Release of cerebral 5-hydroxytryptamine evoked by electrical stimulation of the dorsal and median raphe nuclei: effect of a neurotoxic amphetamine

The Serotonin 4 Receptor Subtype: A Target of Particular Interest, Especially for Brain Disorders

In recent years, particular attention has been paid to the serotonin 4 receptor, which is well expressed in the brain, but also peripherally in various organs. The cerebral distribution of this receptor is well conserved across species, with high densities in the basal ganglia, where they are expressed by GABAergic neurons. The 5-HT4 receptor is also present in the cerebral cortex, hippocampus, and amygdala, where they are carried by glutamatergic or cholinergic neurons. Outside the central nervous system, the 5-HT4 receptor is notably expressed in the gastrointestinal tract. The wide distribution of the 5-HT4 receptor undoubtedly contributes to its involvement in a plethora of functions. In addition, the modulation of this receptor influences the release of serotonin, but also the release of other neurotransmitters such as acetylcholine and dopamine. This is a considerable asset, as the modulation of the 5-HT4 receptor can therefore play a direct or indirect beneficial role in various disorders. One of the main advantages of this receptor is that it mediates a much faster antidepressant and anxiolytic action than classical selective serotonin reuptake inhibitors. Another major benefit of the 5-HT4 receptor is that its activation enhances cognitive performance, probably via the release of acetylcholine. The expression of the 5-HT4 receptor is also altered in various eating disorders, and its activation by the 5-HT4 agonist negatively regulates food intake. Additionally, although the cerebral expression of this receptor is modified in certain movement-related disorders, it is still yet to be determined whether this receptor plays a key role in their pathophysiology. Finally, there is no longer any need to demonstrate the value of 5-HT4 receptor agonists in the pharmacological management of gastrointestinal disorders.

Involvement of serotonin in the ventral tegmental area in thermoregulation of freely moving rats

We have recently reported that the serotonin (5-HT) projections from the midbrain's raphe nuclei that contains 5-HT cell bodies may play a role both in heat production and in heat loss. The purpose of the present study was to clarify the involvement of 5-HT in the ventral tegmental area (VTA), where 5-HT is suggested to participate in thermoregulation, using the combined methods of telemetry, microdialysis, and high performance liquid chromatography, with a special emphasis on regulation of the body temperature (T_b) in freely moving rats. First, we measured changes in T_b, tail skin temperature (T_tail; an index of heat loss), heart rate (HR; an index of heat production), locomotor activity (Act), and levels of extracellular monoamines in the VTA during cold (5°C) or heat (35°C) exposure. Subsequently, we perfused citalopram (5-HT re-uptake inhibitor) into the VTA and measured the thermoregulatory parameters and monoamines release. Although T_b, T_tail, and HR changed during both exposures, significant changes in extracellular level of 5-HT (138.7±12.7% baseline, p<0.01), but not dopamine (DA) or noradrenaline (NA) were noted in the VTA only during heat exposure. In addition, perfusion of citalopram into the VTA increased extracellular 5-HT levels (221.0±52.2% baseline, p<0.01), but not DA or NA, while T_b decreased from 37.4±0.1°C to 36.8±0.2°C (p<0.001),T_tail increased from 26.3±0.4°C to 28.4±0.4°C (p<0.001), and HR and Act remained unchanged. Our results suggest that the VTA is a key area for thermoregulation, and 5-HT, but not DA or NA, modulates the heat loss system through action in the VTA.

Pons to Posterior Cingulate Functional Projections Predict Affective Processing Changes in the Elderly Following Eight Weeks of Meditation Training

Evidence indicates meditation facilitates affective regulation and reduces negative affect. It also influences resting-state functional connectivity between affective networks and the posterior cingulate (PCC)/precuneus, regions critically implicated in self-referential processing. However, no longitudinal study employing active control group has examined the effect of meditation training on affective processing, PCC/precuneus connectivity, and their association. Here, we report that eight-week meditation, but not relaxation, training 'neutralized' affective processing of positive and negative stimuli in healthy elderly participants. Additionally, meditation versus relaxation training increased the positive connectivity between the PCC/precuneus and the pons, the direction of which was largely directed from the pons to the PCC/precuneus, as revealed by dynamic causal modeling. Further, changes in connectivity between the PCC/precuneus and pons predicted changes in affective processing after meditation training. These findings indicate meditation promotes self-referential affective regulation based on increased regulatory influence of the pons on PCC/precuneus, which new affective-processing strategy is employed across both resting state and when evaluating affective stimuli. Such insights have clinical implications on interventions on elderly individuals with affective disorders.

Multiscale single-cell analysis reveals unique phenotypes of raphe 5-HT neurons projecting to the forebrain

Serotonergic neurons of the raphe nuclei exhibit anatomical, neurochemical and elecrophysiological heterogeneity that likely underpins their specific role in multiple behaviors. However, the precise organization of serotonin (5-HT) neurons to orchestrate 5-HT release patterns throughout the brain is not well understood. We compared the electrophysiological and neurochemical properties of dorsal and median raphe 5-HT neurons projecting to the medial prefrontal cortex (mPFC), amygdala (BLA) and dorsal hippocampus (dHP), combining retrograde tract tracing with brain slice electrophysiology and single-cell RT-PCR in Pet1-EGFP mice. Our results show that 5-HT neurons projecting to the dHP and the mPFC and the BLA form largely non-overlapping populations and that BLA-projecting neurons have characteristic excitability and membrane properties. In addition, using an unbiased clustering method that correlates anatomical, molecular and electrophysiological phenotypes, we find that 5-HT neurons with projections to the mPFC and the dHP segregate from those projecting to the BLA. Single-cell gene profiling showed a restricted expression of the peptide galanin in the population of 5-HT neurons projecting to the mPFC. Finally, cluster analysis allowed identifying an atypical subtype of 5-HT neuron with low excitability, long firing delays and preferential expression of the vesicular glutamate transporter type 3. Overall, these findings allow to define correlated anatomical and physiological identities of serotonin raphe neurons that help understanding how discrete raphe cells subpopulations account for the heterogeneous activities of the midbrain serotonergic system.

Tyrosine-free amino acid mixtures reduce physiologically-evoked release of dopamine in a selective and activity-dependent manner

Depletion of the catecholamine precursor tyrosine using tyrosine-free amino acid mixtures is an important tool in neuropsychological studies, and often considered dopamine selective on the basis of neuropharmacological studies. However, little is known of the effects of tyrosine depletion when catecholamine neurons are activated physiologically. Here we investigated the effect of tyrosine-free amino acid mixtures on catecholamine release evoked in vivo using a stimulation paradigm aimed to approximate the phasic firing pattern of these neurons that accompanies cognitive and behavioural change. Dopamine and noradrenaline release was monitored by microdialysis in rat medial prefrontal cortex (mPFC) and striatum (chloral hydrate anaesthesia, perfusion medium containing 1 µM cocaine). Electrical stimulation of the medial forebrain bundle (MFB) caused a short-lasting, frequency-dependent increase in dopamine and noradrenaline. A full tyrosine-free amino acid mixture reduced the release of dopamine in mPFC and striatum, across a range of stimulation frequencies, and the effect was greater as stimulation frequency increased. Similar results were obtained using a smaller tyrosine-free amino acid mixture. In the same experiments showing decreased dopamine, neither tyrosine-free mixture of amino acids significantly altered stimulation-evoked release of noradrenaline. These results show that tyrosine depletion using tyrosine-free amino acid mixtures causes a selective, activity-dependent decrease in dopamine release when dopamine neurons are driven physiologically.

