Light and electron microscopic study of m2 muscarinic acetylcholine receptor in the basal forebrain of the rat

Neurokinin1 - cholinergic receptor mechanisms in the medial Septum-Dorsal hippocampus axis mediates experimental neuropathic pain

The neurokinin-1 receptors (NK1Rs) in the forebrain medial septum (MS) region are localized exclusively on cholinergic neurons that partly project to the hippocampus and the cingulate cortex (Cg), regions implicated in nociception. In the present study, we explored the hypothesis that neurotransmission at septal NK1R and hippocampal cholinergic mechanisms mediate experimental neuropathic pain in the rodent chronic constriction injury model (CCI). Our investigations showed that intraseptal microinjection of substance P (SP) in rat evoked a peripheral hypersensitivity (PH)-like response in uninjured animals that was attenuated by systemic atropine sulphate, a muscarinic-cholinergic receptor antagonist. Conversely, pre-emptive destruction of septal cholinergic neurons attenuated the development of PH in the CCI model that also prevented the expression of cellular markers of nociception in the spinal cord and the forebrain. Likewise, anti-nociception was evoked on intraseptal microinjection of L-733,060, an antagonist at NK1Rs, and on bilateral or unilateral microinjection of the cholinergic receptor antagonists, atropine or mecamylamine, into the different regions of the dorsal hippocampus (dH) or on bilateral microinjection into the Cg. Interestingly, the effect of L-733,060 was accompanied with a widespread decreased in levels of CCI-induced nociceptive cellular markers in forebrain that was not secondary to behaviour, suggesting an active modulation of nociceptive processing by transmission at NK1R in the medial septum. The preceding suggest that the development and maintenance of neuropathic nociception is facilitated by septal NK1R-dH cholinergic mechanisms which co-ordinately affect nociceptive processing in the dH and the Cg. Additionally, the data points to a potential strategy for pain modulation that combines anticholinergics and anti-NKRs.

Saporin as a Commercial Reagent: Its Uses and Unexpected Impacts in the Biological Sciences—Tools from the Plant Kingdom

Saporin is a ribosome-inactivating protein that can cause inhibition of protein synthesis and causes cell death when delivered inside a cell. Development of commercial Saporin results in a technology termed ‘molecular surgery’, with Saporin as the scalpel. Its low toxicity (it has no efficient method of cell entry) and sturdy structure make Saporin a safe and simple molecule for many purposes. The most popular applications use experimental molecules that deliver Saporin via an add-on targeting molecule. These add-ons come in several forms: peptides, protein ligands, antibodies, even DNA fragments that mimic cell-binding ligands. Cells that do not express the targeted cell surface marker will not be affected. This review will highlight some newer efforts and discuss significant and unexpected impacts on science that molecular surgery has yielded over the last almost four decades. There are remarkable changes in fields such as the Neurosciences with models for Alzheimer’s Disease and epilepsy, and game-changing effects in the study of pain and itch. Many other uses are also discussed to record the wide-reaching impact of Saporin in research and drug development.

Profiling Basal Forebrain Cholinergic Neurons Reveals a Molecular Basis for Vulnerability Within the Ts65Dn Model of Down Syndrome and Alzheimer's Disease

Basal forebrain cholinergic neuron (BFCN) degeneration is a hallmark of Down syndrome (DS) and Alzheimer's disease (AD). Current therapeutics have been unsuccessful in slowing disease progression, likely due to complex pathological interactions and dysregulated pathways that are poorly understood. The Ts65Dn trisomic mouse model recapitulates both cognitive and morphological deficits of DS and AD, including BFCN degeneration. We utilized Ts65Dn mice to understand mechanisms underlying BFCN degeneration to identify novel targets for therapeutic intervention. We performed high-throughput, single population RNA sequencing (RNA-seq) to interrogate transcriptomic changes within medial septal nucleus (MSN) BFCNs, using laser capture microdissection to individually isolate ~500 choline acetyltransferase-immunopositive neurons in Ts65Dn and normal disomic (2N) mice at 6 months of age (MO). Ts65Dn mice had unique MSN BFCN transcriptomic profiles at ~6 MO clearly differentiating them from 2N mice. Leveraging Ingenuity Pathway Analysis and KEGG analysis, we linked differentially expressed gene (DEG) changes within MSN BFCNs to several canonical pathways and aberrant physiological functions. The dysregulated transcriptomic profile of trisomic BFCNs provides key information underscoring selective vulnerability within the septohippocampal circuit. We propose both expected and novel therapeutic targets for DS and AD, including specific DEGs within cholinergic, glutamatergic, GABAergic, and neurotrophin pathways, as well as select targets for repairing oxidative phosphorylation status in neurons. We demonstrate and validate this interrogative quantitative bioinformatic analysis of a key dysregulated neuronal population linking single population transcript changes to an established pathological hallmark associated with cognitive decline for therapeutic development in human DS and AD.

