Kerstin Schwabe

Effects of neonatal lesions of the medial prefrontal cortex on adult rat behaviour.

While prefrontal lesions in rodents serve as models for frontal lobe syndromes, neonatal lesions are considered as models for disconnection syndromes, such as schizophrenia. We investigated the effect of neonatal lesions of the rat medial prefrontal cortex (mPFC) together with pubertal dexamethasone-challenge on adult rat behaviour and on apomorphine-induced behavioural changes. Adult lesions were used as controls. Rats with neonatal (postnatal day 7) or adult excitotoxic lesions or sham-lesions of the mPFC were tested 9 weeks after surgery. At postnatal day 49 one group of neonatal operated rats were systemically injected with the glucocorticoid receptor agonist dexamethasone (20 mg/kg), in order to simulate stress-induced glucocorticoid receptor activation. Working memory and perseveration was tested in T-maze tasks (continuous delayed alternation and reversal learning). Additionally, locomotor activity and prepulse inhibition (PPI) of startle was tested with and without apomorphine-treatment. Brain tissue damage was assessed using Nissl-staining and parvalbumine-immunocytochemistry. Pronounced thinning of the prelimbic-infralimbic subregion of the mPFC accompanied by altered cytoarchitecture and reduced number of parvalbumine-immunopositive neurones was found after neonatal lesions while adult lesions resulted in loss of neurones accompanied by gliosis. Neonatal lesions increased perseveration in the T-maze tasks and enhanced PPI, while adult lesions induced a working memory deficit. This differential behavioural outcome presumably reflects neurodevelopmentally induced alterations in neuronal circuits after neonatal lesions versus damage to mPFC alone after adult lesions. Dexamethasone-injection at day 49 did not alter behaviour in these tasks. Motor activity was not affected by neonatal or adult lesions but dexamethasone reduced apomorphine-induced hyperlocomotion.

Behavioural effects of neonatal lesions of the medial prefrontal cortex and subchronic pubertal treatment with phencyclidine of adult rats.

According to the neurodevelopmental hypothesis of schizophrenia, early brain damage renders the brain vulnerable to adverse effects during puberty, which precipitate the disease in young adults. Animal models can be used to test this hypothesis. We investigated the potentially independent or interactive effects of neonatal (postnatal day 7) excitotoxic lesions of the rat medial prefrontal cortex (mPFC) and subchronic pubertal phencyclidine (PCP)-treatment on adult rat behaviour. Sham-lesioned (vehicle-injection) and naive (unoperated) rats served as controls. On postnatal days 42-48 rats were systemically injected with 5 mg/kg PCP or vehicle twice daily. Behavioural testing started at postnatal day 70. Rats were tested for locomotor activity (open field), anxiety (elevated plus maze), social behaviour (conditioned place preference for cage-mates), reward-related operant behaviour [progressive ratio (PR)] and spatial learning (four-arm baited eight-arm radial maze task). Nissl-stained sections revealed considerable regeneration of much of the lesioned tissue in the mPFC, however, with disturbed cytoarchitecture. Locomotor activity was increased by neonatal lesions but reduced after pubertal PCP-treatment. Neonatal lesions alone increased operant behaviour in the PR-test and reduced anxiety in the elevated plus maze. In contrast, PCP-treatment disturbed social behaviour while neonatal lesions had no effect. Different aspects of leaning and memory in the radial maze task were independently disturbed after neonatal lesions and PCP-treatment. Neonatal lesions and pubertal PCP-treatment differentially affected adult rat behaviour and no interactions were found.

Role of the medial prefrontal cortex in N-methyl-d-aspartate receptor antagonist induced sensorimotor gating deficit in rats

The medial prefrontal cortex (mPFC) regulates sensorimotor gating measured as prepulse inhibition (PPI) of startle. We here tested the effect of lesions of the mPFC on the PPI-disruptive effect of the non-competitive NMDA receptor antagonist dizocilpine in rats. Neurotoxic lesions of the mPFC were induced by ibotenic acid. Rats were tested for PPI after systemic injection of dizocilpine (0.15 mg/kg) and after injection of the dopamine receptor agonist apomorphine (2 mg/kg). Dizocilpine failed to disrupt PPI in rats with mPFC lesions while the PPI-disruptive effect of apomorphine was not affected. Startle response magnitude in the absence of prepulses was not affected by mPFC lesions or drugs. These data suggest that the mPFC is an important brain region within the neuronal circuit responsible for NMDA receptor antagonist induced PPI-deficits.

