Blockade of the behavioral effects of 5-HTP by the decarboxylase inhibitor Ro 4-4602

Twenty male rats were trained to press a bar on a fixed-ratio (FR 32) schedule of water reinforcement. The behavioral effects of 5-HTP (50 mg/kg) were studied following pretreatment with small (50 mg/kg) and large (400 mg/kg) doses of the decarboxylase inhibitor Ro 4-4602. It was found that both doses of the pretreatment agent blocked the disruptive effects of 5-HTP. This result suggests that at least some of the effects of 5-HTP may be mediated peripherally.     

Differential effect of benserazide (Ro4-4602) on the concentration of indoleamines in rat pineal and hypothalamus.

1 Low doses (50 and 80 mg/kg) of benserazide (Ro4-4602), an aromatic amino acid decarboxylase inhibitor, markedly reduced 5-hydroxytryptamine and melatonin in the rat pineal gland without affecting hypothalamic 5-hydroxytryptamine. 2 This differential effect shows that inhibition of the pineal gland decarboxylase activity is possible, and confirms that the rat pineal gland is accessible to peripherally acting agents.     

Effects of a decarboxylase inhibitor on the Dopa and 5-HTP induced changes in the locomotor-like discharge pattern of rabbit hind limb nerves

Three Rhesus monkeys were treated daily po with crude marihuana extract (CME) containing ⁹-THC 22–25%, cannabidiol 2–3% and cannabinol 2–3%, but no ⁸-THC. CME with the equivalent of ⁹-THC 12.5 mg/kg or more caused sedation, ptosis, ataxia, huddled posture, spontaneous jerky body movements and increased EEG synchrony without an initial phase of increased motor activity. Thereafter, ⁹-THC 12.5 mg/kg in 2/3 monkeys produced specific EEG changes including the appearance of protracted trains of 20–25 cps rhythmic activity in thalamus and cerebellar nuclei. ⁹-THC 37.5 mg/kg or more, in all 3 monkeys tested, caused 1.5–2 cps slow waves in hippocampus, amygdala and septum. Spikes or other epileptiform EEG activity were not observed. EEG manifestations after oral treatment were therefore different from those previously observed following i.v. injection or smoke inhalation. Behavioral effects consistently preceded and outlasted EEG changes, and tolerance developed more rapidly to specific EEG changes than to behavioral effects. The two monkeys that became tolerant after 50 daily treatments with ⁹-THC 37.5 mg/kg responded to termination of treatment with withdrawal signs manifested by increased aggressiveness. One of the two exhibited hallucinations and increased periods of EEG desynchronization. Withdrawal signs were, therefore, more severe for behavioral than for EEG effects.Supported in part by NIMH Contract HSM 42-71-79.     

A comparison of the discriminative stimulus properties of l-5-hydroxytryptophan in the presence of either citalopram or Ro 4-4602

The establishment of stimulus control by 5-HTP, the amino acid precursor for serotonin (5-HT), has been reported previously [1-3]. In the present investigation, two groups of rats were trained with 5-HTP versus saline in a 2-lever discrimination procedure. Prior to the administration of 5-HTP, subjects were pretreated with either Ro 4-4602, an inhibitor of peripheral decarboxylase (R-HTP), or citalopram, a specific 5-HT reuptake inhibitor (C-HTP). Neither C-HTP nor R-HTP was antagonized completely by either pirenperone or pizotyline. When C-HTP and R-HTP were tested in a third group of rats trained with LSD, complete generalization was not observed. The results of cross tests in the R-HTP and C-HTP groups with LSD, TFMPP, 8-OH-DPAT, C-HTP, and R-HTP indicate that the stimuli induced by R-HTP and C-HTP are similar but not identical. Taken together, these data suggest that 5-HTP produces a compound stimulus that is not readily explained in terms of either 5-HT1 or 5-HT2 receptors alone.     

The disposition of a histidine decarboxylase inhibitor (S)-alpha-fluoromethylhistidine in rats

An amino acid analyser method using ninhydrin was developed for (S)-alpha-fluoromethylhistidine (FMH) with a minimum quantitation limit of 0.2 microgram mL-1. The assay was used to study the kinetics of FMH in rat. After bolus intravenous administration of FMH hydrochloride hemihydrate (50 mg kg-1), plasma concentration decreased biexponentially with half-lives of 4.4 and 32.7 min. The distribution volumes of the central and peripheral compartments were 127.4 and 166.3 mL kg-1, respectively. The tissue concentration of FMH was highest in the kidney and also decreased biphasically. The FMH concentrations in other tissues were lower, but their tissue/plasma ratios of FMH increased continuously after FMH injection, indicating that FMH partitioned into these tissues and was lost from them very slowly.     

S-(5'-deoxy-5'-adenosyl)-1-ammonio-4-(methylsulfonio)-2-cyclopentene: A potent, enzyme-activated irreversible inhibitor of S-adenosylmethionine decarboxylase.

The compound S-(5'-deoxy-5'-adenosyl)-1-ammonio-4-(methylsulfonio)-2- cyclopentene (AdoMac) was prepared and evaluated as an irreversible inhibitor of S-adenosylmethionine decarboxylase (AdoMet-DC). AdoMac was shown to inhibit AdoMet-DC in a time-dependent manner with a KI of 18.3 microM and a kinact of 0.133 min-1. In addition, AdoMet-DC activity could not be restored following extensive dialysis of the enzyme-inhibitor complex, and the enzyme was protected from irreversible inactivation by the known competitive inhibitor methylglyoxal bis(guanylhydrazone). HPLC analysis of the enzymatic reaction products revealed a time-dependent decrease in the peak coeluting with AdoMac, and a corresponding increase in the peak coeluting with (methylthio)adenosine (MTA), a byproduct of the irreversible binding of AdoMac to the enzyme. Thus, AdoMac appears to function as an enzyme-activated, irreversible inhibitor of AdoMet-DC.     

