Development of the cholinergic fibres innervating the cerebral cortex of the rat

Examination of cortically projecting cholinergic neurons following exercise and environmental intervention in a rodent model of fetal alcohol spectrum disorders

BACKGROUND

Up to 1 in 5 infants in the United States are exposed to alcohol prenatally, resulting in neurodevelopmental deficits categorized as fetal alcohol spectrum disorders (FASD). Choline supplementation ameliorates some deficits, suggesting that alcohol exposure (AE) perturbs cholinergic neurotransmission and development. Behavioral interventions, which upregulate cholinergic neurotransmission, rescue cognitive deficits in rodent models of FASD.

METHODS

We investigated the impacts of two interventions (either wheel-running (WR) or "super intervention," WR plus exposure to a complex environment) on cholinergic neuronal morphology in the nucleus basalis of Meynert (NBM), the source of cortical cholinergic input, and prefrontal cortex (PFC) in a rodent model of FASD. One third of the total 47 male pups received intragastric intubation of ethanol in milk substitute during postnatal days (PD) 4-9. Another third served as sham-intubated procedural controls while the final third served as suckle controls. Rats from each group were exposed to either intervention during PD 30-72. Choline acetyltransferase (ChAT⁺ ) and acetylcholinesterase staining were used to quantify cholinergic neuron number, soma volume, and axon number.

RESULTS

Our data indicate a main effect of postnatal treatment on ChAT⁺ neuron number in NBM in adulthood. Post hoc analysis demonstrates that ChAT⁺ neuron number is reduced in AE compared to suckle control rodents (p < .01).

CONCLUSIONS

We examined the cytoarchitectonics of cholinergic neurons in NBM and PFC in adulthood following early postnatal AE and two interventions. We show that AE reduces ChAT⁺ neuron number in NBM, and this is not mitigated by either intervention.

Behavioral states modulate sensory processing in early development

Purpose of Review

Sleep-wake states modulate cortical activity in adults. In infants, however, such modulation is less clear; indeed, early cortical activity comprises bursts of neural activity driven predominantly by peripheral sensory input. Consequently, in many studies of sensory development in rodents, sensory processing has been carefully investigated, but the modulatory role of behavioral state has typically been ignored.

Recent Findings

In the developing visual and somatosensory systems, it is now known that sleep and wake states modulate sensory processing. Further, in both systems, the nature of this modulation shifts rapidly during the second postnatal week, with subcortical nuclei changing how they gate sensory inputs.

Summary

The interactions among sleep and wake movements, sensory processing, and development are dynamic and complex. Now that established methods exist to record neural activity in unanesthetized infant animals, we can provide a more comprehensive understanding of how infant sleep-wake states interact with sensory-driven responses to promote developmental plasticity.

Developmental suppression of forebrain trkA receptors and attentional capacities in aging rats: A longitudinal study

Basal forebrain (BF) cholinergic neurons innervating the cortex regulate cognitive, specifically attentional, processes. Cholinergic atrophy and cognitive decline occur at an accelerated pace in age-related neurodegenerative disorders such as Alzheimer's disease; however, the mechanism responsible for this phenomenon remains unknown. Here we hypothesized that developmental suppression of nerve growth factor signaling, mediated via tropomyosin-related kinase A (trkA) receptors, would escalate age-related attentional vulnerability. An adeno-associated viral vector expressing trkA shRNA (AAV-trkA) was utilized to knockdown trkA receptors in postnatal rats at an ontogenetic time point when cortical cholinergic inputs mature, and the impact of this manipulation on performance was assessed in animals maintained on an operant attention task throughout adulthood and until old (24 months) age. A within-subject comparison across different time points illustrated a gradual age-related decline in attentional capacities. However, the performance under baseline and distracted conditions did not differ between the AAV-trkA-infused and animals infused with a vector expressing shRNA against the control protein luciferase at any time point. Additional analysis of cholinergic measures conducted at 24 months showed that the capacity of cholinergic terminals to release acetylcholine following a depolarizing stimulus, cortical cholinergic fiber density and BF cholinergic cell size remained comparable between the two groups. Contrary to our predictions, these data indicate that developmental BF trkA disruption does not impact age-related changes in attentional functions. It is possible that life-long engagement in cognitive activity might have potentially rescued the developmental insults on the cholinergic system, thus preserving attentional capacities in advanced age.

Ontogeny of mammalian metabolizing enzymes in humans and animals used in toxicological studies

It is well recognized that expression of enzymes varies during development and growth. However, an in-depth review of this acquired knowledge is needed to translate the understanding of enzyme expression and activity into the prediction of change in effects (e.g. kinetics and toxicity) of xenobiotics with age. Age-related changes in metabolic capacity are critical for understanding and predicting the potential differences resulting from exposure. Such information may be especially useful in the evaluation of the risk of exposure to very low (µg/kg/day or ng/kg/day) levels of environmental chemicals. This review is to better understand the ontogeny of metabolizing enzymes in converting chemicals to either less-toxic metabolite(s) or more toxic products (e.g. reactive intermediate[s]) during stages before birth and during early development (neonate/infant/child). In this review, we evaluated the ontogeny of major "phase I" and "phase II" metabolizing enzymes in humans and commonly used experimental animals (e.g. mouse, rat, and others) in order to fill the information gap.

