Tryptophan Hydroxylase 2 Knockout Male Rats Exhibit a Strengthened Oxytocin System, Are Aggressive, and Are Less Anxious

The central serotoninergic system is critical for stress responsivity and social behavior, and its dysregulations have been centrally implicated in virtually all neuropsychiatric disorders. Genetic serotonin depletion animal models could provide a tool to elucidate the causes and mechanisms of diseases and to develop new treatment approaches. Previously, mice lacking tryptophan hydroxylase 2 (Tph2) have been developed, showing altered behaviors and neurotransmission. However, the effect of congenital serotonin deficiency on emotional and social behavior in rats is still largely unknown, as are the underlying mechanisms. In this study, we used a Tph2 knockout (Tph2^-/-) male rat model to study how the lack of serotonin in the rat brain affects anxiety-like and social behaviors. Since oxytocin is centrally implicated in these behaviors, we furthermore explored whether the effects of Tph2 knockout on behavior would relate to changes in the oxytocin system. We show that Tph2^-/- rats display reduced anxiety-like behavior and a high level of aggression in social interactions. In addition, oxytocin receptor expression was increased in the infralimbic and prelimbic cortices, paraventricular nucleus, dorsal raphe nucleus, and some subregions of the hippocampus, which was paralleled by increased levels of oxytocin in the medial frontal cortex and paraventricular nucleus but not the dorsal raphe nucleus, central amygdala, and hippocampus. In conclusion, our study demonstrated reduced anxiety but exaggerated aggression in Tph2^-/- male rats and reveals for the first time a potential involvement of altered oxytocin system function. Meanwhile, the research of oxytocin could be distinguished in almost any psychiatric disorder including anxiety and mental disorders. This research potentially proposes a new target for the treatment of such disorders, from a genetic serotonin deficiency aspect.

“Neural Network Responses to Traumatic Stress Predicting its Longterm Consequences”

Adaptive responding to severe stress or trauma requires an optimized reconfiguration in the activity of large-scale neural networks. In vulnerable individuals, this response can go awry, inducing long-term consequences on mental health, such as posttraumatic stress disorder (PTSD). Improved understanding of the neurobiological mechanisms underlying this maladaptive neural response to trauma might benefit early intervention (i.e., secondary prevention) options in stress-related psychopathology. Yet, because of obvious ethical limitations these acute responses to trauma are inaccessible in humans. Therefore, we here used a mouse model for PTSD to investigate adaptive vs. maladaptive neural responding to trauma, the latter leading to long-term behavioral consequences mimicking symptoms observed in PTSD patients. By using transgenic mice, we were able to fluorescently label all activated neurons during trauma exposure, and relate these activation patterns to later PTSD-like symptomatology. We observed increased neuronal activity in sensory-processing and memory-related areas of mice vulnerable to the long-term consequences of trauma exposure, compared to resilient mice. Moreover, vulnerable mice displayed increased functional connectivity between the default mode network and lateral cortical network (a proxy for the central executive network in humans) during trauma processing relative to resilient mice. As such, these findings provide first insight in how a maladaptive neural response to trauma can result in later symptoms of psychopathology.

Ndufs4 knockout mouse models of Leigh syndrome: pathophysiology and intervention

Mitochondria are small cellular constituents that generate cellular energy (ATP) by oxidative phosphorylation (OXPHOS). Dysfunction of these organelles is linked to a heterogeneous group of multisystemic disorders, including diabetes, cancer, ageing-related pathologies and rare mitochondrial diseases. With respect to the latter, mutations in subunit-encoding genes and assembly factors of the first OXPHOS complex (complex I) induce isolated complex I deficiency and Leigh syndrome. This syndrome is an early-onset, often fatal, encephalopathy with a variable clinical presentation and poor prognosis due to the lack of effective intervention strategies. Mutations in the nuclear DNA-encoded NDUFS4 gene, encoding the NADH:ubiquinone oxidoreductase subunit S4 (NDUFS4) of complex I, induce ‘mitochondrial complex I deficiency, nuclear type 1’ (MC1DN1) and Leigh syndrome in paediatric patients. A variety of (tissue-specific) Ndufs4 knockout mouse models were developed to study the Leigh syndrome pathomechanism and intervention testing.

Here, we review and discuss the role of complex I and NDUFS4 mutations in human mitochondrial disease, and review how the analysis of Ndufs4 knockout mouse models has generated new insights into the MC1ND1/Leigh syndrome pathomechanism and its therapeutic targeting.

