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

Behavioral and pharmacogenetics of aggressive behavior

Serotonin (5-HT) has long been considered as a key transmitter in the neurocircuitry controlling aggression. Impaired regulation of each subtype of 5-HT receptor, 5-HT transporter, synthetic and metabolic enzymes has been linked particularly to impulsive aggression. The current summary focuses mostly on recent findings from pharmacological and genetic studies. The pharmacological treatments and genetic manipulations or polymorphisms of aspecific target (e.g., 5-HT1A receptor) can often result in inconsistent results on aggression, due to "phasic" effects of pharmacological agents versus "trait"-like effects of genetic manipulations. Also, the local administration of a drug using the intracranial microinjection technique has shown that activation of specific subtypes of 5-HT receptors (5-HT1A and 5-HT1B) in mesocorticolimbic areas can reduce species-typical and other aggressive behaviors, but the same receptors in the medial prefrontal cortex or septal area promote escalated forms of aggression. Thus, there are receptor populations in specific brain regions that preferentially modulate specific types of aggression. Genetic studies have shown important gene-environment interactions; it is likely that the polymorphisms in the genes of 5-HT transporters or rate-limiting synthetic and metabolic enzymes of 5-HT (e.g., MAOA) determine the vulnerability to adverse environmental factors that escalate aggression. We also discuss the interaction between the 5-HT system and other systems. Modulation of 5-HT neurons in the dorsalraphe nucleus by GABA, glutamate and CRF profoundly regulate aggressive behaviors. Also, interactions of the 5-HT system with other neuropeptides(arginine vasopressin, oxytocin, neuropeptide Y, opioid) have emerged as important neurobiological determinants of aggression. Studies of aggression in genetically modified mice identified several molecules that affect the 5-HT system directly (e.g., Tph2, 5-HT1B, 5-HT transporter, Pet1, MAOA) or indirectly[e.g., BDNF, neuronal nitric oxide (nNOS), aCaMKII, Neuropeptide Y].The future agenda delineates specific receptor subpopulations for GABA, glutamate and neuropeptides as they modulate the canonical aminergic neurotransmitters in brainstem, limbic and cortical regions with the ultimate outcome of attenuating or escalating aggressive behavior.

Different actions for acute and chronic administration of mirtazapine on serotonergic transmission associated with raphe nuclei and their innervation cortical regions

The atypical antidepressant, mirtazapine enhances noradrenergic transmission, but its effects on serotonergic transmission remain to be clarified. The present study determined the effects of acute and chronic administration of mirtazapine on serotonergic transmissions in raphe nuclei and their innervation regions, frontal and entorhinal cortex, using multiple-probes microdialysis with real-time PCR and western blotting. Acute administration of mirtazapine did not affect extracellular serotonin level in raphe nuclei or cortex; however, chronic administration increased extracellular serotonin level in raphe nuclei without affecting that in cortex. Blockade of 5-HT1A receptor, but not that of the 5-HT2A/2C receptor, enhanced the effects of acute administration of mirtazapine on extracellular serotonin level in raphe nuclei. Chronic mirtazapine administration reduced the inhibitory function associated with somatodendritic 5-HT1A receptor in raphe nuclei, but enhanced postsynaptic 5-HT1A receptor in serotonergic innervated cortical regions. Chronic administration reduced the expression of mRNA and protein of serotonin transporter and 5-HT1A receptor in raphe nuclei, but not in the cortices. These results suggested that acute administration of mirtazapine probably activated serotonergic transmission, but its stimulatory action was abolished by activated inhibitory 5-HT1A receptor. Chronic administration of mirtazapine resulted in increased extracellular serotonin level via reduction of serotonin transporter with reduction of somatodendritic 5-HT1A autoreceptor function in raphe nuclei. These pharmacological actions of mirtazapine include its serotonergic profiles as noradrenergic and specific serotonergic antidepressant (NaSSA).

Context-dependent modulation of auditory processing by serotonin

Context-dependent plasticity in auditory processing is achieved in part by physiological mechanisms that link behavioral state to neural responses to sound. The neuromodulator serotonin has many characteristics suitable for such a role. Serotonergic neurons are extrinsic to the auditory system but send projections to most auditory regions. These projections release serotonin during particular behavioral contexts. Heightened levels of behavioral arousal and specific extrinsic events, including stressful or social events, increase serotonin availability in the auditory system. Although the release of serotonin is likely to be relatively diffuse, highly specific effects of serotonin on auditory neural circuitry are achieved through the localization of serotonergic projections, and through a large array of receptor types that are expressed by specific subsets of auditory neurons. Through this array, serotonin enacts plasticity in auditory processing in multiple ways. Serotonin changes the responses of auditory neurons to input through the alteration of intrinsic and synaptic properties, and alters both short- and long-term forms of plasticity. The infrastructure of the serotonergic system itself is also plastic, responding to age and cochlear trauma. These diverse findings support a view of serotonin as a widespread mechanism for behaviorally relevant plasticity in the regulation of auditory processing. This view also accommodates models of how the same regulatory mechanism can have pathological consequences for auditory processing.

Phenylephrine into the median raphe nucleus evokes an anxiolytic-like effect in free-feeding rats but does not alter food intake in free feeding rats

This study investigated the role of MnR α₁-adrenergic receptors in the control of anxiety-like and feeding behaviors and attempted to reveal a possible functional association between both behaviors. The α₁-adrenergic agonist phenylephrine (PHE) (at doses of 0.2, 2, 6, 20 nmol) or saline was injected into the MnR or into the pontine nucleus (Pn) of free-feeding rats. The animals were exposed to the elevated plus maze to analyse spatial-temporal and ethological variables. Subsequently, the ingestive and non-ingestive behaviors were recorded during 30 min and feeding and drinking behaviors were measured. Both in the elevated plus-maze and in the feeding chamber, all PHE doses injected into the MnR decreased the risk assessment frequency, an ethological parameter of anxiolytic-like effect. The spatial-temporal variables remained unchanged after PHE treatment. Feeding behavior was not affected by PHE into the MnR. The anxiety-like or ingestive behaviors were not affected by PHE treatment in the Pn, an area adjacent to the MnR. These data indicate that α₁-adrenergic receptors within MnR participate in the control of anxiety-like behaviors. The absence of effects on feeding behavior after MnR α₁-adrenergic activation could be due to an elevated α₁-adrenergic tonus and its possible strong facilitatory influence on 5-HT neurons within MnR. Furthermore, the present results suggest that anxiety-like and feeding behaviors controled by MnR adrenergic circuits operate by independent neural pathways.

GABA(B) receptor modulation of serotonin neurons in the dorsal raphé nucleus and escalation of aggression in mice

The serotonin (5-HT) system in the brain has been studied more than any other neurotransmitter for its role in the neurobiological basis of aggression. However, which mechanisms modulate the 5-HT system to promote escalated aggression is not clear. We here explore the role of GABAergic modulation in the raphé nuclei, from which most 5-HT in the forebrain originates, on escalated aggression in male mice. Pharmacological activation of GABA(B), but not GABA(A), receptors in the dorsal raphé nucleus (DRN) escalated aggressive behaviors. In contrast, GABA agonists did not escalate aggressive behaviors after microinjection into the median raphé nucleus. The aggression-heightening effect of the GABA(B) agonist baclofen depended on the activation of 5-HT neurons in the DRN because it was blocked by coadministration of the 5-HT(1A) agonist 8-OH-DPAT [((+/-)-8-hydroxy-2-(di-n-propylamino)tetralin) hydrobromide] (DPAT), which acts on autoreceptors and inhibits 5-HT neural activity. In vivo microdialysis showed that GABA(B) activation in the DRN increased extracellular 5-HT level in the medial prefrontal cortex. This may be attributable to an indirect action via presynaptic GABA(B) receptors. The presynaptic GABA(B) receptors suppress Ca(2+) channel activity and inhibit neurotransmission, and the coadministration of N-type Ca(2+) channel blocker facilitated the effect of baclofen. These findings suggest that the indirect disinhibition of 5-HT neuron activity by presynaptic GABA(B) receptors on non-5-HT neurons in the DRN is one of the neurobiological mechanisms of escalated aggression.