Neural plasticity expressed in central auditory structures with and without tinnitus

Sensory training therapies for tinnitus are based on the assumption that, notwithstanding neural changes related to tinnitus, auditory training can alter the response properties of neurons in auditory pathways. To assess this assumption, we investigated whether brain changes induced by sensory training in tinnitus sufferers and measured by electroencephalography (EEG) are similar to those induced in age and hearing loss matched individuals without tinnitus trained on the same auditory task. Auditory training was given using a 5 kHz 40-Hz amplitude-modulated (AM) sound that was in the tinnitus frequency region of the tinnitus subjects and enabled extraction of the 40-Hz auditory steady-state response (ASSR) and P2 transient response known to localize to primary and non-primary auditory cortex, respectively. P2 amplitude increased over training sessions equally in participants with tinnitus and in control subjects, suggesting normal remodeling of non-primary auditory regions in tinnitus. However, training-induced changes in the ASSR differed between the tinnitus and control groups. In controls the phase delay between the 40-Hz response and stimulus waveforms reduced by about 10° over training, in agreement with previous results obtained in young normal hearing individuals. However, ASSR phase did not change significantly with training in the tinnitus group, although some participants showed phase shifts resembling controls. On the other hand, ASSR amplitude increased with training in the tinnitus group, whereas in controls this response (which is difficult to remodel in young normal hearing subjects) did not change with training. These results suggest that neural changes related to tinnitus altered how neural plasticity was expressed in the region of primary but not non-primary auditory cortex. Auditory training did not reduce tinnitus loudness although a small effect on the tinnitus spectrum was detected.

Methamphetamine exposure during brain development alters the brain acetylcholine system in adolescent mice

Children exposed to methamphetamine during brain development as a result of maternal drug use have long-term hippocampus-dependent cognitive impairments, but the mechanisms underlying these impairments are not understood. The acetylcholine system plays an important role in cognitive function and potential methamphetamine-induced acetylcholine alterations may be related to methamphetamine-induced cognitive impairments. In this study, we investigated the potential long-term effects of methamphetamine exposure during hippocampal development on the acetylcholine system in adolescence mice on postnatal day 30 and in adult mice on postnatal day 90. Methamphetamine exposure increased the density of acetylcholine neurons in regions of the basal forebrain and the area occupied by acetylcholine axons in the hippocampus in adolescent female mice. In contrast, methamphetamine exposure did not affect the density of GABA cells or total neurons in the basal forebrain. Methamphetamine exposure also increased the number of muscarinic acetylcholine receptors in the hippocampus of adolescent male and female mice. Our results demonstrate for the first time that methamphetamine exposure during hippocampal development affects the acetylcholine system in adolescent mice and that these changes are more profound in females than males.

Development of the motivational system during adolescence, and its sensitivity to disruption by nicotine

The brain continues to develop during adolescence, and exposure to exogenous substances such as nicotine can exert long-lasting adaptations during this vulnerable period. In order to fully understand how nicotine affects the adolescent brain it is important to understand normal adolescent brain development. This review summarizes human and animal data on brain development, with emphasis on the prefrontal cortex, for its important function in executive control over behavior. Moreover, we discuss how nicotine exposure during adolescence can disrupt brain development bearing long-term consequences on executive cognitive function in adulthood.

Cholinergic modulation of spindle bursts in the neonatal rat visual cortex in vivo

Acetylcholine (ACh) is known to shape the adult neocortical activity related to behavioral states and processing of sensory information. However, the impact of cholinergic input on the neonatal neuronal activity remains widely unknown. Early during development, the principal activity pattern in the primary visual (V1) cortex is the intermittent self-organized spindle burst oscillation that can be driven by the retinal waves. Here, we assessed the relationship between this early activity pattern and the cholinergic drive by either blocking or augmenting the cholinergic input and investigating the resultant effects on the activity of the rat visual cortex during the first postnatal week in vivo. Blockade of the muscarinic receptors by intracerebroventricular, intracortical, or supracortical atropine application decreased the occurrence of V1 spindle bursts by 50%, both the retina-independent and the optic nerve-mediated spindle bursts being affected. In contrast, blockade of acetylcholine esterase with physostigmine augmented the occurrence, amplitude, and duration of V1 spindle bursts. Whereas direct stimulation of the cholinergic basal forebrain nuclei increased the occurrence probability of V1 spindle bursts, their chronic immunotoxic lesion using 192 IgG-saporin decreased the occurrence of neonatal V1 oscillatory activity by 87%. Thus, the cholinergic input facilitates the neonatal V1 spindle bursts and may prime the developing cortex to operate specifically on relevant early (retinal waves) and later (visual input) stimuli.

Development of Cholinergic Modulation and Graded Persistent Activity in Layer V of Medial Entorhinal Cortex

During muscarinic modulation, principal neurons from layer V of rat medial entorhinal cortex (mEC) respond to repeated applications of a brief stimulus with a graded change in persistent firing frequency. This pattern of discharge has been proposed to represent an intrinsic mechanism for short-term memory operations. To investigate the implementation of persistent activity in mEC during development, we characterized the electrophysiological properties of layer V principal neurons in the mEC over a range of postnatal stages. We observed significant differences in both passive (resistance, time constant, and resting membrane potential) and active properties (threshold, action potential, and adaptation) of principal neurons from rats aged 5–7, 10–13, 16–19, and 21–23 days. We also examined the properties of muscarinic-dependent persistent activity in EC slices from different age groups. Recordings were conducted using the perforated-patch whole cell technique because persistent activity runs down in the ruptured-patch configuration. Although no neuron in the youngest group exhibited graded persistent activity in response to muscarinic receptor activation, this activity was recorded in the 10- to 13-day-old group and its occurrence increased from 69% in the 16- to 19-day-old group to 76% in the 21- to 23-day-old group. This postnatal increase in neurons endowed with persistent firing properties in mEC was found to parallel the increase in density of ChAT-positive immunostaining of fibers and the developmental changes in M1 muscarinic receptor mRNA levels. All these data suggest that the implementation of mnemonic properties in mEC principal neurons matches the ontogenic development of afferent cholinergic circuits and their signaling components.

