Selective cholinergic immunolesioning affects synaptic plasticity in developing visual cortex

Basal Forebrain Cholinergic Neurons Have Specific Characteristics during the Perinatal Period

Cholinergic neurons of the basal forebrain represent the main source of cholinergic innervation of large parts of the neocortex and are involved in adults in the modulation of attention, memory, and arousal. During the first postnatal days, they play a crucial role in the development of cortical neurons and cortical cytoarchitecture. However, their characteristics, during this period have not been studied. To understand how they can fulfill this role, we investigated the morphological and electrophysiological maturation of cholinergic neurons of the substantia innominata-nucleus basalis of Meynert (SI/NBM) complex in the perinatal period in mice. We show that cholinergic neurons, whether or not they express gamma-aminobutyric acid (GABA) as a cotransmitter, are already functional at Embryonic Day 18. Until the end of the first postnatal week, they constitute a single population of neurons with a well developed dendritic tree, a spontaneous activity including bursting periods, and a short-latency response to depolarizations (early-firing). They are excited by both their GABAergic and glutamatergic afferents. During the second postnatal week, a second, less excitable, neuronal population emerges, with a longer delay response to depolarizations (late-firing), together with the hyperpolarizing action of GABAA receptor-mediated currents. This classification into early-firing (40%) and late-firing (60%) neurons is again independent of the coexpression of GABAergic markers. These results strongly suggest that during the first postnatal week, the specific properties of developing SI/NBM cholinergic neurons allow them to spontaneously release acetylcholine (ACh), or ACh and GABA, into the developing cortex.

Intracerebroventricular Administration of 192IgG-Saporin Alters Expression of Microglia-Associated Genes in the Dorsal But Not Ventral Hippocampus

One of important aspects of development of Alzheimer’s disease is degeneration of septal cholinergic neurons that innervate the hippocampus. We took advantage of widely used model of cholinergic deficit in the hippocampus, intracerebroventricular administration of ¹⁹²IgG-saporin (Ig-saporin), to analyze the postponed consequences of cholinergic deficit in different parts of the hippocampus. We studied effects of the immunotoxin on the behavior of rats and gene expression in the dorsal and ventral hippocampus using RNA-seq approach. We found that under normal conditions dorsal and ventral parts of the hippocampus differ in the expression of 1129 protein-coding genes and 49 non-coding RNAs (ncRNAs) and do not differ in the expression of 10 microRNAs, which were detected in both parts of the hippocampus. Ig-saporin-induced degeneration of cholinergic septal neurons did not affect rat behavior in open field, T-maze, and passive avoidance task but impaired memory retention in Morris water maze. To analyze ¹⁹²Ig-saporin-induced changes in the gene expression, we formed the following groups of genes: genes expressed exclusively in certain cell types (neurons, astrocytes, microglia, oligodendrocytes, and vascular cells) and, among universally expressed genes, a group of genes that encode ribosome-forming proteins. For all groups of genes, the alterations in the gene expression produced by the immunotoxin were stronger in the dorsal as compared to the ventral hippocampus. We found that, among groups of universally expressed genes, Ig-saporin increased the expression of ribosome-forming proteins in both dorsal and ventral hippocampus. Ig-saporin also strongly upregulated expression of microglia-specific genes only in the dorsal hippocampus. A subset of affected microglial genes comprised genes associated with inflammation, however, did not include genes related to acute inflammation such as interleukins-1b, -6, -15, and -18 as well as TNF. The expression of other cell-specific genes (genes specific for neurons, astrocytes, oligodendrocytes, and vascular cells) was unaffected. The data obtained suggest that disturbance of memory-associated behavior after administration of Ig-saporin is associated with upregulation of microglia-associated genes in the dorsal but not ventral hippocampus.

