Tolerance to the increased locomotor activity produced by l-5-hydroxytryptophan following peripheral decarboxylase inhibition in mice

The development of tolerance to hyperactivity produced by l-5-hydroxytryptophan (5-HTP) was studied in mice pretreated with the peripheral decarboxylase inhibitor MK-486. The results of Experiment I indicated that partial tolerance developed to 5-HTP given twice daily (i.p.) at a dose of 400 mg/kg, but not at a dose of 800 mg/kg. Sustained hyperactivity at the greater dose (800 mg/kg) apparently resulted from the induction of seizures and stereotypy rather than increased locomotor activity. When 5-HTP (400 mg/kg) or saline was administered three times daily (Experiments II and III), the locomotor activity of saline control groups did not differ significantly from chronic 5-HTP-treated groups, but both differed significantly from that of acute 5-HTP-treated animals. Cessation of treatments resulted in a recovery of 5-HTP-induced hyperactivity for experimental animals when later retested. These findings suggest that mice develop tolerance to the effects of 5-HTP on locomotor activity and agree with the hypothesis that behavior change is more closely correlated with the rate of change in concentration of neurotransmitters than the absolute concentrations.     

Effects of l-tryptophan on motor activity in mice

The effects of l-tryptophan (50–800 mg/kg i.p.) on motor activity in mice were studied. l-tryptophan, 800 mg/kg, caused a reduction of motor activity while lower doses had no significant effect. The role of different metabolites of l-tryptophan in behavioural depression was tested by pretreating groups of animals with inhibitors of tryptophan hydroxylase (p-chlorophenylalanine), peripheral aromatic amino acid decarboxylase (DC) (MK-486), peripheral and central DC (NSD 1015), or tryptophan pyrrolase (allopurinol). None of these pretreatments antagonized the l-tryptophan induced depression of motor activity. Pretreatment with MK-486 or NSD-1015 potentiated the depression. Pretreatment with allopurinol potentiated as well as prolonged the depressive effect. Pretreatment with chlorimipramine had no significant effect on the l-tryptophan induced depression. The elevations of brain tryptophan and 5-hydroxytryptamine (5-HT) concentrations after l-tryptophan (800 mg/kg i.p.) were prolonged by pretreatment with allopurinol. The l-tryptophan induced increases in brain concentrations of 5-HT and 5-hydroxyindoleacetic acid were more pronounced after pretreatment with allopurinol. It is suggested that the l-tryptophan induced reduction of motor activity in mice is mediated via the amino acid itself and not via its metabolites.     

Comparison of tyrosine hydroxylase and dopamine-Β-hydroxylase inhibition with the effects of various 6-hydroxydopamine treatments on d-amphetamine induced motor activity

The significance of central noradrenergic and dopaminergic neural systems for the locomotor stimulant effects of d-amphetamine were investigated in rats with depletions of norepinephrine, dopamine, or both catecholamines produced by treatment with either reserpine, L--methyl-tyrosine (-MPT), 6-hydroxydopamine (6-OHDA), or the dopamine--hydroxylase inhibitor 1-phenyl-3-(2-thiazolyl)-2-thiourea (U-14,624). In animals pretreated with reserpine, amphetamine-stimulated locomotor activity was blocked by -MPT but not by U-14,624 when amphetamine was given l h after these catecholamine synthesis inhibitors. In rats with chronic depletions of brain norepinephrine, dopamine, or both catecholamines produced by different 6-OHDA treatments, both amphetamine-stimulated motor activity and stereotyped behavior were antagonized by treatments reducing dopamine or both catecholamines but not in animals in which brain norepinephrine was reduced. Results are consistent with the view that the locomotor stimulation and stereotyped behaviors produced by d-amphetamine are dependent upon functional dopaminergic neural systems in brain.     

l-DOPA induced effects on motor activity in mice after inhibition of dopamine-β-hydroxylase

The modification by FLA-63 bis-(4-methyl-1-homopiperazinyl thiocarbonyl)-disulfide (Florvall and Corrodi, 1970), an inhibitor of dopamine (DA)--hydroxylase, of the effect of different doses of l-DOPA on motor activity and monoamine levels in the caudate nucleus and the rest of the brain, was studied. Throughout the experiment extracerebral decarboxylase was inhibited by means of Mk-485. FLA-63 did not alter the depressant effect on motility of a medium dose of l-DOPA, although the characteristic hyperexcitability was abolished. However, the strong motor stimulant effect of a high dose of l-DOPA was partly antagonized by FLA-63. l-DOPA caused an accumulation of DA both in the caudate nucleus and in the rest of the brain, a small but significant decrease in central noradrenaline (NA) and a decrease in 5-hydroxytryptamine in both brain parts. This pattern was mainly unaffected by FLA-63, which by itself caused a decrease in NA. It was concluded, that both NA and DA are propably of importance for the motor stimulant effect of l-DOPA in the mouse.     

