Effect of ergot drugs on central 5-hydroxytryptamine neurons: evidence for 5-hydroxytryptamine release or 5-hydroxytryptamine receptor stimulation

Systematic Elucidation of the Mechanism of Sappan Lignum in the Treatment of Diabetic Peripheral Neuropathy Based on Network Pharmacology

Background

Diabetic peripheral neuropathy (DPN) is one of the most common chronic complications of diabetes, which seriously affects the physical and mental health of patients. Sappan Lignum (SL) is effective in treating DPN. Previous reports have shown that SL has a clear hypoglycemic and anti-inflammatory effect. However, the study of SL in the treatment of DPN is still limited and rare.

Objective

To investigate the mechanism of SL in the treatment of DPN based on network pharmacology.

Methods

The active ingredients of SL were screened by related databases. The compound targets were collected by the target prediction platforms. The DPN-related targets were gathered through disease databases. The intersection targets were obtained by uploading the compound targets and disease targets to Venny 2.1.0, and a compound-target network was constructed by Cytoscape3.7.2. The protein-protein interaction (PPI) relationships were obtained by the STRING11.0 database. Genome Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses were performed using the DAVID6.8 database. Molecular docking of key compounds and core targets was performed by DockThor.

Results

A total of 29 compounds and 51 intersection targets with potential therapeutic effects on DPN were obtained. The compound-target network construction resulted in four key compounds: protostemonine, 3-deoxysappanchalcone, 7,3′,4′-trihydroxy-3-benzyl-2H-chromene, and o-12′-methylergocornine. PPI network analysis yielded 10 core targets: AKT1, MAPK3, CXCL8, TNF, OPRM1, MTOR, STAT3, MAPK8, SIRT1, and HSP90AA1. KEGG analysis resulted in 82 signaling pathways (P < 0.05), including insulin resistance, HIF-1 signaling pathway, and type II diabetes. The docking results indicated that the main active compounds could stably bind to core targets.

Conclusion

SL had the mechanism of multiple ingredients, multiple targets, and multiple pathways in the treatment of DPN. This study provided a scientific basis for further research on the treatment of DPN with SL and its extracts.

Evidence of the existence of two different intraneuronal pools from which pharmacological agents can release serotonin

The effect of various drugs on the release of [(3)H]-serotonin from synaptosomes of reserpine-treated rats was compared with that obtained with synaptosomes of untreated animals. The increase in [(3)H]-serotonin release induced by d-fenfluramine was virtually abolished by reserpine; the effect of d-norfenfluramine, the main metabolite of fenfluramine, was instead enhanced in synaptosomes of reserpine treated animals. [(3)H]-serotonin release induced by l-isomers of fenfluramine or norfenfluramine was increased or not affected, respectively, after reserpine treatment. The effects of other drugs, known to activate serotonin mechanisms such as metachlorophenylpiperazine and quipazine, like d-norfenfluramine, were increased by the reserpine treatment. The present data show that [(3)H]-serotonin can be released by drugs from two pools with different sensitivity to reserpine. The reserpinized synaptosomes could provide useful information on the mechanisms of action of drugs acting on brain serotonin.

Bromocriptine and dopamine receptor stimulation

The response to different doses of bromocriptine (12.5, 25, 50 and 100 mg) has been established in six patients with Parkinson's disease. Bromocriptine, like levodopa, causes improved mobility in patients with Parkinsonism, emesis, hallucinations, a fall in supine and erect blood pressure, increase of plasma growth hormone and suppression of prolactin concentration. Bromocriptine (50 or 100 mg) has as great an anti-Parkinsonian effect as average therapeutic doses of levodopa, and a longer duration of action, 6-10 hours. In the dose range studied, bromocriptine appears to be a complete dopamine agonist, although 100 mg was less effective than 50 mg in two patients. The different actions of bromocriptine and other dopamine agonist drugs may result from stimulation of different types of dopamine receptor.

