Albert Adell

Origin and functional role of the extracellular serotonin in the midbrain raphe nuclei.

There is considerable interest in the regulation of the extracellular compartment of the transmitter serotonin (5-hydroxytryptamine, 5-HT) in the midbrain raphe nuclei because it can control the activity of ascending serotonergic systems and the release of 5-HT in terminal areas of the forebrain. Several intrinsic and extrinsic factors of 5-HT neurons that regulate 5-HT release in the dorsal (DR) and median (MnR) raphe nucleus are reviewed in this article. Despite its high concentration in the extracellular space of the raphe nuclei, the origin of this pool of the transmitter remains to be determined. Regardless of its origin, is has been shown that the release of 5-HT in the rostral raphe nuclei is partly dependent on impulse flow and Ca(2+) ions. The release in the DR and MnR is critically dependent on the activation of 5-HT autoreceptors in these nuclei. Yet, it appears that 5-HT autoreceptors do not tonically inhibit 5-HT release in the raphe nuclei but rather play a role as sensors that respond to an excess of the endogenous transmitter. Both DR and MnR are equally responsive to the reduction of 5-HT release elicited by the local perfusion of 5-HT(1A) receptor agonists. In contrast, the effects of selective 5-HT(1B) receptor agonists are more pronounced in the MnR than in the DR. However, the cellular localization of 5-HT(1B) receptors in the raphe nuclei remains to be established. Furthermore, endogenous noradrenaline and GABA tonically regulate the extracellular concentration of 5-HT although the degree of tonicity appears to depend upon the sleep/wake cycle and the behavioral state of the animal. Glutamate exerts a phasic facilitatory control over the release of 5-HT in the raphe nuclei through ionotropic glutamate receptors. Overall, it appears that the extracellular concentration of 5-HT in the DR and the MnR is tightly controlled by intrinsic serotonergic mechanisms as well as afferent connections.

Differential effects of clomipramine given locally or systemically on extracellular 5-hydroxytryptamine in raphe nuclei and frontal cortex. An in vivo brain microdialysis study

SummaryThe antidepressant drug clomipramine (CIM) blocks 5-hydroxytryptamine (5-HT) uptake in vitro. Electrophysiological studies have shown that CIM also reduces the firing of serotonergic neurons in the dorsal raphe nucleus. In order to assess the effects of CIM on serotonergic transmission in vivo, the technique of intracerebral microdialysis was used. CIM was administered either through the dialysis probe or i. p., and dialysate 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) were determined in frontal cortex and/or raphe nuclei. In addition, the action of extracellular 5-HT in raphe nuclei on the release of 5-HT in frontal cortex was studied.The administration of CIM through the dialysis probe increased dialysate 5-HT in frontal cortex in a dose-dependent fashion. An actual ED50 of 3.15 μM CIM for the in vivo inhibition of 5-HT uptake can be calculated in this brain area. When given systemically (10 or 20 mg/kg i. p.), CIM did not increase dialysate 5-HT in the frontal cortex. The occurrence of extracellular 5-HT in the raphe area was demonstrated. This pool of 5-HT increased markedly after local (10 or 40 μM) or systemic (20 mg/kg i. p.) administration of CIM. We also examined the effect of CIM applied locally in the raphe nuclei on extracellular 5-HT in the frontal cortex. The increased dialysate 5-HT in raphe after 10 or 40 μM CIM paralleled a decrease of dialysate 5-HT in the frontal cortex. Values of dialysate 5-HT in the two areas correlated negatively. The administration of CIM through the dialysis probe slightly decreased dialysate 5-HIAA in the frontal cortex. When given systemically, CIM also decreased dialysate 5-HIAA in the frontal cortex, but significantly only after the highest dose tested (20 mg/kg i.p.). Furthermore, the local application of CIM into the raphe nuclei produced a decrease of dialysate 5-HIAA in the frontal cortex.These results prove the in vivo inhibitory activity of CIM on 5-HT uptake. This effect is much more pronounced in the raphe nuclei than in the frontal cortex when CIM is given systemically. However, both areas are equally sensitive to the local application of the drug. The effect of CIM on raphe nuclei partly antagonizes that on the frontal cortex, providing evidence for a functional link between these two brain regions.

Comparative study in the rat of the actions of different types of stress on the release of 5-HT in raphe nuclei and forebrain areas.

The effects of several stress procedures on the release of 5-HT in the dorsal and median raphe nuclei (DRN and MRN, respectively) and in forebrain structures of the rat brain innervated by both nuclei have been studied using intracerebral microdialysis. Handling for 30 sec, a saline injection and forced swimming for 5 min elevated significantly the 5-HT output in the MRN. The 5-HT output in the DRN was also enhanced by a saline injection. With regard to the forebrain structure examined, handling and forced swimming increased dialysate 5-HT in the amygdala. The injection of saline induced a slight, but significant, elevation of 5-HT in the medial prefrontal cortex. In contrast, the outflow of 5-HT was significantly reduced in the ventral hippocampus and medial prefrontal cortex following forced swimming and this effect persisted well beyond the cessation of the swim session. These results indicate that the efflux of 5-HT in the MRN appears to respond to different forms of stress, whereas that in the DRN only increases after the injection of saline. The release of 5-HT in the forebrain structures is also dependent on the type of stress procedure and the region studied.