Serotonergic pharmacology in animal models: from behavioral disorders to dyskinesia

Serotonin (5-HT) dysfunction has been involved in both movement and behavioral disorders. Serotonin pharmacology improves dyskinetic movements as well as depressive, anxious, aggressive and anorexic symptoms. Animal models have been useful to investigate more precisely to what extent 5-HT is involved and whether drugs targeting the 5-HT system can counteract the symptoms exhibited. We review existing rodent and non-human primate (NHP) animal models in which selective 5-HT or dual 5-HT-norepinephrine (NE) transporter inhibitors, as well as specific 5-HT receptors agonists and antagonists, monoamine oxidase A inhibitors (IMAO-A) and MDMA (Ecstasy) have been used. We review overlaps between the various drug classes involved. We confront behavioral paradigms and treatment regimen. Some but not all animal models and associated pharmacological treatments have been extensively studied in the litterature. In particular, the impact of selective serotonin reuptake inhibitors (SSRI) has been extensively investigated using a variety of pharmacological or genetic rodent models of depression, anxiety, aggressiveness. But the validity of these rodent models is questioned. On the contrary, few studies did address the potential impact of targeting the 5-HT system on NHP models of behavioral disorders, despite the fact that those models may match more closely to human pathologies. Further investigations with carefull behavioral analysis will improve our understanding of neural bases underlying the pathophysiology of movement and behavioral disorders.

Effect of Mimosa pudica (Linn.) extract on anxiety behaviour and GABAergic regulation of 5-HT neuronal activity in the mouse

Mimosa pudica (Linn.) (M. pudica L.) is a plant used in some countries to treat anxiety and depression. In the present study we investigated the effects of an aqueous extract of M. pudica L. on mouse anxiety-like behaviour using the elevated T maze, and on regulation of dorsal raphe nucleus (DRN) 5-hydroxytryptamine (5-HT) neuronal activity using an in-vitro mouse brain slice preparation. Acute treatment with M. pudica L. extract had an anxiolytic effect on behaviour in the elevated T maze, specifically on inhibitory avoidance behaviour. Acute application of the extract alone had no effect on the activity of DRN 5-HT neurones. However, when co-applied with the GABA(A) receptor agonist THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol), the extract enhanced the inhibitory effect of the THIP on DRN 5-HT neurones. These observed effects of M. pudica L. on both behaviour and GABA modulation of 5-HT neuronal activity are similar to the effects of diazepam, the established anxiolytic and positive modulator of the GABA(A) receptor. This study suggests that the aqueous extract of M. pudica L. contains a positive modulator of GABA(A) receptor function and provides impetus for further investigation of the neuropharmacologically active constituents of the extract.

A combined in vivo neurochemical and electrophysiological analysis of the effect of high-frequency stimulation of the subthalamic nucleus on 5-HT transmission

Movement disability in advanced Parkinson's disease (PD) can be treated by high frequency stimulation (HFS) of the subthalamic nucleus (STN) but some patients experience psychiatric side-effects including depression, which is strongly linked to decreases in 5-hydroxytryptamine (5-HT). The current study investigated the effect of bilateral STN HFS on extracellular 5-HT in brain regions of anesthetized and freely moving rats as measured with microdialysis. Parallel in vivo electrophysiological experiments allowed a correlation of changes in extracellular 5-HT with the firing of 5-HT neurons. Bilateral STN HFS decreased (by up to 25%) extracellular levels of 5-HT in both striatum and medial prefrontal cortex of anesthetized rats. STN HFS also decreased extracellular 5-HT in the medial prefrontal cortex of freely moving rats. This decrease in extracellular 5-HT persisted after turning off the stimulation, and was present in dopamine-denervated rats. As with changes in extracellular 5-HT, in anesthetized rats STN HFS evoked a decrease in the in vivo firing of midbrain raphe 5-HT neurons that also persisted after cessation of stimulation. These data provide neurochemical evidence for an inhibition of 5-HT neurotransmission by STN HFS, which may contribute to its psychiatric side effects and guide therapeutic options.

Serotonin and prefrontal cortex function: neurons, networks, and circuits

Higher-order executive tasks such as learning, working memory, and behavioral flexibility depend on the prefrontal cortex (PFC), the brain region most elaborated in primates. The prominent innervation by serotonin neurons and the dense expression of serotonergic receptors in the PFC suggest that serotonin is a major modulator of its function. The most abundant serotonin receptors in the PFC, 5-HT1A, 5-HT2A and 5-HT3A receptors, are selectively expressed in distinct populations of pyramidal neurons and inhibitory interneurons, and play a critical role in modulating cortical activity and neural oscillations (brain waves). Serotonergic signaling is altered in many psychiatric disorders such as schizophrenia and depression, where parallel changes in receptor expression and brain waves have been observed. Furthermore, many psychiatric drug treatments target serotonergic receptors in the PFC. Thus, understanding the role of serotonergic neurotransmission in PFC function is of major clinical importance. Here, we review recent findings concerning the powerful influences of serotonin on single neurons, neural networks, and cortical circuits in the PFC of the rat, where the effects of serotonin have been most thoroughly studied.

Functional organization of the dorsal raphe efferent system with special consideration of nitrergic cell groups

The serotonin (5HT) system of the brain is involved in many CNS functions including sensory perception, stress responses and psychological disorders such as anxiety and depression. Of the nine 5HT nuclei located in the mammalian brain, the dorsal raphe nucleus (DRN) has the most extensive forebrain connectivity and is implicated in the manifestation of stress-related psychological disturbances. Initial investigations of DRN efferent connections failed to acknowledge the rostrocaudal and mediolateral organization of the nucleus or its neurochemical heterogeneity. More recent studies have focused on the non-5HT contingent of DRN cells and have revealed an intrinsic intranuclear organization of the DRN which has specific implications for sensory signal processing and stress responses. Of particular interest are spatially segregated subsets of nitric oxide producing neurons that are activated by stressors and that have unique efferent projection fields. In this regard, both the midline and lateral wing subregions of the DRN have emerged as prominent loci for future investigation of nitric oxide function and modulation of sensory- and stressor-related signals in the DRN and coinciding terminal fields.

