Isabelle Leroux-Nicollet

Behavioral, neurochemical, and electrophysiological characterization of a genetic mouse model of depression

Depression is a multifactorial illness and genetic factors play a role in its etiology. The understanding of its physiopathology relies on the availability of experimental models potentially mimicking the disease. Here we describe a model built up by selective breeding of mice with strikingly different responses in the tail suspension test, a stress paradigm aimed at screening potential antidepressants. Indeed, “helpless” mice are essentially immobile in the tail suspension test, as well as the Porsolt forced-swim test, and they show reduced consumption of a palatable 2% sucrose solution. In addition, helpless mice exhibit sleep–wakefulness alterations resembling those classically observed in depressed patients, notably a lighter and more fragmented sleep, with an increased pressure of rapid eye movement sleep. Compared with “nonhelpless” mice, they display higher basal seric corticosterone levels and lower serotonin metabolism index in the hippocampus. Remarkably, serotonin1A autoreceptor stimulation induces larger hypothermia and inhibition of serotoninergic neuronal firing in the nucleus raphe dorsalis in helpless than in nonhelpless mice. Thus, helpless mice exhibit a decrease in serotoninergic tone, which evokes that associated with endogenous depression in humans. Finally, both the behavioral impairments and the serotoninergic dysfunction can be improved by chronic treatment with the antidepressant fluoxetine. The helpless line of mice may provide an opportunity to approach genes influencing susceptibility to depression and to investigate neurophysiological and neurochemical substrates underlying antidepressant effects.

Brain-derived neurotrophic factor-deficient mice exhibit a hippocampal hyperserotonergic phenotype

Growing evidence supports the involvement of brain-derived neurotrophic factor (BDNF) in mood disorders and the mechanism of action of antidepressant drugs. However, the relationship between BDNF and serotonergic signalling is poorly understood. Heterozygous mutants BDNF +/- mice were utilized to investigate the influence of BDNF on the serotonin (5-HT) system and the activity of the serotonin transporter (SERT) in the hippocampus. The zero net flux method of quantitative microdialysis revealed that BDNF +/- heterozygous mice have increased basal extracellular 5-HT levels in the hippocampus and decreased 5-HT reuptake capacity. In keeping with these results, the selective serotonin reuptake inhibitor paroxetine failed to increase hippocampal extracellular 5-HT levels in BDNF +/- mice while it produced robust effects in wild-type littermates. Using in-vitro autoradiography and synaptosome techniques, we investigated the causes of attenuated 5-HT reuptake in BDNF +/- mice. A significant decrease in [3H]citalopram-binding-site density in the CA3 subregion of the ventral hippocampus and a significant reduction in [3H]5-HT uptake in hippocampal synaptosomes, revealed mainly a decrease in SERT function. However, 5-HT1A autoreceptors were not desensitized in BDNF +/- mice. These results provide evidence that constitutive reductions in BDNF modulate SERT function reuptake in the hippocampus.

Autoradiographic localization of [3H]nociceptin binding sites from telencephalic to mesencephalic regions of the mouse brain

The binding sites of [3H]nociceptin (also named Orphanin FQ), the endogenous ligand of the ORL1 (opiate receptor like 1) receptor, were localized in the central nervous system of the mouse using an autoradiographic procedure. A high density of binding sites was seen in the cerebral cortex, paraventricular nucleus of the thalamus, amygdaloid complex, suprachiasmatic nucleus, medial thalamus and medial geniculate nucleus. Moderate binding was observed in the nucleus accumbens, lateral septum, lateral thalamus, hippocampus, periaqueductal grey matter and pons. Finally, low levels of binding were seen in the striatum, olfactory tubercle, hypothalamus and substantia nigra. Thus, it appears that the ORL1 receptor is particularly abundant in the cerebral cortex and limbic system of the mouse brain.

Consequences of changes in BDNF levels on serotonin neurotransmission, 5-HT transporter expression and function: studies in adult mice hippocampus.

