Neonatal 192 IgG-saporin lesions of basal forebrain cholinergic neurons selectively impair response to spatial novelty in adult rats

Perinatal 192 IgG-Saporin as Neuroteratogen

The immunotoxin 192 IgG-saporin selectively destroys basal forebrain cholinergic neurons that provide cholinergic input to the hippocampus, entire cortical mantle, amygdala, and olfactory bulb. Perinatal immunotoxic lesions by 192 IgG-saporin induce long-lasting cholinergic depletion mimicking a number of developmental disorders reported in humans. The perinatal injection of 192 IgG-saporin induces several brain modifications, which are observed in neocortex and hippocampus at short and long term. These plastic changes involve both structural (alterations in brain volume, neuronal morphology, and neurogenesis) and molecular (modulations of the levels of neurotransmitters and other proteins related to neurodegeneration) levels. Moreover, the perinatal injection of 192 IgG-saporin may interact with the brain plastic capacity to react to other injuries. Perinatal 192 IgG-saporin lesions allowed investigating the role of the basal forebrain cholinergic system in modulating behavioral functions in developing as well as adult rats. After perinatal cholinergic depletion, rats display reduced ultrasonic vocalizations as neonates, learning and exploratory deficits as juveniles, altered discriminative abilities, impulsive and perseverative behaviors, and memory deficits as adults. Overall, these findings underline the importance of cholinergic system integrity for the development of specific structural and functional features.

Using the Morris water maze to assess spatial learning and memory in weanling mice

Mouse models have been indispensable for elucidating normal and pathological processes that influence learning and memory. A widely used method for assessing these cognitive processes in mice is the Morris water maze, a classic test for examining spatial learning and memory. However, Morris water maze studies with mice have principally been performed using adult animals, which preclude studies of critical neurodevelopmental periods when the cellular and molecular substrates of learning and memory are formed. While weanling rats have been successfully trained in the Morris water maze, there have been few attempts to test weanling mice in this behavioral paradigm even though mice offer significant experimental advantages because of the availability of many genetically modified strains. Here, we present experimental evidence that weanling mice can be trained in the Morris water maze beginning on postnatal day 24. Maze-trained weanling mice exhibit significant improvements in spatial learning over the training period and results of the probe trial indicate the development of spatial memory. There were no sex differences in the animals’ performance in these tasks. In addition, molecular biomarkers of synaptic plasticity are upregulated in maze-trained mice at the transcript level. These findings demonstrate that the Morris water maze can be used to assess spatial learning and memory in weanling mice, providing a potentially powerful experimental approach for examining the influence of genes, environmental factors and their interactions on the development of learning and memory.

Cholinergic systems are essential for late-stage maturation and refinement of motor cortical circuits

Previous studies reported that early postnatal cholinergic lesions severely perturb early cortical development, impairing neuronal cortical migration and the formation of cortical dendrites and synapses. These severe effects of early postnatal cholinergic lesions preclude our ability to understand the contribution of cholinergic systems to the later-stage maturation of topographic cortical representations. To study cholinergic mechanisms contributing to the later maturation of motor cortical circuits, we first characterized the temporal course of cortical motor map development and maturation in rats. In this study, we focused our attention on the maturation of cortical motor representations after postnatal day 25 (PND 25), a time after neuronal migration has been accomplished and cortical volume has reached adult size. We found significant maturation of cortical motor representations after this time, including both an expansion of forelimb representations in motor cortex and a shift from proximal to distal forelimb representations to an extent unexplainable by simple volume enlargement of the neocortex. Specific cholinergic lesions placed at PND 24 impaired enlargement of distal forelimb representations in particular and markedly reduced the ability to learn skilled motor tasks as adults. These results identify a novel and essential role for cholinergic systems in the late refinement and maturation of cortical circuits. Dysfunctions in this system may constitute a mechanism of late-onset neurodevelopmental disorders such as Rett syndrome and schizophrenia.

Effects of brief stress exposure during early postnatal development in balb/CByJ mice: I. Behavioral characterization

Early life stress has been linked to the etiology of mental health disorders. Rodent models of neonatal maternal separation stress frequently have been used to explore the long-term effects of early stress on changes in affective and cognitive behaviors. However, most current paradigms risk metabolic deprivation, due to prolonged periods of pup removal from the dam. We have developed a new paradigm in Balb/CByJ mice, that combines very brief periods of maternal separation with temperature stress to avoid the confound of nutritional deficiencies. We have also included a within-litter control group of pups that are not removed from the dam. The present experiments provide an initial behavioral characterization of this new model. We show that neonatally stressed mice display increased anxiety and aggression along with increased locomotion but decreased exploratory behavior. In contrast, littermate controls show increased exploration of novelty, compared to age-matched, colony-reared controls. Behavioral changes in our briefly stressed mice substantially concur with the existing literature, except that we were unable to observe any cognitive deficits in our paradigm. However, we show that within litter control pups also sustain behavioral changes suggesting complex and long-lasting interactions between different environmental factors in early postnatal life.