Endocytosis of Activated Muscarinic m2 Receptor (m2R) in Live Mouse Hippocampal Neurons Occurs via a Clathrin-Dependent Pathway

Our aim was to examine the dynamics of the muscarinic m2 receptor (m2R), a G-protein coupled receptor (GPCR), after agonist activation in living hippocampal neurons, and especially clathrin dependency endocytosis. We have previously shown that the m2R undergoes agonist-induced internalization in vivo. However, the nature of the endocytotic pathway used by m2R after activation is still unknown in living neurons. Using live cell imaging and quantitative analyses, we have monitored the effect of stimulation on the fate of the membrane-bound m2R and on its redistribution in intraneuronal compartments. Shortly (6 min) after activation, m2R is internalized into clathrin immunopositive structures. Furthermore, after clathrin-dependent endocytosis, m2R associates with early and late endosomes and with subcellular organelles involved in degradation. Together, these results provide, for the first time, a description of m2R trafficking in living neurons and prove that m2R undergoes clathrin-dependent endocytosis before being degraded.

Receptor visualization and the atomic bomb. A historical account of the development of the chemical neuroanatomy of receptors for neurotransmitters and drugs during the Cold War

This is a historical account of how receptors for neurotransmitters and drugs got to be seen at the regional, cellular, and subcellular levels in brain, in the years going from the end of the World War II until the collapse of the Soviet Union, the Cold War (1945-1991). The realization in the US of the problem of mental health care, as a consequence of the results of medical evaluation for military service during the war, let the US Government to act creating among other things the National Institute for Mental Health (NIMH). Coincident with that, new drug treatments for these disorders were introduced. War science also created an important number of tools and instruments, such as the radioisotopes, that played a significant role in the development of our story. The scientific context was marked by the development of Biochemistry, Molecular Biology and the introduction in the early 80's of the DNA recombinant technologies. The concepts of chemical neurotransmission in the brain and of receptors for drugs and transmitters, although proposed before the war, where not generally accepted. Neurotransmitters were identified and the mechanisms of biosynthesis, storage, release and termination of action by mechanisms such as reuptake, elucidated. Furthermore, the synapse was seen with the electron microscope and more important for our account, neurons and their processes visualized in the brain first by fluorescence histochemistry, then using radioisotopes and autoradiography, and later by immunohistochemistry (IHC), originating the Chemical Neuroanatomy. The concept of chemical neurotransmission evolved from the amines, expanded to excitatory and inhibitory amino acids, then to neuropeptides and finally to gases and other "atypical" neurotransmitters. In addition, coexpression of more than one transmitter in a neuron, changed the initial ideas of neurotransmission. The concept of receptors for these and other messengers underwent a significant evolution from an abstract chemical concept to their physical reality as gene products. Important steps were the introduction in the 70's of radioligand binding techniques and the cloning of receptor genes in the 80's. Receptors were first visualized using radioligands and autoradiography, and analyzed with the newly developed computer-assisted image analysis systems. Using Positron Emission Tomography transmitters and receptors were visualized in living human brain. The cloning of receptor genes allowed the use of in situ hybridization histochemistry and immunohistochemistry to visualize with the light and electron microscopes the receptor mRNAs and proteins. The results showed the wide heterogeneity of receptors and the diversity of mode of signal transmission, synaptic and extra-synaptic, again radically modifying the early views of neurotransmission. During the entire period the interplay between basic science and Psychopharmacology and Psychiatry generated different transmitter or receptor-based theories of brain drug action. These concepts and technologies also changed the way new drugs were discovered and developed. At the end of the period, a number of declines in these theories, the use of certain tools and the ability to generate new diagnostics and treatments, the end of an era and the beginning of a new one in the research of how the brain functions.