Serotonin synthesis, release and reuptake in terminals: a mathematical model

BACKGROUND

Serotonin is a neurotransmitter that has been linked to a wide variety of behaviors including feeding and body-weight regulation, social hierarchies, aggression and suicidality, obsessive compulsive disorder, alcoholism, anxiety, and affective disorders. Full understanding of serotonergic systems in the central nervous system involves genomics, neurochemistry, electrophysiology, and behavior. Though associations have been found between functions at these different levels, in most cases the causal mechanisms are unknown. The scientific issues are daunting but important for human health because of the use of selective serotonin reuptake inhibitors and other pharmacological agents to treat disorders in the serotonergic signaling system.

METHODS

We construct a mathematical model of serotonin synthesis, release, and reuptake in a single serotonergic neuron terminal. The model includes the effects of autoreceptors, the transport of tryptophan into the terminal, and the metabolism of serotonin, as well as the dependence of release on the firing rate. The model is based on real physiology determined experimentally and is compared to experimental data.

RESULTS

We compare the variations in serotonin and dopamine synthesis due to meals and find that dopamine synthesis is insensitive to the availability of tyrosine but serotonin synthesis is sensitive to the availability of tryptophan. We conduct in silico experiments on the clearance of extracellular serotonin, normally and in the presence of fluoxetine, and compare to experimental data. We study the effects of various polymorphisms in the genes for the serotonin transporter and for tryptophan hydroxylase on synthesis, release, and reuptake. We find that, because of the homeostatic feedback mechanisms of the autoreceptors, the polymorphisms have smaller effects than one expects. We compute the expected steady concentrations of serotonin transporter knockout mice and compare to experimental data. Finally, we study how the properties of the the serotonin transporter and the autoreceptors give rise to the time courses of extracellular serotonin in various projection regions after a dose of fluoxetine.

CONCLUSIONS

Serotonergic systems must respond robustly to important biological signals, while at the same time maintaining homeostasis in the face of normal biological fluctuations in inputs, expression levels, and firing rates. This is accomplished through the cooperative effect of many different homeostatic mechanisms including special properties of the serotonin transporters and the serotonin autoreceptors. Many difficult questions remain in order to fully understand how serotonin biochemistry affects serotonin electrophysiology and vice versa, and how both are changed in the presence of selective serotonin reuptake inhibitors. Mathematical models are useful tools for investigating some of these questions.

Serotonin in the inferior colliculus fluctuates with behavioral state and environmental stimuli

Neuromodulation by serotonin (5-HT) could link behavioral state and environmental events with sensory processing. Within the auditory system, the presence of 5-HT alters the activity of neurons in the inferior colliculus (IC), but the conditions that influence 5-HT neurotransmission in this region of the brain are unknown. We used in vivo voltammetry to measure extracellular 5-HT in the IC of behaving mice to address this issue. Extracellular 5-HT increased with the recovery from anesthesia, suggesting that the neuromodulation of auditory processing is correlated with the level of behavioral arousal. Awake mice were further exposed to auditory (broadband noise), visual (light) or olfactory (2,5-dihydro-2,4,5-trimethylthiazoline, TMT) stimuli, presented with food or confined in a small arena. Only the auditory stimulus or restricted movement increased the concentration of extracellular 5-HT in the IC. Changes occurred within minutes of stimulus onset, with the auditory stimulus increasing extracellular 5-HT by an average of 5% and restricted movement increasing it by an average of 14%. These findings suggest that the neuromodulation of auditory processing by 5-HT is a dynamic process that is dependent on internal state and behavioral conditions.

Activity-dependent vesicular monoamine transporter-mediated depletion of the nucleus supports somatic release by serotonin neurons

Packaging by the vesicular monoamine transporter (VMAT) is essential for mood-controlling serotonin transmission but has not been assayed during activity. Here, two-photon imaging of the fluorescent serotonin analog 5,7-dihydroxytryptamine and three-photon imaging of endogenous serotonin were used to study vesicular packaging as it supports release from the soma of serotonin neurons. Glutamate receptor activation in dorsal raphe brain slice evoked somatic release that was mediated solely by vesicle exocytosis. This release was accompanied by VMAT-mediated serotonin depletion from the nucleus, a large compartment free of monoaminergic degradation pathways that has not been implicated in neurotransmission previously. Finally, while some monoamine packaged at rest was held in reserve, monoamine packaged during stimulation was released completely. Hence, somatic vesicles loaded by VMAT during activity rapidly undergo exocytosis. In the absence of active zones and with limited neurotransmitter reuptake, somatic release by serotonin neurons is supported by recruitment from a large pool of extravesicular serotonin in the nucleus and cytoplasm, and preferential release of the newly packaged transmitter.

In vivo evidence that constitutive activity of serotonin2C receptors in the medial prefrontal cortex participates in the control of dopamine release in the rat nucleus accumbens: differential effects of inverse agonist versus antagonist

Control of the mesoaccumbens dopamine (DA) pathway by central serotonin(2C) receptors (5-HT(2C)Rs) involves different 5-HT(2C)R populations located within multiple brain areas. Here, using in vivo microdialysis in halothane-anesthetized rats, we assessed the role of medial prefrontal cortex (mPFC) 5-HT(2C)Rs in the control of basal and activated accumbal DA outflow, to identify the modalities of their recruitment and the role of 5-HT(2C)R constitutive activity. Intra-mPFC injection of the 5-HT(2C)R inverse agonist SB 206553 (0.5 microg/0.2 microL), without effect by itself, decreased accumbal DA outflow induced by morphine (2.5-10 mg/kg, s.c.), haloperidol (0.01 mg/kg, s.c.) or GBR 12909 (2.5 mg/kg, i.p.). Conversely, intra-mPFC injection of the 5-HT(2C)R antagonist SB 242084 (0.5 microg/0.2 microL), without effect by itself, decreased the effect of 10 mg/kg morphine, the only drug enhancing basal 5-HT outflow in the mPFC. The inhibitory effect of SB 206553 on 2.5 mg/kg morphine-stimulated DA outflow was suppressed by the concomitant intra-mPFC injection of SB 242084. Finally, changes of basal DA outflow induced by the 5-HT(2C)R agonist Ro 60-0175 (3 mg/kg, i.p.) or SB 206553 (5 mg/kg, i.p.) were unaffected by intra-mPFC injection of SB 242084. These results, showing that 5-HT(2C)R antagonist and inverse agonist behave differently in vivo, demonstrate that mPFC 5-HT(2C)Rs facilitate activated accumbal DA outflow and that 5-HT(2C)R constitutive activity participates in this interaction.

