Moderately aged OFA rats as a novel model for mild age-related alterations in learning and memory.

Aged rodents have been used as preclinical models of age-associated cognitive decline. Most of those models displayed substantial impairments in learning and memory. The initial, more subtle changes that precede more severe losses in cognitive abilities have not been well characterized. Here, we established a model detecting initial subtle cognitive changes by comparing the performance of moderately aged Oncins France Strain A Sprague Dawley rats with young rats in the Morris water maze (MWM) and the Open Field (OF) test. Both age groups improved their performance during the training period at a similar rate; however, the older rats performed worse in several parameters measured in the MWM. Our results suggest that already at the age of 18-20 months rats show changes in their approach to solve the spatial memory task while their ability to learn is not yet diminished. The disparate spatial information processing of the moderately aged rats provides a novel animal model for early age-related cognitive alterations that could be useful to test the effect of early intervention strategies. Moreover, our results suggest that the sensitivity of cognitive tests in the elderly could be substantially enhanced if they assess both the improvement after several trials, and the strategy used to solve a certain task.

Treatment with the calcineurin inhibitor and immunosuppressant cyclosporine A impairs sensorimotor gating in Dark Agouti rats.

RATIONALE: Calcineurin is a protein regulating cytokine expression in T lymphocytes and calcineurin inhibitors such as cyclosporine A (CsA) are widely used for immunosuppressive therapy. It also plays a functional role in distinct neuronal processes in the central nervous system. Disturbed information processing as seen in neuropsychiatric disorders is reflected by deficient sensorimotor gating, assessed as prepulse inhibition (PPI) of the acoustic startle response (ASR). OBJECTIVE: Patients who require treatment with immunosuppressive drugs frequently display neuropsychiatric alterations during treatment with calcineurin inhibitors. Importantly, knockout of calcineurin in the forebrain of mice is associated with cognitive impairments and symptoms of schizophrenia-like psychosis as seen after treatment with stimulants. METHODS: The present study investigated in rats effects of systemic acute and subchronic administration of CsA on sensorimotor gating. Following a single injection with effective doses of CsA, adult healthy male Dark Agouti rats were tested for PPI. For subchronic treatment, rats were injected daily with the same doses of CsA for 1 week before PPI was assessed. Since calcineurin works as a modulator of the dopamine pathway, activity of the enzyme tyrosine hydroxylase was measured in the prefrontal cortex and striatum after accomplishment of the study. RESULTS: Acute and subchronic treatment with the calcineurin inhibitor CsA disrupted PPI at a dose of 20 mg/kg. Concomitantly, following acute CsA treatment, tyrosine hydroxylase activity was reduced in the prefrontal cortex, which suggests that dopamine synthesis was downregulated, potentially reflecting a stimulatory impact of CsA on this neurotransmitter system. CONCLUSIONS: The results support experimental and clinical evidence linking impaired calcineurin signaling in the central nervous system to the pathophysiology of neuropsychiatric symptoms. Moreover, these findings suggest that therapy with calcineurin inhibitors may be a risk factor for developing neurobehavioral alterations as observed after the abuse of psychomotor stimulant drugs.

Male rats treated with subchronic PCP show intact olfaction and enhanced interest for a social odour in the olfactory habituation/dishabituation test.

The olfactory system participates in many sensory processes, and olfactory endophenotypes appear in a variety of neurological disorders such as Alzheimer's and Parkinson's disease, depression and schizophrenia. Social withdrawal is a core negative symptom of schizophrenia and animal models have proven to be invaluable for studying the neurobiological mechanisms and cognitive processes behind the formation of social relationships. The subchronic phencyclidine (PCP) rat model is a validated model for negative symptoms of schizophrenia, such as impaired sociability. However, the complete range of social behaviour and deficits in the model are still not fully understood. Intact rodent olfaction is essential for a wide range of social behaviour and disrupted olfactory function could have severe effects on social communication and recognition. In order to examine the olfactory ability of male rats treated with subchronic PCP, we conducted an olfactory habituation/dishabituation test including both non-social and social odours. The subchronic PCP-treated rats successfully recognized and discriminated among the odours, indicative of intact olfaction. Interestingly, the subchronic PCP-treated rats showed greater interest for a novel social odour compared to the saline-treated rats and the rationale remains to be elucidated. Our data indicate that subchronic PCP treatment does not disrupt olfactory function in male rats. By ruling out impaired olfaction as cause for the poor social interaction performance in subchronic PCP-treated rats, our data supports the use of NMDA receptor antagonists to model the negative symptoms of schizophrenia.

