Amine formation from L-tryptophan in brain slices

Effect of different photoperiods on the diurnal rhythm of 5-methoxytryptamine in the pineal gland of golden hamsters (Mesocricetus auratus)

This study tested the photo-dependency of the rhythmic synthesis of 5-methoxytryptamine (5-MT) in the pineal gland of golden hamsters. After pargyline administration, pineal 5-methoxytryptamine and melatonin were measured by HPLC in male golden hamsters kept under short and long photoperiod. In both photoperiodic regimes, a clear 5-MT rhythm was observed which fitted a sinusoidal function with high values occurring during the daytime and low values occurring during the night-time. The duration of the low night-time levels was clearly proportional to the length of the dark phase. A marked rhythm of melatonin synthesis was also seen with low daytime levels and high night-time values. An inverse relationship between 5-MT and melatonin levels was observed. Thus, after pargyline administration, the rhythms of 5-MT and melatonin in the pineal gland of golden hamsters are photoperiod-dependent and show a reciprocal relationship.

Effects of L-tryptophan and other amino acids on electroencephalographic sleep in the rat

Electroencephalographic sleep was quantitated in adult male Sprague-Dawley rats following single injections of the methylesters of tryptophan, valine or alanine. The amino acids were administered at the onset of the daily light period (09.00 h); electrographic data were collected for the succeeding 6-h period. Saline served as the injection control, and fluoxetine, a serotonin-reuptake blocker, as a positive control. The injection of tryptophan methylester (125 mg/kg) caused a delay in rapid eye movement (REM) sleep onset, and significantly reduced the amount of REM sleep during the first 2 h postinjection. Tryptophan produced no effect on sleep onset, nor did it influence total sleep time. Fluoxetine (2.5 mg/kg) produced similar effects, as previously observed. The methylesters of valine and alanine were without effect on REM sleep, when injected at a molar dose equivalent to that for tryptophan. No consistent effects of any of the test substances were noted on non-REM (NREM) sleep or waking time, or on any of the other sleep indices quantitated. Together, the data indicate that tryptophan selectively reduces REM sleep; the effect is not due to a non-specific action of amino acids or their methylesters. The effect on REM sleep may be the consequence of a tryptophan-induced stimulation of 5-HT synthesis and release, since it is like that produced by fluoxetine, a drug that enhances transmission across serotonin synapses.

Analysis of tryptamine at the femtomole level in tissue using negative ion chemical ionization gas chromatography-mass spectrometry

An ultra sensitive method for the detection of tryptamine, an endogenous amine in mammalian neuronal systems, at the femtomole level has been developed using negative chemical ionization gas chromatography-mass spectrometry (NCI-GC-MS). The amine is converted into a perfluorinated spirocyclic derivative, e.g. 1-pentafluoro-2-methylenepyrrolidine-3-spiro-3'-(3H-indole) which is detected using selected-ion monitoring of the (M-2HF) ions of the endogenous and deuterated internal standard compounds. Two mass spectrometers were compared; they gave minimum detectable quantities from tissue samples of 40 pg (VG-7070F) and 0.9 pg (VG-70S) respectively. These detection levels are approximately 5-200 times lower than have been obtained by previous MS methods.

Determination of tryptamine in rat brain by gas chromatography—mass spectrometry

Tryptamine (TA) occurs in trace levels in the brain, but its role in the central nervous system is not clear. However, there is evidence that TA may be a neuromodulator since it binds to specific binding sites in the brain. TA was measured as a diheptafluorobutyryl derivative in rat whole brain by capillary gas chromatography-mass spectrometry using negative chemical ionization (NCI) and single ion monitoring (SIM). d(4)-TA was used as the internal standard. The ions m/z 532 and m/z 536 were monitored to identify TA and d(4)-TA, respectively and to calculate the concentration of TA in rat whole brain which was found to be 0.19 +/- 0.08 ng g(-1) (n = 8). The results confirm the earlier TA concentrations measured by GC-MS using positive electron impact ionization. However, NCI improved the signal/noise ratio of the method increasing its sensitivity for TA.

