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

Acetylcholine and noradrenaline differentially regulate hippocampus-dependent spatial learning and memory

Severe loss of cholinergic neurons in the basal forebrain nuclei and of noradrenergic neurons in the locus coeruleus are almost invariant histopathological hallmarks of Alzheimer's disease. However, the role of these transmitter systems in the spectrum of cognitive dysfunctions typical of the disease is still unclear, nor is it yet fully known whether do these systems interact and how. Selective ablation of either neuronal population, or both of them combined, were produced in developing animals to investigate their respective and/or concurrent contribution to spatial learning and memory, known to be severely affected in Alzheimer's disease. Single or double lesions were created in 4-8 days old rats by bilateral intraventricular infusion of two selective immunotoxins. At about 16 weeks of age, the animals underwent behavioural tests specifically designed to evaluate reference and working memory abilities, and their brains were later processed for quantitative morphological analyses. Animals with lesion to either system alone showed no significant reference memory deficits which, by contrast, were evident in the double-lesioned subjects. These animals could not adopt an efficient search strategy on a given testing day and were unable to transfer all relevant information to the next day, suggesting deficits in acquisition, storage and/or recall. Only animals with single noradrenergic or double lesions exhibited impaired working memory. Interestingly, ablation of cholinergic afferents to the hippocampus stimulated a robust ingrowth of thick fibres from the superior cervical ganglion which, however, did not appear to have contributed to the observed cognitive performance. Ascending cholinergic and noradrenergic afferents to the hippocampus and neocortex appear to be primarily involved in the regulation of different cognitive domains, but they may functionally interact, mainly at hippocampal level, for sustaining normal learning and memory. Moreover, these transmitter systems are likely to compensate for each other, but apparently not via ingrowing sympathetic fibres.

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.

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.

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.

Pro- and anti-apoptotic evidence for cholinergic denervation and hippocampal sympathetic ingrowth in rat dorsal hippocampus

In rat, injection of the specific cholinotoxin, 192 IgG-saporin, into the medial septum results not only in a selective cholinergic denervation of hippocampus, but in an ingrowth of peripheral sympathetic fibers, originating from the superior cervical ganglion, into the hippocampus. A similar process, in which peripheral noradrenergic axons invade hippocampus, may also occur in Alzheimer's disease. Since apoptotic cell death has been demonstrated in the selective neuronal loss found in Alzheimer's disease, the aim of this study was to measure apoptotic protein expression and DNA fragmentation in hippocampal sympathetic ingrowth and cholinergic denervation. Western blot, TdT-mediated dUTP nick end labeling, and oligo ligation techniques were used. Choline acetyltransferase activity and norepinephrine concentrations were also measured. As seen in our previous results, an increase in apoptotic markers was induced by cholinergic denervation alone (medial septum lesion + ganglionectomy), while hippocampal sympathetic ingrowth (medial septum + sham ganglionectomy) reduced or normalized apoptotic effects to control group levels. A decrease in choline acetyltransferase activity was also found in the dorsal hippocampus of hippocampal sympathetic ingrowth and cholinergic denervation groups. An increase in norepinephrine concentration was found in hippocampal sympathetic ingrowth but not in cholinergic denervation group. Results of this study suggest that cholinergic denervation is responsible for most of the proapoptotic responses, while hippocampal sympathetic ingrowth produces a protective effect in the process of programmed cell death in rat dorsal hippocampus. This effect may be a secondary to an altered relationship between norepinephrine-acetylcholine.

The effect of central cholinergic and noradrenergic denervation on hippocampal sympathetic ingrowth and apoptosis-like reactivity in the rat

In this study, the effect of intraseptal injection of specific cholinotoxin 192-IgG saporin (SAP) +/- intraperitoneal injection of N-[chloroethyl]-N-ethyl-2-bromobenzylamine (DSP-4) (noradrenergic fiber neurotoxin) was examined in rat hippocampus. Medial septal lesions resulted not only in selective cholinergic denervation of hippocampus (Medial septal lesion + ganglionectomy; SAP + Gx) but also in hippocampal sympathetic ingrowth (IG) of adrenergic fibers (Medial septal lesion + sham ganglionectomy; SAP + IG). Saporin-induced septal lesions produced a significant reduction in hippocampal choline acetyltransferase activity in all tested groups (SAP + IG +/- DSP-4 and SAP + Gx +/- DSP-4), and an increase in noradrenaline concentration in the SAP + IG group. Visualization of noradrenergic fibers by histofluorescence revealed a mixture of fine and thick varicosities in the SAP + IG but only fine fibers in control and SAP + Gx animals. SAP + IG + DSP-4 lesions produced significant reduction in noradrenaline concentration in all groups with a concomitant decrease in visualization of central noradrenergic fibers in dorsal and ventral hippocampus. Treatment of SAP + IG animals with DSP-4 left mostly thick fibers, probably derived from peripheral sympathetic ingrowth. No fluorescence was seen in either the control + DSP-4 or SAP + Gx + DSP-4 animals. Apoptotic-like changes, using in situ oligonucleotide ligation techniques, were also assessed. Proapoptotic changes were seen in the SAP + Gx +/- DSP-4 group as compared to CON +/- DSP-4 groups. SAP + IG regardless of DSP-4 treatment protected hippocampal cells from apoptotic cell death when compared to positive control and SAP + Gx +/- DSP-4 groups. In summary, elevated noradrenaline concentration following specific cholinergic denervation probably reflects compensatory hippocampal ingrowth originating from the peripheral sympathetic system which may be responsible for neuroprotective effects, i.e., antiapoptosis-like effect. Since cholinergic and noradrenergic systems are known to be involved in Alzheimer's disease and related cognitive function, knowing how these neurotransmitters work after specific lesions may be of importance as an animal model of Alzheimer's disease and as a potential target for Alzheimer's disease drug therapies.

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.

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.

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.

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.