Age-Related Brain Cholinesterase Inhibition Kinetics following In Vitro Incubation with Chlorpyrifos-Oxon and Diazinon-Oxon

Chlorpyrifos and diazinon are two commonly used organophosphorus insecticides (OPs), and their primary mechanism of action involves the inhibition of acetylcholinesterase by their metabolites chlorpyrifos-oxon (CPO) and diazinon-oxon (DZO), respectively. The study objectives were to assess the in vitro age-related inhibition kinetics of neonatal rat brain cholinesterase (ChE) for CPO and DZO by estimating the bimolecular inhibitory rate constant (k(i)) values. Brain ChE inhibition and k(i) values following CPO and DZO incubation with neonatal Sprague-Dawley rat brain homogenates were determined at postnatal day (PND) 5, 12, and 17 and compared with the corresponding inhibition and k(i) values obtained in the adult rat. A modified Ellman method was utilized for measuring the ChE activity. CPO caused a greater ChE inhibition than DZO as evidenced from the estimated k(i) values of both compounds. Neonatal brain ChE inhibition kinetics exhibited a marked age-related sensitivity to CPO, with the order of ChE inhibition being PND 5 > PND 7 > PND 17 with k(i) values of 0.95, 0.50, and 0.22 nM(-1)hr(-1), respectively. In contrast, DZO ChE inhibition was not age related in the neonatal brain, and the estimated k(i) value at all PND ages was 0.02 nM(-1)hr(-1). These results demonstrated an age- and OP-selective inhibition of rat brain ChE, which may be critically important in understanding the potential sensitivity of juveniles to specific OPs exposures.

Developmental changes in presynaptic muscarinic modulation of excitatory and inhibitory neurotransmission in rat piriform cortex in vitro: relevance to epileptiform bursting susceptibility

Suppression of depolarizing postsynaptic potentials and isolated GABA-A receptor-mediated fast inhibitory postsynaptic potentials by the muscarinic acetylcholine receptor agonist, oxotremorine-M (10 microM), was investigated in adult and immature (P14-P30) rat piriform cortical (PC) slices using intracellular recording. Depolarizing postsynaptic potentials evoked by layers II-III stimulation underwent concentration-dependent inhibition in oxotremorine-M that was most likely presynaptic and M2 muscarinic acetylcholine receptor-mediated in immature, but M1-mediated in adult (P40-P80) slices; percentage inhibition was smaller in immature than in adult piriform cortex. In contrast, compared with adults, layer Ia-evoked depolarizing postsynaptic potentials in immature piriform cortex slices in oxotremorine-M, showed a prolonged multiphasic depolarization with superimposed fast transients and spikes, and an increased 'all-or-nothing' character. Isolated N-methyl-d-aspartate receptor-mediated layer Ia depolarizing postsynaptic potentials (although significantly larger in immature slices) were however, unaffected by oxotremorine-M, but blocked by dl-2-amino-5-phosphonovaleric acid. Fast inhibitory postsynaptic potentials evoked by layer Ib or layers II-III-fiber stimulation in immature slices were significantly smaller than in adults, despite similar estimated mean reversal potentials ( approximately -69 and -70 mV respectively). In oxotremorine-M, only layer Ib-fast inhibitory postsynaptic potentials were suppressed; suppression was again most likely presynaptic M2-mediated in immature slices, but M1-mediated in adults. The degree of fast inhibitory postsynaptic potential suppression was however, greater in immature than in adult piriform cortex. Our results demonstrate some important physiological and pharmacological differences between excitatory and inhibitory synaptic systems in adult and immature piriform cortex that could contribute toward the increased susceptibility of this region to muscarinic agonist-induced epileptiform activity in immature brain slices.

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.

The acetylcholine fiber density of the neocortex is altered by isolated rearing and early methamphetamine intoxication in rodents

Alterations in the cholinergic physiology of the brain were the first to be observed when research on environmental influences on postnatal brain development began 35 years ago. Since then, the effects of isolated rearing (IR) or early pharmacological insults have been shown not only on the physiology, but also the anatomy of a variety of transmitter systems. The cholinergic fiber density, however, still remained to be assessed. We therefore used a histochemical procedure to stain cholinergic fibers in the brains of young adult gerbils reared either in groups in enriched environments or isolated in standard makrolon cages. Half of the animals from each rearing condition had received a single high dose of methamphetamine on postnatal day 14. Fiber densities were measured by computerized image analysis in the medial and orbital prefrontal cortex (PFC), dysgranular and granular insular cortex, sensorimotor cortices, and the entorhinal cortex of both hemispheres. Isolation rearing increased the cholinergic fiber densities in the prefrontal cortices of the left hemisphere and in the entorhinal cortex of the right hemisphere by about 10%, with no effect in the respective contralateral side. The early methamphetamine intoxication showed no influence in prefrontal and entorhinal cortices, but diminished the acetylcholine (ACh) innervation of the forelimb area of cortex in both hemispheres in IR gerbils and of the left hemisphere in ER gerbils, and reduced the acetylcholine innervation in the hindlimb area in both sides in both rearing groups. These results demonstrate that (a) cholinergic fiber density is differentially regulated in different cortical areas and (b) the plasticity of the cholinergic system can only be understood in the interplay with other neuromodulatory innervations.

Functional nicotinic acetylcholine receptors on subplate neurons in neonatal rat somatosensory cortex

The establishment of cortical synaptic circuits during early development requires the presence of subplate neurons (SPn's), a heterogeneous population of neurons capable to integrate and process synaptic information from the thalamus, cortical plate, and neighboring SPn's. An accumulation of cholinergic afferents and nicotinic acetylcholine receptors (nAChRs) has been documentated in the subplate around birth. To assess the developmental role of the cholinergic innervation onto SPn's, we used whole cell patch-clamp recordings of visually identified and biocytin-labeled SPn's in neonatal rat somatosensory cortex. Functional nAChRs were present in 92% of the investigated SPn's. Activation of postsynaptic nAChRs by local application of agonists elicited a brief membrane depolarization associated with a barrage of action potentials and large inward currents reversing around 0 mV. According to our pharmacological data, excitation of SPn's is mediated by alpha4beta2 receptors. In contrast, functional alpha7 nAChRs could not be identified on SPn's. Activation of nAChRs affected neither the spontaneous synaptic activity of SPn's nor the synaptic connections between thalamus and SPn's and within subplate. Nicotine, at concentrations reaching the developing brain by maternal smoking, induced a severe desensitization of nAChRs and an increase in the baseline noise. These results indicate that nAChR-mediated excitation of SPn's may stabilize the developing synaptic circuits and suggest the involvement of nAChRs located on SPn's in the fetal tobacco syndrome.