Evidence for M2 muscarinic receptor modulation of axon terminals and dendrites in the rodent basolateral amygdala: An ultrastructural and electrophysiological analysis

The basolateral amygdala receives a very dense cholinergic innervation from the basal forebrain that is important for memory consolidation. Although behavioral studies have shown that both M₁ and M₂ muscarinic receptors are critical for these mnemonic functions, there have been very few neuroanatomical and electrophysiological investigations of the localization and function of different types of muscarinic receptors in the amygdala. In the present study we investigated the subcellular localization of M₂ muscarinic receptors (M₂Rs) in the anterior basolateral nucleus (BLa) of the mouse, including the localization of M₂Rs in parvalbumin (PV) immunoreactive interneurons, using double-labeling immunoelectron microscopy. Little if any M₂R-immunoreactivity (M₂R-ir) was observed in neuronal somata, but the neuropil was densely labeled. Ultrastructural analysis using a pre-embedding immunogold-silver technique (IGS) demonstrated M₂R-ir in dendritic shafts, spines, and axon terminals forming asymmetrical (excitatory) or symmetrical (mostly inhibitory) synapses. In addition, about one-quarter of PV+ axon terminals and half of PV+ dendrites, localized using immunoperoxidase, were M₂R+ when observed in single thin sections. In all M₂R+ neuropilar structures, including those that were PV+, about one-quarter to two-thirds of M₂R+ immunoparticles were plasma-membrane-associated, depending on the structure. The expression of M₂Rs in PV+ and PV-negative terminals forming symmetrical synapses indicates M₂R modulation of inhibitory transmission. Electrophysiological studies in mouse and rat brain slices, including paired recordings from interneurons and pyramidal projection neurons, demonstrated M₂R-mediated suppression of GABA release. These findings suggest cell-type-specific functions of M₂Rs and shed light on organizing principles of cholinergic modulation in the BLa.

Structural basis for cholinergic regulation of neural circuits in the mouse olfactory bulb

Odor information is regulated by olfactory inputs, bulbar interneurons, and centrifugal inputs in the olfactory bulb (OB). Cholinergic neurons projecting from the nucleus of the horizontal limb of the diagonal band of Broca and the magnocellular preoptic nucleus are one of the primary centrifugal inputs to the OB. In this study, we focused on cholinergic regulation of the OB and analyzed neural morphology with a particular emphasis on the projection pathways of cholinergic neurons. Single-cell imaging of a specific neuron within dense fibers is critical to evaluate the structure and function of the neural circuits. We labeled cholinergic neurons by infection with virus vector and then reconstructed them three-dimensionally. We also examined the ultramicrostructure of synapses by electron microscopy tomography. To further clarify the function of cholinergic neurons, we performed confocal laser scanning microscopy to investigate whether other neurotransmitters are present within cholinergic axons in the OB. Our results showed the first visualization of complete cholinergic neurons, including axons projecting to the OB, and also revealed frequent axonal branching within the OB where it innervated multiple glomeruli in different areas. Furthermore, electron tomography demonstrated that cholinergic axons formed asymmetrical synapses with a morphological variety of thicknesses of the postsynaptic density. Although we have not yet detected the presence of other neurotransmitters, the range of synaptic morphology suggests multiple modes of transmission. The present study elucidates the ways that cholinergic neurons could contribute to the elaborate mechanisms involved in olfactory processing in the OB. J. Comp. Neurol. 525:574-591, 2017. © 2016 Wiley Periodicals, Inc.

Localization of the M2 muscarinic cholinergic receptor in dendrites, cholinergic terminals, and noncholinergic terminals in the rat basolateral amygdala: An ultrastructural analysis