A reminder before extinction failed to prevent the return of conditioned threat responses irrespective of threat memory intensity in rats

After retrieval, reactivated memories may destabilize and require restabilization processes to persist, referred to as reconsolidation. The reminder-extinction procedure has been proposed as a behavioral reconsolidation-based intervention to persistently attenuate threat-conditioned memories. After the presentation of a single reminder trial, the conditioned threat memory may enter a labile state, and extinction training during this window can prevent the return of conditioned threat responses. However, findings on this reminder-extinction procedure are mixed and its effectiveness may be subject to boundary conditions, including memory strength. Here, we systematically investigate whether more intense threat memories are less susceptible to disruption through a reminder-extinction procedure. Using a Pavlovian auditory threat conditioning procedure at three different shock intensities, rats acquired conditioned threat responses of variable "strength." Rats subsequently underwent either extinction preceded by a reminder or standard extinction. Although different shock intensities led to different strength threat memories, all groups showed reinstatement of conditioned threat responses irrespective of shock intensity or reminder condition. Hence, regardless of the intensity of the threat memory, the reminder procedure was ineffective in preventing the return of threat responses in rats. We thus find no evidence that threat memory intensity is a potential modulator of the effectiveness of the reminder-extinction procedure. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Long access to cocaine self-administration dysregulates the glutamate synapse in the nucleus accumbens core of serotonin transporter knockout rats

Background and Purpose

It is well established that the nucleus accumbens and glutamate play a critical role in the motivation to take drugs of abuse. We have previously demonstrated that rats with ablation of the serotonin (5‐HT) transporter (SERT^−/− rats) show increased cocaine intake reminiscent of compulsivity.

Experimental Approach

By comparing SERT^−/− to SERT^+/+ rats, we set out to explore whether SERT deletion influences glutamate neurotransmission under control conditions as well as after short access (1 h/session) or long access (6 h/session) to cocaine self‐administration.

Key Results

Rats were killed at 24 h after the final self‐administration session for ex vivo molecular analyses of the glutamate system (vesicular and glial transporters, post‐synaptic subunits of NMDA and AMPA receptors and their related scaffolding proteins). Such analyses were undertaken in the nucleus accumbens core. In cocaine‐naïve animals, SERT deletion evoked widespread abnormalities in markers of glutamatergic neurotransmission that, overall, indicate a reduction of glutamate signalling. These results suggest that 5‐HT is pivotal for the maintenance of accumbal glutamate homeostasis. We also found that SERT deletion altered glutamate homeostasis mainly after long access, but not short access, to cocaine.

Conclusion and Implications

Our findings reveal that SERT deletion may sensitize the glutamatergic synapses of the nucleus accumbens core to the long access but not short access, intake of cocaine.

LINKED ARTICLES

This article is part of a themed issue on New discoveries and perspectives in mental and pain disorders. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v179.17/issuetoc

Knockout serotonin transporter in rats moderates outcome and stimulus generalization

Understanding the common dimension of mental disorders (such as anxiety, depression, and drug addiction) might contribute to the construction of biological frameworks (Research Domain Criteria, RDoC) for novel ways of treatment. One common dimension at the behavioral level observed across these disorders is a generalization. Testing generalization in serotonin transporter (5-HTT) knockout (KO) rats, an animal model showing depression/anxiety-like behaviors and drug addiction-like behaviors, could therefore provide more insights into this framework. We tested the outcome and stimulus generalization in wild-type (WT) and 5-HTT KO rats. Using a newly established touchscreen-based task, subjects directly responded to visual stimuli (Gabor patch images). We measured the response time and outcome in a precise manner. We found that 5-HTT KO rats processed visual information faster than WT rats during outcome generalization. Interestingly, during stimulus generalization, WT rats gradually responded faster to the stimuli as the sessions progressed, while 5-HTT KO rats responded faster than WT in the initial sessions and did not change significantly as the sessions progressed. This observation suggests that KO rats, compared to WT rats, may be less able to update changes in information. Taken together, KO 5-HTT modulates information processing when the environment changes.

An open-source high-speed infrared videography database to study the principles of active sensing in freely navigating rodents

Background

Active sensing is crucial for navigation. It is characterized by self-generated motor action controlling the accessibility and processing of sensory information. In rodents, active sensing is commonly studied in the whisker system. As rats and mice modulate their whisking contextually, they employ frequency and amplitude modulation. Understanding the development, mechanisms, and plasticity of adaptive motor control will require precise behavioral measurements of whisker position.