Inhibition of brain dopa-decarboxylase by RO 4-4602 infused ICV blocks alcohol drinking induced in rats by cyanamide

Following the stereotaxic implantation of chronic cannulae for intracerebroventricular (ICV) infusion, rats were given an alcohol preference test to establish their preferred concentration in comparison with water. After alcohol was removed, 15 mg/kg cyanamide was then injected subcutaneously for 4 days in order to maximize volitional intake of single solutions of alcohol, which in these animals ranged from 7 to 15%. The l-dopa-decarboxylase inhibitor benserazide (Ro 4-4602) injected subcutaneously twice daily in doses of 50–100 mg/kg failed to alter the rats' alcohol consumption either in terms of g/kg or proportional values. However, when given ICV twice daily in concentrations of 10 ng–2.0 g per 5.0 l volume, benserazide attenuated the rats' alcohol drinking significantly. This reduction occurred in a dose-dependent manner in terms of both absolute and proportional intakes of alcohol. Pre-treatment of the animals with 1.0 g benserazide given ICV, when alcohol was removed from the test situation, did not abolish the subsequent ingestion of alcohol but its peripheral administration (50 mg/kg) enhanced drinking. These results suggest that the interference with the metabolic pathway of dopamine or serotonin synthesis, possibly through the mechanism of reduced formation of aldehyde adducts in the brain, markedly alters the pattern of voluntary drinking in the rat. Alternatively, benserazide could act by its central inhibition of aldehyde dehydrogenase, which in turn would concomitantly elevate levels of acetaldehyde and thereby reduce alcohol drinking.     

Involvement of leptin in hypophagia induced by the serotonin precursor 5-hydroxytryptophan (5-HTP) in mice

We previously demonstrated that a serotonin (5-HT) precursor 5-hydroxytryptophan (5-HTP) increases serum leptin levels in mice. It was reported that administration of 5-HTP elicits hypophagia in rodents and humans. In the present study, we examined involvement of leptin in 5-HTP-elicited decreases in the milk intake of fasted mice. Serum leptin levels increased with increases in milk intake in mice, while 5-HTP strongly decreased milk intake in fasted mice compared to that in the control group. Serum leptin levels in fasted mice treated with 5-HTP were similar to those control mice after milk intake. As leptin is a powerful anorectic signal, 5-HTP-induced anorexia may be mediated by facilitation of leptin secretion.     

Cardiocirculatory effects of Ro 11-2464, a selective 5-HT (serotonin) uptake inhibitor,in healthy volunteers

Summary The imipramine derivative Ro 11-2465, a potent, selective 5-HT (serotonin) uptake inhibitor, is being developed as an antidepressant agent. The effects of Ro 11-2465 on heart rate, blood pressure, electrocardiogram and systolic time intervals were assessed in nine normotensive volunteers. Ro 11-2465 1 and 2 mg and a placebo were given in a single blind, cross over design study. The placebo did not induce any significant changes. With Ro 11-2465, the ECG-intervals (P,PQ,QRS,QTc) did not change, the blood pressure increased 3–6 h after administration of either dose, and the heart rate was increased 4–6 h after the 2 mg dose. There was also evidence of increased ventricular automaticity in one subject. The total electromechanical systole (QS₂-index) was significantly shortened 4–8 h after administration of 2 mg, whereas neither 1 mg the dose nor the placebo had any such effect. This finding suggests the presence of a positive inotropic effect, which is probably due to a stimulatory effect of serotonin, and is not mediated by an adrenergic mechanism. The findings suggest that Ro 11-2465, as a potential new tricyclic antidepressant, might have favourable cardiocirculatory side effects, particularly in patients with pre-existing heart disease.     

Inhibition of firing of raphe neurones by tryptophan and 5-hydroxytryptophan: blockade by inhibiting serotonin synthesis with Ro-4-4602.

In these studies, serotonergic ncuronal activity was monitored by extracellular recording from identified cells in the midbrain dorsal raphe nucleus. The systemic administration of l-tryptophan. the initial precursor in the biosynthesis of brain serotonin (5-HT). was found to depress the firing of serotonergic neurones. The systemic administration of l-5-hydroxytryptophan, the immediate precursor of 5-HT. in combination with a low dose of Ro4-4602 (50 mg/kg, which inhibits mainly peripheral decarboxylase) was also found to depress raphe firing. The tryptophan-induced depression of raphe activity was prevented by pretreatment of the animal with an l-aromatic amino acid decarboxylase inhibitor, Ro4-4602, in a dose (800 mg/kg) sufficient to block the formation of 5-HT in raphe neurones. p-Chlorophenylalanine. another 5-HT synthesis inhibitor, was found to be ineffective in preventing the depression of raphe firing by tryptophan. However, previous studies have shown that p-chlorophenylalanine does not prevent the accumulation of 5-HT within the raphe perikarya following loading doses of tryptophan (Aghajanian, Kuhar and Roth. 1973). Raphe neurones were also inhibited by the local (microiontophoretic) administration of l-tryptophan or l-5-hydroxytryptophan. We conclude that (1) both 5-HT precursors (tryptophan and 5-hydroxytryptophan) inhibit raphe firing through their enzymatic conversion to 5-HT: (2) a local increase in 5-HT within the raphe perikarya is sufficient to lead to a decrease in raphe cellular activity.