In vivo intracerebral microdialysis is an important neurochemical technique that has been applied extensively in genetic and pharmacological studies aimed at investigating the relationship between neurotransmitters. Among the main interests of microdialysis application is the infusion of drugs through the microdialysis probe (reverse dialysis) in awake, freely moving animals. As an example of the relevance of intracerebral microdialysis, this review will focus on our recent neurochemical results showing the impact of Brain-Derived Neurotrophic Factor (BDNF) on serotonergic neurotransmission in basal and stimulated conditions. Indeed, although the elevation of 5-HT outflow induced by chronic administration of selective serotonin reuptake inhibitors (SSRIs) causes an increase in BDNF protein levels and expression (mRNA) in the hippocampus of rodents, the reciprocal interaction has not been demonstrated yet. Thus, the neurochemical sight of this question will be addressed here by examining the consequences of either a constitutive decrease or increase in brain BDNF protein levels on hippocampal extracellular levels of 5-HT in conscious mice.

A chronic treatment with fluoxetine decreases 5-HT1A receptors labeling in mice selected as a genetic model of helplessness

Two lines of mice were bred for their opposite helpless behavior in the tail suspension test, i.e., helpless (HL) mice and non helpless (NHL) mice. The 5-HT(1A) receptor labeling was quantified by means of autoradiography with (3)H-8-OH-DPAT on brain sections from mice of these two lines. We observed a significantly higher level of (3)H-8-OH-DPAT binding sites density in HL mice comparatively to NHL mice, in the medial prefrontal, cingulate, motor and sensorial cortices, in several regions of the limbic system, such as CA3 field of hippocampus, dentate gyrus, medial and baso-medial amygdala, and in dorsal and median raphe nuclei. A chronic 21-day treatment with the antidepressant fluoxetine (10 mg/kg, i.p. daily) attenuated significantly the spontaneous helplessness in HL mice but did not alter the behavior of NHL mice. In the brain of HL mice chronically injected with fluoxetine, the elevated (3)H-8-OH-DPAT binding sites density was no longer observed after treatment in several regions, among which the raphe nuclei. Conversely, the antidepressant treatment did not modify the (3)H-8-OH-DPAT binding sites density in NHL mice. The variation of 5-HT(1A) receptors binding density in the HL mice in response to a chronic fluoxetine treatment parallels the attenuation of the spontaneous helplessness observed in the tail suspension test, and may underlie this behavior.

Comparisons between anxiety tests for selection of anxious and non anxious mice

Male Swiss albinos mice were submitted to two behavioural tests intended to determine their anxiety level: the elevated plus-maze test as well as the black and white compartments test. In addition they were submitted to the hole-board test. It was observed: (i) that the correlation between scores in the two first tests was weak, suggesting that they explore different components of anxiety; (ii) that the score on the latter test was better correlated with the response in the elevated plus-maze test than in the black and white compartments test. From these data three groups of animals were constituted, considered, respectively, as anxious, non anxious and intermediates. It was observed that both horizontal and vertical locomotion in an unfamiliar environment differed between groups, with higher activity in non anxious than in anxious. In the hole-board test, only animals classified as anxious displayed an obvious response to the anxiolytic drug diazepam (0.5mg/kg). Finally in the forced-swimming test, the three groups demonstrated a similar immobility time, suggesting that the operated segregation was not depending on a helpless component. It is proposed that the selection of mice from a combination of either elevated plus-maze and black and white compartments tests or a combination of hole-board test and black and white compartments test, allows to distinguish high or low anxiety animals among a population of mice.

Differential effects of chronic antidepressant treatments on μ- and δ-opioid receptors in rat brain