Inhibition by 5-HT of the synaptic responses evoked by callosal fibers on cortical neurons in the mouse

We have studied the modulation by 5-HT of the synaptic excitatory responses evoked by callosal fibers on cortical pyramidal neurons. We have used a mouse brain slice preparation that preserves the callosal fibers and allows their selective activation. EPSCs evoked by callosal stimulation (ccEPSCs) were recorded with patch electrodes from pyramidal neurons identified visually. We observed that 5-HT (10-40 μM) inhibited the ccEPSCs peak amplitude in 64% of the neurons; 5-HT had no effect in the remaining neurons. 5-HT also increased the frequency and amplitude of spontaneous EPSCs. This inhibition was accompanied with an increase in the coefficient of variation of the fluctuations of the ccEPSCs amplitude and with an increase in the ratio of the amplitudes of paired ccEPSCs. Agonists of 5-HT receptor subtypes 5-HT(1A) (8-OH-DPAT) and 5-HT(2A) (DOI) mimicked the effect of 5-HT; also, the effect of 8-OH-DPAT and DOI was blocked in the presence of specific blockers of 5-HT(1A) (WAY 100135) and 5-HT(2A) (MDL 11,939) receptors. Application of 5-HT did not change the amplitude of currents evoked by direct application of glutamate to neurons in which 5-HT decreased the amplitude of ccEPSC. The effects of 5-HT on ccEPSCs and on the synaptic currents evoked by intracortical stimulation were not correlated; this suggests that the effect of 5-HT was specific to particular synaptic inputs to a neuron. These results demonstrate the presynaptic modulation of the callosal synaptic responses by 5-HT and the implication of 5-HT(1A) and 5-HT(2A) receptors in this effect.

Serotonin modulates fast-spiking interneuron and synchronous activity in the rat prefrontal cortex through 5-HT1A and 5-HT2A receptors

Alterations of the serotonergic system in the prefrontal cortex (PFC) are implicated in psychiatric disorders such as schizophrenia and depression. Although abnormal synchronous activity is observed in the PFC of these patients, little is known about the role of serotonin (5-HT) in cortical synchrony. We found that 5-HT, released by electrical stimulation of the dorsal raphe nucleus (DRN) in anesthetized rats, regulates the frequency and the amplitude of slow (<2 Hz) waves in the PFC via 5-HT(2A) receptors (5-HT(2A)Rs). 5-HT also modulates prefrontal gamma (30-80 Hz) rhythms through both 5-HT(1A)Rs and 5-HT(2A)Rs, but not 5-HT(2C)Rs, inducing an overall decrease in the amplitude of gamma oscillations. Because fast-spiking interneurons (FSi) are involved in the generation of gamma waves, we examined serotonergic modulation of FSi activity in vivo. Most FSi are inhibited by serotonin through 5-HT(1A)Rs, while a minority is activated by 5-HT(2A)Rs, and not 5-HT(2C)Rs. In situ hybridization histochemistry confirmed that distinct populations of FSi in the PFC express 5-HT(1A)Rs and 5-HT(2A)Rs, and that the number of FSi expressing 5-HT(2C)Rs is negligible. We conclude that 5-HT exerts a potent control on slow and gamma oscillations in the PFC. On the one hand, it shapes the frequency and amplitude of slow waves through 5-HT(2A)Rs. On the other hand, it finely tunes the amplitude of gamma oscillations through 5-HT(2A)R- and 5-HT(1A)R-expressing FSi, although it primarily downregulates gamma waves via the latter population. These results may provide insight into impaired serotonergic control of network activity in psychiatric illnesses such as schizophrenia and depression.

Decrease in REM latency and changes in sleep quality parallel serotonergic damage and recovery after MDMA: a longitudinal study over 180 days

The recreational drug ecstasy [3,4-methylenedioxymethamphetamine (MDMA)], has been found to selectively damage brain serotonin neurons in experimental animals, and probably in human MDMA users, but detailed morphometric analyses and parallel functional measures during damage and recovery are missing. Since there is evidence that serotonin regulates sleep, we have compared serotonergic markers parallel with detailed analysis of sleep patterns at three time-points within 180 d after a single dose of 15 mg/kg MDMA in male Dark Agouti rats. At 7 d and 21 d after MDMA treatment, significant(30-40%), widespread reductions in serotonin transporter (5-HTT) density were detected in the cerebral cortex, hippocampus, most parts of the hypothalamus, and some of the brainstem nuclei. With the exception of the hippocampus, general recovery was observed in the brain 180 d after treatment. Transient increases followed by decreases were detected in 5-HTT mRNA expression of dorsal and median raphe nuclei at 7 d and 21 d after the treatment. Significant reductions in rapid eye movement (REM) sleep latency, increases in delta power spectra in non-rapid eye movement sleep and increased fragmentation of sleep were also detected, but all these alterations disappeared by the 180th day. The present data provide evidence for long-term, albeit, except for the hippocampus, transient changes in the terminal and cellular regions of the serotonergic system after this drug. Reduced REM latency and increased sleep fragmentation are the most characteristic alterations of sleep consistently described in depression using EEG sleep polygraphy.

Altered depression-related behavior and neurochemical changes in serotonergic neurons in mutant R406W human tau transgenic mice

Mutant R406W human tau was originally identified in frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) and causes a hereditary tauopathy that clinically resembles Alzheimer's disease (AD). In the current study, we examined the performance of R406W transgenic (Tg) mice in the forced swimming test, a test with high predictivity of antidepressant efficacy in human depression, and found an enhancement of the immobility time. In contrast, the motor function and anxiety-related emotional response of R406W Tg mice were normal. Furthermore, a selective serotonin reuptake inhibitor (SSRI), fluvoxamine (100 mg/kg, p.o.), significantly reduced this enhancement of the immobility time, whereas a noradrenaline reuptake inhibitor, desipramine, had no effect. In an in vivo microdialysis study, R406W Tg mice exhibited a significantly decreased extracellular 5-hydroxyindoleacetic acid (5-HIAA) level in the frontal cortex and also exhibited a tendency toward a decreased extracellular 5-hydroxytryptamine (5-HT) level. Moreover, fluvoxamine, which reduced the enhancement of the immobility time, significantly increased the extracellular 5-HT level in R406W Tg mice. These results suggest that R406W Tg mice exhibit changes in depression-related behavior involving serotonergic neurons and provide an animal model for investigating AD with depression.

Differential involvement of serotonin and dopamine systems in cost-benefit decisions about delay or effort

RATIONALE

Although tasks assessing the role of dopamine in effort-reward decisions are similar to those concerned with the role of serotonin in impulsive choice in that both require analysis of the costs and benefits of possible actions, they have never been directly compared.

OBJECTIVES

This study investigated the involvement of serotonin and dopamine in two cost-benefit paradigms, one in which the cost was delay and the other in which it was physical effort.

METHODS

Sixteen rats were trained on a T-maze task in which they chose between high and low reward arms. In one version, the high reward arm was obstructed by a barrier, in the other, delivery of the high reward was delayed by 15 s. Serotonin and dopamine function were manipulated using systemic pCPA and haloperidol injections, respectively.

RESULTS

Haloperidol-treated rats were less inclined either to exert more effort or to countenance a delay for a higher reward. pCPA had no effect on the performance of the rats on the effortful task, but significantly increased the rats' preference for an immediate but smaller reward. All animals (drug treated and controls) chose the high reward arm on the majority of trials when the delay or effort costs were matched in both high and low reward arms.

CONCLUSION

A dissociation was found between the neurotransmitter systems involved in different types of cost-benefit decision making. While dopaminergic systems were required for decisions about both effort and delay, serotonergic systems were only needed for the latter.

