An electrophysiological and neuroanatomical study of the medial prefrontal cortical projection to the midbrain raphe nuclei in the rat

Increased striatal serotonin synthesis following cortical resection in children with intractable epilepsy

BACKGROUND AND PURPOSE

Serotonin is a major regulator of structural brain plasticity, which may occur following cortical resection in humans. In this study we used positron emission tomography (PET) with alpha[11C]methyl-l-tryptophan (AMT) to evaluate serotonergic alterations in subcortical structures following cortical resection in children with intractable epilepsy.

METHODS

AMT uptake in the thalamus and lentiform nucleus was evaluated postoperatively (1-89 months following resection) in 19 children (mean age: 8.7 years) with a previous cortical resection due to intractable epilepsy. Ten children with partial epilepsy but without resection and seven normal children served as controls.

RESULTS

There was an increased AMT uptake in the lentiform nucleus ipsilateral to the resection as compared to the contralateral side (mean asymmetry: 4.2+/-3.0%), and the asymmetries were significantly higher than those measured in the control groups (p<or=0.001). Post-resection asymmetry indices in the lentiform nucleus correlated inversely with postoperative time (r=-0.67; p=0.002), but not with age (p=0.29) or the extent of resection (p=0.77). In contrast, thalamic AMT uptake asymmetries were not different among the three groups (p=0.63).

CONCLUSIONS

Cortical resection results in a sustained increase of AMT uptake in the lentiform nucleus, suggesting increased serotonin synthesis. Serotonergic activation in the deafferented striatum may play a role in the functional reorganization of cortico-striatal projections in humans.

Important messages in the 'post': recent discoveries in 5-HT neurone feedback control

The neurotransmitter 5-hydroxytryptamine (5-HT, serotonin) mediates important brain functions and contributes to the pathophysiology and successful drug treatment of many common psychiatric disorders, especially depression. It is established that a key mechanism involved in the control of 5-HT neurones is feedback inhibition by presynaptic 5-HT autoreceptors, which are located on 5-HT cell bodies and nerve terminals. However, recent experiments have discovered an unexpected complexity of 5-HT neurone control, specifically in the form of postsynaptic 5-HT feedback mechanisms. These mechanisms have the physiological effects of 5-HT autoreceptors but use additional 5-HT receptor subtypes and operate through neural inputs to 5-HT neurones. A postsynaptic feedback system that excites 5-HT neurones has also been reported. This article discusses current knowledge of the pharmacology and physiology of these new found 5-HT feedback mechanisms and considers their possible contribution to depression pathophysiology and utility as a resource of novel antidepressant drug strategies.

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.

The synaptic and nonsynaptic glycine transporter type-1 inhibitors Org-24461 and NFPS alter single neuron firing rate in the rat dorsal raphe nucleus. Further evidence for a glutamatergic-serotonergic interaction and its role in antipsychotic action

Single neuron firing rate was recorded from dorsal raphe nucleus of anesthetized rats. The firing rate of raphe neurons varied from 4 to 8 discharge per second before drug administration and this neuronal activity was decreased by L-701,324 (2 mg/kg i.v. injection), a competitive antagonist of glycineB binding site of N-methyl-D-aspartate (NMDA) receptors. The glycine transporter type-1 (GlyT1) antagonists Org-24461 (10 mg/kg i.v.) and NFPS (3 mg/kg i.v.) reversed the inhibitory effect of L-701,324 on single neuron activity recorded from dorsal raphe nucleus of the rat. Org-24461 and NFPS both tended to increase the raphe neuronal firing rate also when given alone but their effect was not significant. This finding serves further evidence that glutamate released from axon terminals of the cortico-striatal projection neurons stimulates serotonergic neurons in the raphe nuclei and this effect is mediated at least in part by postsynaptic NMDA receptors. Thus, GlyT1 inhibitors are able to reverse the hypofunctional state of NMDA receptors, suggesting that these drugs may have beneficial therapeutic effects in neurological and psychiatric disorders characterized with impaired NMDA receptor-mediated transmission.

AMPA and NMDA receptor regulation of firing activity in 5-HT neurons of the dorsal and median raphe nuclei

The glutamatergic regulation of 5-hydroxytryptamine (5-HT) neuronal activity has not been extensively studied. Here, we used extracellular single unit recording in midbrain slices to examine glutamate receptor mediated effects on 5-HT neuronal activity in the dorsal raphe nucleus (DRN) and the median raphe nucleus (MRN). Alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA; 1 and 3 microm) concentration-dependently increased firing in 5-HT neurons in both the DRN and the MRN. The response to AMPA was blocked by the AMPA receptor antagonist, 6,7-dinitroquinoxaline-2,3(1H-4H)-dione (DNQX; 10 microm) but not the N-methyl-d-aspartate (NMDA) receptor antagonist, 2-amino-5-phosphonopentanoic acid (AP-5; 50 microm). NMDA (10-100 microm) also increased 5-HT neuronal firing in a concentration-dependent manner in both the DRN and MRN; a response that was blocked by AP-5 (50 microm). In some DRN neurons the NMDA response was partially antagonized by DNQX (10 microm) suggesting that NMDA, as well as directly activating 5-HT neurons, evokes local release of glutamate, which indirectly activates AMPA receptors on 5-HT neurons. Responses of DRN 5-HT neurons to AMPA and NMDA were enhanced by the gamma-amino-butyric acid (GABA)(A) receptor antagonist, bicuculline (50 microm), suggesting that both AMPA and NMDA increase local release of GABA. Finally in the DRN the 5-HT(1A) receptor antagonist, WAY100635 (100 nm), failed to enhance the response of 5-HT neurons to AMPA and caused only a small increase in the excitatory response to NMDA suggesting a low degree of tonic activation of 5-HT(1A) autoreceptors even when 5-HT neuronal firing rate is high.

Projection patterns from the amygdaloid nuclear complex to subdivisions of the dorsal raphe nucleus in the rat

The goal of the present study was to identify the projection from the subdivisions of the amygdaloid nuclear complex to specified subregions of the dorsal raphe (DR) nucleus and to attempt to compare the density of amygdaloid input to the DR with that of inputs from other limbic structures. Use of a retrograde tracer, gold-conjugated and inactivated wheatgerm agglutinin-horseradish peroxidase (WGA-apo-HRP-gold), demonstrated that amygdaloid input to midline DR subdivision originates mainly from the medial portion of the medial amygdaloid nucleus, whereas that to lateral wing subdivision derives from the region extending from the lateral portion of the medial amygdaloid nucleus to the commissural stria terminalis. Use of the retrograde tracer Fluorogold (FG) produced relatively large but circumscribed injection sites comprising midline DR as well as portions of lateral wing subdivision and confirmed that the medial amygdaloid nucleus provides the major input to the DR. We also demonstrated that although amygdaloid input was not as extensive as inputs from other limbic structures such as the medial prefrontal cortex or the lateral habenular nucleus, it was comparable to input from the lateral septal nucleus. Based on these observations, we suggest that the medial amygdaloid nucleus provides substantial input to the DR and may contribute an emotional influence on sleep-wakefulness cycle or pain-stress modulation. Furthermore, it seems that the medial amygdaloid-DR projection might be anatomically and functionally distinct from the well-characterized central amygdaloid-periaqeductal gray (PAG) circuit which is essential for conditioned fear.

Identification of the cannabinoid receptor type 1 in serotonergic cells of raphe nuclei in mice

The endocannabinoid system (ECS) possesses neuromodulatory functions by influencing the release of various neurotransmitters, including GABA, noradrenaline, dopamine, glutamate and acetylcholine. Even though there are studies indicating similar interactions between the ECS and the serotonergic system, there are no results showing clear evidence for type 1 cannabinoid receptor (CB1) location on serotonergic neurons. In this study, we show by in situ hybridization that a low but significant fraction of serotonergic neurons in the raphe nuclei of mice contains CB1 mRNA as illustrated by the coexpression with the serotonergic marker gene tryptophane hydroxylase 2, the rate limiting enzyme for the serotonin synthesis. Furthermore, by double immunohistochemistry and confocal microscopy, we were able to detect CB1 protein on serotonergic fibers and synapses expressing the serotonin uptake transporter in the hippocampus and the amygdala. Our findings indicate that the CB1-mediated regulation of serotonin release can depend in part on a direct cross-talk between the two systems at single cell level, which might lead to functional implications in the modulation of emotional states.