Effects of brief stress exposure during early postnatal development in Balb/CByJ mice: II. Altered cortical morphology

Early life experience can significantly determine later mental health status and cognitive function. Neonatal stress, in particular, has been linked to the etiology of mental health disorders as divergent as mood disorder, schizophrenia, and autism. Our study uses a Balb/CByJ mouse model to test the hypothesis, that neonatal stress will alter development and subsequent environmental modulation of neocortex. Using a split litter design, we generated stressed mice (STR) and within litter controls (LMC) along with age-matched, untreated animals (AMC), to serve as across litter controls. Short, daily exposure to a psychosocial/physical stressor, during the first week of life, resulted by adulthood in significant changes in neocortical thickness and architecture, which were further modulated by exposure to behavioral testing. Surprisingly, cortical size in LMC mice was also affected. These observations were compared to the effects of environmental enrichment in the same mouse strain. Our data indicate that LMC and STR males share with environmentally enriched males, an increase in thickness in infra-granular cortical layers, while STR also display a stress selective decrease in supragranular layers, in response to behavioral training as adults.

Cognitive and social functions and growth factors in a mouse model of Rett syndrome

Rett syndrome (RTT) is an autism-spectrum disorder caused by mutations in the X-linked gene encoding methyl-CpG-binding protein 2 (MeCP2). Abnormalities in social behavior, stereotyped movements, and restricted interests are common features in both RTT and classic autism. While mouse models of both RTT and autism exist, social behaviors have not been explored extensively in mouse models of RTT. Here, we report cognitive and social abnormalities in Mecp2(1lox) null mice, an animal model of RTT. The null mice show severe deficits in short- and long-term object recognition memories, reminiscent of the severe cognitive deficits seen in RTT girls. Social behavior, however, is abnormal in that the null mice spend more time in contact with stranger mice than do wildtype controls. These findings are consistent with reports of increased reciprocal social interaction in RTT girls relative to classic autism. We also report here that the levels of the neurotrophins brain-derived neurotrophic factor (BDNF), insulin-like growth factor-1 (IGF-1), and nerve growth factor (NGF) are decreased in the hippocampus of the null mice, and discuss how this may provide an underlying mechanism for both the cognitive deficits and the increased motivation for social contact observed in the Mecp2(1lox) null mice. These studies support a differential etiology between RTT and autism, particularly with respect to sociability deficits.

Developmental forebrain cholinergic lesion and environmental enrichment: behaviour, CA1 cytoarchitecture and neurogenesis

Intraventricular injections of 192 IgG saporin in 7-day-old rat severely reduced hippocampal cholinergic innervation as reflected by both decreased acetylcholinesterase staining and immunoreactivity for the p75 neurotrophin receptor. It was determined if this altered the effects of environmental enrichment on spatial learning, hippocampal CA1 cell cytoarchitecture as reflected by the Golgi stain, and neurogenesis in the dentate gyrus as indicated by doublecortin immunoreactivity. At weaning, lesioned and control rats were either group housed in large, environmentally enriched cages or housed two per standard cage for 42 days. When subsequently assessed with a working-memory spatial navigation task, both lesioned and control rats showed enhanced learning as a result of enrichment. Quantitative analysis of Golgi stained sections indicated that enrichment did not affect CA1 dendritic branching, total dendritic length or dendritic spine density. However, the lesion reduced the number of apical branches, spine density on intermediate to distal apical dendrites, and the length of basal branches. It also reduced the number of doublecortin immunoreactive neurons in the dentate gyrus and appeared to prevent their increase due to environmental enrichment. It is concluded that developmental cholinergic lesioning does not attenuate neurobehavioral plasticity, at least as reflected by the behavioral consequences of enrichment. It does, however, attenuate neurogenesis in the dentate gyrus, like adult-inflicted cholinergic lesions. As previously found for cortical neurons, it also reduces CA1 pyramidal cell dendritic complexity and spine density in adulthood. The results have implications for the loss of synapses that occurs in both developmental and aging-related brain disorders involving cholinergic dysfunction.

Sex differences in water maze performance and cortical neurotrophin levels of LHX7 null mutant mice

Mice lacking both alleles of the LIM-homeobox gene Lhx7 display dramatically reduced number of forebrain cholinergic neurons. Given the fact that sex differences are consistently observed in forebrain cholinergic function, in the present study we investigated whether the absence of LHX7 differentially affects water maze performance in the two sexes. Herein we demonstrate that LHX7 null mutants display a sex-dependent impairment in water maze, with females appearing more affected than males. Moreover, neurotrophin assessment revealed a compensatory increase of brain-derived neurotrophic factor and neurotrophin 3 in the neocortex of both male and female mutants and an increase of nerve growth factor levels only in the females. Nevertheless, the compensatory increase of cortical neurotrophin levels did not restore cognitive abilities of Lhx7 homozygous mutants. Finally, our analysis revealed that cortical neurotrophin levels correlate negatively with water maze proficiency, indicating that there is an optimal neurotrophin level for successful cognitive performance.