Characterization and quantitative autoradiography of [3H]tryptamine binding sites in rat brain

[3H]Tryptamine binds with high affinity (Kd = 9.1 nM, Bmax = 54 fmol/mg wet wt.) to tissue sections of rat brain. The binding occurs rapidly and is reversible. Low concentrations of the beta-carbolines harmaline (IC50 = 25 nM) and tetrahydronorharman (tetrahydro-beta-carboline, IC50 = 50 nM) inhibit [3H]tryptamine binding. Serotonin (5-HT, IC50 = 2600 nM) as well as the 5-HT receptor antagonists methysergide and metergoline displace [3H]tryptamine at much higher concentrations from brain slices. The distribution of [3H]tryptamine binding sites in sections of rat brain has been analyzed by quantitative autoradiography. The highest density of binding sites is found in the nucleus (n.) interpeduncularis, a slightly lower one in the locus coeruleus. Moderately labelled are the n. accumbens septi, n. septi lateralis, n. medialis habenulae, n. tractus olfactorii lateralis, the central region of the amygdala, n. caudatus/putamen, n. reuniens and the hippocampal formation. A low density of binding sites is detected in the cerebral cortex and the subiculum. Even less binding sites are found in the n. dorsalis raphe and the substantia nigra. The pattern of distribution of [3H]tryptamine binding sites differs from that of [3H]5-HT (5-HT1), [3H]ketanserin (5-HT2) as well as [3H]imipramine binding sites. These data suggest unique tryptamine binding sites.

Increased total activity in the rat after L-tryptophan plus the monoamine oxidase-A inhibitor amiflamine but not after L-tryptophan plus clorgyline

The effect of pretreatment with either saline or the monoamine oxidase-A inhibitors clorgyline and amiflamine upon the total activity, locomotion and rearing behaviour of the rat induced by various doses of the monoamine precursor L-tryptophan was studied by use of automated activity boxes. Amiflamine (2.5 and 5.0 mg kg-1, i.p.) increased in a dose-dependent manner total activity and to a lesser extent, locomotion when given 60 min before L-tryptophan (100 mg kg-1, i.p.). The increased activity was seen after amiflamine plus either 25 or 75 mg kg-1 L-tryptophan. Rearing behaviour was not affected. Analysis of 5-hydroxytryptamine (5-HT) and its deaminated metabolite 5-hydroxyindoleacetic acid (5-HIAA) by high performance liquid chromatography with electrochemical detection indicated that in both frontal cortex and hypothalamus, amiflamine (at both doses) increased 5-HT and reduced 5-HIAA concentrations. Combination of amiflamine with L-tryptophan (100 mg kg-1, i.p.) resulted in a higher 5-HT concentration being found than after amiflamine alone. L-Tryptophan treatment alone did not change 5-HT concentrations but increased 5-HIAA concentrations. Clorgyline, at a dose of either 1 or 5 mg kg-1 i.p. plus L-tryptophan (25 or 100 mg kg-1, i.p.) did not increase total activity, locomotion or behaviour. A number of possible explanations for the differences in the behavioural effects of clorgyline and amiflamine when given with L-tryptophan are discussed. It is concluded that in addition to monoamine oxidase-A inhibition, other pharmacological effects of the drugs, such as 5-HT release (amiflamine) and inhibition of tryptophan hydroxylation (clorgyline) may be of importance in determining the magnitude of the increase in activity when the compounds are given together with L-tryptophan.

Measurement of endogenous tryptamine in rat brain by capillary column gas chromatography mass spectrometry

A capillary column gas chromatographic mass spectrometric method, involving the use of a deuterated analogue as internal standard, was developed and used to identify and quantify tryptamine in the rat brain. The mean level of tryptamine in rat whole brain was 0.54 pmol g-1.