Basal Forebrain Cholinergic System Is Involved in Rapid Nerve Growth Factor (NGF)-Induced Plasticity in the Barrel Cortex of Adult Rats

We have previously reported that topical application of nerve growth factor (NGF) to the barrel cortex of an adult rat rapidly augmented a whisker functional representation (WFR) by increasing its area and height within minutes after NGF application. In addition, we found that TrkA, the high-affinity NGF receptor, was only found on fibers projecting into the barrel cortex. Here we use a combination of techniques including chronic intrinsic signal optical imaging, neuronal fiber tracking and immunohistological techniques, to test the hypothesis that NGF-induced rapid cortical plasticity is mediated by the cortical projections of the basal forebrain cholinergic system (BFCS). Our studies localize the source of the cells in the BFCS that project to a single WFR and also demonstrate that TrkA-immunoreactive fibers in the cortex are also cholinergic and likely arise from the BFCS. In addition, by selectively lesioning the BFCS cortical fibers with the immunotoxin 192 IgG-saporin, we show that NGF-induced WFR-cortical plasticity is eliminated. These results, taken together with our previously reported imaging results that demonstrated that agonists of the cholinergic system (particularly nicotine) showed transient NGF-like augmentations of a WFR, implicate the BFCS cortical projections as necessary for NGF's rapid plasticity in the adult rat somatosensory cortex.

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.

The cholinergic innervation develops early and rapidly in the rat cerebral cortex: a quantitative immunocytochemical study

A recently developed method for determining the length of cholinergic axons and number of cholinergic axon varicosities (terminals) in brain sections immunostained for choline acetyltransferase was used to estimate the areal and laminar densities of the cholinergic innervation in rat frontal (motor), parietal (somatosensory) and occipital (visual) cortex at different postnatal ages. This cortical innervation showed an early beginning, a few immunostained fibers being already present in the cortical subplate at birth. In the first two postnatal weeks, it developed rapidly along three parameters: a progressive increase in the number of varicosities per unit length of axon, and a lengthening and branching of the axons. Between postnatal days 4 and 16, the number of varicosities increased steadily from two to four per 10 microm of cholinergic axon. The mean densities of cholinergic axons increased from 1.4 to 9.6, 1.7 to 9.3 and 0.7 to 7.2 m/mm(3), and the corresponding densities of varicosities from 0.4 to 3.9, 0.4 to 3.5, and 0.2 to 2.6x10(6)/mm(3) in the frontal, parietal and occipital areas, respectively. The rate of growth was maximal during these first two weeks, after which the laminar pattern characteristic of each area appeared to be established. Adult values were almost reached by postnatal day 16 in the parietal cortex, but maturation proceeded further in the frontal and particularly in the occipital cortex. These quantitative data on the ingrowth and maturation of the cholinergic innervation in postnatal rat cerebral cortex substantiate a role for acetylcholine in the development of this brain region and emphasize the striking growth capacity of individual cholinergic neurons.

Functional nicotinic acetylcholine receptor expression in stem and progenitor cells of the early embryonic mouse cerebral cortex

The adult cerebral cortex contains nicotinic acetylcholine (ACh) receptors vital to cortical function. However, little is known about the assembly of embryonic nicotinic receptor subunits into functional receptors or whether they play an active role in cortical development. We now report evidence of functional nicotinic acetylcholine receptor channels in fetal mouse cerebral cortex as early as embryonic day 10 (E10), when the cortex consists of dividing stem and progenitor cells. Patch-clamp electrophysiological measurements indicate that nicotine and ACh evoke sizable inward currents characteristic of nicotinic receptors, that are strongly rectifying with a reversal potential near 0 mV. Three different nicotinic agonists, ACh, nicotine, and dimethylphenylpiperazinium, evoked cytosolic Ca(2+) signals. Agonist-evoked Ca(2+) signals and electrophysiological responses were found in greater than 70% of all E10-E11 cells tested and were blocked by nicotinic receptor antagonists. The Ca(2+) response to nicotinic agonists was markedly prolonged in cells from early embryonic stages relative to later stages of development. alpha3, alpha4, and alpha7 receptor subunit proteins were detected immunocytochemically in cortical cells from E10 to birth. The incidence of each subunit declined with embryonic age, suggesting a role in early development. We discuss the possible function of nicotinic receptors in early cortical development and their role as a target for nicotine in the developmental pathologies associated with the fetal tobacco syndrome.

Postnatal development of cholinergic system in mouse basal forebrain: acetylcholinesterase histochemistry and choline-acetyltransferase immunoreactivity

The distribution of acetylcholinesterase histochemistry and choline-O-acetyltransferase immunohistochemistry in the basal forebrain was studied in newborn mice (P0) and until 60 days of postnatal life (P60). A weak acetylcholinesterase activity was found at P0 and P2 in the anterior and intermediate parts of the basal forebrain, and higher in the posterior region. The intensity of labeling, neuronal size and dendritic growth seems to increase progressively in all regions of basal forebrain from P4 to P10. The AChE+ cell count shows that in the anterior portion of the magnocellular basal nucleus the number of cells does not vary significantly from birth to the second month of postnatal life. However, in the intermediate and posterior portions of the nucleus the mean number of labeled cells increases significantly from birth to the end of the second week of postnatal life (P13). The choline-acetyltransferase immunoreactivity appears only detectable at the end of the first week (P6) as a slight immunoreaction, which increases progressively in intensity at P8, and at P10 seems to attain the same intensity of labeling found at P60. These results seem to indicate that the acetylcholinesterase could have a non-classic cholinergic role in the first stages of postnatal development, acting as a growth and cellular differentiation factor.

