Basal forebrain cholinergic cell attachment and neurite outgrowth on organotypic slice cultures of hippocampal formation

Cholinergic influences on cortical development and adult neurogenesis

In this review, we focus on immature neurons and their regulation by the cholinergic system, both during cortical development as well as during adult neurogenesis. We discuss various studies that indicate roles for acetylcholine in precursor development and neuronal differentiation. Cholinergic neurons projecting from the basal forebrain innervate the cerebral cortex during critical periods of neuronal development. Acetylcholine stimulation may help to promote a favourable environment for neuronal maturation. Afferents and their cortical target cells interact and are likely to influence each other during the establishment and refinement of connections. Intracortical cholinergic interneurons similarly have a local effect on cortical circuits. Reduced cholinergic innervation during development hence leads to reduced cortical thickness and dendritic abnormalities. Acetylcholine is also likely to play a critical role in neuronal plasticity, as shown in the visual and barrel cortices. Spontaneous nicotinic excitation is also important during a brief developmental window in the first postnatal weeks leading to waves of neural activity, likely to have an effect on neurite extension, target selection and synaptogenesis. In the hippocampus such activity plays a role in the maturation of GABAergic synapses during the developmental shift from depolarizing to hyperpolarizing transmission. The cholinergic system also seems likely to regulate hippocampal neurogenesis in the adult, positively promoting proliferation, differentiation, integration and potentially survival of newborn neurons.

Spontaneous neuronal burst discharges as dependent and independent variables in the maturation of cerebral cortex tissue cultured in vitro: a review of activity-dependent studies in live 'model' systems for the development of intrinsically generated bioelectric slow-wave sleep patterns

A survey is presented of recent experiments which utilize spontaneous neuronal spike trains as dependent and/or independent variables in developing cerebral cortex cultures when synaptic transmission is interfered with for varying periods of time. Special attention is given to current difficulties in selecting suitable preparations for carrying out biologically relevant developmental studies, and in applying spike-train analysis methods with sufficient resolution to detect activity-dependent age and treatment effects. A hierarchy of synchronized nested burst discharges which approximate early slow-wave sleep patterns in the intact organism is established as a stable basis for isolated cortex function. The complexity of reported long- and short-term homeostatic responses to experimental interference with synaptic transmission is reviewed, and the crucial role played by intrinsically generated bioelectric activity in the maturation of cortical networks is emphasized.

Optic nerve transection affects development and use-dependent plasticity in neocortex of the rat: Quantitative acetylcholinesterase imaging

We investigated the effects of neonatal optic nerve transection on cortical acetylcholinesterase (AChE) activity in hooded rats during postnatal development and following behavioral manipulation after weaning. AChE reaction product was quantified on digitized images of histochemically stained sections in layer IV of primary somatic sensory, primary visual and visual association cortex. Rats with optic nerve transection were compared to sham-operated littermates. In all cortical regions of both types of animal, AChE reaction product was increased to peak 2 weeks after birth and decreased thereafter, reaching adult levels at the end of the third postnatal week. During postnatal development, reaction product in primary visual cortex was lower in rats deprived of retinal input than in sham-operated littermates and the area delineated by reaction product was smaller. However, optic nerve transection did not modify the time course of postnatal development or statistically significantly diminish adult levels of AChE activity. Behavioral manipulations after weaning statistically significantly increased enzyme activity in sham-operated rats in all cortical areas examined. Compared with cage rearing, training in a discrimination task with food reward had a greater impact than environmental enrichment. By contrast, in the rats with optic nerve transection enrichment and training resulted in statistically significantly increased AChE activity only in lateral visual association cortex. Our findings provide evidence for intra- and supramodal influences of the neonatal removal of retinal input on neural activity- and use-dependent modifications of cortical AChE activity. The laminar distribution of the AChE reaction product suggests that the observed changes in AChE activity were mainly related to cholinergic basal forebrain afferents. These afferents may facilitate the stabilization of transient connections between the somatic sensory and the visual pathway.

