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

Deep Brain Stimulation of the dorsal raphe abolishes serotonin 1A facilitation of AMPA receptor-mediated synaptic currents in the ventral hippocampus

In a previous study we showed that Deep Brain Stimulation (DBS) of the rat dorsal subregion of the dorsal raphe (DRD), which sends serotonergic projections to forebrain areas, such as the ventral hippocampus, induces anxiolytic-like effects. The purpose of the present study was to investigate neurobiological alterations which might underline these behavioral effects. For that, we tested the influence of DBS upon the neuromodulatory action of serotonin on excitatory post-synaptic currents (EPSCs) in the ventral hippocampus. Male Wistar rats were submitted to high-frequency stimulation (100 μA, 100 Hz) of the DRD for 1 h during three consecutive days. On the third day, immediately after the DBS procedure, animals were euthanized. Slices of the ventral hippocampus were processed for whole cell patch clamp recordings of AMPA-receptor (AMPAR) mediated EPSCs in the CA1 area. As reported by others, we confirmed that in pre-weaning rats a high affinity 5-HT1A receptor agonist (8-OH-PIPAT, 0.5-5nM) inhibits EPSCs. However, in adult rats (non-operated or sham-operated), 8-OH-PIPAT (0.5-5 nM) increased EPSC amplitude, an effect blocked by the 5-HT1A antagonist WAY-100,635 (200 nM). Importantly, in adult rats exposed to DBS, the 5-HT1A agonist was devoid of effect. Taken together these results show that: 1) changes in 5-HT1A receptor-mediated hippocampal synaptic transmission occur with age; 2) these changes lead to a facilitatory effect of 5-HT1A receptors; 3) DBS blocks this serotonergic facilitatory action. These observations suggest that an alteration in serotonin modulation of limbic areas may underlie the psychotherapeutic effects of DBS.

Serotonin 2C receptors in the basolateral amygdala mediate the anxiogenic effect caused by serotonergic activation of the dorsal raphe dorsomedial subnucleus

BACKGROUND

Stimulation of serotonergic neurons within the dorsal raphe dorsomedial subnucleus facilitates inhibitory avoidance acquisition in the elevated T-maze. It has been hypothesized that such anxiogenic effect is due to serotonin release in the basolateral nucleus of the amygdala, where facilitation of serotonin 2C receptor-mediated neurotransmission increases anxiety. Besides the dorsal raphe dorsomedial subnucleus, the dorsal raphe caudal subnucleus is recruited by anxiogenic stimulus/situations. However, the behavioral consequences of pharmacological manipulation of this subnucleus are still unknown.

AIMS

Investigate whether blockade of serotonin 2C receptors in the basolateral nucleus of the amygdala counteracts the anxiogenic effect caused by the stimulation of dorsal raphe dorsomedial subnucleus serotonergic neurons. Evaluate the effects caused by the excitatory amino acid kainic acid or serotonin 1A receptor-modulating drugs in the dorsal raphe caudal subnucleus.

METHODS

Male Wistar rats were tested in the elevated T-maze and light-dark transition tests after intra-basolateral nucleus of the amygdala injection of the serotonin 2C receptor antagonist SB-242084 (6-chloro-2,3-dihydro-5-methyl-N-[6-[(2-methyl-3-pyridinyl)oxy]-3-pyridinyl]-1H-indole-1-carboxyamide dihydrochloride) followed by intra-dorsal raphe dorsomedial subnucleus administration of the serotonin 1A receptor antagonist WAY-100635 (N-[2-[4-2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinil-cyclohexanecarboxamide maleate). In the dorsal raphe caudal subnucleus, animals were injected with kainic acid, WAY-100635 or the serotonin 1A receptor agonist 8-OH-DPAT ((±)-8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide) and tested in the elevated T-maze.

RESULTS

SB-242084 in the basolateral nucleus of the amygdala blocked the anxiogenic effect caused by the injection of WAY-100635 in the dorsal raphe dorsomedial subnucleus. Kainic acid in the dorsal raphe caudal subnucleus increased anxiety, but also impaired escape expression in the elevated T-maze. Neither WAY-100635 nor 8-OH-DPAT in the dorsal raphe caudal subnucleus affected rat's behavior in the elevated T-maze.

CONCLUSION

Serotonin 2C receptors in the basolateral nucleus of the amygdala mediate the anxiogenic effect caused by the stimulation of serotonergic neurons in the dorsal raphe dorsomedial subnucleus. The dorsal raphe caudal subnucleus regulates anxiety- and panic-like behaviors, presumably by a serotonin 1A receptor-independent mechanism.

Functional neuroanatomy in panic disorder: Status quo of the research

AIM

To provide an overview of the current research in the functional neuroanatomy of panic disorder.

METHODS

Panic disorder (PD) is a frequent psychiatric disease. Gorman et al (1989; 2000) proposed a comprehensive neuroanatomical model of PD, which suggested that fear- and anxiety-related responses are mediated by a so-called “fear network” which is centered in the amygdala and includes the hippocampus, thalamus, hypothalamus, periaqueductal gray region, locus coeruleus and other brainstem sites. We performed a systematic search by the electronic database PubMed. Thereby, the main focus was laid on recent neurofunctional, neurostructural, and neurochemical studies (from the period between January 2012 and April 2016). Within this frame, special attention was given to the emerging field of imaging genetics.

RESULTS

We noted that many neuroimaging studies have reinforced the role of the “fear network” regions in the pathophysiology of panic disorder. However, recent functional studies suggest abnormal activation mainly in an extended fear network comprising brainstem, anterior and midcingulate cortex (ACC and MCC), insula, and lateral as well as medial parts of the prefrontal cortex. Interestingly, differences in the amygdala activation were not as consistently reported as one would predict from the hypothesis of Gorman et al (2000). Indeed, amygdala hyperactivation seems to strongly depend on stimuli and experimental paradigms, sample heterogeneity and size, as well as on limitations of neuroimaging techniques. Advanced neurochemical studies have substantiated the major role of serotonergic, noradrenergic and glutamatergic neurotransmission in the pathophysiology of PD. However, alterations of GABAergic function in PD are still a matter of debate and also their specificity remains questionable. A promising new research approach is “imaging genetics”. Imaging genetic studies are designed to evaluate the impact of genetic variations (polymorphisms) on cerebral function in regions critical for PD. Most recently, imaging genetic studies have not only confirmed the importance of serotonergic and noradrenergic transmission in the etiology of PD but also indicated the significance of neuropeptide S receptor, CRH receptor, human TransMEMbrane protein (TMEM123D), and amiloride-sensitive cation channel 2 (ACCN2) genes.

CONCLUSION

In light of these findings it is conceivable that in the near future this research will lead to the development of clinically useful tools like predictive biomarkers or novel treatment options.