Enhancement of cortical extracellular 5-HT by 5-HT1A and 5-HT2C receptor blockade restores the antidepressant-like effect of citalopram in non-responder mice

We recently found that the response of DBA/2 mice to SSRIs in the forced swim test (FST) was impaired and they also had a smaller basal and citalopram-stimulated increase in brain extracellular serotonin (5-HT) than 'responder' strains. We employed intracerebral microdialysis, FST and selective antagonists of 5-HT1A and 5-HT2C receptors to investigate whether enhancing the increase in extracellular 5-HT reinstated the anti-immobility effect of citalopram in the FST. WAY 100635 (0.3 mg/kg s.c.) or SB 242084 (1 mg/kg s.c.), respectively a selective 5-HT1A and 5-HT2C receptor antagonist, raised the effect of citalopram (5 mg/kg) on extracellular 5-HT in the medial prefrontal cortex of DBA/2N mice (citalopram alone 5.2+/-0.3 fmol/20 microl, WAY 100635+citalopram 9.9+/-2.1 fmol/20 microl, SB 242084+ citalopram 7.6+/-1.0 fmol/20 microl) to the level reached in 'responder' mice given citalopram alone. The 5-HT receptor antagonists had no effect on the citalopram-induced increase in extracellular 5-HT in the dorsal hippocampus. The combination of citalopram with WAY 100635 or SB 242084 significantly reduced immobility time in DBA/2N mice that otherwise did not respond to either drug singly. Brain levels of citalopram in mice given citalopram alone or with 5-HT antagonists did not significantly differ. The results confirm that impaired 5-HT transmission accounts for the lack of effect of citalopram in the FST and suggest that enhancing the effect of SSRIs on extracellular 5-HT, through selective blockade of 5-HT1A and 5-HT2C receptors, could be a useful strategy to restore the response in treatment-resistant depression.

That warm fuzzy feeling: brain serotonergic neurons and the regulation of emotion

Whether lying on the beach in the midday sun on a Caribbean island, grabbing a few minutes in the sauna or spa after work, or sitting in a hot bath or Jacuzzi in the evening, we often associate feeling warm with a sense of relaxation and well-being. Even 'working up a good sweat', exercising or performing manual labour in the garden can have its rewards. Although we take these feelings for granted, convergent lines of evidence suggest that sensations of 'warmth' may alter neural circuits controlling cognitive function and mood, including serotonergic circuits, in addition to those directly involved in thermoregulatory cooling. One mechanism through which sensations of warmth may modulate neural circuits controlling cognitive function and mood is the activation of temperature-activated transient receptor potential (TRP) ion channels, including TRPv3 and TRPv4 which are active in the non-noxious thermal range, 27-42 degrees C, and subsequent activation of a subpopulation of brainstem serotonergic neurons. In this article, we explore the hypothesis that a subpopulation of serotonergic neurons are thermosensitive and form part of a thermoafferent pathway regulating physiology and behaviour. We also propose the novel hypothesis that dysregulation of this thermosensitive population of serotonergic neurons plays an important role in stress-related neuropsychiatric disorders, including anxiety and affective disorders.

Role of different monoamine receptors controlling MK-801-induced release of serotonin and glutamate in the medial prefrontal cortex: relevance for antipsychotic action

Several studies have demonstrated that systemically administered N-methyl-d-aspartate (NMDA) receptor antagonists increase serotonin (5-HT) and glutamate release in the medial prefrontal cortex (mPFC). Previously we showed that the perfusion of clozapine in the mPFC prevented the MK-801-induced increase in extracellular glutamate and 5-HT whereas haloperidol blocked only the effect of MK-801 on glutamate. To study the contribution of different monoaminergic receptors (for which clozapine and haloperidol exhibit distinct affinities) to these effects, here we used in-vivo microdialysis to examine the role of local blockade of dopamine D2, 5-HT2A and alpha1-adrenergic receptors as well as agonism at dopamine D1 and 5-HT1A receptors in the mPFC on the increased efflux of glutamate and 5-HT elicited by MK-801. The results show that M100907 (5-HT2A antagonist), BAY x 3702 (5-HT1A agonist) and prazosin (alpha1-adrenergic antagonist) blocked the MK-801-induced increase of 5-HT and glutamate in the mPFC. However, raclopride, eticlopride (dopamine D2 antagonists) and SKF-38393 (dopamine D1 agonist) were able to prevent the increased efflux of glutamate (but not that of 5-HT) elicited by MK-801. We propose that D2 receptor antagonists and D1 agonists would act predominantly on a subpopulation of GABAergic interneurons of the mPFC, thus leading to an enhanced cortical inhibition that would prevent an excessive glutamatergic transmission. On the other hand, atypical antipsychotic drugs might further act upon 5-HT2A, 5-HT1A and alpha1-adrenoceptors present in pyramidal cells (including those projecting to the dorsal raphe nucleus), which would directly inhibit an excessive excitability of these cells.

Partial regulation of serotonin transporter function by gamma-synuclein

Human alpha-synuclein (alpha-Syn) is instrumental in maintaining homeostasis of monoamine neurotransmitters in brain, through its trafficking, and regulation of the cell surface expression and, thereby, activity of dopamine, serotonin and norepinephrine transporters. Here we have investigated whether other members of the synuclein family of proteins, gamma-synuclein (gamma-Syn) and beta-synuclein (beta-Syn) can similarly modulate the serotonin transporter (SERT). In Ltk(-) cells co-transfected with SERT and gamma-Syn, gamma-Syn reduced [(3)H]5-HT uptake, in a manner dependent on its expression levels. The decrease in SERT activity was via decreased V(max) of the transporter, without change in K(m), compared to cells expressing only SERT. By contrast, beta-Syn co-expression failed to alter SERT uptake activity, and neither the V(max) nor the K(m) was changed in the presence of beta-Syn. gamma-Syn modulation of SERT was only partial, with a maximal approximately 27% decrease in SERT activity seen even at high expression levels of gamma-Syn. By contrast, alpha-Syn attenuated SERT activity by approximately 65% at identical expression levels as gamma-Syn. Co-immunoprecipitation studies showed the presence of heteromeric protein:protein complexes between gamma-Syn or alpha-Syn and SERT, while beta-Syn failed to physically interact with SERT. Both alpha-Syn and gamma-Syn colocalized with SERT in rat primary raphae nuclei neurons. These studies document a novel physiological role for gamma-Syn in regulating 5-HT synaptic availability and homeostasis, and may be of relevance in depression and mood disorders, where SERT function is dysregulated.

Relationships among body mass, brain size, gut length, and blood tryptophan and serotonin in young wild-type mice

Background

The blood hyperserotonemia of autism is one of the most consistent biological findings in autism research, but its causes remain unclear. A major difficulty in understanding this phenomenon is the lack of information on fundamental interactions among the developing brain, gut, and blood in the mammalian body. We therefore investigated relationships among the body mass, the brain mass, the volume of the hippocampal complex, the gut length, and the whole-blood levels of tryptophan and 5-hydroxytryptamine (5-HT, serotonin) in young, sexually immature wild-type mice.

Results

Three-dimensional reconstructions of the hippocampal complex were obtained from serial, Nissl-stained sections and the gut was allowed to attain its maximal relaxed length prior to measurements. The tryptophan and 5-HT concentrations in the blood were assessed with high-performance liquid chromatography (HPLC) and the sex of mice was confirmed by genotyping. Statistical analysis yielded information about correlative relationships among all studied variables. It revealed a strong negative correlation between blood 5-HT concentration and body mass and a strong negative correlation between the brain mass/body mass ratio and gut length. Also, a negative correlation was found between the volume of the hippocampal complex and blood tryptophan concentration.