Hyperalgesia produced by the intrathecal administration of tryptamine to rats

Tryptamine was applied directly into the spinal subarachnoid space of rats via permanently indwelling cannulas. Changes in pain-perception were measured by changes in the latency of the tail-flick in response to a radiant heat source of low intensity. While an intrathecal injection of serotonin has been previously shown to be analgesic, exogenous tryptamine produced dual effects on the pain-threshold, depending on the dose of tryptamine injected. Low doses of tryptamine (100 and 200 micrograms/rat) injected intrathecally onto the sacral area of the spinal cord appeared to be hyperalgesic by significantly decreasing the average tail-flick latency by 5 min after injection. Administration of the serotonin antagonist methysergide alone was without effect on the average tail-flick reaction time when injected either intrathecally or subcutaneously. However, pretreatment with either methysergide or cinanserin not only failed to inhibit tryptamine's potentiation of nociception, but actually enhanced the hyperalgesia produced by tryptamine. In contrast, a dose of 400 micrograms of tryptamine significantly increased the average tail-flick latency, suggesting an analgesic effect at this higher dose. This analgesic effect of 400 micrograms of tryptamine was completely inhibited by subcutaneously administered methysergide, while intrathecally injected methysergide produced even greater decreases in the tail-flick latencies after this high dose of tryptamine. These results suggest that tryptamine, although it differs from serotonin by only one hydroxyl group, may play a role in nociception which is opposite that played by serotonin.

Tryptamine-induced myoclonus in guinea-pigs pretreated with a monoamine oxidase inhibitor indicates pre- and post-synaptic actions of tryptamine upon central indoleamine systems

Tryptamine (1-320 mg/kg) evoked only slight muscle jerking in naive guinea-pigs but, in animals pretreated with pargyline (75 mg/kg; 1 hr previously), tryptamine induced a dose-dependent (6-160 mg/kg) myoclonus. The myoclonus induced by tryptamine (40 mg/kg) plus pargyline (75 mg/kg) was differentially inhibited by the indoleamine receptor antagonists, methergoline (5 mg/kg) which was more potent than methysergide (10 mg/kg), mianserin (10 mg/kg) which was more potent that cyproheptadine (10 mg/kg) and propranolol (20 mg/kg) which was more potent than cinanserin (10 mg/kg). This rank order of potency differed from that observed for the order of potency of these drugs in inhibiting the myoclonus induced by L-5-hydroxytryptophan (5HTP) plus carbidopa in guinea-pigs (Luscombe, Jenner and Marsden, Neuropharmacology, 1981), perhaps indicating involvement of pharmacologically distinct indoleamine receptors. Manipulation of presynaptic function of 5-hydroxytryptamine (5HT) by tryptophan hydroxylase inhibition with p-chlorophenylalanine to produce depletion of cerebral 5HT, or by an L-tryptophan load to elevate 5HT in brain, suggested that the functional integrity of serotonergic neurones is required for the expression of myoclonus induced by tryptamine plus pargyline. A range of blockers of 5HT re-uptake did not alter the jerking produced by tryptamine (40 mg/kg) in guinea pigs pretreated with pargyline (75 mg/kg; 1 hr previously), or the threshold myoclonus induced by a smaller dose of tryptamine (10 mg/kg; plus pargyline 75 mg/kg). It is suggested that myoclonus induced by tryptamine in guinea pigs pretreated with pargyline involves activation of post-synaptic indoleamine receptors by tryptamine by a mechanism which requires intact presynaptic function of 5HT.

Further studies on the role of indoleamines in the responses of cortical neurones to stimulation of nucleus raphe medianus: effects of indoleamine precursor loading

Responses of cortical neurones to stimulation of nucleus raphe medianus (RM) were determined before and after parenteral administration of l-tryptophan or l-5-hydroxytryptophan (5HTP). The short latency inhibitory effects of stimulation and the longer latency excitation were enhanced by l-tryptophan. 5-Hydroxytryptophan, on the other hand, enhanced the excitatory effects but had no effects on the initial inhibition. Both precursors were able to induce a third type of response, a long-latency inhibition which succeeded the excitation. Inhibition of tryptophan hydroxylase abolished the facilitatory effects of tryptophan on the excitatory and long-latency inhibitory effects of stimulation of the raphé medianus but the enhancement of the short-latency inhibition remained intact. Inhibition of tryptophan hydroxylase failed to alter any of the effects of 5HTP on evoked responses to stimulation of the raphé medianus. Finally, inhibition of aromatic amino acid decarboxylase abolished the effects of both indoleamine precursors on all response phases. The results are consistent with a previous suggestion that the short-latency inhibition may be tryptamine-mediated while the other response phases are mediated by 5-hydroxytryptamine.

