Role of GABAA receptors in dorsal raphe nucleus in stress-induced reinstatement of morphine-conditioned place preference in rats

Rationale

The serotonin (5-hydroxytryptamine, 5-HT) system plays an important role in stress-related psychiatric disorders and substance abuse. Our data indicate that stress inhibits the dorsal raphe nucleus (DRN)-5-HT system via stimulation of GABA synaptic activity by the stress neurohormone corticotropin-releasing factor and, more recently, that morphine history sensitizes DRN-5-HT neurons to GABAergic inhibitory effects of stress.

Objectives

We tested the hypothesis that DRN GABA_A receptors contribute to stress-induced reinstatement of morphine-conditioned place preference (CPP).

Methods

First, we tested if activation of GABA_A receptors in the DRN would reinstate morphine CPP. Second, we tested if blockade of GABA_A receptors in the DRN would attenuate swim stress-induced reinstatement of morphine CPP. CPP was induced by morphine (5 mg/kg) in a 4-day conditioning phase followed by a conditioning test. Upon acquiring conditioning criteria, subjects underwent 4 days of extinction training followed by an extinction test. Upon acquiring extinction criteria, animals underwent a reinstatement test. For the first experiment, the GABA_A receptor agonist muscimol (50 ng) or vehicle was injected into the DRN prior to the reinstatement test. For the second experiment, the GABA_A receptor antagonist bicuculline (75 ng) or vehicle was injected into the DRN prior to a forced swim stress, and then, animals were tested for reinstatement of CPP.

Results

Intraraphe injection of muscimol reinstated morphine CPP, while intraraphe injection of bicuculline attenuated swim stress-induced reinstatement.

Conclusions

These data provide evidence that GABA_A receptor-mediated inhibition of the serotonergic DRN contributes to stress-induced reinstatement of morphine CPP.     

A combination of mirtazapine and milnacipran augments the extracellular levels of monoamines in the rat brain

Mirtazapine, an antidepressant, antagonizes α(2)-adrenergic autoreceptors and heteroreceptors, which leads to enhanced noradrenergic and serotonergic transmission without inhibiting monoamine transporters. Using a microdialysis technique, we investigated whether co-administration of mirtazapine and a serotonin noradrenaline reuptake inhibitor (SNRI), milnacipran, augments the effects of each drug on the extracellular levels of monoamines by pharmacological synergy. Mirtazapine increased the extracellular levels of noradrenaline and serotonin in the dorsal hippocampus. In contrast, it increased the levels of noradrenaline and dopamine without changing serotonin levels in the prefrontal cortex. Milnacipran increased the levels of all monoamines evaluated in both areas, and the combined treatment with mirtazapine augmented these changes. The combined treatment with idazoxan, an α(2) adrenoceptor antagonist, and milnacipran also increased all monoamine levels in the prefrontal cortex. Ketanserin, a serotonin 5-HT(2A) receptor antagonist, showed no effect in combination with milnacipran, while SB242084, a 5-HT(2C) receptor antagonist, augmented the effects of milnacipran on the levels of serotonin and dopamine in the prefrontal cortex. These results suggest that combined treatment with mirtazapine and milnacipran augments the extracellular levels of noradrenaline, serotonin and dopamine through the blockade of α(2) adrenoceptors without regional specificity, whereas mirtazapine enhances serotonergic transmission in a region-specific manner. 5-HT(2C) receptor antagonism may also partly contribute to the amplification effects of mirtazapine on serotonin and dopamine levels. These neurochemical changes could play a role in reported advantageous clinical effects in patients treated with an SNRI and mirtazapine.     

α2-Adrenoceptors are targets for antipsychotic drugs

Rationale

Almost all antipsychotic drugs (APDs), irrespective of whether they belong to the first-generation (e.g. haloperidol) or second-generation (e.g. clozapine), are dopamine D₂ receptor antagonists. Second-generation APDs, which differ from first-generation APDs in possessing a lower propensity to induce extrapyramidal side effects, target a variety of monoamine receptors such as serotonin (5-hydroxytryptamine) receptors (e.g. 5-HT_1A, 5-HT_2A, 5-HT_2C, 5-HT₆, 5-HT₇) and α₁- and α₂-adrenoceptors in addition to their antagonist effects at D₂ receptors.

Objective

This short review is focussed on the potential role of α₂-adrenoceptors in the antipsychotic therapy.