Behavioral control, the medial prefrontal cortex, and resilience

The degree of control that an organism has over a stressor potently modulates the impact of the stressor, with uncontrollable stressors producing a constellation of outcomes that do not occur if the stressor is behaviorally controllable. It has generally been assumed that this occurs because uncontrollability actively potentiates the effects of stressors. Here it will be suggested that in addition, or instead, the presence of control actively inhibits the impact of stressors. At least in part this occurs because (i) the presence of control is detected by regions of the ventral medial prefrontal cortex (mPFCv); and (ii) detection of control activates mPFCv output to stress-responsive brain stem and limbic structures that actively inhibit stress-induced activation of these structures, Furthermore, an initial experience with control over stress alters the mPFCv response to subsequent stressors so that mPFCv output is activated even if the subsequent stressor is uncontrollable, thereby making the organism resilient. The general implications of these results for understanding resilience in the face of adversity are discussed.

Antipsychotic drugs reverse the AMPA receptor-stimulated release of 5-HT in the medial prefrontal cortex

The prefrontal cortex (PFC) is involved in the pathophysiology of schizophrenia. PFC neuronal activity is modulated by monoaminergic receptors for which antipsychotic drugs display moderate-high affinity, such as 5-HT(2A) and alpha(1)-adrenoceptors. Conversely, PFC pyramidal neurons project to and modulate the activity of raphe serotonergic neurons and serotonin (5-HT) release. Under the working hypothesis that atypical antipsychotic drugs may partly exert their action in PFC, we assessed their action on the in vivo 5-HT release evoked by increasing glutamatergic transmission in rat medial PFC (mPFC). This was achieved by applying S-AMPA in mPFC (reverse dialysis) or by disinhibiting thalamic excitatory afferents to mPFC with bicuculline. The application of haloperidol, chlorpromazine, clozapine and olanzapine in mPFC by reverse dialysis (but not reboxetine or diazepam) reversed the S-AMPA-evoked local 5-HT release. Likewise, the local (in mPFC) or systemic administration of these antipsychotic drugs reversed the increased prefrontal 5-HT release produced by thalamic disinhibition. These effects were shared by the 5-HT(2A) receptor antagonist M100907 and the alpha(1)-adrenoceptor antagonist prazosin. However, raclopride (DA D2 antagonist) had very modest effects. These results suggest that, besides their action in limbic striatum, antipsychotic drugs may attenuate glutamatergic transmission in PFC, possibly by interacting with 5-HT(2A) and/or alpha(1)-adrenoceptors.

The temporal dynamics model of emotional memory processing: a synthesis on the neurobiological basis of stress-induced amnesia, flashbulb and traumatic memories, and the Yerkes-Dodson law

We have reviewed research on the effects of stress on LTP in the hippocampus, amygdala and prefrontal cortex (PFC) and present new findings which provide insight into how the attention and memory-related functions of these structures are influenced by strong emotionality. We have incorporated the stress-LTP findings into our "temporal dynamics" model, which provides a framework for understanding the neurobiological basis of flashbulb and traumatic memories, as well as stress-induced amnesia. An important feature of the model is the idea that endogenous mechanisms of plasticity in the hippocampus and amygdala are rapidly activated for a relatively short period of time by a strong emotional learning experience. Following this activational period, both structures undergo a state in which the induction of new plasticity is suppressed, which facilitates the memory consolidation process. We further propose that with the onset of strong emotionality, the hippocampus rapidly shifts from a "configural/cognitive map" mode to a "flashbulb memory" mode, which underlies the long-lasting, but fragmented, nature of traumatic memories. Finally, we have speculated on the significance of stress-LTP interactions in the context of the Yerkes-Dodson Law, a well-cited, but misunderstood, century-old principle which states that the relationship between arousal and behavioral performance can be linear or curvilinear, depending on the difficulty of the task.

Serotonin and Dopamine Interactions in Rodents and Primates: Implications for Psychosis and Antipsychotic Drug Development

Since the late 1950s, appreciation of dopamine receptor blockade has played a primary role in understanding the mechanism underlying the therapeutic effects of antipsychotic drugs in schizophrenic patients in treating the positive symptoms of schizophrenia (e.g., delusions and hallucinations). Development of the second generation of antipsychotic drugs, otherwise known as atypical antipsychotic drugs, has resulted in treatments with improved subjective tolerability but relatively modest improvements in the negative symptoms of schizophrenia such as avolition, flat affect, and anhedonia. The major current challenge is to develop medications which can further improve negative symptoms treatment and also tackle the intractable clinical problems of cognitive impairment associated with schizophrenia. Further advances along these lines with respect to the dopaminergic and serotonergic neurostransmitter systems will be aided by an appreciation of the interaction between dopamine and serotonin receptor subtypes in a range of key brain structures, such as the prefrontal cortex, thalamus, striatum, amygdala, hippocampus, and the brain stem nuclei, from which the cell bodies of monoaminergic-containing neurons originate. Increasing emphasis on the use of animal models which are homologous to critical aspects of the pathophysiology in the brains of schizophrenic patients will also be required, especially as negative symptoms and cognitive impairment become an important focus for generating novel therapeutics.

Propagation of spike and wave activity to the medial prefrontal cortex and dorsal raphe nucleus of WAG/Rij rats

Although there is pharmacological evidence for the involvement of the serotonergic system in the expression of spike and wave discharges (SWDs) in experimental absence epilepsy, no direct investigation of this paroxysm in the dorsal raphe nucleus (DRN), one of the main serotonergic nuclei, has been carried out. We have now recorded the EEG simultaneously with local field potentials and unit activity in DRN from WAG/Rij rats, one of the best established models of absence epilepsy during spontaneous SWDs. We have also compared this activity to that in the thalamocortical networks, where SWDs are generated, and in the medial prefrontal cortex (mPFC), as this brain area is reciprocally connected to the DRN. We have found that SWDs propagate to the DRN with a short delay, and that the firing rate of its neurons changes during this type of paroxysm. These results provide the first direct evidence for clear alterations in the firing properties of mPFC and DRN neurons during spontaneous SWDs.

The anxiogenic drug FG-7142 increases serotonin metabolism in the rat medial prefrontal cortex

The neural mechanisms underlying anxiety states are believed to involve interactions among forebrain limbic circuits and brainstem serotonergic systems. Consistent with this hypothesis, FG-7142, a partial inverse agonist at the benzodiazepine allosteric site of the GABAA receptor, increases c-Fos expression within a subpopulation of brainstem serotonergic neurons. Paradoxically, FG-7142 has no effect on extracellular serotonin concentrations, as measured using in vivo microdialysis, in certain anxiety-related brain structures. This study tested the hypothesis that FG-7142 alters serotonin metabolism within one or more nodes of a defined anxiety-related forebrain circuit. Rats received one of four treatments (vehicle, 1.9, 3.8, or 7.5 mg/kg FG-7142, i.p.) and brains were collected 1 h following treatment. Thirteen forebrain regions were microdissected and analyzed for l-tryptophan, serotonin, and 5-hydroxyindoleacetic acid concentrations using high pressure liquid chromatography with electrochemical detection. FG-7142 (7.5 mg/kg) increased l-tryptophan, serotonin, and 5-hydroxyindoleacetic acid concentrations in the prelimbic cortex but not in several other regions studied including subdivisions of the amygdala and bed nucleus of the stria terminalis. These data demonstrate that FG-7142 alters brain tryptophan concentrations and serotonin metabolism in specific components of an anxiety-related forebrain circuit including the medial prefrontal cortex, an important structure involved in executive function and the regulation of emotional behavior.

Corticotropin-releasing factor in the dorsal raphe elicits temporally distinct serotonergic responses in the limbic system in relation to fear behavior

The neurotransmitters serotonin and corticotrophin-releasing factor are thought to play an important role in fear and anxiety behaviors. This study aimed to determine the relationship between corticotrophin-releasing factor-evoked changes in serotonin levels within discrete regions of the limbic system and the expression of fear behavior in rats. The effects of corticotrophin-releasing factor administration to the serotonin cell body regions of the dorsal raphe nucleus on fear behavior, behavioral activity, and extracellular serotonin levels were assessed in freely moving rats with microdialysis probes implanted into the central nucleus of the amygdala and the medial prefrontal cortex. Infusion of corticotrophin-releasing factor (0.5 microg) into the dorsal raphe rapidly induced freezing behavior, which was positively correlated with an immediate increase in serotonin release in the central nucleus of the amygdala. In contrast, cessation of freezing behavior correlated with a delayed and prolonged increase in serotonin release within the medial prefrontal cortex. Our findings suggest that corticotrophin-releasing factor-induced freezing behavior is associated with regionally and temporally distinct serotonergic responses in the limbic system that may reflect differing roles for these regions in the expression of fear/anxiety behavior.