Neonatal dopamine depletion induces changes in morphogenesis and gene expression in the developing cortex

The mesocorticolimbic dopamine (DA) system is implicated in mental health disorders affecting attention, impulse inhibition and other cognitive functions. It has also been involved in the regulation of cortical morphogenesis. The present study uses focal injections of 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle of BALB/c mice to examine morphological, behavioral and transcriptional responses to selective DA deficit in the fronto-parietal cortex. Mice that received injections of 6-OHDA on postnatal day 1 (PND1) showed reduction in DA levels in their cortices at PND7. Histological analysis at PND120 revealed increased fronto-cortical width, but decreased width of somatosensory parietal cortex. Open field object recognition suggested impaired response inhibition in adult mice after 6-OHDA treatment. Transcriptional analyses using 17K mouse microarrays showed that such lesions caused up-regulation of 100 genes in the cortex at PND7. Notably, among these genes are Sema3A which plays a repulsive role in axonal guidance, RhoD which inhibits dendritic growth and tubulin beta-5 microtubule subunit. In contrast, 127 genes were down-regulated, including CCT-epsilon and CCT-zeta that play roles in actin and tubulin folding. Thus, neonatal DA depletion affects transcripts involved in control of cytoskeletal formation and pathway finding, instrumental for normal differentiation and synaptogenesis. The observed gene expression changes are consistent with histological cortical and behavioral impairments in the adult mice treated with 6-OHDA on PND1. Our results point towards specific molecular targets that might be involved in disease process mediated by altered developmental DA regulation.

Neonatal serotonin depletion alters behavioral responses to spatial change and novelty

Multiple brain disorders that show serotonergic imbalances have a developmental onset. Experimental models indicate a role for serotonin as a morphogen in brain development. To selectively study the effects of serotonin depletions on cortical structural development and subsequent behavior, we developed a mouse model in which a serotonin neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT), is injected into the medial forebrain bundle (mfb) on the day of birth. Littermates with saline injections into the mfb and age matched mice served as controls. This study characterized the extent and duration of serotonergic denervation after the selective neonatal lesion and investigated effects on exploratory behavior, spatial learning and anxiety in mice of both sexes. We report significant decreases in the serotonergic (5-HT) innervation to cortex and hippocampus, but not to subcortical forebrain structures in 5,7-DHT-lesioned mice. The depletion of 5-HT fibers in cortical areas was long lasting in lesioned mice but autoradiographic binding to high affinity 5-HT transporters was only transiently reduced. Male but not female lesioned mice reduced their exploration significantly in response to spatial rearrangement and object novelty, suggesting increased anxiety in response to change but normal spatial cognition. Our data show that developmental disruptions in the serotonergic innervation of cortex and hippocampus are sufficient to induce permanent, sex specific, behavioral alterations. These results may have significant implications for understanding brain disorders presenting with cortical morphogenetic abnormalities and altered serotonin neurotransmission, such as autism, schizophrenia and affective disorders.

Behavioral and anatomical abnormalities in Mecp2 mutant mice: a model for Rett syndrome

Over 90% of Rett syndrome (RTT) cases have a mutation in the X-linked gene encoding methyl CpG binding-protein 2 (MeCP2). A mouse model that reprises clinical manifestations of the disease would be valuable for examining disease mechanisms. Here, we characterize physical and behavioral measures, as well as brain region volumes in young adult mice that have mutations in mouse methyl CpG binding-protein 2 gene (Mecp2) to serve as a baseline for other studies. Hemizygous males, which produce no functional protein, exhibit hypoactivity and abnormalities in locomotion, stereotypies, and anxiety reminiscent of the clinical condition. The mutant males also exhibit cognitive deficits in fear conditioning and object recognition relative to wildtypes. Volumetric analyses of male brains revealed a 25% reduction in whole brain volume in mutants relative to wildtypes; regional differences were also apparent. Mutants had decreased volumes in three specific brain regions: the amygdala (39%), hippocampus (21%), and striatum (29%). Heterozygous females, which produce varying amounts of functional protein, displayed a less severe behavioral phenotype. The mutant females exhibit abnormalities in locomotion, anxiety measures, and cognitive deficits in object recognition in an open field. This study provides the first evidence that the abnormal motor and cognitive behavioral phenotype in Mecp2 mice is consistent with specific volume reductions in brain regions associated with these behaviors, and shows how these data parallel the human condition. The Mecp2 mutant mice provide a very good model in which to examine molecular and behavioral mechanisms, as well as potential therapeutic interventions in RTT.