Generation of Cholinergic and Dopaminergic Interneurons from Human Pluripotent Stem Cells as a Relevant Tool for In Vitro Modeling of Neurological Disorders Pathology and Therapy

The cellular and molecular bases of neurological diseases have been studied for decades; however, the underlying mechanisms are not yet fully elucidated. Compared with other disorders, diseases of the nervous system have been very difficult to study mainly due to the inaccessibility of the human brain and live neurons in vivo or in vitro and difficulties in examination of human postmortem brain tissue. Despite the availability of various genetically engineered animal models, these systems are still not adequate enough due to species variation and differences in genetic background. Human induced pluripotent stem cells (hiPSCs) reprogrammed from patient somatic cells possess the potential to differentiate into any cell type, including neural progenitor cells and postmitotic neurons; thus, they open a new area to in vitro modeling of neurological diseases and their potential treatment. Currently, many protocols for generation of various neuronal subtypes are being developed; however, most of them still require further optimization. Here, we highlight accomplishments made in the generation of dopaminergic and cholinergic neurons, the two subtypes most affected in Alzheimer's and Parkinson's diseases and indirectly affected in Huntington's disease. Furthermore, we discuss the potential role of hiPSC-derived neurons in the modeling and treatment of neurological diseases related to dopaminergic and cholinergic system dysfunction.

Distribution and effects of the muscarinic receptor subtypes in the primary visual cortex

Muscarinic cholinergic receptors modulate the activity and plasticity of the visual cortex. Muscarinic receptors are divided into five subtypes that are not homogeneously distributed throughout the cortical layers and cells types. This distribution results in complex action of the muscarinic receptors in the integration of visual stimuli. Selective activation of the different subtypes can either strengthen or weaken cortical connectivity (e.g., thalamocortical vs. corticocortical), i.e., it can influence the processing of certain stimuli over others. Moreover, muscarinic receptors differentially modulate some functional properties of neurons during experience-dependent activity and cognitive processes and they contribute to the fine-tuning of visual processing. These functions are involved in the mechanisms of attention, maturation and learning in the visual cortex. This minireview describes the anatomo-functional aspects of muscarinic modulation of the primary visual cortex's (V1) microcircuitry.

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

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

Nicotinic acetylcholine receptors in attention circuitry: the role of layer VI neurons of prefrontal cortex

Cholinergic modulation of prefrontal cortex is essential for attention. In essence, it focuses the mind on relevant, transient stimuli in support of goal-directed behavior. The excitation of prefrontal layer VI neurons through nicotinic acetylcholine receptors optimizes local and top-down control of attention. Layer VI of prefrontal cortex is the origin of a dense feedback projection to the thalamus and is one of only a handful of brain regions that express the α5 nicotinic receptor subunit, encoded by the gene chrna5. This accessory nicotinic receptor subunit alters the properties of high-affinity nicotinic receptors in layer VI pyramidal neurons in both development and adulthood. Studies investigating the consequences of genetic deletion of α5, as well as other disruptions to nicotinic receptors, find attention deficits together with altered cholinergic excitation of layer VI neurons and aberrant neuronal morphology. Nicotinic receptors in prefrontal layer VI neurons play an essential role in focusing attention under challenging circumstances. In this regard, they do not act in isolation, but rather in concert with cholinergic receptors in other parts of prefrontal circuitry. This review urges an intensification of focus on the cellular mechanisms and plasticity of prefrontal attention circuitry. Disruptions in attention are one of the greatest contributing factors to disease burden in psychiatric and neurological disorders, and enhancing attention may require different approaches in the normal and disordered prefrontal cortex.

Visual cortex plasticity: a complex interplay of genetic and environmental influences

The central nervous system architecture is highly dynamic and continuously modified by sensory experience through processes of neuronal plasticity. Plasticity is achieved by a complex interplay of environmental influences and physiological mechanisms that ultimately activate intracellular signal transduction pathways regulating gene expression. In addition to the remarkable variety of transcription factors and their combinatorial interaction at specific gene promoters, epigenetic mechanisms that regulate transcription have emerged as conserved processes by which the nervous system accomplishes the induction of plasticity. Experience-dependent changes of DNA methylation patterns and histone posttranslational modifications are, in fact, recruited as targets of plasticity-associated signal transduction mechanisms. Here, we shall concentrate on structural and functional consequences of early sensory deprivation in the visual system and discuss how intracellular signal transduction pathways associated with experience regulate changes of chromatin structure and gene expression patterns that underlie these plastic phenomena. Recent experimental evidence for mechanisms of cross-modal plasticity following congenital or acquired sensory deprivation both in human and animal models will be considered as well. We shall also review different experimental strategies that can be used to achieve the recovery of sensory functions after long-term deprivation in humans.