Conclusion

The study provides information on the covariance structure of several central and peripheral variables related to the body serotonin systems. In particular, the results indicate that body mass should be included as a covariate in studies on platelet 5-HT levels and they also suggest a link between brain growth and gut length.

Three-photon microscopy shows that somatic release can be a quantitatively significant component of serotonergic neurotransmission in the mammalian brain

Recent experiments on monoaminergic neurons have shown that neurotransmission can originate from somatic release. However, little is known about the quantity of monoamine available to be released through this extrasynaptic pathway or about the intracellular dynamics that mediate such release. Using three-photon microscopy, we directly imaged serotonin autofluorescence and investigated the total serotonin content, release competence, and release kinetics of somatic serotonergic vesicles in the dorsal raphe neurons of the rat. We found that the somata of primary cultured neurons contain a large number of serotonin-filled vesicles arranged in a perinuclear fashion. A similar distribution is also observed in fresh tissue slice preparations obtained from the rat dorsal raphe. We estimate that the soma of a cultured neuron on an average contains about 9 fmoles of serotonin in about 450 vesicles (or vesicle clusters) of < or =370 nm average diameter. A substantial fraction (>30%) of this serotonin is released with a time scale of several minutes by K(+)-induced depolarization or by para-chloroamphetamine treatment. The amount of releasable serotonin stored in the somatic vesicles is comparable to the total serotonin content of all the synaptic vesicles in a raphe neuron, indicating that somatic release can potentially play a major role in serotonergic neurotransmission in the mammalian brain.

Unilateral lesion of the nigrostriatal pathway induces an increase of neuronal firing of the midbrain raphe nuclei 5-HT neurons and a decrease of their response to 5-HT(1A) receptor stimulation in the rat

Several studies have shown that the 5-hydroxytryptamine (serotonin, 5-HT) system is severely affected after degeneration of nigrostriatal dopaminergic neurons. In the present study, we examined the changes in the firing rate and firing pattern of the dorsal and median raphe nuclei (DRN and MRN) 5-HT neurons, and the effect of the selective 5-HT(1A) receptor agonist (R)-(+)-8-hydroxy-2-(dipropylamino)tetralin hydrobromide (8-OH-DPAT) and antagonist (N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-2-pyridylcyclohexane carboxamide maleate salt (WAY-100635) on the neuronal firing in rats with 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra pars compacta by using extracellular recording. The unilateral lesion of the nigrostriatal pathway significantly increased the mean firing rate of DRN and MRN 5-HT neurons compared with normal rats, and the firing pattern of these neurons also changed significantly towards a more bursty one. The lower dose of 8-OH-DPAT, 4 microg/kg (cumulative doses, i.v.), completely inhibited the firing activity of all DRN and MRN 5-HT neurons examined in normal and sham rats. In contrast to normal and sham rats, only the higher doses of 8-OH-DPAT, 128 and 64 microg/kg, completely inhibited the firing rate of DRN and MRN 5-HT neurons in 6-OHDA-lesioned rats, respectively. Furthermore, the local application of 8-OH-DPAT, 1.5 microg, in the DRN completely inhibited the firing rate of 5-HT neurons in normal and sham rats, while having no effect on firing rate in the lesioned rats. Altogether, these results indicate that lesion of the nigrostriatal pathway leads to hyperactivity of DRN and MRN 5-HT neurons, suggesting the implication of the DRN and MRN in the pathophysiology of Parkinson's disease, and the decreased response of these 5-HT neurons to 5-HT(1A) receptor stimulation, reflecting 5-HT(1A) receptor dysfunction in 6-OHDA-lesioned rats.

Functional interrelations between nucleus raphé dorsalis and nucleus raphé medianus: a dual probe microdialysis study of glutamate-stimulated serotonin release

Dual-probe in vivo microdialysis was used to explore the relationships between the two midbrain raphé nuclei, raphé dorsalis (DRN) and raphé medianus (MRN). Infusion of the excitatory neurotransmitter glutamate (10 mM) into the dorsal raphé nucleus produced a large increase in the extracellular 5-HT (5-HT(ext)) in the dorsal raphé (1400% of control values) that was limited to the time of infusion. This was followed by a significant decrease in extracellular 5-HT below baseline levels that continued for the duration of the experiment (3 h). Extracellular 5-HT (5-HT(ext)) was also increased to 500% of control values in the median raphé nucleus following infusion of 10 mM glutamate (GLU) into the dorsal raphé nucleus. Infusion of the competitive NMDA receptor antagonist AP5 prior to and during infusion of GLU into the DRN resulted in a decrease in the response to GLU in the DRN and an antagonism of the increase of 5-HT(ext) in the MRN. Infusion of 10mM GLU into the lateral midbrain tegmentum, an area of the brain just lateral to the DRN, also increased 5-HT(ext) in the probe in the lateral midbrain tegmentum (900% of control) but did not alter 5-HT(ext) in the MRN. When glutamate was infused into the MRN, 5-HT(ext) was also increased to 1400% of control in a time course similar to that seen with infusion of GLU into the DRN. Infusion of glutamate into the MRN, however, did not alter the 5-HT(ext) in the DRN. These data suggest a serotonergic innervation of the median raphé nucleus by the dorsal raphé nucleus. A reciprocal innervation from the median raphé to the dorsal raphé is not mediated by glutamate, does not appear to be serotonergic, and does not regulate extracellular serotonin in the dorsal raphé.

Corticotropin-releasing factor in the dorsal raphe nucleus increases medial prefrontal cortical serotonin via type 2 receptors and median raphe nucleus activity

Interactions between central corticotropin-releasing factor (CRF) and serotonergic systems are believed to be important for mediating fear and anxiety behaviors. Recently we demonstrated that infusions of CRF into the rat dorsal raphe nucleus result in a delayed increase in serotonin release within the medial prefrontal cortex that coincided with a reduction in fear behavior. The current studies were designed to study the CRF receptor mechanisms and pathways involved in this serotonergic response. Infusions of CRF (0.5 microg/0.5 microL) were made into the dorsal raphe nucleus of urethane-anesthetized rats following either inactivation of the median raphe nucleus by muscimol (25 ng/0.25 microL) or antagonism of CRF receptor type 1 or CRF receptor type 2 in the dorsal raphe nucleus with antalarmin (25-50 ng/0.5 microL) or antisauvagine-30 (2 microg/0.5 microL), respectively. Medial prefrontal cortex serotonin levels were measured using in-vivo microdialysis and high-performance liquid chromatography with electrochemical detection. Increased medial prefrontal cortex serotonin release elicited by CRF infusion into the dorsal raphe nucleus was abolished by inactivation of the median raphe nucleus. Furthermore, antagonism of CRF receptor type 2 but not CRF receptor type 1 in the dorsal raphe nucleus abolished CRF-induced increases in medial prefrontal cortex serotonin. Follow-up studies involved electrical stimulation of the central nucleus of the amygdala, a source of CRF afferents to the dorsal raphe nucleus. Activation of the central nucleus increased medial prefrontal cortex serotonin release. This response was blocked by CRF receptor type 2 antagonism in the dorsal raphe. Overall, these results highlight complex CRF modulation of medial prefrontal cortex serotonergic activity at the level of the raphe nuclei.