Activation of M2 muscarinic receptors (M2Rs) in the rat anterior basolateral nucleus (BLa) is critical for the consolidation of memories of emotionally arousing events. The present investigation used immunocytochemistry at the electron microscopic level to determine which structures in the BLa express M2Rs. In addition, dual localization of M2R and the vesicular acetylcholine transporter protein (VAChT), a marker for cholinergic axons, was performed to determine whether M2R is an autoreceptor in cholinergic axons innervating the BLa. M2R immunoreactivity (M2R-ir) was absent from the perikarya of pyramidal neurons, with the exception of the Golgi complex, but was dense in the proximal dendrites and axon initial segments emanating from these neurons. Most perikarya of nonpyramidal neurons were also M2R-negative. About 95% of dendritic shafts and 60% of dendritic spines were M2 immunoreactive (M2R(+) ). Some M2R(+) dendrites had spines, suggesting that they belonged to pyramidal cells, whereas others had morphological features typical of nonpyramidal neurons. M2R-ir was also seen in axon terminals, most of which formed asymmetrical synapses. The main targets of M2R(+) terminals forming asymmetrical (putative excitatory) synapses were dendritic spines, most of which were M2R(+) . The main targets of M2R(+) terminals forming symmetrical (putative inhibitory or neuromodulatory) synapses were unlabeled perikarya and M2R(+) dendritic shafts. M2R-ir was also seen in VAChT(+) cholinergic terminals, indicating a possible autoreceptor role. These findings suggest that M2R-mediated mechanisms in the BLa are very complex, involving postsynaptic effects in dendrites as well as regulating release of glutamate, γ-aminobutyric acid, and acetylcholine from presynaptic axon terminals. J. Comp. Neurol. 524:2400-2417, 2016. © 2016 Wiley Periodicals, Inc.

M3 muscarinic receptor in the ventral medial prefrontal cortex modulating the expression of contextual fear conditioning in rats

RATIONALE

Basal forebrain cholinergic neurons modulate the activation of cortical neurons by several stimuli such as fear and anxiety. However, the role of the muscarinic receptor in the medial prefrontal cortex (MPFC) in the modulation of the conditioned emotional response (CER) evoked in the model contextual conditioned fear remains unclear.

OBJECTIVES

The objective of this study is to test the hypothesis that inhibition of the muscarinic receptor in ventral MPFC modulates CER observed during animal's re-exposure to the aversive context.

METHODS

Rats implanted with cannulae aimed at the prelimbic (PL) or the infralimbic (IL) were submitted to a high-intensity contextual fear conditioning protocol. Before the test session, they received microinjections of the hemicholinium (choline reuptake blocker), atropine (muscarinic antagonist), J104129 fumarate (M1-M3 muscarinic antagonists), pirenzepine (M1 muscarinic antagonist), neostigmine (inhibitor acetylcholinesterase enzyme), or the systemic administration of the FG7142 (inverse benzodiazepine agonist). Additional independent groups received the neostigmine or FG7142 before the ineffective doses of J104129 fumarate in the low-intensity protocol of contextual fear conditioning.

RESULTS

In the high-intensity protocol, the administration of hemicholinium (1 nmol), atropine (0.06-6 nmol), J104129 fumarate (6 nmol), or pirenzepine (6 nmol) attenuated the expression of CER in rats. However, in the low-intensity protocol, only J10129 fumarate (0.06 nmol) reduced the expression of the CER. Finally, neostigmine (0.1-1 nmol) or FG7142 (8 mg/Kg) increased CER expression, an effect inhibited by the low dose of the J10129 fumarate.

CONCLUSIONS

These results indicated that the blockade of M3 muscarinic receptor in the vMPFC attenuates the CER expression.

Cholinergic interneurons in the dorsal and ventral striatum: anatomical and functional considerations in normal and diseased conditions

Striatal cholinergic interneurons (ChIs) are central for the processing and reinforcement of reward-related behaviors that are negatively affected in states of altered dopamine transmission, such as in Parkinson's disease or drug addiction. Nevertheless, the development of therapeutic interventions directed at ChIs has been hampered by our limited knowledge of the diverse anatomical and functional characteristics of these neurons in the dorsal and ventral striatum, combined with the lack of pharmacological tools to modulate specific cholinergic receptor subtypes. This review highlights some of the key morphological, synaptic, and functional differences between ChIs of different striatal regions and across species. It also provides an overview of our current knowledge of the cellular localization and function of cholinergic receptor subtypes. The future use of high-resolution anatomical and functional tools to study the synaptic microcircuitry of brain networks, along with the development of specific cholinergic receptor drugs, should help further elucidate the role of striatal ChIs and permit efficient targeting of cholinergic systems in various brain disorders, including Parkinson's disease and addiction.

