephrine from its storage sites is essential for the behavioral stimulation elicited by tetrabenazine in DMI pretreated rats.