Effects of 2-bromoterguride, a dopamine D2 receptor partial agonist, on cognitive dysfunction and social aversion in rats.

RATIONALE: 2-Bromoterguride, a dopamine D2 receptor partial agonist with antagonist properties at serotonin 5-HT2A receptors and α2C-adrenoceptors, meets the prerequisites of a putative atypical antipsychotic drug (APD). We recently showed that 2-bromoterguride is effective in tests of positive symptoms of schizophrenia in rats without inducing extrapyramidal side effects or metabolic changes. OBJECTIVE: In continuation of our recent work, we now investigated the effect of 2-bromoterguride on apomorphine and phencyclidine (PCP)-induced disruptions of prepulse inhibition (PPI) of the acoustic startle response, a measure of sensory gating. In addition, we used subchronic PCP treatment to produce cognitive deficits and social aversion, and assessed the effect of 2-bromoterguride on the performance in the novel object recognition (NOR) task (model for studying cognitive deficit symptoms of schizophrenia) and the social interaction test (model for studying negative symptoms of schizophrenia). Finally, we extended the side effect profile of 2-bromoterguride by measuring the prolactin response to systemic administration of the drug in rats. RESULTS: Treatment with 2-bromoterguride (0.1 and 0.3 mg/kg) reversed PPI deficits induced by apomorphine and PCP, respectively. Subchronic PCP induced impairments in object memory and social interaction behavior which were ameliorated by 2-bromoterguride but not by clozapine and aripiprazole, respectively. Prolactin concentration in blood serum was not elevated at 1, 2, or 4 h post-2-bromoterguride treatment, which further supports the safe and effective use of this drug. CONCLUSIONS: Our data support 2-bromoterguride as a promising APD candidate due to its beneficial effect on cognitive impairments and negative symptoms of schizophrenia.

Enhanced Stress Response in 5-HT1AR Overexpressing Mice: Altered HPA Function and Hippocampal Long-Term Potentiation.

Postsynaptic 5-HT1A receptors (5-HT1AR) play an important role in anxiety and stress, although their contribution is still controversial. Previous studies report that mice overexpressing postsynaptic 5-HT1ARs show no changes in basal anxiety, though the influence of stress conditions has not been addressed yet. In this study, we used this animal model to evaluate the role of 5-HT1ARs in anxiety response after pre-exposure to an acute stressor. Under basal conditions, 5-HT1AR overexpressing animals presented high corticosterone levels and a lower mineralocorticoid/glucocorticoid receptor ratio. After pre-exposure to a single stressor, they showed a high anxiety-like response, associated with a blunted increase in corticosterone levels and higher c-Fos activation in the prefrontal cortex. Moreover, these mice also presented a lack of downregulation of hippocampal long-term potentiation after stress exposure. Therefore, higher postsynaptic 5-HT1AR activation might predispose to a high anxious phenotype and an impaired stress coping behavior.

2-Bromoterguride-a potential atypical antipsychotic drug without metabolic effects in rats.

RATIONALE: Recently, we showed that 2-bromoterguride acted as a dopamine D2 receptor partial agonist, a serotonin 5-HT2A and α2C-adrenergic receptor antagonist, and exhibited antidopaminergic efficacy in amphetamine-induced locomotion (AIL) in rats without inducing catalepsy. OBJECTIVE: To extend our knowledge on the antipsychotic effects of 2-bromoterguride, we used convergent preclinical animal models and tests; i.e., conditioned avoidance response (CAR), predictive of antipsychotic-like effects; Fos protein expression, a molecular marker for (atypical) antipsychotic activity; wet dog shake behavior, a test for the in vivo effects of drugs acting on central 5-HT2A receptors; and investigated metabolic changes as a common side effect of atypical antipsychotic drugs (APDs). RESULTS: Acute treatment with 2-bromoterguride (0.1 and 0.3 mg/kg) decreased the CAR at 30, 90, and 270 min post-injection in rats without inducing escape failures at any time. Fos protein expression, as shown by Western blotting, was enhanced by 2-bromoterguride in the nucleus accumbens (NAc), the dorsolateral striatum (dStr), and the medial prefrontal cortex (mPFC). (±)-2,5-Dimethoxy-4-iodoamphetamine (DOI)-induced wet dog shakes in rats were reduced by 2-bromoterguride. Chronic treatment with 2-bromoterguride did not affect metabolic parameters such as body weight development and body fat composition as well as behavioral parameters such as food intake and locomotor activity. CONCLUSIONS: Our data suggest that 2-bromoterguride is a promising candidate in the treatment of schizophrenia due to its atypical antipsychotic-like activity and its inability to induce weight gain.