Cell-cycle kinetics of neocortical precursors are influenced by embryonic thalamic axons

Thalamic afferents are known to exert a control over the differentiation of cortical areas at late stages of development. Here, we show that thalamic afferents also influence early stages of corticogenesis at the level of the ventricular zone. Using an in vitro approach, we show that embryonic day 14 mouse thalamic axons release a diffusable factor that promotes the proliferation of cortical precursors over a restricted developmental window. The thalamic mitogenic effect on cortical precursors (1) shortens the total cell-cycle duration via a reduction of the G(1) phase; (2) facilitates the G(1)/S transition leading to an increase in proliferative divisions; (3) is significantly reduced by antibodies directed against bFGF; and (4) influences the proliferation of both glial and neuronal precursors and does not preclude the action of signals that induce differentiation in these two lineages. We have related these in vitro findings to the in vivo condition: the organotypic culture of cortical explants in which anatomical thalamocortical innervation is preserved shows significantly increased proliferation rates compared with cortical explants devoid of subcortical afferents. These results are in line with a number of studies at subcortical levels showing the control of neurogenesis via afferent fibers in both vertebrates and invertebrates. Specifically, they indicate the mechanisms whereby embryonic thalamic afferents contribute to the known early regionalization of the ventricular zone, which plays a major role in the specification of neocortical areas.

Effect of neonatal ethanol exposure on parvalbumin-expressing GABAergic neurons of the rat medial septum and cingulate cortex

This study was performed to determine the long-term effects of ethanol exposure during the brain growth spurt (postnatal days 4-10) on the number of parvalbumin-immunoreactive (PA+) GABAergic neurons in the adult (P60) rat medial septum and anterior cingulate cortex. Significant loss of neurons within each of these populations has previously been demonstrated following prenatal ethanol exposure. In the present study, no significant differences in the number of PA+ neurons were found in either the medial septum or the cingulate cortex when control and ethanol-exposed animals were compared. The cellular densities and volumetric measures in both brain regions were also similar in the two groups. We speculate that compensatory up-regulative mechanisms may have accounted for the protection of the PA neuronal populations in these two areas following the early neonatal exposure.

The alpha7 nicotinic acetylcholine receptor in neuronal plasticity

A growing body of evidence indicates that neuronal nicotinic acetylcholine receptors (nAChRs), in addition to promoting fast cholinergic transmission, may modulate other neuronal activities within the central nervous system (CNS). In particular, the alpha7 nAChR is highly permeable to Ca2+ and may serve a distinct role in regulating neuronal plasticity. By elevating intracellular Ca2+ levels in discrete neuronal locations, these ligand-gated ion channels may influence numerous physiological processes in developing and adult CNS. In this article, we review evidence that both pre- and postsynaptic alpha7 nAChRs modulate transmitter release in the brain and periphery through Ca2+-dependent mechanisms. The possible role of alpha7 nAChRs in regulating neuronal growth and differentiation in developing CNS is also evaluated. We consider an interaction between cholinergic and glutamatergic transmission and propose a hypothesis on the possible coregulation of intracellular Ca2+ by N-methyl-D-aspartate (NMDA) receptors and alpha7 nAChRs. Finally, the clinical significance of alterations in the normal function of alpha7 nAChRs is discussed as it pertains to prenatal nicotine exposure, schizophrenia, and epilepsy.

Developmental and aging aspects of the cholinergic innervation of the olfactory bulb

The olfactory bulb is a limbic paleocortex which receives monosynaptic sensory afferents from the olfactory mucosa, and a strong direct cholinergic input from the basal forebrain. This review focuses on the rat olfactory bulb as a suitable model to study cholinergic involvements in cortical processing, during development, adulthood and aging. Anatomical and biochemical data show that cholinergic influences upon the bulbar neuronal network are exerted through several types of target cells and receptors (muscarinic and nicotinic). Functional data indicate that cholinergic afferents to the olfactory bulb are involved in local events related to olfactory learning. Neurodegenerative disorders such as Alzheimer's disease involve early olfactory deficits and typical histopathological lesions in the olfactory bulb. In summary, with its exclusively extrinsic cholinergic innervation and direct sensory input, the rat olfactory bulb offers the opportunity to study the cellular and molecular mechanisms of cholinergic influences on cortical processing, in both normal and pathological conditions.

Time course of changes in cholinergic and neurotrophin-related markers after infusion of colchicine into the basal forebrain

After bilateral infusions of colchicine or vehicle in the rat nucleus basalis magnocellularis, the time course of changes in several cholinergic and neurotrophin-related markers were assessed. Animals were sacrificed at 3, 7, 14, 28, 35 and 84 days post-lesion, and both the NBM and cortical areas were assessed. Sections were stained immunohistochemically for choline acetyltransferase (ChAT) or p140trk (trk) or histochemically for acetylcholinesterase (AChE). ChAT activity and neurotrophin protein levels were assessed regionally. The number of ChAT immunoreactive NBM neuronal profiles decreased beginning 3 days post-lesion and reach maximal loss by 28 days post-lesion, with no recovery. Examination of trk-IR around the NBM revealed a time-dependent decrease in trk-IR of magnocellular neuron and an increase in trk-IR of astrocytes at 14 and 28 days post-lesion. The density of AChE-stained cortical fibers was maximally decreased 3 days post-lesion followed by an increase in fiber staining across the remaining time points. Cortical ChAT activity showed the largest decrease at 7 days followed by recovery 84 days after colchicine infusion. There was an increase in NGF in the parietal cortex after colchicine infusion but no change in BDNF level. These patterns of changes in the cholinergic and neurotrophin-related markers suggest an association between NGF and lesion-induced compensatory responses in the basal forebrain cholinergic system.