Bromocriptine-induced hypothermia in Balb/C mice: its possible mechanism of action

1. Administration of bromocriptine (0.1-2.5 mg/kg, i.p.) to mice produced hypothermia. 2. Pretreatment with the alpha 1-adrenoceptor blocker, prazosin (2.0 mg/kg, i.p.) or the alpha 2-adrenoceptor antagonist yohimbine (2.5 mg/kg, i.p.) had no effect on this response. 3. The inhibitor of catecholamine synthesis, alpha-methyl-p-tyrosine, and the muscarinic antagonist, atropine (10 mg/kg, i.p.) failed to alter the hypothermia. 4. Pretreatment with the dopamine (DA) receptor antagonists haloperidol (0.02 mg/kg, i.p.) or cis-flupenthixol (0.01 mg/kg, i.p.) completely blocked this response while trans-flupenthixol (0.05 mg/kg, i.p.) was inactive. 5. Depletion of 5-HT in the brain by p-chlorophenylalanine reduced the hypothermic response. 6. Similarly, pretreatment with the serotonergic (5-HT) receptor blocker ketanserin (1 mg/kg, i.p.) attenuated the hypothermia and at a dose of 2 mg/kg (i.p.) it completely blocked the hypothermic response. 7. Methysergide (5 mg/kg, i.p.) was also effective in antagonizing the hypothermia. 8. It was concluded that both DA and 5-HT mechanisms are involved in bromocriptine-induced hypothermia in mice.

The pharmacology of Parkinson's disease: basic aspects and recent advances

Basic aspects and recent advances in the understanding of the pharmacological mechanism of action of the clinically most used antiparkinson drugs are reviewed. Recent human and animal biochemical investigations clearly confirm and extend previous findings indicating that benserazide is much more potent than carbidopa as peripheral decarboxylase inhibitor. L-DOPA in combination with benserazide or carbidopa constitutes the best available therapy for Parkinson's disease (PD). To reduce peaks and rapid fluctuations of L-DOPA plasma levels (possibly responsible for peak-dose dyskinesias and end-of-dose deterioration) a slow-release formulation of L-DOPA in combination with benserazide or with benserazide plus catechol-O-methyltransferase inhibitors should be developed. In parkinsonian patients under long-term L-DOPA therapy monoamine oxidase inhibitors type B (MAO-B) e.g. (-)deprenyl and direct dopamine receptor agonists (bromocriptine, lisuride, pergolide etc.), due to their L-DOPA-sparing effects, alleviate in some cases L-DOPA-induced side-effects e.g. dyskinesias and on-off phenomena. However, since (-)deprenyl, due to its metabolism to (-)methamphetamine and (-)amphetamine, seem to have indirect sympathomimetic activity, new selective MAO-B inhibitors devoid of indirect sympathomimetic effects should be tested clinically to assess the functional role of pure MAO-B inhibition in the therapy of PD. The auxiliary therapy with direct dopamine receptor agonists of the D-2 subtype represents another valid approach which should be further investigated in order to find novel dopamine agonists, less expensive than bromocriptine, and strictly selective for D-2 receptor sites.

The ergolene derivative MPME induces in the rat a behavioural syndrome associated with activation of dopamine D-1 receptors belonging to the dotted type of forebrain dopamine nerve terminals

The ergolene derivative MPME (PTR 17402; (5R,8R)-8-(4-p-methoxyphenyl)-1-piperazynylmethyl-6- methylergolene], a dopamine (DA) receptor agonist acting mainly at DA D-1 receptors linked to dotted types of forebrain DA nerve terminals, induces a characteristic behavioural syndrome consisting of increased locomotion, head-bobbing and sniffing activity without oral stereotypies or increased rearing. Haloperidol and cis-flupenthixol, nonselective DA receptor antagonists, but not selective D-2 receptor antagonists such as remoxipride and sulpiride, could significantly counteract the locomotion and head-bobbing behaviour induced by MPME. In contrast, the sniffing behaviour induced by MPME was only marginally affected by haloperidol and cis-flupenthixol pretreatment. These results suggest that activation of D-1 receptors in the forebrain mainly linked to the dotted types of DA nerve terminals in the striatum, and in the limbic forebrain, can result in behavioural effects which differ from those caused by stimulation of D-2 receptors located mainly within the diffuse types of DA nerve terminal systems.