Effect of vagus nerve stimulation on serotonergic and noradrenergic transmission

Vagus nerve stimulation (VNS) is an antiepileptic treatment, which has recently shown promise as an antidepressant. Yet, its antidepressant mechanisms of action are unknown. Serotonergic [5-hydroxytryptamine (5-HT, serotonin)] and noradrenergic [norepinephrine (NE)] systems are involved in the pathophysiology of depression and in the mechanisms of action of antidepressants. The present study analyzes 5-HT and NE neuronal firing rates in their brainstem nuclei: the dorsal raphe nucleus (DRN) and locus coeruleus (LC), respectively. The basal firing rates in the DRN and LC were significantly increased after long-term treatments with VNS. After short-term VNS treatments, firing rates were significantly higher for LC (at 1 h and 3 days). As changes in their firing rate may have been due to altered autoreceptor sensitivities, the responses of autoreceptors to the acute administration of their respective agonists were assessed. However, no significant difference was seen in the DRN. No significant differences in dose response curves for 5-HT(1A) somatodendritic and alpha 2-adrenergic autoreceptors were noticed between long-term VNS and controls. VNS appears to have a novel mechanism of antidepressant action, enabling its effectiveness in treatment-resistant depression. LC firing rates significantly increase earlier than the DRN basal firing. As the LC has an excitatory influence on DRN, it is possible that the increased DRN firing rate is secondary to an initial increased LC firing rate from VNS.

Electrophysiological diversity of the dorsal raphe cells across the sleep-wake cycle of the rat

Through their widespread projections to the entire brain, dorsal raphe cells participate in many physiological functions and are associated with neuropsychiatric disorders. In previous studies, the width of action potentials was used as a criterion to identify putative serotonergic neurons, and to demonstrate that cells with broad spikes were more active in wakefulness, slowed down their activity in slow wave sleep and became virtually silent during paradoxical sleep. However, recent studies reported that about half of these presumed serotonergic cells were not immunoreactive for tyrosine hydroxylase. Here, we re-examine the electrophysiological properties of dorsal raphe cells across the sleep-wake cycle in rats by the extracellular recording of a large sample of single units (n = 770). We identified two major types of cells, which differ in spike waveform: a first population characterized by broad, mostly positive spikes, and a second one displaying symmetrical positive-negative spikes with a large distribution of spike durations (0.6-3.2 ms). Although we found classical broad-spike cells that were more active in wakefulness, we also found that about one-third of these cells increased or did not change their firing rate during sleep compared with wakefulness. Moreover, 62% of the latter cells were active in paradoxical sleep when most of raphe cells were silent. Such a diversity in the neuronal firing behaviour is important in the light of the recent controversy regarding the neurochemical identity of dorsal raphe cells exhibiting broad spikes. Our results also suggest that the dorsal raphe contains subpopulations of neurons with reciprocal activity across the sleep-wake cycle.

Severe deficits in 5-HT2A-mediated neurotransmission in BDNF conditional mutant mice

BDNF is thought to provide critical trophic support for serotonin neurons. In order to determine postnatal effects of BDNF on the serotonin system, we examined a line of conditional mutant mice that have normal brain content of BDNF during prenatal development but later depletion of this neurotrophin in the postnatal period. These mice show a behavioral phenotype that suggests serotonin dysregulation. However, as shown here, the presynaptic serotonin system in the adult conditional mutant mice appeared surprisingly normal from histological, biochemical, and electrophysiological perspectives. By contrast, a dramatic and unexpected postsynaptic 5-HT2A deficit in the mutant mice was found. Electrophysiologically, serotonin neurons appeared near normal except, most notably, for an almost complete absence of expected 5-HT2A -mediated glutamate and GABA postsynaptic potentials normally displayed by these neurons. Further analysis showed that BDNF mutants had much reduced 5-HT2A receptor protein in dorsal raphe nucleus and a similar deficit in prefrontal cortex, a region that normally shows a high level of 5-HT2A receptor expression. Recordings in prefrontal slice showed a marked deficit in 5-HT2A -mediated excitatory postsynaptic currents, similar to that seen in the dorsal raphe. These findings suggest that postnatal levels of BDNF play a relatively limited role in maintaining presynaptic aspects of the serotonin system and a much greater role in maintaining postsynaptic 5-HT2A and possibly other receptors than previously suspected.

Modulation of anxiety circuits by serotonergic systems

Anxiety is a complex emotional state associated with sustained heightened autonomic and behavioral arousal and an increase in avoidance behavior. Anxiety-related behavior is a form of risk assessment behavior that is associated with a level of uncertainty or unpredictability regarding the outcome of emotionally salient events, often when both rewarding and aversive outcomes are possible. In this review, we highlight recent advances in our understanding of the neural circuits regulating anxiety states and anxiety-related behavior with an emphasis on the role of brainstem serotonergic systems in modulating anxiety-related circuits. In particular, we explore the possibility that the regulation of anxiety states and anxiety-related behavior by serotonergic systems is dependent on a specific, topographically organized mesolimbocortical serotonergic system that originates in the mid-rostrocaudal and caudal parts of the dorsal raphe nucleus.

Involvement of 5-HT1A receptors in prefrontal cortex in the modulation of dopaminergic activity: role in atypical antipsychotic action

Atypical antipsychotics increase dopamine (DA) release in the medial prefrontal cortex (mPFC), an effect possibly involved in the superior effects of atypical versus classical antipsychotics on cognitive/negative symptoms. We examined the role of 5-HT1A receptors in the mPFC on the modulation of dopaminergic activity and the mesocortical DA release in vivo. The highly selective 5-HT1A agonist BAY x 3702 (BAY; 10-40 microg/kg, i.v.) increased the firing rate and burst firing of DA neurons in the ventral tegmental area (VTA) and DA release in the VTA and mPFC. The increase in DA release in both areas was potentiated by nomifensine coperfusion. The selective 5-HT1A antagonist WAY-100635 reversed the effects of BAY in both areas, and the changes in the VTA were prevented by frontocortical transection. The application of BAY in rat and mouse mPFC by reverse dialysis increased local extracellular DA at a low concentration (3 microM) and reduced it at a higher concentration (30 microM). Both effects disappeared in 5-HT1A knock-out mice. In the presence of bicuculline, BAY reduced DA release at all concentrations. The atypical antipsychotics clozapine, olanzapine, and ziprasidone (but not haloperidol) enhanced DA release in the mPFC of wild-type but not 5-HT1A knock-out mice after systemic and local (clozapine and olanzapine) administration in the mPFC. Likewise, bicuculline coperfusion prevented the elevation of DA release produced by local clozapine or olanzapine application. These results suggest that the activation of mPFC 5-HT1A receptors enhances the activity of VTA DA neurons and mesocortical DA release. This mechanism may be involved in the elevation of extracellular DA produced by atypical antipsychotics.

Two populations of glutamatergic axons in the rat dorsal raphe nucleus defined by the vesicular glutamate transporters 1 and 2

Most glutamatergic neurons in the brain express one of two vesicular glutamate transporters, vGlut1 or vGlut2. Cortical glutamatergic neurons highly express vGlut1, whereas vGlut2 predominates in subcortical areas. In this study immunohistochemical detection of vGlut1 or vGlut2 was used in combination with tryptophan hydroxylase (TPH) to characterize glutamatergic innervation of the dorsal raphe nucleus (DRN) of the rat. Immunofluorescence labeling of both vGlut1 and vGlut2 was punctate and homogenously distributed throughout the DRN. Puncta labeled for vGlut2 appeared more numerous then those labeled for vGlut1. Ultrastructural analysis revealed axon terminals containing vGlut1 and vGlut2 formed asymmetric-type synapses 80% and 95% of the time, respectively. Postsynaptic targets of vGlut1- and vGlut2-containing axons differed in morphology. vGlut1-labeled axon terminals synapsed predominantly on small-caliber (distal) dendrites (42%, 46/110) or dendritic spines (46%, 50/110). In contrast, vGlut2-containing axons synapsed on larger caliber (proximal) dendritic shafts (> 0.5 microm diameter; 48%, 78/161). A fraction of both vGlut1- or vGlut2-labeled axons synapsed onto TPH-containing dendrites (14% and 34%, respectively). These observations reveal that different populations of glutamate-containing axons innervate selective dendritic domains of serotonergic and non-serotonergic neurons, suggesting they play different functional roles in modulating excitation within the DRN.