Results

Schizophrenia is characterised by three categories of symptoms: positive symptoms, negative symptoms and cognitive deficits. α₂-Adrenoceptors are classified into three distinct subtypes in mammals, α_2A, α_2B and α_2C. Whereas the α_2B-adrenoceptor seems to play only a minor role in the brain, activation of postsynaptic α_2A-adrenoceptors in the prefrontal cortex improves cognitive functions. Preclinical models such as D-amphetamine-induced locomotion, the conditioned avoidance response and the pharmacological N-methyl-d-aspartate receptor hypofunction model have shown that α_2C-adrenoceptor blockade or the combination of D₂ receptor antagonists with idazoxan (α_2A/2C-adrenoceptor antagonist) could be useful in schizophrenia. A potential benefit of a treatment combination of first-generation APDs with the α_2A/2C-adrenoceptor antagonists idazoxan or mirtazapine was also demonstrated in patients with schizophrenia.

Conclusions

It is concluded that α₂-adrenoceptors may be promising targets in the antipsychotic therapy.     

Differential effects of the antidepressant mirtazapine on amphetamine- and dizocilpine-induced PPI deficits

Prepulse inhibition (PPI) refers to the decrease in motor startle response to salient sensory stimuli (pulses) when they are closely preceded in time by another more modest sensory stimulus (prepulse). PPI deficits can be induced by stimulation of dopamine receptors (e.g., amphetamine or apomorphine) or blockade of NMDA glutamate receptors (e.g., dizocilpine or PCP). Previously we found that antagonists of α₂-noradrenergic and H₁-histaminergic receptors significantly attenuate PPI impairments caused by amphetamine or dizocilpine. In the current study we assessed the effects of the antidepressant mirtazapine, which has combined antagonist effects at α₂-noradrenergic, H₁-histaminergic and 5-HT serotonergic receptors, on amphetamine- and dizocilpine-induced PPI deficits. In Experiment 1, rats were tested for PPI of the startle response to a tactile air-puff stimulus after auditory prepulses of three different intensities. Drug treatments consisted of combinations of amphetamine (0 and 1 mg/kg) and mirtazapine (0, 0.5, 1, 2, and 5 mg/kg), with all rats receiving all drug doses and combinations with different counterbalanced orders. In Experiment 2, a different group of rats was tested with drug treatments consisting of combinations of dizocilpine (0 and 0.05 mg/kg) and mirtazapine (0, 0.5, 1, 2, and 5 mg/kg). In Experiment 1 amphetamine (1 mg/kg) significantly reduced PPI whereas mirtazapine caused the opposite effect, with the highest dose of mirtazapine (5 mg/kg) effectively reversing the amphetamine-induced PPI deficit. In Experiment 2 dizocilpine (0.05 mg/kg) significantly reduced PPI, but mirtazapine did not have a significant effect on the inhibition of the startle response. These results indicate that the potential beneficial effects of combined α-adrenergic, 5-HT, and H₁ receptor blockade in counteracting PPI deficits may be associated to cases of sensorimotor gating disorders mediated by dopamine, but not necessarily to NMDA glutamate-induced PPI impairments.     

Evaluation of the emotional phenotype and serotonergic neurotransmission of fatty acid amide hydrolase-deficient mice

Rationale

By enhancing brain anandamide tone, inhibitors of fatty acid amide hydrolase (FAAH) induce anxiolytic-like effects in rodents and enhance brain serotonergic transmission. Mice lacking the faah gene (FAAH^?/?) show higher anandamide levels. However, their emotional phenotype is still debated and their brain serotonergic tone remained unexplored.

Objectives and methods

In this study, we tested FAAH^?/? mice in the social interaction and the open field tests performed under different lighting conditions (dim and bright) since variations of the experimental context were proposed to explain opposite findings. Moreover, by microdialysis performed under dim light, we analyzed their serotonergic transmission in frontal cortex (FC) and ventral hippocampus (vHIPP).

Results

In both light conditions, FAAH^?/? mice showed reduced emotionality, compared to wt controls, as suggested by the increased rearing and reduced thigmotaxis displayed in the open field and by the longer time spent in social interaction. Basal serotonergic tone was higher in the FC of mutant mice as compared to control mice, while no difference was observed in the vHIPP. K⁺-induced depolarization produced similar increases of serotonin in both areas of both genotypes. An acute treatment with the CB1 antagonist rimonabant completely abolished the emotional phenotype of FAAH^?/? mice and prevented the K⁺-stimulated release of serotonin in their FC and vHIPP, without producing any effect in wt mice.