Is receptor activation involved in the mechanism by which (+)-fenfluramine and (+)-norfenfluramine deplete 5-hydroxytryptamine in the rat brain?

1 The effects of (+)-fenfluramine, (+)-norfenfluramine and reserpine on the concentrations of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in brainstem and telencephalon were studied in rats treated with methergoline, a 5-HT antagonist. 2 Methergoline significantly reduced the effect of (+)-norfenfluramine (5 mg/kg) on 5-HT levels in telencephalon and brainstem but did not modify the effect of (+)-norfenfluramine (2.5 mg/kg). 3 Neither the effect of (+)-fenfluramine on 5-HT levels nor the decrease of 5-HT metabolism caused by (+)-fenfluramine and (+)-norfenfluramine was significantly modified by methergoline treatment. 4 Methergoline potentiated the effects of reserpine on brain indoles. The effects was particularly evident on 5-HIAA levels in the brainstem, although significant effects were found on 5-HT in the brainstem and 5-HIAA in the telencephalon depending on the dose reserpine used. 5 The results show that postsynaptic receptor activation may partially contribute to the depletion of brain 5-HT caused by (+)-norfenfluramine in the rat. This mechanism does not seem to play a significant role in the effect of (+)-fenfluramine.

EEG sleep patterns in Parkinsonian patients treated with bromocryptine and L-dopa: a comparative study

The nocturnal sleep patterns of six Parkinsonian patients treated with Bromocryptine (2-Br-L-ergocryptine CB-154), a dopamine-like agonist, were compared with those of the same patients under L-DOPA treatment. No significant differences were found between the two groups. It is suggested that Bromocryptine, acting on dopamine receptors in the sleep regulating systems at the reticular level in the midbrain has the same effect on sleep patterns of Parkinsonian patients as L-DOPA.

Effects of ergot drugs on serotonergic function: behavior and neurochemistry

Several new ergot drugs were tested for behavioral and neurochemical effects related to serotonergic function. Lergotrile and bromocriptine potentiated the so-called "5-HT syndrome", a set of behaviours associated with increased serotonergic neurotransmission consequent to monoamine oxidase inhibition and tryptophan loading. Metergoline antagonized this behavior. In studies of receptor binding using 3H-5-HT or 3H-LSD, metergoline was the most potent at displacing specific ligand binding. Since the ergots also affect dopaminergic function, these results are discussed for their information on both dopaminergic and serotonergic actions of these drugs and their implications for clinical use of ergots.

Effects of morphine, physostigmine and raphe nuclei stimulation on 5-hydroxytryptamine release from the cerebral cortex of the cat

1. The release of 5-hydroxytryptamine (5-HT) from the cerebral cortex and caudate nucleus of brainstem-transected cats and from the cerebral cortex of rats anaesthetized with urethane was determined by radioenzymatic and biological assay. 2. The stimulation of nucleus linearis intermedius of raphe doubles the basal 5-HT release in the caudate nucleus and increases it 3 fold in the cerebral cortex. The effects of the electrical stimulation of the raphe are potentiated by chlorimipramine. 3. Brain 5-HT release is greatly increased by morphine hydrochloride (6 mg/kg i.v.) and by physostigmine (100 microgram/kg i.v.), but not by DL-DOPA (50 mg/kg i.v.). 4. It is suggested that the 5-HT releasing action of physostigmine can contribute to some of its pharmacological effects such as the analgesic effect so far attributed exclusively to its indirect cholinomimetic activity. 5. The 5-HT releasing action of physostigmine seems unrelated to its anticholinesterase activity.