Relative contribution of the dorsal raphe nucleus and ventrolateral periaqueductal gray to morphine antinociception and tolerance in the rat

The dorsal raphe nucleus (DRN) is embedded in the ventral part of the caudal periaqueductal gray (PAG). Electrical or chemical activation of neurons throughout this region produces antinociception. The objective of this manuscript is to determine whether the ventrolateral PAG and DRN are distinct antinociceptive systems. This hypothesis was tested by determining the antinociceptive potency of microinjecting morphine into each structure (Experiment 1), creating a map of effective microinjection sites that produce antinociception (Experiment 2) and comparing the development of antinociceptive tolerance to repeated microinjections of morphine into the ventrolateral PAG and DRN (Experiment 3). Morphine was more potent following cumulative injections (1.0, 2.2, 4.6 & 10 μg/0.2 μL) into the ventrolateral PAG (D₅₀  = 3.3 μg) compared to the lateral (4.3 μg) or medial DRN (5.8 μg). Antinociception occurred following 94% of the morphine injections into the ventrolateral PAG, whereas only 68.3% and 78.3% of the injections into the lateral and medial aspects of the DRN produced antinociception. Repeated microinjections of morphine into the ventrolateral PAG produced tolerance as indicated by a 528% difference in potency between morphine and saline pretreated rats. In contrast, relatively small changes in potency occurred following repeated microinjections of morphine into the lateral and medial aspects of the DRN (107% and 49%, respectively). These data indicate that the ventrolateral PAG and DRN are distinct antinociceptive structures. Antinociception is greater with injections into the ventrolateral PAG compared to the DRN, but this antinociception disappears rapidly because of the development of tolerance.

Rat exposure in mice with neuropathic pain induces fear and antinociception that is not reversed by 5-HT2C receptor activation in the dorsal periaqueductal gray

Previous studies have demonstrated that serotonin 5-HT2C receptors in the dorsal periaqueductal gray (dPAG) mediate both anxiety and antinociception in mice submitted to the elevated plus maze. The present study examined the effects of intra-dPAG infusion of the serotonin 5-HT2C receptor agonist (MK-212) in the defensive reactions and antinociception in mice with neurophatic pain confronted by a predator. Neuropathic pain was induced by chronic constriction injury (CCI) of the sciatic nerve, and predator confrontation was performed using the rat exposure test (RET). Our results demonstrated that both sham-operated and CCI mice exhibited intense defensive reactions when confronted by rats. However, rat-exposed CCI mice showed reduced pain reactivity in comparison to CCI mice exposed to a toy rat. Intra-dPAG infusion of MK-212 prior to predator exposure did not significantly alter defensive or antinociceptive responses. To our knowledge, our results represent the first evidence of RET-induced antinociception in mice. Moreover, the results of the present study suggest that 5-HT2C receptor activation in the dPAG is not critically involved in the control of predator-evoked fearful or antinociceptive responses.

5-HT1A receptors of the rat dorsal raphe lateral wings and dorsomedial subnuclei differentially control anxiety- and panic-related defensive responses

The dorsal raphe nucleus (DR), the main source of 5-HT projections to brain areas involved in anxiety regulation, is composed by 5 subnuclei that differ morphologically, functionally and neurochemically. Based on immunohistochemical evidence, it has been proposed that whereas 5-HT cells of the dorsomedial (dmDR) and caudal subnuclei are implicated in the pathophysiology of generalized anxiety disorder (GAD), neurons of the lateral wings (lwDR) are associated with panic disorder (PD). We here tested this hypothesis from a behavioral perspective by investigating the consequences of the non-selective stimulation of neurons within the dmDR and lwDR, or the pharmacological manipulation of 5-HT1A receptors located in these nuclei, of male Wistar rats exposed to the elevated T-maze. This test allows the measurement of both a GAD- (i.e. inhibitory avoidance) and a PD- (i.e. escape) related response in the same animal. Intra-dmDR injection of either the excitatory amino acid kainic acid or the 5-HT1A receptor antagonist WAY-100635 facilitated inhibitory avoidance acquisition, suggesting an anxiogenic effect, and inhibited escape expression, a panicolytic-like effect. Microinjection of the 5-HT1A receptor agonist 8-OH-DPAT caused the opposite effect. Administration of the same drugs into the lwDR only altered escape performance. Whereas kainic acid and 8-OH-DPAT facilitated its expression, WAY-100635 inhibited it. At higher doses, kainic acid administration evoked vigorous escape reactions as measured in an open-field. These findings implicate 5-HT neurons of the dmDR in the regulation of both GAD- and PD-related defensive behaviors. They also support a primary role of the lwDR in the mediation of PD-associated responses.

Nociceptin/orphanin FQ induces simultaneously anxiolytic and amnesic effects in the mouse elevated T-maze task

Studies have shown a close relationship between anxiety and aversive memory processing, but few animal models are suitable for investigating the effects of a given compound on anxiety and memory simultaneously. A growing body of evidence suggests anxiolytic and amnesic effects of nociceptin/orphanin FQ (N/OFQ). The mouse elevated T-maze (ETM) has been shown to detect the effects of drugs on anxiety and memory at the same time. In this study, the effects of intracerebroventricular N/OFQ injected before or immediately after training session were assessed in the ETM task. When pretraining injected, N/OFQ 0.1 nmol significantly decreased the latency to enter an open arm in the training session compared to control, which is suggestive of anxiolysis. In addition, N/OFQ (0.1 and 1 nmol) significantly reduced the latency to enter an open arm during the test session compared to control, thus suggesting memory impairments. However, when N/OFQ was administered posttraining, it did not affect memory retrieval. No alterations in locomotion were detected in N/OFQ-treated mice in the open field test. In conclusion, these findings are discussed considering the simultaneous anxiolytic and amnesic effects of N/OFQ.

The Deakin/Graeff hypothesis: focus on serotonergic inhibition of panic

The Deakin/Graeff hypothesis proposes that different subpopulations of serotonergic neurons through topographically organized projections to forebrain and brainstem structures modulate the response to acute and chronic stressors, and that dysfunction of these neurons increases vulnerability to affective and anxiety disorders, including panic disorder. We outline evidence supporting the existence of a serotonergic system originally discussed by Deakin/Graeff that is implicated in the inhibition of panic-like behavioral and physiological responses. Evidence supporting this panic inhibition system comes from the following observations: (1) serotonergic neurons located in the 'ventrolateral dorsal raphe nucleus' (DRVL) as well as the ventrolateral periaqueductal gray (VLPAG) inhibit dorsal periaqueductal gray-elicited panic-like responses; (2) chronic, but not acute, antidepressant treatment potentiates serotonin's panicolytic effect; (3) contextual fear activates a central nucleus of the amygdala-DRVL/VLPAG circuit implicated in mediating freezing and inhibiting panic-like escape behaviors; (4) DRVL/VLPAG serotonergic neurons are central chemoreceptors and modulate the behavioral and cardiorespiratory response to panicogenic agents such as sodium lactate and CO2. Implications of the panic inhibition system are discussed.

Functional topography of serotonergic systems supports the Deakin/Graeff hypothesis of anxiety and affective disorders

Over 20 years ago, Deakin and Graeff hypothesized about the role of different serotonergic pathways in controlling the behavioral and physiologic responses to aversive stimuli, and how compromise of these pathways could lead to specific symptoms of anxiety and affective disorders. A growing body of evidence suggests these serotonergic pathways arise from topographically organized subpopulations of serotonergic neurons located in the dorsal and median raphe nuclei. We argue that serotonergic neurons in the dorsal/caudal parts of the dorsal raphe nucleus project to forebrain limbic regions involved in stress/conflict anxiety-related processes, which may be relevant for anxiety and affective disorders. Serotonergic neurons in the "lateral wings" of the dorsal raphe nucleus provide inhibitory control over structures controlling fight-or-flight responses. Dysfunction of this pathway could be relevant for panic disorder. Finally, serotonergic neurons in the median raphe nucleus, and the developmentally and functionally-related interfascicular part of the dorsal raphe nucleus, give rise to forebrain limbic projections that are involved in tolerance and coping with aversive stimuli, which could be important for affective disorders like depression. Elucidating the mechanisms through which stress activates these topographically and functionally distinct serotonergic pathways, and how dysfunction of these pathways leads to symptoms of neuropsychiatric disorders, may lead to the development of novel approaches to both the prevention and treatment of anxiety and affective disorders.