The effects of dl-p-chlorophenylalanine and dl-α-methyl-p-tyrosine on the brain catecholamine,serotonin and free amino acid contents in rat

The changes in the brain levels of catecholamines, serotonin and free amino acids have been considered as a factor in the development of the physical dependence on and tolerance to some drugs. In order to show the relationships among the brain levels of these substances, three groups of rats were given i.p. dl-MpT alone, dl-pCPA alone, or dl-MpT and dl-pCPA together, twice a day for three days. Another group was kept as control. The brain levels of catecholamines, serotonin and free amino acids were determined. Although dl-MpT and dl-pCPA alone caused a decrease in the levels of catecholamines and serotonin respectively, the administration of dl-MpT and dl-pCPA together did not. The changes in the levels of free amino acids which were found were related to the metabolism of catecholamines and serotonin.     

Discriminative stimulus properties of l-5-hydroxytryptophan: Behavioral evidence for multiple serotonin receptors

Rats were trained to discriminate the stimulus properties of l-5-hydroxytryptophan (l-5-HTP) (30 mg/kg SC), the immediate precursor of serotonin (5-HT). The peripheral decarboxylase inhibitor R04-4602, administered prior to l-5-HTP, greatly attenuated the disruptive effects observed on responding when l-5-HTP alone was injected. Following acquisition, the discrimination was dosedependent and generalized to fenfluramine, a 5-HT-releasing drug, but not to amphetamine, a catecholamine-releasing agent. Further evidence for the involvement of 5-HT receptor stimulation in mediating the discrimination was that pretreatment with fluoxetine, a highly specific 5-HT uptake inhibitor, markedly potentiated the cue. Nevertheless, the classical 5-HT antagonists methysergide, cyproheptadine, metergoline, and methiothepin did not block the l-5-HTP-related discriminative stimulus. This finding suggested that the cue properties of l-5-HTP might be mediated by a population of 5-HT receptors previously identified electrophysiologically in limbic structures. As in the present experiment, the putative 5-HT antagonists did not block the synaptic effects of 5-HT in these structures.     

Stereospecific actions of baclofen on sociosexual behavior,locomotor activity and motor execution

The behavioral effects of racemic baclofen and the r and s enantiomers were studied in order to determine whether the stereospecificity found in receptor binding studies also applies to the behavioral actions of the drug. Racemic and r-baclofen inhibited sexual behavior, locomotor activity and motor execution at relatively low doses while s-baclofen was completely inactive even when a dose 40 times higher than the minimum effective dose of r-baclofen was used. The r enantiomer seems to be twice as active as racemic baclofen. These data strongly suggest that the behavioral effects of baclofen are the result of an action at the GABA-B receptor. In order to differentiate the effects of baclofen on sexual interactions from those on nonspecific social interactions, the sociosexual behavior was observed with a castrated male or a receptive female as stimulus animal. r, s-baclofen had effects only upon sociosexual interaction with a receptive female. Moreover, the inhibitory effects of baclofen were restricted to behavioral items related to sexual interactions, primarily those constituting precopulatory behaviors. Since no effect was observed in social interactions with a castrated male, it is suggested that the inhibition of sociosexual behavior is not a consequence of impairment of motor execution. Rather it appears that baclofen has a specific inhibitory effect on behaviors associated with the initiation of copulatory activity. Therefore, once initiated, sexual behavior was not significantly modified by baclofen.     