Conclusions

Our results support the role of FAAH in the regulation of emotional reactivity and suggest that anandamide-mediated hyperactivation of CB1 is responsible for the emotional phenotype of FAAH^?/? mice and for their enhanced serotonergic tone.     

In vivo control of 5-hydroxytryptamine release by terminal autoreceptors in rat brain areas differentially innervated by the dorsal and median raphe nuclei

Selective serotonin reuptake inhibitors (SSRIs) reduce the 5-HT release in vivo. This effect is due to the activation of somatodendritic 5-HT_1A receptors and it displays a regional pattern comparable to that of selective 5-HT_1A agonists, i.e., preferentially in forebrain areas innervated by the dorsal raphe nucleus (DRN). However, despite a comparatively lower 5-HT_1A-mediated inhibition of 5-HT release and a greater density of serotonergic uptake sites in hippocampus, the net elevation produced by the systemic administration of SSRIs is similar in various forebrain areas, regardless of the origin of serotonergic fibres. As terminal autoreceptors may also limit the SSRI-induced elevations of 5-HT in the extracellular brain space, we reasoned that a differential control of 5-HT release by terminal autoreceptors in DRN- and median raphe-innervated areas might be accountable. To examine this possibility, we have conducted a regional microdialysis study in the DRN, MRN and four forebrain regions preferentially innervated either by the DRN (frontal cortex, striatum) or the median raphe nucleus (MRN; dorsal and ventral hippocampus) using freely moving rats. Dialysis probes were perfused with 1 μM of the SSRI citalopram to augment the endogenous tone on terminal 5-HT autoreceptors. The non-selective 5-HT₁ antagonist methiothepin (10 and 100 μM, dissolved in the dialysis fluid) increased extracellular 5-HT in frontal cortex and dorsal hippocampus in a concentration-dependent manner. The 5-HT_1B/1D antagonist GR 127935 was ineffective at 10 μM and tended to reduce 5-HT in dorsal hippocampus at 100 μM. The local infusion of 100 μM methiothepin significantly elevated the extracellular 5-HT concentration to 142–173% of baseline (mean values of 260 min post-administration) in the DRN, MRN, frontal cortex, striatum and hippocampus (dorsal and ventral). Comparable elevations were noted in the four forebrain regions examined. As observed in frontal cortex and dorsal hippocampus, the perfusion of 10 μM GR 127935 did not elevate 5-HT in DRN, MRN, striatum or ventral hippocampus. Because the stimulated 5-HT release in the DRN has been suggested to be under control of 5-HT_1B/1D receptors, we examined the possible contribution of these receptor subtypes to the effects of methiothepin in the DRN. The perfusion of sumatriptan (0.01–10 μM) or GR 127935 (0.01–10 μM) did not significantly modify the 5-HT concentration in dialysates from the DRN. Thus, the present data suggest that the comparable effects of SSRIs in DRN- and MRN-innervated forebrain regions are not explained by a preferential attenuation of 5-HT release by terminal 5-HT_1B autoreceptors in hippocampus, an area with a low inhibitory influence of somatodendritic 5-HT_1A receptors. Methiothepin-sensitive autoreceptors (possibly 5-HT_1B) appear to play an important role not only in the projection areas but also with respect to the control of 5-HT release in the DRN and MRN. In addition, our findings indicate that GR 127935 is not an effective antagonist of the actions of 5-HT at rat terminal autoreceptors. Received: 27 February 1998 / Accepted: 12 June 1998     

Differential involvement of hippocampal serotonin1A receptors and re-uptake sites in non-cognitive behaviors of Alzheimer��s disease

Rationale

Previous studies have shown extensive serotonergic deficits in the hippocampus of Alzheimer??s disease (AD) patients. However, it is unclear whether such deficits play a role in non-cognitive, neuropsychiatric behaviors that occur frequently in AD and cause significant caregiver distress.

Objectives

In this study, we aimed to correlate serotonergic markers in the AD hippocampus with neuropsychiatric behaviors.