Specificity of attachment and neurite outgrowth of dissociated basal forebrain cholinergic neurons seeded on to organotypic slice cultures of forebrain

Development and differentiation of basal forebrain-derived cholinergic neurons were studied using a new technique that combines dissociated cell cultures with organotypic slice cultures. Slices of cerebral cortex or entire forebrain hemispheres were taken from early postnatal rat pups and maintained as organotypic cultures on membranes. Dissociated cell suspensions of basal forebrain tissue, taken from rat or mouse fetuses at gestational day 15-17, were seeded on to the slice cultures. Combined cultures were maintained for two to 14 days in vitro. Cultures processed for acetylcholinesterase histochemical staining demonstrated that stained neurons display regional variation in attachment to the slice, with most attachment occurring on cortex and with no detectable attachment on the caudate-putamen. Regional differences in attachment occur between cortical areas, with medial (cingulate) cortex showing much denser cell attachment than lateral (parietal) cortex, and across cortical layers, with layer I and deep layers showing more attachment than middle cortical layers. Similar patterns were observed on slices from rat brain irrespective of whether rat or mouse dissociated cells were used. Tyrosine hydroxylase-stained dissociated cells from ventral midbrain displayed a different pattern of attachment, with prominent attachment to the caudate putamen and less apparent specificity of regional and cortical laminar attachment. Little evidence of neurite outgrowth occurred during the first two days in vitro, but by four days, acetylcholinesterase-positive basal forebrain cells displayed several short and thick neurites that appeared to be dendrites, and one long process that appeared to be an axon. By seven days in vitro, dendrites are well developed and the presumed axon has extended branches over wide areas of cortex. These studies revealed several different types of cell-tissue interaction. The degree of cell growth and differentiation ranged from robust growth when dissociated cells were seeded on to slice cultures of normal target tissue, to apparently no attachment or growth when cells were seeded on to non-target tissue. This combined technique appears to be a useful method for studies of specificity of cell attachment and patterns of neurite outgrowth.

Thalamic and basal forebrain afferents modulate the development of parvalbumin and calbindin D28k immunoreactivity in the barrel cortex of the rat

In the adult barrel cortex of the rat the calcium-binding proteins calbindin D28k (CALB) and parvalbumin (PARV) are found in separate populations of GABAergic nonpyramidal neurons. In layers II to IV of the barrel cortex most PARV-immunoreactive neurons are likely to derive from a subpopulation of CALB-immunoreactive neurons whose CALB immunoreactivity ceases when they begin to express PARV between the second and third postnatal weeks. The aim of this study was to investigate the influence of subcortical afferents on the neurochemical differentiation of cortical PARV- and CALB-immunoreactive nonpyramidal neurons during development of the barrel cortex. We produced unilateral excitotoxic lesions with a single injection of ibotenic acid (0.5 microl, 0.05 M) in different subcortical nuclei in 7- to 8-day-old rats. Lesions involving the ventroposterior thalamic nuclei resulted in delayed development of PARV and CALB immunoreactivity in the barrel cortex. One week after ibotenic acid injections a transient decrease in the number of PARV-immunoreactive neurons in layer IV was observed, together with increased numbers of CALB-immunoreactive neurons in all cortical layers. The number of nonpyramidal neurons displaying coexistence of PARV and CALB in the lesioned hemisphere also increased compared with the numbers in the control hemisphere or control littermates. In contrast, lesions affecting the globus pallidus, zona incerta and reticular thalamic nucleus transiently increased the number of PARV-immunoreactive neurons in layers II and III, but had no effect on the number of CALB-positive cells. From 3 weeks onwards no differences were found between control and lesioned hemispheres after injections into either the ventroposterior thalamic nuclei or the magnocellular basal forebrain. These results suggest that CALB and PARV expression in nonpyramidal cortical neurons can be reversibly modulated in opposite directions by different cortical afferents during postnatal development.

Regulation of alpha7 nicotinic acetylcholine receptors in the developing rat somatosensory cortex by thalamocortical afferents

Distributions of alpha7 nicotinic acetylcholine receptor (nAChR) mRNA and [125]alpha-bungarotoxin (alpha-BTX) binding sites in the developing rat somatosensory cortex were characterized in relation to acetylcholinesterase (AChE) histochemical staining of thalamocortical terminals to investigate the role of this receptor in cortical development. Using quantitative in situ hybridization and receptor autoradiography, elevated levels of mRNA and binding-site expression were first detected at post-natal day 1 (P1) in deep and superficial layers, just beneath the AChE-stained thalamocortical terminals. Onset of expression occurred approximately 1 d after ingrowth of AChE-stained thalamocortical afferents. By P5, mRNA and binding-site expression exhibited a disjunctive, barrel-like pattern in layer IV and, more clearly, in layer VI. The mRNA and binding-site expressions peaked at approximately 1 week postnatal and then declined to adult levels. Unilateral electrolytic or cytochemical lesions placed in the thalamic ventrobasal complex at P0 (just as thalamocortical afferents are innervating the cortex) and at P6 (when the somatotopic map is well established) resulted in a marked reduction of alpha7 nAChR mRNA and [125]alpha-BTX binding-site levels in layers IV and VI, indicating their regulation by thalamocortical afferents. With P6 lesions, this reduction was observed as early as 6 hr postlesion. These results suggest that alpha7 nAChRs are localized primarily on cortical cells in rat somatosensory cortex and provide further evidence for thalamocortical influence on cortical ontogeny. These data also suggest a role for cholinergic systems during a critical period of cortical synaptogenesis.

Cholinergic innervation of cerebral cortex in organotypic slice cultures: sustained basal forebrain and transient striatal cholinergic projections

Slices of entire forebrain hemispheres were taken from early postnatal rat pups and maintained as organotypic slice cultures. Basal forebrain cholinergic neurons, identified by histochemical staining for acetylcholinesterase, develop axons that grow rapidly into cerebral cortex. Ingrowth occurs by two routes: some axons course laterally from the basal forebrain region to reach lateral neocortex; others course dorsally from the septum to reach medial cortex. By one to two weeks in vitro, acetylcholinesterase-positive axons have extended throughout most of the cortical territory. In addition to basal forebrain cholinergic axons, the normally local circuit cholinergic neurons of the striatum also send axons into cerebral cortex. These striatum-derived axons can be distinguished from basal forebrain axons by their distinct morphological characteristics and by their different response to excision of the striatum or basal forebrain. Further, acetylcholinesterase-positive axons in cortex that originate from striatum appear to retract or degenerate after about one week in culture, while those from basal forebrain remain present and apparently healthy beyond two weeks. These data document the basal forebrain cholinergic ingrowth into cerebral cortex using this whole hemisphere slice culture system and also demonstrate different degrees of maintenance of cortical afferents that are derived from different subcortical sources.