Long-term treatment of Parkinson's disease with bromocriptine

BRomocriptine (15-75 mg per day) alone or with L-dopa was studied during five to 29 months on 44 patients with Parkinson's disease. Used as sole therapeutic agent, it was found excellent in 12 patients who had never received regular L-dopa treatment either because it was never attempted or because of intolerance from the outset. Its anti-Parkinsonism activity was comparable with L-dopa. The gain was stable in the long term until this report. The side effects of L-dopa were not seen after bromocriptine. In cases where L-dopa had ceased to be active, bromocriptine produced a further improvement if mental deterioration was not associated. In very advanced forms of Parkinson's disease with associated dementia, bromocriptine did not produce durable results. Bromocriptine did not improve the "on-off" effects but reduced a number of the side effects of L-dopa, in particular cardiac, painful contractions, and dyskinesia without "on-off" effects.

Brain dopamine receptor stimulation and the relief of Parkinsonism: relationship between bromocriptine and levodopa

The relationship between dopamine receptor stimulation by bromocriptine or levodopa and the relief of parkinsonism was studied in 24 patients with Parkinson disease. Bromocriptine, 30 mg daily for 20 weeks, elicited an improvement in the parkinsonian clinical features, but this was less than the subsequent improvement with levodopa and benserazide, 800 mg and 200 mg daily, respectively. There was a negative correlation between the pretreatment severity of the disease or changes in cerebrospinal fluid homovanillic acid (HVA) and improvement in parkinsonian disability during bromocriptine treatment. Futhermore, it was found that clinical improvement and HVA responses in the cerebrospinal fluid after dopamine receptor stimulation by bromocriptine may predict the clinical response to levodopa.

Inhibition of reserpine-induced PGO waves in the cat by ergot derivatives

The number of reserpine-induced PGO waves in the cat is decreased by administration of ergot derivatives. The inhibition is dose-dependent and the various ergot derivatives show differing potencies. The action of the ergot derivatives may result from stimulation of central serotonin receptors. In addition, possible involvement of dopaminergic systems is discussed.

Dopamine receptors and ergot drugs. Evidence that an ergolene derivative is a differential agonist at subcortical limbic dopamine receptors

The actions of a new type of ergolene derivative, MPME ((5R, 8R)-8-(4-p-methoxyphenyl-1-piperazinyl methyl)-6-methylergolene), have been evaluated on central catecholamine (CA) neurons in the rat by means of a combined biochemical, histochemical and behavioural analysis. The evidence suggests that this ergolene derivative is a preferential agonist at subcortical limbic dopamine (DA) receptors and at DA receptors belonging to the neostriatal DA islands. (1) MPME does not change the DA and noradrenaline (NA) levels 4 h after the injection in doses ranging from 0.1 to 5 mg/kg. MPME significantly reduced DA turnover in doses from 0.5 to 5 mg/kg, whereas the NA turnover was increased in the same dose range. (2) Histochemically, using quantitative microfluorometry, DA levels were unchanged, except in the islands of the nucleus caudatus following administration of MPME. The drug selectively reduced DA turnover in the subcortical limbic regions (tuberculum olfactorium and nucleus accumbens) and in the DA terminal islands of the nucleus caudatus in doses of 1-5 mg/kg, whereas the large diffuse DA terminal systems of the nucleus caudatus were unaffected. Using this ergolene derivative the islandic small neostriatal DA system can be excellently demonstrated also in the adult rat. The effects of MPME on DA turnover, are blocked by haloperiodol but not by methergoline (which blocks 5-HT receptors). (3) Studies on uptake of tritiated DA in the nucleus caudatus and tuberculum olfactorium reveal a weak inhibition of DA uptake and retention only in high concentrations (10(-5)-10(-6) M). Such actions therefore can probably not explain the changes in DA turnover observed. (4) Behavioural effects of MPME were evaluated in the rotometer model of Ungerstedt25. This model will reveal actions on supersensitive striatal DA receptors. MPME was found to mimic the action of apomorphine and cause a prolonged rotational behaviour towards the nonoperated side in doses of 0.25-0.5 mg/kg. A marked potentiation of the action of MPME was obtained by means of pretreatment with phosphodiesterase inhibitors suggesting that the effect of MPME might be mediated by cyclic AMP. Studies with the DA receptor blocking agent pimozide indicated a high affinity of MPME for the supersensitive striatal DA receptors, since only very high doses of pimozide (15 mg/kg) were capable of blocking the actions of MPME. (5) Studies on the effect of MPME on DA sensitive adenylate cyclase in the nucleus caudatus and the subcortical limbic system (mainly tuberculum olfactorium and nucleus accumbens) suggested that MPME is a partial DA receptor agonist with different intrinsic activity on the DA receptors of the subcortical limbic system and of the nucleus caudatus, the effects in the subcortical limbic system being considerably larger than in the nucleus caudatus. Thus, the present paper gives evidence that the various DA receptor populations in the brain are sufficiently different to allow their preferential activation by drugs.

