Bart A. Ellenbroek

Early maternal deprivation reduces the expression of BDNF and NMDA receptor subunits in rat hippocampus

It is well accepted that events that interfere with the normal program of neuronal differentiation and brain maturation may be relevant for the etiology of psychiatric disorders, setting the stage for synaptic disorganization that becomes functional later in life. In order to investigate molecular determinants for these events, we examined the modulation of the neurotrophin brain-derived neurotrophic factor (BDNF) and the glutamate NMDA receptor following 24 h maternal separation (MD) on postnatal day 9. We found that in adulthood the expression of BDNF as well as of NR-2A and NR-2B, two NMDA receptor forming subunits, were significantly reduced in the hippocampus of MD rats whereas, among other structures, a slight reduction of NR-2A and 2B was detected only in prefrontal cortex. These changes were not observed acutely, nor in pre-weaning animals. Furthermore we found that in MD rats the modulation of hippocampal BDNF in response to an acute stress was altered, indicating a persistent functional impairment in its regulation, which may subserve a specific role for coping with challenging situations. We propose that adverse events taking place during brain maturation can modulate the expression of molecular players of cellular plasticity within selected brain regions, thus contributing to permanent alterations in brain function, which might ultimately lead to an increased vulnerability for psychiatric diseases.

The effects of an early stressful life event on sensorimotor gating in adult rats

There is increasing evidence that patients suffering from schizophrenia have disturbances in the brain and other parts of the body indicative of a disturbed development. These findings have led to the so-called neurodevelopmental hypotheses of schizophrenia, which state that schizophrenia (or a predisposition for this disease) results from perinatal disturbances which affect the normal development of the central nervous system. In order to study such a possible relationship we have used early short-lasting (24 h) maternal deprivation, and studied the influence of this life event on prepulse inhibition of the acoustic startle at adult age in rats, since it has been shown that schizophrenic patients show a disruption of prepulse inhibition. The results show that early maternal deprivation significantly reduced prepulse inhibition when the animals were tested at postnatal day (pnd) 69 (birth being pnd 0). The effects were qualitatively similar when deprivation took place on pnd 3, 6 or 9, although at the later days the effects were stronger. There was little influence on baseline startle response (except for a small reduction seen after deprivation on pnd 6). In separate experiments it was shown that the effect of maternal deprivation on prepulse inhibition was not seen before puberty and was similar for male and female offspring. Moreover, the effects could be reversed by treatment with the classical antipsychotic, haloperidol, or the putative atypical antipsychotic, quetiapine (both given 15 min before the prepulse inhibition experiment). In summary, the results show that an early stressful life event can have a delayed influence on prepulse inhibition in rats, qualitatively similar to the disturbances seen in schizophrenic patients. These data suggest that maternal deprivation (i.e., a 24 h separation of rat pups from their mother early in life) may represent an interesting animal model for investigating the influence of early life events on the information processing and general functioning of an individual at adult age.

Animal behavior models of the mechanisms underlying antipsychotic atypicality

This review describes the animal behavior models that provide insight into the mechanisms underlying the critical differences between the actions of typical vs. atypical antipsychotic drugs. Although many of these models are capable of differentiating between antipsychotic and other psychotropic drugs, only a few seem to be able to differentiate between typical and atypical antipsychotics, such as the paw test and the phencyclidine (PCP)-induced disruption of prepulse inhibition (PPI) of startle in rats. Moreover, there is an urgent need for animal models focusing more on the negative and the cognitive symptoms. Hence, improved animal models are crucial for developing better treatments for schizophrenia.

Characterization of the serotonin transporter knockout rat: a selective change in the functioning of the serotonergic system.

Serotonergic signaling is involved in many neurobiological processes and disturbed 5-HT homeostasis is implicated in a variety of psychiatric and addictive disorders. Here, we describe the functional characterization of the serotonin transporter (SERT) knockout rat model, that is generated by N-ethyl-N-nitrosurea (ENU)-driven target-selected mutagenesis. Biochemical characterization revealed that SERT mRNA and functional protein are completely absent in homozygous knockout (SERT-/-) rats, and that there is a gene dose-dependent reduction in the expression and function of the SERT in heterozygous knockout rats. As a result, 5-HT homeostasis was found to be severely affected in SERT-/- rats: 5-HT tissue levels and depolarization-induced 5-HT release were significantly reduced, and basal extracellular 5-HT levels in the hippocampus were ninefold increased. Interestingly, we found no compensatory changes in in vitro activity of tryptophan hydroxylase and monoamine oxidase, the primary enzymes involved in 5-HT synthesis and degradation, respectively. Similarly, no major adaptations in non-serotonergic systems were found, as determined by dopamine and noradrenaline transporter binding, monoamine tissue levels, and depolarization-induced release of dopamine, noradrenaline, glutamate and GABA. In conclusion, neurochemical changes in the SERT knockout rat are primarily limited to the serotonergic system, making this novel rat model potentially very useful for studying the behavioral and neurobiological consequences of disturbed 5-HT homeostasis.