Food Deprivation, Body Weight Loss and Anxiety-Related Behavior in Rats.

In behavioral studies, food deprivation protocols are routinely used to initiate or maintain motivational states that are required in a particular test situation. However, there is limited evidence as to when food deprivation compromises animal welfare. This study investigated the effects of different lengths of food deprivation periods and restricted (fixed-time) feeding on body weight loss as well as anxiety-related and motivated behavior in 5-6 month old male and female Wistar rats. The observed body weight loss was not influenced by sex and ranged between 4% (16 h deprivation) to approximately 9% (fixed-time feeding). Despite significant body weight loss in all groups, the motivation to eat under the aversive test conditions of the modified open field test increased only after 48 h of food deprivation. Long-lasting effects on anxiety as measured in the elevated plus maze test 24 h after refeeding have not been observed, although fixed-time feeding could possibly lead to a lasting anxiogenic effect in female rats. Overall, female rats showed a more anxiolytic profile in both tests when compared to male rats. Despite these sex differences, results suggest that food deprivation is not always paralleled by an increased motivation to feed in a conflict situation. This is an important finding as it highlights the need for tailored pilot experiments to evaluate the impact of food deprivation protocols on animals in regard to the principles of the 3Rs introduced by Russell and Burch.

Modulatory Role of Postsynaptic 5-Hydroxytryptamine Type 1A Receptors in (±)-8-Hydroxy-N,N-dipropyl-2-aminotetralin-Induced Hyperphagia in Mice.

Brain serotonin (5-HT) is involved in the control of food intake. The ingestive effects of 5-HT are mediated by various receptor subtypes, among others the 5-HT1A receptor. While the involvement of presynaptic 5-HT1A receptors is regarded as certain, the role of postsynaptic 5-HT1A receptors is rather vague. Here, we studied the role of the 5-HT1A receptor on feeding in non-food-deprived and food-deprived (young adult and adult, both sexes) wild-type NMRI mice as well as transgenic NMRI mice, which are characterized by a distinct overexpression of postsynaptic 5-HT1A receptors. The known hyperphagic effect of the 5-HT1A receptor full agonist 8-OH-DPAT ((±)-8-hydroxy-N,N-dipropyl-2-aminotetralin) in non-food-deprived animals was demonstrated in male NMRI wild-type mice and could be antagonized by the selective 5-HT1A receptor antagonist WAY100635. In transgenic mice, this hyperphagic response was induced at lower doses, with an earlier onset and even in females. However, in adult male transgenic mice, the hyperphagic effect did not occur. In food-deprived NMRI wild-type as well as transgenic mice, 8-OH-DPAT first induced a hypophagic and subsequently a hyperphagic effect. Again, in transgenic animals most responses occurred at lower doses and with an earlier onset. The results indicate that postsynaptic 5-HT1A receptors exert a modulatory function in food intake in free-feeding and fasted mice, which for the first time shows an involvement of postsynaptic 5-HT1A receptors in feeding behavior. Understanding the function of pre- and postsynaptic 5-HT1A receptors may help to achieve new insights into the regulation of food intake and foster prospective treatment strategies for eating disorders.

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-HT1A, 5-HT2A, 5-HT2C, 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, α2A, α2B and α2C. Whereas the α2B-adrenoceptor seems to play only a minor role in the brain, activation of postsynaptic α2A-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 α2C-adrenoceptor blockade or the combination of D2 receptor antagonists with idazoxan (α2A/2C-adrenoceptor antagonist) could be useful in schizophrenia. A potential benefit of a treatment combination of first-generation APDs with the α2A/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.