Effects of l-dihydroxyphenylalanine (l-Dopa) and d,l,5-hydroxytryptophan (d,l,5-HTP) on reserpine-induced amnesia

In a series of experiments the effects of reserpine, l-Dopa, and d,l,5-hydroxytryptophan (d,l,5-HTP) on retention of a passive avoidance training in mice were investigated. Reserpine (2.5 mg/kg) produced amnesia when given at 120 min before but not at 30 min before or at 0, 10, 30, or 90 min following training. This time-dependent reserpine effect did not appear to be due to either an alteration in footshock sensitivity during training or to the drug producing state-dependent learning. The amnesic effect of reserpine could be blocked when both l-Dopa and d,l,5-HTP were also administered up to 10 min but not at 30 or 90 min following training. The drugs, l-Dopa or d,l,5-HTP, given alone or in higher doses, could not at any time counteract the reserpine effect. The retrograde effects of the combined administration of these biogenic amine precursors on the reserpine-induced amnesia are discussed in terms of the possible role of biogenic amines in memory formation.     

Comparison of behavioral and cardiovascular effects of l-DOPA and 5-HTP in conscious dogs

Moderate doses of l-DOPA (30–40 mg/kg) alone produced depressant effects, emesis and a mild hypertensive response in dogs. These depressant effects included decreased alertness, reduced response to overt stimuli such as whistling or sharp noises, reduced response to handling and a degree of flaccidity. Haloperidol completely blocked the behavioral and emetic effects of l-DOPA and, in most cases, significantly reduced the hypertensive response. Methysergide prevented the depressive effects and significantly attenuated emesis but had variable effect on the hypertensive response of l-DOPA. HMD and Ro 4-4062, in doses which selectively inhibit peripheral decarboxylase, completely blocked most of the behavioral effects, markedly attenuated emesis and prevented or reduced the hypertensive response, of l-DOPA. HMD, in addition, converted the hypertensive response of l-DOPA to a hypotensive response. dl-Parachlorophenylalanine completely blocked the behavioral effects and significantly reduced the cardiovascular effect of l-DOPA but had no effect on emesis induced by l-DOPA. dl-5-HTP, in contrast to l-DOPA, produced behavioral excitation in the dogs. It also produced a hypertensive response. Haloperidol had no significnat effect on the behavioral responses but significantly reduced the cardiovascular response produced by dl-5-HTP. Methysergide, similarly, had no significant effect on the behavioral responses of dl-5-HTP but had variable effects on the hypertensive response. Ro 4-4602 and HMD also had no effect on the behavioral and cardiovascular responses due to dl-5-HTP. l-DOPA failed to prevent the behavioral effects of 5-HTP. 5-HTP completely blocked or significantly attenuated the emetic response produced by l-DOPA although there was no significant change in the blood pressure when compared with controls.This work was supported by USPHS Grant MH 12383.     

A comparative study of the effects of reserpine and p-chlorophenylalanine on morphine analgesia in mice

The potentiating effect of reserpine on morphine analgesia in mice was studied by means of the hot plate method and compared with the anti-analgesic action produced by p-chlorophenylalanine (p-Clphe) in the same strain of mice. The administration of DOPA or 5-hydroxytryptophan did not alter the potentiation, while the combination of both monoamine precursors markedly increased the morphine effect in reserpine-treated mice. p-Clphe showed no significant ability to antagonize the potentiating effect of reserpine. -Methyl tyrosine attenuated the analgesic effect of morphine in both untreated and reserpinized mice. These results suggest that the enhanced effect of morphine observed in reserpinized mice is not mediated by the depletion of brain monoamines produced by reserpine.     

Role of peripheral mechanisms in the behavioral effects of 5-hydroxytryptophan.

The effects of 5-hydroxytryptophan (5-HTP) were studied in rats trained to press a lever under a fixed-ratio (Fr-32) schedule of water presentation. d-, l-and d,l-5-HTP all decreased responding in a dose-related manner. The levo isomer (12.5-25 mg/kg) was twice as potent as the racemic form (25-50 mg/kg) in this respect. Moderate doses of d-5-HTP (less than 100 mg/kg) did not affect responding, whereas 200 mg/kg produced almost complete suppression. The response decrement produced by 25 mg/kg l-5-HTP was completely antagonized by pretreatment with either 50 mg/kg or 400 mg/kg of the decarboxylase inhibitor, benserazide (Ro4-4602). The specific peripheral decarboxylase inhibitor, carbidopa (MK-486) (50 mg/kg) and the peripheral serotonergic antagonist, xylamidine tosylate (1 mg/kg) also antagonized the effects of 25 mg/kg l-5-HTP. These results suggest that at least some of the behavioral effects of 5-HTP are due to increases in levels or turnover of 5-HTP in peripheral serotonergic neuronal systems.     