Dopamine in the prefrontal cortex regulates rats behavioral flexibility to changing reward value.

Prefrontocortical dopamine (DA) plays an essential role in the representation of reward value and is implicated in behavioral flexibility. We here tested the effect of systemic and local blockade of DA D1- and D2-receptors in the medial prefrontal cortex (mPFC) and orbitofrontal cortex (OFC) by using an operant paradigm, where rats have to adjust their behavior to changing reward value. Rats were trained in a Skinner box, where different numbers of lever-presses for pellet-rewards were assigned to and switched between two levers. After rats commit to the efficient lever the lever-occupancy reversed and rats had to switch to the now efficient one. Rats were either intraperitoneally injected with the DA D1-receptor antagonist SCH23390 (40 microg/kg), the DA D2-receptor antagonist sulpiride (10mg/kg), or phosphate buffered saline (PBS). Two other groups received bilateral local mPFC- or OFC-infusions of SCH23390, sulpiride (both 3 microg/0.5 microl), or PBS (0.5 microl) through previously implanted cannulae. After initial detection of reverse of lever-occupancy, systemic and local blockade of D1-receptors increased the number of switches back to the previously efficient lever, thus reducing the total number of reverses completed. D2-receptor blockade deteriorated this measure after local mPFC-infusion. Notably, initial detection of reverse of lever-occupancy was not affected. Blockade of DA receptors within the prefrontal cortex do not deteriorate the detection of changes in reward value, whereas maintenance of behavioral adaptation is disturbed. Interestingly, blockade of DA receptors in the mPFC and OFC had similar effects, i.e., these regions apparently act in a cooperative manner.

Role of glutamate receptors in nucleus accumbens core and shell in spatial behaviour of rats

The nucleus accumbens (NAC) is considered to be an important neural interface between corticolimbic and motor systems of the brain. Several studies have shown that the NAC is not only involved in motivation and reward-related processes but also in spatial behavior. We here investigated the involvement of different glutamate receptor subclasses within NAC core and shell subregions on behavior in a radial-maze. Rats were first trained in a four-arm-baited eight-arm radial maze task for baseline performance. Thereafter, the effects of microinjection of the nonselective glutamate receptor antagonist kynurenic acid (4.5 microg), the NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (1 microg) and the non-NMDA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (0.75 microg) in NAC core and shell were tested on reference memory errors (RME) and working memory errors (WME). Moreover, the choice pattern of entries and duration of arm-entries were evaluated. Microinjection of all drugs increased RME. Additionally, non-NMDA receptor blockade in NAC shell but not core increased WME. After microinjection of all drugs into NAC core and shell rats preferentially choose the arms next to the previously visited arm. This work shows that glutamate receptors in both NAC subregions are important for spatial behavior. The deficits seen after glutamate receptor blockade may not be working- or reference memory-related but caused by a switch from a memory-dependent allocentric strategy to an egocentric response strategy.

Neonatal medial prefrontal cortex lesion enhances the sensitivity of the mesoaccumbal dopamine system.

Neurodevelopmental models of schizophrenia posit that early brain damage leads to dys‐ or misconnection effects possibly altering synaptic transmission in brain sites distal of the lesion. We tested the hypothesis that neonatal medial prefrontal cortex (mPFC) lesions affect the sensitivity of the mesoaccumbal dopamine (DA) system. Using extracellular single‐unit recordings combined with systemic application of the DA agonist apomorphine, followed by the D2 receptor antagonist haloperidol or the D1 receptor antagonist SCH23390, we compared electrophysiological properties of nucleus accumbens core and shell neurons after bilateral excitotoxic lesions of mPFC induced at postnatal day 7 or in adult rats. Whereas animals with adult mPFC lesions showed an altered discharge pattern within the core region, neonatal mPFC lesions altered the discharge pattern within the shell region. Subcutaneous administration of apomorphine (4 mg/kg) reduced accumbal firing rate in 77% of all neurons. Onset and magnitude of apomorphine‐induced inhibition of neuronal activity was faster and stronger in rats with neonatal but not adult mPFC lesions in both core and shell regions. Apomorphine‐induced inhibition was partially reversed by 0.1 mg/kg haloperidol only in core region of neonatal lesioned rats. Apomorphine‐induced excitation of neuronal activity (in 21% of all neurons) was reversed by the D1 receptor antagonist SCH23390 (0.1 mg/kg) in all excited neurons. These data support the hypothesis that neonatal but not adult lesions of mPFC alter cortico‐striatal networks and suggest that disturbance of mPFC development leads to neurodevelopmental changes in mesoaccumbal DA system during adulthood.