Blockade of photically induced epilepsy by 'dopamine agonist' ergot alkaloids

The effect of the intravenous administration of ergot alkaloids on epileptic responses to intermittent photic stimulation )IPS) has been studied in adolescent baboons, Papio papio, from Senegal. Ergocornine, 1--2 mg/kg, produced marked autonomic and behavioural effects, slowed the EEG, and abolished myoclonic responses to IPS for 30--90 min. Ergometrine, 1 mg/kg, activated the EEG and blocked the induction of myoclonic responses for 1--3 h. Bromocriptine, 0.5--4 mg/kg, did not consistently prevent myoclonic responses to IPS. After pretreatment with a subconvulsant dose of allylglycine (180--200 mg/kg), lysergic acid diethylamide, 0.1 mg/kg, retained the capacity to block myoclonic responses to IPS, and ergocornine 1 mg/kg reduced such responses. The convulsant effect of allylglycine was enhanced, however, so that prolonged seizure sequences began 19--96 min after ergocornine administration. The protective action of ergot alkaloids against epileptic responses induced by sensory stimulation is interpreted in terms of effects at several sites, including dopaminergic and serotoninergic synapses.

Myoclonic attacks induced by L-dopa and bromocryptin in Parkinson patients: a sleep EEG study

Six patients with Parkinson's disease developed nocturnal myoclonic attacks after prolongued treatment with L-Dopa which were electroencephalographically recorded. These symptoms persisted after treatment with 2 bromo-alpha-ergocryptin (Bromocryptin), a dopamine receptor agonist, which was substituted for L-Dopa. Bromocryptin is known to have no pre- or postsynaptic effect on serotonin metabolism. It is proposed that these myoclonic phenomena are the expression of the hypersensitivity of denervated catecholamine receptors in the brainstem to the stimulation of L-Dopa and Bromocryptin. This thesis differs with previous suggestions that serotonin plays a major role in the genesis of myoclonic seizures in Parkinsonian patients treated with L-Dopa.

Bromocriptine in Parkinsonism: long-term treatment, dose response, and comparison with levodopa

Thirty-seven patients with Parkinsonism were treated with bromocriptine 2.5-300 mg daily. Bromocriptine, alone or combined with levodopa, caused a 20-30% reduction in disability scores in 11 patients treated for one year. Tolerance did not develop during this period. Bromocriptine treatment was not of value in six patients who had previously not responded or who had lost their response to levodopa. However, in four of five patients with response swings on levodopa due to rapid changes in plasma dopa levels, the addition of bromocriptine caused a more stable response. Dose response curves to bromocriptine 12.5, 25, 50, and 100 mg and to levodopa 250, 500, 1000, and 2000 mg were studied in seven patients. Levodopa 2 g had a greater therapeutic effect and caused a greater rise in plasma growth hormone concentration than bromocriptine 100 mg. Levodopa caused emesis more commonly and hallucinations less commonly than bromocriptine. Bromocriptine appears to be a less potent stimulant than dopamine, and has both pre- and post-synaptic effects. Metoclopramide 60 mg oral was given 30 minutes before bromocriptine or levodopa to establish whether this caused dopamine-receptor blockade. Metoclopramide acted as a competitive antagonist to the anti-Parkinsonism and growth hormone effect of both drugs and in individual cases prevented emesis and hallucinations. The fall in blood pressure due to bromocriptine or levodopa was not antagonised by metoclopramide. Central and peripheral vascular dopamine receptors may be different in nature.