Comparative ontogenic profile of cholinergic markers, including nicotinic and muscarinic receptors, in the rat brain

The ontogenic profiles of several cholinergic markers were assessed in the rat brain by using quantitative in vitro receptor autoradiography. Brain sections from animals at different stages of development were processed with [3H]AH5183 (vesamicol; vesicular acetylcholine transport sites), [3H]N-methylcarbamylcholine (alpha(4)beta(2) nicotinic receptor sites), [3H]hemicholinium-3 (high-affinity choline uptake sites), [3H]3-quinuclidinyl benzilate (total population of muscarinic receptor sites), [3H]4-DAMP (muscarinic M1/M3 receptor sites), [3H]pirenzepine (muscarinic M1 receptor sites), and [3H]AF-DX 116 and [3H]AF-DX 384 (muscarinic M2 receptor sites) as radiolabeled probes. The results revealed that, by the end of the prenatal period (embryonic day 20), the densities of nicotinic receptor and vesicular acetylcholine transport sites already represented a considerable proportion of those observed in adulthood (postnatal day 60) in different laminae of the frontal, parietal, and occipital cortices, in the layers of Ammon's horn fields and the dentate gyrus of the hippocampal formation, as well as in the amygdaloid body, the olfactory tubercle, and the striatum. In contrast, at that stage, the densities of total muscarinic, M1/M3, M1, and possibly M2 receptor and high-affinity choline uptake sites represent only a small proportion of levels seen in the adult. Differences were also observed in the postnatal ontogenic profiles of nicotinic, muscarinic, vesamicol, and high-affinity choline uptake sites. For example, between postnatal weeks 3 and 5, the levels of M1/M3 and M1 sites were at least as high as in the adult, whereas M2 and high-affinity choline uptake site densities appeared to be delayed and to reach adult values only after postnatal week 5. With regard to cholinergic innervation in the developing rat brain, the present findings suggest a temporal establishment of several components of the cholinergic systems. The first components are the vesicular acetylcholine transporter and nicotinic sites; these are followed by M1/M3 and M1 sites and, finally, by M2 and high-affinity choline uptake sites.

Hypoxia and brain development

Hypoxia threatens brain function during the entire life-span starting from early fetal age up to senescence. This review compares the short-term, long-term and life-spanning effects of fetal chronic hypoxia and neonatal anoxia on several behavioural paradigms including novelty-induced spontaneous and learning behaviours. Furthermore, it reveals that perinatal hypoxia is an additional threat to neurodegeneration and decline of cognitive and other behaviours during the aging process. Prenatal hypoxia evokes a temporary delay of ingrowth of cholinergic and serotonergic fibres into the hippocampus and neocortex, and causes an enhanced neurodegeneration of 5-HT-ir axons during aging. Neonatal anoxia suppresses hippocampal ChAT activity and up-regulates muscarinic receptor sites for 3H-QNB and 3H-pirenzepine binding in the hippocampus in the early postnatal age. The altered development of axonal arborization and pre- and postsynaptic cholinergic functions may be an important underlying mechanism to explain the behavioural deficits. As far as the cellular mechanisms of perinatal hypoxia is concerned, our primary aim was to study the putative importance of Ca2+ homeostasis of developing neurons by means of pharmacological interventions and by measuring the development of immunoexpression of Ca(2+)-binding proteins. We assessed that nimodipine, an L-type calcium channel blocker, prevented or attenuated the adverse behavioural and neurochemical effects of perinatal hypoxias, while it enhanced the early postnatal development of ir-Ca(2+)-binding proteins. The results are discussed in the context of different related research areas on brain development and hypoxia and ischaemia.

Developmental expression of alpha 7 neuronal nicotinic receptor messenger RNA in rat sensory cortex and thalamus

The distribution of alpha 7 messenger RNA expression was characterized in developing rat cortex and thalamus. Northern blot analysis of neonatal and adult cortex revealed a single messenger RNA transcript of 5.7 kb. Using in situ hybridization with both full length and short 35S-labeled alpha 7 riboprobes, a distinct transient expression of messenger RNA within sensory cortex and thalamus, during early postnatal development, was observed. alpha 7 transcripts were expressed in low levels as early as embryonic day 13 in the ventricular zone of the neocortex, and as early as embryonic day 15 in the thalamic neuroepithelium. A marked increase in messenger RNA levels was observed during the late prenatal period in both sensory and non-sensory regions of the cortex and thalamus. Moderate to high levels of messenger RNA were maintained into the first postnatal week, followed by a decline into adulthood. alpha 7 messenger RNA expression was significantly higher in the anterodorsal, lateral dorsal, ventral posterior medial and ventral posterior lateral thalamic nuclei of postnatal day 7 pups than in adult brains. Expression of messenger RNA within dorsal lateral geniculate, ventral lateral geniculate and medial geniculate did not show a significant reduction with age. Within the developing cortex, messenger RNA expression delineated the primary somatosensory, auditory and visual cortices in a unique laminar pattern that was consistently and significantly higher than in the adult in superficial layer VI. Higher levels of expression were also observed in retrosplenial cortex at postnatal day 7 than in the adult. Tangential sections through postnatal day 7 cortex revealed low levels of alpha 7 messenger RNA expression delineating the primary sensory areas in layer IV, corresponding to acetylcholinesterase-labeled thalamocortical afferents. However, these sensory areas exhibited higher levels of alpha 7 messenger RNA expression and were more clearly defined in layer VI, but not by acetylcholinesterase staining. The distribution of alpha 7 messenger RNA within the developing thalamocortical system parallels the distribution of alpha-bungarotoxin binding sites and suggests that the receptor is localized on both thalamic cells and their cortical target neurons. This transient and distinct pattern of distribution of the alpha 7 neuronal nicotinic receptor, which coincides with the major phase of thalamocortical development, suggests that it may play a functional role in the development of cortical circuitry.