A study in male and female 5-HT transporter knockout rats: an animal model for anxiety and depression disorders.

Human studies have shown that a reduction of 5-HT transporter (SERT) increases the vulnerability for anxiety and depression. Moreover, women are more vulnerable to develop depression and anxiety disorders than men. For that reason we hypothesized that homozygous 5-HT transporter knockout rat (SERT(-/-)) models, especially female, are valuable and reliable animal models for humans with an increased vulnerability for anxiety- and depression-related disorders. As rats are extensively used in neuroscience research, we used the unique 5-HT transporter knockout rat, that was recently generated using N-ethyl-N-nitrosurea (ENU) -driven mutagenesis, to test this hypothesis. Behavioral testing revealed that male and female SERT(-/-) rats spent less time in the center of the open field and spent less time on the open arm of the elevated plus maze compared with wild-type 5-HT transporter knockout rats (SERT(+/+)). In the novelty suppressed feeding test, only male SERT(-/-) rats showed a higher latency before starting to eat in a bright novel arena compared with SERT(+/+) controls. Both male and female SERT(-/-) rats showed a higher escape latency from their home cage than SERT(+/+) littermates. Moreover, SERT(-/-) rats were less mobile in the forced swim test, and sucrose consumption was reduced in SERT(-/-) rats relative to SERT(+/+) rats. Both effects were sex-independent. Neurochemically, basal extracellular 5-HT levels were elevated to a similar extent in male and female SERT(-/-) rats, which was not influenced by the selective 5-HT reuptake inhibitor citalopram. 5-HT immunostaining revealed no difference between SERT(+/+) and SERT(-/-) rats in the dorsal raphe nuclei, in both males and females. These findings demonstrate that SERT(-/-) rats show anxiety and depression-related behavior, independent of sex. Genetic inactivation of the SERT has apparently such a great impact on behavior, that hardly any differences are found between male and female rats. This knockout rat model may provide a valuable model to study anxiety- and depression-related disorders in male and female rats.

Role of cannabis and endocannabinoids in the genesis of schizophrenia

RationaleCannabis abuse and endocannabinoids are associated to schizophrenia.ObjectivesIt is important to discern the association between schizophrenia and exogenous Cannabis sativa, on one hand, and the endogenous cannabinoid system, on the other hand.ResultsOn one hand, there is substantial evidence that cannabis abuse is a risk factor for psychosis in genetically predisposed people, may lead to a worse outcome of the disease, or it can affect normal brain development during adolescence, increasing the risk for schizophrenia in adulthood. Regarding genetic predisposition, alterations affecting the cannabinoid CNR1 gene could be related to schizophrenia. On the other hand, the endogenous cannabinoid system is altered in schizophrenia (i.e., increased density of cannabinoid CB1 receptor binding in corticolimbic regions, enhanced cerebrospinal fluid anandamide levels), and dysregulation of this system can interact with neurotransmitter systems in such a way that a “cannabinoid hypothesis” can be integrated in the neurobiological hypotheses of schizophrenia. Finally, there is also evidence that some genetic alterations of the CNR1 gene can act as a protectant factor against schizophrenia or can induce a better pharmacological response to atypical antipsychotics.ConclusionsCannabis abuse is a risk factor for psychosis in predisposed people, it can affect neurodevelopment during adolescence leading to schizophrenia, and a dysregulation of the endocannabinoid system can participate in schizophrenia. It is also worth noting that some specific cannabinoid alterations can act as neuroprotectant for schizophrenia or can be a psychopharmacogenetic rather than a vulnerability factor.

Early maternal deprivation and prepulse inhibition: The role of the postdeprivation environment

Early postnatal maternal deprivation leads to a variety of biochemical and behavioural alterations in the offspring, some of which do not develop until adulthood, like deficits in prepulse inhibition. Since a number of these deficits are similar to abnormalities observed in schizophrenic patients, maternal deprivation has been proposed as an interesting model for schizophrenia. However, little is still known about the processes that determine these long-term consequences. Previous experiments showed that the strain of rats and the deprivation procedure are important factors. In the present set of experiments, we focussed on the postdeprivation period. We showed that rearing normal Wistar rats in social isolation from weaning disrupts prepulse inhibition. However, if maternally deprived Wistar rats were reared in social isolation, the prepulse inhibition was normal. We further showed that if only half of the litters were maternally deprived at postnatal day 9, the animals had only a small disruption in prepulse inhibition compared to animals that came from litters where all the animals were deprived. In a final experiment, we crossfostered maternally deprived mothers to nondeprived pups and vice versa. This experiment showed that both the nondeprived pups raised by a deprived mother and the deprived pups raised by a nondeprived mother had small deficits in prepulse inhibition. Taken together, these data clearly show that the postdeprivation period is of crucial importance for the development of prepulse inhibition deficits in maternally deprived rats. We present a working model in order to explain the long-term behavioural consequences of maternal deprivation.

