Influence of apomorphine on brain serotonin turnover rate

Brain structures involved in the behavioral stimulant effect of central serotonin release

Drugs such as p-chloroamphetamine or a combination of tranylcypromine and tryptophan release serotonin in the central nervous system and produce a behavioral serotonin syndrome. However, in the presence of methysergide or following destruction of descending spinal serotonergic projections by 5,7-dihydroxytryptamine, central serotonin release produces hyperlocomotion. This supports the hypothesis that release of serotonin in the brain promotes locomotion but that the expression of this effect can be blocked by concomitant intraspinal effects of serotonin release. Hyperlocomotion induced by serotonin release is attenuated or blocked by: (a) pretreatment with p-chlorophenylalanine; (b) acute surgical lesions of the basal diencephalon; (c) chronic lesions of the ventromedial midbrain tegmentum by local injection of 5,7-dihydroxytryptamine; and (d) acute surgical decortication. Medial decortication tends to be more effective then lateral decortication. Hyperlocomotion produced by methamphetamine is also attenuated or blocked by acute basal diencephalic lesions or decortication. It is suggested that ascending serotonergic and dopaminergic projections collaborate in the generation of spontaneous voluntary motor activity.

Striatal depth EEG reveals postsynaptic activity of striatal neurons following dopamine receptor stimulation and blockade

This paper demonstrates a technique for measuring depth electroencephalographic (EEG) recordings from the freely moving mouse. This technique minimizes electrical artifact associated with gross movements by amplifying the current of the EEG signal directly at permanently indwelling electrodes. Stable EEG signals, with high signal-to-noise ratios, can be obtained from these animals while their movement inside the testing cage remains relatively unrestricted. We used this technique to examine the effects of dopamine (DA) receptor agonist and antagonist treatments on depth EEG signals generated within the striatum. Baseline measures of spontaneous striatal EEG activity were obtained prior to drug administration and post-drug measures of striatal activity were subsequently obtained. Apomorphine treatment resulted in desynchronization of striatal EEG signals while haloperidol or sulpiride treatment induced slow wave synchronization. Fast Fourier analysis of EEG signals revealed that DA agonist and antagonist treatment altered spontaneous striatal EEG activity in an opposite manner--relative to baseline signals, apomorphine attenuated low frequency components and augmented higher frequency components of the signal while haloperidol augmented low frequency components and attenuated higher frequency components of the signal. Moreover, mice pretreated with unilateral intracerebral injections of sulpiride and subsequently administered systemic apomorphine simultaneously demonstrated EEG synchronization on the side ipsilateral to the injection of sulpiride and EEG desynchronization on the contralateral side. The population of neurons examined in the medial striatum appear to have the properties of being excitatory to DA agonist stimulation and show decreased activity following DA antagonist treatment. These results suggest that striatal EEG activity may be used as measure of postsynaptic activity of dopaminergic neurons.

Determination of levels of biogenic amines and their metabolites and both forms of monoamine oxidase in prefrontal cortex of aged rats

Levels of dopamine, noradrenaline and serotonin and their metabolites and the activities of monoamine oxidase A and B, have been determined in the prefrontal cortex of the rat during ageing. Serotonin turnover rate has been measured as 5-hydroxytryptophan accumulation rate after central decarboxylase inhibition. The major changes were an increase of dopamine and noradrenaline levels and a decrease of serotonin in aged animals compared with control animals. A decrease of the MAO-A to MAO-B ratio was found in aged rats.

L-tryptophan in neuroleptic-induced tardive dyskinesia

Administration of the serotonin precursor L-tryptophan in a patient with neuroleptic-induced tardive dyskinesia, produced a dramatic reduction in the severity of the abnormal movements within 24 hours. This report supports our hypothesis that alterations in the function of serotoninergic neurotransmission are implicated in the pathophysiology of neuroleptic-induced tardive dyskinesia.