Impairment of cognitive performance after reelin knockdown in the medial prefrontal cortex of pubertal or adult rats.

The glycoprotein reelin is important for embryonic neuronal migration. During adulthood reelin possibly acts as a modulator of synaptic plasticity. Several studies link reduced levels of reelin messenger RNA and protein to the pathophysiology of certain neuropsychiatric disorders. However, little is known about reelin's role for behavioral and cognitive functions in vivo. Therefore, the effect of a reelin knockdown in the medial prefrontal cortex (mPFC) of Wistar rats was examined in behavioral tasks related to neuropsychiatric disorders, such as schizophrenia. Rats treated with reelin antisense phosphothioate oligonucleotides in the mPFC during puberty or adulthood were tested for prepulse inhibition (PPI) of the acoustic startle reflex, spatial working memory, object recognition, and locomotor activity. Reelin quantification in the mPFC was assessed by Western blotting. Local reelin knockdown during puberty or adulthood induced (1) a PPI deficit as well as (2) an impairment of spatial working memory and object recognition following pubertal injections. Western blot analyses showed a distinct and highly selective reelin knockdown in the rats' mPFC. These results indicate that mPFC reelin signaling plays an important role in behavioral tasks with relevance to e.g. schizophrenia. Understanding reelin's function as a neurotrophic modulator of the extracellular matrix may help to achieve new insights into the etiology of certain neuropsychiatric diseases and foster prospective treatment strategies.

Pharmacological and parametrical investigation of prepulse inhibition of startle and prepulse elicited reactions in Wistar rats.

Prepulse inhibition (PPI) is the inhibition of an acoustic startle response (ASR) that is observed when a weak prepulse is presented shortly before a startling stimulus. Here we studied in Wistar rats the dependence of PPI on variations of the interstimulus interval (ISI; from 25-1020ms) after treatment with various drugs that are known to disrupt PPI. The motor response to the prepulse itself (prepulse elicited reaction, PER) was also studied. The direct dopamine receptor agonist apomorphine, the non-competitive NMDA glutamate receptor antagonist MK-801, and the cannabinoid CB1 receptor agonist WIN 55,212-2 all reduced PPI, depending on the ISI, with different effects on the PER and/or pulse alone. The serotonin 2A receptor agonist DOI tended to reduce PPI. The cannabinoid CB1 receptor antagonist AM 251 did neither affect PPI nor the responses to prepulses or startling noise pulses. Taken together this study supports the current notion of a pharmacologically complex pattern of regulation of PPI at different ISIs and suggests that the PER is a miniature ASR that does, however, not predict the level of PPI.

Effects of haloperidol and clozapine on sensorimotor gating deficits induced by 5-hydroxytryptamine depletion in the brain.

Evidence is increasing for a role of brain 5-hydroxytryptamine (5-HT) systems in schizophrenia. We previously showed that brain 5-HT depletion causes disruption of prepulse inhibition, a measure of sensorimotor gating that is deficient in schizophrenia. Antipsychotic treatment has been reported to reverse these deficits in patients with schizophrenia. The present study was designed to investigate the ability of antipsychotic drugs to reverse prepulse inhibition deficits caused by lesions of the brain 5-HT system in rats. In male Sprague-Dawley rats, selected parts of the brain 5-HT systems were lesioned by micro-injection of the 5-HT neurotoxin 5,7-dihydroxytryptamine into the dorsal raphe nucleus (DRN) or median raphe nucleus (MRN). The effects of antipsychotic drugs on lesion-induced changes in prepulse inhibition were examined 2 weeks after the surgery. There was significant disruption of prepulse inhibition in the MRN-lesioned group compared to sham-operated controls. This deficiency in prepulse inhibition was restored by clozapine (1 and 5 mg kg(-1)) treatment, and by treatment with a relatively high dose of haloperidol (0.25 mg kg(-1)). There was no significant effect of the DRN lesions on prepulse inhibition compared with sham-operated controls. These results indicate that 5-HT depletion in MRN-innervated brain structures leads to disruption of prepulse inhibition. Treatment with both antipsychotic drugs, haloperidol and clozapine, significantly increased prepulse inhibition in these animals back to the level seen in sham-operated controls. The present findings highlight the importance of the 5-HT systems in cognitive models of schizophrenia.