Methods

Using postmortem hippocampal homogenates from aged controls as well as a cohort of longitudinally assessed AD patients, measurements of 5-HT_1A receptors, 5-HT_2A receptors, and serotonin re-uptake (5-HTT) sites were performed by binding with ³H-labeled 8-OH-DPAT, ketanserin, and citalopram, respectively.

Results

Alterations of 5-HT_1A receptors and 5-HTT were found to be differentially involved in neuropsychiatric behaviors, with loss of 5-HT_1A receptors specifically correlated with depressive symptoms, while 5-HTT sites were preserved or up-regulated in patients with aggressive behaviors.

Conclusions

Our data suggest that neuropsychiatric behaviors in AD share certain neurochemical features with psychiatric disorders like major depression and that serotonergic drugs used in psychiatric disorders may also be efficacious against behavioral symptoms in AD.     

Social instigation and aggression in postpartum female rats: role of 5-Ht1A and 5-Ht1B receptors in the dorsal raph�� nucleus and prefrontal cortex

Rationale

5-HT_1A and 5-HT_1B receptor agonists effectively reduce aggressive behavior in males that has been escalated by social instigation. Important sites of action for these drugs are the receptors in dorsal raphé nuclei (DRN) and the ventral?Corbital prefrontal cortex (VO PFC). DRN and VO PFC areas are particularly relevant in the inhibitory control of escalated aggressive and impulsive behavior.

Objectives

The objectives of this study are to assess the anti-aggressive effects of 5-HT_1A (8-OH-DPAT) and 5-HT_1B (CP-93,129) receptor agonists microinjected into DRN and VO PFC, respectively, and to study the aggressive behavior in postpartum female Wistar rats using the social instigation protocol to increase aggression.

Methods and Results

8-OH-DPAT (0.56???g) in the DRN increased aggressive behavior in postpartum female rats. By contrast, CP-93,129 (1.0???g) microinjected into VO PFC decreased the number of attack bites and lateral threats. 5-HT_1A and 5-HT_1B receptor agonists differed in their effects on non-aggressive activities, the former decreasing rearing and grooming and the latter increasing these acts. When 8-OH-DPAT was microinjected into DRN and CP-93,129 was microinjected into VO PFC in female rats at the same time, maternal aggression decreased. Specific participation of 5-HT_1B receptors was verified by reversal of the anti-aggressive effects using the selective antagonist SB-224,289 (1.0???g).

Conclusions

The decrease in maternal aggressive behavior after microinjections of 5-HT_1B receptor agonists into the VO PFC and DRN of female postpartum rats that were instigated socially supports the hypothesis that activation of these receptors modulates high levels of aggression in a behaviorally specific manner, due to activation of 5-HT_1B receptors at the soma and terminals.     

Modulation of the subthalamic nucleus activity by serotonergic agents and fluoxetine administration

Rationale

Within the basal ganglia, the subthalamic nucleus (STN) is the only glutamatergic structure and occupies a central position in the indirect pathway. In rat, the STN receives serotonergic input from the dorsal raphe nucleus and expresses serotonergic receptors.

Objective

This study examined the consequences of serotonergic neurotransmission modulation on STN neuron activity.

Methods

In vivo single-unit extracellular recordings, HPLC determination, and rotarod and bar test were performed in control, 4-chloro-DL-phenylalanine methyl ester hydrochloride- (pCPA, a serotonin synthesis inhibitor) and chronically fluoxetine-treated rats.

Results

The pCPA treatment and the administration of serotonin (5-HT) receptor antagonists increased number of bursting neurons in the STN. The systemic administration of the 5-HT_1A agonist, 8-OH-DPAT, decreased the firing rate and increased the coefficient of variation of STN neurons in pCPA-treated rats but not in control animals. Additionally, microinjection of 8-OH-DPAT into the STN reduced the firing rate of STN neurons, while microinjection of the 5-HT_2C agonist, Ro 60-0175, increased the firing rate in both control and fluoxetine-treated animals. Finally, the fluoxetine challenge increased the firing rate of STN neurons in fluoxetine-treated rats and induced catalepsy.

Conclusions

Our results indicate that the depletion and the blockage of 5-HT modify STN neuron firing pattern. STN neuron activity is under the control of 5-HT_1A and 5-HT_2C receptors located both inside and outside the STN. Finally, fluoxetine increases STN neuron activity in chronically fluoxetine-treated rats, which may explain the role of this nucleus in fluoxetine-induced extrapyramidal side effects.     