The distribution of gamma-hydroxybutyrate-induced Fos expression in rat brain: comparison with baclofen

gamma-Hydroxybutyrate (GHB) is a euphoric, prosocial and sleep inducing drug that binds with high affinity to its own GHB receptor site and also more weakly to GABA(B) receptors. GHB is efficacious in the treatment of narcolepsy and alcoholism, but heavy use can lead to dependence and withdrawal. Many effects of GHB (sedation, hypothermia, catalepsy) are mimicked by GABA(B) receptor agonists (e.g. baclofen). However other effects (euphoric and prosocial effects and a therapeutic effect in narcolepsy) are not. The present study used Fos immunohistochemistry to assess the neural activation produced in rat brain by medium to high doses of GHB (250, 500 and 1000 mg/kg) and a high dose of baclofen (10 mg/kg) that produced similar sedation to 500 mg/kg GHB. Results showed many common regions of activation with these two drugs including the supraoptic, paraventricular, median preoptic and ventral premammillary nuclei of the hypothalamus, the central nucleus of the amygdala, Edinger-Westphal nucleus, lateral parabrachial nucleus, locus coeruleus, and nucleus of the solitary tract. GHB (500 mg/kg), but not baclofen (10 mg/kg), induced significant Fos expression in the median raphe nucleus and lateral habenula, while a higher dose of GHB (1000 mg/kg) induced additional Fos expression in the islands of Calleja, dentate gyrus (polymorphic layer) and arcuate nucleus, and in various regions implicated in rapid and non-rapid eye movement sleep (laterodorsal tegmental nucleus, tuberomammillary nucleus and the ventrolateral and anterodorsal preoptic nuclei). Surprisingly, Fos immunoreactivity was not observed with either GHB or baclofen in reward-relevant regions such as the nucleus accumbens, striatum and ventral tegmental area. Overall these results indicate a distinctive signature of brain activation with GHB that may be only partly due to GABA(B) receptor effects. This confirms a unique neuropharmacological profile for GHB and indicates key neural substrates that may underlie its characteristic influence on sleep, body temperature, sociability and endocrine function.

The absence of 5-HT(1A) receptors has minor effects on dopamine but not serotonin release evoked by MK-801 in mice prefrontal cortex

RATIONALE

Non-competitive NMDA receptor antagonists markedly increase neuronal activity in medial prefrontal cortex (mPFC), an effect which partly underlies their schizomimetic actions. Projection pyramidal neurons and local GABAergic interneurons in mPFC express 5-HT(1A) receptors, whose activation modulates dopaminergic (DA) and serotonergic (5-HT) activity in midbrain and the cortical release of both monoamines.

OBJECTIVE

To examine whether the presence of 5-HT(1A) receptors can modulate the effect of NMDA receptor blockade with MK-801 (dizocilpine) on DA and 5-HT release in mouse mPFC.

MATERIALS AND METHODS

Brain microdialysis and locomotor activity measures in wild-type and 5-HT(1A) receptor knockout mice.

RESULTS

Systemic MK-801 administration (0.125, 0.25, 0.50, and 1 mg/kg i.p.) induced a dose-dependent increase in mPFC 5-HT output, which was independent of the genotype. MK-801 increased DA output in a dose-dependent manner with a significant effect of genotype on low doses (0.125, 0.25 mg/kg). These differences were not paralleled by differences in gross locomotor activity. Overall, MK-801 increased more markedly DA than 5-HT output in both genotypes. Finally, the local perfusion of MK-801 in mPFC (30, 100, 300 muM) by reverse dialysis did not elevate dialysate DA or 5-HT concentrations in mPFC.

CONCLUSION

5-HT(1A) receptors partly modulate the increase in mPFC DA (but not 5-HT) release produced by NMDA receptor blockade. The lack of effect observed after the local MK-801 application suggests that the change in cortical monoamines is mainly driven by subcortical NMDA receptor blockade, without a significant involvement of PFC 5-HT(1A) receptors.

The effects of intra-cerebral drug infusions on animals' unconditioned fear reactions: a systematic review

Intra-cerebral (i.c.) microinfusion of selective receptor agonists and antagonists into behaving animals can provide both neuroanatomical and neurochemical insights into the neural mechanisms of anxiety. However, there have been no systematic reviews of the results of this experimental approach that include both a range of unconditioned anxiety reactions and a sufficiently broad theoretical context. Here we focus on amino acid, monoamine, cholinergic and peptidergic receptor ligands microinfused into neural structures previously implicated in anxiety, and subsequent behavioral effects in animal models of unconditioned anxiety or fear. GABAA receptor agonists and glutamate receptor antagonists produced the most robust anxiolytic-like behavioral effects, in the majority of neural substrates and animal models. In contrast, ligands of the other receptor systems had more selective, site-specific anti-anxiety effects. For example, 5-HT1A receptor agonists produced anxiolytic-like effects in the raphe nuclei, but inconsistent effects in the amygdala, septum, and hippocampus. Conversely, 5-HT3 receptor antagonists produced anxiolytic-like effects in the amygdala but not in the raphe nuclei. Nicotinic receptor agonists produced anxiolytic-like effects in the raphe and anxiogenic effects in the septum and hippocampus. Unexpectedly, physostigmine, a general cholinergic agonist, produced anxiolytic-like effects in the hippocampus. Neuropeptide receptors, although they are popular targets for the development of selective anxiolytic agents, had the least reliable effects across different animal models and brain structures, perhaps due in part to the fact that selective receptor ligands are relatively scarce. While some inconsistencies in the microinfusion data can easily be attributed to pharmacological variables such as dose or ligand selectivity, in other instances pharmacological explanations are more difficult to invoke: e.g., even the same dose of a known anxiolytic compound (midazolam) with a known mechanism of action (the benzodiazepine-GABAA receptor complex), can selectively affect different fear reactions depending upon the different subregions of the nucleus into which it is infused (CeA versus BLA). These particular functional dissociations are important and may depend on the ability of a GABAA receptor agonist to interact with distinct isoforms and combinations of GABAA receptor subunits (e.g., alpha1-6, beta1-3, Upsilon1-2, delta), many of which are unevenly distributed throughout the brain. Although this molecular hypothesis awaits thorough evaluation, the microinfusion data overall give some support for a model of "anxiety" that is functionally segregated along different levels of a neural hierarchy, analogous in some ways to the organization of sensorimotor systems.

Serotonin transporter gene variation in sudden infant death syndrome

AIM

To investigate polymorphisms in the serotonin transporter (5-HTT) gene in cases of sudden infant death syndrome (SIDS) and controls, and further to elucidate a possible relationship between 5-HTT genotypes and external risk factors for SIDS.

METHOD

The subjects investigated consist of 163 SIDS cases and 243 controls. Polymorphisms in both the promoter and intron 2 of the 5-HTT gene were investigated, and the genotypes were determined using polymerase chain reaction (PCR) and gel electrophoresis.

RESULTS

In the promoter, there was a tendency for the L allele and L/L genotype to be found more often in the SIDS cases than in the controls (p=0.05 and p=0.07, respectively). Regarding the intron 2 polymorphism, there were no differences between the groups, and the SIDS cases were not found to have a higher frequency of either the L/L-12/12 genotype or the L-12 haplotype than the controls. When investigating possible correlations between genotype and risk factors for SIDS, there was a tendency towards different distribution of the promoter genotypes in cases found dead prone compared to cases found dead in other sleeping positions (p=0.06).

