TIPS review

Modulation of serotonin release in the brain via presynaptic receptors

This review contains background information on the serotonin system, furthermore the suggestion to introduce the term ContemporaryWitness Report (CWR) for a novel type of review and, as the main part, an overview over the history of serotonin discovery as a basis for the identification of its receptor heterogeneity and the increase in complexity by genetic and allosteric variation. The present article conforms to CWRs in historical and autobiographical elements, in more emphasis on the author's work than in conventional reviews and in aspects neglected in previous reviews, but not in the main feature namely the work of a scientist with comprehensive expertise in a field in which, over long time, he/she continuously performed research and published. Ascientist complying with these requirements is a contemporary witness in that field. His report on the scientific achievements in that period, a CWR, comprises confirmation and putative re-interpretation of data from a superior viewpoint. Identification of serotonin's vascular properties (publication year: 1912) as an “adrenaline mimicking substance” (without attempt to isolate it) by O’Connor preceded the discovery of serotonin in the gastrointestinal tract by Erspamer [1937] and in blood by Rapport [1948, 1949], who identified its structure as 5-hydroxytryptamine [1949]. Detection as a neurotransmitter in invertebrates suggested its occurrence in vertebrate CNS as well. This was verified by finding it in dog, rat and rabbit brain [1953]. The Falck-Hillarp technique [1962] visualized serotonin neurones as fluorescent structures. The neurotoxin 5,7-dihydroxytryptamine [1972] indirectly proved the involvement of 5-HT in multiple CNS functions.

To explore if prolonged – as opposed to acute – 5-HT uptake blockade can lead to changes in the function of ATP-dependent potassium (K_ATP) channels, we investigated in rat and mouse neocortical slices the effects of K_ATP channel blockers on electrically evoked [³H]-serotonin ([³H]-5-HT) release after short- and long-term exposure to 5-HT uptake blockers. Glibenclamide (1 μM), a K_ATP channel blocker, enhanced the electrically evoked [³H]-5-HT release by 66 and by 77%, respectively, in rat and in mouse neocortex slices. This effect was confirmed in the rat by tolbutamide (1 μM), another K_ATP channel antagonist. After short-term blockade (45 min) of 5-HT uptake, glibenclamide still increased the release of [³H]-5-HT in the rat. Glibenclamide, however, failed to enhance [³H]-5-HT release after long-term uptake blockade (210 min). In the mouse, however, both short- and long-term inhibition of 5-HT reuptake by citalopram (1 μM) prevented the facilitatory effect of glibenclamide. The Na⁺/K⁺-ATPase inhibitor ouabain (3.2 μM) abolished the glibenclamide-induced increase in [³H]-5-HT release in both rat and mouse, suggesting that an operative Na⁺/K⁺-ATPase is a prerequisite for activation of K_ATP channels. The terminal 5-HT_1B autoreceptor-mediated feedback control was involved in the glibenclamide-induced increase in [³H]-5-HT release only in mouse neocortical tissue, as evident from the use of the 5-HT_1B autoreceptor ligands metitepin (1 μM) and cyanopindolol (1 μM). These results suggest that in the rat long-term uptake blockade leads to an impaired activity of the Na⁺/K⁺-ATPase, which increases intracellular ATP and consequently closes K_ATP channels. In the mouse, however, short-term uptake blockade seems to already reduce the activity of the Na⁺/K⁺-ATPase and thereby the consumption of ATP. Blockade of 5-HT transporters thus may close K_ATP channels through increased intracellular ATP. The following slight depolarisation seems to facilitate 5-HT release. These results may contribute to a better understanding of the mechanisms involved in the clinical time latency of antidepressant efficacy of monoamine uptake blockers.