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.

Effects of neonatal excitotoxic lesions of the entorhinal cortex on cognitive functions in the adult rat

The entorhinal cortex (EC) is involved in a variety of cognitive functions by virtue of its neuronal input from the neocortex and projection to the hippocampal formation and the limbic-striatal system. Neonatal lesions are increasingly considered useful models for disconnection syndromes such as schizophrenia. Therefore, we investigated the effects of neonatal EC lesions on adult rat behavior. Neonatal (postnatal day 7) lesions were inflicted by bilateral injections of ibotenate into the EC. Sham-lesioned (vehicle injection) and naive (unoperated) rats served as controls. Locomotor activity was measured in prepubertal and young adult rats. Adult rats were then tested for spatial learning in an eight-arm radial maze (reinforced delayed alternation) and for motivation (progressive ratio schedule of operant behavior). Finally, prepulse inhibition (PPI) of the acoustic startle reflex and locomotor activity were investigated with and without apomorphine (APO) challenge. Brain tissue damage was assessed using Nissl-staining. The total volume of the adult rat EC was reduced after neonatal ibotenate-injection. Neonatal EC-lesions increased perseveration only in a delayed task in the radial maze and induced a leftward-shift of breakpoints in operant responding. Lesions did not alter baseline locomotor activity, but enhanced the locomotor stimulating effect of APO. PPI was not affected by neonatal lesions of the EC with and without APO challenge. Neonatal lesions of the EC impaired the ability to hold information during delays and reduced motivation during operant behavior which reflects a state of anhedonia. Thus, they may serve as an animal model for certain aspects of schizophrenia.

Expression of Hepatoma-derived growth factor family members in the adult central nervous system

BackgroundHepatoma-derived growth factor (HDGF) belongs to a polypeptide family containing five additional members called HDGF related proteins 1–4 (HRP-1 to -4) and Lens epithelial derived growth factor. Whereas some family members such as HDGF and HRP-2 are expressed in a wide range of tissues, the expression of others is very restricted. HRP-1 and -4 are only expressed in testis, HRP-3 only in the nervous system. Here we investigated the expression of HDGF, HRP-2 and HRP-3 in the central nervous system of adult mice on the cellular level by immunohistochemistry. In addition we performed Western blot analysis of various brain regions as well as neuronal and glial cell cultures.ResultsHDGF was rather evenly expressed throughout all brain regions tested with the lowest expression in the substantia nigra. HRP-2 was strongly expressed in the thalamus, prefrontal and parietal cortex, neurohypophysis, and the cerebellum, HRP-3 in the bulbus olfactorius, piriform cortex and amygdala complex. HDGF and HRP-2 were found to be expressed by neurons, astrocytes and oligodendrocytes. In contrast, strong expression of HRP-3 in the adult nervous system is restricted to neurons, except for very weak expression in oligodendrocytes in the brain stem. Although the majority of neurons are HRP-3 positive, some like cerebellar granule cells are negative.ConclusionThe coexpression of HDGF and HRP-2 in glia and neurons as well as the coexpression of all three proteins in many neurons suggests different functions of members of the HDGF protein family in cells of the central nervous system that might include proliferation as well as cell survival. In addition the restricted expression of HRP-3 point to a special function of this family member for neuronal cells.

Deficient sensorimotor gating induced by selective breeding in rats is improved by entopeduncular nucleus lesions

Deficient prepulse inhibition (PPI) of startle reflects disturbed sensorimotor gating found in certain neuropsychiatric disorders, such as Tourettes syndrome, ADHD, Huntingtons and schizophrenia. We here tested, whether lesions of the entopeduncular nucleus (EPN) would improve a PPI-deficit induced by selective breeding. Rats with breeding induced high and low expression of PPI were stereotaxically microinjected with ibotenate (0.2 microg in 0.3 microl phosphate buffered saline) or vehicle into the EPN and two weeks later tested for PPI of the acoustic startle response (ASR) and motor activity. Lesions of the EPN counteracted the breeding-induced PPI-deficit and reduced ASR in the PPI low group without affecting their motor activity. In the PPI high group EPN lesions did not affect PPI, ASR, and motor activity. This work indicates an important role of the EPN in the modulation of sensorimotor gating. Additionally, PPI low rats may provide a non-pharmacological model that can be used to develop new therapeutic strategies for neuropsychiatric disorders.