Effects of p-chlorophenylalanine and alpha-methyltryptophan on behaviour and brain 5-hydroxyindoles

Anxiolytic effect of serotonin depletion in the novelty-induced hypophagia test

RATIONALE

Relatively little is known about the neural mechanisms underlying anxiety in the novelty-induced hypophagia test, the only known anxiety test that is responsive to chronic but not acute or subchronic antidepressant treatment.

OBJECTIVES

The goal of the present experiment was to characterize the role of serotonin in the ability of novelty to suppress feeding.

MATERIALS AND METHODS

Pair-housed male Sprague-Dawley rats were trained to eat graham cracker crumbs individually in their home cage (15 min/day). After stable daily intakes were obtained, the animals were depleted of serotonin using 4-chloro-DL -phenylalanine (150 mg kg(-1) day(-1) x 2 days). Forty-eight hours later, central serotonin was restored by the administration of the peripheral L -aromatic amino acid decarboxylase inhibitor, benserazide (10 mg/kg), followed 15 min later with the immediate precursor of serotonin, 5-hydroxy-L -tryptophan (30 mg/kg). Thirty minutes later, the animals were given access to graham cracker crumbs in a novel environment.

RESULTS

The animals demonstrated increased latencies to approach the food and reduced food intake in the novel environment. This effect was attenuated by serotonin depletion. Repletion of central serotonin restored the inhibitory response to novelty. The analysis of serotonin content in different brain regions confirmed that serotonin was depleted by greater than 90%, whereas the repletion treatment resulted in serotonin levels similar to nondepleted animals.

CONCLUSIONS

Acute depletion of serotonin acts to reduce anxiety behavior as measured by an inhibitory anxiety response during exposure to novel stimuli. These findings are in agreement with the proposed general role for serotonin in behavioral inhibition and that reductions of serotonin facilitate the adoption of more active coping responses to stress.

Effect of a 5-HT(1A) receptor agonist, flesinoxan, on the extracellular noradrenaline level in the hippocampus and on the locomotor activity of rats

We have studied effects of 5-hydroxytryptamine 1A (5-HT(1A)) receptor-selective compounds on the extracellular noradrenaline (NA) level in the hippocampus of rats using microdialysis and on their locomotor activity. A selective 5-HT(1A) receptor agonist, flesinoxan (5 mg/kg, i.p.) increased the extracellular NA level in the hippocampus, and increased the locomotor activity. Both responses were blocked by pretreatment with a 5-HT(1A) receptor antagonist, WAY100635 (1 mg/kg, i.p.) and an alpha(2) adrenoceptor agonist, clonidine (50 microg/kg, i.p.). Bilateral intrahippocampal injection of flesinoxan (200 nmol in 2 microl, respectively) increased the locomotor activity of rats and the intrahippocampal perfusion of flesinoxan (1 mM, 2 microl/min) increased the extracellular NA level in the hippocampus. Bilateral intrahippocampal injections of a small amount of WAY100635 (0.1 nmol in 2 microl, respectively) blocked the flesinoxan (5 mg/kg, i.p.)-induced hyperactivity. Flesinoxan (5 mg/kg, i.p.) did not significantly influence the level of serotonin or its major metabolite in the hippocampus, or dopamine or its metabolites in the striatum. In conclusion, these behavioral as well as pharmacological results indicate that postsynaptic 5-HT(1A) receptor activation by flesinoxan increase the extracellular NA level in the hippocampus, which may be the cause of the increase of the locomotor activity.

I. Serotonin (5-HT) within dopamine reward circuits signals open-field behavior. II. Basis for 5-HT--DA interaction in cocaine dysfunctional behavior