Selective cholinergic depletion in medial septum leads to impaired long term potentiation and glutamatergic synaptic currents in the hippocampus

Cholinergic depletion in the medial septum (MS) is associated with impaired hippocampal-dependent learning and memory. Here we investigated whether long term potentiation (LTP) and synaptic currents, mediated by alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) and N-methyl-D-aspartate (NMDA) receptors in the CA1 hippocampal region, are affected following cholinergic lesions of the MS. Stereotaxic intra-medioseptal infusions of a selective immunotoxin, 192-saporin, against cholinergic neurons or sterile saline were made in adult rats. Four days after infusions, hippocampal slices were made and LTP, whole cell, and single channel (AMPA or NMDA receptor) currents were recorded. Results demonstrated impairment in the induction and expression of LTP in lesioned rats. Lesioned rats also showed decreases in synaptic currents from CA1 pyramidal cells and synaptosomal single channels of AMPA and NMDA receptors. Our results suggest that MS cholinergic afferents modulate LTP and glutamatergic currents in the CA1 region of the hippocampus, providing a potential synaptic mechanism for the learning and memory deficits observed in the rodent model of selective MS cholinergic lesioning.

The controlled generation of functional basal forebrain cholinergic neurons from human embryonic stem cells

An early substantial loss of basal forebrain cholinergic neurons (BFCN) is a constant feature of Alzheimer's disease and is associated with deficits in spatial learning and memory. The ability to selectively control the differentiation of human embryonic stem cells (hESCs) into BFCN would be a significant step toward a cell replacement therapy. We demonstrate here a method for the derivation of a predominantly pure population of BFCN from hESC cells using diffusible ligands present in the forebrain at developmentally relevant time periods. Overexpression of two relevant human transcription factors in hESC-derived neural progenitors also generates BFCN. These neurons express only those markers characteristic of BFCN, generate action potentials, and form functional cholinergic synapses in murine hippocampal slice cultures. siRNA-mediated knockdown of the transcription factors blocks BFCN generation by the diffusible ligands, clearly demonstrating the factors both necessary and sufficient for the controlled derivation of this neuronal population. The ability to selectively control the differentiation of hESCs into BFCN is a significant step both for understanding mechanisms regulating BFCN lineage commitment and for the development of both cell transplant-mediated therapeutic interventions for Alzheimer's disease and high-throughput screening for agents that promote BFCN survival.

Localization of pre- and postsynaptic cholinergic markers in rodent forebrain: a brief history and comparison of rat and mouse

Rat and mouse models are widely used for studies in cognition and pathophysiology, among others. Here, we sought to determine to what extent these two model species differ for cholinergic and cholinoceptive features. For this purpose, we focused on cholinergic innervation patterns based on choline acetyltransferase (ChAT) immunostaining, and the expression of muscarinic acetylcholine receptors (mAChRs) detected immunocytochemically. In this brief review we first place cholinergic and cholinoceptive markers in a historic perspective, and then provide an overview of recent publications on cholinergic studies and techniques to provide a literature survey of current research. Next, we compare mouse (C57Bl/J6) and rat (Wistar) cholinergic and cholinoceptive systems simultaneously stained, respectively, for ChAT (analyzed qualitatively) and mAChRs (analyzed qualitatively and quantitatively). In general, the topographic cholinergic innervation patterns of both rodent species are highly comparable, with only considerable (but region specific) differences in number of detectable cholinergic interneurons, which are more numerous in rat. In contrast, immunolabeling for mAChRs, detected by the monoclonal antibody M35, differs markedly in the forebrain between the two species. In mouse brain, basal levels of activated and/or internalized mAChRs (as a consequence of cholinergic neurotransmission) are significantly higher. This suggests a higher cholinergic tone in mouse than rat, and hence the animal model of choice may have consequences for cholinergic drug testing experiments.