Apomorphine Susceptibility and Animal Models for Psychopathology: Genes and Environment

Many years ago we found a bimodal distribution of a number of different behaviors in our regular outbred Wistar stock. This was observed in the response to novelty, the response in a resident-intruder test as well as in the stereotypy response to the dopamine agonist apomorphine. On the basis of that, we decided to selectively breed these animals, which resulted in the the APO-SUS and APO-UNSUS lines. The APO-SUS rats show a strong, stereotyped gnawing response, whereas APO-UNSUS show only a weak gnawing response. Follow-up studies have shown that the phenotypical expression of these rats depend on genetic and early and late environmental factors. Because these rats were not selected on the basis of a specific behavioral trait, but rather on the basis of a difference in susceptibility for a specific neurotransmitter, it is not surprising that these animals show major differences in the neurochemical state of the central nervous system. In fact, in many respects they represent mirror images of each other. Moreover, these animals show clear differences in their endocrine and immunological systems. APO-SUS rats can be characterized as having a hyper-reactive hypothalamus-pituitary-adrenal axis, and a dominance of the TH2 system. Apart from discussing the main differences between APO-SUS and APO-UNSUS rats, the review specifically focuses on the former as a potential model for schizophrenia. We have been able to show that APO-SUS rats indeed share a large number of behavioral, neurochemical, endocrinological, and immunological similarities with patients suffering from schizophrenia. Because schizophrenia is also likely to result from an interaction between genetic and early stressful life events, the APO-SUS rat might represent a promising animal model for studying this severe mental disorder.

Differential effects of ketamine on gating of auditory evoked potentials and prepulse inhibition in rats

Abstract Schizophrenic patients suffer from deficits in information processing. Patients show both a decrease in P50 gating [assessed in the conditioning-testing (C-T) paradigm] and prepulse inhibition (PPI), two paradigms that assess gating. These two paradigms might have a related underlying neural substrate. Gating, as measured in both the C-T paradigm (the gating of a component of the auditory evoked potential (AEP)], and PPI can easily be measured in animals as well as in humans. This offers the opportunity to model these information processing paradigms in animals in order to investigate the effects of neurotransmitter manipulations in the brain. In order to validate the animal model for disturbances in AEP gating, d-amphetamine (0.5 and 1 mg/kg, IP) was administered. Gating of an AEP component was changed due to injection of d-amphetamine (1 mg/kg) in the same way as seen in schizophrenic patients: both the amplitude to the conditioning click and the gating were significantly reduced. Next, the effect of the N-methyl-D-aspartate (NMDA) antagonist ketamine (2.5 and 10 mg/kg, IP) was investigated to assess its effects in the two gating paradigms. It was found that ketamine (10 mg/kg) did not affect gating as measured with components of the AEP. However, ketamine (10 mg/kg) disrupted PPI of the startle response to the extent that prepulse facilitation occurred. Firstly, it is concluded that AEP gating was disrupted by d-amphetamine and not by ketamine. Secondly, PPI and the C-T paradigm reflect distinct inhibitory sensory processes, since both paradigms are differentially influenced by ketamine.

Early maternal deprivation alters hippocampal levels of neuropeptide Y and calcitonin-gene related peptide in adult rats

Stressful events early in life are reported to be more prevalent among patients with an adult life psychiatric disorder. Early maternal deprivation is considered an animal model of early life stress. Maternally deprived adult rats display long-term alterations in the neuroendocrine system, brain and behavior that are in many ways analogous to depressive and schizophrenic symptomatology. Neuropeptide Y (NPY) and calcitonin-gene related peptide (CGRP) have been implicated in both disorders and also been suggested to play a role in the neuroadaptational response to stress. Consequently, male Wistar rat-pups were subjected to early maternal deprivation or control handling, on postnatal day (pnd) 9. On pnd 21, pups were weaned and split into two groups that were reared either on a saw-dust floor or on a grid-floor, considered to be a mild stressor. On pnd 67, all animals were subjected to the prepulse inhibition test. One week later, the animals were sacrificed, the brains removed and dissected on ice. Levels of NPY-like immunoreactivity (LI) and CGRP-LI were quantified by radioimmunoassay in brain regional extracts. Maternal deprivation led to a significant reduction in basal startle amplitude and disruption of prepulse inhibition. These findings were paralleled by significantly reduced levels of NPY and CGRP in the hippocampus and occipital cortex. It is hypothesised that these changes may be of relevance to aspects of schizophrenic and affective symptomatology. The present study further shows that brain NPY and, in particular, CGRP are sensitive to long-term mild stress and further implicate the involvement of these peptides in the neuroendocrine stress response.