Region-specific regulation of 5-HT1B receptors in the rat brain by chronic venlafaxine treatment

Rationale

Venlafaxine is a non-selective serotonin and noradrenaline reuptake inhibitor antidepressant drug for which clinical studies have suggested a high level efficacy and a possible early action onset compared to the classical antidepressants. Its therapeutic effects might be due, at least in part, to adaptive changes in serotonergic neurotransmission, through the activation of the different 5-HT receptor subtypes. 5-HT_1B receptors are located in the axon terminals of both serotonergic and non-serotonergic neurons, where they act as inhibitory autoreceptors or heteroreceptors, respectively. However, the information about the involvement of this subtype in the mechanism of action of antidepressants is limited and quite controversial.

Objectives

The aim of this study was to evaluate the effect of venlafaxine (10 mg kg^?1 day^?1, p.o.) after 21 days of treatment on the density of 5-HT_1B receptors and their functionality in rat brain.

Methods

Effects of chronic venlafaxine were evaluated at different levels of 5-HT_1B receptor by using receptor autoradiography, [³⁵S]GTPγS binding, and the regulation of body temperature induced by selective 5-HT_1B agonist.

Results

Our results show that venlafaxine induced an increase in sensitivity of 5-HT_1B receptors in hypothalamus both at G-protein level and the control of core temperature without affecting the receptor density.

Conclusions

These results demonstrate that adaptive changes on 5-HT_1B receptors induced by chronic administration of venlafaxine exhibit regional differences suggesting that the hypothalamus might be an important site of drug action.     

Chlorpheniramine exerts anxiolytic-like effects and activates prefrontal 5-HT systems in mice

Rationale

The traditional antihistamine chlorpheniramine ameliorates panic attacks, phobias, and lowered mood, and this therapeutic effect is independent of the blockade of histamine H₁ receptors. Since chlorpheniramine inhibits the reuptake of serotonin (5-HT), the anxiolytic-like effect of chlorpheniramine may be produced by an increase in serotonergic function.

Objective

To elucidate the mechanisms underlying the anxiolytic-like effects of chlorpheniramine in mice, we examined the involvement of 5-HT systems in the prefrontal cortex that is a crucial region in the regulation of emotional function.

Results

Chlorpheniramine (0.05?C5?mg/kg, i.p.) dose-dependently and significantly decreased the duration of freezing behavior in both the elevated open-platform and conditioned fear tests. The anti-freezing effects of chlorpheniramine (5?mg/kg, i.p.) in these tests were inhibited by pretreatment with the non-selective antagonist at 5-HT receptors, methiothepin (0.01?mg/kg, s.c.). In addition, the local injection of chlorpheniramine (10?C100?ng/mouse) and 5-HT (1?C10???g/mouse) into the medial part of the prefrontal cortex (mPFC) dose-dependently and significantly decreased the duration of freezing behavior in the elevated open-platform test. In a microdialysis study, chlorpheniramine (0.5 and 5?mg/kg, i.p.) dose-dependently and significantly increased the extracellular 5-HT level in the mPFC. In addition, the local perfusion of chlorpheniramine (10 and 30???M), but not of the selective H1 receptor antagonist, cetirizine, into the mPFC markedly increased the extracellular 5-HT level in the mPFC.

Conclusion

The anxiolytic-like effect of chlorpheniramine is produced, at least in part, by the facilitation of serotonergic neurotransmission in the PFC.     

Inhibition of median and dorsal raphe neurones following administration of the selective serotonin reuptake inhibitor paroxetine