A comparison between amantadine and bromocriptine using the stereotyped behaviour response test (SBR) in the rat

Amantadine (100 mg/kg), apomorphine (2.5-10 mg/kg) and bromocriptine (10-50 mg/kg) all produced stereotyped behaviour in the rat. Apomorphine was rapid in onset and of short duration, amantadine was slower to reach a maximum and bromocriptine had a delayed onset of 50 min and a prolonged action. Amantadine and bromocriptine were antagonised by pimozide (1 mg/kg for 30 min) suggesting an action on dopamine receptors, and by D,L-alpha-methyl-p-tyrosine (150 mg/kg for 3 h) suggesting an indirect action. Amantadine, though not bromocriptine, antagonised apomorphine and amantadine also reversed the SBR due to bromocriptine. Pretreatment of rats with p-chlorophenylalanine (100 mg/kg twice daily for 2 days) had no effect on bromocriptine. The significance of these results is discussed with reference to the proposed mechanism of action of bromocriptine and to the use of multiple drug therapy in Parkinsonism.

Correlation of behavioural inhibition or excitation produced by bromocriptine with changes in brain catecholamine turnover

The dopamine agonist, bromocriptine, produced either inhibition or stimulation of motor behaviour in rats depending upon the dose and time after administration. Stimulation of motor activity occurred only with high doses after a 1-2 h delay. Both inhibition and stimulation were associated with decreased turnover of dopamine in the brain. Release of noradrenaline in brain and noradrenaline plus adrenaline in adrenal varied with motor activity. It is suggested that low doses of bromocriptine inhibit behaviour by activating an inhibitory presynaptic receptor, resulting in reduced synthesis and release of dopamine, whilst high doses cause behavioural excitation by activating the post-synaptic dopamine receptor.

Ergot alkaloids as dopamine agonists: comparison in two rodent models

A series of ergot alkaloids, together with the DA agonists apomorphine and piribedil, were tested for protective effects against audiogenic seizures in an inbred strain of mice (DBA/2) and for induction of circling behaviour in mice with unilateral destruction of one nigrostriatal DA pathway. The order of potency against audiogenic seizures was apomorphine greater than ergocornine greater than bromocryptine greater than ergometrine greater than LSD greater than methysergide greater than piribedil while that observed in the rotating mouse model was apomorphine greater than ergometrine greater than ergocornine greater than bromocryptine greater than piribedil. LSD caused only weak circling behaviour even when administered in high doses (greater than 1 mg/kg). Methysergide was ineffective. Prior administration of the neuroleptic agent haloperidol blocked the effect of DA agonists and of ergot alkaloids in both animal models. The possible action of ergot alkaloids as DA agonists is discussed.

Inhibition of ovulation in rats by antagonists to serotonin and by a new tricyclic compound

The ovulation inhibiting activity in adult rats of the 5HT-antagonists cyproheptadine, mianserin and methysergide is shown. Furthermore the activity of a newly synthetized Cycloheptathiophenederivative, compound 26-921, which inhibits LH-secretion and consequently ovulation, is described.