Long-term effects of neonatal basal forebrain cholinergic lesions on radial maze learning and impulsivity in rats

We examined long-term behavioural effects of neonatal lesions of the cholinergic basal forebrain obtained by intracerebroventricular injections of 192 IgG saporin (192 IgG-Sap). Five-month-old Wistar male rats (injected with 192 IgG-Sap or phosphate-buffered saline on postnatal day 7) were tested using operant chambers with two nose-poking holes, delivering one food pellet immediately or five pellets after a delay. The length of delay progressively increased over days (from 0 to 100 s). When compared with controls, 192 IgG-Sap rats showed a slight preference for smaller immediate over larger delayed rewards, thus indicating elevated intolerance to delay (i.e. more impulsivity). Sibling animals were tested in a computerized radial maze (baited vs. nonbaited arm procedure). 192 IgG-Sap rats appeared slower than controls in accomplishing the task. The neonatal 192 IgG-Sap lesion did not alter cortical levels of serotonin and/or its metabolites, but induced a marked cortical cholinergic loss. Our data suggest that a prolonged basal forebrain cholinergic hypofunction produces (i) an impairment in cognitive performances that is detectable only when highly complex tasks are used; (ii) a slight enhancement of the impulsive behavioural profile. This animal model may thus be useful to investigate some cognitive deficits and other secondary symptoms seen in Alzheimer's disease.

Diisopropylfluorophosphate administration in the pre-weanling period induces long-term changes in anxiety behavior and passive avoidance in adult mice

The developing brain may be particularly vulnerable to exposure to acetylcholinesterase (AChE) inhibitors because of the role of AChE on neuronal development and the effects of cholinergic pathways in mediating behavioral and hormonal responses to stress. C57BL/65 mice of both sexes were injected with 1 mg/kg s.c. diisopropylfluorophosphate (DFP) or saline in three separate experiments, on postnatal days (PNDs) 4-10, 14-20, or 30-36. Anxiety and conditioned avoidance were assessed on the elevated-plus maze (EPM) and step-down passive avoidance (PA) paradigms, respectively, at age 4-5 months. In addition, locomotion and reactivity to pain on the hot plate were assessed. Mice treated on PNDs 4-10 or PNDs 14-20 spent relatively more time and made more entries to the open arms on the first, but not second, exposure to the EPM. Females, but not males, treated with DFP showed deficits in PA retention after 24 h when treated on PNDs 4-10 and on PNDs 14-20. Mice treated on PNDs 30-36 were not impaired in either behavior. Administration of DFP in the preweanling period did not affect locomotor activity or pain reactivity. The results suggest that preweanling exposure to DFP results in anxiolysis in novel conflict situations but exacerbated context-enhanced anxiety.

Scoring learning and memory in developing rodents

Learning and memory abilities can be reliably measured in rodents starting from very early phases of postnatal development. In particular, in the study of learning and memory in periweanling or adolescent (from around postnatal day 20 to 50) mice or rats, two experimental protocols are appropriate for a reliable characterization of cognitive ability development: the water maze and the spatial open-field with objects. These experimental protocols have been developed to study the behavior of adult rodents, but are easily adapted to the peculiar physiological and behavioral responses of immature animals by applying selected modifications to both test apparatuses as well as protocols. It is worth noting that these two experimental protocols have been proven to be complementary because they provide different information on possible cognitive deficits; thus, use of both is recommended for an exhaustive behavioral characterization.

Overview of behavioral teratology

The discipline of behavioral teratology is based on the study of behavior as a powerful tool to identify and disentangle noxious effects of compounds known or suspected to be potentially neurotoxic early during development. Indeed, because behavior is the ultimate output of the brain, behavioral assessments can provide critical information on the noxious actions exerted by selected chemicals-information which is different from, and complementary to, the information provided by neurochemical, cellular, and histological evaluations. In particular, behavioral assessments allow one to investigate the integrity of a number of brain processes, helping to identify selected structural and/or functional alterations. A large number of behavioral protocols are available which are rapid, simple, quantitative, easily replicable, and reliable as indices of brain dysfunction. This unit provides an overview of the field of behavioral teratology.

Susceptibility to seizure-induced injury and acquired microencephaly following intraventricular injection of saporin-conjugated 192 IgG in developing rat brain

To study the role of neurotrophin-responsive neurons in brain growth and developmental resistance to seizure-induced injury, we infused saporin-conjugated 192-IgG (192 IgG-saporin), a monoclonal antibody directed at the P75 neurotrophin receptors (p75(NTR)), into the ventricles of postnatal day 8 (P8) rat pups. 7-10 days after immunotoxin treatment, loss of p75(NTR) immunoreactivity was associated with depletion of basal forebrain cholinergic projection to the neocortex and hippocampus. Kainic acid (KA)-induced seizures on P15 resulted in hippocampal neuronal injury in the majority of toxin-treated animals (13/16), but only rarely in saline-injected controls (2/25) (P < 0.001). In addition, widespread cerebral atrophy and a significant decrease in brain weight with preserved body weight were observed. Volumetric analysis of the hippocampal hilar region revealed a 2-fold reduction in perikaryal size and a 1.7-fold increase in cell packing density after 192 IgG-saporin injection. These observations indicate that neurotrophin-responsive neurons including basal forebrain magnocellular cholinergic neurons may be critical for normal brain growth and play a protective role in preventing excitotoxic neuronal injury during development.