Acute systemic injection of selective serotonin reuptake inhibitors (SSRIs) decreases 5-HT neuronal firing in the dorsal raphe nucleus (DRN). Recent data, however, question whether these drugs also inhibit the firing of 5-HT neurones in the median raphe nucleus (MRN). Using in vivo extracellular electrophysiological recording techniques in the chloral hydrate anaesthetised rat, we have tested the effect of acute administration of the SSRI, paroxetine, on 5-HT neuronal activity in the MRN and DRN. Presumed 5-HT neurones in the MRN displayed the same electrophysiological characteristics as those in the DRN, the only detectable difference being that MRN neurones showed a significantly (p < 0.001) slower mean ( ± SEM(n)) spontaneous firing rate (MRN, 5.6 ± 0.9 (14) spikes/10 s; DRN, 13.5 ± 1.6 (24) spikes/10 s). Paroxetine caused a dose-related (0.1–0.8 mg/kg i.v.) inhibition of all MRN neurones tested (n = 8), producing a complete cessation of cell-firing at the highest doses. DRN neurones (n = 9) responded in a similar fashion. Furthermore, paroxetine inhibited MRN and DRN neurones with almost identical potency (MRN ED₅₀ 259 ± 57 g/kg i.v.: DRN ED₅₀ 243 ± 49 g/kg i.v.). In the majority of cells tested, the effect of paroxetine was reversed by the 5-HT_1A receptor antagonists spiperone or (+)WAY100135, implicating the involvement of the 5-HT_1A autoreceptor. The selective 5-HT_1A receptor agonist 8-OH-DPAT also inhibited the firing of MRN (n = 5) and DRN (n = 12) neurones and with equal potency (MRN ED₅₀, 1.32 ± 0.40 g/kg i.v.: DRN ED₅₀, 1.19 ± 0.23 g/kg i.v.). Our data indicate that paroxetine not only inhibits the firing of 5-HT neurones in the MRN but does so with equal potency to those in the DRN.     

Dissociable roles for the dorsal and median raphé in the facilitatory effect of 5-HT1A receptor stimulation upon cocaine-induced locomotion and sensitization.

A distinct role for serotonin transmission from the dorsal and median raphé nuclei (DRN and MRN, respectively) was identified in regulating the behavioral and neurochemical effects of acute and repeated cocaine administration. Serotonin 1A (5-hydroxytryptophan (5-HT)1A) receptors were stimulated by intraraphé microinjection of 8-hydroxy-2-(di-n-propylamino)tetralin (DPAT; 5 or 10 microg) and behavior, as well as extracellular neurotransmitter content in the nucleus accumbens was measured. Pretreatment of the DRN with DPAT caused a sensitization-like potentiation of acute cocaine-induced motor activity and an elevation in extracellular dopamine and glutamate. In contrast, DPAT microinjection into the MRN did not alter acute cocaine-induced motor activity or extracellular levels of dopamine or glutamate. Acutely, DPAT microinjection into either raphé nucleus reduced the basal and acute cocaine-stimulated levels of extracellular serotonin. Pretreatment with DPAT before systemic cocaine administration was continued for 5 days, and 3 weeks after the last injection, all rats were administered a cocaine challenge injection. The sensitized behavioral and neurochemical response produced by repeated cocaine in control subjects was unaffected by the intra-DRN administration of DPAT. However, in animals administered DPAT into the MRN, both the sensitized motor response and the increase in glutamate were augmented, while the sensitized serotonin response was blocked, without altering dopamine sensitization. These data show a differential role for 5-HT1A receptors in the DRN and MRN in the acute and sensitized effects of cocaine. While the DRN is involved in the acute effects of cocaine, neuroadaptations in the MRN may regulate the long-term consequences of repeated cocaine exposure.     

Rasgrf2 controls noradrenergic involvement in the acute and subchronic effects of alcohol in the brain

Rationale

Alcohol addiction is a major psychiatric disease, and yet, the underlying molecular adaptations in the brain remain unclear. Recent evidence suggests a functional role for the ras-specific guanine-nucleotide releasing factor 2 (Rasgrf2) in alcoholism. Rasgrf2^?/? mice consume less alcohol and show entirely absent dopamine responses to an alcohol challenge compared to wild types (WT).

Objective

In order to further investigate how Rasgrf2 modifies the acute and subchronic effects of alcohol in the brain, we investigated its effects on the noradrenergic and serotonergic systems.

Methods

We measured noradrenaline and serotonin activity in the brain by in vivo microdialysis and RNA expression by chip analysis and RT-PCR after acute and sub-chronic alcohol exposure in Rasgrf2^?/? and WT mice.

Results

In vivo microdialysis showed a significantly reduced noradrenergic response and an absent serotonergic response in the nucleus accumbens (NAcc) and caudate putamen (CPu) after an alcohol challenge in Rasgrf2^?/? mice. A co-expression analysis showed that there is a high correlation between Rasgrf2 and α2 adrenoceptor RNA expression in the ventral striatum in naïve animals. Accordingly, we further assessed the role of Rasgrf2 in the response of the noradrenergic system to subchronic alcohol exposure. A decrease in β1 adrenoceptor gene expression was seen in Rasgrf2^+/+, but not Rasgrf2^?/? mice following alcohol exposure. Conversely, alcohol resulted in a decrease in both β2 and α2 adrenoceptor gene expression in knockout but not WT Rasgrf2 mice.

