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

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.

Acetylcholine modulates cortical synaptic transmission via different muscarinic receptors, as studied with receptor knockout mice

The central cholinergic system plays a crucial role in synaptic plasticity and spatial attention; however, the roles of the individual cholinergic receptors involved in these activities are not well understood at present. In the present study, we show that acetylcholine (ACh) can facilitate or depress synaptic transmission in occipital slices of mouse visual cortex. The precise nature of the ACh effects depends on the ACh concentration, and is input specific, as shown by stimulating different synaptic pathways. Pharmacological blockade of muscarinic receptor (mAChR) subtypes and the use of M1-M5 mAChR-deficient mice showed that specific mAChR subtypes, together with the activity of the cholinesterases (ChEs), mediate facilitation or depression of synaptic transmission. The present data suggest that local ACh, acting through mAChRs, regulates the cortical dynamics making cortical circuits respond to specific stimuli.

Molecular ontogeny of major neurotransmitter receptor systems in the mammalian central nervous system: norepinephrine, dopamine, serotonin, acetylcholine, and glycine

Neurotransmitter receptors are critical elements in intercellular signaling within the central nervous system and are divided into two major types based on their molecular structure and biophysical properties. The first are ionotropic receptors--ligand-gated ion channels that directly affect the membrane potential via passage of permeant ions (such as sodium and calcium) and mediate fast synaptic transmission. The second type are slower metabotropic receptors that are also ligand gated but depend on an interaction with guanine nucleotide-binding proteins and mediate signal transduction by activating second-messenger systems within the cell. In the past two decades, a wealth of information has emerged regarding the molecular biology and pharmacology of classic neurotransmitter receptors (including adrenergic, dopaminergic, serotonergic, cholinergic, glycine, gamma-aminobutyric acid [GABA(A)], and glutamate receptors). Further, the distribution of subunits comprising these receptors has been extensively studied. This review focuses on the molecular ontogeny of several of the major neurotransmitter receptor systems in the mammalian central nervous system, highlighting the role that some of these may play during brain development and in certain pathologic states.

Muscarinic acetylcholine receptors in the hippocampus, neocortex and amygdala: a review of immunocytochemical localization in relation to learning and memory

Immunocytochemical mapping studies employing the extensively used monoclonal anti-muscarinic acetylcholine receptor (mAChR) antibody M35 are reviewed. We focus on three neuronal muscarinic cholinoceptive substrates, which are target regions of the cholinergic basal forebrain system intimately involved in cognitive functions: the hippocampus; neocortex; and amygdala. The distribution and neurochemistry of mAChR-immunoreactive cells as well as behaviorally induced alterations in mAChR-immunoreactivity (ir) are described in detail. M35+ neurons are viewed as cells actively engaged in neuronal functions in which the cholinergic system is typically involved. Phosphorylation and subsequent internalization of muscarinic receptors determine the immunocytochemical outcome, and hence M35 as a tool to visualize muscarinic receptors is less suitable for detection of the entire pool of mAChRs in the central nervous system (CNS). Instead, M35 is sensitive to and capable of detecting alterations in the physiological condition of muscarinic receptors. Therefore, M35 is an excellent tool to localize alterations in cellular cholinoceptivity in the CNS. M35-ir is not only determined by acetylcholine (ACh), but by any substance that changes the phosphorylation/internalization state of the mAChR. An important consequence of this proposition is that other neurotransmitters than ACh (especially glutamate) can regulate M35-ir and the cholinoceptive state of a neuron, and hence the functional properties of a neuron. One of the primary objectives of this review is to provide a synthesis of our data and literature data on mAChR-ir. We propose a hypothesis for the role of muscarinic receptors in learning and memory in terms of modulation between learning and recall states of brain areas at the postsynaptic level as studied by way of immunocytochemistry employing the monoclonal antibody M35.

Phospholipase C-beta1 expression correlates with neuronal differentiation and synaptic plasticity in rat somatosensory cortex

Receptor-mediated signal transduction is thought to play an important role in neuronal differentiation and the modification of synaptic connections during brain development. The intracellular signalling molecule phospholipase C-beta1 (PLC-beta1), which is activated via specific neurotransmitter receptors, has recently been implicated in activity-dependent plasticity in the cat visual cortex. PLC-beta1 has been shown to be concentrated in an intermediate compartment-like organelle, the botrysome, which is present in 5-week-old, but not adult, cat cortical neurons. We have characterized the spatial and temporal regulation of PLC-beta1 expression in the developing rat cerebral cortex. PLC-beta1-positive botrysome-like organelles are observed during early postnatal cortical development, but not at postnatal day 14 or later stages. In the postnatal somatosensory cortex, there is also striking spatial variation in diffuse neuropilar immunoreactivity of layer IV and above, in a pattern corresponding to the thalamocortical recipient zones known as barrels. This expression pattern is specific to the developing barrel field and is most distinct at postnatal days 4-7, when cellular components of barrels are capable of activity-dependent modification. During later stages of cortical maturation, stained botrysomes disappear, expression of PLC-beta1 is down-regulated and only diffuse immunoreactivity remains in dendritic processes. Our results are consistent with a role for PLC-beta1 in activity-dependent, receptor-mediated neuronal plasticity during development of the somatosensory cortex.

Non-selectivity of the monoclonal antibody M35 for subtypes of muscarinic acetylcholine receptors

The monoclonal antibody M35, one of the first monoclonal antibodies successfully raised against muscarinic acetylcholine receptors, has been widely used to study the distribution of this protein in a variety of tissues and cell types of different species. It is not fully known, however, to which muscarinic acetylcholine receptor subtypes M35 binds. Knowledge of subtype-selectivity of M35 is a necessary step towards a functional interpretation of the obtained immunocytochemical data. The aim of the present study was to determine the subtype-selectivity of M35 employing transfected CHO-K1 cells stably expressing human m1-m5 muscarinic acetylcholine receptors separately, and to study M35 immunoreactivity in areas of rat central and peripheral tissues known to be specifically enriched in a single muscarinic acetylcholine receptor subtype. The results show that (a) all five transfected cell lines were immunopositive for M35, (b) nontransfected control cells were immunonegative, (c) the number of mAChRs expressed per cell correlated positively with the intensity of M35 immunoreactivity, and (d) cell types in aldehyde-fixed rat tissue enriched in a single m1-m4 subtypes revealed clear M35 immunoreactivity. Taken together, the present results show that M35 does not discriminate between muscarinic acetylcholine receptor subtypes. Evidently, the epitope of M35 on the receptor-protein is preserved on all muscarinic acetylcholine receptor subtypes. The epitope for M35 must, therefore, be localized on a homologous part of each subtype.

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.