Chronic stress increases serotonin and noradrenaline in rat brain and sensitizes their responses to a further acute stress

Abstract: The effects of 1 h/day restraint in plastic tubes for 24 days on the levels of serotonin (5‐HT), 5‐hydroxyindole‐acetic acid (5‐HIAA), tryptophan (TP), and noradrenaline (NA) in six regions of rat brain 20 h after the last restraint period were investigated. The levels of 5‐HT, 5‐HIAA, and NA but not TP increased in several regions. The effects of 1 h of immobilization on both control and chronically restrained rats were also studied. Immobilization per se did not alter brain 5‐HT, 5‐HIAA, and TP levels, but decreased NA in the pons plus medulla oblongata and hypothalamus. However, immobilization after chronic restraint decreased 5‐HT, increased 5‐HIAA, and decreased NA in most brain regions in comparison with values for the chronically restrained rats. We suggest that chronic restraint leads to compensatory increases of brain 5‐HT and NA synthesis and sensitizes both monoaminergic systems to an additional acute stress. These changes may affect coping with stress demands.

The somatodendritic release of dopamine in the ventral tegmental area and its regulation by afferent transmitter systems

The release of dopamine in the ventral tegmental area (VTA) plays an important role in the autoinhibition of the dopamine neurons of the mesocorticolimbic system through the activation of somatodendritic dopamine D2 autoreceptors. Accordingly, the intra-VTA application of dopamine D2 receptor agonists reduces the firing rate and release of dopamine in the VTA, and this control appears to possess a tonic nature because the corresponding antagonists enhance the somatodendritic release of the transmitter. In addition, the release of dopamine in the VTA is increased by potassium or veratridine depolarization and abolished by tetrodotoxin and calcium omission. Overall, it appears that the somatodendritic release of dopamine is consistently lower than that in nerve endings. Apart from intrinsic dopaminergic mechanisms, other transmitter systems such as serotonin, noradrenaline, acetylcholine, GABA and glutamate play a role in the control of the activity of dopaminergic neurons of the VTA, although the final action depends on the particular receptor involved as well as the neuronal type where it is localized. Given the involvement of the mesocorticolimbic dopaminergic systems in the pathogenesis of severe neuropsychiatric disorders such as schizophrenia, the knowledge of the factors that regulate the release of dopamine in the VTA could provide new insight into the ethiogenesis of the disease as well as its implication on the mechanisms of action of therapeutic drugs.

How does pindolol improve antidepressant action

Since 1994, the beta-adrenoceptor and 5-HT(1A/1B) receptor ligand pindolol has been used to accelerate or enhance the clinical effects of antidepressant drugs, such as the selective 5-HT reuptake inhibitors (SSRIs), that act primarily on 5-HT-containing neurones. Pindolol was initially thought to act by preventing the inhibition of 5-HT release, elicited by SSRIs and other 5-HT-acting drugs, as a result of its ability to antagonize the action of 5-HT at midbrain raphe 5-HT(1A) autoreceptors that control the activity of ascending 5-HT-mediated pathways. However, the partial agonist properties of pindolol at 5-HT(1A) receptors and beta-adrenoceptors suggest that other explanations for its action are also possible. In this article, recent controversial data on the mechanism of action of pindolol, which are crucial for the development of more rapid and efficient antidepressant therapies, will be discussed.

Clozapine and haloperidol differently suppress the MK-801-increased glutamatergic and serotonergic transmission in the medial prefrontal cortex of the rat

The administration of noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists such as phencyclidine and ketamine has been shown to increase the extracellular concentration of glutamate and serotonin (5-HT) in the medial prefrontal cortex (mPFC). In the present work, we used in vivo microdialysis to examine the effects of the more potent noncompetitive NMDA receptor antagonist, MK-801, on the efflux of glutamate and 5-HT in the mPFC, and whether the MK-801-induced changes in the cortical efflux of both transmitters could be blocked by clozapine and haloperidol given systemically or intra-mPFC. The systemic, but not the local administration of MK-801, induced an increased efflux of 5-HT and glutamate, which suggests that the NMDA receptors responsible for these effects are located outside the mPFC, possibly in GABAergic neurons that tonically inhibit glutamatergic inputs to the mPFC. The MK-801-induced increases of extracellular glutamate and 5-HT were dependent on nerve impulse and the activation of mPFC AMPA/kainate receptors as they were blocked by tetrodotoxin and NBQX, respectively. Clozapine and haloperidol blocked the MK-801-induced increase in glutamate, whereas only clozapine was able to block the increased efflux of 5-HT. The local effects of clozapine and haloperidol paralleled those observed after systemic administration, which emphasizes the relevance of the mPFC as a site of action of these antipsychotic drugs in offsetting the neurochemical effects of MK-801. The ability of clozapine to block excessive cortical 5-HT efflux elicited by MK-801 might be related to the superior efficacy of this drug in treating negative/cognitive symptoms of schizophrenia.