Basal forebrain cholinergic lesions in 7-day-old rats alter ultrasound vocalisations and homing behaviour

We analysed the effects of 192 IgG-saporin lesions on pnd 7 upon neonatal behavioural responses. Number of ultrasonic vocalisations (USVs) were recorded on pnds 9, 11 and 13. On pnd 13 rats underwent a homing test to measure olfactory orientation towards nest material. 192 IgG-saporin reduced the number of USVs at all ages considered while increasing number of entrancies into the nest area. These data suggest that early damage to the basal forebrain cholinergic nuclei induces significant changes in the rat behavioural repertoire as early as the second-postnatal week.

Cognitive and neurological deficits induced by early and prolonged basal forebrain cholinergic hypofunction in rats

In the present study we examined the long-term effects of neonatal lesion of basal forebrain cholinergic neurons induced by intracerebroventricular injections of the immunotoxin 192 IgG saporin. Animals were then characterised behaviourally, electrophysiologically and molecularly. Cognitive effects were evaluated in the social transmission of food preferences, a non-spatial associative memory task. Electrophysiological effects were assessed by recording of cortical electroencephalographic (EEG) patterns. In addition, we measured the levels of proteins whose abnormal expression has been associated with neurodegeneration such as amyloid precursor protein (APP), presenilin 1 and 2 (PS-1, PS-2), and cyclooxygenases (COX-1 and COX-2). In animals lesioned on postnatal day 7 and tested 6 months thereafter, memory impairment in the social transmission of food preferences was evident, as well as a significant reduction of choline acetyltransferase activity in hippocampus and neocortex. Furthermore, similar to what observed in Alzheimer-like dementia, EEG cortical patterns in lesioned rats presented changes in alpha, beta and delta activities. Levels of APP protein and mRNA were not affected by the treatment. Levels of hippocampal COX-2 protein and mRNA were significantly decreased whereas COX-1 remained unaltered. PS-1 and PS-2 transcripts were reduced in hippocampus and neocortex. These findings indicate that neonatal and permanent basal forebrain cholinergic hypofunction is sufficient to induce behavioural and neuropathological abnormalities. This animal model could represent a valid tool to evaluate the role played by abnormal cholinergic maturation in later vulnerability to neuropathological processes associated with cognitive decline and, possibly, to Alzheimer-like dementia.

Progressive behavioural changes in the spatial open-field in the quinolinic acid rat model of Huntington's disease

Huntington's disease (HD) is a progressive neurodegenerative disorder, characterized by severe degeneration of basal ganglia, motor abnormalities, impaired cognitive functions and emotional disturbances. Intrastriatal injection of the excitotoxin quinolinic acid (QA), an N-methyl-D-aspartate receptor agonist, appears to reproduce in rats some of the clinical features of human HD, included motor and behavioural deficits. Aim of this study was to assess whether the behavioural alterations described in the QA rat model of HD progressed over time. We analysed the effects of bilateral striatal injection of QA (300 nmol/1 microl) to adult rats in the spatial open-field test, a nonaversive task in which exploratory activity and responses to both spatial rearrangement of familiar objects and object novelty are measured. Rats were tested 2 weeks, 2 and 6 months after the QA lesion. Lesioned rats showed progressive alterations in performance in this task. Whereas sham and QA rats did not markedly differ 2 weeks post-lesion, lesioned rats were significantly more active than controls 2 and 6 months after surgery. Specifically, frequency and duration of rearing and wall rearing increased progressively over time, while grooming was enhanced at 2 months post-lesion only. Spatial and object novelty discrimination was not affected. These results show that a single injection of QA excitotoxin can induce behavioural changes that progress over time. The main implication of these findings is that, besides genetic mice models of HD, QA-lesioned rats may represent a suitable mean to test the ability of new drugs to slow down disease progression.

The '3Rs' model and the concept of alternatives in animal research: a questionnaire survey

It has been 45 years since Russell and Burch first proposed the concept of the '3Rs', yet it remains unclear how those individuals involved in animal research view and implement these concepts. The authors used a questionnaire survey to determine how well-known experts judged issues related to the 3Rs.

Sex differences in the behavioral response to spatial and object novelty in adult C57BL/6 mice

The present studies examined sex differences in object localization and recognition in C57BL/6 mice. Experiment 1 measured responses to spatial novelty (object displacement) and object novelty (object substitution). Males strongly preferred displaced and substituted objects over unchanged objects, whereas females showed a preference in only 1 measure of object novelty. Experiment 2 further examined object recognition by presenting mice with 2 identical objects, followed 24 hr or 7 days later by testing with a familiar and a novel object. After 24 hr, males preferentially explored the novel object, whereas females exhibited no such preference. Neither sex displayed a preference for the novel object after 7 days. The data suggest that male mice are superior to females at localizing and recognizing objects.