Serotonergic neural links from the dorsal raphe nucleus modulate defensive behaviours organised by the dorsomedial hypothalamus and the elaboration of fear-induced antinociception via locus coeruleus pathways

Decrease of γ-aminobutyric acid (GABA)-mediated neurotransmission in the dorsomedial hypothalamus (DMH) evokes instinctive fear-like responses. The aim of the present study was to investigate the involvement of the serotonin (5-HT)- and norepinephrine-mediated pathways of the endogenous pain inhibitory system, including the dorsal raphe nucleus (DRN) and the locus coeruleus (LC), in the defensive responses and antinociceptive processes triggered by the blockade of GABAergic receptors in the DMH. The intra-hypothalamic microinjection of the GABA(A) receptor antagonist bicuculline (40 ng/200 nL) elicited elaborate defensive behaviours interspersed with exploratory responses. This escape behaviour was followed by significantly increased pain thresholds, a phenomenon known as fear-induced antinociception. Furthermore, at 5 and 14 days after DRN serotonin-containing neurons were damaged using the selective neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), the frequency and duration of alertness and escape behaviour evoked by the GABA(A) receptor blockade in the DMH decreased, as well as fear-induced antinociception. Pre-treatment with the non-selective 5-HT receptor antagonist methysergide, the 5-HT(2A/2C) receptor antagonist ketanserin and the 5-HT(2A) receptor selective antagonist R-96544 in the LC also decreased fear-induced antinociception, without significant changes in the expression of defensive behaviours. These data suggest that the serotonergic neurons of the DRN are directly involved in the organisation of defensive responses as well as in the elaboration of the innate fear-induced antinociception. However, serotonin-mediated inputs from the NDR to the LC modulate only fear-induced antinociception and not the defensive behaviours evoked by GABA(A) receptor blockade in the DMH.

Involvement of dorsal raphe nucleus and dorsal periaqueductal gray 5-HT receptors in the modulation of mouse defensive behaviors

Previous findings point to the involvement of the dorsal raphe nucleus (DRN) and dorsal periaqueductal gray (dPAG) serotonergic receptors in the mediation of defensive responses that are associated with specific subtypes of anxiety disorders. These studies have mostly been conducted with rats tested in the elevated T-maze, an experimental model of anxiety that was developed to allow the measurement, in the same animal, of two behaviors mentioned: inhibitory avoidance and one-way escape. Such behavioral responses have been respectively related to generalized anxiety disorder (GAD) and panic disorder (PD). In order to assess the generality of these findings, in the current study we investigated the effects of the injection of 5-HT-related drugs into the DRN and dPAG of another rodent species, mouse, on the mouse defense test battery (MDTB), a test of a range of defensive behaviors to an unconditioned threat, a predator. Male CD-1 mice were tested in the MDTB after intra-DRN administration of the 5-HT(1A) receptor antagonist WAY-100635 or after intra-dPAG injection of two serotonergic agonists, the 5-HT(1A) receptor agonist 8-OH-DPAT and the 5-HT(2A/2C) receptor agonist DOI. Intra-DRN injection of WAY-100635 did not change behavioral responses of mice confronted with a rat in the MDTB. In the dPAG, both 8-OH-DPAT and DOI consistently impaired mouse escape behavior assessed in the MDTB. Intra-dPAG infusion of 8-OH-DPAT also decreased measures of mouse risk assessment in the rat exposure test. In conclusion, the current findings are in partial agreement with previous results obtained with rats tested in the elevated T-maze. Although there is a high level of similarity between the behavioral effects obtained in rats (elevated T-maze) and mice (MDTB and RET) with the infusion of 5-HT agonists into the dPAG, the same is not true regarding the effects of blockade of DRN 5-HT(1A) receptors in these rodent species. These data suggest that there may be differences between mice and rats regarding the involvement of the DRN in the mediation of defensive behaviors.

Serotonin-1A receptors in the dorsal periaqueductal gray matter mediate the panicolytic-like effect of pindolol and paroxetine combination in the elevated T-maze

The β-adrenergic blocker and 5-HT(1A) receptor antagonist pindolol has been combined with selective serotonin reuptake inhibitors (SSRIs) in patients with depressive and anxiety disorders to shorten the onset of the clinical action and/or increase the proportion of responders. The results of a previous study have shown that pindolol potentiates the panicolytic effect of paroxetine in rats submitted to the elevated T-maze (ETM). Since reported evidence has implicated the 5-HT(1A) receptors of the dorsal periaqueductal gray matter (DPAG) in the panicolytic effect of antidepressants, rats treated with pindolol (5.0mg/kg, i.p.) and paroxetine (1.5mg/kg, i.p.) received a previous intra-DPAG injection of the selective 5-HT(1A) antagonist, WAY-100635 (0.4 μg) and were submitted to the ETM. Pretreatment with WAY-100635 reversed the increase in escape latency, a panicolytic effect, determined by the pindolol-paroxetine combination. These results implicate the 5-HT(1A) receptors of the DPAG in the panicolytic effect of the pindolol-paroxetine combination administered systemically. They also give further preclinical support for the use of this drug combination in the treatment of panic disorder.

Activation patterns of cells in selected brain stem nuclei of more and less stress responsive rats in two animal models of PTSD - predator exposure and submersion stress

This study had two purposes. First: compare predator and water submersion stress cFos activation patterns in dorsal raphe (DR), locus coeruleus (LC) and periaqueductal gray (PAG). Second: identify markers of vulnerability to stressors within these areas. Rats were either predator or submersion stressed and tested 1.75 h later for anxiety-like behavior. Immediately thereafter, rats were sacrificed and cFos expression examined. In DR, serotonergic cells expressing or not expressing cFos were also counted. Predator and submersion stress increased anxiety-like behavior (in the elevated plus maze- EPM) equally over controls. Moreover, stressed rats spent equally less time in the center of the hole board than handled controls, another indication of increased anxiety-like behavior. To examine vulnerability, rats which were less anxious (LA) and more (highly) anxious (MA) in the EPM were selected from among handled control and stressed animals. LA rats in the stressed groups were considered stress non-responsive and MA stressed rats were considered stress responsive. LA and MA rats did not differ in cFos expression in any brain area, though stressors did increase cFos cell counts in all areas over controls. Intriguingly, the number of serotonergic DR neurons not activated by stress predicted degree of anxiety response to submersion stress only. LA submersion stressed rats had more serotonergic cells than all other groups, and MA submersion stressed rats had fewer serotonergic cells than all other groups, which did not differ. Moreover, these cell counts correlated with EPM anxiety. We conclude that a surplus of such cells protects against anxiogenic effects of submersion, while a paucity of such cells enhances vulnerability to submersion stress. Other data suggest serotonergic cells may exert their effects via inhibition of dorsolateral PAG cells during submersion stress. Findings are discussed with respect to serotonergic transmission in vulnerability to predator stress and relevance of findings for post traumatic stress disorder (PTSD). This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Facilitation of 5-HT1A-mediated neurotransmission in dorsal periaqueductal grey matter accounts for the panicolytic-like effect of chronic fluoxetine