CONCLUSION

Polymorphisms in the promoter of the 5-HTT gene may be of importance with regard to SIDS.

Origin of the blood hyperserotonemia of autism

BACKGROUND

Research in the last fifty years has shown that many autistic individuals have elevated serotonin (5-hydroxytryptamine, 5-HT) levels in blood platelets. This phenomenon, known as the platelet hyperserotonemia of autism, is considered to be one of the most well-replicated findings in biological psychiatry. Its replicability suggests that many of the genes involved in autism affect a small number of biological networks. These networks may also play a role in the early development of the autistic brain.

RESULTS

We developed an equation that allows calculation of platelet 5-HT concentration as a function of measurable biological parameters. It also provides information about the sensitivity of platelet 5-HT levels to each of the parameters and their interactions.

CONCLUSION

The model yields platelet 5-HT concentrations that are consistent with values reported in experimental studies. If the parameters are considered independent, the model predicts that platelet 5-HT levels should be sensitive to changes in the platelet 5-HT uptake rate constant, the proportion of free 5-HT cleared in the liver and lungs, the gut 5-HT production rate and its regulation, and the volume of the gut wall. Linear and non-linear interactions among these and other parameters are specified in the equation, which may facilitate the design and interpretation of experimental studies.

Going with the flow: trafficking-dependent and -independent regulation of serotonin transport

Antidepressant-, cocaine- and 3,4-methylenedioxymethamphetamine-sensitive serotonin (5-hydroxytryptamine, 5-HT) transporters (SERTs) are expressed on presynaptic membranes of 5-HT-secreting neurons to provide efficient uptake of the biogenic amine after release. SERTs also support 5-HT transport across platelet, placental, gastrointestinal and pulmonary membranes and thus play a critical role in central nervous system and peripheral nervous system 5-HT signaling. SERTs are subject to multiple levels of posttranslational regulation that can rapidly alter 5-HT uptake and clearance rates. Specific cell surface receptors are now known to regulate SERT trafficking and/or catalytic function, with pathways activating protein kinase C, protein kinase G and p38 mitogen-activated protein kinase receiving the greatest attention. Remarkably, disease-associated mutations in SERT not only impact basal SERT activity but also selectively impact one or more SERT regulatory pathway(s). In this review, we describe both trafficking-dependent and trafficking-independent modes of SERT regulation and also the suspected roles played in regulation by SERT-associated proteins. Elucidation of the SERT 'regulome' provides important depth to our understanding of the likely origins of 5-HT-associated disorders and may help orient research to develop novel therapeutics.

Serotonin 5-HT2B receptors are required for 3,4-methylenedioxymethamphetamine-induced hyperlocomotion and 5-HT release in vivo and in vitro

The "club drug" 3,4-methylenedioxymethamphetamine (MDMA; also known as ecstasy) binds preferentially to and reverses the activity of the serotonin transporter, causing release of serotonin [5-hydroxytryptamine (5-HT)] stores from nerve terminals. Subsequent activation of postsynaptic 5-HT receptors by released 5-HT has been shown to be critical for the unique psychostimulatory effects of MDMA. In contrast, the effects of direct activation of presynaptic and/or postsynaptic receptors by MDMA have received far less attention, despite the agonist actions of the drug itself at 5-HT(2) receptors, in particular the 5-HT(2B) receptor. Here we show that acute pharmacological inhibition or genetic ablation of the 5-HT(2B) receptor in mice completely abolishes MDMA-induced hyperlocomotion and 5-HT release in nucleus accumbens and ventral tegmental area. Furthermore, the 5-HT(2B) receptor dependence of MDMA-stimulated release of endogenous 5-HT from superfused midbrain synaptosomes suggests that 5-HT(2B) receptors act, unlike any other 5-HT receptor, presynaptically to affect MDMA-stimulated 5-HT release. Thus, our findings reveal a novel regulatory component in the actions of MDMA and represent the first demonstration that 5-HT(2B) receptors play an important role in the brain, i.e., modulation of 5-HT release. As such, 5-HT(2B) receptor antagonists may serve as promising therapeutic drugs for MDMA abuse.

Intermolecular association provides specific optical and NMR signatures for serotonin at intravesicular concentrations

Neurotransmitter vesicles contain biomolecules at extraordinarily high concentrations (hundreds of millimoles/liter). Such concentrations can drive intermolecular associations, which may affect vesicular osmolarity and neuronal signaling. Here we investigate whether aqueous serotonin (a monoamine neurotransmitter) forms oligomers at intravesicular concentrations and whether these oligomers have specific spectroscopic signatures that can potentially be used for monitoring neuronal storage and release. We report that, as serotonin concentration is increased from 60 microM to 600 mM, the normalized fluorescence spectrum of serotonin displays a growing long-wavelength tail, with an isoemissive point at 376 nm. The fluorescence decay is monoexponential with a lifetime of 4 ns at low concentrations but is multiexponential with an average lifetime of 0.41 ns at 600 mM. A 600 mM serotonin solution has 30% less osmolarity than expected for monomeric serotonin, indicating oligomer formation. The proton NMR chemical shifts move upfield by as much as 0.3 ppm at 600 mM compared to those at 10 mM, indicating a stacking of the serotonin indole moieties. However, no intermolecular crosspeak is evident in the two-dimensional NMR rotating frame Overhauser effect spectroscopy spectrum even at 600 mM, suggesting that oligomeric structures are possibly weakly coupled. The appearance of a single peak for each proton suggests that the rate of interconversion between the monomeric and the oligomeric structures is faster than 240 Hz. A stopped-flow kinetic experiment also confirms that the rate of dissociation is faster than 100 ms. We conclude that serotonin forms oligomers at intravesicular concentrations but becomes monomeric quickly on dilution. NMR signatures of the oligomers provide potential contrast agents for monitoring the activity of serotonergic neurons in vivo.

The cannabinoid CB1 receptor is expressed on serotonergic and dopaminergic neurons

The endocannabinoid system is involved in memory, cognition, and pain perception by the presynaptic cannabinoid CB(1) receptor, which is expressed at high levels in many brain regions. Functional studies have shown that activation of cannabinoid CB(1) receptors inhibits the synaptic release of many neurotransmitters such as gamma-aminobutyric acid, glutamate, acetylcholine and monoamines. Monoamines, however, are known not only to be released from and taken back up at nerve terminals but also at extrasynaptic axonal and somatodendritic sites. Here we present immunocytochemical data documenting cannabinoid CB(1) receptor expression on neurite extensions and over cell bodies of serotonergic and dopaminergic neurons.

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.

Temporal neurotransmitter conditioning restores the functional activity of adult spinal cord neurons in long-term culture

The ability to culture functional adult mammalian spinal cord neurons represents an important step in the understanding and treatment of a spectrum of neurological disorders including spinal cord injury. Previously, the limited functional recovery of these cells, as characterized by a diminished ability to initiate action potentials and to exhibit repetitive firing patterns, has arisen as a major impediment to their physiological relevance. In this report, we demonstrate that single temporal doses of the neurotransmitters serotonin, glutamate (N-acetyl-DL-glutamic acid) and acetylcholine-chloride lead to the full electrophysiological functional recovery of adult mammalian spinal cord neurons, when they are cultured under defined serum-free conditions. Approximately 60% of the neurons treated regained their electrophysiological signature, often firing single, double and, most importantly, multiple action potentials.