μ-Opioids disinhibit and κ-opioids inhibit serotonin efflux in the dorsal raphe nucleus

The relative importance of GABAergic and glutamatergic afferents in mediating the effects of mu- and kappa-opioids on serotonin (5-HT) efflux in vivo has not been firmly established. Thus, we used microdialysis in the dorsal raphe nucleus (DRN) of freely behaving rats to study the effect of GABA and glutamate receptor antagonists on opioid-induced changes in 5-HT efflux. Infusing the mu-opioid agonist DAMGO (300 microM) increased extracellular 5-HT in the DRN by approximately 70%. During infusion of the GABA(A) receptor blocker bicuculline (100 microM), extracellular 5-HT increased by approximately 250%, and subsequent infusion of DAMGO decreased 5-HT to approximately 70% above the pre-bicuculline baseline. These data are consistent with the hypothesis that mu-opioids disinhibit 5-HT neurons, an effect attenuated by direct inhibition of 5-HT efflux or inhibition of excitatory influences on 5-HT efflux. To further test this hypothesis, glutamate receptor blockers, AP-5 (1 mM) and DNQX (300 microM), were co-infused with DAMGO. The glutamate receptor antagonists prevented decreases in 5-HT elicited by DAMGO in the presence of bicuculline. This indicates that DAMGO inhibits glutamatergic afferents, which partly offsets the disinhibitory influence of mu-opioids on 5-HT efflux. In contrast, the kappa-opioid agonist, U-50,488 (300 microM), decreased 5-HT by approximately 30% in the DRN. Glutamate and GABA receptor antagonists did not block this effect. In conclusion, mu-opioids inhibit GABAergic and glutamatergic afferents, thereby indirectly affecting 5-HT efflux in the DRN. In contrast, kappa-opioids inhibit 5-HT efflux independent of effects on glutamatergic and GABAergic afferents.

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.

More tryptophan hydroxylase in the brainstem dorsal raphe nucleus in depressed suicides

Deficient serotonin neurotransmission in suicide is indicated by reduced brainstem serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA), fewer 5-HT(1A) autoreceptors and reduced cortical serotonin transporter binding in suicide victims. Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in the synthesis of 5-HT, and alterations in TPH could explain some of these findings. We sought to determine the amount of TPH immunoreactivity (TPH-IR) in the dorsal (DRN) and median (MRN) raphe nuclei in suicides and controls. Brainstems of suicide victims and controls (n = 11 pairs) were collected at autopsy, matched for age, sex and postmortem interval, frozen and sectioned (20 microm). Immunoautoradiography, using an antibody to label TPH, was performed, slides exposed to film and autoradiograms quantified by a computer-based image analysis system. We examined sections every 1000 microm throughout the whole length of the nucleus, performing statistical analysis only on those subjects for whom the raphe was complete (n = 8 pairs). TPH-IR (microCi/g) was higher in suicides than controls (S: 300.8 +/- 70.8 vs. C: 259.6 +/- 40.7, t = 2.57, df = 7, P = 0.04) in the dorsal raphe nucleus (DRN), and not different between suicides and controls (S: 251.3 +/- 44.2 vs. C: 235.9 +/- 27.4, t = 1.49, df = 7, P = 0.18) in the MRN. DRN TPH-IR was higher in male suicide victims (MS) compared to male controls (MC; MS: 318.4 +/- 54.4 vs. MC: 271.9 +/- 22.5, t = 2.66, df = 6, P = 0.03). The analysis of TPH-IR area and density at each DRN rostrocaudal levels showed higher area and density in suicides compared to controls in the rostral DRN and lower area and density in the caudal DRN. TPH-IR, an index of the amount of TPH enzyme, in the DRN is higher in depressed suicides. More TPH may be an upregulatory homeostatic response to impaired serotonin release or less autoreceptor activation. Alternatively, the serotonin impairment in suicide may be due to hypofunctional serotonin-synthesizing enzyme.

The role of sigma receptors in depression

Behavioral models used to test potential antidepressants have shown that ligands that bind to sigma receptors possess "antidepressant-like" properties. The focus of this review is to discuss the literature concerning sigma receptors and their ligands, with respect to their antidepressants properties. In addition to the behavioral data, we discuss electrophysiological and biochemical models demonstrating sigma receptors' ability to modulate important factors in the pathophysiology of depression and/or the mechanisms of action of antidepressants such as the serotonergic neurotransmission in the dorsal raphe nucleus (DRN) and the glutamatergic transmission in the hippocampus. We also discuss the significance of these two systems in the mechanism of action of antidepressants. Sigma ligands have potential as antidepressant medications with a fast onset of action as they produce a rapid modulation of the serotonergic system in the DRN and the glutamatergic transmission in the hippocampus. As these effects of sigma ligands may produce antidepressant properties by completely novel mechanisms of action, they may provide an alternative to the antidepressants currently available and may prove to be beneficial for treatment-resistant depressed patients.

Participation of dorsal raphe nucleus in the behavioral alteration observed after discontinuation of chronic diazepam administration: Possible neural circuitry involved

Previous findings from our laboratory have demonstrated a positive correlation between the development of tolerance to diazepam (DZ) 5 mg/kg/day over 4 days, and increased hippocampal synaptic plasticity. It seems likely that a similar plastic phenomenon may occur on hippocampal formation after chronic (18 days) DZ administration. We postulate hippocampal long-term potentiation (LTP) underlying substrate to the behavioral alteration observed after chronic DZ administration. In the present study, we investigated the involvement of the serotonergic (5-HT) system in the possible neural circuitry recruited during DZ withdrawal and in the increased hippocampal synaptic plasticity associated with the discontinuation of chronic DZ administration. The results of the current research demonstrate an increased neuronal activity in the dorsal raphe nucleus (DRN) during withdrawal. Previous MK-801 administration impairs the development of anxiety signs observed during withdrawal and the concomitant increased electrical activity on 5-HT neurons on DRN. These results are discussed in terms of the participation of 5-HT system in the modulation of hippocampal plasticity developed on DZ withdrawal.

Medial prefrontal cortex determines how stressor controllability affects behavior and dorsal raphe nucleus

The degree of behavioral control that an organism has over a stressor is a potent modulator of the stressor's impact; uncontrollable stressors produce numerous outcomes that do not occur if the stressor is controllable. Research on controllability has focused on brainstem nuclei such as the dorsal raphe nucleus (DRN). Here we find that the infralimbic and prelimbic regions of the ventral medial prefrontal cortex (mPFCv) in rats detect whether a stressor is under the organism's control. When a stressor is controllable, stress-induced activation of the DRN is inhibited by the mPFCv, and the behavioral sequelae of uncontrollable stress are blocked. This suggests a new function for the mPFCv and implies that the presence of control inhibits stress-induced neural activity in brainstem nuclei, in contrast to the prevalent view that such activity is induced by a lack of control.

Phasic alterations in dopamine and serotonin release in striatum and prefrontal cortex in response to cocaine predictive cues in behaving rhesus macaques

The ability of environmental cues associated with cocaine availability to cause relapse may result from conditioned activation of dopamine (DA) release. We examined this hypothesis in macaque monkeys by conducting microdialysis studies in animals during exposure to a cocaine predictive compound cue. In addition to studying DA release in mesolimbic and sensorimotor striatum, both DA and serotonin levels were determined in the prefrontal cortex (medial orbitofrontal and anterior cingulate). The compound cue employed visual, auditory, and olfactory components, and was salient to the animals as demonstrated by anticipatory lever pressing in the absence of cocaine. During a 10-min period of exposure prior to cocaine availability, there was no significant increase in striatal or cortical DA. The addition of a DA uptake inhibitor to the striatal perfusate to reduce the potential interference of neuronal uptake did not alter the results. In contrast to the lack of any change in striatal DA, a significant decrease in extracellular serotonin in the prefrontal cortex during the 10 min of cue exposure was observed.

Effects of the Potential Antidepressant OPC-14523 [1-[3-[4-(3-chlorophenyl)-1-piperazinyl]propyl]-5-methoxy-3,4-dihydro-2-quinolinone Monomethanesulfonate] a Combined σ and 5-HT1A Ligand: Modulation of Neuronal Activity in the Dorsal Raphe Nucleus

OPC-14523 (OPC; [1-[3-[4-(3-chlorophenyl)-1-piperazinyl]propyl]-5-methoxy-3,4-dihydro-2-quinolinone monomethanesulfonate)] is a novel compound with high affinity for sigma and 5-HT(1A) receptors as well as for the 5-HT transporter. OPC has previously been shown to produce antidepressant-like effects in animal models of depression. This project set out to determine the effect of OPC on serotonergic neurotransmission and to shed light on its mechanism(s) of action. In an electrophysiological model of in vivo extracellular recordings in anesthetized rats, a 2-day treatment (1 mg/kg/day) with OPC induced a significant increase in dorsal raphe nucleus (DRN) putative 5-HT neurons' firing activity. This increase was blocked by the coadministration of NE-100 [N,N-dipropyl-2-(4-methoxy-3-(2-phenylethoxy)phenyl)-thylamine], a selective sigma(1) antagonist (10 mg/kg/day). Furthermore, after 2-day treatments with OPC, the 5-HT(1A) autoreceptor response was altered, as demonstrated by the dramatically reduced response to an increase of endogenous 5-HT induced by the acute administration of paroxetine (500 microg/kg, i.v.). However, the 5-HT(1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (4 microg/kg, i.v.) maintained its ability to decrease 5-HT firing activity, an effect that was reversible by the subsequent administration of the 5-HT(1A) antagonist WAY 100635 [N-[2-(4-[2-methoxyphenyl]-1-piperazinyl)ethyl]-N-2-pyridinylcyclohexanecarboxamide] (100 microg/kg, i.v.). As 8-OH-DPAT has been shown to act preferentially through postsynaptic 5-HT(1A) receptors, our data suggests that this effect of OPC is mediated primarily by the 5-HT(1A) autoreceptor. The decreased response of the 5-HT(1A) autoreceptor to paroxetine was not blocked by the coadministration of NE-100 indicating that sigma(1) receptors are not involved in this effect. Thus, both sigma and 5-HT(1A) receptors play a role in the "antidepressant-like" effects produced by OPC, which is in keeping with previously published behavioral data. In addition, the current series of experiments suggest that OPC might have potential as an antidepressant with a rapid onset of action compared with selective serotonin reuptake inhibitor treatments, which initially suppress the firing activity of putative 5-HT neurons and require at least 2 to 3 weeks to restore the firing activity to baseline neuronal firing activity through a desensitization of the 5-HT(1A) autoreceptor.