Tryptamine: a neuromodulator or neurotransmitter in mammalian brain?

Tryptamine synthesized by decarboxylation of L-tryptophan occurs as an endogenous constituent of mammalian brain albeit at very low concentrations (low ng/g range). It is primarily metabolized by oxidative deamination by MAO and possesses an extremely rapid turnover and half-life. Subcellular localization appears to be in nerve terminals and it is releasable by electrical or potassium evoked depolarization. Neuropharmacological and electrophysiological data strongly suggest the existence of post-synaptic receptors for tryptamine independent of those for 5HT. There may exist a rostrally projecting neuronal tryptamine containing system arising from cell bodies in or close to the nucleus raphé medianus. The demonstration of specific receptors for tryptamine in the CNS strongly indicates a transmitter role, although a strong case can be made for a role as a modifier of central 5HT systems. The possibility also exists that 5HT and tryptamine may be mediators of functionally opposite neuronal pathways. Whatever the role of tryptamine in the CNS it is clear that it not simply present as an accident of metabolism or a "biological artefact." The indications are that it possesses important functions in central neurotransmission.

Effect of nerve activity on the in vivo release of [3H]serotonin continuously formed from L-[3H]tryptophan in the caudate nucleus of the cat

A new isotopic approach has been developed to study the in vivo release of serotonin (5-HT). 'Encéphale isolé' cats were implanted with a push-pull cannula in the ventrocaudal part of the head of the caudate nucleus to estimate the release of [3H]5-HT continuously synthesized from L-[3H]tryptophan. Both [3H]5-HT and [3H]tryptamine were found in superfusates. Resting steady state in the release of [3H]indoleamines was observed as soon as 20 min after the beginning of the superfusion with L-[3H]tryptophan; the levels of [3H]5-HT in superfusates were 2.5 times those of [3H]tryptamine and about 6 times the blank value. They were markedly enhanced in the presence of fluoxetine (5 x 10(-6)M), a blocker of the 5-HT uptake process. A marked increase in the release of [3H]5-HT was seen during the local depolarization of 5-HT terminals with potassium chloride (60 mM) or batrachotoxin (10(-6)M) or during the stimulation of 5-HT cell bodies in the nucleus raphe dorsalis with L-glutamic acid (5 x 10(-5)M). These treatments did not enhance the efflux of [3H]tryptamine. The potassium-evoked release of [3H]5-HT was reduced by LSD (10(-5)M). LSD added alone in the superfusing fluid was without effect. The batrachotoxin-evoked release of [3H]5-HT was inhibited in the presence of tetrodotoxin (9 x 10(-6)M). The spontaneous release of [3H]5-HT and [3H]tryptamine was markedly reduced in the presence of a calcium-free medium containing cobalt (10 mM). A transient slight reduction in the spontaneous release of [3H]5-HT was observed in the presence of tetrodotoxin (9 x 10(-6)M). The local cooling of 5-HT cell bodies with a cryoelectrode induced a slight reversible decrease in [3H]5-HT release. These last two treatments were without significant effect on [3H]tryptamine efflux in superfusates. These results indicate that the release of [3H]5-HT endogenously formed from [3H]tryptophan is dependent on nerve activity and that this is not the case for [3H]tryptamine. The advantages of the isotopic approach for in vivo studies on the release of 5-HT are discussed.

Kinetics of tryptophan accumulation into synaptosomes of various regions of rat brain

The kinetics of synaptosomal tryptophan accumulation has been determined in five regions of the rat brain. For tryptophan concentrations ranging from 2.5 -- 20 microM, we found an active uptake in all the structures studied, i.e.: Corpus striatum, midbrain, brainstem, hypothalamus and cerebral cortex + hippocampus. The Vm of tryptophan uptake was highest in the cortex, followed in descending order by corpus striatum, hypothalamus, midbrain and brainstem, while the Km was highest in the cortex, then in descending order corpus striatum, brainstem, midbrain and hypothalamus. In spite of the possible nonspecific high affinity tryptophan uptake into serotoninergic neurons, we found a correlation between the Vm of tryptophan uptake and the different results in the literature concerning uptake and release of serotonin. These observations might indicate a correlation between the Vm of tryptophan uptake and the functional activity of serotonergic neurons.