Effects of galanin subchronic treatment on memory and muscarinic receptors

The cholinergic pathways, which originate in the basal forebrain and are responsible for the control of different cognitive processes including learning and memory, are also regulated by some neuropeptides. One of these neuropeptides, galanin (GAL), is involved in both neurotrophic and neuroprotective actions. The present study has evaluated in rats the effects on cognition induced by a subchronic treatment with GAL by analyzing the passive avoidance response, and the modulation of muscarinic cholinergic receptor densities and activities. [(3)H]-N-methyl-scopolamine, [(3)H]-oxotremorine, and [(3)H]-pirenzepine were used to quantify the density of muscarinic receptors (MRs) and the stimulation of the binding of guanosine 5'-(γ-[(35)S]thio)triphosphate by the muscarinic agonist, carbachol, to determine their functionality. Some cognitive deficits that were induced by the administration of artificial cerebrospinal fluid (aCSF) (i.c.v. aCSF 2 μl/min, once a day for 6 days) were not observed in the animals also treated with GAL (i.c.v. 1.5 mmol in aCSF, 2 μl/min, once a day for 6 days). GAL modulates the changes in M1 and M2 MR densities observed in the rats treated with aCSF, and also increased their activity mediated by G(i/o) proteins in specific areas of the dorsal and ventral hippocampus. The subchronic administration of the vehicle was also accompanied by an increased number of positive fibers and cells for GAL around the cortical tract of the cannula used, but that was not the case in GAL-treated rats. In addition, the increase of GAL receptor density in the ventral hippocampus and entorhinal cortex in the aCSF group was avoided when GAL was administered. The number of acetylcholinesterase (AChE)-positive neurons was decreased in the nucleus basalis of Meynert of both GAL- and aCSF-treated animals. In summary, GAL improves memory-related abilities probably through the modulation of MR density and/or efficacy in hippocampal areas.

The integrated role of ACh, ERK and mTOR in the mechanisms of hippocampal inhibitory avoidance memory

The purpose of this review is to summarize the present knowledge on the interplay among the cholinergic system, Extracellular signal-Regulated Kinase (ERK) and Mammalian Target of Rapamycin (mTOR) pathways in the development of short and long term memories during the acquisition and recall of the step-down inhibitory avoidance in the hippocampus. The step-down inhibitory avoidance is a form of associative learning that is acquired in a relatively simple one-trial test through several sensorial inputs. Inhibitory avoidance depends on the integrated activity of hippocampal CA1 and other brain areas. Recall can be performed at different times after acquisition, thus allowing for the study of both short and long term memory. Among the many neurotransmitter systems involved, the cholinergic neurons that originate in the basal forebrain and project to the hippocampus are of crucial importance in inhibitory avoidance processes. Acetylcholine released from cholinergic fibers during acquisition and/or recall of behavioural tasks activates muscarinic and nicotinic acetylcholine receptors and brings about a long-lasting potentiation of the postsynaptic membrane followed by downstream activation of intracellular pathway (ERK, among others) that create conditions favourable for neuronal plasticity. ERK appears to be salient not only in long term memory, but also in the molecular mechanisms underlying short term memory formation in the hippocampus. Since ERK can function as a biochemical coincidence detector in response to extracellular signals in neurons, the activation of ERK-dependent downstream effectors is determined, in part, by the duration of ERK phosphorylation itself. Long term memories require protein synthesis, that in the synapto-dendritic compartment represents a direct mechanism that can produce rapid changes in protein content in response to synaptic activity. mTOR in the brain regulates protein translation in response to neuronal activity, thereby modulating synaptic plasticity and long term memory formation. Some studies demonstrate a complex interplay among the cholinergic system, ERK and mTOR. It has been shown that co-activation of muscarinic acetylcholine receptors and β-adrenergic receptors facilitates the conversion of short term to long term synaptic plasticity through an ERK- and mTOR-dependent mechanism which requires translation initiation. It seems therefore that the complex interplay among the cholinergic system, ERK and mTOR is crucial in the development of new inhibitory avoidance memories in the hippocampus.