The adrenaline microinjection into the median raphe nucleus induced hypophagic effect in rats submitted to food restriction regimen

The effect of the equimolar doses (6, 20 and 60 nmol) of either adrenaline (AD) or noradrenaline (NA) microinjected into the median raphe nucleus (MR) on feeding behavior of food-restricted rats (15 g/day/rat) was investigated. The data indicated that 20 nmol AD microinjection, but not NA, into the MR decreased the animal food intake. This hypophagic effect induced by AD may be ascribed to a feeding bout conclusion (satiation process) and not to any changes in non-ingestive behaviors induced by drug microinjection. Since equimolar doses of NA failed to change the animal feeding behavior, it is possible to say that AD-induced hypophagia may be due to either changes in tonic stimulatory control exerted by endogenous noradrenaline on MR or to AD-beta(2) receptor activation in the MR. We claim that such activation may be much more importantly exerted by adrenaline-containing afferents to MR neurons involved with ingestive behavior than by noradrenergic inputs.

Activity of the serotonergic system during isoflurane anesthesia

BACKGROUND

Microdialysis studies have demonstrated that the release of serotonin (5-hydroxytryptamine, 5-HT) in the serotonergic projection areas increases during waking and decreases during sleep in rat and cat, suggesting that 5-HT plays an important role in modulation of sleep. Although it might be expected that 5-HT release is also decreased during general anesthesia, the functional contribution of serotonergic neurons in pharmacological effects of volatile anesthetics has not been fully investigated.

METHODS

Using an in vivo microdialysis technique, we measured extracellular 5-HT in rat frontal cortex during waking, slow-wave sleep, and isoflurane anesthesia. To assess the involvement of the serotonergic system in the hypnotic action of isoflurane, the concentration of isoflurane required for loss of righting reflex was determined with or without pretreatment of fluoxetine hydrochloride, a selective 5-HT reuptake inhibitor.

RESULTS

During slow-wave sleep and isoflurane anesthesia (0.1-1.5 MAC), 5-HT release decreased to 21%-44% of that during the waking state. Loss of righting reflex occurred at significantly higher isoflurane concentrations in fluoxetine-treated rats (0.76% +/- 0.03% [n = 8]) than in control rats (0.60% +/- 0.01% [n = 8]).

CONCLUSIONS

It is suggested that a change in the activity of the serotonergic system in the brain is involved in the hypnotic action of isoflurane.

Characterization of rat rostral raphe primary cultures: multiplex quantification of serotonergic markers

Previous reports establishing raphe cultures typically yield less than 1% serotonin (5-HT)-positive neurons and are impractical for transcriptional studies. In this study, we have established primary cultures enriched in 5-HT neurons and quantified the proportion of cells expressing serotonergic and non-serotonergic markers. We have also shown the feasibility of using the multiplex real-time PCR technique to measure the relative amounts of RNA for some of these markers. Rostral raphe cells derived from E13-15 rat embryos were cultured for 7 days and analyzed by quantitative immunofluorescence and western blot analysis. In these cultures, approximately 8% of neurons were immunopositive for serotonergic markers (5-HT or tryptophan hydroxylase (TPH)). The percentage of cells labeled for GFAP (glial marker), tyrosine hydroxylase (catecholaminergic), and GAD65/67 (GABAergic) was 5, 1, and 54%, respectively. Transcription factors REST/NRSF and Deaf-1 were present in 9 and 98% of cells, respectively. Multiplex quantitative RT-PCR (Q-PCR) analysis was done for TPH2, 5-HT1A receptor or Deaf-1 RNAs paired with GAPDH RNA as control. Using this approach, standard curves for each RNA were obtained over 200-fold concentration range of dilution with r2 values >0.99. The relative abundances determined by Q-PCR are consistent with the expression of TPH2>Deaf-1>5-HT1A receptor RNA in serotonergic raphe cells. The standard error of TPH2 RNA levels between cultures was <20%, indicating a consistent purity of 5-HT neurons. Thus, we have generated a highly consistent and reproducible model system that is enriched in 5-HT neurons and that will be valuable in future investigation of serotonergic regulation.

Selective 5-HT receptor inhibition of glutamatergic and GABAergic synaptic activity in the rat dorsal and median raphe

The dorsal (DR) and median (MR) raphe nuclei contain 5-hydroxytryptamine (5-HT) cell bodies that give rise to the majority of the ascending 5-HT projections to the forebrain. The DR and MR have differential roles in mediating stress, anxiety and depression. Glutamate and GABA activity sculpt putative 5-HT neuronal firing and 5-HT release in a seemingly differential manner in the MR and DR, yet isolated glutamate and GABA activity within the DR and MR has not been systematically characterized. Visualized whole-cell voltage-clamp techniques were used to record excitatory and inhibitory postsynaptic currents (EPSC and IPSC) in 5-HT-containing neurons. There was a regional variation in action potential-dependent (spontaneous) and basal [miniature (m)] glutamate and GABAergic activity. mEPSC activity was greater than mIPSC activity in the DR, whereas in the MR the mIPSC activity was greater. These differences in EPSC and IPSC frequency indicate that glutamatergic and GABAergic input have distinct cytoarchitectures in the DR and MR. 5-HT(1B) receptor activation decreased mEPSC frequency in the DR and the MR, but selectively inhibited mIPSC activity only in the MR. This finding, in concert with its previously described function as an autoreceptor, suggests that 5-HT(1B) receptors influence the ascending 5-HT system through multiple mechanisms. The disparity in organization and integration of glutamatergic and GABAergic input to DR and MR neurons and their regulation by 5-HT(1B) receptors may contribute to the distinction in MR and DR regulation of forebrain regions and their differential function in the aetiology and pharmacological treatment of psychiatric disease states.

Elevated mazes as animal models of anxiety: effects of serotonergic agents

This article reviews reported results about the effects of drugs that act upon the serotonergic neurotransmission measured in three elevated mazes that are animal models of anxiety. A bibliographic search has been performed in MEDLINE using different combinations of the key words X-maze, plus-maze, T-maze, serotonin and 5-HT, present in the title and/or the abstract, with no time limit. From the obtained abstracts, several publications were excluded on the basis of the following criteria: review articles that did not report original results, species other than the rat, intracerebral drug administration alone, genetically manipulated rats, and animals having any kind of experimental pathology. The reported results indicate that the effect of drugs on the inhibitory avoidance task performed in the elevated T-maze and on the spatio temporal indexes of anxiety measured in the X and plus mazes correlate with their effect in patients diagnosed with generalized anxiety disorder. In contrast, the drug effects on the one-way escape task in the elevated T-maze predict the drug response of panic disorder patients. Overall, the drug effects assessed with the avoidance task in the T-maze are more consistent than those measured through the anxiety indexes of the X and plus mazes. Therefore, the elevated T-maze is a promising animal model of generalized anxiety and panic disorder.