Light microscopic immunocytochemical studies, using a sensitive silver intensification procedure, show that dopamine (DA) and serotonin (5-HT) axons terminate on neurons in the nucleus accumbens (NAcc) (A10) terminals and also in dorsal striatum (DSTr) (A9) terminals. The data demonstrate a prominent endogenous anatomic interaction at these distal presynaptic sites between the neurotransmitters 5-HT and DA; the pattern of the 5-HT-DA interaction differs between A10 and A9 terminals. Moreover, in distinction to the variance shown anatomically between 5-HT--DA interactions at distal A9 and A10 sites, the 5-HT--DA interactions at the level of DA somatodendrites, the proximal site, are similar, i.e. 5-HT terminals in the midbrain tegmentum are profuse and have a massive overlap with DA neurons in both ventral tegmental area (VTA) and substantia nigra pars compacta (SNpc). We suggest with reference to the DA neurons of A10 and A9 pathways, inclusive of somatodendrites (sites of proximal presynaptic interactions in the midbrain) and axons (sites of distal presynaptic interactions), that 5-HT--DA interactions in A10 terminals are more likely to exceed those in the DStr arrangement. Furthermore, our neuroanatomic data show that axonally released DA at A10 terminals may originate from proximal 5-HT somatodendrites, i.e. dorsal raphe (DR) or the proximal DA somatodendrites, VTA. In vivo microvoltammetric studies were done with highly sensitive temporal and spatial resolution; the studies demonstrate basal (endogenous) real time 5-HT release at distal A10 and distal A9 terminal fields and real time 5-HT release at proximal A10 VTA somatodendrites. In vivo microvoltammetric studies were performed concurrently and on line with studies of DA release, also at distal A10 and distal A9 terminal fields and at proximal A10 somatodendrites. Serotonin release was detected in a separate voltammetric peak from the DA voltammetric peak. The electrochemical signal for 5-HT release was detected within 10-12 s and that for DA release within 12-15 s, after each biogenic amine diffused through the synaptic environment onto the microelectrode surface. The electrochemical signal for 5-HT and a separate electrochemical signal for DA are detected on the same voltammogram within 22-27 s; each electrochemical signal represents current changes in picoamperes, within seconds of detection time. The amplitude of each electrochemical signal reflects the changes in diffusion of each biogenic amine to the microelectrode surface. Each neurotransmitter has a distinct potential at which oxidation occurs; this results in a recording which has a distinct peak for a specific neurotransmitter. The concentration of each neurotransmitter within the synaptic environment is directly related to the electrochemical signal detected via the Cottrell equation. Voltammograms were recorded every 5 min. At the time that basal 5-HT release and basal DA release were recorded within same animal control, open-field behavioral studies were performed, also concurrently, by infrared photocell beams. The frequency of each behavioral parameter was monitored every 100 ms; the number of behavioral events, were summated every 5 min during the time course of study. Thus, the detection of neurotransmitters occurs in real time, while simultaneously monitoring the animal's behavior by infrared photocell beams. The results from the in vivo microvoltammetric and behavioral data from this study show that basal 5-HT release at distal A10 and A9 terminals dramatically increased with DA release. Moreover, each increase in basal 5-HT release, at both A10 and at A9 terminal fields occurred consistently and at the same time as each increase in open-field locomotion and stereotypy occurred naturally during the animal's exploration in a novel chamber. Thus, the terminology 'synchronous and simultaneous' describes aptly the correlation between 5-HT release at distal A10 and A9 terminal fields and open-field locomo

Alpha-methyltryptophan as a therapeutic agent

1. Alpha-methyltryptophan, on administration to experimental animals gives rise to cerebral alpha-methylserotonin, which substitutes for serotonin in certain behavioral and functional tests. 2. Such results are consistent with the similarity of the properties of the two indoleamines with respect to storage, uptake and release, as well as with the 5HT2-receptor agonist activity of alpha-methylserotonin. 3. It is proposed that alpha-methyltryptophan be regarded as a candidate drug for the provision of alpha-methylserotonin as a substitute for serotonin in disorders where the latter amine is thought to be deficient.

Alpha-methylserotonin, a substitute transmitter for serotonergic neurons

1. Administration to rats of alpha-methyltryptophan (AMTP) gives rise to alpha-(AM5HT) in the brain along with a decrease of cerebral 5HT. 2. Analysis of fractions prepared from brains of AMTP-injected rats shows that AM5HT occurs mainly in the synaptosomes. 3. The synaptosomal content of AM5HT in proportion to the total AM5HT in the brain represents the same ratio as for the corresponding fractions of 5HT.

Relation between yawning behavior and central serotonergic neuronal system in rats

Subcutaneously (s.c.) administered apomorphine (0.0125-0.4 mg/kg) or physostigmine (0.025-0.4 mg/kg) to rats elicited yawning. The dose-response curves were bell-shaped. The peak effects of apomorphine and physostigmine were observed with a dose of 0.1 mg/kg of each drug. Yawning elicited by apomorphine (0.1 mg/kg) or physostigmine (0.1 mg/kg) was reduced by intraperitoneally (i.p.) administered 5-hydroxytryptophan (5-HTP, 50-200 mg/kg, given 30 min before). Yawning elicited by apomorphine but not by physostigmine was enhanced by p-chlorophenylalanine (p-CPA, 25-400 mg/kg i.p., given 24 h before). Apomorphine elicited but not physostigmine-elicited yawning was enhanced by pretreatment with 5,7-dihydroxytryptamine (5,7-DHT, 8 micrograms/rat, given 14 days before into the dorsal raphe). This treatment led to a 35% depletion of serotonin (5-HT) in the striatum. 5-HTP, p-CPA or 5,7-DHT given alone did not elicit yawning. Bilateral, intrastriatal microinjection of apomorphine (1.5-50 micrograms/site) but not physostigmine (5-50 micrograms/site) elicited yawning. The dose-response curve was also bell-shaped. These results indicate that central serotonergic pathways play an important role in modulating drug-elicited yawning in rats.