The present study employed a combined neurochemical and behavioural approach to address the question of whether blockade of (presynaptic) 5-HT_1B or 5-HT_1D receptors enhances the facilitatory influence of 5-HT_1A autoreceptor antagonism upon the actions of selective serotonin re-uptake inhibitors (SSRI). In the presence of the selective 5-HT_1A antagonist, WAY100,635, the fluoxetine-induced increase in dialysate levels of 5-HT in the frontal cortex (FCX) of freely-moving rats was significantly potentiated. The selective 5-HT_1B antagonist, SB224,289, likewise potentiated the increase in 5-HT levels evoked by fluoxetine. Further, administered together, WAY100,635 and SB224,289, at least additively, potentiated the influence of fluoxetine upon 5-HT levels. This effect was selective inasmuch as, either alone or together, WAY100,635 and SB224,289 did not modify the influence of fluoxetine upon FCX levels of dopamine (DA) or noradrenaline (NA) quantified in the same dialysis samples. Co-administration of SB224,289 also enhanced the ability of WAY100,635 to potentiate the induction of head-twitches (HTW) by fluoxetine. This response reflects activation of 5-HT_2A sites in FCX and was abolished by the selective 5-HT_2A antagonist, MDL100,907. In contrast to SB224,289, the 5-HT_1D antagonist, BRL15572, failed to enhance the facilitatory influence of WAY100,635 upon the neurochemical or behavioural actions of fluoxetine. In conclusion, co-joint blockade of 5-HT_1B — but not 5-HT_1D — with 5-HT_1A autoreceptors markedly potentiates the neurochemical and functional actions of the SSRI, fluoxetine.

1. The influences of the purported serotonergic agonist 5-methoxy-N,N-dimethyltryptamine (MeODMT) on noradrenaline release and metabolism were investigated in a rat spinal cord release model and a monoamine oxidase (MAO) assay.

2. MeODMT inhibited the basal outflow of tritium from rat spinal cord slices preincubated with [³H]noradrenaline and enhanced the electrically-evoked overflow.

3. Effects on basal outflow were not observed, when monoamine oxidase (MAO) was inhibited by pargyline. Effects on the evoked overflow were not observed in the presence of metitepine or phentolamine.

4. Preferential inhibition by MeODMT of MAO A-type enzyme activity was found in a direct assay.

5. The results provide evidence for two different effects by which MeODMT reinforces noradrenergic neurotransmission in the rat spinal cord: facilitation of stimulation-evoked noradrenaline release and inhibition of noradrenaline metabolism by MAO inhibition.

In vivo microdialysis was used to examine the effects of 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-indole (RU24969) and 1-(m-trifluoromethylphenyl)piperazine (TFMPP) on extracellular 5-hydroxytryptamine (5-HT) in the diencephalon of unanesthetized rats. Both RU24969 and TFMPP are potent 5-HT autoreceptor agonists but both compounds caused a dose-dependent increase in extracellular 5-HT, when infused into the diencephalon at micromolar concentrations. The piperazine compound, TFMPP, also caused an increase in 5-HT when administered peripherally (2.5–10 mg/kg i.p.). In contrast, peripheral administration of RU24969 (2.5 mg/kg i.p.) caused a decrease in extracellular 5-HT. Since the effects of local infusion with RU24969 and TFMPP were not additive with the increase produced by the inhibitor of the uptake of 5-HT, fluoxetine, these compounds may be acting at the site of the membrane carrier. These results suggest that direct 5-HT₁ agonist activity is not the only factor involved in the physiological and behavioral consequences of peripheral administration of TFMPP.

Using a batch method for incubation of hippocampal slices, we have examined the effects of 5-methoxy-3-(l,2,3,6-tetrahydro-4-pyridinyl)-1H-indole (RU24969) and (m-trifluoromethylphenyl)piperazine (TFMPP) on release of endogenous 5-hydroxytryptamine (5-HT). Release of 5-HT from slices was enhanced by RU24969 and TFMPP at concentrations from 1 to 10 μmol. The 5-HT uptake inhibitors imipramine and fluoxetine, but not the autoreceptor antagonist methiothepui, blocked the enhancement in 5-HT. These results suggest that RU24969 and TFMPP, previously identified as potent agonists at the nerve terminal autoreceptor, also interact at higher concentrations with the reuptake carrier to enhance extracellular levels of 5-HT.