Findings

Advances in high-speed videography and analytical methods now permit collection and systematic analysis of large datasets. Here, we provide 6,642 videos as freely moving juvenile (third to fourth postnatal week) and adult rodents explore a stationary object on the gap-crossing task. The dataset includes sensory exploration with single- or multi-whiskers in wild-type animals, serotonin transporter knockout rats, rats received pharmacological intervention targeting serotonergic signaling. The dataset includes varying background illumination conditions and signal-to-noise ratios (SNRs), ranging from homogenous/high contrast to non-homogenous/low contrast. A subset of videos has been whisker and nose tracked and are provided as reference for image processing algorithms.

Conclusions

The recorded behavioral data can be directly used to study development of sensorimotor computation, top-down mechanisms that control sensory navigation and whisker position, and cross-species comparison of active sensing. It could also help to address contextual modulation of active sensing during touch-induced whisking in head-fixed vs freely behaving animals. Finally, it provides the necessary data for machine learning approaches for automated analysis of sensory and motion parameters across a wide variety of signal-to-noise ratios with accompanying human observer-determined ground-truth.

The relation between gray matter volume and the use of alcohol, tobacco, cocaine and cannabis in male polysubstance users

BACKGROUND

Neuroimaging studies have demonstrated gray matter (GM) volume abnormalities in substance users. While the majority of substance users are polysubstance users, very little is known about the relation between GM volume abnormalities and polysubstance use.

METHODS

In this study we assessed the relation between GM volume, and the use of alcohol, tobacco, cocaine and cannabis as well as the total number of substances used, in a sample of 169 males: 15 non-substance users, 89 moderate drinkers, 27 moderate drinkers who also smoke tobacco, 13 moderate drinkers who also smoke tobacco and use cocaine, 10 heavy drinkers who smoke tobacco and use cocaine and 15 heavy drinkers who smoke tobacco, cannabis and use cocaine.

RESULTS

Regression analyses showed that there was a negative relation between the number of substances used and volume of the dorsal medial prefrontal cortex (mPFC) and the ventral mPFC. Without controlling for the use of other substances, the volume of the dorsal mPFC was negatively associated with the use of alcohol, tobacco, and cocaine. After controlling for the use of other substances, a negative relation was found between tobacco and cocaine and volume of the thalami and ventrolateral PFC, respectively.

CONCLUSION

These findings indicate that mPFC alterations may not be substance-specific, but rather related to the number of substances used, whereas, thalamic and ventrolateral PFC pathology is specifically associated with tobacco and cocaine use, respectively. These findings are important, as the differential alterations in GM volume may underlie different cognitive deficits associated with substance use disorders.

Enhanced Amygdala-Striatal Functional Connectivity during the Processing of Cocaine Cues in Male Cocaine Users with a History of Childhood Trauma

BACKGROUND AND AIMS

Childhood trauma is associated with increased levels of anxiety later in life, an increased risk for the development of substance use disorders, and neurodevelopmental abnormalities in the amygdala and frontostriatal circuitry. The aim of this study was to investigate the (neurobiological) link among childhood trauma, state anxiety, and amygdala-frontostriatal activity in response to cocaine cues in regular cocaine users.

METHODS

In this study, we included 59 non-treatment seeking regular cocaine users and 58 non-drug using controls. Blood oxygenation level-dependent responses were measured using functional magnetic resonance imaging while subjects performed a cue reactivity paradigm with cocaine and neutral cues. Psychophysiological interaction analyses were applied to assess functional connectivity between the amygdala and other regions in the brain. Self-report questionnaires were used to measure childhood trauma, state anxiety, drug use, drug use severity, and craving.

RESULTS

Neural activation was increased during the presentation of cocaine cues, in a widespread network including the frontostriatal circuit and amygdala in cocaine users but not in controls. Functional coupling between the amygdala and medial prefrontal cortex was reduced in response to cocaine cues, in both cocaine users and controls, which was further diminished with increasing state anxiety. Importantly, amygdala-striatal connectivity was positively associated with childhood trauma in regular cocaine users, while there was a negative association in controls. At the behavioral level, state anxiety was positively associated with cocaine use severity and craving related to negative reinforcement.