Increased behavioural activity of rats in forced swimming test after partial denervation of serotonergic system by parachloroamphetamine treatment

The present study aimed at characterizing the effect of partial 5-HT denervation by parachloroamphetamine (PCA), a 5-HT selective neurotoxin, on forced swimming behaviour and monoamine levels in several rat brain regions. PCA was administered intraperitoneally in two independent experiments in doses of 2, 4 and 6 mg/kg and in doses 1, 2, 4 mg/kg, respectively. PCA (2 mg/kg) reduced immobility in the forced swimming test in the Experiment 1 and according to Experiment 2 this is explained by increased swimming time. Dose-dependent reductions in 5-HT and 5-HIAA levels were found in all brain regions studied, and the maximal effects were of a similar magnitude. In septum, the effect of PCA took more time to develop. The effects of the lowest dose of PCA suggest that the neurotoxin affects not only the dorsal raphe projection areas but also the fine axons which arise from the median raphe. alpha2-Adrenoceptors and beta-adrenoceptors in cerebral cortex were not affected by the PCA treatment. Binding affinity of the 5-HT(1A) receptors was higher after all doses of PCA. On the second exposure to the forced swimming the time spent in swimming was found to be negatively and the time spent in immobile posture positively correlated with serotonin turnover in frontal cortex. The time spent in struggling on the second exposure to test was found to be negatively correlated with KD of beta-adrenoceptor binding in cerebral cortex. These data suggest that partial 5-HT denervation with low doses of PCA, which elicits a specific pattern of neurodegeneration, results in an increased behavioural activity, and that the traditional interpretation of the measures in forced swimming test, despite of the test's predictive power in revealing antidepressants acting on monoaminergic systems, is not adequate for studies on the neurochemical basis of depression.

Modulation of dorsal spinocerebellar responses to limb movement. I. Effect of serotonin

Spinocerebellar neurons (DSCT) receive converging sensory information from various sensory receptors in the hindlimbs and lower trunk. Previous studies have shown that sensory processing by DSCT neurons results in a representation of global hindlimb kinematic parameters such as the length and the orientation of the limb axis. In addition to the sensory input, the DSCT circuitry also receives a descending input from the raphe nuclei in the brain stem. Recent studies have demonstrated that the raphe serotonergic terminals synapse directly on DSCT neurons and exert a differential modulatory influence on their sensory inputs. We examined the role of serotonergic modulation on the DSCT representation of hindlimb kinematic parameters by recording DSCT activity during passive hindlimb movements before and after perturbing serotonergic transmission. We used two types of perturbation: electrical stimulation of the raphe areas in the brain stem to release serotonin in the spinal cord (42 neurons) and intravenous administration of serotonergic agonists or antagonists, mostly the 5HTP2 antagonist ketanserin (30 neurons). We found that movement responses were altered in approximately 70% of the DSCT units studied with each protocol. Changes could include shifts in mean firing rate, increases or decreases in response amplitude, and changes in response waveform. We used a principal component analysis (PCA) to examine waveform components and to determine how they contributed to the response waveform changes caused by serotonin perturbation. Such changes could be explained by new or different response components that might indicate a modification in the data processing or by a different weighting of existing components that might indicate a modification of synaptic weighting. The results were consistent with the second alternative. We found that the same underlying response components could account for both control responses and those altered by serotonin perturbations. The observed changes in waveform could be entirely accounted for by a re-weighting of response components. In particular, the changes observed after raphe stimulation could be accounted for by selective changes in the weighting of the first principal component (PC) with only minor changes of the weighting of the second PC. Because these response components were shown previously to correlate with the limb axis orientation and length trajectories respectively, the finding is consistent with the idea that limb axis length and orientation information are processed separately within the spinal circuitry.

Displacement of the PET ligand 18F-MPPF by the electrically evoked serotonin release in the rat hippocampus

The effects of the electrically evoked serotonin release were evaluated on the binding of (18)F-MPPF in the hippocampus of anesthetized rats. The specific binding of (18)F-MPPF was measured by an implanted beta-microprobe and the serotonin (5-HT) extracellular concentration was measured by microdialysis under the same conditions. Our results showed that the 10-, 20-, or 30-min electrical stimulation of the raphe nucleus elicited a significant increase in extracellular 5-HT, only detectable in the presence of a 5-HT reuptake inhibitor in the perfusate (5 microM clomipramine). Interestingly, the raphe stimulations were associated with a 27-76% reversible decrease of the (18)F-MPPF specific binding in the hippocampus, but an unchanged extracellular (18)F-MPPF collected in dialysates. Considered together, these observations suggest that (18)F-MPPF binding is sensitive to 5-HT released at a neuronal level. This compartment, explored by the beta-microprobe, is probably distinct from the extracellular compartment, explored by microdialysis.

Effects of low-frequency cranial electrostimulation on the rest-activity rhythm and salivary cortisol in Alzheimer's disease

OBJECTIVE

In previous studies, cranial electrostimulation (CES) had positive effects on sleep in depressed patients and in patients with vascular dementia. The present study examined the effects of low-frequency CES on the rest-activity rhythm and cortisol levels of patients with probable Alzheimer's disease (AD).

METHOD

It was hypothesised that a decreased level of cortisol would parallel a positive effect of low-frequency CES on nocturnal restlessness. Sixteen AD patients were randomly assigned to an experimental group (n = 8) or a control group (n = 8). The experimental group was treated with CES, whereas the control group received sham stimulation, for 30 minutes a day, during 6 weeks. The rest-activity rhythm was assessed by actigraphy. Cortisol was measured repeatedly in the saliva throughout the day by means of salivette tubes.

RESULTS

Low-frequency CES did not improve the rest-activity rhythm in AD patients. Moreover, both groups showed an increase instead of a decrease in the level of cortisol.

CONCLUSIONS

These preliminary results suggest that low-frequency CES has no positive effect on the rest-activity rhythm in AD patients. An alternative research design with high-frequency CES in AD is discussed.

Rapid habituation of hippocampal serotonin and norepinephrine release and anxiety-related behaviors, but not plasma corticosterone levels, to repeated footshock stress in rats

Prior stress exposure is known to alter the activation response to a subsequent stressor. In the present study, we examined neurochemical, neuroendocrinological, and behavioral correlates of short-term adaptation to homotypic stressors administered 60 min apart. An initial electric footshock significantly induced extracellular levels of both serotonin (5-HT) and norepinephrine (NE) in the rat hippocampus (650% and 200% above baseline, respectively), as measured by in vivo microdialysis. A rapid habituation in this response was evident in the inability of a second footshock to evoke similar increases. In contrast, the hypothalamic-pituitary-adrenal (HPA) response was augmented further after the second shock session: plasma corticosterone (CORT) levels were 18.1, 316.5, and 441.6 mg/ml in nonstressed, one-footshock-, or two-footshock-treated rats, respectively. In a social interaction paradigm, rats subjected to a single footshock showed several fear- and anxiety-related behaviors such as increases in freezing and decreases in rearing and active approach for social interaction. Exposure to a second footshock completely blocked the freezing response and restored rearing behavior without affecting the disruption in social interactions. Taken together, these data raise the possibility that neurochemical and neuroendocrine adaptations to short-term homotypic stressors differentially contribute to expression of different fear and anxiety-like responses in the rat.