A morphogenetic role for acetylcholine in mouse cerebral neocortex

Recently, cholinergic afferents to cerebral cortex have met renewed attention regarding the regulation of plasticity as well as cognitive processing. My laboratory has developed a mouse neonatal basal forebrain lesion paradigm that has contributed considerably to the understanding of cholinergic mechanisms in cortical development. We have shown that transient cholinergic deafferentation, beginning at birth, precipitates alterations in neuronal differentiation and synaptic connectivity that persist into maturity, and contribute to altered cognitive behavior. These data are in general agreement with studies in rats in which the cholinergic basal forebrain is lesioned very early in development but contrast with effects of later developmental lesions. Moreover, in mouse, both morphological and behavioral consequences of the lesion are sex dependent. Studies of receptors and secondary messengers that are instrumental in morphogenesis and plasticity suggest that sex dependent molecular alterations occur within days if not hours following cortical cholinergic deafferentation.

The cholinergic basal forebrain system during development and its influence on cognitive processes: important questions and potential answers

This review seeks to address, though perhaps not answer fully, four important questions about the cholinergic basal forebrain (BF) system in developing mammals. First, what role does the cholinergic basal forebrain system play in the development of cognitive functions? Second, does the cholinergic BF system play a fundamentally similar role in development vs. adulthood? Third, does sexual dimorphism of the developing cholinergic BF system influence cognition differently in the two sexes? Finally, what role does the developing cholinergic BF system play in developmental disorders such as Down syndrome and Rett syndrome? Examples from the literature, primarily studies in mice and rats, are given in an attempt to answer these important questions.

Behavioral patterns under cholinergic control during development: lessons learned from the selective immunotoxin 192 IgG saporin

The immunotoxin 192 IgG saporin (192 IgG-sap) offers a valuable tool to investigate the role of the developing basal forebrain cholinergic system in modulating behavioral functions in developing, as well as adult rats. After neonatal 192 IgG-sap lesions, rats display reduced ultrasonic vocalizations as neonates, deficits in passive avoidance learning as juveniles, and altered reactions to spatial novelty as adults. These data suggest that neonatal cholinergic depletion affects cognitive performance in juvenile and adult rats. Additionally, neonatal cholinergic depletion alters ultrasonic vocalizations, which could then alter establishing normal mother-infant relationships, and thus compound the pup's cognitive deficits. These findings underscore the importance of assessing behavior during ontogeny, as well as in adulthood.

Neonatal cholinergic lesions and development of exploration upon administration of the GABAa receptor agonist muscimol in preweaning rats

Neonatal rats were administered 192 IgG-saporin (192 IgG-Sap), a selective cholinergic immunotoxin, on postnatal day (PND) 7. Behavioural responsiveness to muscimol, a GABAa receptor agonist, was then assessed using locomotor activity and object exploration tests on PND 18. In Experiment 1, 192 IgG-Sap-lesioned and control rats were injected with the GABAa agonist, muscimol, on PND 18 and tested in a standard open field test. Muscimol reduced rearing responses in both control and 192 IgG-Sap-lesioned animals whereas reduced wall-rearing responses occurred in control animals only. 192 IgG-Sap also reduced rearing and wall-rearing responses. In Experiment 2, muscimol effects were evaluated on PND 18 in a spatial open field test in which object exploration in addition to locomotion and rearing responses was assessed. Neonatal cholinergic lesion per se increased locomotion during object exploration while decreasing time spent exploring objects. Depressant effects of muscimol on object exploration were also evident. As a whole, these data provide evidence for (i) basal forebrain (BF) cholinergic control on locomotor activity and object exploration and (ii) GABAa-mediated regulation of selective behavioural patterns associated with locomotion and exploration in weaning rats. Neonatal cholinergic lesions, however, do not appear to alter reactivity to GABAergic agonists in juvenile rats.

Effects of lesions of basal forebrain cholinergic neurons in newborn rats on susceptibility to seizures

The cholinergic system modulates cerebral excitability. We recently reported that immunolesions of the basal forebrain (BF) cholinergic neurons in adult rats increase the susceptibility to generalized seizures. In this study we investigated the effects of lesions of the BF cholinergic neurons in neonatal rats on seizure susceptibility and cognitive function. Neonatal rats at postnatal day (P) 7 received intracerebroventricular (i.c.v.) injections of 192 IgG-saporin (SAP) or phosphate-buffered saline. Following 3 weeks after the injection the first group of rats was implanted with hippocampal electrodes for electroencephalogram (EEG) recordings while the second group of rats was tested for visual spatial memory using the hidden platform version of the water maze test. The first group of rats was then tested for seizure susceptibility using flurothyl 1 week after the electrode implantation. Rats that received immunolesions of the BF cholinergic neurons at P7 had significantly shorter latencies to onset of myoclonic jerks and tonic-clonic seizures than controls. However, no significant differences were found in the duration of seizures, or EEG ictal duration. No significant deficits in spatial learning were found between rats that received i.c.v. injections of SAP at P7 and controls. As in adult rats, lesions of the BF cholinergic system in rat pups result in subsequent increase in seizure susceptibility.