CONCLUSION

Childhood trauma is associated with enhanced amygdala-striatal connectivity during cocaine cue reactivity in regular cocaine users, which may contribute to increased habit behavior and poorer cognitive control. While we cannot draw conclusions on causality, this study provides novel information on how childhood trauma may contribute to the development and persistence of cocaine use disorder.

d-Cycloserine enhanced extinction of cocaine-induced conditioned place preference is attenuated in serotonin transporter knockout rats

d-Cycloserine (DCS), a partial NMDA receptor agonist, has been proposed as a cognitive enhancer to facilitate the extinction of drug-related memories. However, it is unknown whether there are individual differences in the efficacy of DCS. Here, we set out to investigate the influence of serotonin transporter (5-HTT) genotype on DCS treatment outcome and the underlying neural mechanism. To that end, we first determined the mRNA levels of several NMDA receptor subunits and observed a reduction in NR1/NR2C receptors in the ventromedial prefrontal cortex and nucleus accumbens of 5-HTT^-/- compared with 5-HTT^+/+ rats. Based on this finding, we hypothesized a lower sensitivity to DCS in the 5-HTT^-/- rats. To test this, rats were trained in a cocaine-induced conditioned place preference (CPP) paradigm. A significant extinction of CPP was observed in 5-HTT^+/+ rats receiving 1 mg/kg i.v. DCS, while a similar effect was found in the 5-HTT^-/- rats only after 5 mg/kg. Following CPP, we tested if DCS were able to reduce FosB/∆FosB protein expression, a molecular switch for cocaine-seeking behaviour. We observed an overall lower number of FosB/∆FosB positive cells in 5-HTT^-/- ventromedial prefrontal cortex and amygdala and an overall effect of DCS treatment on the number of positive cells in the nucleus accumbens. In conclusion, in this study, we show that the dosing of DCS to facilitate the extinction of cocaine-seeking behaviour is, at least partially, determined by 5-HTT genotype.

Hyperresponsiveness of the Neural Fear Network During Fear Conditioning and Extinction Learning in Male Cocaine Users

OBJECTIVE

The authors investigated whether cocaine use disorder is associated with abnormalities in the neural underpinnings of aversive conditioning and extinction learning, as these processes may play an important role in the development and persistence of drug abuse.

METHOD

Forty male regular cocaine users and 51 male control subjects underwent a fear conditioning and extinction protocol during functional MRI. Skin conductance response was measured throughout the experiment as an index of conditioned responses.

RESULTS

Cocaine users showed hyperresponsiveness of the amygdala and insula during fear conditioning, as well as hyporesponsiveness of the dorsomedial prefrontal cortex during extinction learning. In cocaine users, but not in control subjects, skin conductance responses were positively correlated with responsiveness of the insula, amygdala, and dorsomedial prefrontal cortex during fear conditioning but negatively correlated with responsiveness of the ventromedial prefrontal cortex during extinction learning.

CONCLUSIONS

Increased sensitivity to aversive conditioned cues in cocaine users might be a risk factor for stress-relief craving in cocaine use disorder. These results support the postulated role of altered aversive conditioning in cocaine use disorder and may be an important step in understanding the role of aversive learning in the pathology of cocaine use disorder.

Aversive Counterconditioning Attenuates Reward Signaling in the Ventral Striatum

Appetitive conditioning refers to the process of learning cue-reward associations and is mediated by the mesocorticolimbic system. Appetitive conditioned responses are difficult to extinguish, especially for highly salient reward such as food and drugs. We investigate whether aversive counterconditioning can alter reward reinstatement in the ventral striatum in healthy volunteers using functional magnetic resonance imaging (fMRI). In the initial conditioning phase, two different stimuli were reinforced with a monetary reward. In the subsequent counterconditioning phase, one of these stimuli was paired with an aversive shock to the wrist. In the following extinction phase, none of the stimuli were reinforced. In the final reinstatement phase, reward was reinstated by informing the participants that the monetary gain could be doubled. Our fMRI data revealed that reward signaling in the ventral striatum and ventral tegmental area following reinstatement was smaller for the stimulus that was counterconditioned with an electrical shock, compared to the non-counterconditioned stimulus. A functional connectivity analysis showed that aversive counterconditioning strengthened striatal connectivity with the hippocampus and insula. These results suggest that reward signaling in the ventral striatum can be attenuated through aversive counterconditioning, possibly by concurrent retrieval of the aversive association through enhanced connectivity with hippocampus and insula.