Efficacy of various dithiol compounds in acute As2O3 poisoning in mice

The efficacy ofdl-dimercaptopropanol (British Anti-Lewisite, BAL),dl-dimercaptopropanesulfonate (DMPS), and meso-dimercaptosuccinic acid (DMS A) was compared in reducing the acute As₂O₃ toxicity in mice. Mice were treated with a single equimolar dose of a dithiol compound (0.7 mmol/kg i.p.) 0.5 or 30 min after the s.c. injection of various doses of As₂O₃. Both DMPS and DMSA were significantly (p<0.05) more effective in mice treated 0.5 min after the poisoning if compared to BAL on an equimolar level. The highest potency ratio (PR) (LD50 with treatment/LD5o without treatment) was found in animals injected with DMSA (PR=8.6). The corresponding value for DMPS was 4.2, and for BAL 2.1, respectively. In animals treated 30 min after poisoning the efficacy of DMPS (PR = 2.6) was similar to the efficacy of DMSA 2.4, both being only slightly superior to BAL 2.O. DMPS and DMSA were found to be much less toxic than BAL. The LD₅₀ of arsenic was 0.057 mmol/kg. The efficacy of BAL, DMPS, and DMSA in reducing the tissue content of arsenic following acute As₂O₃ poisoning was investigated in mice (n=6/group) and guinea pigs (n=3-4/group). The animals were injected s.c. with 0.043 mmol/kg As₂O₃ (containing a tracer dose of⁷⁴As(III)). Thirty minutes later the antidotes were administered A were more effective in reducing the arsenic content of tissues than BAL. Moreover, BAL caused accumulation of the toxicant in the brain. It is concluded that the recommendation of BAL as drug of choice in acute arsenic poisoning needs to be carefully re-evaluated.     

Supersensitivity to l-5-hydroxytryptophan after 5,7-dihydroxytryptamine injections in desmethylimipramine- and nomifensine-pretreated rats: Behavioral evidence for postsynaptic supersensitivity

The behavioral syndrome induced by l-5-hydroxytryptophan (l-5-HTP) in rats was used to study the supersensitivity to l-5-HTP and 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) which develops after unilateral intracerebroventricular (ICV) injections of 200 g 5,7-dihydroxytryptamine (5,7-DHT). Pretreatment of the animals with a combination of desipramine and nomifensine was found to protect dopamine neurones better than desipramine alone. Maximal behavioral supersensitivity to l-5-HTP and 5-MeODMT was found as early as 24 h after injection of the neurotoxin, even in the presence of the specific 5-HT uptake inhibitor CGP 6085 A, or the MAO-A inhibitor clorgyline. The results indicate that a quickly occurring postsynaptic event contributes to the development of behavioral supersensitivity after ICV injections of 5,7-DHT.     

Possible involvement of 5-hydroxytryptamine and cyclic-AMP in tolerance to tremorine analgesia in mice

The phenomenon of tolerance to the analgesic action of tremorine in mice was studied by the hot-plate and tail-clip methods. Reduction in 5-HT levels in brain by parachlorophenylalanine pretreatment decreased the ED₅₀ of tremorine analgesia in tremorine tolerant mice. 5-Hydroxytryptophan, l-Dopa or -methyl-para-tyrosine did not influence the analgesic response to tremorine in tremorine tolerant animals. However, theophylline was found to enhance the tolerance to tremorine analgesia.Brain 5-HT and cAMP are probably involved in tremorine tolerance, whereas neither noradrenaline nor dopamine is involved in the phenomenon.     

