Neural and behavioral effects of intracranial 192 IgG-saporin in neonatal rats: sexually dimorphic effects?

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.

Characterization of spatial performance in male and female Long-Evans rats by means of the Morris water task and the ziggurat task

Sex differences are prominent influences on spatial performance. One of the most common tasks to assess sex differences in spatial navigation in rodents is the Morris water task (MWT). In this task rats swim in a pool of water to locate a hidden platform employing the topographical relationships among the distal visual cues, pool wall, and goal location. Some evidence suggests that male rats display superior performance relative to females in the MWT. It is unknown, however, to what extent the sex difference in rats is task-dependent. This study compared the performance of male and female Long-Evans rats in the wet-land MWT versus the dry-land ziggurat task (ZT). The ZT represents a new dry-land task in which rats explore an arena with 16 ziggurat pyramids to locate food rewards. Several behavioural parameters, including latency, path length, path speed, probe trial performance, errors, and the number of returns were used as indices of spatial learning and memory. While males and females did not display significant differences in the traditional measures of spatial navigation within MWT, they displayed a robust sex difference in all measures of the ZT. These results indicate task-specific sex differences in spatial performance. Our findings suggest that males and females may employ different learning strategies in the MWT and ZT and that the latter task provides a more favourable task for assessing sex differences in rats.

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.

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.

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.

Neonatal lesion of forebrain cholinergic neurons: Further characterization of behavioral effects and permanency

Intraventricular injections of 192 IgG-saporin in the neonatal rat caused severe loss of basal forebrain cholinergic neurons and ectopic hippocampal ingrowths. These were evident at 24 months of age and thus, were lifelong consequences of the 192 IgG-saporin treatment. When tested as young adults on a novel water-escape radial arm maze, the rats with this lesion were slower to learn the task, committing significantly more working and reference memory errors before they achieved control level of performance. It is unlikely that this was a result of attentional impairment as the lesioned rats performed as vigilantly as controls in a five choice serial reaction time task. When tested in the Morris water maze at 22 months of age, they were slower at learning the hidden platform location. This contrasts with previous studies which have repeatedly shown that they normally acquire this task as young adults. It was concluded that this neonatal cholinergic lesion has modest but discernable effects on problem solving in young adulthood that are consistent with the reported effects of the lesion on cortical pyramidal neurons. The cognitive effects of the lesion may become more severe in aging, perhaps as a result of the added effects of aging on these neurons.

Selective acetylcholine and dopamine lesions in neonatal rats produce distinct patterns of cortical dendritic atrophy in adulthood

Acetylcholine and dopamine afferents reach their cortical targets during periods of synaptogenesis, and are in position to influence the cytoarchitectural development of cortical neurons. To determine the effect of removing these afferents on dendritic development, we lesioned rat pups at 7 days of age with the selective immunotoxins 192 IgG-saporin, or 6-hydroxydopamine, or both. One group of rats was killed in adulthood for neurochemistry and another was prepared for morphology using Golgi-Cox staining. Changes in morphology were compared in layer V pyramidal cells from medial prefrontal cortex, which sustained the greatest dopamine depletion, and in layer II/III pyramidal cells from retrosplenial cortex, which sustained the greatest choline acetyltransferase depletion. In rats with acetylcholine lesions, layer V medial prefrontal cells had smaller apical tufts and fewer basilar dendritic branches. Both apical and basilar spine density was substantially reduced. Layer II/III retrosplenial cells also had smaller apical tufts and substantially smaller basilar dendritic trees. Apical and basilar spine density did not change. In rats with dopamine lesions, layer V medial prefrontal cells had fewer oblique apical dendrites and atrophied basilar trees. Layer II/III retrosplenial cells had fewer apical dendritic branches. In neither area were spine densities significantly different from control. Neurons from rats with combined lesions were always smaller and less complex than those from singly lesioned rats. However, these cells were simple, additive composites of the morphology produced by single lesions. These data demonstrate that ascending acetylcholine and dopamine afferents play a vital role in the development of cortical cytoarchitecture.

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.

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 192 IgG-saporin lesion of forebrain cholinergic neurons: focus on the life span?