Serotonin transporter is not required for the development of severe pulmonary hypertension in the Sugen hypoxia rat model

Increased serotonin serum levels have been proposed to play a key role in pulmonary arterial hypertension (PAH) by regulating vessel tone and vascular smooth muscle cell proliferation. An intact serotonin system, which critically depends on a normal function of the serotonin transporter (SERT), is required for the development of experimental pulmonary hypertension in rodents exposed to hypoxia or monocrotaline. While these animal models resemble human PAH only with respect to vascular media remodeling, we hypothesized that SERT is likewise required for the presence of lumen-obliterating intima remodeling, a hallmark of human PAH reproduced in the Sugen hypoxia (SuHx) rat model of severe angioproliferative pulmonary hypertension. Therefore, SERT wild-type (WT) and knockout (KO) rats were exposed to the SuHx protocol. SERT KO rats, while completely lacking SERT, were hemodynamically indistinguishable from WT rats. After exposure to SuHx, similar degrees of severe angioproliferative pulmonary hypertension and right ventricular hypertrophy developed in WT and KO rats (right ventricular systolic pressure 60 vs. 55 mmHg, intima thickness 38 vs. 30%, respectively). In conclusion, despite its implicated importance in PAH, SERT does not play an essential role in the pathogenesis of severe angioobliterative pulmonary hypertension in rats exposed to SuHx.

Relationship between trait impulsivity and cortical volume, thickness and surface area in male cocaine users and non-drug using controls

BACKGROUND

Trait impulsivity is commonly associated with cocaine dependence. The few studies that have investigated the relation between trait impulsivity and cortical morphometry, have shown a distinct relation between impulsivity and cortical volume (CV) of temporal, frontal and insula cortex. As CV is the function of cortical surface area (SA) and cortical thickness (CT) impulsivity may be differently associated to SA than to CT.

METHOD

Fifty-three cocaine users (CU) and thirty-five controls (HC) (males aged 18-55 years) completed the Barrat impulsiveness scale and a structural scan was made on a 3T MRI scanner. CV, SA and CT were measured using Freesurfer. Multivariate analysis was used to test for group differences and group by impulsivity interaction effects in CV, SA and ST across nine regions of interest in the temporal, frontal and insular cortices. Possible confounding effects of drug- and alcohol exposure were explored.

RESULTS

Compared to HC, CU had a smaller SA of the superior temporal cortex but a larger SA of the insula. There were divergent relations between trait impulsivity and SA of the superior temporal cortex and insula (positive in HC, negative in CU) and CT of the anterior cingulate cortex (negative in HC, positive in CU). Within CU, there was a negative association between monthly cocaine use and CT of the insula and superior temporal cortex.

DISCUSSION

The distinct relation between trait impulsivity and cortical morphometry in CU and HC might underlie inefficient control over behavior resulting in maladaptive impulsive behaviour such as cocaine abuse.

Cross-species approaches to pathological gambling: a review targeting sex differences, adolescent vulnerability and ecological validity of research tools

Decision-making plays a pivotal role in daily life as impairments in processes underlying decision-making often lead to an inability to make profitable long-term decisions. As a case in point, pathological gamblers continue gambling despite the fact that this disrupts their personal, professional or financial life. The prevalence of pathological gambling will likely increase in the coming years due to expanding possibilities of on-line gambling through the Internet and increasing liberal attitudes towards gambling. It therefore represents a growing concern for society. Both human and animal studies rapidly advance our knowledge on brain-behaviour processes relevant for understanding normal and pathological gambling behaviour. Here, we review in humans and animals three features of pathological gambling which hitherto have received relatively little attention: (1) sex differences in (the development of) pathological gambling, (2) adolescence as a (putative) sensitive period for (developing) pathological gambling and (3) avenues for improving ecological validity of research tools. Based on these issues we also discuss how research in humans and animals may be brought in line to maximize translational research opportunities.

High serotonin levels during brain development alter the structural input-output connectivity of neural networks in the rat somatosensory layer IV