Brain serotonin depletion by lesions of the median raphe nucleus enhances the psychotomimetic action of phencyclidine, but not dizocilpine (MK-801), in rats.

We have previously shown that brain serotonin depletion by lesions of the median raphe nucleus (MRN) causes enhancement of phencyclidine-induced locomotor hyperactivity [S. Kusljic, D.L. Copolov, M. van den Buuse, Differential role of serotonergic projections arising from the dorsal and median raphe nuclei in locomotor hyperactivity and prepulse inhibition, Neuropsychopharmacology 28 (2003) 2138-2147]. In this study, we extend our previous work by (1) comparing the effect of phencyclidine with that of another NMDA receptor antagonist, dizocilpine (MK-801); (2) investigate behavioral changes in more detail; (3) assess in detail the effect of raphe lesions on regional serotonin levels in the brain. Male Sprague-Dawley rats received microinjection of the serotonergic neurotoxin 5,7-dihydroxytryptamine into the MRN or dorsal raphe nucleus (DRN). The effects of treatment with saline, phencyclidine and MK-801 on locomotor activity were determined 2 weeks after the surgery. MRN lesions caused serotonin depletion in the dorsal hippocampus, whereas DRN lesions caused serotonin depletion in the frontal cortex, striatum and ventral hippocampus. There was a significant increase in phencyclidine-induced locomotor hyperactivity in the MRN-lesioned group compared to sham-operated controls. Further analysis of behavior showed that phencyclidine-induced hyperambulation, but not stereotypy or rearing, was significantly higher in MRN-lesioned rats compared to controls. In contrast, there was no significant effect of the lesions on the psychotomimetic effect of MK-801. These results indicate that a hyposerotonergic state induced by destruction of projections from the MRN leads to altered brain circuitry that is responsible for the regulation of phencyclidine-but not MK-801-induced locomotor hyperactivity. Thus, MRN projections may play an inhibitory role in mechanisms involved in symptoms of schizophrenia.

Clozapine enhances disruption of prepulse inhibition after sub-chronic dizocilpine- or phencyclidine-treatment in Wistar rats.

Sensitisation (i.e. progressive enhancement) of behavioural abnormalities induced by repeated treatment with non-competitive NMDA receptor antagonists in animals is considered an animal model for schizophrenia. Here, male Wistar rats were treated for 11 days with either dizocilpine (0.1 mg/kg), phencyclidine (PCP, 2 mg/kg), or saline and tested for prepulse inhibition (PPI) of the acoustic startle response (ASR). The aims of this study were twofold: First, we tested whether sensitisation of PPI deficits previously found in Sprague-Dawley rats were also found in Wistar rats, and, second, whether these effects can be ameliorated by the atypical antipsychotic clozapine. PPI is a paradigm for the assessment of sensorimotor gating (and its deficits) and is impaired in schizophrenic patients. After the sub-chronic treatment the rats were tested drug-free (day 12), and on the following days after drug challenge by PCP (2 mg/kg), combinations of PCP (2 mg/kg) and clozapine (5 and 10 mg/kg), or clozapine (5 mg/kg) alone. PPI was significantly reduced by both NMDA receptor antagonists. This effect was not further enhanced by the daily treatment. Startle magnitude was increased after eight days of dizocilpine-treatment only, indicating sensitisation of startle-potentiation by this drug. Testing the rats drug-free on day 12 revealed enhanced PPI and reduced startle (compared to the matching test on day 0) irrespective of previous treatment. Drug challenge with PCP (2 mg/kg) again reduced PPI in all groups. Clozapine (5 and 10 mg/kg) failed to antagonise the PPI-disruptive effects of PCP and even enhanced the PCP-induced PPI-deficits in rats pretreated with PCP or dizocilpine. These findings suggest: (1) that PPI and startle are influenced differently by non-competitive NMDA receptor antagonists, (2) that PCP and dizocilpine reduce PPI in Wistar rats, but do not lead to a sensitisation of this effect; and (3) that under the present schedule of treatments, the antipsychotic compound clozapine does not antagonise but rather enhances PPI-disruptive effects of non-competitive NMDA receptor antagonists, pointing towards a complex interaction of the brain processes underlying the action of psychotomimetic and atypical antipsychotic drugs.