Development of basal forebrain projections to visual cortex: DiI studies in rat

We performed experiments using retrograde and anterograde labeling with DiI to examine the development of basal forebrain (BFB) projections to the visual cortex in postnatal rats. DiI placed in occipital cortex led to retrograde labeling of BFB neurons as early as postnatal day 0 (P0); labeled cells were found mainly in the diagonal band complex but also in the medial septum, globus pallidus, and substantia innominata. The retrogradely labeled BFB cells displayed remarkably well-developed dendritic arbors, even in younger animals, and showed increases in soma size, dendritic arbors, and dendritic spines over the first 2 postnatal weeks. DiI placements in the diagonal band led to anterogradely labeled axons in cortex. At early ages (P0-P1), labeled axons were largely confined to white matter. With increasing age, greater numbers of labeled axons were seen in the white matter and in deep cortical layers, and labeled axons extended into superficial layers. The leading edge of labeled fibers reached layer V of visual cortex by P2 and layer IV by P4 and were found throughout the cortical layers by P6. Numbers and densities of labeled axons in visual cortex were greater in older animals, at least through P14. The time of ingrowth of labeled BFB axons into visual cortex indicates that these afferents grow into particular cortical layers after those layers have differentiated from the cortical plate. These data indicate that basal forebrain projections arrive in occipital cortex after cortical lamination is well underway and after the entry of primary thalamocortical projections.

Immunocytochemical demonstration of developmental distribution of muscarinic acetylcholine receptors in rat parietal cortex

The present investigation reveals many cortical neurons immunopositive for M35, the monoclonal antibody raised against purified muscarinic acetylcholine receptor (mAChR) proteins, in the early postnatal rat brain. The ontogeny of mAChR expression, exemplified on the parietal neocortex, was studied in a series of rat pups from postnatal days (PD) 1, 3, 7, 14 and 21. Immunoprecipitation in the parietal somatosensory cortex was manifest in the population of pyramidal neurons during postnatal development. In particular during the early postnatal ages, until 2 weeks after birth, M35 immunoreactivity (M35-ir) was present in all neuronal compartments, indicating transportation of mAChR protein in axonal and dendritic processes as observed in light and electron microscopic analysis. The immunoprecipitation in the apical dendrites yielded dense labeling in layer 1 where the distal processes of the pyramidal dendrites branched extensively forming a plexus that intermingled with horizontal fibers in this superficial layer. At PD21, immunolabeling in layer 1 and in axons of pyramidal cells was reduced compared to earlier ages suggesting a transient expression of mAChRs in these neuronal structures. The development of M35-ir in the cortex appeared to antedate that of its cholinergic afferentation as indicated by AChE histochemical study.

Postnatal development of hippocampal and neocortical cholinergic and serotonergic innervation in rat: effects of nitrite-induced prenatal hypoxia and nimodipine treatment

Postnatal development of ingrowing cholinergic and serotonergic fiber patterns were studied in the rat hippocampus and parietal cortex employing a histochemical procedure for acetylcholinesterase as a cholinergic fiber marker, and immunocytochemistry of serotonin for serotonergic fiber staining. The rat pups were killed at postnatal days 1, 3, 5, 7, 10, and 20. The development of cholinergic and serotonergic innervation was described and the fiber density quantified under normal conditions and after long-term prenatal anemic hypoxia induced by chronic exposure to sodium nitrite. Furthermore, a third group was studied in which the nitrite hypoxia was combined with a simultaneous treatment with the Ca(2+)-entry blocker nimodipine to test the neuroprotective potential of this drug. Quantitative measurement of fiber density from postnatal day 1 to day 20 yielded the following results: (i) both neurotransmitter systems revealed an age-dependent and an anatomically-organized developmental pattern; (ii) the serotonergic innervation of the dorsal hippocampus preceded that of cholinergic afferentation in postnatal days 1-3; (iii) prenatal hypoxia induced a transient delay in the innervation of parietal neocortex and dentate gyrus for both neurotransmitter systems, but left the innervation of the cornu ammonis unaffected; and (iv) the hypoxia-induced retardation of cholinergic and serotonergic fiber development was prevented by concomitant application of the Ca(2+)-antagonist nimodipine during the hypoxia. The results indicate that prenatal hypoxia evokes a temporary delay in the cholinergic and serotonergic fiber outgrowth in cortical target areas in a region-specific manner. The hypoxia-induced growth inhibition is prevented by the calcium antagonist nimodipine, which supports the importance of the intracellular Ca2+ homeostasis of cells and growth cones in regulating axonal proliferation.

Formation of synapses between basal forebrain afferents and cerebral cortex neurons: an electron microscopic study in organotypic slice cultures

Co-cultures of rat basal forebrain and cerebral cortex were maintained from 1 to 5 weeks in vitro with serum-free defined medium. The formation of synaptic connections between basal forebrain afferent fibres and cortical neurons was studied by specific labelling with three staining techniques, including (i) neuronal tract tracing with the fluorescent dye 1,1'-dioctodecyl-3,3,3'3'- tetramethylindocarbocyanine perchlorate, (ii) acetylcholinesterase histochemistry, and (iii) choline acetyltransferase immunocytochemistry. Both basal forebrain and cerebral cortex tissue displayed organotypic characteristics in culture. Cerebral cortex revealed a dense innervation by axonal projections from the basal forebrain. All three labelling techniques produced similar results at the light microscopic level, with densest innervation located in the marginal zone. At the fine structural level, the 1,1'-dioctodecyl-3,3,3'3'-tetramethylindocarbocyanine perchlorate-, acetylcholinesterase- and choline acetyltransferase-stained basal forebrain afferents all revealed a number of synaptic contacts with cortical neurons. The contacts displayed consistent synaptic features, including presynaptic accumulation of small round vesicles, cleft widening, and postsynaptic densities forming symmetric synapses. These morphological characteristics of connections formed in vitro are similar to basal forebrain cholinergic projections to cerebral cortex in normal brain. Based on these results, this tissue culture model appears to be an useful tool for investigations of the development of cholinergic innervation of cerebral cortex.