Early neonatal 192 IgG saporin induces learning impairments and disrupts cortical morphogenesis in rats

We have shown previously that neonatal intraventricular injections of the selective cholinergic immunotoxin 192 IgG saporin on postnatal day 7 (pnd 7) induce marked cholinergic loss in hippocampus and neocortex and a learning impairment on pnd 15. In the present study, we analysed the behavioural, morphological and neurochemical effects of earlier intraventricular injection of the immunotoxin 192 IgG saporin (pnd 1 and 3). We hypothesised that these earlier lesions would interrupt a critical stage in neocortical maturation, and impair behavior more profoundly than the later lesions. Passive avoidance (PA) learning and locomotor activity during the PA test were assessed on pnd 15. Retention of the PA task was assessed on pnd 16. Reactivity to spatial and object novelty was assessed on pnd 180 in a spatial open field test with five objects. Choline acetyltransferase (ChAT) activity was measured in basal forebrain targets on pnd 20 and pnd 180. Neonatal administration of 192 IgG saporin resulted in a slower acquisition of the PA task in females; retention and locomotor activity were not affected. On pnd 180, reaction to spatial novelty was mildly impaired in lesioned rats of both sexes. There was a marked reduction of ChAT in the hippocampus and neocortex of lesioned rats of both sexes, at both ages. Morphological analysis of the somatosensory cortex of lesioned rats revealed alterations in cortical development with sex specific variations in total cortical thickness. These results suggest that interrupting cholinergic basal forebrain innervation of neocortex and hippocampus during the first postnatal days affects the development of cognitive behaviour, neurochemistry and cortical organisation in a sex specific manner. Furthermore, the alterations in cortical organization are more profound than those noted after a lesion later in postnatal development. These behavioural and morphological abnormalities could be considered a model for several neurodevelopmental disorders associated with mental retardation.

NGF induces appearance of adult-like response to spatial novelty in 18-day male mice

We investigated the effects of Nerve Growth Factor (NGF) administration on the maturation of reactivity to spatial and non-spatial novelty in developing mice. CD-1 mice of both sexes received intracerebral administration of NGF on postnatal day (pnd) 15, and their response to object displacement (spatial novelty) and object substitution (object novelty) were assessed in a spatial open-field with four objects on pnd 18 or 28. On pnd 18, NGF induced only in males precocious appearance of spatial novelty discrimination, while increasing choline acetyltransferase activity in neocortex and hippocampus of both sexes. The behavioral and neurochemical effects disappeared by pnd 28. NGF triggers adult-like responding to spatial novelty in developing mice and such effect is gender-specific.

Transgenic and knock-out mouse pups: the growing need for behavioral analysis

Few laboratories working with transgenic and knockout mice analyze the neurobehavioral consequences of genetic manipulation in early ontogeny. However, the study of behavioral endpoints during the early postnatal period in genetically modified mice is important not only to assess possible developmental abnormalities, but also to better understand and disentangle the effects of genetic manipulations in adulthood. We propose that the assessment of neurobehavioral development represents an appropriate strategy to identify possible compensatory and/or unexpected effects. Nowadays, a large number of experimental protocols that take into account the practical constraints imposed by the peculiar physiological and behavioral responses of an immature subject are available to assess the neurobehavioral profile of developing mice. While this knowledge should be applied to the field of transgenic and knock-out mice in general, it should be recommended, in particular, for the study of mouse models of neurodevelopmental disorders.

Reactivity to object and spatial novelty is normal in older Ts65Dn mice that model Down syndrome and Alzheimer's disease

Ts65Dn mice, a model for Down syndrome and Alzheimer's disease, have a spontaneous age-related reduction of cholinergic markers in medial septal neurons, hippocampal abnormalities, and an age-related learning deficit in a task that requires an intact hippocampus. Others have shown that when normal rodents explored an open field with objects, they detected the displacement of some of the familiar objects within the arena (spatial novelty) and the presence of a new object (object novelty); whereas rodents with hippocampal, fornix, or neonatal selective basal forebrain cholinergic lesions were impaired in detecting spatial, but not object, novelty. In this study, both control and Ts65Dn mice responded to both the spatial and object changes. This unexpected finding could have several explanations. One may be related to recent studies that suggest that only rats with neonatal, but not adult, basal forebrain cholinergic 192 IgG-saporin lesions are impaired in reacting to spatial novelty.

Changes in the septohippocampal cholinergic system following removal of molar teeth in the aged SAMP8 mouse

We investigated the effect of dysfunctional teeth on age-related changes in the septohippocampal cholinergic system by assessing acetylcholine (ACh) release and choline acetyltransferase (ChAT) activity in the hippocampus and ChAT immunohistochemistry in the medial septal nucleus and the vertical limb of the diagonal band in young-adult and aged SAMP8 mice after removal of their upper molar teeth (molarless condition). Aged molarless mice showed decreased ACh release and ChAT activity in the hippocampus and a reduced number of ChAT-immunopositive neurons in the medial septal nucleus compared to age-matched control mice, whereas these effects were not seen in young-adult mice. The results suggest that the molarless condition in aged SAMP8 mice may enhance an age-related decline in the septohippocampal cholinergic system.