Hypocretins (orexins) regulate serotonin neurons in the dorsal raphe nucleus by excitatory direct and inhibitory indirect actions

The hypocretins (hcrt1 and hcrt2) are expressed by a discrete population of hypothalamic neurons projecting to many regions of the CNS, including the dorsal raphe nucleus (DRN), where serotonin (5-HT) neurons are concentrated. In this study, we investigated responses to hcrts in 216 physiologically identified 5-HT and non-5-HT neurons of the DRN using intracellular and whole-cell recording in rat brain slices. Hcrt1 and hcrt2 induced similar amplitude and dose-dependent inward currents in most 5-HT neurons tested (EC50, approximately 250 nm). This inward current was not blocked by the fast Na+ channel blocker TTX or in a Ca2+-free solution, indicating a direct postsynaptic action. The hcrt-induced inward current reversed near -18 mV and was primarily dependent on external Na+ but not on external or internal Ca2+, features typical of Na+/K+ nonselective cation channels. At higher concentrations, hcrts also increased spontaneous postsynaptic currents in 5-HT neurons (EC50, approximately 450-600 nm), which were TTX-sensitive and mostly blocked by the GABA(A) antagonist bicuculline, indicating increased impulse flow in local GABA interneurons. Accordingly, hcrts were found to increase the basal firing of presumptive GABA interneurons. Immunolabeling showed that hcrt fibers projected to both 5-HT and GABA neurons in the DRN. We conclude that hcrts act directly to excite 5-HT neurons primarily via a TTX-insensitive, Na+/K+ nonselective cation current, and indirectly to activate local inhibitory GABA inputs to 5-HT cells. The greater potency of hcrts in direct excitation compared with indirect inhibition suggests a negative feedback function for the latter at higher levels of hcrt activity.

Electrical stimulation of the dorsal and median raphe nuclei increases extracellular noradrenaline in rat hippocampus: Evidence for a 5-HT-independent mechanism

Recent studies have used raphe stimulation combined with in vivo measurements of extracellular dopamine to investigate interactions between the 5-hydroxytryptamine (5-HT) and dopamine systems. Here we have tested whether the same approach can be used to investigate interactions between the 5-HT and noradrenaline systems. Electrical stimulation of the dorsal raphe nucleus (DRN) or median raphe nucleus (MRN) was performed in anaesthetised rats implanted with microdialysis probes in the hippocampus and locus coeruleus (LC). DRN stimulation (3, 5 and 10 Hz) evoked a frequency-dependent increase in extracellular noradrenaline in the hippocampus. MRN stimulation had a similar effect. Both DRN and MRN stimulations enhanced extracellular 5-HT levels in the LC and previous studies have demonstrated that extracellular 5-HT also increases in the hippocampus. However, the increase in hippocampal noradrenaline evoked by DRN stimulation was not altered by 5-HT neuronal lesions, which reduced 5-HT metabolite levels by 90%. In conclusion, electrical stimulation of the midbrain raphe increases extracellular noradrenaline in the hippocampus, however, experiments in 5-HT-lesioned animals suggest that this response is not mediated by 5-HT. Although raphe stimulation may be useful to investigate interactions between 5-HT and dopamine, our data indicate that the same approach may not be feasible for 5-HT and noradrenaline.

Neurochemical and electrophysiological studies on the functional significance of burst firing in serotonergic neurons

We have previously described a population of 5-hydroxytryptamine neurons which repetitively fires bursts of usually two (but occasionally three or four) action potentials, with a short (<20 ms) interspike interval within a regular low-frequency firing pattern. Here we used a paradigm of electrical stimulation comprising twin pulses (with 7- or 10-ms inter-pulse intervals) to mimic this burst firing pattern, and compared the effects of single- and twin-pulse electrical stimulations in models of pre- and postsynaptic 5-hydroxytryptamine function. Firstly, we measured the effect of direct electrical stimulation (2 Hz for 2 min) of rat brain slices on efflux of preloaded [3H]5-hydroxytryptamine. In this in vitro model, twin-pulse stimulation increased the efflux of tritium by about twice as much as did single-pulse stimulation. This effect was evident in the medial prefrontal cortex (area under the curve: 2. 59+/-0.34 vs 1.28+/-0.22% relative fractional release), as well as in the caudate-putamen (3.93+/-0.65 vs 2.17+/-0.51%) and midbrain raphe nuclei (5.42+/-1.05 vs 2.51+/-0.75%). Secondly, we used in vivo microdialysis to monitor changes in endogenous extracellular 5-hydroxytryptamine in rat medial prefrontal cortex in response to electrical stimulation (3 Hz for 10 min) of the dorsal raphe nucleus. In this model, twin-pulse stimulation of the dorsal raphe nucleus increased 5-hydroxytryptamine by approximately twice as much as did single-pulse stimulation at the same frequency (area under the curve: 50.4+/-9.0 vs 24.2+/-4.4 fmol). Finally, we used in vivo extracellular recording to follow the response of postsynaptic neurons in the rat medial prefrontal cortex to 5-hydroxytryptamine released by dorsal raphe stimulation. Electrical stimulation of the dorsal raphe nucleus (1 Hz) induced a clear-cut poststimulus inhibition in the majority of cortical neurons tested. In these experiments, the duration of poststimulus inhibition following twin-pulse stimulation was markedly longer than that induced by single-pulse stimulation (200+/-21 vs 77+/-18.5 ms). Taken together, the present in vitro and in vivo data suggest that in 5-hydroxytryptamine neurons, short bursts of action potentials will propagate along the axon to the nerve terminal and will enhance both the release of 5-hydroxytryptamine and its postsynaptic effect.

Regulation of serotonin release by GABA and excitatory amino acids

Regulation of serotonin release by gamma-aminobutyric acid (GABA) and glutamate was examined by microdialysis in unanaesthetized rats. The GABA(A) receptor agonist muscimol, or the glutamate receptor agonists kainate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolaproprionate or N-methyl-D-aspartate were infused into the dorsal raphe nucleus (DRN) while extracellular serotonin was measured in the DRN and nucleus accumbens. Muscimol produced decreases, and the glutamate receptor agonists produced increases in serotonin. To determine if these receptors have a tonic influence on serotonergic neurons, glutamate or GABA(A) receptor antagonists were infused into the DRN. Kynurenate, a nonselective glutamate receptor blocker, produced a small, 30% decrease in serotonin. A similar decrease was obtained with combined infusion of AP-5 and DNQX into the DRN. The GABAA receptor blocker bicuculline produced an approximately three-fold increase in DRN serotonin. In conclusion, glutamate neurotransmitters have a weak tonic excitatory influence on serotonergic neurons in the rat DRN. However, the predominate influence is mediated by GABA(A) receptors.