The cholinergic immunotoxin 192 IgG-saporin can be used to effect selective, substantial and permanent lesions of basal forebrain neurons in the neonatal rat. Human neurodevelopmental disorders such as Rett and Down syndromes are characterized by early cholinergic dysfunction and cognitive impairment. Hence, the study of the neonatal 192 IgG-saporin lesioned rat should illuminate the role of cholinergic dysfunction in these human disorders. To date, we and others have failed to observe notable effects of this neonatal lesion on learning and memory, even when combined with a severe lesion of noradrenergic forebrain innervation. As well, attention seems not to be affected. However, complex problem solving (intelligence?) is compromised by the cholinergic lesion. There also appears to be reduced cortical dendritic branching indicative of synapse loss but further research is needed to characterize this. Even if the synapse loss due to neonatal cholinergic lesion is modest and thus insufficient to cause a significant neurodevelopmental dysfunction, its consequences may be devastating during old age.

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.

Combined 192 IgG-saporin and 5,7-dihydroxytryptamine lesions in the male rat brain: a neurochemical and behavioral study

In a previous experiment [Eur J Neurosci 12 (2000) 79], combined intracerebroventricular injections of 5,7-dihydroxytryptamine (5,7-DHT; 150 microg) and 192 IgG-saporin (2 microg) in female rats produced working memory impairments, which neither single lesion induced. In the present experiment, we report on an identical approach in male rats. Behavioral variables were locomotor activity, T-maze alternation, beam-walking, Morris water-maze (working and reference memory) and radial-maze performances. 192 IgG-saporin reduced cholinergic markers in the frontoparietal cortex and the hippocampus. 5,7-DHT lesions reduced serotonergic markers in the cortex, hippocampus and striatum. Cholinergic lesions induced motor deficits, hyperactivity and reduced T-maze alternation, but had no other effect. Serotonergic lesions only produced hyperactivity and reduced T-maze alternation. Beside the deficits due to cholinergic lesions, rats with combined lesions also showed impaired radial-maze performances. We confirm that 192 IgG-saporin and 5,7-DHT injections can be combined to produce concomitant damage to cholinergic and serotonergic neurons in the brain. In female rats, this technique enabled to show that interactions between serotonergic and basal forebrain cholinergic mechanisms play an important role in cognitive functions. The results of the present experiment in male rats are not as clear-cut, although they are not in obvious contradiction with our previous results in females.

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.

Ectopic noradrenergic hyperinnervation does not functionally compensate for neonatal forebrain acetylcholine lesion

Adult rats who have undergone neonatal 192 IgG-saporin induced lesions of forebrain acetylcholine (ACH) neurons are normal on many behavioral tasks. In this study we determined whether ectopic hippocampal ingrowths, a documented consequence of these neonatal cholinergic lesions, functionally compensate for ACH denervation in these rats. Neonatal rats underwent systemic 6-hydroxydopamine (6-OHDA) injections on postnatal days (PND) 1-3 to prevent the ingrowths, and/or intraventricular 192 IgG-saporin injections on PND 7. The 192 IgG-saporin profoundly reduced basal forebrain p75 neurotrophin receptor (p75(NTR)) immunoreactive (IR) neurons. The 6-OHDA treatment abolished hippocampal and cortical dopamine-beta-hydroxylase (DBH) IR terminals, indicating the absence of normal norepinephrine (NE) innervation. Ectopic DBH IR and p75(NTR) IR varicosities which occurred in the hippocampus of 192 IgG-saporin treated rats were also eliminated by 6-OHDA treatment. Behavioral testing in adulthood indicated no effect of the treatments on the Morris water maze. 192 IgG-saporin treatment caused perseveration during delayed spatial alternation (DSA) and increased working but not reference memory errors on the radial arm maze (RAM). The 6-OHDA plus 192 IgG-saporin treated rats did not differ from the 192 IgG-saporin only rats on any task. These results indicate that ectopic hippocampal NE ingrowths do not functionally compensate for neonatal ACH lesions. Neonatal forebrain ACH lesion impairs working memory on the RAM but the absence of an effect on DSA contraindicates a basic dysfunction of short term memory. Despite severe combined neonatal loss of forebrain ACH and NE innervation, behavior is remarkably intact.