Microtubule-associated targets in chlorpyrifos oxon hippocampal neurotoxicity

Prolonged exposure to organophosphate (OP) pesticides may produce cognitive deficits reflective of hippocampal injury in both humans and rodents. Recent work has indicated that microtubule trafficking is also adversely affected by exposure to the OP pesticide chlorpyrifos, suggesting a novel mode of OP-induced neurotoxicity. The present studies examined effects of prolonged exposure to chlorpyrifos oxon (CPO) on acetylcholinesterase (AChE) activity, immunoreactivity (IR) of microtubule-associated proteins, neuronal injury, and tubulin polymerization using in vitro organotypic slice cultures of rat hippocampus and bovine tubulin. Cultures were exposed to CPO (0.1-10 microM) in cell culture medium for 1-7 days, a regimen producing progressive reductions in AChE activity of 15-60%. Cytotoxicity (somatic uptake of the non-vital marker propidium iodide), as well as IR of alpha-tubulin and microtubule-associated protein-2 (a/b) [MAP-2], was assessed 1, 3, and 7 days after the start of CPO exposure. As early as 24 h after the start of exposure, CPO-induced deficits in MAP-2 IR were evident and progressive in each region of slice cultures at concentrations as low as 0.1 microM. CPO exposure did not alter alpha-tubulin IR at any time point. Concentration-dependent injury in the cornu ammonis (CA)1 pyramidal cell layer and to a lesser extent, CA3 and dentate cells, was evident 3 days after the start of CPO exposure (>or=0.1 microM) and was greatest after 7 days. Tubulin polymerization assays indicated that CPO (>or=0.1 microM) markedly inhibited the polymerization of purified tubulin and MAP-rich tubulin, though effects on MAP-rich tubulin were more pronounced. These data suggest that exposure to CPO produces a progressive decrease in neuronal viability that may be associated with impaired microtubule synthesis and/or function.

A role for neurotrophin-3 in targeting developing cholinergic axon projections to cerebral cortex

This study examined the relationship between expression of neurotrophin-3 (NT-3) and the ingrowth of cholinergic axonal projections in cerebral cortex. Patterns of expression of NT-3 (defined by beta-galactosidase reporter expression in heterozygous offspring of transgenic NT-3(lacZneo/+) mice) revealed that limbic cortical regions (including frontal, cingulate, and insular cortex, as well as the dentate gyrus) express NT-3 and that these cortical regions receive early and relatively dense cholinergic axons (stained for acetylcholinesterase, AChE). Using the dentate gyrus as a model system, studies revealed that expression of the NT-3 reporter parallels, and precedes by approximately 2 days, the ingrowth of AChE positive cholinergic axons. Studies of forebrain organotypic slice cultures demonstrate that basal forebrain-derived cholinergic axons extend into cortical regions in a pattern that mimics the pattern of expression of the NT-3 reporter. Similarly, chimeric co-cultures, combining wild type septum with a slice of hippocampus from heterozygous NT-3(lacZneo/+) mice, demonstrate that cholinergic axons grow into regions of the dentate gyrus that express the NT-3 reporter. Hemisphere slice cultures made from NT-3 knockout mice reveal cholinergic axonal growth into cortex, but these axons do not form the regional pattern characteristic of slice cultures made from wild type or heterozygous NT-3(lacZneo/+) mice. Further, chimeric co-cultures made using slices of wild type septum combined with slices of hippocampus from NT-3 knockout mice demonstrate robust cholinergic axonal growth into the hippocampus, but the cholinergic axons do not form the characteristic preterminal pattern associated with the dentate gyrus. Slice cultures from limbic cortical tissue from the NT-3 null mice do not display exaggerated levels of cell death. In aggregate, these data support the hypothesis that expression of NT-3 by cortical neurons serves to attract basal forebrain cholinergic projections to their target cells in cerebral cortex.

Patterns of afferent projections to the dentate gyrus studied in organotypic co-cultures

Cholinergic axons originating from the septum form a characteristic layer of preterminal axons and apparent termination in the molecular layer of the hippocampal dentate gyrus. The present study explored the specificity of this characteristic axonal pattern, through the use of organotypic slice co-cultures. Slices of hippocampus were co-cultured with a slice from one of a variety of other potential sources of afferents, and the afferent axons were labeled histochemically or immunocytochemically to determine which afferents distribute within the dentate molecular layer in a pattern similar to that formed by septal cholinergic projections. Acetylcholinesterase (AChE) histochemistry demonstrated that cholinergic axons from septum, substantia innominata, and striatum all consistently targeted the inner molecular layer of the dentate gyrus. AChE-labeled cholinergic axons from dorsal lateral pontine tegmentum and from spinal cord sometimes formed this pattern, while axons from the habenula failed to extend into the dentate gyrus. Immunocytochemically identified monoaminergic axons from the substantia nigra, locus coeruleus, and raphe extended into co-cultured hippocampus; each of these afferent systems displayed a prominent axonal plexus within the hilus of the dentate, but only the raphe axons projected prominently to the molecular layer. These data demonstrate that the molecular layer of the dentate gyrus provides an attractive target zone for some cholinergic and monoaminergic afferents, but not all. Commonalities between neuronal populations that preferentially project to the molecular layer in vitro may offer clues regarding the axon guidance mechanisms that normally direct cholinergic axons to target sites in the dentate gyrus molecular layer.