Depletion of brain serotonin by 5,7-dihydroxytryptamine alters the response to amphetamine and the habituation of locomotor activity in rats

Awake Sprague-Dawley rats were depleted of brain serotonin (5HT) by intraventricular injections of 50 micrograms 5,7-dihydroxytryptamine (5,7-DHT) through chronically implanted cannulae. Oral pretreatment with 25 mg/kg desmethylimipramine was used to protect brain noradrenergic neurons from 5,7-DHT. In a separate set of animals, liquid chromatographic assays revealed that this treatment did not significantly alter catecholamine levels but depleted hippocampal 5HT by 80-90% and caudate 5HT by 30-42% as early as 24 h after administration of 5,7-DHT. One or 3 days after lesioning, locomotor and exploratory behavior was characterized with a Behavioral Pattern Monitor (BPM). Relative to controls, lesioned rats exhibited a decreased rate of habituation of both locomotor activity and investigatory holepokes. Although the amount of locomotor activity elicited by amphetamine (1.0 mg/kg) was unchanged by the 5HT depletion, lesioned animals exhibited highly stereotyped patterns of locomotion during the last 30-min test session, in contrast to the relatively random patterns characteristic of control animals given amphetamine. These results show that central serotonergic pathways play an important role in modulating both spontaneous and amphetamine-elicited activity in rats.

An automated method to measure monoamines and metabolites using elevated temperature reversed phase HPLC with electrochemical detection. Application to striatal dopamine and hippocampal serotonin turnover

High pressure liquid chromatography with electrochemical detection has been successfully used for the analysis of noradrenaline, dopamine(DA), serotonin(5-HT), and selected metabolites in brain. Automated sample injection allows up to 100 samples per day to be analyzed; precise thermostatic control of the chromatography at 45 degrees C increases both method reproducibility and separation efficiency while increasing column life. The method requires minimal sample pretreatment and is rapid and inexpensive. It has been applied to the analysis of rat and mouse whole brain and, in particular, to milligram samples of rat striatum and hippocampus, thus permitting the measurement of regional DA and 5-HT turnover. Effects of selected psychotropic drugs on these processes illustrate the value of the method to either DA or 5-HT turnover studies.

Effect of p-chlorophenylalanine and tryptophan on sleep, EEG and motor activity in the rat

The effects of a single injection of parachlorophenylalanine (PCPA, 300 mg/kg i.p.) on sleep, motor activity and consummatory behavior were investigated in unrestrained rats which were continuously recorded with telemetric techniques on two control days and six drug days. Slow wave sleep (SWS) was defined as the non-REM sleep (NREMS) fraction with a low predominant EEG frequency. In the 24 h following PCPA administration, motor activity and food intake were reduced and sleep was increased. SWS was massively enhanced, while REM sleep (REMS) was depressed. The initial phase of sedation was followed by a phase of partial insomnia lasting 1-2 days. SWS and REMS were particularly depressed. A rebound phenomenon was observed at the end of recovery period when some of the SWS and REMS values exceeded the control level. The administration of tryptophan (Trp, 150 mg/kg i.p.) 28 h after PCPA pretreatment, causing a significant rise in the brain serotonin (5-HT) concentration, produced a temporary increase in SWS and REMS, and a reduction of motor activity. The experiments show that the depression of SWS and REMS, and the hyperactivity 1-2 days after PCPA administration, are a consequence of the reduced 5-HT level, whereas the involvement of serotonergic mechanisms in the initial sedative phase and in the recovery phase is less clear. The persistence of the daily distribution of sleep and activity, and of the specific pattern of SWS and REMS, indicates that the circadian sleep organization is little affected by 5-HT depletion.

The effects of 5,7-dihydroxytryptamine lesions of extrapyramidal and mesolimbic sites on spontaneous motor behaviour, and amphetamine-induced stereotypy

5,7-Dihydroxytryptamine lesions of the nucleus accumbens septi, or substantia nigra, resulted in a twofold increase in spontaneous locomotor activity. Striatal 5HT depletion also raised basal activity levels, as well as increasing rearing behaviour in an open field. The sterotyped responses to all doses of amphetamine tested (2.5--10 mg/kg, i.p.) were enhanced by lesions of the nucleus accumbens or substantia nigra. Striatal lesions only affected the response to the lowest dose of amphetamine. Lesions of the tuberculum olfactorium were without effect on spontaneous or amphetamine induced responses. The results support the concept of a modulatory 5HT influence on nigro-striatal function, and suggest that 5HT in the nucleus accumbens has an antagonistic role with respect to dopamine function in this site.

A study of the sites of interaction between dopamine and 5-hydroxytryptamine for the production of fluphenazine-induced catalepsy

The effect of reducing 5-hydroxytryptamine (5-HT) concentration within various areas of the central dopamine (DA) system on catalepsy has been investigated. The neurotoxin 5,7-dihydroxytryptamine was used to selectively deplete 5-HT in the striatum, nucleus accumbens septi, tuberculum olfactorium or substantia nigra. Localised depletion of 5-HT within the nucleus accumbens septi and substantia nigra reduced the cataleptic effects of the neuroleptic agent fluphenazine, while lesions of the striatum or tuberculum olfactorium were without effect. Each injection of neurotoxin resulted in a 38--47% depletion of 5-HT in the target site: DA levels were not significantly altered. The results suggest that varied dopamine/5-hydroxytryptamine interactions within the nucleus accumbens may contribute to the action of the neuroleptic. The reduction of fluphenazine-induced catalepsy produced by 5-HT depletion within the substantia nigra supports the concept of a controlling influence of 5-HT on nigro-striatal DA function.