Frequency-dependent, cell type-divergent signaling in the hippocamposeptal projection

Hippocampal oscillations are critical for information processing, and are strongly influenced by inputs from the medial septum. Hippocamposeptal neurons provide direct inhibitory feedback from the hippocampus onto septal cells, and are therefore likely to also play an important role in the circuit; these neurons fire at either low or high frequency, reflecting hippocampal network activity during theta oscillations or ripple events, respectively. Here, we optogenetically target the long-range GABAergic projection from the hippocampus to the medial septum in rats, and thereby simulate hippocampal input onto downstream septal cells in an acute slice preparation. In response to optogenetic activation of hippocamposeptal fibers at theta and ripple frequencies, we elicit postsynaptic GABAergic responses in a subset (24%) of septal cells, most predominantly in fast-spiking cells. In addition, in another subset of septal cells (19%) corresponding primarily to cholinergic cells, we observe a slow hyperpolarization of the resting membrane potential and a decrease in input resistance, particularly in response to prolonged high-frequency (ripple range) stimulation. This slow response is partially sensitive to GIRK channel and D2 dopamine receptor block. Our results suggest that two independent populations of septal cells distinctly encode hippocampal feedback, enabling the septum to monitor ongoing patterns of activity in the hippocampus.

Nicotinic and muscarinic acetylcholine receptors shape ganglion cell response properties

The purpose of this study was to evaluate the expression patterns of nicotinic and muscarinic ACh receptors (nAChRs and mAChRs, respectively) in relation to one another and to understand their effects on rabbit retinal ganglion cell response properties. Double-label immunohistochemistry revealed labeled inner-retinal cell bodies and complex patterns of nAChR and mAChR expression in the inner plexiform layer. Specifically, the expression patterns of m1, m4, and m5 muscarinic receptors overlapped with those of non-α7 and α7 nicotinic receptors in presumptive amacrine and ganglion cells. There was no apparent overlap in the expression patterns of m2 muscarinic receptors with α7 nicotinic receptors or of m3 with non-α7 nicotinic receptors. Patch-clamp recordings demonstrated cell type-specific effects of nicotinic and muscarinic receptor blockade. Muscarinic receptor blockade enhanced the center responses of brisk-sustained/G4 On and G4 Off ganglion cells, whereas nicotinic receptor blockade suppressed the center responses of G4 On-cells near the visual streak but enhanced the center responses of nonstreak G4 On-cells. Blockade of muscarinic or nicotinic receptors suppressed the center responses of brisk-sustained Off-cells and the center light responses of subsets of brisk-transient/G11 On- and Off-cells. Only nicotinic blockade affected the center responses of G10 On-cells and G5 Off-cells. These data indicate that physiologically and morphologically identified ganglion cell types have specific patterns of AChR expression. The cholinergic receptor signatures of these cells may have implications for understanding visual defects in disease states that result from decreased ACh availability.

Control of insulin secretion by cholinergic signaling in the human pancreatic islet

Acetylcholine regulates hormone secretion from the pancreatic islet and is thus crucial for glucose homeostasis. Little is known, however, about acetylcholine (cholinergic) signaling in the human islet. We recently reported that in the human islet, acetylcholine is primarily a paracrine signal released from α-cells rather than primarily a neural signal as in rodent islets. In this study, we demonstrate that the effects acetylcholine produces in the human islet are different and more complex than expected from studies conducted on cell lines and rodent islets. We found that endogenous acetylcholine not only stimulates the insulin-secreting β-cell via the muscarinic acetylcholine receptors M3 and M5, but also the somatostatin-secreting δ-cell via M1 receptors. Because somatostatin is a strong inhibitor of insulin secretion, we hypothesized that cholinergic input to the δ-cell indirectly regulates β-cell function. Indeed, when all muscarinic signaling was blocked, somatostatin secretion decreased and insulin secretion unexpectedly increased, suggesting a reduced inhibitory input to β-cells. Endogenous cholinergic signaling therefore provides direct stimulatory and indirect inhibitory input to β-cells to regulate insulin secretion from the human islet.