In Vivo Brain Dialysis Study of the Somatodendritic Release of Serotonin in the Raphe Nuclei of the Rat: Effects of 8-Hydroxy-2-(Di-n-Propylamino)tetralin

Abstract: The characteristics of the serotonin (5‐HT) output in the dorsal and median raphe nuclei of the rat were studied using in vivo microdialysis. The basal output of 5‐HT increased after KC1 was added to the perfusion fluid. In contrast, neither the omission of calcium ions nor the addition of 0.5 nM tetrodotoxin affected dialysate 5‐HT or 5‐hy‐droxyindoleacetic acid (5‐H1AA). Reserpine did not decrease the output of 5‐HT and 5‐HIAA 24 h later and p‐chloroamphetamine increased 5‐HT in both vehicle‐ and reserpine‐treated rats severalfold. 8‐Hydroxy‐2‐(di‐n‐pro‐pylamino)tetralin (8‐OH‐DPAT), at 1 or 10 μM, perfused into the raphe did not change the outputs of 5‐HT or 5‐HIAA. Higher doses (0.1, Land 10 mM) increased extracellular 5‐HT in the raphe, probably via an inhibition of uptake. In animals bearing two probes (raphe nuclei and ventral hippocampus), only the 10 vaM dose of 8‐OH‐DPAT perfused into the raphe decreased the hippocampal output of 5‐HT and 5‐HIAA. The systemic injection of 0.1 mg/kg 8‐OH‐DPAT decreased dialysate 5‐HT and 5‐HIAA in the raphe and hippocampus. These results suggest that extracellular 5‐HT in raphe nuclei originates from a cytoplasmic pool and is not dependent on either nerve impulse of 5‐HT neurons or local activation of 5‐HT1A receptors.

Strategies for producing faster acting antidepressants

Existing antidepressant treatments exhibit limited efficacy and a slow onset of action. Several neurobiological adaptive mechanisms might delay the clinical effects of antidepressants, whose therapeutic action is primarily triggered by an increase of serotonergic and noradrenergic neurotransmission. Here, we review several potential mechanisms that could be useful to increase the speed of current antidepressant drugs, such as additional blockade of aminergic autoreceptors or antagonism of certain postsynaptic (5-HT2A, 5-HT2C) receptors. The potential use of strategies not based on monoaminergic transmission, such as CRF and NK1 receptor antagonists, or more novel strategies, based on glutamatergic or GABAergic transmission or on intracellular messengers, are also reviewed.

The role of 5‐HT1B receptors in the regulation of serotonin cell firing and release in the rat brain

The release of 5‐HT in terminal areas of the rodent brain is regulated by 5‐HT1B receptors. Here we examined the role of 5‐HT1B receptors in the control of 5‐HT output and firing in the dorsal raphe nucleus (DR), median raphe nucleus (MnR) and forebrain of the rat in vivo. The local perfusion (30–300 µm) of the selective 5‐HT1B receptor agonist CP‐93,129 to freely moving rats decreased 5‐HT release in the DR and more markedly in the MnR. Likewise, 300 µm CP‐93,129 reduced 5‐HT output in substantia nigra pars reticulata, ventral pallidum, lateral habenula and the suprachiasmatic nucleus. The effect of CP‐93,129 was prevented by SB‐224289, but not by WAY‐100635, selective 5‐HT1B and 5‐HT1A receptor antagonists, respectively. SB‐224289 did not alter dialysate 5‐HT in any raphe nuclei. The intravenous administration of the brain‐penetrant selective 5‐HT1B receptor agonist CP‐94,253 (0.5–2.0 mg/kg) to anesthetized rats decreased dialysate 5‐HT in dorsal hippocampus and globus pallidus, increased it in MnR and left it unaltered in the DR and medial prefrontal cortex. SB‐224289, at a dose known to block 5‐HT1B autoreceptor‐mediated effects (5 mg/kg), did not prevent the effect of CP‐94,253 on MnR 5‐HT. The intravenous administration of CP‐94,253 (0.05–1.6 mg/kg) to anesthetized rats increased the firing rate of MnR, but not DR‐5‐HT neurons. The local perfusion of CP‐94,253 in the MnR showed a biphasic effect, with 5‐HT reductions at 0.3–3 µm and increase at 300 µm. These results suggest that 5‐HT cell firing and release in midbrain raphe nuclei (particularly in the MnR) are under control of 5‐HT1B receptors. The activation of 5‐HT1B autoreceptors (possibly located on 5‐HT nerve endings and/or varicosities within DR and MnR) reduces 5‐HT release. The effects of higher concentrations of 5‐HT1B receptor agonists seem more compatible with the activation of 5‐HT1B heteroreceptors on inhibitory neurons.