Sympathetic sprouting in visual cortex stimulated by cholinergic denervation rescues expression of two forms of long-term depression at layer 2/3 synapses

Cholinergic innervation of hippocampus and cortex is required for some forms of learning and memory. Several reports have shown that activation of muscarinic m1 receptors induces a long-term depression (mLTD) at glutamate synapses in hippocampus and in several areas of cortex, including perirhinal and visual cortices. This plasticity likely contributes to cognitive function dependent upon the cholinergic system. In rodent models, degeneration of hippocampal cholinergic innervation following lesion of the medial septum stimulates sprouting of adrenergic sympathetic axons, originating from the superior cervical ganglia (SCG), into denervated hippocampal subfields. We previously reported that this adrenergic sympathetic sprouting occurs simultaneously with a reappearance of cholinergic fibers in hippocampus and rescue of mLTD at CA3-CA1 synapses. Because cholinergic neurons throughout basal forebrain degenerate in aging and Alzheimer's disease, it is critical to determine if this compensatory sprouting occurs in other regions impacted by cholinergic cell loss. To this end, we investigated whether lesion of the nucleus basalis magnocellularis (NbM) to cholinergically denervate cortex stimulates adrenergic sympathetic sprouting and the accompanying increase in cholinergic innervation. Further, we assessed whether the presence of sprouting positively correlates with the ability of glutamate synapses in acute visual cortex slices to express mLTD and low frequency stimulation induced LTD (LFS LTD), another cholinergic dependent form of plasticity in visual cortex. We found that both mLTD and LFS LTD are absent in animals when NbM lesion is combined with bilateral removal of the SCG to prevent possible compensatory sprouting. In contrast, when the SCG remain intact to permit sprouting in animals with NbM lesion, cholinergic fiber density is increased concurrently with adrenergic sympathetic sprouting, and mLTD and LFS LTD are preserved. Our findings suggest that autonomic compensation for central cholinergic degeneration is not specific to hippocampus, but is a general repair mechanism occurring in other brain regions important for normal cognitive function.

Between molecules and experience: role of early patterns of coordinated activity for the development of cortical maps and sensory abilities

Sensory systems processing information from the environment rely on precisely formed and refined neuronal networks that build maps of sensory receptor epithelia at different subcortical and cortical levels. These sensory maps share similar principles of function and emerge according to developmental processes common in visual, somatosensory and auditory systems. Whereas molecular cues set the coarse organization of cortico-subcortical topography, its refinement is known to succeed under the influence of experience-dependent electrical activity during critical periods. However, coordinated patterns of activity synchronize the cortico-subcortical networks long before the meaningful impact of environmental inputs on sensory maps. Recent studies elucidated the cellular and network mechanisms underlying the generation of these early patterns of activity and highlighted their similarities across species. Moreover, the experience-independent activity appears to act as a functional template for the maturation of sensory networks and cortico-subcortical maps. A major goal for future research will be to analyze how this early activity interacts with the molecular cues and to determine whether it is permissive or rather supporting for the establishment of sensory topography.

Developmental sex differences in nicotinic currents of prefrontal layer VI neurons in mice and rats

Background

There is a large sex difference in the prevalence of attention deficit disorder; yet, relatively little is known about sex differences in the development of prefrontal attention circuitry. In male rats, nicotinic acetylcholine receptors excite corticothalamic neurons in layer VI, which are thought to play an important role in attention by gating the sensitivity of thalamic neurons to incoming stimuli. These nicotinic currents in male rats are significantly larger during the first postnatal month when prefrontal circuitry is maturing. The present study was undertaken to investigate whether there are sex differences in the nicotinic currents in prefrontal layer VI neurons during development.

Methodology/Principal Findings

Using whole cell recording in prefrontal brain slice, we examined the inward currents elicited by nicotinic stimulation in male and female rats and two strains of mice. We found a prominent sex difference in the currents during the first postnatal month when males had significantly greater nicotinic currents in layer VI neurons compared to females. These differences were apparent with three agonists: acetylcholine, carbachol, and nicotine. Furthermore, the developmental sex difference in nicotinic currents occurred despite male and female rodents displaying a similar pattern and proportion of layer VI neurons possessing a key nicotinic receptor subunit.

Conclusions/Significance

This is the first illustration at a cellular level that prefrontal attention circuitry is differently affected by nicotinic receptor stimulation in males and females during development. This transient sex difference may help to define the cellular and circuit mechanisms that underlie vulnerability to attention deficit disorder.