Effect of CB 154 (2-bromo-alpha-ergocryptine) on paralysis agitans compared with Madopar in a double-blind, cross-over trial

Twenty patients with paralysis agitans took part in a double-blind, cross-over investigation of CB 154 (2-bromo-alpha-ergocryptine) and Madopar (L-Dopa + benserazid (a peripheral decarboxylase inhibitor), dose ratio 4:1). Each treatment phase lasted for 8 weeks. Modapar was found to be significantly superior to CB 154 in the treatment of the Parkinson state as a whole (Webster total score) and the individual symptoms of hypokinesia, rigidity and tremor. Compared with pretreatment score, CB 154 had a weak, but significant effect on tremor, but not on the Webster total score, hypokinesia and rigidity. The effect of CB 154, however, varied: four patients preferred CB 154 to Madopar on account of its satisfactory therapeutic effect and fewer side-effects ("on-off" phenomena, hyperkinesia, psychiatric complications); other patients showed neither therapeutic effect nor side-effects of CB 154, which in some cases may be related to too low a dose-level of CB 154 (median 30 mg daily, range 20-60 mg). In the four cases first mentioned which showed a good effect of CB 154, the ratio between the dose of CB 154 and the dose of L-Dopa (in Madopar) was 3.5-10 mg/100 mg, i.e. in certain cases it must be assumed that the maximum dose of CB 154 lies around 120 mg daily.

The effect of mepiprazole on central monoamine neurons. Evidence for increased 5-hydroxytryptamine and dopamine receptor activity

In combined biochemical, histochemical and functional studies on central monoamine neurons it has been shown that a pyrozolyl derivative with a phenyl piperazine side chain (PAP) exerts marked effects on central dopamine (DA) and particularly 5-hydroxytryptamine (5-HT) neurons. The brain 5-HT turnover was reduced with doses down to 0.25 mg/kg, and spontaneous overflow of radioactivity from 3H-5-HT-labelled cortical slices was markedly increased by PAP in a concentration of 10(-6) M. PAP may therefore cause extragranular release of 5-HT stores, since the 5-HT levels were not affected. In agreement with this view, sexual behaviour in the female rat, which is controlled by an inhibitory 5-HT pathway, was inhibited by low doses (0.1-0.5 mg/kg) of PAP. The extensor hindlimb reflex, which is dependent on 5-HT receptor activity, was only increased with higher doses (2.5-10 mg/kg), suggesting that the spinal 5-HT nerve terminals are less sensitive to the releasing action of PAP. A certain direct activation of spinal 5-HT receptors may also be involved, since the actions of PAP in the spinal cord were independent of presynaptic 5-HT stores. The actions of PAP on the DA neurons mainly involve a presynaptic action in the DA nerve terminals leading to increased DA receptor activity. This action may primarily involve a blockade of DA uptake (50% inhibition at 10(-6) M) and/or an extragranular release of DA (two-fold increase in spontaneous overflow at 10(-6) M). The DA turnover was not clearly affected, although a trend to a reduction was observed especially in the nuc, accumbens, probably as a result of a compensatory nervous feedback reducing nervous impulse flow. In agreement with the view mentioned above, PAP mimics amphetamine and not apomorphine in the rotometer model which reveals changes in DA receptor activity. PAP in doses of 0.5-1 mg/kg causes a turning towards the denervated side. The brain noradrenaline (NA) turnover is only significantly increased with somewhat higher doses (5-10 mg/kg) and may be related to NA receptor blockade, since the L-DOPA-induced increase in flexor activity is blocked by PAP in doses down to 0.5 mg/kg. It is suggested that the extragranular release of 5-HT caused by PAP is partly responsible for the inhibition of conditioned avoidance behaviour and the reduction of threatening behaviour found after PAP in low doses (0.05-0.5 mg/kg). In the clinic, PAP may prove to be a new therapeutic tool in the treatment of depressions due to 5-HT deficiency. Its actions on DA terminals may also prove helpful in this respect. When combined with L-DOPA, PAP may also help to alleviate the motor deficits in parkinsonian patients with a moderate degree of degeneration of the DA system in view of its action on DA uptake and/or release.