Chronic administration of antidepressants such as fluoxetine and imipramine increases the responsiveness of 5-HT(1A) receptors in dorsal periaqueductal grey matter (DPAG), a midbrain area consistently implicated in the pathogenesis of panic disorder. This effect has been related to the clinically relevant anti-panic action of these drugs. In this study we determined whether long-term administration of fluoxetine also affects 5-HT efflux in DPAG. As a comparison, the effect of chronic treatment with the anxiolytic 5-HT(1A) receptor agonist buspirone on DPAG 5-HT levels was assessed. We also investigated whether the inhibitory effect of chronic fluoxetine on escape behaviour in the rat elevated T-maze, considered as a panicolytic-like effect, is counteracted by intra-DPAG injection of the 5-HT(1A) receptor antagonist WAY 100635. Male Wistar rats were treated (1 or 21 d, i.p.) with fluoxetine, buspirone or vehicle, once daily. After treatment, 5-HT in DPAG was measured by in-vivo microdialysis coupled to HPLC. In another study, rats treated (21 d, i.p.) with either fluoxetine or vehicle also received intra-DPAG injection of WAY 100635 or saline 10 min before being tested in the elevated T-maze. Chronic, but not acute, administration of fluoxetine significantly raised extracellular levels of 5-HT in DPAG. Long-term treatment with buspirone was ineffective. In the elevated T-maze, intra-DPAG injection of WAY 100635 fully blocked the anti-escape effect of chronic administration of fluoxetine. Therefore, chronic fluoxetine facilitates 5-HT(1A)-mediated neurotransmission within DPAG and this effect accounts for the panicolytic-like effect of this antidepressant in the elevated T-maze.

Individual differences in the sensitivity to serotonergic drugs: a pharmacobehavioural approach using rats selected on the basis of their response to novelty

RATIONALE

The mechanisms underlying individual differences in the response to serotonergic drugs are poorly understood. Rat studies may contribute to our knowledge of the neuronal substrates that underlie these individual differences.

OBJECTIVES

A pharmacobehavioural study was performed to assess individual differences in the sensitivity to serotonergic drugs in rats that were selected based on their response to a novel environment.

METHODS

Low responders (LR) and high responders (HR) to novelty rats were tested on the elevated T-maze following systemic injections of increasing doses of various serotonergic agents. The duration of avoidance of the open arms was scored for five trials.

RESULTS

The duration of avoidance behaviour was larger in saline-treated LR rats compared to saline-treated HR rats. The 5-HT1A agonist 8-OH-DPAT and the 5-HT2 agonists mCPP and DOI decreased the duration of avoidance behaviour in LR rats, but increased it in HR rats. The 5-HT3 agonist SR57227A and the 5-HT releaser/reuptake inhibitor d-fenfluramine increased the duration of avoidance behaviour in both types of rat. However, higher doses of SR57227A were required to alter avoidance behaviour in HR than in LR rats. The onset of the effects of SR57227A, d-fenfluramine and WAY100635 was faster in LR than in HR rats. The described effects were receptor specific. A model explaining the data is presented.

CONCLUSIONS

These data demonstrate that LR and HR rats differ in their sensitivity to serotonergic drugs that act at 5-HT3, 5-HT2 and 5-HT1A receptors. The implications of these individual differences for individual-specific treatment of substance abuse are briefly discussed.

Disruption of GABAergic tone in the dorsomedial hypothalamus attenuates responses in a subset of serotonergic neurons in the dorsal raphe nucleus following lactate-induced panic

Panic patients are vulnerable to induction of panic attacks by sub-threshold interoceptive stimuli such as intravenous (i.v.) sodium lactate infusions. Facilitation of serotonergic signaling with selective serotonin reuptake inhibitors can suppress anxiety and panic-like responses, but the mechanisms involved are not clearly defined. We investigated the effects of i.v. 0.5 M sodium lactate or saline, in control and panic-prone rats on c-Fos expression in serotonergic neurons within subdivisions of the midbrain/pontine raphe nuclei. Rats were chronically infused with either the GABA synthesis inhibitor l-allylglycine into the dorsomedial hypo thalamus to make them panic-prone, or the enantiomer d-allylglycine (d-AG) in controls. Lactate increased c-Fos expression in serotonergic neurons located in the ventrolateral part of the dorsal raphe nucleus (DRVL) and ventrolateral periaqueductal gray (VLPAG) of control, but not panic-prone, rats. The distribution of lactate-sensitive serotonergic neurons in d-AG-treated rats is virtually identical to previously defined pre-sympathomotor serotonergic neurons with multisynaptic projections to peripheral organs mediating 'fight-or-flight'-related autonomic and motor responses. We hypothesized that serotonergic neurons within the DRVL/VLPAG region represent a 'sympathomotor control system' that normally limits autonomic/behavioral responses to innocuous interoceptive and exteroceptive stimuli, and that dysfunction of this serotonergic system contributes to an anxiety-like state and increases vulnerability to panic in animals and humans.

Pharmacology of the beta-carboline FG-7,142, a partial inverse agonist at the benzodiazepine allosteric site of the GABA A receptor: neurochemical, neurophysiological, and behavioral effects

Given the well-established role of benzodiazepines in treating anxiety disorders, beta-carbolines, spanning a spectrum from full agonists to full inverse agonists at the benzodiazepine allosteric site for the GABA(A) receptor, can provide valuable insight into the neural mechanisms underlying anxiety-related physiology and behavior. FG-7,142 is a partial inverse agonist at the benzodiazepine allosteric site with its highest affinity for the alpha1 subunit-containing GABA(A) receptor, although it is not selective. FG-7,142 also has its highest efficacy for modulation of GABA-induced chloride flux mediated at the alpha1 subunit-containing GABA(A) receptor. FG-7,142 activates a recognized anxiety-related neural network and interacts with serotonergic, dopaminergic, cholinergic, and noradrenergic modulatory systems within that network. FG-7,142 has been shown to induce anxiety-related behavioral and physiological responses in a variety of experimental paradigms across numerous mammalian and non-mammalian species, including humans. FG-7,142 has proconflict actions across anxiety-related behavioral paradigms, modulates attentional processes, and increases cardioacceleratory sympathetic reactivity and neuroendocrine reactivity. Both acute and chronic FG-7,142 treatment are proconvulsive, upregulate cortical adrenoreceptors, decrease subsequent actions of GABA and beta-carboline agonists, and increase the effectiveness of subsequent GABA(A) receptor antagonists and beta-carboline inverse agonists. FG-7,142, as a partial inverse agonist, can help to elucidate individual components of full agonism of benzodiazepine binding sites and may serve to identify the specific GABA(A) receptor subtypes involved in specific behavioral and physiological responses.

Panic attacks after treatment with zuclopenthixol decanoate

The association between neuroleptics and anxiety is unclear: neuroleptics have been used to treat anxiety though may also themselves be anxiogenic. We present the case of a man who developed new onset panic attacks after treatment with zuclopenthixol decanoate, a commonly administered depot antipsychotic. We review the literature on the association between antipsychotics and anxiety and present possible pathophysiological mechanisms.

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.