Neurochemical and anatomical identification of fast- and slow-firing neurones in the rat dorsal raphe nucleus using juxtacellular labelling methods in vivo

GABA neurones in the dorsal raphe nucleus (DRN) influence ascending 5-hydroxytryptamine (5-HT) neurones but are not physiologically or anatomically characterised. Here, in vivo juxtacellular labelling methods in urethane-anaesthetised rats were used to establish the neurochemical and morphological identity of a fast-firing population of DRN neurones, which recent data suggest may be GABAergic. Slow-firing, putative 5-HT DRN neurones were also identified for the first time using this approach. Fast-firing, DRN neurones were successfully labelled with neurobiotin (n=10) and the majority (n=8/10) were immunoreactive for the GABA synthetic enzyme glutamic acid decarboxylase. These neurones were located in the DRN (mainly lateral regions), and consistently fired spikes with short width (1.1+/-0.1 ms) and high frequency (12.1+/-2.0 Hz). In most cases spike trains were regular but displayed low frequency oscillations (1-2 Hz). These neurones were morphologically heterogeneous but commonly had branching axons with varicosities and dendrites that extended across DRN subregions and the midline. Slow-firing DRN neurones were also successfully labelled with neurobiotin (n=24). These neurones comprised a population of neurones immunopositive for 5-HT and/or tryptophan hydroxylase (n=12) that fired broad spikes (2.2+/-0.2 ms) with high regularity and low frequency (1.7+/-0.2 Hz). However, a slow-firing, less regular population of neurones immunonegative for 5-HT/tryptophan hydroxylase (n=12) was also apparent. In summary, this study chemically identifies fast- and slow-firing neurones in the DRN and establishes for the first time that fast-firing DRN neurones are GABAergic. The electrophysiological and morphological properties of these neurones suggest a novel function involving co-ordination between GABA and 5-HT neurones dispersed across DRN subregions.

Prefrontal cortical projections to the rat dorsal raphe nucleus: ultrastructural features and associations with serotonin and gamma-aminobutyric acid neurons

Studies of human brain indicate that both the ventromedial prefrontal cortex (PFC) and the dorsal raphe nucleus (DRN) may be dysfunctional in major depressive illness, making it important to understand the functional interactions between these brain regions. Anatomical studies have shown that the PFC projects to the DRN, although the synaptic targets of this excitatory pathway have not yet been identified. Electrophysiological investigations in the rat DRN report that most serotonin neurons are inhibited by electrical stimulation of the PFC, suggesting that this pathway is more likely to synapse onto neighboring gamma-aminobutyric acid (GABA) neurons than onto serotonin cells. We tested this hypothesis by electron microscopic examination of DRN sections dually labeled for biotin dextran amine anterogradely transported from the PFC and immunogold-silver labeling for tryptophan hydroxylase (TrH) or for GABA. In the DRN, the majority of PFC axons either synapsed onto unlabeled dendrites or failed to form detectable synapses in single sections. Other PFC axons synapsed onto either TrH- or GABA-immunolabeled processes. Considerably more tissue sampling was necessary to detect PFC synapses onto TrH- than onto GABA-labeled dendrites, suggesting that the latter connections are more common. In other cases, PFC terminals and TrH- or GABA-immunoreactive dendrites either were closely apposed, without forming detectable synapses, or were separated by glial processes. These results provide potential anatomical substrates whereby the PFC can both directly and indirectly regulate the activity of serotonin neurons in the DRN and possibly contribute to the pathophysiology of depression.

Stimulation of α1-adrenoceptors in the rat medial prefrontal cortex increases the local in vivo 5-hydroxytryptamine release: reversal by antipsychotic drugs

Pyramidal neurons of the medial prefrontal cortex (mPFC) project to midbrain serotonergic neurons and control their activity. The stimulation of prefrontal 5-HT2A and AMPA receptors increases pyramidal and serotonergic cell firing, and 5-hydroxytryptamine (5-HT) release in mPFC. As the mPFC contains abundant alpha1-adrenoceptors whose activation increases the excitability of pyramidal neurons, we examined the effects of their stimulation on local 5-HT release, using microdialysis. The application of the alpha1-adrenoceptor agonist cirazoline by reverse dialysis increased the prefrontal 5-HT release in a concentration-dependent manner, an effect antagonized by coperfusion of TTX, prazosin (alpha1-adrenoceptor antagonist), BAY x 3702 (5-HT1A agonist), NBQX (AMPA/KA antagonist) and 1S,3S-ACPD (mGluR II/III agonist), but not by MK-801 (NMDA antagonist). Cirazoline also enhanced the increase in 5-HT release induced by DOI (5-HT2A/2C agonist) and AMPA. In addition, M100907 (5-HT2A antagonist) but not SB-242084 (5-HT2C antagonist) reversed the cirazoline- and AMPA-induced 5-HT release. These results suggest that the stimulation of prefrontal alpha1-adrenoceptors activates pyramidal afferents to ascending serotonergic neurons. The effect of cirazoline was also reversed by coperfusion of classical (chlorpromazine, haloperidol) and atypical (clozapine, olanzapine) antipsychotics, which suggests that a functional antagonism of the alpha1-adrenoceptor-mediated activation of prefrontal neurons may partly underlie their therapeutic action.

Do rats have a prefrontal cortex?

The lack of a single anatomical or functional definition of 'prefrontal cortex' has led to different and, in some respects, controversial views on the existence of a prefrontal cortex in non-primate mammals, in particular in rats. Until the classic paper by Rose and Woolsey [Res. Publ. Assoc. Nerv. Ment. Dis. 27 (1948) 210], the general idea was that a prefrontal cortex is unique to primate species. Rose and Woolsey's 'prefrontal cortex' definition was based upon a single anatomical criterion, i.e. the cortical projection area of the mediodorsal thalamic nucleus. Single criteria, however, do not appear to be sufficient for defining the prefrontal cortex. Therefore, other anatomical and functional characteristics are currently used to identify the prefrontal cortex in different species. Yet, recently the debate about the nature of the prefrontal cortex in non-primate species has been resumed. In the present paper we will compare the structural and functional characteristics of the prefrontal cortex of nonhuman primates and rats. We will argue that rats have a functionally divided prefrontal cortex that includes not only features of the medial and orbital areas in primates, but also some features of the primate dorsolateral prefrontal cortex.

The neurobiology of social anxiety disorder: the relevance of fear and anxiety

OBJECTIVE

Social anxiety disorder (SAD) is a ubiquitous anxiety disorder. Despite being the third most common psychiatric disorder, little is known about the interaction between genetic predisposition and environmental factors in the development of SAD. The available literature on SAD has been compared with data on the genetics and environmental impact on the phenotypic expression of fear and anxiety, and its implicated neurobiology, in order to explore the neurobiology of SAD as understood through the neurochemical dysregulation expressed in fear and anxiety.

METHOD

A systematic review of the literature was employed for the years from 1966 to 2001.

RESULTS

SAD does indeed have much overlap with fear and anxiety. This is best demonstrated by the interactions of the noradrenergic and serotonergic systems with each other and the hypothalamic-pituitary-adrenal axis.

CONCLUSION

SAD may well be understood as one potential outcome for predisposed individuals who are exposed to the proverbial 'second hit', or environmental insult, in childhood. Behavioral inhibition may be an early expression of this predisposition, with natural progression to SAD occurring via a disruption of neurochemical homeostasis. Through animal and human data it has become evident that fear and anxiety have shared, as well as distinct, neurochemical and neuroanatomical pathways. These similarities are expressed as symptoms and objective signs that are common to many individuals with social anxiety disorder.