Apomorphine in the evaluation of dopaminergic function in man

1. Apomorphine (Apo), a short acting dopamine (DA) receptor agonist, stimulates growth hormone (GH) secretion, decreases prolactin secretion, induces yawning, penile erections and other physiological effects in man. An effect on behavior, movement disorders and alcoholism has also been described. 2. Apo-mediated responses are used to evaluate DA function in psychiatric and neurological disorders. Many of the studies in schizophrenia using the GH response to Apo as an index of central DA function are difficult to interpret because of failure to control for key variables. 3. The GH response to Apo is a useful system to evaluate the effects of various drugs including peptides which may not cross the blood brain barrier on DA function in man. 4. Apo is a potent sedative. Specific antimanic, antischizophrenic, and anticraving effects in alcoholics have not been convincingly demonstrated. Side effects of Apo and failure to use active placebo make double-blind studies difficult. 5. Apo improves parkinsonian symptoms and certain forms of reflex epilepsy but beneficial effects in other involuntary movement disorders requires further documentation. 6. Apo may be a useful agent to evaluate DA function in impotent patients and predict a therapeutic response to long-acting dopaminergic agents. 7. Impairment of DA function may play a role in diabetic impotence. 8. The development of a simple polygraphic method to monitor the yawning response to Apo may facilitate clinical studies on the basic physiology of yawning in man and the use of the yawning response as a measure of central DA function in schizophrenia and other clinical disorders. 9. The use of Apo with 18F-fluorodeoxyglucose positron emission tomography to examine regional DA function in man opens up a promising area of research. 10. Though long-acting orally active aporphine DA agonists and antagonists have been developed the problem of tolerance may limit their therapeutic potential.

Gabaergic interneurons in the dorsal raphe mediate the effects of apomorphine on serotonergic system

Apomorphine (APO) has been shown to elevate the concentrations of serotonin (5-HT) and its major metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the mesostriatal but not the mesolimbic serotonergic systems. We have previously demonstrated that the serotonergic actions of APO were secondary to dopamine (DA) autoreceptor stimulation in the substantia nigra. Using picrotoxin as a pharmacological tool, we have presently found that these effects of APO were also indirectly mediated through gamma-aminobutyric acid (GABA) neurons. In examination of the exact anatomical locus of GABA neurons responsible for the observed effects of APO, the results indicate that bilateral lateral habenular lesions did not block the effects of APO on 5-HT neurons, while direct picrotoxin infusion to the dorsal raphe, at a dose having no significant influence by itself, antagonized APO's actions. Together with the anatomical, biochemical and histofluorescent findings, it is suggested that APO influences dorsal raphe 5-HT by stimulation of DA autoreceptors in the substantia nigra; therefore, inhibition of DA neuron activity and the nigro-raphe pathway. Normally, DA probably exerts an excitatory influence on gabaergic interneurons in the dorsal raphe, and these inhibitory interneurons then synapse on 5-HT neurons in the same area. Activation of 5-HT neurons were explained by a disinhibitory effect as a result of reduced release of GABA due to feedback inhibition of DA neuron firing following APO activation of DA autoreceptors in the substantia nigra. The striatal presynaptic and postsynaptic DA receptors, however, do not appear to mediate the above effects of APO.

Additive effects of apomorphine and clonidine on serotonin neurons in the dorsal raphe

Effects of apomorphine (APO) and clonidine (CLON) on the mesostriatal and mesolimbic serotonergic systems were examined in the present study. Both drugs selectively elevated serotonin (5-HT) concentrations in the dorsal raphe and the striatum without significantly altering 5-HT measures in the median raphe and the hippocampus. Apomorphine also increased tryptophan and 5-hydroxyindoleacetic acid (5-HIAA) levels in the dorsal raphe and 5-HIAA level in the striatum. Clonidine did not markedly alter tryptophan and 5-HIAA measures, while it decreased 5-HT turnover rate in both region, as indicated by the ratio of 5-HIAA/5-HT levels. Co-administration of APO and CLON, at doses of each drug exerted maximum effects on 5-HT alone, produced an additive effect on 5-HT in the dorsal raphe, while their effects on 5-HT and 5-HIAA in the striatum were counteracting each other. Effects of APO on 5-HT and 5-HIAA were attributed to the elevation of 5-HT precursor tryptophan, while effects of CLON on 5-HT and 5-HIAA were due to a decreased rate of 5-HT turnover. Therefore, the present results support the hypothesis that the additive effects of APO and CLON on dorsal raphe 5-HT are mediated through different receptors and neuropharmacological mechanisms.