Conclusions

These findings suggest that adaptations in the noradrenergic system contribute to the Rasgrf2 enhanced risk of alcoholism.     

A fatal case of venlafaxine overdose

Introduction

Based on its primary action of serotonin reuptake inhibition, venlafaxine overdose would be expected to result in serotonergic effects.

Case Report

A 40 year old male ingested venlafaxine without co-ingestants in a suicide attempt. The patient developed refractory ventricular fibrillation and expired approximately 9 hours post-ingestion. ECG monitoring revealed significant QRS and QT_C interval prolongation prior to his demise.

Discussion

A literature review of venlafaxine overdose cases and investigation into its mechanism of action was conducted. The potential for sodium channel blockade and implications for therapy are discussed.     

Activation of metabotropic glutamate 2/3 receptors attenuates methamphetamine-induced hyperlocomotion and increase in prefrontal serotonergic neurotransmission

Rationale

Metabotropic glutamate (mGlu) 2/3 receptor agonists inhibit amphetamine- and phencyclidine-induced hyperlocomotion. The mechanism for the antipsychotic effect of mGlu2/3 receptor agonists was studied in a hypoglutamatergic model, but not a hyperdopaminergic model.

Objectives

To study the mechanism for the antipsychotic effect of the agonist in the hyperdopaminergic model, this study examined the effects of the selective mGlu2/3 receptor agonist MGS0028 on methamphetamine-induced hyperlocomotion and the increases in extracellular levels of serotonin, dopamine, noradrenaline, and glutamate in the prefrontal cortex and nucleus accumbens of mice.

Results

Systemic administration of MGS0028 attenuated methamphetamine-induced hyperlocomotion in a dose-dependent manner. Microdialysis studies showed that MGS0028 significantly inhibited methamphetamine-induced increases in the extracellular serotonin, but not dopamine and noradrenaline, levels in the prefrontal cortex, and it did not affect methamphetamine-induced increases in the extracellular amine levels in the nucleus accumbens. Methamphetamine did not affect the glutamate release in the prefrontal cortex and nucleus accumbens. Local application of MGS0028 into the prefrontal cortex also attenuated methamphetamine-induced hyperlocomotion and increases in the extracellular serotonin levels in the prefrontal cortex. Moreover, MGS0028 did not affect methamphetamine-induced hyperlocomotion in the mice pretreated with p-chlorophenylalanine, a serotonin synthesis inhibitor.

Conclusions

Activation of prefrontal mGlu2/3 receptors inhibits the psychomotor stimulant effect of methamphetamine in mice, and the prefrontal serotonergic system may be involved in this effect. The finding provides evidence that prefrontal mGlu2/3 receptors are functionally coupled with the serotonergic system.     

Involvement of serotonin 5-HT3 receptors in the modulation of noradrenergic transmission by serotonin reuptake inhibitors: a microdialysis study in rat brain

Rationale

Selective serotonin reuptake inhibitors (SSRIs), in addition to being able to enhance serotonergic neurotransmission, are able to modulate other brain systems involved in depression.

Objectives

This study evaluates the neurochemical effect of the SSRI citalopram on brain noradrenergic activity and the serotonin receptor involved in this effect.

Methods

Dual-probe microdialysis in the locus coeruleus (LC) and prefrontal cortex (PFC) was performed in freely awake rats.

Results

Systemic citalopram (10 mg/kg, i.p.) increased noradrenaline (NA) in the LC (E _max?=?141?±?13 %) and simultaneously decreased NA in the PFC (E_max?=??46?±?7 %). In the local presence into the LC of the α₂-adrenoceptor antagonist RS79948 (1 μM), systemic citalopram increased NA in the LC (E_max?=?157?±?25 %) and PFC (E_max?=?175?±?24 %). Local citalopram (0.1–100 μM) into the LC induced NA increase in the LC (E_max?=?210?±?25 %) and decrease in the PFC (E_max?=??38?±?9 %). Local LC citalopram effect was abolished by LC presence of the 5-HT₃ receptor antagonist MDL72222 (1 μM) but not the 5-HT_1/2 receptor antagonist methiothepin (1 μM). Systemic citalopram in the LC presence of MDL72222 did not modify NA in the LC but increased NA in the PFC (E_max?=?158?±?26 %). Local citalopram into the PFC enhanced NA (E_max?=?376?±?18 %) in the area, which was prevented by MDL72222.