Differential projections of the infralimbic and prelimbic cortex in the rat

The medial prefrontal cortex has been associated with diverse functions including attentional processes, visceromotor activity, decision-making, goal-directed behavior, and working memory. The present report compares and contrasts projections from the infralimbic (IL) and prelimbic (PL) cortices in the rat by using the anterograde anatomical tracer, Phaseolus vulgaris-leucoagglutinin. With the exception of common projections to parts of the orbitomedial prefrontal cortex, olfactory forebrain, and midline thalamus, PL and IL distribute very differently throughout the brain. Main projection sites of IL are: 1) the lateral septum, bed nucleus of stria terminalis, medial and lateral preoptic nuclei, substantia innominata, and endopiriform nuclei of the basal forebrain; 2) the medial, basomedial, central, and cortical nuclei of amygdala; 3) the dorsomedial, lateral, perifornical, posterior, and supramammillary nuclei of hypothalamus; and 4) the parabrachial and solitary nuclei of the brainstem. By contrast, PL projects at best sparingly to each of these structures. Main projection sites of PL are: the agranular insular cortex, claustrum, nucleus accumbens, olfactory tubercle, the paraventricular, mediodorsal, and reuniens nuclei of thalamus, the capsular part of the central nucleus and the basolateral nucleus of amygdala, and the dorsal and median raphe nuclei of the brainstem. As discussed herein, the pattern of IL projections is consistent with a role for IL in the control of visceral/autonomic activity homologous to the orbitomedial prefrontal cortex of primates, whereas those of PL are consistent with a role for PL in limbic-cognitive functions homologous to the dorsolateral prefrontal cortex of primates.

Female rats show an increased sensibility to the forced swim test depressive-like stimulus in the hippocampus and frontal cortex 5-HT1A receptors

Affective disorders are more common in women. The forced swim test acts like a depressive stimulus. Hippocampus and frontal cortex 5-HT1A receptors of female and male Wistar rats subjected to the forced swim test were compared with a sham group. The forced swim test diminishes (P<0.05) the hippocampus 3H-8OH-DPAT bound in the female rats (184+/-16 fmol/mg protein) with respect to the male rats (309+/-41 fmol/mg protein) and to the female sham rats (255+/-20 fmol/mg protein). The forced swim test increases the frontal cortex 5-HT1A receptors in the female rats with respect to the female sham group (40.4+/-5 versus 24.7+/-4 fmol/mg protein, P<0.05). An increased sensibility of the 5-HT1A receptors to depressive-stimulus may be one mechanism underlying the higher prevalence of depression in female.

The medial prefrontal cortex in the rat: evidence for a dorso-ventral distinction based upon functional and anatomical characteristics

The prefrontal cortex in rats can be distinguished anatomically from other frontal cortical areas both in terms of cytoarchitectonic characteristics and neural connectivity, and it can be further subdivided into subterritories on the basis of such criteria. Functionally, the prefrontal cortex of rats has been implicated in working memory, attention, response initiation and management of autonomic control and emotion. In humans, dysfunction of prefrontal cortical areas with which the medial prefrontal cortex of the rat is most likely comparable is related to psychopathology including schizophrenia, sociopathy, obsessive-compulsive disorder, depression, and drug abuse. Recent literature points to the relevance of conducting a functional analysis of prefrontal subregions and supports the idea that the area of the medial prefrontal cortex in rats is characterized by its own functional heterogeneity, which may be related to neuroanatomical and neurochemical dissociations. The present review covers recent findings with the intent of correlating these distinct functional differences in the dorso-ventral axis of the rat medial prefrontal cortex with anatomical and neurochemical patterns.

Stressor controllability modulates stress-induced dopamine and serotonin efflux and morphine-induced serotonin efflux in the medial prefrontal cortex

It has previously been shown that inescapable (IS) but not escapable (ES) stress potentiates the rewarding properties of morphine as measured by conditioned place preference and psychomotor activation, and that this potentiation may be mediated by dorsal raphe nucleus (DRN) serotonin (5-HT) neurons. The medial prefrontal cortex (mPFC) has been implicated in both reward and stress, and is a projection region of the DRN. The mPFC also contains dopaminergic afferents from the ventral tegmental area, which has been the focus of many studies exploring both the rewarding properties of drugs and the aversive properties of stress. The role of the mPFC in stress/drug reactivity interactions is largely unknown. The present study used in vivo microdialysis to examine 5-HT and dopamine (DA) efflux in the mPFC of rats during IS, ES or no stress (NS). IS and ES rats received the stressor in yoked pairs. The stressor consisted of tailshocks that could be terminated for both rats by the ES rats. Large increases in 5-HT and DA levels were observed during IS but not ES or NS. DA and 5-HT efflux were also measured 24 h later in the same rats in response to morphine (3 mg/kg) or saline. Sustained increases in 5-HT levels were observed after morphine in rats that had previously received IS but not in rats that had received ES or NS. No changes in DA efflux were observed after morphine. Thus, 5-HT and DA in the mPFC may be involved in stressor controllability effects, and the sensitization of 5-HT neurons by IS extends to the mPFC and to morphine as a challenge.

In vivo modulation of 5-hydroxytryptamine release in mouse prefrontal cortex by local 5-HT(2A) receptors: effect of antipsychotic drugs

In the rat, postsynaptic 5-hydroxytryptamine2A receptors medial prefrontal cortex control the activity of the serotonergic system through changes in the activity of pyramidal neurons projecting to the dorsal raphe nucleus. Here we extend these observations to mouse brain. The prefrontal cortex expresses abundant 5- hydroxytryptamine2A receptors, as assessed by immunohistochemistry, Western blots and in situ hybridization procedures. The application of the 5-hydroxytryptamine2A/2C agonist DOI (100 microm) by reverse dialysis in the medial prefrontal cortex doubled the local release of 5-hydroxytryptamine. This effect was reversed by coperfusion of tetrodotoxin, and by the selective 5-hydroxytryptamine2A receptor antagonist M100907, but not by the 5-hydroxytryptamine2C antagonist SB-242084. The effect of DOI was also reversed by prazosin (alpha1-adrenoceptor antagonist), BAY x 3702 (5-hydroxytryptamine1A receptor agonist), NBQX (alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionate/kainic acid antagonist) and 1S,3S-ACPD (mGluR II/III agonist), but not by dizocilpine (N-methyl-d-aspartate antagonist). alpha-Amino-3-hydroxy-5-methyl-4-isoxazole-4-propionate mimicked the 5-hydroxytryptamine elevation produced by DOI, an effect also reversed by BAY x 3702. Likewise, the coperfusion of classical (chlorpromazine, haloperidol) and atypical antipsychotic drugs (clozapine, olanzapine) fully reversed the 5-hydroxytryptamine elevation induced by DOI. These observations suggest that DOI increases 5-hydroxytryptamine release in the mouse medial prefrontal cortex through the activation of local 5-hydroxytryptamine2A receptors by an impulse-dependent mechanism that involves/requires the activation of local alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionate receptors. This effect is reversed by ligands of receptors present in the medial prefrontal cortex, possibly in pyramidal neurons, which are involved in the action of antipsychotic drugs. In particular, the reversal by classical antipsychotics may involve blockade of alpha1-adrenoceptors, whereas that of atypical antipsychotics may involve 5-hydroxytryptamine2A receptors and alpha1-adrenoceptors.

Hyperlocomotive and discriminative stimulus effects of cocaine are under the control of serotonin(2C) (5-HT(2C)) receptors in rat prefrontal cortex

The serotonin2C (5-hydroxytryptamine2C; 5-HT2C) receptor (5-HT2CR) is found in abundance in dopamine (DA) mesocorticolimbic pathways and is one of the important target proteins that modulates the behavioral effects of cocaine. In the present study, the hypothesis was tested that the 5-HT2CR in the prefrontal cortex (PFC) may control either spontaneous or cocaine-evoked locomotor activity as well as the discriminative stimulus properties of cocaine. In male Sprague-Dawley rats implanted with bilateral cannulae aimed at the PFC, local microinjections of the preferential 5-HT2CR agonist 6-chloro-2-(1-piperazinyl)pyrazine hydrochloride (MK 212) (0.05-0.5 microg/side) did not alter spontaneous activity, but dose-dependently decreased horizontal hyperactivity evoked by cocaine (10 mg/kg i.p.). Given alone, the selective 5-HT2CR antagonist 8-[5-(2,4-dimethoxy-5-(4-trifluorophenylsulfonamido)phenyl-5-oxopentyl]-1,3,8-triazo-spiro[4.5]decane-2,4-dione hydrochloride (RS 102221) (5 microg/side) increased basal locomotor activity of rats expressed in the vertical plane. Microinjections of RS 102221 (5 microg/side, but not 0.15-1.5 microg/side) significantly enhanced the horizontal activity induced by cocaine (10 mg/kg). In rats trained to discriminate cocaine (10 mg/kg i.p.) from saline (i.p.) in a two-lever, water-reinforced fixed ratio 20 task, intra-PFC microinjections of MK 212 (0.05 and 0.5 microg/side) did not substitute for cocaine, but attenuated the stimulus effects of cocaine. However, intra-PFC microinjections of RS 102221 (1.5 and 5 microg/side) evoked 13 and 40% cocaine-lever responding when tested alone and enhanced the recognition of cocaine. These data indicate that the PFC is a brain site at which the 5-HT2CR exerts an inhibitory control over the hyperactive and discriminative stimulus effects of cocaine known to be dependent upon activation of the DA mesoaccumbens circuit.