Methadone disrupts performance on the working memory version of the Morris water task

The aim of the study was to examine if administration of the mu-opiate agonist methadone hydrochloride resulted in deficits in performance on the Morris water tank task, a widely used test of spatial cognition. To this end, after initial training on the task, Long-Evans rats were administered saline or methadone at either 1.25, 2.5 or 5 mg/kg ip 15 min prior to testing. The performance of the highest-dose methadone group was inferior to that of the controls on the working memory version of the Morris task. There were also differences between the groups on the reference memory version of the task, but this result cannot be considered reliable. These data show that methadone has its most profound effect on cognition in rats when efficient performance on the task requires attention to and retention of new information, in this case, the relationship between platform location and the extramaze cues.

Facilitation of dopamine-mediated locomotor activity in adult rats following cholinergic denervation

The dopamine hypothesis of schizophrenia postulates hyperactivity of dopaminergic neurotransmission in the mesolimbic system. However, the possible underlying causes for this dopaminergic overfunction are not well understood. Therefore, the main aim of this study was to examine the effect of central cholinergic denervation on dopamine-mediated functions. We also examined the effect of neonatal cholinergic denervation upon adult brain function. The immunotoxin 192 IgG-saporin causes severe lesions of the basal forebrain cholinergic system when infused into the lateral ventricles by targeting neurons expressing the p75 neurotrophin receptor. The toxin may also damage p75-expressing Purkinje neurons in the cerebellum. We have compared the behavioral effects of intracerebroventricular injections of 192 IgG-saporin to adult rats with that of injections to neonate rats. As expected, adult treated rats displayed an almost complete cholinergic denervation of forebrain corticohippocampal areas concomitant with a marked impairment in the Morris water maze. When tested as adults, neonatally treated animals had a less complete cholinergic denervation and showed lesser impairments in water maze behaviors. Interestingly, adult treated rats showed increased spontaneous horizontal activity and a remarkable increase in locomotor response to d-amphetamine as evidenced by increased horizontal and vertical activity. There were no marked changes of spontaneous or drug-induced locomotor activity in adult rats treated with 192 IgG-saporin as neonates. These results suggest that cholinergic denervation of the forebrain causes a marked enhancement of behavioral responses related to dopaminergic activity, probably mainly mediated presynaptically. However, it cannot be fully excluded that damage to noncholinergic systems, e.g., Purkinje cells, might contribute to the effects. The striking overreaction to dopaminergic stimuli, presumably caused by the cholinergic deficit, is discussed in relation to the suggested role of cholinergic malfunctioning in schizophrenia.

Clinical, neurophysiological and immunological correlations in classical Rett syndrome

Rett syndrome (RTT) is neurodevelopmental disorder with the onset at critical period of postnatal ontogenesis and age dependent occurrence of clinical manifestations. The aim of the present study was to investigate possible correlations of the age of disease onset with clinical manifestations at the stage 3 of illness and neurobiological parameters. The study was carried out in 38 girls with classical RTT, aged from 3 to 7 years, and twenty and eighteen patients with the disease onset before and after the age of one year were divided into the groups 1 and 2 (Gr1 and Gr2), respectively. Quantitative EEG (QEEG) and measurement of the serum levels of autoantibodies (AAB) to nerve growth factor (NGF) were performed. Clinically, speech and motor functions were significantly more severely affected in the Gr1 than in the Gr2. In QEEG, spectral density of theta activity was significantly higher in Gr1 than in the Gr2. The titer of AAB to NGF was significantly increased in comparison with healthy controls, and the titer in Gr2 was higher than in Gr1. The data obtained suggests that patients with the classical RTT can be divided into subgroups according to the age of disease onset and genetic factors such as mosaicism of MeCP2 mutation may be associated with the heterogeneity of phenotype in RTT patients.

Enrichment-dependent differences in novelty exploration in rats can be explained by habituation

In rats, exploratory activity and emotional reactivity towards novel stimuli reflect independent biological functions that are modulated differently by rearing experiences. Environmental enrichment is known to improve performance in exploratory tasks, while having inconsistent effects on emotionality. This study examined the effect of environmental enrichment on the behaviour of rats in two exploratory tasks. Male rats were reared under one of four conditions, differing in social and non-social complexity. At 9 weeks of age, exploration of a novel open field, and exploration of novel objects in the same open field following 24 h habituation, was assessed. Differences in social and non-social complexity of the rearing environment had inconsistent effects on exploration in the novel open field. In contrast, when rats were faced with novel objects in an otherwise familiar environment, exploration habituated faster with increasing stimulus complexity of the non-social environment. The social environment had no effect on this latter test. These findings indicate that environmental enrichment affects exploratory activity primarily through its effect on habituation to novelty. This effect depends on relative stimulus complexity of the rearing environment, but is independent of social factors. The present results further suggest that aversive tasks can obscure the expression of enrichment-dependent differences in habituation to novelty.