Homeostatic regulation of serotonin (5-HT) concentration is critical for “normal” topographical organization and development of thalamocortical (TC) afferent circuits. Down-regulation of the serotonin transporter (SERT) and the consequent impaired reuptake of 5-HT at the synapse, results in a reduced terminal branching of developing TC afferents within the primary somatosensory cortex (S1). Despite the presence of multiple genetic models, the effect of high extracellular 5-HT levels on the structure and function of developing intracortical neural networks is far from being understood. Here, using juvenile SERT knockout (SERT^−/−) rats we investigated, in vitro, the effect of increased 5-HT levels on the structural organization of (i) the TC projections of the ventroposteromedial thalamic nucleus toward S1, (ii) the general barrel-field pattern, and (iii) the electrophysiological and morphological properties of the excitatory cell population in layer IV of S1 [spiny stellate (SpSt) and pyramidal cells]. Our results confirmed previous findings that high levels of 5-HT during development lead to a reduction of the topographical precision of TCA projections toward the barrel cortex. Also, the barrel pattern was altered but not abolished in SERT^−/− rats. In layer IV, both excitatory SpSt and pyramidal cells showed a significantly reduced intracolumnar organization of their axonal projections. In addition, the layer IV SpSt cells gave rise to a prominent projection toward the infragranular layer Vb. Our findings point to a structural and functional reorganization of TCAs, as well as early stage intracortical microcircuitry, following the disruption of 5-HT reuptake during critical developmental periods. The increased projection pattern of the layer IV neurons suggests that the intracortical network changes are not limited to the main entry layer IV but may also affect the subsequent stages of the canonical circuits of the barrel cortex.

A critical review of sex differences in decision-making tasks: focus on the Iowa Gambling Task

It has been observed that men and women show performance differences in the Iowa Gambling Task (IGT), a task of decision-making in which subjects through exploration learn to differentiate long-term advantageous from long-term disadvantageous decks of cards: men choose more cards from the long-term advantageous decks than women within the standard number of 100 trials. Here, we aim at discussing psychological mechanisms and neurobiological substrates underlying sex differences in IGT-like decision-making. Our review suggests that women focus on both win-loss frequencies and long-term pay-off of decks, while men focus on long-term pay-off. Furthermore, women may be more sensitive to occasional losses in the long-term advantageous decks than men. As a consequence hereof, women need 40-60 trials in addition before they reach the same level of performance as men. These performance differences are related to differences in activity in the orbitofrontal cortex and dorsolateral prefrontal cortex as well as in serotonergic activity and left-right hemispheric activity. Sex differences in orbitofrontal cortex activity may be due to organisational effects of gonadal hormones early in life. The behavioural and neurobiological differences in the IGT between men and women are an expression of more general sex differences in the regulation of emotions. We discuss these findings in the context of sex differences in information processing related to evolutionary processes. Furthermore we discuss the relationship between these findings and real world decision-making.

Developmental influence of the serotonin transporter on the expression of npas4 and GABAergic markers: modulation by antidepressant treatment

Alterations of the serotonergic system are involved in the pathophysiology of mood disorders and represent an important target for its pharmacological treatment. Genetic deletion of the serotonin transporter (SERT) in rodents leads to an anxious and depressive phenotype, and is associated with reduced neuronal plasticity as indicated by decreased brain-derived neurotrophic factor (Bdnf) expression levels. One of the transcription factors regulating Bdnf is the neuronal PAS domain protein 4 (Npas4), which regulates activity-dependent genes and neuroprotection, and has a critical role in the development of GABA synapses. On the basis of these premises, we investigated the expression of Npas4 and GABAergic markers in the hippocampus and prefrontal cortex of homozygous (SERT(-/-)) and heterozygous (SERT(+/-)) knockout rats, and analyzed the effect of long-term duloxetine treatment on the expression of these targets. We found that Npas4 expression was reduced in both the brain structures of adult SERT(+/-) and SERT(-/-) animals. This effect was already present in adolescent SERT(-/-), and could be mimicked by prenatal exposure to the antidepressant fluoxetine. Moreover, SERT(-/-) rats showed a strong impairment of the GABAergic system, as indicated by the reduction of several markers, including the vesicular transporter (Vgat), glutamic acid decarboxylase-67 (Gad67), the receptor subunit GABA A receptor, gamma 2 (GABA(A)-γ2), and calcium-binding proteins that label subgroups of the GABAergic neurons. Interestingly, chronic treatment with the antidepressant duloxetine was able to restore the physiological levels of Npas4 and GABAergic markers in SERT(-/-) rats, although some differences in the modulation of GABAergic genes exist between hippocampus and prefrontal cortex. Our results demonstrate that SERT knockout rats, an animal model of mood disorders, have reduced Npas4 expression that correlates with decreased expression of Bdnf exon I and IV. These changes lead to an impairment of the GABAergic system that may contribute to the anxious and depressive phenotype associated with inherited SERT downregulation.