Abstract We performed an autoradiographic study of [ d -Ala 2 ,MePhe 4 ,Gly-ol 5 ]enkephalin (DAMGO)-sensitive [ 3 H]naloxone binding to μ-opioid receptors and of [ 3 H][ d -Pen 2 , d -Pen 5 ]enkephalin (DPDPE) binding to δ-opioid receptors in the rat brain after 4- or 21-day treatments with paroxetine, reboxetine and moclobemide to investigate the participation of these receptors in the adaptive mechanisms occurring during the delay of action of new generation antidepressants. Paroxetine increased μ-opioid receptor binding site density in cingulate and insular cortices, dorsal endopiriform nucleus (4 days) and olfactory tubercle (21 days) and decreased it in thalamus (21 days). Reboxetine increased it in amygdala (4 days), hippocampus and thalamus (21 days) and decreased it in dorsal raphe (4 days). Moclobemide increased it in hippocampus (4 days) and decreased it in anterior olfactory nucleus, frontal cortex, amygdala and hypothalamus (21 days). Moclobemide increased δ-opioid receptor binding site density in frontal cortex and amygdala (4 days) and decreased it in amygdala and colliculi (21 days). Opioid receptors displayed distinct patterns of adaptations in response to the three antidepressants studied.

RESERPINE AFFECTS DIFFERENTIALLY THE DENSITY OF THE VESICULAR MONOAMINE TRANSPORTER AND DIHYDROTETRABENAZINE BINDING SITES

We have studied the effect of a single injection of reserpine (5 mg/kg, s.c.) on the synaptic vesicle monoamine transporter (VMAT) density in the rat striatum, using two labelling procedures: radioimmunolabelling with an antibody against VMAT, and binding of the specific ligand [3H]dihydrotetrabenazine ([3H]TBZOH). In the rostral and medial striatum, the distribution of VMAT immunoreactivity displayed the highest density in the lateral subregions. In the caudal part of the striatum, VMAT immunoreactivity showed increasing density from dorsal to ventral subregions. The VMAT immunoreactivity was not altered 2 and 30 days after the reserpine injection, whereas [3H]TBZOH binding site density, measured on adjacent slices, showed a dramatic decrease at day 2 and a moderate recovery at day 30, suggesting that despite a persistent blockade of [3H]TBZOH binding sites, VMAT protein density was unchanged.

Kainic acid lesion of the striatum increases dopamine release but reduces 3-methoxytyramine level

Kainic acid lesion of the striatum leads to the disruption of the striatonigral regulatory loop. Although microdialysis showed an increase in dopamine release, two days after kainic acid injection, 3-methoxytyramine levels are dramatically decreased. This suggests that the postsynaptic membrane-bound catechol-O-methyltransferase (COMT), but not the glial COMT, is responsible for the formation of measured 3-methoxytyramine.

Opioid Receptor-Like 1 (NOP) Receptors in the Rat Dorsal Raphe Nucleus: Evidence for Localization on Serotoninergic Neurons and Functional Adaptation After 5,7-Dihydroxytryptamine Lesion

A high density of opioid receptor‐like 1 (ORL1) receptor (also referred to as NOP receptor) is found in limbic areas and in regions containing monoamines, which are implicated in emotional activity and physiopathology of depression and anxiety. We aimed at defining precisely the localization of ORL1 receptors in dorsal raphe nucleus, by means of a lesion strategy and autoradiographic studies. In control rats, [3H]nociceptin and nociceptin‐stimulated [35S]GTPγS bindings were found to be correlated in several brain regions. We performed in rats a selective destruction of serotoninergic neurons by surgical stereotaxic injection of 5,7‐dihydroxytryptamine (5,7‐DHT) in dorsal raphe nucleus. This led to a marked decrease in serotonin contents in striata and frontal cortices (about –60%) and in autoradiographic [3H]citalopram binding in posterior regions. In dorsal raphe nucleus, [3H]nociceptin binding was decreased to the same extent as [3H]citalopram binding, whereas it was unchanged in the other regions studied. Nevertheless, in the dorsal raphe, nociceptin‐stimulated [35S]GTPγS binding was decreased to a lesser extent than [3H]nociceptin binding in 5,7‐DHT‐lesioned rats. The ratio between nociceptin‐stimulated [35S]GTPγS binding and [3H]nociceptin binding was significantly increased in 5,7‐DHT‐lesioned rats compared with controls in this region. These data demonstrate 1) that ORL1 receptors are located on serotoninergic neurons in the dorsal raphe nucleus and 2) that, after a lesion, the functionality of remaining ORL1 receptors appears to be up‐regulated, which could correspond to a compensatory mechanism.