Comparison of motor depressant effects of caerulein and N-propylnorapomorphine in mice

The motor depressant effects of caerulein and N-propylnorapomorphine (NPA) were compared in male mice. Caerulein (1-50 micrograms/kg SC) in a dose dependent manner depressed the exploratory activity, whereas NPA in lower doses (0.5-10 micrograms/kg SC) decreased the motor activity, but in higher doses (over 50 micrograms/kg) had stimulating effect on the exploratory behavior. In mice selected according to their motor response after administration of 100 micrograms/kg NPA to weak and strong responders, the low dose of NPA (1 microgram/kg) similarly suppressed motor activity in both selected groups, while the effect of caerulein (2 micrograms/kg) was apparently higher in weak responders. Destruction of catecholaminergic terminals by 6-hydroxydopamine (60 micrograms ICV) reversed completely the motor depressant effect of NPA, whereas degeneration of serotoninergic terminals (5,7-dihydroxytryptamine 60 micrograms ICV or p-chloroamphetamine 2 X 15 mg/kg IP) enhanced the sedative effect of NPA. The motor depressant effect of caerulein remained unchanged after lesions of monoaminergic terminals in forebrain. Subchronic haloperidol (0.25 mg/kg IP, twice daily during 14 days) treatment, reducing significantly the density of high-affinity dopamine2- and serotonin2-receptors, decreased the motor depressant action of caerulein. It is possible that motor depressant effect of caerulein, differently from the action of NPA, is mediated through the high-affinity dopamine2-receptors and in lesser extent through the high-affinity serotonin2-receptors.

Joint cholinergic-serotonergic control of neocortical and hippocampal electrical activity in relation to behavior: effects of scopolamine, ditran, trifluoperazine and amphetamine

Previous research has indicated that low voltage fast activity (LVFA) in the neocortex and rhythmical slow activity (RSA) in the hippocampus can result from activity in either (or both) the cholinergic corticipetal projections from the basal forebrain and the serotonergic corticipetal projections from the brainstem raphe. These inputs appear to give rise, respectively, to atropine-sensitive LVFA and RSA and atropine-resistant LVFA and RSA. The atropine-sensitive and atropine-resistant waveforms have been shown to have distinctive behavioral correlates. The present experiments extend these findings by providing dose-response data on the effects of scopolamine and Ditran on neocortical activity in relation to behavior in the rat. In addition, new evidence is presented which indicates that neuroleptic drugs reduce activity in the atropine-resistant (presumably serotonergic) inputs to the hippocampus and neocortex by an indirect action involving dopamine receptors. A single dose of d-amphetamine or apomorphine appears to increase activity in the same pathway by a similar indirect action. These findings may be relevant to the psychiatric effects of neuroleptic drugs.

Selective effects of apomorphine on dorsal raphe neurons: a cytofluorimetric study

Using a quantitative cytofluorimetric method to detect changes in the intracellular levels of serotonin (5-HT) in individual neurons in rat brain, we have found that the dopaminergic agonist apomorphine increases 5-HT content of dorsal raphe cell bodies without affecting cells in the median raphe nucleus. Liquid chromatographic studies revealed that apomorphine also elevated the concentrations of both 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in striatum, the projection site for dorsal raphe neurons. Conversely, the dopaminergic antagonist haloperidol, at a dose of 0.8 mg/kg, decreased 5-HT levels in dorsal raphe cells. A lower dose of haloperidol (0.4 mg/kg), which had no significant effect alone, completely blocked the effect of apomorphine in the dorsal raphe. These results support the hypothesis that the effects of apomorphine on serotonergic neurons are secondary to dopamine receptor stimulation.

The effects of quipazine, fenfluramine and apomorphine on the morphine potentiation of tonic immobility

Acute administration of morphine (2 mg/kg, IM) enhanced tonic immobility (TI) durations in three-week old chickens. This effect could be reversed with the 5-HT receptor agonist quipazine. Similarly, promoting 5-HT release by fenfluramine antagonized the morphine potentiation of the response. Both 5-HT agonists reduced TI durations. Finally, the DA receptor agonist apomorphine produced decrements in TI duration and blocked the effect of morphine. The results suggest the involvement of serotonergic and dopaminergic mechanism in the morphine potentiation of the response. The findings are also discussed in terms of a revised serotonergic model of tonic immobility.