Identification of genetic modifiers of behavioral phenotypes in serotonin transporter knockout rats

Background

Genetic variation in the regulatory region of the human serotonin transporter gene (SLC6A4) has been shown to affect brain functionality and personality. However, large heterogeneity in its biological effects is observed, which is at least partially due to genetic modifiers. To gain insight into serotonin transporter (SERT)-specific genetic modifiers, we studied an intercross between the Wistar SERT^-/- rat and the behaviorally and genetically divergent Brown Norway rat, and performed a QTL analysis.

Results

In a cohort of >150 intercross SERT^-/- and control (SERT^+/+) rats we characterized 12 traits that were previously associated with SERT deficiency, including activity, exploratory pattern, cocaine-induced locomotor activity, and abdominal and subcutaneous fat. Using 325 genetic markers, 10 SERT^-/--specific quantitative trait loci (QTLs) for parameters related to activity and exploratory pattern (Chr.1,9,11,14), and cocaine-induced anxiety and locomotor activity (Chr.5,8) were identified. No significant QTLs were found for fat parameters. Using in silico approaches we explored potential causal genes within modifier QTL regions and found interesting candidates, amongst others, the 5-HT1D receptor (Chr. 5), dopamine D2 receptor (Chr. 8), cannabinoid receptor 2 (Chr. 5), and genes involved in fetal development and plasticity (across chromosomes).

Conclusions

We anticipate that the SERT^-/--specific QTLs may lead to the identification of new modulators of serotonergic signaling, which may be targets for pharmacogenetic and therapeutic approaches.

Deletion of the serotonin transporter in rats disturbs serotonin homeostasis without impairing liver regeneration

The serotonin transporter is implicated in the uptake of the vasoconstrictor serotonin from the circulation into the platelets, where 95% of all blood serotonin is stored and released in response to vascular injury. In vivo studies indicated that platelet-derived serotonin mediates liver regeneration after partial hepatectomy. We have recently generated serotonin transporter knockout rats and demonstrated that their platelets were almost completely depleted of serotonin. Here we show that these rats exhibit impaired hemostasis and contain about 1-6% of wild-type serotonin levels in the blood. Despite the marked reduction of serotonin levels in blood and platelets, efficient liver regeneration and collagen-induced platelet aggregation occur in rats lacking the serotonin transporter. These results provide evidence that liver regeneration is not dependent on the release of serotonin from platelets. Our findings indicate that very low levels of serotonin in blood are sufficient for liver regeneration.

Generation of gene knockouts and mutant models in the laboratory rat by ENU-driven target-selected mutagenesis

OBJECTIVE

The rat is one of the most important model organisms for biomedical and pharmacological research. However, the generation of novel models for studying specific aspects of human diseases largely depends on selection for specific traits using existing rat strains, thereby solely depending on naturally occurring variation. This study aims to provide the tools to manipulate the rat genome in a more directed way.

METHODS

We developed robust, automated, and scaleable reverse genetic methodology based on ENU (N-ethyl-N-nitrosourea)-driven target-selected mutagenesis. Optimal mutagenesis conditions have been determined in three different rat strains and a universal, rapid, and cost-effective dideoxy resequencing-based screening setup was established for mutation discovery. The effectiveness of the approach is illustrated by the identification of 120 induced mutations in a set of genes of interest, including six that result in unique rat knockout models due to the introduction of premature stop codons. In addition, 56 mutations were found that change amino acids, including critical residues in transmembrane domains of receptors and channels.

CONCLUSIONS

The approach described here allows for the systematic generation of knockout and protein function altering alleles in the rat. The resulting induced rat models will be powerful tools for studying many aspects of a wide variety of human diseases.

Expression of sensitization to amphetamine and dynamics of dopamine neurotransmission in different laminae of the rat medial prefrontal cortex

The present study investigated the effect of acute and repeated administrations of amphetamine (AMPH) on dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) in the two main cytoarchitectonic subterritories of the medial prefrontal cortex (mPFC) (anterior cingulate and dorsocaudal prelimbic cortices vs ventral prelimbic and rostral infralimbic cortices). Both the acute locomotor effects of AMPH and the expression of behavioral sensitization following its repeated administration were also simultaneously assessed. The repeated, intermittent administration of AMPH over five consecutive days led to a significant sensitized locomotor response to a subsequent challenge that occurred following a 48-h withdrawal period. Basal dialysate DA levels were higher in the ventral mPFC compared with its dorsal counterpart in naive animals, that is prior to the acute administration of AMPH. However, the inverse relationship was observed in animals that had developed sensitization: basal dialysate DA levels were significantly lower in the ventral mPFC compared with the dorsal mPFC. In naïve animals, AMPH produced a significant decrease in DA levels in both the ventral and dorsal subregions of the mPFC. However, the inverse relationship was observed in animals that had developed sensitization: dialysate DA levels in response to AMPH remained significantly decreased in the dorsal mPFC, whereas DA went back to baseline levels in the ventral mPFC. Given that a critical concentration of DA is required for normal function of the mPFC, our results suggest that AMPH-induced changes in DA levels in different subregions of the mPFC are critical for both the acute effects of the drug and the expression of behavioral sensitization to its repeated administration by producing either less or more selectivity or sharpening of stimuli to cortico-cortical dendrites and subcortical synaptic afferents to the pyramidal cells located in the dorso-ventral axis of the mPFC.