Serotonin and the sleep/wake cycle: special emphasis on microdialysis studies

Several areas in the brainstem and forebrain are important for the modulation and expression of the sleep/wake cycle. Even if the first observations of biochemical events in relation to sleep were made only 40 years ago, it is now well established that several neurotransmitters, neuropeptides, and neurohormones are involved in the modulation of the sleep/wake cycle. Serotonin has been known for many years to play a role in the modulation of sleep, however, it is still very controversial how and where serotonin may operate this modulation. Early studies suggested that serotonin is necessary to obtain and maintain behavioral sleep (permissive role on sleep). However, more recent microdialysis experiments provide evidence that the level of serotonin during W is higher in most cortical and subcortical areas receiving serotonergic projections. In this view the level of extracellular serotonin would be consistent with the pattern of discharge of the DRN serotonergic neurons which show the highest firing rate during W, followed by a decrease in slow wave sleep and by virtual electrical silence during REM sleep. This suggests that during waking serotonin may complement the action of noradrenaline and acetylcholine in promoting cortical responsiveness and participate to the inhibition of REM-sleep effector neurons in the brainstem (inhibitory role on REM sleep). The apparent inconsistency between an inhibitory and a facilitatory role played by serotonin on sleep has at least two possible explanations. On the one hand serotonergic modulation on the sleep/wake cycle takes place through a multitude of post-synaptic receptors which mediate different or even opposite responses; on the other hand the achievement of a behavioral state depends on the complex interaction between the serotonergic and other neurotransmitter systems. The main aim of this commentary is to review the role of brain serotonin in relation to the sleep/wake cycle. In particular we highlight the importance of microdialysis for on-line monitoring of the level of serotonin in different areas of the brain across the sleep/wake cycle.

The effects of median raphé electrical stimulation on serotonin release in the dorsal hippocampal formation of prenatally protein malnourished rats

Our previous work had shown an enhanced inhibition in the hippocampal formation of prenatally protein malnourished rats. We have also found a diminishment in 5-hydroxytryptamine (5-HT) fibers in the hippocampal formation of malnourished rats as well as increased levels of 5-HT in the brain. The purpose of the present study was to determine 5-HT release in the dorsal hippocampal formation following electrical stimulation of the median raphé nucleus (MRN) in unanesthetized prenatally malnourished rats. Stimulation of this nucleus at 20 Hz in malnourished rats resulted in a significantly diminished release of 5-HT compared to well-nourished rats. The latter group showed a lesser, though still significant, decrease in 5-HT release following raphé stimulation. Basal release of 5-HT prior to stimulation was significantly higher in malnourished rats as compared to well-nourished controls. This may be the result of a decreased density of 5-HT neurons leading to a diminished control of release. Stimulation of the MRN in behaving malnourished animals may markedly affect the recurrent negative feedback collaterals onto somatodendritic 5-HT(1A) and 5-HT(1D) autoreceptors thus enhancing the inhibitory effects of stimulation of the median raphé on 5-HT release. Studies are underway to examine the sensitivity of both the somatodendritic and terminal 5-HT autoreceptors in malnourished animals, in order to understand possible mechanisms for our findings.

Opposite change of in vivo dopamine release in the rat nucleus accumbens and striatum that follows electrical stimulation of dorsal raphe nucleus: role of 5-HT3 receptors

In the present study we investigate, using in vivo microdialysis, the involvement of central 5-HT3 receptors in the effect of dorsal raphe nucleus (DRN) electrical stimulation on dopamine (DA), 3, 4-dihydroxyphenylacetic acid (DOPAC), and 5-hydroxyindole-3-acetic acid (5-HIAA) extracellular levels monitored in the nucleus accumbens and the striatum of halothane-anesthetized rats. DRN stimulation (300 microA, 1 msec at 3, 5, 10, and 20 Hz for 15 min) induced a frequency-dependent increase of accumbal DA release and a concomitant reduction of DA release in the ipsilateral striatum at 20 Hz. In both structures DOPAC and 5-HIAA dialysate contents were enhanced in a frequency-dependent manner. Central serotonin (5-HT) depletion, induced by intra-raphe injections of 5, 7-dihydroxytryptamine neurotoxin, abolished the effect of 20 Hz DRN stimulation on DA, DOPAC, and 5-HIAA extracellular levels in both regions. The 5-HT synthesis inhibitor para-chlorophenylalanine (3 x 400 mg/kg, i.p., for 3 d), although preventing the effect on DA release, failed to modify significantly the effect of 20 Hz DRN stimulation on DOPAC and 5-HIAA outflow in both structures. Ondansetron (0.1 and 1 mg/kg) and (S)-zacopride (0.1 mg/kg), two 5-HT3 antagonists, significantly impaired the increase of accumbal DA release induced by 20 Hz DRN stimulation but did not affect either the decrease of striatal DA release or the increase in DOPAC outflow in both structures. These results indicate that an enhancement of central 5-HT transmission induced by DRN stimulation differentially affects striatal and accumbal DA release and that endogenous 5-HT, via its action on 5-HT3 receptors, exerts a facilitatory control restricted to the mesoaccumbal DA pathway.

Serotonin neuronal release from dorsal hippocampus following electrical stimulation of the dorsal and median raphé nuclei in conscious rats

We have studied 5-hydroxytryptamine (5-HT) release in the hippocampal formation following electrical stimulation of the dorsal and median raphé nuclei in the behaving rat. The primary finding in this study is a decrease in neuronal release of serotonin in the dorsal hippocampal formation following electrical stimulation of either the dorsal or median raphé nucleus in conscious rats. At no time did electrical stimulation of either raphé nucleus result in behavioral, including vigilance state, changes. The amount of 5-HT released was found to be frequency dependent with higher frequencies (20 Hz) producing larger decreases in release of 5-HT. However, the pattern of release differs between the two raphé nuclei. Extracellular levels of 5-HT decrease during stimulation of the dorsal raphé, whereas levels decrease only following cessation of stimulation of the median raphé nucleus. This may relate to the patterns of innervation of the dorsal hippocampal formation by these two midbrain raphé nuclei and also may reflect an inhibition of median raphé cell firing during stimulation of the dorsal raphé. Electrical stimulation of the dorsal raphé in anesthetized animals resulted in an enhanced release of 5-HT. The suppression of 5-HT release in the dorsal hippocampal formation in behaving animals was long-lasting (over 2 h), suggesting that the control mechanisms that regulate 5-HT release operate over a long time-course. This difference in release between non-anesthetized and anesthetized animals may relate to anesthesia blocking long- and/or short-loop serotonin recurrent axonal collaterals negatively feeding back onto 5-HT1A and 5-HT1D somatodendritic autoreceptors on raphé neurons. Further, the anesthetized animal has diminished monoaminergic "gating" influences on the hippocampal formation, whereas the behaving animal is more complex with behavioral (vigilance) states associated with different patterns of gating of information flow through the hippocampal formation.

Differential regional antagonism of 8-OH-DPAT-induced decrease in serotonin synthesis by two 5-HT1A receptor antagonists

The effects of two 5-HT1A receptor antagonists, (R)-3-N, N-dicyclobutylamino-8-fluoro-3,4-dihydro-2 H-1-benzopyran-5-carboxamide hydrogen (2 R,3 R)-tartrate monohydrate (NAD-299) and N-(2-(1-(2-methoxyphenyl)-piperazinyl))ethyl)-N-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride (WAY-100635) on the decrease in 5-hydroxytryptophan (5-HTP) accumulation evoked by (RS)-2-dipropylamino-8-hydroxy-1,2,3,4-tetrahydronaphthalene (8-OH-DPAT) in rats treated with the decarboxylase inhibitor, 3-hydroxyphenylhydrazine (NSD 1015) were studied in four rat brain regions: hippocampus, hypothalamus, striatum and frontal cortex. Dose-response studies revealed differential effects of both antagonists in the areas examined. Both antagonists were significantly more potent in antagonising the effect of 0.30 and 0.76 micromol/kg s.c. 8-OH-DPAT in hippocampus than in hypothalamus, striatum and frontal cortex in mentioned order. This order of potency was the opposite to that found for 8-OH-DPAT in decreasing the 5-HTP accumulation. Since previous studies by others have indicated that the reserve of somatodendritic 5-HT1A receptors is greater in dorsal raphe nucleus innervating frontal cortex and striatum than in median raphe nucleus which mainly innervates hippocampus, the observed different regional potency of the two 5-HT1A receptor antagonists may be explained by this difference in the 5-HT1A receptor reserve.