Conclusions

The SSRI citalopram modulates central noradrenergic neurotransmission by activation, through endogenous serotonin, of 5-HT₃ receptors expressed in the somatodendritic (LC) and terminal (PFC) areas, which subsequently promote an enhancement of local NA. Therefore, 5-HT₃ receptors and somatodendritic α₂-adrenoceptors in the LC play an important role in the global effect of SSRIs.     

Behavioral characterization of escalated aggression induced by GABAB receptor activation in the dorsal raphe nucleus

Rationale

Pharmacological activation of GABA_B receptors in the dorsal raphe nucleus (DRN) can escalate territorial aggression in male mice.

Objectives

We characterized this escalated aggression in terms of its behavioral and environmental determinants.

Methods

Aggressive behavior of resident male (CFW or ICR mouse) was assessed in confrontations with a group-housed intruder. Either baclofen (0.06 nmol/0.2 μl) or vehicle (saline) was microinjected into the DRN 10 min before the confrontation. We examined baclofen-heightened aggression in five situations: aggression in a neutral arena and after social instigation (experiment 1), aggression during the light phase of the cycle (experiment 2), aggression without prior fighting experience (experiment 3), aggression toward a female (experiment 4), and aggression after defeat experiences (experiment 5). In addition, we examined the body targets towards which bites are directed and the duration of aggressive bursts after baclofen treatment.

Results

Regardless of the past social experience, baclofen escalated aggressive behaviors. Even in the neutral arena and after defeat experiences, where aggressive behaviors were inhibited, baclofen significantly increased aggression. Baclofen increased attack bites directed at vulnerable body areas of male intruders but not toward a female and only in the dark. Also, baclofen prolonged the duration of aggressive bursts.

Conclusions

For baclofen to escalate aggression, specific stimulation (male intruder) and tonic level of serotonin (dark cycle) are required. Once aggressive behavior is triggered, intra-DRN baclofen escalates the level of aggression to abnormal levels and renders it difficult to terminate. Also, baclofen counteracts the effects of novelty or past experiences of defeat.     

Guanfacine modulates the influence of emotional cues on prefrontal cortex activation for cognitive control

Rationale

Functional interactions between limbic regions that process emotions and frontal networks that guide response functions provide a substrate for emotional cues to influence behavior. Stimulation of postsynaptic α₂ adrenoceptors enhances the function of prefrontal regions in these networks. However, the impact of this stimulation on the emotional biasing of behavior has not been established.

Objectives

This study tested the effect of the postsynaptic α₂ adrenoceptor agonist guanfacine on the emotional biasing of response execution and inhibition in prefrontal cortex.

Methods

Fifteen healthy young adults were scanned twice with functional magnetic resonance imaging while performing a face emotion go/no-go task following counterbalanced administration of single doses of oral guanfacine (1 mg) and placebo in a double-blind, cross-over design.

Results

Lower perceptual sensitivity and less response bias for sad faces resulted in fewer correct responses compared to happy and neutral faces but had no effect on correct inhibitions. Guanfacine increased the sensitivity and bias selectively for sad faces, resulting in response accuracy comparable to happy and neutral faces, and reversed the valence-dependent variation in response-related activation in left dorsolateral prefrontal cortex (DLPFC), resulting in enhanced activation for response execution cued by sad faces relative to happy and neutral faces, in line with other frontoparietal regions.

Conclusions

These results provide evidence that guanfacine stimulation of postsynaptic α₂ adrenoceptors moderates DLPFC activation associated with the emotional biasing of response execution processes. The findings have implications for the α₂ adrenoceptor agonist treatment of attention-deficit hyperactivity disorder.     

GABAA receptors in the dorsal raphé nucleus of mice: escalation of aggression after alcohol consumption

Rationale  

The dorsal raphé nucleus (DRN), the origin for serotonin (5-HT) in forebrain areas, has been implicated in the neural control of escalated aggression. Gamma aminobutyric acid type-A (GABA_A) and type-B (GABA_B) receptors are expressed in the DRN and modulate 5-HT neuronal activity, and both play a role in the behavioral effect of alcohol.