Postsynaptic muscarinic m2 receptors at cholinergic and glutamatergic synapses of mouse brainstem motoneurons

In many brain areas, few cholinergic synapses are identified. Acetylcholine is released into the extracellular space and acts through diffuse transmission. Motoneurons, however, are contacted by numerous cholinergic terminals, indicating synaptic cholinergic transmission on them. The muscarinic m2 receptor is the major acetylcholine receptor subtype of motoneurons; therefore, we analyzed the localization of the m2 receptor in correlation with synapses by electron microscopic immunohistochemistry in the mouse trigeminal, facial, and hypoglossal motor nuclei. In all nuclei, m2 receptors were localized at the membrane of motoneuronal perikarya and dendrites. The m2 receptors were concentrated at cholinergic synapses located on the perikarya and most proximal dendrites. However, m2 receptors at cholinergic synapses represented only a minority (<10%) of surface m2 receptors. The m2 receptors were also enriched at glutamatergic synapses in both motoneuronal perikarya and dendrites. A relatively large proportion (20-30%) of plasma membrane-associated m2 receptors were located at glutamatergic synapses. In conclusion, the effect of acetylcholine on motoneuron populations might be mediated through a synaptic as well as diffuse type of transmission.

The anterior cingulate cortex may enhance inhibition of lateral prefrontal cortex via m2 cholinergic receptors at dual synaptic sites

The anterior cingulate cortex (ACC) and dorsolateral prefrontal cortices (DLPFC) share robust excitatory connections. However, during rapid eye movement (REM) sleep, when cortical activity is dominated by acetylcholine, the ACC is activated but DLPFC is suppressed. Using pathway tracing and electron microscopy in nonhuman primates (Macaca mulatta), we tested the hypothesis that the opposite states may reflect specific modulation by acetylcholine through strategic synaptic localization of muscarinic m2 receptors, which inhibit neurotransmitter release presynaptically, but are thought to be excitatory postsynaptically. In the ACC pathway to DLPFC (area 32 to area 9), m2 receptors predominated in ACC axon terminals and in more than half of the targeted dendrites of presumed inhibitory neurons, suggesting inhibitory cholinergic influence. In contrast, in a pathway linking the DLPFC area 46 to DLPFC area 9, postsynaptic m2 receptors predominated in targeted spines of presumed excitatory neurons, consistent with their mutual activation in working memory. These novel findings suggest that presynaptic and postsynaptic specificity of m2 cholinergic receptors may help explain the differential engagement of ACC and DLPFC areas in REM sleep for memory consolidation and synergism in awake states for cognitive control.

Muscarinic and nicotinic acetylcholine receptor agonists and allosteric modulators for the treatment of schizophrenia

Muscarinic and nicotinic acetylcholine (ACh) receptors (mAChRs and nAChRs) are emerging as important targets for the development of novel treatments for the symptoms associated with schizophrenia. Preclinical and early proof-of-concept clinical studies have provided strong evidence that activators of specific mAChR (M(1) and M(4)) and nAChR (α(7) and α(2)β(4)) subtypes are effective in animal models of antipsychotic-like activity and/or cognitive enhancement, and in the treatment of positive and cognitive symptoms in patients with schizophrenia. While early attempts to develop selective mAChR and nAChR agonists provided important preliminary findings, these compounds have ultimately failed in clinical development due to a lack of true subtype selectivity and subsequent dose-limiting adverse effects. In recent years, there have been major advances in the discovery of highly selective activators for the different mAChR and nAChR subtypes with suitable properties for optimization as potential candidates for clinical trials. One novel strategy has been to identify ligands that activate a specific receptor subtype through actions at sites that are distinct from the highly conserved ACh-binding site, termed allosteric sites. These allosteric activators, both allosteric agonists and positive allosteric modulators, of mAChR and nAChR subtypes demonstrate unique mechanisms of action and high selectivity in vivo, and may provide innovative treatment strategies for schizophrenia.

Muscarinic receptor pharmacology and circuitry for the modulation of cognition

The muscarinic cholinergic system constitutes an important part of the neuronal circuitry that modulates normal cognition. Muscarinic receptor antagonists are well known to produce or exacerbate impairments in attention, learning, and memory. Conversely, both direct-acting muscarinic receptor agonists and indirect-acting muscarinic cholinergic agonists, such as acetylcholinesterase inhibitors, have shown cognition-enhancing properties, including improvements in normal cognitive function, reversal of cognitive deficits induced by muscarinic receptor antagonists, and attenuation of cognitive deficits in psychiatric and neurological disorders, such as Alzheimer's disease and schizophrenia. However, until recently, the lack of small molecule ligands that antagonize or activate specific muscarinic acetylcholine receptor (mAChR) subtypes with high selectivity has been a major obstacle in defining the relative contributions of individual mAChRs to different aspects of cognitive function and for the development of novel therapeutic agents. These limitations may be potentially overcome by the recent discovery of novel mAChR subtype-selective compounds, notably allosteric agonists and positive allosteric modulators, which exhibit greater selectivity for individual mAChR subtypes than previous mAChR orthosteric agonists. In preclinical studies, these novel ligands have shown promising efficacy in several models for the enhancement of cognition. In this chapter, we will review the muscarinic cholinergic circuitry and pharmacology of mAChR agonists and antagonists relevant to the modulation of different aspects of cognition in animals and clinical populations.