On-line detection of extracellular levels of serotonin in dorsal raphe nucleus and frontal cortex over the sleep/wake cycle in the freely moving rat

We used in vivo microdialysis coupled with polygraphic recording to monitor 5-hydroxytryptamine levels in the dorsal raphe nucleus and frontal cortex across waking, slow-wave sleep and rapid eye-movement sleep. Male Sprague-Dawley rats were prepared with electroencephalogram and electromyogram electrodes. Microdialysis probes were placed in dorsal raphe nucleus and/or frontal cortex. Dialysate samples were manually collected during polygraphically-defined behavioural states and the level of serotonin was assayed by means of microbore high-performance liquid chromatography separation and electrochemical detection. Samples from microdialysis probes histologically localized to the dorsal raphe nucleus and frontal cortex showed different levels of extracellular 5-hydroxytryptamine in waking, slow-wave sleep and rapid eye-movement sleep. In dorsal raphe nucleus the extracellular level of serotonin was highest in waking, decreased in slow-wave sleep to 69% and in rapid eye-movement sleep to 39% of waking mean level (waking 3.2 +/- 0.9; slow-wave sleep 2.2 +/- 0.8; rapid eye-movement sleep 1.3 +/- 0.4 fmol/sample). Mean extracellular levels of serotonin in frontal cortex displayed a similar pattern (waking 1.7 +/- 0.4; slow-wave sleep 1.0 +/- 0.3; rapid eye-movement 0.5 +/- 0.05 fmol/sample). In frontal cortex, rapid eye-movement sleep samples were only obtained in three animals. Our findings are consistent with previous results in cats, and suggest that in rats also, extracellular 5-hydroxytryptamine levels in dorsal raphe nucleus and frontal cortex across the sleep/wake cycle might reflect serotonergic neuronal activity. The findings stress the importance of controlling for behavioural state when investigating neurochemical correlates of serotonergic function.

Acute effects of 3,4-methylenedioxymethamphetamine (MDMA) on 5-HT cell firing and release: comparison between dorsal and median raphe 5-HT systems

It is proposed that 3,4-methylenedioxymethamphetamine (MDMA; Ecstasy) is more toxic to 5-HT neurones projecting from the dorsal raphe nucleus (DRN) than to those from the median raphe nucleus (MRN). Since increased 5-HT release has been associated with MDMA-induced neurotoxicity, MDMA may have a DRN-selective 5-HT releasing effect. Here we have compared the effects of acute MDMA on DRN and MRN 5-HT pathways using in vivo electrophysiological and neurochemical techniques. MDMA inhibited the firing of 5-HT neurones in both the DRN and the MRN, and did so with similar potency (ED50 values, 0.589 +/- 0.151 (8) and 0.588 +/- 0.207 (6) mg/kg i.v., respectively). In both nuclei this inhibitory effect was reversed by the selective 5-HT1A receptor antagonist, WAY 100635 (0.1 mg/kg i.v.). Microdialysis measurements were made in the frontal cortex and dorsal hippocampus, regions which receive a DRN- and an MRN-selective 5-HT innervation, respectively. A dose of 1 mg/kg i.v. MDMA increased extracellular 5-HT 3-fold in both the frontal cortex and dorsal hippocampus. A higher dose (3 mg/kg i.v.) increased 5-HT levels 8-fold in both regions. Overall, our data suggest that MDMA releases 5-HT from the cell body and terminal regions of both DRN and MRN 5-HT pathways, and does so in a qualitatively and quantitatively similar fashion. We conclude that any DRN-selectivity in the neurotoxic effects of MDMA is not due to a DRN-selective, acute 5-HT releasing action of the drug.

Characterization of the extracellular serotonin release in the substantia nigra of the freely moving rat using microdialysis

The characteristics of the serotonin release were investigated in the substantia nigra (SN) of the freely moving rat using microdialysis. We also examined whether the delay between surgery and microdialysis experiments might influence these characteristics by implanting rats with a guide cannula 1 or 2 days prior to microdialysis experiments. In the first group, the tissue was not punctured until the microdialysis probe was inserted the evening before the experiment. In the second group, the nigral tissue was punctured with an extended obturator which was then replaced by a microdialysis probe the evening before the experiment. After administration of 60 mM K+ a more pronounced increase in serotonin was observed in the first group (260%) compared to the second group (159%). Calcium-free and tetrodotoxin (TTX, a sodium channel blocker) (1 microM) perfusion reduced extracellular serotonin to respectively 77% and 80% in the first group and 70% and 64% in the second group. These results suggest that vesicular release of nigral serotonin only occurs partially in this region and that minimizing the damage caused by implantation of the probe results only in 10% more vesicular release of serotonin. However, blockade of the serotonin reuptake carrier caused more TTX sensitivity of the serotonin release. Also, stimulation of the dorsal raphe by locally perfusing 60 mM K+ decreased serotonin in the SN, confirming the anatomical and functional link between both areas.

The effect of the selective 5-HT1A agonists alnespirone (S-20499) and 8-OH-DPAT on extracellular 5-hydroxytryptamine in different regions of rat brain

1. We have examined the effects of the systemic administration of the selective 5-HT1A agonist alnespirone (S-20499) on in vivo 5-hydroxytryptamine (5-HT) release in the dorsal raphe nucleus, the median raphe nucleus and four forebrain areas innervated differentially by both (dorsal striatum, frontal cortex, ventral hippocampus and dorsal hippocampus). 2. Alnespirone (0.1-3 mg kg(-1), s.c.) dose-dependently reduced extracellular 5-HT in the six areas examined. In forebrain, the maximal reductions occurred in striatum and frontal cortex (maximal reduction to 23 and 29% of baseline, respectively). Those in dorsal and ventral hippocampus were more moderate (to ca 65% of baseline). In contrast, the decrease in 5-HT elicited in the median raphe nucleus was more marked than that in the dorsal raphe nucleus (to ca 30 and 60% of baseline, respectively). The selective 5-HT1A antagonist WAY-100635 (0.5 mg kg(-1), s.c.) prevented the decrease in 5-HT induced by alnespirone (0.3 mg kg(-1), s.c.) in frontal cortex. 3. 8-OH-DPAT (0.025, 0.1 and 0.3 mg kg(-1), s.c.) also reduced extracellular 5-HT in a regionally-selective manner (e.g., to 32% of baseline in striatum and to 69% in dorsal hippocampus at 0.1 mg kg(-1), s.c.). In midbrain, 8-OH-DPAT reduced the dialysate 5-HT slightly more in the median than in the dorsal raphe nucleus at all doses examined. 4. Doses of both compounds close to their respective ED50 values (0.3 mg kg(-1) alnespirone, 0.025 mg kg(-1) 8-OH-DPAT) reduced 5-HT to a comparable extent in all regions examined. However, the reductions attained at higher doses were more pronounced for 8-OH-DPAT. 5. These data show that the reduction of 5-HT release elicited by alnespirone and 8-OH-DPAT is more important in forebrain areas innervated by 5-hydroxytryptaminergic neurones of the dorsal raphe nucleus. This regional selectivity seems unlikely to be accounted for by differences in the sensitivity of 5-HT1A autoreceptors controlling 5-HT release, given the dissimilar effects of these two 5-HT1A agonists in regions rich in cell bodies and nerve terminals. This suggests the presence of complex mechanisms of control of 5-HT release by 5-HT1A receptors.