Regulation of septo-hippocampal activity by 5-hydroxytryptamine(2C) receptors

It is established that the serotonin system modulates hippocampal functions by regulating neuronal activity of both the medial septum and hippocampus. Inhibition of serotonin neurons leads to theta oscillation of septal neurons and theta wave activity in the hippocampus, indicating a tonic regulation of the septo-hippocampal system by serotonin neurons. Because the postsynaptic 5-hydroxytryptamine (5-HT) receptor subtypes mediating this tonic inhibition have not been identified, a putative role of 5-HT2C receptors has been evaluated in the present study. Extracellular single units were recorded from the medial septum/vertical limb of diagonal band (MS/DBv) and hippocampal CA1 or dentate gyrus with simultaneous hippocampal EEG recordings from anesthetized rats. Intravenous administration of 5-HT2C receptor agonists 1-(3-chlorophenyl)piperazine dihydrochloride (m-CPP) and [S]-2-(chloro-5-fluoro-indol-1-yl)-1-methyl-ethylamine fumarate (Ro 60-0175) dose dependently inhibited firing activity most of the recorded MS/DBv neurons and abolished theta oscillation in all tested MS/DBv and hippocampal neurons. Parallel to inhibition of theta oscillation of MS/DBv neurons, hippocampal EEG activity was desynchronized and its power spectrum was shifted to lower frequencies. The selective 5-HT2C receptor antagonist 6-chloro-5-methyl-1-[2-(2-methylpyridyl-3-oxy)-pyrid-5-yl carbomyl] indoline (SB-242084) [but not the 5-HT2B antagonist 2-amino-4-(4-fluoronaphth-1-yl)-6-isopropyl-pyrimidine (RS-127445) or 5-HT2A antagonist R-(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)-ethyl]-4-piperidinemethanol (MDL-100907)] reversed the action of 5-HT2C receptor agonists. Furthermore, in control rats 5-HT2C receptor antagonists [SB-242084 and 5-methyl-1-(3-pyridil-carbamoyl)-1,2,3,5-tetrahydropyr-rolo[2,3-f]indole hydrochloride (SB-206553)] induced or enhanced theta oscillation in MS/DBv and hippocampal neurons and theta wave activity of the hippocampus. These findings provide evidence for a tonic regulation of the activity and theta oscillation of the septo-hippocampal system via 5-HT2C receptors in the anesthetized rat, indicating that pharmacological interactions with these receptors could modulate various physiological and pathological processes associated with limbic theta activity.

Evidence that central 5-HT2A and 5-HT2B/C receptors regulate 5-HT cell firing in the dorsal raphe nucleus of the anaesthetised rat

1. Systemic administration of phenethylamine-derived, 5-hydroxytryptamine(2) (5-HT(2)) receptor agonists inhibits the firing of midbrain 5-HT neurones, but the 5-HT receptors involved are poorly defined, and the contribution of peripheral mechanisms is uncertain. This study addresses these issues using extracellular recordings of 5-HT neurones in the dorsal raphe nucleus of anaesthetised rats. 2. The 5-HT(2) receptor agonists DOI ((+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride) and DOB ((+/-)-2,5-dimethoxy-4-bromoamphetamine hydrobromide), caused a dose-related (10-100 micro g kg(-1) i.v.) inhibition of 5-HT neuronal activity, with the highest dose reducing firing rates by >80%. 3. Pretreatment with the 5-HT(2) receptor antagonist ritanserin (1 mg kg(-1) i.v.) completely blocked the action of DOI. The 5-HT(2A) receptor antagonist MDL 100,907 (0.2 mg kg(-1) i.v.) blocked the action of both DOI and DOB. In comparison, the 5-HT(2B/C) receptor antagonist SB 206553 (0.5 mg kg(-1) i.v.) caused a small, but statistically significant, shift to the right in the dose response to DOI and DOB. 4. Pretreatment with the peripherally acting 5-HT(2) receptor antagonist BW 501C67 (0.1 mg kg(-1) i.v.) had no effect on the DOI-induced inhibition of 5-HT cell firing, but completely blocked the DOI-induced rise in mean arterial blood pressure. 5. These data indicate that the inhibition of 5-HT cell firing induced by systemic administration of DOI and DOB is mediated predominantly by the 5-HT(2A) receptor-subtype, but that 5-HT(2B/C) receptors also play a minor role. Moreover, central and not peripheral mechanisms are involved. Given evidence that 5-HT(2) receptors are not located on 5-HT neurones, postsynaptic 5-HT feedback mechanisms are implicated.

Use of Arc expression as a molecular marker of increased postsynaptic 5-HT function after SSRI/5-HT1A receptor antagonist co-administration

An increase in central postsynaptic 5-hydroxytryptamine (5-HT) function activates expression of activity-related cytoskeletal protein (Arc). Here, Arc expression was used to test whether, in rats, co-administration of a 5-HT re-uptake inhibitor (paroxetine) and a 5-HT1A receptor antagonist (WAY 100635) increases postsynaptic 5-HT function. After pre-treatment with WAY 100635 (0.3 mg/kg s.c.), paroxetine (5 mg/kg s.c.) caused a threefold increase in 5-HT in prefrontal cortex microdialysates. In situ hybridization studies found that neither paroxetine (5 mg/kg s.c.) nor WAY 1000635 (0.3 mg/kg s.c.) altered Arc mRNA abundance in any region examined. In contrast, paroxetine (5 mg/kg s.c.) increased Arc mRNA after pre-treatment with WAY 100635 (0.3 mg/kg s.c.). This increase was apparent in cortical regions (frontal, parietal and cingulate) and caudate nucleus but was absent in hippocampus (CA1). Increases in Arc mRNA were accompanied by an increase in c-fos mRNA. The increase in Arc expression induced by paroxetine/WAY 100635 was abolished by the 5-HT synthesis inhibitor, p-chlorophenylalanine (300 mg/kg i.p., daily for two days). In conclusion, paroxetine and WAY 100635 injected in combination (but not alone) caused a region-specific, 5-HT-mediated increase in Arc expression. These data provide molecular evidence that co-administration of a 5-HT re-uptake inhibitor and 5-HT1A receptor antagonist increases 5-HT function at the postsynaptic level.

Intra-prefrontal 8-OH-DPAT and M100907 improve visuospatial attention and decrease impulsivity on the five-choice serial reaction time task in rats

RATIONALE

The central serotonergic systems are a major target for drugs used to treat neuropsychiatric disorders such as depression and schizophrenia in which disruption of frontal cortex function has been implicated. However, it is not known precisely how serotonin (5-HT) modulates the medial prefrontal cortex (mPFC) to affect cognitive function and behaviour.

OBJECTIVE

To investigate the roles of 5-HT(1A) and 5-HT(2A) receptors in mPFC on performance of the five-choice serial reaction time task (5CSRT), which assesses visuospatial attention, impulsivity and motivational processes.

METHODS

Following training on the 5CSRT, rats were implanted with bilateral guide cannulae aimed at the mPFC. Rats received intra-mPFC infusions of either 8-OH-DPAT (10, 30 and 100 ng) or M100907 (30, 100 and 300 ng) according to a Latin square design.

RESULTS

Both 8-OH-DPAT and M100907 selectively enhanced accuracy of target detection. When the stimulus duration was shortened, infusions of 8-OH-DPAT continued to improve accuracy, whereas M100907 decreased premature responding and omissions, thus partly dissociating the effects of these two compounds. Similar effects were obtained following systemic administration of M100907 and 8-OH-DPAT. The effects of 8-OH-DPAT were blocked by the 5-HT(1A) antagonist WAY 100635, at a dose that itself had no significant effects on behaviour.

CONCLUSIONS

These results indicate that modulation of 5-HT function within the mPFC via distinct receptors can enhance performance on the 5CSRT. These findings suggest a mechanism by which serotonergic agents improve cognitive function, which may be relevant to their therapeutic benefit in the treatment of neuropsychiatric disorders.