Acquisition and extinction of a conditioned passive avoidance reflex in mice with genetic knockout of monoamine oxidase A

We report here the results obtained from comparative analysis of learning and the dynamics of extinction of a conditioned passive avoidance response in mice with genetic knockout of monoamine oxidase A (MAO A) and the progenitor line C3H. Mice of both lines acquired the conditioned passive avoidance reaction efficiently. Mice with genetic knockout of MAO A were characterized by prolonged retention of reproduction of the memory trace, as compared with rapid extinction in C3H mice. Smaller numbers of transfers, and vertical rearings on days 7-13 and the numbers of glances into and rom the dark sector on days 11-13 of extinction in MAO A-knockout mice appear to reflect their more marked fear reactions when confronted with the "dangerous" sector, along with increased anxiety, these facilitating longer-lasting retention of the memory trace.

5-HT(1A) and 5-HT(2A) receptors in the rat dorsal periaqueductal gray mediate the antipanic-like effect induced by the stimulation of serotonergic neurons in the dorsal raphe nucleus

RATIONALE

It has been proposed that the serotonergic pathway that connects the dorsal raphe nucleus (DRN) to the dorsal periaqueductal gray (DPAG) is implicated in the regulation of escape, a behavior that has been related to panic.

OBJECTIVES

We further evaluated this hypothesis by investigating whether intra-DRN injection of the 5-HT(1A) receptor antagonist WAY-100635 changes the escape response of rats submitted to the elevated T-maze. This test also measures inhibitory avoidance, which has been associated with generalized anxiety disorder. We also investigated whether the 5-HT(1A) and 5-HT(2A) receptors in the DPAG mediate the behavioral consequences induced by the injection of WAY-100635 into the DRN.

RESULTS

Intra-DRN injection of WAY-100635 facilitated inhibitory avoidance, while impairing escape. The same effect was obtained after intra-DRN injection of the glutamate receptor agonist kainic acid. Preadministration of WAY-100635 into the DPAG counteracted the effect induced by intra-DRN injection of WAY-100635 and of kainic acid on escape, but not on inhibitory avoidance. Preadministration of the preferential 5-HT(2A) receptor antagonist ketanserin into the DPAG abolished the effects of intra-DRN injection of WAY-100635 on both elevated T-maze tasks.

CONCLUSION

The results are indicative that 5-HT(1A) autoreceptors in the DRN are under tonic inhibitory influence by endogenous 5-HT. The effects of 5-HT release in the DPAG after intra-DRN injection of WAY-100635 and kainic acid on inhibitory avoidance and escape involve different 5-HT receptor subtypes. Whereas 5-HT(2A) receptors in the DPAG seem to mediate the effect of 5-HT on both behaviors, 5-HT(1A) receptors are only involved in the regulation of escape.

Behavioral effects of systemically administered MK-212 are prevented by ritanserin microinfusion into the basolateral amygdala of rats exposed to the elevated plus-maze

RATIONALE

Although 5-HT2 receptors seem to play an important role in anxiety, results from numerous studies are still highly variable. Moreover, little is known about the behavioral effects of centrally administered 5-HT2 compounds in animal models of anxiety.

OBJECTIVE

The current study was performed to: (1) further investigate the effects of 5-HT2 receptor activation in rats exposed to the elevated plus-maze (EPM) and the open-field arena, two widely used animal models for studying anxiety and locomotor activity; and (2) evaluate the involvement of the 5-HT2 receptors within the basolateral nucleus of the amygdala (BLA) in the modulation of such effects.

METHODS

In the first experiment, male Wistar rats were exposed for 5 min to the EPM 27 min following intraperitoneal (i.p.) (1.0 ml/kg) injections of the preferential 5-HT2C receptor agonist 6-chloro-2[1-piperazinyl]pyrazine (MK-212) at doses of 1.0, 2.0, or 4.0 mg/kg. Control animals were injected with saline. The percentage of open-arm entries and the percentage of time spent in these arms were employed as anxiety indexes, whereas the number of closed-arm entries was calculated as indicative of locomotor activity. In the second experiment, rats were exposed for 10 min in an open-field arena to further assess the interference of the same MK-212 doses upon locomotor activity. In Experiment 3, rats were microinjected (0.2 microl) either with the mixed 5-HT 2A/2C receptor antagonist ritanserin (0.5, 1.25, 2.5, and 5.0 microg) or its vehicle into the BLA 12 min following i.p. injections of saline or the intermediate dose of MK-212 (2.0 mg/kg). Fifteen minutes later, each animal was exposed to the EPM as before.

RESULTS

Whereas the highest dose of MK-212 (4.0 mg/kg) induced motor-suppressant effects in both EPM and open-field arena, the intermediate dose of the drug (2.0 mg/kg) reduced open-arm exploration without significantly affecting the number of closed-arm entries. This behavioral profile, consistent with selective anxiogenic effect in the EPM, was dose-dependently prevented by ritanserin microinfusion into the BLA. In saline-pretreated animals, however, ritanserin (all doses) was ineffective.

CONCLUSIONS

MK-212 increases anxiety and decreases locomotor activity. The anxiogenic-like profile of 5-HT2 receptor activation is prevented by the blockade of 5-HT2 receptors within the BLA, which does not have an effect by itself upon basal anxiety levels triggered by the EPM.

Acute hypercarbic gas exposure reveals functionally distinct subpopulations of serotonergic neurons in rats

Although increasing evidence suggests that anatomically defined subpopulations of serotonergic neurons have unique stress-related functional properties, the topographical distribution of the serotonergic neurons involved in responses to stress-related stimuli have not been well-defined. Inspiration of air containing elevated concentrations of carbon dioxide (CO(2); hypercarbic gas exposure) at high concentrations activates both hypothalamic-pituitary-adrenal axis and sympathetic responses in rats and humans. In order to determine the effects of acute hypercarbic gas exposure on subpopulations of topographically organized serotonergic neurons, conscious adult male rats were placed in flow cages and exposed to either atmospheric air or increasing environmental CO2 concentrations (from baseline concentrations up to 20% CO2) for 5min. The presence of immunoreactivity for the protein product of the immediate-early gene c-fos was used as a measure, at the single cell level, of functional cellular responses within subpopulations of serotonergic, noradrenergic and adrenergic neurons. Rats exposed to hypercarbic gas had increased numbers of c-Fos/tryptophan hydroxylase immunoreactive (ir) and c-Fos/tyrosine hydroxylase-ir neurons in specific topographically organized subdivisions of brainstem nuclei, compared to control rats. Within serotonergic cell groups (B1-B9), the most striking effects occurred in a subpopulation of large, multipolar serotonergic neurons within the ventrolateral periaqueductal grey and ventrolateral part of the dorsal raphe nucleus, a region implicated in serotonin-dependent suppression of stress-induced sympathetic outflow and serotonin-dependent inhibition of 'fight or flight' behaviour. These findings have important implications for understanding the role of serotonergic systems in the modulation of stress-related physiology and behaviour and stress-related neuropsychiatric disorders.