Cholinergic pairing with visual activation results in long-term enhancement of visual evoked potentials

Acetylcholine (ACh) contributes to learning processes by modulating cortical plasticity in terms of intensity of neuronal activity and selectivity properties of cortical neurons. However, it is not known if ACh induces long term effects within the primary visual cortex (V1) that could sustain visual learning mechanisms. In the present study we analyzed visual evoked potentials (VEPs) in V1 of rats during a 4–8 h period after coupling visual stimulation to an intracortical injection of ACh analog carbachol or stimulation of basal forebrain. To clarify the action of ACh on VEP activity in V1, we individually pre-injected muscarinic (scopolamine), nicotinic (mecamylamine), α7 (methyllycaconitine), and NMDA (CPP) receptor antagonists before carbachol infusion. Stimulation of the cholinergic system paired with visual stimulation significantly increased VEP amplitude (56%) during a 6 h period. Pre-treatment with scopolamine, mecamylamine and CPP completely abolished this long-term enhancement, while α7 inhibition induced an instant increase of VEP amplitude. This suggests a role of ACh in facilitating visual stimuli responsiveness through mechanisms comparable to LTP which involve nicotinic and muscarinic receptors with an interaction of NMDA transmission in the visual cortex.

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.

Muscarinic receptor dependent long-term depression in rat visual cortex is PKC independent but requires ERK1/2 activation and protein synthesis

Intact cholinergic innervation of visual cortex is critical for normal processing of visual information and for spatial memory acquisition and retention. However, a complete description of the mechanisms by which the cholinergic system modifies synaptic function in visual cortex is lacking. Previously it was shown that activation of the m1 subtype of muscarinic receptor induces an activity-dependent and partially N-methyl-d-aspartate receptor (NMDAR)-dependent long-term depression (LTD) at layer 4-layer 2/3 synapses in rat visual cortex slices in vitro. The cellular mechanisms downstream of the Galphaq coupled m1 receptor required for induction of this LTD (which we term mLTD) are currently unknown. Here, we confirm a role for m1 receptors in mLTD induction and use a series of pharmacological tools to study the signaling molecules downstream of m1 receptor activation in mLTD induction. We found that mLTD is prevented by inhibitors of L-type Ca(2+) channels, the Src kinase family, and the mitogen-activated kinase/extracellular kinase. mLTD is also partially dependent on phospholipase C but is unaffected by blocking protein kinase C. mLTD expression can be long-lasting (>2 h) and its long-term maintenance requires translation. Thus we report the signaling mechanisms underlying induction of an m1 receptor-dependent LTD in visual cortex and the requirement of protein synthesis for long-term expression. This plasticity could be a mechanism by which the cholinergic system modifies glutamatergic synapse function to permit normal visual system processing required for cognition.

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.

Perinatal kynurenine 3-hydroxylase inhibition in rodents: pathophysiological implications

The kynurenine pathway (KP) of tryptophan degradation contains three neuroactive metabolites: the neuroinhibitory agent kynurenic acid (KYNA) and, in a competing branch, the free radical generator 3-hydroxykynurenine (3-HK) and the excitotoxin quinolinic acid (QUIN). These three "kynurenines" derive from a common precursor, L-kynurenine, and are recognized for their role in brain physiology and pathophysiology. Inhibition of kynurenine 3-hydroxylase, the enzyme responsible for 3-HK formation, shifts KP metabolism in the mature brain toward enhanced KYNA formation. We now tested the cerebral effects of kynurenine 3-hydroxylase inhibition in immature rodents. Rat pups treated with the kynurenine 3-hydroxylase inhibitor UPF 648 (30 mg/kg, i.p.) 10 min after birth showed substantial increases in cerebral and liver kynurenine and KYNA levels up to 24 hr later, whereas 3-HK and QUIN levels were simultaneously decreased. Administered to pregnant rats or mice on the last day of gestation, UPF 648 (50 mg/kg, i.p.) produced qualitatively similar changes (i.e., large increases in kynurenine and KYNA and reductions in 3-HK and QUIN) in the brain and liver of the offspring. Rat pups delivered by UPF 648-treated mothers and immediately exposed to neonatal asphyxia showed further enhanced brain KYNA levels. These studies demonstrate that acute kynurenine 3-hydroxylase inhibition effectively shifts cerebral KP metabolism in neonatal rodents toward increased KYNA formation. Selective inhibitors of this enzyme may therefore provide neuroprotection in newborns and will also be useful for the experimental evaluation of the long-term effects of perinatal KP impairment.