The effects of 5-HT uptake- and MAO-inhibitors on l-5-HTP-induced excitation in rats

Summary The behavioural syndrome caused by l-5-HTP in rats was used for the study of effects of selective 5-HT uptake inhibitors and inhibitors of MAO on central 5-HT receptors. A good correlation was found between the relative potencies of drugs in inhibiting the 5-HT uptake in the rat brain and in intensifying l-5-HTP-induced behavioural stimulation. The potentiation of the l-5-HTP syndrome by the MAO inhibitors correlated with the inhibition of the A- but not of the B-form of the brain monoamine oxidase.In rats treated with the maximally inhibiting dose of a 5-HT uptake inhibitor, MAO inhibitors were still able to increase the intensity of the l-5-HTP syndrome, while the combination of maximal doses of two 5-HT uptake inhibitors did not produce a more intense syndrome than that produced by one 5-HT uptake inhibitor alone.The l-5-HTP-induced behavioural syndrome in rats seems to afford an experimental model allowing the quantification and characterization of the interaction of drugs with serotonin metabolism in the brain.     

l-Dopa effects on motor and “central programming” deficits in Parkinsonism

A study was made to test the effects of treatment by l-Dopa on both motor and central programming deficits in Parkinsonism.Nine patients with Parkinsonism were tested on psychomotor tasks, involving both unimanual and bimanual performance, before and after treatment. Control data was obtained from 28 age-matched subjects. These were only tested on one occasion.As expected, there was a very significant improvement in motor performance of the Parkinsonism group after treatment by l-Dopa. The evidence of improvement in the central programming deficit was less conclusive, but seemed substantial enough to warrant further investigation.     

Catecholamines and memory: Enhanced verbal learning during l-DOPA administration

Verbal learning was measured during the administration of l-DOPA in large oral doses to depressed patients. Longer-term memory on two different tasks improved during treatment, while short-term memory (immediate recall) was unaffected. In contrast, the catecholamine synthesis inhibitor -methyl-p-tyrosine did not alter either memory process.The effects of l-DOPA on learning may be related to increased arousal produced by this drug.     

Pre- and postsynaptic effects of yohimbine stereoisomers on noradrenergic transmission in the pulmonary artery of the rabbit

Summary Effects on noradrenergic neurotransmission of five stereoisomers of yohimbine and of the closely related compound yohimbol were studied in strips of the pulmonary artery of the rabbit. In some experiments the tissue was preincubated with ³H-noradrenaline. Three effects were observed. Firstly, antagonism to the contractile effect of noradrenaline and of sympathetic nerve stimulation; the antagonism reflected competitive blockade of postsynaptic -adrenoceptors. Secondly, an increase in the stimulation-evoked overflow of total tritium and ³H-noradrenaline; the increase appeared to be due to blockade of presynaptic -adrenoceptors. Thirdly, an increase in the basal outflow of ³H-3,4-dihydrophenylglycol, presumably by impairment of the vesicular storage of ³H-noradrenaline. According to their relative potencies in eliciting these effects, the drugs could be divided into three groups. Rauwolscine, -yohimbine and yohimbol preferentially blocked the presynaptic -adrenoceptor; rauwolscine and -yohimbine, like yohimbine, at low concentrations increased the contractile response to sympathetic nerve stimulation. Corynanthine preferentially blocked the postsynaptic -adrenoceptor. Pseudoyohimbine and 3-epi--yohimbine were very weak antagonists at either receptor; they mainly accelerated the basal outflow of ³H-3,4-dihydroxyphenylglycol.From these results and those of a previous study it is concluded that, in a series of twelve -adrenolytic drugs, rauwolscine shows the greatest preference for presynaptic and corynanthine the greatest preference for postsynaptic -adrenoceptors. In view of the chemical similarity of the two compounds these opposite properties are striking. Corynanthine and rauwolscine might be useful tools for the subclassification of -adrenoceptors.     

Treatment of Sydenham's chorea with a combination of l-Dopa and a peripheral Dopa decarboxylase inhibitor

Two male patients suffering from Sydenham's chorea were treated with a combination of l-Dopa and a peripheral decarboxylase inhibitor. This treatment markedly reduced in one patient and totally suppressed in the other the abnormal involuntary movements present in this disease.     

Effect of monoamine precursors on the forced-swimming test in mice

The antidepressant properties of monoamine precursors were evaluated by the forced-swimming test for mice developed by Porsolt et al. DOPA but not 5-hydroxy-tryptophan (5HTP) shortened immobility at doses that did not increase locomotor activity. Although l-threo-dihydroxyphenylserine (DOPS), an artificial norepinephrine (NE) precursor, did not change immobility in intact mice, DOPS significantly reduced immobility in mice pretreated with the selective NE neurotoxin DSP4. These results suggest possible antidepressant properties of DOPA and DOPS, the latter of which may act as an antidepressant in a certain NE-depleting condition.