A functional subset of serotonergic neurons in the rat ventrolateral periaqueductal gray implicated in the inhibition of sympathoexcitation and panic

The ability of serotonin (5-HT) to facilitate or attenuate autonomic, endocrine, and behavioral responses to stressful stimuli has received much attention. The effects of 5-HT on physiologic and behavioral responses to stressful stimuli seem to depend on the brain region where it is released and the effector system it acts upon. This and the distinct morphology and topographic organization of subpopulations of serotonergic neurons have led to the hypothesis that subpopulations of serotonergic neurons are functionally distinct. Serotonin's role as a modulator of the "fight-or-flight" response is mediated in part by 5-HT release in the dorsolateral periaqueductal gray (DLPAG) and in the rostral ventrolateral medulla (RVLM), an area that contains sympathoexcitatory C1 adrenergic (A) neurons. The release of 5-HT in either region inhibits stress-induced sympathetic activity in part via actions on 5-HT(1A) receptors. In addition, 5-HT release in the DLPAG inhibits fight-or-flight or "Go" behaviors. The origin of endogenous 5-HT in the DLPAG and RVLM seems to be a subpopulation of serotonergic neurons within the ventrolateral PAG, a region implicated in "freezing" or "No Go" behaviors. These serotonergic neurons are located in the lateral "wings" of the dorsal raphe nucleus (DRN) a region also referred to as the ventrolateral DRN. The existence of a functional subpopulation of serotonergic neurons capable of inhibiting sympathoexcitation and fight-or-flight behavioral responses may be clinically relevant for explaining in part the efficacy of serotonergic drugs in the treatment of hypertension and panic attacks in panic disorder patients.

Serotonin, the periaqueductal gray and panic

This article reviews experimental evidence and theoretical constructs that implicate serotonin (5-HT) modulation of defensive behavior within the midbrain periaqueductal gray in panic disorder (PD). Evidence with conflict tests in experimental animals indicates that 5-HT enhances anxiety, whereas results with aversive stimulation of the dorsal periaqueductal gray point to an anxiolytic role of 5-HT. To solve this contradiction, it has been suggested that the emotional states determined by the two types of animal model are different. Conflict tests would generate conditioned anxiety, whereas periaqueductal gray stimulation would produce unconditioned fear, as evoked by proximal threat. Clinically, the former would be related to generalized anxiety while the latter to PD. Thus, 5-HT is supposed to facilitate anxiety, but to inhibit panic. This hypothesis has been tested in the animal model of anxiety and panic named the elevated T-maze, in two procedures of human experimental anxiety applied to healthy volunteers or panic patients, and in CO2-induced panic attacks. Overall, the obtained results have shown that drugs that enhance 5-HT function increase different indexes of anxiety, but decrease indexes of panic. Drugs that impair 5-HT function have the opposite effects. Thus, so far the predictions derived from the above hypothesis have been fulfilled.

Tonic and phasic alteration in amygdala 5-HT, glutamate and GABA transmission after prefrontal cortex damage in rats

The relationship between the ventromedial prefrontal cortex and the amygdala during the presentation of an unconditioned fear stimulus was assessed. Rats underwent bilateral ibotenic acid or vehicle administration into the ventromedial prefrontal cortex. Five weeks later, the behavior as well as the neurochemical changes in the amygdala was evaluated before and after a brief cat presentation. Lesioned animal freezing behavior increased 10 times when compared to controls. In the right basolateral amygdala, basal concentrations of 5-HT, 5-HIAA, glutamate and serine were elevated but basal level of GABA was diminished in lesioned animals relative to controls. Sham but not lesioned animals increased 5-HT and decreased GABA and serine levels after cat presentation. Phasic changes in glutamate were not detected either in lesioned or shams but the difference in amygdala glutamate between lesioned and shams persisted after cat presentation. These data show that increased serotonin and glutamate tone and decreased gabaergic tone in the amygdala correlate to elevated fear and anxiety after prefrontal cortex ibotenic acid lesion. The lesion also seems to produce a failure of adaptive changes in neurotransmitter systems revealing lost of control of the ventromedial prefrontal cortex over the amygdala in frightening situations.

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.

Stimulation of neurons in rat ARC inhibits gastric acid secretion via hypothalamic CRF1/2- and NPY-Y1 receptors

Neuropeptide Y (NPY) neuronal projections from the arcuate nucleus (ARC) have been proposed to target corticotropin-releasing factor (CRF)-positive neurons in the paraventricular nucleus (PVN) as part of the ARC-PVN axis. The existence of a positive feedback loop involving CRF receptors in the PVN has been suggested. Exogenous NPY and CRF in the PVN have been shown to inhibit gastric acid secretion. Recently, we have demonstrated that activation of ARC neurons inhibits gastric acid secretion via vagal pathways. To what extent NPY- and CRF-mediated mechanisms in the PVN contribute to the CNS modulation of gastric acid secretion is still an open question. In the present study, we performed consecutive bilateral microinjections of antagonists to NPY receptor subtypes Y1 and Y2 and to CRF1/2 receptors in the PVN and of the excitatory amino acid kainate in the ARC to assess the role of NPY- and CRF-mediated mechanisms in the kainate-induced effects on gastric acid secretion. Gastric acid secretion was measured at the basal condition and during pentagastrin (16 microg/kg body wt) stimulation. Microinjection of vehicle in the PVN and kainate in the ARC decreased gastric acid secretion. Microinjection of the specific NPY-Y1 receptor antagonist BIBP-3226 (200 pmol) and the nonspecific CRF1/2 antagonist astressin (30 pmol) in the PVN abolished the inhibitory effect of neuronal activation in the ARC by kainate on gastric acid secretion. The CRF antagonist astressin was more effective. Pretreatment with the NPY-Y2 receptor antagonist BIIE-0246 (120 pmol) in the PVN had no significant effect. Our results indicate that activation of neurons in the ARC inhibits gastric acid secretion via CRF1/2 and NPY-Y1 receptor-mediated pathways in the PVN.

The dorsal raphe nucleus exerts opposed control on generalized anxiety and panic-related defensive responses in rats

It has been proposed that the ascending dorsal raphe (DR)-serotonergic (5-HT) pathway facilitates conditioned avoidance responses to potential or distal threat, while the DR-periventricular 5-HT pathway inhibits unconditioned flight reactions to proximal danger. Dysfunction on these pathways would be, respectively, related to generalized anxiety (GAD) and panic disorder (PD). To investigate this hypothesis, we microinjected into the rat DR the benzodiazepine inverse receptor agonist FG 7142, the 5-HT(1A) receptor agonist 8-OH-DPAT or the GABA(A) receptor agonist muscimol. Animals were evaluated in the elevated T-maze (ETM) and light/dark transition test. These models generate defensive responses that have been related to GAD and PD. Experiments were also conducted in the ETM 14 days after the selective lesion of DR serotonergic neurons by 5,7-dihydroxytriptamine (DHT). In all cases, rats were pre-exposed to one of the open arms of the ETM 1 day before testing. The results showed that FG 7142 facilitated inhibitory avoidance, an anxiogenic effect, while impairing one-way escape, an anxiolytic effect. 8-OH-DPAT, muscimol, and 5,7-DHT-induced lesions acted in the opposite direction, impairing inhibitory avoidance while facilitating one-way escape from the open arm. In the light/dark transition, 8-OH-DPAT and muscimol increased the time spent in the lighted compartment, an anxiolytic effect. The data supports the view that distinct DR-5-HT pathways regulate neural mechanisms underlying GAD and PD.