Evidence of the modulatory role of serotonin in acetylcholine release from striatal interneurons

The release of acetylcholine was studied in isolated striatal slices of the rat. The spontaneous and ouabain-stimulated release of acetylcholine was higher in those slices where serotonergic input was somehow impaired: raphe nuclei lesion or p-chlorophenylalanine pretreatment or 5, 7-dihydroxytryptamine pretreatment resulted in a higher release. L-(m-chlorophenyl)-piperazine, a pure serotonin receptor stimulant and D-fenfluramine, a serotonin releaser significantly reduced the release of acetylcholine evoked by ouabain. Serotonin antagonists (cyproheptadine, mianserine and methysergide) prevented the effect of serotonin agonists. When the serotonergic neurons were destroyed either by p-chlorophenylalanine or by 5, 7-dihydroxytryptamine pretreatment D-fenfluramine had no inhibitory action; however, the effect of L-(m-chlorophenyl)-piperazine was not affected. It is suggested that there is a link between serotonergic and cholinergic neurons in the striatum: serotonin released from raphe-striatal neurons is able to inhibit the release of acetylcholine from striatal interneurons.

Tonic immobility in domestic fowl: possible interaction of serotonergic and dopaminergic mechanisms

Treatment with the dopamine (DA) receptor blocker, haloperidol, enhanced tonic immobility (TI) duration. Fenfluramine, a receptor agonist for serotonin (5-HT), reversed this effect. Tryptophan produced long TI reactions, and is believed to do so due to impaired synaptic transmission of 5-HT following its direct inhibitory effects on 5-HT neurons. DA receptor stimulation by apomorphine prevented the tryptophan potentiation of tonic immobility. The results suggest that serotonergic and dopaminergic systems may interact with respect to tonic immobility.

5,7-Dihydroxytryptamine lesions of the amygdala reduce amphetamine- and apomorphine-induced stereotyped behaviour in the rat

5,7-Dihydroxytryptamine lesions directed towards the central nucleus of the amygdala produced an apparently selective and localised destruction of 5HT terminals within the amygdaloid complex, without damage to dopamine neurons. Such lesions attenuated the biting responses produced by both amphetamine and apomorphine, suggesting that 5HT within this area may play an important role in the mediation of these behavioural effects.

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.

Potentiation of haloperidol-induced catalepsy by dopamine agonists: possible involvement of central 5-hydroxytryptamine

Apomorphine (0.12--2 mg/kg, SC) and d-amphetamine (1--8 mg/kg, IP) were each able, at certain doses, to potentiate the cataleptic state produced by the neuroleptic agent, haloperidol (1 mg/kg, IP). In subsequent biochemical experiments, in which the effects of combinations of apomorphine or d-amphetamine and haloperidol on brain monoamine levels were studied, this behavioural observation was seen to be related to an enhanced utilisation of 5-hydroxytryptamine (5-HT) in certain brain regions. The results suggest not only the possible involvement of 5-HT in the production of catalepsy, but also that the effects of these 'classical' dopamine agonists on other central transmitter systems should be considered when interpreting their various behavioural responses.

Effect of nomifensine on central 5-hydroxytryptamine neurons

Effects of nomifensine (8-amino-1, 2, 3, 4-tetrahydro-2-methyl-4-phenyl-isoquinoline) (NF), an antidepressant drug which inhibits dopamine uptake, on central serotonergic structures were studied in rats. NF affects of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid levels in the whole brain as well as in the separate rat brain structures. The 5-HT turnover is decreased in the whole brain and the striatum but increased in the midbrain and the hippocamp as judged from experiments with tryptophan hydroxylase inhibitor, alpha-propyldopacetamide. Pretreatment with dopamine-receptor blocking agent, spiperone, antagonized the increase of 5-HT turnover rate in brain regions mentioned above. NF stimulates the hind limb flexor reflex in spinal rat, a preparation regarded as a model for evaluation of drug action on central 5-HT neurons. This stimulatory effect was antogonized by cyproheptadine, by reserpine and imipramine. The obtained results indicate that NF activates central 5-HT neurons both directly and indirectly, via stimulation of dopamine receptors as described previously for other dopamine agonists.