Kainate microinjection into the dorsal raphe nucleus induces 5-HT release in the amygdala and periaqueductal gray

Earlier results obtained in one of our laboratories showed that microinjection into the dorsal raphe nucleus (DRN) of the excitatory amino acid kainic acid, the benzodiazepine (BZD) inverse agonist FG 7142, and the 5-HT1A receptor agonist 8-OHDPAT changed the behavior of rats in the elevated T-maze, an animal model of anxiety. The present study investigates biochemical correlates of these results in awake rats by measuring 5-HT release with in vivo microdialysis in two brain structures innervated by the DRN-the amygdala (Am) and the dorsal periaqueductal gray matter (DPAG)-that have been implicated in anxiety. Microinjection of kainic acid (60 pmol) into the DRN significantly increased 5-HT release in both the Am and the DPAG. In the DPAG, the increase was 14-fold higher with respect to the baseline and occurred only at the first sample, which was collected 30 min after the injection. In the Am, the increase was less pronounced (nearly fourfold) but persistent, lasting until the fourth sample, which was collected 120 min from the injection. FG 7142 (40 pmol) and 8-OH-DPAT (8 nmol) were ineffective. Because only intra-DRN kainate both increased inhibitory avoidance and decreased one-way escape in the elevated T-maze, the present behavioral results support the suggestion that 5-HT facilitates conditioned fear in the Am and inhibits unconditioned fear in the DPAG.

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.

Regional differences in the effect of the combined treatment of WAY 100635 and fluoxetine: an in vivo microdialysis study

We studied the changes in extracellular serotonin (5-HT) levels in the frontal cortex (FC) and ventral hippocampus (vHi) in conscious rats, induced by the combined administration of a highly selective 5-HT1A receptor antagonist, WAY 100635 (0.1 mg/kg, i.v.), and fluoxetine (1 mg/kg, i.p.), a selective 5-HT reuptake inhibitor (SSRI). In the two brain areas studied, no change in extracellular 5-HT concentrations was observed following fluoxetine administration over the 210 min post-injection period. However, in animals co-administered with [WAY 100635 + fluoxetine], the maximal increase in 5-HT levels in the FC was to 215% of the respective basal value (100%), while no significant change in 5-HT was observed in dialysates from the vHi. Furthermore, the [norfluoxetine]-to-[fluoxetine] ratio in the FC was significantly higher than in the hippocampus as measured in homogenates of animals treated with either fluoxetine alone or a prior administration of WAY 100635. Thus, WAY 100635 made the fluoxetine short-lasting effect apparent in the FC, but not by interfering with pharmacokinetic parameters of fluoxetine. Taken together, our data suggest the possibility, that either 5-HT1A autoreceptor sensitivity or uptake carrier density or higher [metabolite]-to-[parent drug] ratios in the FC than in the hippocampus may be involved in regional specific responses to SSRIs.

Effects of repeated oral doses of dexnorfenfluramine on 5-HT levels and 5-HT uptake sites in rat brain

The effects of oral dexnorfenfluramine (DNF; 1-4 mg/kg, twice daily for 4 days), the active metabolite of dexfenfluramine, were examined on rat regional brain indole contents and [3H]citalopram binding. Two hours after the last dose, serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were dose-dependently lowered at doses above 1.5 mg/kg, with slight regional differences. Cortical 5-HT uptake sites were reduced only at the highest dose. Above 2 mg/kg DNF also caused a more lasting reduction (4 weeks) of regional indoles and cortical 5-HT uptake sites. At this longer time while the decrease in hippocampal 5-HT levels and cortical 5-HT uptake sites remained essentially constant, cortical and striatal 5-HT levels were lowered less than at 2 h, suggesting a return toward control values.

P-chloroamphetamine induces c-fos in rat brain: a study of serotonin2A/2C receptor function

Recent studies suggest that a class of substituted amphetamines, which includes p-chloroamphetamine, causes an acute release of serotonin (5-hydroxytryptamine) and appears to act preferentially on axons arising from the dorsal raphe nucleus. The postsynaptic targets of these axons are not well characterized, but they have been localized in close proximity to the distribution of serotonin2A receptor binding sites. In the present study, c-fos immunocytochemistry has been used to investigate this anatomical relationship further. Administration of p-chloroamphetamine or the serotonin2A/2C receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane to rats resulted in similar patterns of Fos-like immunoreactivity in some, but not all, forebrain areas. Areas which expressed Fos following either treatment included cerebral cortex, claustrum, amygdala and nucleus accumbens. A particularly close match was seen in layer Va of the somatosensory cortex. No specificity of p-chloroamphetamine for dorsal raphe nucleus-innervated areas was noted. Prior treatment of animals with p-chloroamphetamine two weeks before a second challenge with the same drug, or with the serotonin2A/2C receptor antagonist ritanserin 30 min before p-chloroamphetamine challenge, resulted in an attenuation of p-chloroamphetamine-induced Fos-like immunoreactivity in the olfactory tubercle, the islands of Calleja and the caudate-putamen. The reduction was most noticeable in layer Va of the somatosensory cortex. The results of this study indicate that a close anatomical correlation may exist between the fine serotonin axon terminals that show vulnerability to the neurotoxic effects of p-chloroamphetamine and serotonin2A receptors in some brain regions. This association may prove to be important in explaining the actions of certain psychotropic drugs, for example in the control of affective states.

A 5-hydroxytryptamine lesion markedly reduces the incidence of burst-firing dorsal raphe neurones in the rat

Recently, we described neurones in the rat dorsal raphe nucleus (DRN) with electrophysiological characteristics typical of 5-hydroxytryptamine (5-HT) neurones except that the neurones fired brief bursts. Here we report the effect of 5-HT lesions on the incidence of these burst-firing neurones. In vivo extracellular recordings revealed that in rats pretreated with the selective 5-HT neurotoxin, 5,7-dihydroxytryptamine, the occurrence of typical 5-HT neurones was significantly decreased (80%) compared to controls. In these 5-HT lesioned animals, the number of the burst-firing DRN neurones was also significantly reduced (91%). Neurones previously characterised as not containing 5-HT, were not altered by the lesion. These data are further evidence to support our hypothesis that burst-firing neurones in the DRN contain 5-HT.