Serotonergic regulation of inhibitory avoidance and one-way escape in the rat elevated T-maze

It has been proposed that distinct 5-HT pathways modulate different types of anxiety. Activation of the ascending dorsal raphe (DR)-5-HT pathway, innervating the amygdala and frontal cortex, would facilitate learned defensive behaviors. On the other hand, activation of the DR-periventricular 5-HT pathway, which innervates the dorsal periaqueductal gray matter (DPAG), would inhibit innate flight or fight reactions. Dysfunction of these pathways has been suggested to relate to generalized anxiety disorder (GAD) and panic disorder (PD) in humans, respectively. The elevated T-maze has been developed to separate conditioned (inhibitory avoidance) from unconditioned (escape) defensive responses in the same rat. Pharmacological validation of this model has shown that the GAD-effective serotonergic anxiolytic buspirone or the putative anxiolytic ritanserin selectively impaired inhibitory avoidance while leaving one-way escape unchanged. Chronic injection of the 5-HT/noradrenaline reuptake inhibitor imipramine impaired inhibitory avoidance and prolonged escape, an effect that may be related to the therapeutic action of this drug on both GAD and PD. Like imipramine, intra-DPAG injection of the 5-HT(1A) agonist 8-OH-DPAT impaired both inhibitory avoidance and one-way escape. Intra-DPAG administration of the 5-HT(2A/2C) agonist DOI prolonged escape, without affecting inhibitory avoidance. The reversible inactivation of the DRN by muscimol impaired inhibitory avoidance, while facilitating escape from the open arm. Taken together, these results suggest that 5-HT exerts differential control on inhibitory avoidance and escape response in the elevated T-maze, mobilizing different types of 5-HT receptors in key structures implicated in fear/anxiety.

Comparison between two models of experimental anxiety in healthy volunteers and panic disorder patients

To further investigate the role of serotonin (5-HT) in anxiety, two tests were used in human subjects. The first was the conditioning of skin conductance response (CSCR) that associates a tone to a loud noise. The second was simulated public speaking (SPS), which is believed to represent unconditioned fear. In healthy volunteers the 5-HT(2A) receptor blocker and 5-HT reuptake inhibitor nefazodone reduced subjective anxiety and the number of spontaneous fluctuations of skin conductance during CSCR, but enhanced anxiety induced by SPS. Opposite effects had been reported with the 5-HT releasing and uptake-inhibiting agent D-fenfluramine. Panic patients behaved like controls in the CSCR. However, they had a higher level of baseline anxiety and were insensitive to SPS. This profile resembles the reported effect of the non-selective 5-HT receptor blocker metergoline in healthy volunteers. Therefore, panic patients seem to process unconditioned fear abnormally, which may be due to lack of 5-HT inhibition in brain structures commanding flight from proximal danger stimuli.

Serotonin, testosterone and alcohol in the etiology of domestic violence

In a previous study we administered the panicogenic agent sodium lactate to a select group of perpetrators of domestic violence and comparison groups. Results of that study showed that perpetrators exhibited exaggerated lactate-induced fear, panic and rage. In this current study, we compared the cerebral spinal fluid (CSF) concentrations of 5-hydroxyindoleacetic acid (5-HIAA) and testosterone obtained from perpetrators of domestic violence and a group of healthy comparison subjects. All subjects were assessed for DSM-III-R diagnoses. Perpetrators with alcohol dependence (DV-ALC) (n=13), perpetrators without alcohol dependence (DV-NALC) (n=10) and healthy comparison subjects (HCS) (n=20) were clinically assessed using the Spielberger Trait Anxiety, Brown-Goodwin Aggression Scale, Buss Durkee Hostility Inventory and Straus Conflict Tactics. Following an overnight fast and bed rest, subjects received a lumbar puncture to obtain CSF concentrations of 5-HIAA and testosterone. Perpetrators scored significantly higher on measures of aggression than HCS. DV-NALC had significantly lower concentrations of CSF 5-HIAA and higher Straus Conflict Tactics (CT) physical violence scores than DV-ALC and HCS. DV-ALC had significantly higher concentrations of CSF testosterone than DV-NALC. DV-ALC also had significantly higher Straus CT physical violence scores than HCS. DV-NALC and DV-ALC differed on 5-HIAA concentrations, testosterone concentrations, Straus CT physical violence scores and alcohol dependence. These results suggest that DV-NALC and DV-ALC groups could have different biological mechanisms mediating domestic violence.

Excitatory stimulation of neurons in the arcuate nucleus inhibits gastric acid secretion via vagal pathways in anesthetized rats

It is well established that autonomic control of gastrointestinal function is modulated by central autonomic neurotransmission. In this context it has been shown that gastrointestinal motility and secretion can be modulated by exogenous neuropeptides microinjected into the paraventricular nucleus of the hypothalamus (PVN). Furthermore, there is considerable evidence suggesting that neurons projecting from the arcuate nucleus (Arc) to the PVN may be the source of endogenous neuropeptide release in the PVN. This poses the question whether stimulation of neurons in the arcuate nucleus, e.g. by an excitatory amino acid, alters gastrointestinal function. In the present study, we investigated the effect of an excitatory amino acid, kainate, microinjected into the arcuate nucleus on gastric acid secretion in urethane-anesthetized rats. Kainate (140 pmol/rat) bilaterally microinjected into the Arc induced an significant inhibition of pentagastrin (PG) stimulated (16 mg/kg per h) gastric acid secretion throughout an observation period of 120 min after microinjection. Microinjection of kainate into hypothalamic areas outside the arcuate nucleus did not modify gastric secretion. Bilateral cervical vagotomy blocked the effect of kainate injected into the Arc on PG-stimulated gastric acid secretion. These data show that gastric secretory function can be modulated by stimulation of neuronal activity in the Arc via efferent vagal pathways. The results suggest that the arcuate nucleus is a forebrain area involved in the CNS regulation of gastrointestinal function.

Seizure activity and hyperthermia potentiate the increases in dopamine and serotonin extracellular levels in the amygdala during exposure to d-amphetamine

Behavioral stereotypy, hyperthermia, and convulsive activity produced by exposure to multiple doses of d-amphetamine (AMPH) were related to changes in the extracellular levels of dopamine and serotonin (5-HT) in the amygdala, using the technique of microdialysis in awake and freely moving rats. Hyperactivity and stereotypy, as well as increases in microdialysis dopamine levels ranging from 100-300% of pre-AMPH basal microdialysate levels (BL), occurred during exposure to 3 doses of 2.5 mg/kg (3 x 2.5 mg/kg) AMPH. Three doses of 5 mg/kg produced a more intense stereotypic behavior as well as hyperthermia, and resulted in large increases in the peak dopamine levels (700% BL) while 5-HT levels were increased to a lesser extent (300% BL). The highest doses tested of 3 x 15 mg/kg produced convulsive activity, seizures, intense stereotypy and hyperthermia with peak microdialysate dopamine (1300% BL) and 5-HT levels (1800% BL) that were 2-fold and 6-fold greater, respectively, than those at the 3 x 5-mg/kg doses. Microdialysate glutamate levels were not changed by AMPH exposure. Rats that did not become hyperthermic when dosed with 15 mg/kg AMPH in a cold environment (10 degrees C) exhibited some hyperactivity and stereotypic behavior, but not overt convulsive behavior. Dopamine and 5-HT levels in these rats were significantly reduced by about 75% and 60%, respectively, compared to the room-temperature group. Inclusion of 2 microM tetrodotoxin (TTX) in the microdialysis buffer significantly reduced the 15-mg/kg AMPH-induced increases in dopamine by 30% and the increase in 5-HT levels by 70% at room temperature. These results indicate that a smaller portion of the dopamine release evoked by doses of AMPH that induce seizure activity is neuronal impulse-dependent while the majority of 5-HT released is impulse-dependent. Irrespective of impulse activity, the hyperthermia alone markedly potentiated dopamine release but had a lesser effect on 5-HT release. Thus, there are differences in the regulation of dopamine and serotonin release in the amygdala from high doses of AMPH, which are known to produce neurotoxicity. Further studies are necessary to determine the impact of these differences in release on AMPH neurotoxicity.