Agonist--antagonist interaction on dopamine receptors in brain, as reflected in the rates of tyrosine and tryptophan hydroxylation

The effect of haloperidol and apomorphine, and both drugs in combination, on the first steps in the synthesis of catecholamines and 5-hydroxytryptamine (5-HT) has been studied in three rat brain regions. The rate of formation of dopa and 5-hydroxytryptophan (5-HTP) was studied by measuring the accumulation of these amino acids during 30 min after administration of the inhibitor of the aromatic L-amino acid decarboxylase, NSD 1015 (3-hydroxybenzylhydrazine HCl). Haloperidol caused an increase in dopa and no change in 5-HTP formation. The threshold dose was severalfold higher in the noradrenaline-predominated hemisphere portion than in the dopamine-rich striatal and limbic regions, suggesting a higher affinity of haloperidol for dopamine than for noradrenaline receptors. Apomorphine caused a decrease in dopa formation in all three brain regions studied, although the effect was much more pronounced in the regions dominated by dopamine. The threshold dose was about 30 microng/kg, i.e. an order of magnitude lower than the threshold dose for apparent postsynaptic dopaminergic receptor activation. This discrepancy is suggested to be due to preferential activation of inhibitory dopaminergic autoreceptors by low apomorphine doses. This phenomenon may also contribute to explain the complex dose-response curves of apomorphine. Low doses of apomorphine caused a decrease and high doses an increase in 5-HTP formation. These effects, like those on noradrenaline synthesis, are suggested to be secondary to activation of dopaminergic pre- and post-synaptic receptors. The interaction between apomorphine and haloperidol with respect to dopa formation appears to be largely explicable on the assumption of a competition between an agonist and an antagonist for dopaminergic receptors. However, very large doses of apomorphine cause a haloperidol-resistant inhibition of tyrosine, and probably also tryptophan, hydroxylation, which may be due to a direct inhibition of the aromatic amino acid hydroxylase involved.

Dopamine agonists and interaction with other neurotransmitter systems

Studies on the interaction of two dopamine (DA) agonists (selected as most specific): apomorphine (APO) and 1,3-dimethyl-5-aminoadamantane (DMAA) with serotonin (5-HT) and noradrenaline (NA) brain systems have revealed that the drugs induce the activation of 5-HT and NA neurons in an indirect way (via primary DA stimulation). These interactions are relevant for some pharmacological effects of DA agonists.

The distribution and metabolism of chlorpromazine in rats and the relationship to effects on cerebral monoamine metabolism

Rats were injected with chlorpromazine (CPZ), 21 mumoles/kg (7.5 mg/kg) i.p., and killed after different time intervals up to 24 h. Mass fragmentographic methods were used to determine the levels of CPZ, monodemethyl-chlorpromazine (nor1-CPZ) and 7-hydroxy-chlorpromazine (7-OH-CPZ) in brain and blood and dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxy-3-indoleacetic acid (5-HIAA) in the striatum. Significant correlations between the brain and blood concentrations of CPZ as well as its active metabolites and the levels of DOPAC and HVA were obtained at several time points. The similarity between the time curves for CPZ dominantly the unchanged drug in the brain which determines the acceleration of DA metabolism.

Apomorphine in the rat nucleus accumbens: effects on the synthesis of 5-hydroxytryptamine and noradrenaline, the motor activity and the body temperature

Apomorphine (10 mug) was injected bilaterally into the nucleus accumbens or the neostriatum of rats. The application of apomorphine to the nucleus accumbens, but not to the neostriatum, enhanced the accumulation of 5-hydroxytryptophan in the central nervous system following inhibition of the aromatic L-amino acid decarboxylase. The effect was greatest in the brain stem particularly in the pons plus medulla oblongata. Similar but smaller rises were observed for the accumulation of DOPA in noradrenalinerich regions. The motor activity was increased and the body temperature was decreased by apomorphine in the nucleus accumbens, whereas smaller or no effects were obtained from the neostriatum.