Norepinephrine but not serotonin reuptake inhibitors enhance theta and gamma activity of the septo-hippocampal system

Current neurobiological concepts attribute a central role of the hippocampal formation in cognitive and affective processes. Recent studies indicate that the hippocampus is affected in human depression, and antidepressant drugs induce hippocampal adaptive changes that are thought to be associated with their therapeutic action. In the present study, we investigated the action of various antidepressant drugs on the activity of the septo-hippocampal system, its oscillatory activity in particular. The acute effects of the norepinephrine (NE) reuptake inhibitors reboxetine and desipramine, and the selective serotonin reuptake inhibitor fluvoxamine were evaluated. Extracellular single-unit recordings were performed from the medial septum/diagonal band of Broca (MS/DBv), with simultaneous hippocampal EEG recordings of anesthetized rats. Systemic administration of reboxetine synchronized hippocampal EEG, resulting in a significant increase in power at theta frequency, and an increase in frequency and power of gamma-wave activity. Parallel to EEG synchrony, reboxetine induced or enhanced theta oscillation of MS/DBv neurons. Oscillatory frequencies of MS/DBv neurons were identical, and phase locked to the corresponding hippocamapal theta frequencies. Under the same experimental conditions, reboxetine induced a two-fold increase in extracellular NE (but not serotonin) levels in the hippocampus as revealed by microdialysis. Desipramine, but not the serotonin reuptake inhibitor fluvoxamine, evoked responses similar to those of reboxetine regarding septo-hippocampal theta activity. The present findings indicate that even though both NE and serotonin reuptake inhibitors are clinically effective antidepressant drugs, their action on the septo-hippocampal oscillatory behavior is different. It is presumed that selective NE reuptake inhibitors could modulate various cognitive processes associated with hippocampal oscillatory activity.

Control of 5-hydroxytryptamine release in the dorsal raphe nucleus by the noradrenergic system in rat brain. Role of alpha-adrenoceptors

The interactions between the brainstem serotonergic (5-hydroxytryptamine, 5-HT) and noradrenergic (NA) systems are important for the pathophysiology and treatment of affective disorders. We examined the influence of alpha-adrenoceptors on 5-HT and NA release in the rat dorsal raphe nucleus (DR) using microdialysis. 5-HT and NA concentrations in DR dialysates were virtually suppressed by TTX and increased by veratridine. The local and systemic administration of the alpha(1)-adrenoceptor antagonist prazosin reduced the DR 5-HT output but not that of NA. The maximal 5-HT reduction induced by local prazosin administration (-78% at 100 microM) was more marked than by its systemic administration (-43% at 0.3 mg/kg). The local application of NA and desipramine, to increase the tone on DR alpha(1)-adrenoceptors, did not enhance 5-HT release. The local (100 microM) or systemic (0.1-1 mg/kg s.c.) administration of clonidine reduced 5-HT and NA release (-48 and -79%, respectively, at 1 mg/kg), an effect reversed by RX-821002, which by itself increased both amines when given systemically. DSP-4 pretreatment prevented the effects of clonidine on 5-HT, suggesting the participation of alpha(2)-adrenoceptors on NA elements. Moreover, the systemic effect of clonidine on 5-HT (but not NA) was cancelled by lesion of the lateral habenula and by anesthesia, and was slightly enhanced by cortical transection. These data support the view that alpha(1)-adrenoceptors in the DR tonically stimulate 5-HT release, possibly at nearly maximal tone. Likewise, the 5-HT release is modulated by alpha(2)-adrenoceptors in NA neurons and in forebrain areas involved in the distal control of 5-HT neurons.

Modulation of serotonergic function in rat brain by VN2222, a serotonin reuptake inhibitor and 5-HT1A receptor agonist

VN2222 (1-(benzo[b]thiophen-3-yl)-3-[4-(2-methoxiphenyl piperazin-1-yl]propan-1-ol) is a potential antidepressant with high affinity for the serotonin transporter and 5-HT(1A) receptors. Locally applied, VN2222 enhanced the extracellular 5-hydroxytryptamine (5-HT) concentration (5-HT(ext)) in rat striatum to 780% of baseline whereas its systemic administration (1-10 mg/kg s.c.) reduced 5-HT(ext). In the presence of citalopram, 8-OH-DPAT or VN2222 applied in medial prefrontal cortex reduced 5-HT(ext). Fluoxetine, VN2222, and 8-OH-DPAT suppressed the firing rate of dorsal raphe 5-HT neurons (ED(50): 790, 14.9, and 0.8 microg/kg i.v., respectively). These effects were antagonized by WAY 100635. Administration of VN2222 for 2 weeks desensitized 5-HT(1A) receptors as assessed by microdialysis and single-unit recordings (ED(50) values for 8-OH-DPAT were 0.45 and 2.34 microg/kg i.v. for controls and rats treated with 6 mg/kg day VN2222). These results show that VN2222 is a mixed 5-HT reuptake inhibitor/5-HT(1A) agonist that markedly desensitizes 5-HT(1A) autoreceptors. These properties suggest that it may be a clinically effective dual action antidepressant drug.

Glutamatergic afferent projections to the dorsal raphe nucleus of the rat

Based on WGA-apo-HRP-gold (WG) retrograde tracing, the present study revealed that different subdivisions of the dorsal raphe (DR) such as dorsomedial, ventromedial, lateral wing, and caudal regions receive unique, topographically organized afferent inputs, that are more restricted than previously reported. Phaseolus vulgaris leucoagglutinin anterograde tracing studies confirmed that the medial prefrontal cortex provides the major afferent input to each subdivision of the DR. Double-labeling studies combining WG tracing and glutamate immunostaining indicated that the medial prefrontal cortex, various hypothalamic nuclei including perifornical, lateral, and arcuate nuclei, and several medullary regions such as lateral and medial parabrachial nuclei, and laterodorsal tegmental nucleus provide the major glutamatergic input to each subregion of the DR. It should be noted that the degree of glutamatergic input from these afferent sites was specific for each DR subdivision. The present findings indicated that dorsomedial, ventromedial, lateral wing, and caudal subdivisions of the DR receive excitatory inputs from both cortical and subcortical sites which might be involved in regulation or modulation of a broad range of systems, including sensory and motor functions, arousal and sleep-wake cycle, biorhythmic, cognitive, and affective behaviors.

Nicotine induces glutamate release from thalamocortical terminals in prefrontal cortex

It has been proposed that activation of nicotinic acetylcholine receptors (nAChRs) can activate the prefrontal cortex, enhancing attention and cognition. Nicotine can stimulate the release of several different neurotransmitters in many brain regions. In the present study, we found that stimulation of nAChRs by nicotine or the endogenous agonist, acetylcholine (ACh), induces a large spontaneous increase in glutamate release onto layer V pyramidal neurons of the prefrontal cortex. This release of glutamate, measured by spontaneous excitatory postsynaptic currents (sEPSCs) in the prefrontal cortical slice, depends on intact thalamocortical terminals. It can be suppressed by mu-opioids or eliminated by blocking action potentials. The increase in sEPSCs is sensitive to low concentrations of nicotine, suggesting the involvement of high-affinity (eg alpha(4)beta(2)) nAChRs. Recent work has shown alterations in prefrontal alpha(4)beta(2) nAChRs in autism and schizophrenia, two conditions that are distinguished by abnormal prefrontal cortical activation as well as difficulty in certain aspects of cognition and integrating social and emotional cues. We show that mice lacking the beta(2) nAChR subunit do not show increased sEPSCs with either nicotine or ACh, again implicating high-affinity nicotinic receptors. These findings give new insight into the mechanism by which nicotine affects excitatory neurotransmission to the output neurons of the cerebral cortex in a pathway that is critical for cognitive function and reward expectation.

Influence of inhibitory and excitatory inputs on serotonin efflux differs in the dorsal and median raphe nuclei

The dorsal (DRN) and median raphe nuclei (MRN) are two major sources of serotonergic projections to forebrain that are involved in regulation of behavioral state and motor activity, and implicated in affective disorders such as depression and schizophrenia. To investigate afferent influences on serotonergic neurons, this study compared the role of endogenous GABA and glutamate in the DRN and MRN using microdialysis and measurement of locomotor activity in freely behaving rats. Local infusion of the GABA(A) receptor antagonist bicuculline increased serotonin (5-HT) efflux in the DRN but not the MRN. In contrast, infusion of glutamate receptor antagonists produced larger decreases in 5-HT efflux in the MRN compared with the DRN. Moreover, glutamate receptor antagonists attenuated the increase in 5-HT efflux produced by GABA receptor blockade in the DRN. Thus, the disinhibitory effect of GABA blockers could be ascribed in part to an enhanced influence of glutamate. Measurements of locomotor activity indicate that changes in 5-HT were not simply correlated with behavioral activity induced by drug infusion. In summary, the role of inhibitory and excitatory afferents was strikingly different in the DRN and MRN. GABA afferents were the predominant tonic influence on serotonergic neurons in the DRN. In contrast, glutamatergic but not GABAergic afferents had a strong tonic influence on serotonergic neurons in the MRN.