Muscarinic acetylcholine receptor knockout mice show distinct synaptic plasticity impairments in the visual cortex

In the present report, we focused our attention on the role played by the muscarinic acetylcholine receptors (mAChRs) in different forms of long-term synaptic plasticity. Specifically, we investigated long-term potentiation (LTP) and long-term depression (LTD) expression elicited by theta-burst stimulation (TBS) and low-frequency stimulation (LFS), respectively, in visual cortical slices obtained from different mAChR knockout (KO) mice. A normal LTP was evoked in M(1)/M(3) double KO mice, while LTP was impaired in the M(2)/M(4) double KO animals. On the other hand, LFS induced LTD in M(2)/M(4) double KO mice, but failed to do so in M(1)/M(3) KO mice. Interestingly, LFS produced LTP instead of LTD in M(1)/M(3) KO mice. Analysis of mAChR single KO mice revealed that LTP was affected only by the simultaneous absence of both M(2) and M(4) receptors. A LFS-dependent shift from LTD to LTP was also observed in slices from M(1) KO mice, while LTD was simply abolished in slices from M(3) KO mice. Using pharmacological tools, we showed that LTP in control mice was blocked by pertussis toxin, an inhibitor of G(i/o) proteins, but not by raising intracellular cAMP levels. In addition, the inhibition of phospholipase C by U73122 induced the same shift from LTD to LTP after LFS observed in M(1) single KO and M(1)/M(3) double KO mice. Our results indicate that different mAChR subtypes regulate different forms of long-term synaptic plasticity in the mouse visual cortex, activating specific G proteins and downstream intracellular mechanisms.

Developmental modulation of synaptic transmission by acetylcholine in the primary visual cortex

Despite the evidence that cortical synaptic organization and cognitive functions are influenced by the activity of the cholinergic system during postnatal development, so far no information is available on the effects produced by acetylcholine (ACh) on synaptic transmission. In the present article, we show that the ability of visual cortex slices to respond to ACh depends on postnatal age. In adulthood, ACh exerts mainly a facilitatory action on synaptic transmission, depressing field potential (FP) amplitude only if applied at high concentrations (millimolar range). During early postnatal development, at postnatal day 13 (P13), facilitation by ACh was lacking, with depression of FP observed with concentration of ACh in the micromolar range. The magnitude of ACh facilitatory effects increases with age. The time course of ACh-dependent facilitation overlaps the developmental maturation of acetylcholinesterase (AChE), suggesting a close relationship between ACh action and AChE activity. Thus, age-dependent modification of the cholinergic modulatory action may affect cortical maturation by regulating the magnitude of synaptic transmission.

Multiple receptors coupled to phospholipase C gate long-term depression in visual cortex

Long-term depression (LTD) in sensory cortices depends on the activation of NMDA receptors. Here, we report that in visual cortical slices, the induction of LTD (but not long-term potentiation) also requires the activation of receptors coupled to the phospholipase C (PLC) pathway. Using immunolesions in combination with agonists and antagonists, we selectively manipulated the activation of alpha1 adrenergic, M1 muscarinic, and mGluR5 glutamatergic receptors. Inactivation of these PLC-coupled receptors prevents the induction of LTD, but only when the three receptors were inactivated together. LTD is fully restored by activating any one of them or by supplying intracellular D-myo-inositol-1,4,5-triphosphate (IP3). LTD was also impaired by intracellular application of PLC or IP3 receptor blockers, and it was absent in mice lacking PLCbeta1, the predominant PLC isoform in the forebrain. We propose that visual cortical LTD requires a minimum of PLC activity that can be supplied independently by at least three neurotransmitter systems. This essential requirement places PLC-linked receptors in a unique position to control the induction of LTD and provides a mechanism for gating visual cortical plasticity via extra-retinal inputs in the intact organism.