5,7-dihydroxytryptamine lesion of the dorsal raphe nucleus alters neuronal activity of the subthalamic nucleus in normal and 6-hydroxydopamine-lesioned rats

The subthalamic nucleus receives serotonergic projections from the dorsal raphe nucleus. However, the role of serotonergic innervation in the activity of subthalamic neurons in vivo is unknown. The aim of the present work is to study the changes in the firing of subthalamic neurons in rats with 5,7-dihydroxytryptamine lesions of the dorsal raphe nucleus and rats with combined 5,7-dihydroxytryptamine lesions in the dorsal raphe nucleus and 6-hydroxydopamine lesions in the substantia nigra pars compacta by using single-unit extracellular recordings. In rats with 5,7-dihydroxytryptamine lesions of the dorsal raphe nucleus, the firing rate of subthalamic neurons increased significantly compared with normal rats and the firing pattern changed significantly towards a more bursting firing in the majority of the neurons observed. In rats with combined dorsal raphe nucleus and substantia nigra pars compacta lesions, the firing rate and firing pattern of subthalamic neurons did not show a significant difference compared to rats with lesions of the substantia nigra pars compacta. However, dorsal raphe nucleus and substantia nigra pars compacta lesions combined increased significantly the percentage of subthalamic neurons with burst-firing pattern compared to normal rats, while having no effect on their firing rate. These results show that the serotonergic efferent projections of the dorsal raphe nucleus significantly influence on the activity of subthalamic neurons and that the loss of dopaminergic projection by substantia nigra pars compacta lesion decreases the effect of the lesions of the dorsal raphe nucleus on subthalamic nucleus neuronal activity, suggesting that the role of the dorsal raphe nucleus may be exerted by the dorsal raphe nucleus-substantia nigra pars compacta-subthalamic nucleus pathway.

Serotonin and psychostimulant addiction: Focus on 5-HT1A-receptors

Serotonin(1A)-receptors (5-HT(1A)-Rs) are important components of the 5-HT system in the brain. As somatodendritic autoreceptors they control the activity of 5-HT neurons, and, as postsynaptic receptors, the activity in terminal areas. Cocaine (COC), amphetamine (AMPH), methamphetamine (METH) and 3,4-methylenedioxymethamphetamine ("Ecstasy", MDMA) are psychostimulant drugs that can lead to addiction-related behavior in humans and in animals. At the neurochemical level, these psychostimulant drugs interact with monoamine transporters and increase extracellular 5-HT, dopamine and noradrenalin activity in the brain. The increase in 5-HT, which, in addition to dopamine, is a core mechanism of action for drug addiction, hyperactivates 5-HT(1A)-Rs. Here, we first review the role of the various 5-HT(1A)-R populations in spontaneous behavior to provide a background to elucidate the contribution of the 5-HT(1A)-Rs to the organization of psychostimulant-induced addiction behavior. The progress achieved in this field shows the fundamental contribution of brain 5-HT(1A)-Rs to virtually all behaviors associated with psychostimulant addiction. Importantly, the contribution of pre- and postsynaptic 5-HT(1A)-Rs can be dissociated and frequently act in opposite directions. We conclude that 5-HT(1A)-autoreceptors mainly facilitate psychostimulant addiction-related behaviors by a limitation of the 5-HT response in terminal areas. Postsynaptic 5-HT(1A)-Rs, in contrast, predominantly inhibit the expression of various addiction-related behaviors directly. In addition, they may also influence the local 5-HT response by feedback mechanisms. The reviewed findings do not only show a crucial role of 5-HT(1A)-Rs in the control of brain 5-HT activity and spontaneous behavior, but also their complex role in the regulation of the psychostimulant-induced 5-HT response and subsequent addiction-related behaviors.

Brain serotonin transporter binding in depressed patients with bipolar disorder using positron emission tomography

CONTEXT

Depression in bipolar disorder is clinically indistinguishable from that observed in major depressive disorder. As in major depression, selective serotonin reuptake inhibitors targeting brain serotonin transporters are first-line treatments for bipolar depression. Associations of serotonin transporter promoter polymorphisms and bipolarity have been reported; however, research on alterations in serotonergic neurotransmission in bipolar depression remains scant.

OBJECTIVES

To assess in vivo brain serotonin transporter binding potential (BP(1), proportional to serotonin transporter number) in patients with bipolar depression and controls and to examine the relationship between serotonin transporter binding and genotype.

DESIGN

Case-control study.

SETTING

University hospital.

PARTICIPANTS

A sample of 18 medication-free patients with bipolar depression and 41 controls.

MAIN OUTCOME MEASURES

In vivo brain serotonin transporter binding was measured using positron emission tomography and radiolabeled trans-1,2,3,5,6,10-beta-hexahydro-6-[4-(methylthio)phenyl]pyrrolo-[2,1-a]-isoquinoline ([(11)C](+)-McNeil 5652). Participants were genotyped assessing biallelic and triallelic 5-HTTLPR polymorphisms.

RESULTS

Patients with bipolar disorder had 16% to 26% lower serotonin transporter BP(1) in the midbrain, amygdala, hippocampus, thalamus, putamen, and anterior cingulate cortex. Triallelic 5-HTTLPR genotypes were unrelated to serotonin transporter BP(1).

CONCLUSIONS

Lower serotonin transporter BP(1) in bipolar depression overlaps with that observed in major depression and suggests that serotonergic dysfunction is common to depressive conditions.

Desensitization of 5-HT(1A) autoreceptors induced by neonatal DSP-4 treatment

To examine the effect of noradrenergic lesion on the reactivity of central 5-HT(1A) receptors, DSP-4 (50 mg/kg) was administered neonatally 30 min after zimelidine (10 mg/kg) administration. 5-HT(1A) autoreceptors are involved in the regulation of serotonin (5-HT) synthesis. In HPLC assay R-(+)-8-OH-DPAT (0.03 mg/kg) significantly decreased 5-HT synthesis rate in striatum, hypothalamus and frontal cortex of control, whilst nonsignificantly in DSP-4-lesioned adult rats (10-12 weeks old). To determine which type of receptor, pre- or postsynaptically located, is involved in the attenuated response to 5-HT(1A) receptors' agonist, behavioral tests were conducted. R-(+)-8-OH-DPAT (0.015 mg/kg) caused hyperphagia of control rats, but did not change feeding of DSP-4 treated rats. R-(+)-8-OH-DPAT (0.1 mg/kg) induced hypothermia and "5-HT(1A) syndrome" in both control and DSP-4-lesioned animals. The nature of this phenomenon is attributable to the presynaptic adaptive mechanism and suggests the desensitization of 5-HT(1A) autoreceptors of rats with neonatal lesion of the central noradrenergic system.

[18F]MPPF as a tool for the in vivo imaging of 5-HT1A receptors in animal and human brain

Serotonin (5-hydroxytryptamine, 5-HT) and its various receptors are involved in numerous CNS functions and psychiatric disorders. 5-HT(1A), the best-characterized subtype of currently known 5-HT receptors, is tightly implicated in the pathogenesis of depression, anxiety, epilepsy and eating disorders. It thus represents an important target for drug therapy. Specific radioligands and positron emission tomography (PET) allow for a quantitative imaging of brain 5-HT(1A) receptor distribution in living animals and humans. Recently, the selective 5-HT(1A) receptor antagonist, MPPF, has been successfully labeled with [(18)F]fluorine ([(18)F]MPPF), and an increasing number of academic and industry centres have used this radiotracer in preclinical and clinical studies. After a brief account of some of the structural, distributional and electrophysiological characteristics of brain 5-HT(1A) receptors, this review focuses on studies conducted with [(18)F]MPPF, with emphasis on preclinical results illustrating the actual and potential value of this PET radioligand for clinical research and drug development.