Behavioral, neurochemical and endocrinological characterization of the early social isolation syndrome

Rearing rats in isolation has been shown to be a relevant paradigm for studying early life stress and understanding the genesis of depression and related affective disorders. Recent studies from our laboratory point to the relevance of studying the social isolation syndrome as a function of home caging conditions. Accordingly, the present series of experiments assessed the contribution of each condition to the expression of the prepulse inhibition of the acoustic startle, food hoarding and spontaneous locomotor activity. In addition, ex vivo neurochemical changes in the brains of isolated and grouped rats reared either in sawdust-lined or in grid-floor cages were determined by measuring dopamine and serotonin as well as their major metabolites in a "psychosis circuit" that includes mainly the hippocampus and selected hippocampal efferent pathways projecting towards the anterior cingulate and infralimbic cortices, nucleus accumbens, dorsolateral caudate nucleus, amygdala and entorhinal cortex. The results of the present study demonstrate that rearing rats in isolation (i) produces a syndrome of generalized locomotor hyperactivity; (ii) increases the startle response; (iii) impairs prepulse inhibition; (iv) tends to increase food hoarding behavior; (v) increases basal dopamine turnover in the amygdaloid complex; (vi) decreases basal dopamine turnover in the infralimbic part of the medial prefrontal cortex; and (vii) decreases basal turnover of serotonin in the nucleus accumbens. In the entorhinal cortex, dopamine neurotransmission seemed to be more sensitive to the caging conditions since a decreased basal turnover of dopamine was observed in grid-reared animals. Plasma corticosterone levels were also increased in grid-reared animals compared with rats reared in sawdust cages. Finally, isolates reared on grids showed a significant positive correlation between plasma corticosterone levels and dopamine in the left nucleus accumbens.Altogether, these results support the contention that there is a link between social isolation, attention deficit, spontaneous locomotor hyperactivity and reduced dopamine turnover in the medial prefrontal cortex. Furthermore, our data demonstrate that rearing rats in grid-floor cages represents a form of chronic mild stress associated with increased corticosterone levels, decreased basal turnover of entorhinal dopamine and increased dopamine activity in the left nucleus accumbens. Finally, a significant and selective decrease in the basal turnover of serotonin in the nucleus accumbens of isolated rats may be linked to the isolation-induced locomotor hyperactivity.

Effect of amphetamine on extracellular acetylcholine and monoamine levels in subterritories of the rat medial prefrontal cortex

The present study sought to investigate the contributions of the dorsal prelimbic/anterior cingulate and ventral prelimbic/infralimbic cortices to the reverse microdialysis of amphetamine (1, 10, 100, 500, and 1000 microM) on dialysate acetylcholine, choline, norepinephrine, and serotonin levels. The results demonstrate that basal levels of acetylcholine, choline, and serotonin were homogeneous within subregions of the medial prefrontal cortex. In contrast, dialysate norepinephrine levels were significantly higher in the anterior cingulate cortex compared with the infralimbic cortex. Reverse microdialysis of amphetamine in both subareas of the medial prefrontal cortex produced a dose-dependent increase in norepinephrine and serotonin levels; the magnitude of this effect was similar in both subterritories of the medial prefrontal cortex. Microinfusion of amphetamine increased dialysate acetylcholine levels in a dose-dependent manner only in the infralimbic cortex. Finally, amphetamine decreased choline levels in both subregions of the medial prefrontal cortex. The magnitude of this effect was larger in the anterior cingulate cortex compared with its infralimbic counterpart. Since depletions of frontal cortical acetylcholine result in severe cognitive deficits, the present data raise the possibility that the type of neural integrative processes that acetylcholine mediates depends, at least in part, on the subterritories that characterize the medial prefrontal cortex.