5-HT(1A) receptor-mediated inhibition and 5-HT(2) as well as 5-HT(3) receptor-mediated excitation in different subdivisions of the rat amygdala

The techniques of extracellular single cell recording and microiontophoresis were used to study the effects of serotonin (5-HT) and of 5-HT(1A), 5-HT(2A/2C) and 5-HT(3) receptor agonists on the spontaneous activity of amygdaloid neurons in rats anesthetized with urethane. The background discharge rate was modified by 5-HT as well as by 5-HT agonists in about two-thirds of neurons tested in different nuclei of the amygdaloid complex. Whereas the 5-HT(2) and 5-HT(3) agonists significantly increased the neuronal discharge rate in nearly all subdivisions of the amygdala, the 5-HT(1A) agonist significantly inhibited the firing rate. Co-administration of bicuculline and 5-HT receptor agonists prevented the 8-OH-DPAT-induced increases in the firing rate in most cases tested, as well as the inhibitory effects of DOI or 2-methyl-5HT. Therefore, GABAergic interneurons seem to be involved in the mediation of serotonergic effects. The action of 5-HT agonists on the neuronal discharge rate was blocked by different receptor-specific antagonists. The results support the hypothesis that 5-HT exerts control throughout the amygdala by acting at least on 5-HT(1A), 5-HT(2A/2C) and 5-HT(3) receptors seemingly located both on projection and interneurons.

Androgen actions on central serotonin neurotransmission: relevance for mood, mental state and memory

Sex steroids exert potent effects on mood and mental state in the human. Our previous experimental findings in female rats suggest that these effects may be mediated, in part, by the action of estrogen on the 5-hydroxytryptamine2A receptor (5-HT(2A)R) and serotonin transporter (SERT) in brain. Here we review our recent findings on the effect of acute (approximately 32 h) testosterone manipulation on central 5-HT(2A)R and SERT in male rats. Castration decreased while testosterone or estrogen, but not 5alpha-dihydrotestosterone (5alpha-DHT), increased significantly the content of 5-HT(2A)R mRNA and SERT mRNA in the dorsal raphe nucleus (DR) and the density of 5-HT(2A)R and SERT binding sites in higher centers of the brain. The lack of effect of 5alpha-DHT, a potent androgen which cannot be converted to estrogen, suggests that the action of testosterone depends upon its conversion to estrogen by aromatase. This may also explain why estrogen, but not testosterone or 5alpha-DHT, increased the density of 5-HT(2A)R binding sites in the caudate-putamen, a brain region where aromatase is scarce. The estrogen induction of SERT mRNA is most prominent in the rostral DR and this together with the correlation between sensitivity of DR serotonin neurons to estrogen and neurotoxic amphetamine derivatives provides a potential topochemical handle with which to investigate testosterone/estrogen regulation of SERT gene expression. These findings are discussed in relation to the possible role of interactions between sex steroids and serotonin mechanisms in mood disorders, schizophrenia and Alzheimer's disease.

Long-lasting inhibitory avoidance acquisition in rats submitted to the elevated T-maze model of anxiety

The present study was outlined in order to provide additional behavioral validation to the elevated T-maze as a model of anxiety and memory. In this model, rats acquire an inhibitory avoidance response after successive exposures, and memory can be estimated by retesting later. Two experiments were carried out with rats initially tested at the ages of 30, 60, 90 and 120 days. Experiment 1 showed that, while 100% of subjects studied learned inhibitory avoidance, regardless of their ages, 60-day-old rats needed a higher number of trials in order to reach the criterion to stay 300 s inside the enclosed arm. Experiment 2 explored the relevancy of the aversive stimulus in maintaining the learned experience by retesting the subjects monthly. The results showed that after acquisition of inhibitory avoidance there is remarkable long-lasting memory retrieval. Risk assessment behavior, increasing in the first trial of the retest day, revealed also the approach-avoidance conflict while on the enclosed arm. The results are discussed in terms of age effects and the implications of an enduring learned emotional response in animal models of anxiety and memory.

Serotonin transporter (SERT) mRNA and binding site densities in male rat brain affected by sex steroids

Estrogen increases serotonin transporter (SERT) mRNA and binding sites in female rat brain. In order to determine whether changes in SERT are gender- and steroid-specific we have now carried out studies on adult male Wistar rats which were either intact or castrated (under halothane anesthesia) and injected with arachis oil, estradiol benzoate (EB), testosterone propionate (TP) or the non-aromatizable androgen, 5alpha-dihydrotestosterone (5alpha-DHT). The number of SERT mRNA-expressing cells in the dorsal raphe (DR) nucleus was decreased by castration and increased by treatment (for approximately 32 h) with EB or TP, but not 5alpha-DHT. Sex steroids had no effect on the number of SERT mRNA-expressing cells in the median raphe nucleus. The density of SERT sites, assessed by autoradiography of [3H]paroxetine binding, was significantly reduced in arcuate nucleus and median raphe after castration, and increased in arcuate, basolateral amygdala and ventromedial hypothalamic nucleus by treatment with EB or TP, but not 5alpha-DHT. Estradiol, but not testosterone or 5alpha-DHT reduced the density of SERT sites in midbrain central grey. These data show that testosterone as well as estrogen affects SERT expression in male brain, and that the action of testosterone probably depends upon its enzymatic conversion, by aromatase, to estradiol. Our findings may have implications for sex steroid control of mood and behavior, and the action of neurotoxic derivatives of amphetamine, such as 3, 4-methylenedioxymethamphetamine, in the human.

Escapable and inescapable stress differentially alter extracellular levels of 5-HT in the basolateral amygdala of the rat

The effects of escapable and yoked inescapable electric tailshocks on extracellular levels of serotonin (5-HT) in the basolateral amygdala were measured by in vivo microdialysis. Inescapable, but not escapable, shock increased extracellular 5-HT in the amygdala relative to restrained controls. Basal levels of 5-HT were elevated 24 h after inescapable shock, and previously inescapably shocked subjects exhibited an exaggerated 5-HT response to two brief footshocks. Levels of extracellular 5-HIAA did not follow any particular pattern and were not correlated with the changes in 5-HT.

Clinical utility of the selective serotonin reuptake inhibitors in the spectrum of anxiety

The selective serotonin reuptake inhibitors (SSRIs) are now being employed in the treatment of the full spectrum of anxiety disorders. In comparative trials, the SSRIs are proving to be equal or superior in efficacy to traditional antianxiety medications. Due to their favorable side effect profile, safety, and tolerability, they are rapidly replacing older agents in the treatment of anxiety. Neuroanatomical pathways that may be important in the antianxiety effect of the SSRIs are outline and discussed, followed by a review of the clinical evidence supporting the efficacy of this class of medications in the treatment of anxiety disorders.