Neuronal localization of M2 muscarinic receptor immunoreactivity in the rat amygdala

Muscarinic cholinergic neurotransmission in the amygdala is critical for memory consolidation in emotional/motivational learning tasks, but little is known about the neuronal distribution of different receptor subtypes. Immunohistochemistry was used in the present investigation to localize the m2 receptor (M2R). Differential patterns of M2R-immunoreactivity (M2R-ir) were observed in the somata and neuropil of the various amygdalar nuclei. Neuropilar M2R-ir was strongest in rostral portions of the basolateral nuclear complex (BLC). M2R-positive (M2R+) somata were seen in low numbers in all nuclei of the amygdala. Most M2R+ neurons associated with the BLC were in the lateral nucleus and external capsule. These cells were nonpyramidal neurons that contained glutamatic acid decarboxylase (GAD), somatostatin (SOM), and neuropeptide Y (NPY), but not parvalbumin (PV), calretinin (CR), or cholecystokinin (CCK). Little or no M2R-ir was observed in GAD+, PV+, CR+, or CCK+ axons in the BLC, but it was seen in some SOM+ axons and many NPY+ axons. M2R-ir was found in a small number of spiny and aspiny neurons of the central nucleus that were mainly located along the lateral and ventral borders of its lateral subdivision. Many of these cells contained SOM and NPY. M2R+ neurons were also seen in the medial nucleus, including a distinct subpopulation of neurons that surrounded its anteroventral subdivision. The latter neurons were negative for all neuronal markers analyzed. The intercalated nuclei (INs) were associated with two types of large M2R+ neurons, spiny and aspiny. The small principal neurons of the INs were M2R-negative. The somata and dendrites of the large spiny neurons, which were actually found in a zone located just outside of the rostral INs, expressed SOM and NPY, but not GAD. These findings indicate that acetylcholine can modulate a variety of discrete neuronal subpopulations in various amygdalar nuclei via M2Rs, especially neurons that express SOM and NPY.

Number and type of synapses on the distal dendrite of a rat striatal cholinergic interneuron: a quantitative, ultrastructural study

Knowledge of the innervation of interneurons within the striatum is critical to determining their role in the functioning of the striatal network. To this end, the synaptic innervation of a distal dendrite of a rat striatal cholinergic interneuron was quantified for the first time. These synaptic data were compared to three other dendrites from rat striatal interneurons and to published data from dendrites in the mammalian cerebral cortex. To label the cholinergic interneurons and their distal dendrites, a male Wistar rat was perfused and the striatum was double-immunolabelled with an antibody to choline acetyltransferase (ChAT) and an antibody to m2 muscarinic receptor. After processing for transmission electron microscopy, a cholinergic interneuron was located and an m2-labelled distal dendrite identified by tracing it through serial ultrathin sections to this double-immunolabelled soma. Two interneuronal distal dendrites in the same tissue, and another from a second rat, were used for comparison. The widths and lengths of the four distal dendrites, the total number and type of synapses, and the number of synapses per mum for each distal dendrite were measured. Symmetric synapses were the most common type on all four dendrites. There were 0.73 synapses per mum on the distal dendrite of the identified striatal cholinergic interneuron. Two other interneuronal dendrites that were positive for the m2 muscarinic receptor antibody showed similar synaptic densities of 0.62 and 0.83 synapses per microm of distal dendrite, respectively. On a third unlabelled interneuronal distal dendrite located in the lateral striatum, there were 2.17 synapses per microm. This interneuron was thought to be a parvalbumin interneuron rather than a calretinin interneuron, which would more likely be medially located. These data suggest that the number of synapses per microm on the distal dendrite of the cholinergic interneuron, and possibly two other cholinergic interneurons, is three times lower than that of a likely parvalbumin interneuron in the rat striatum. The number of synapses per microm of distal dendrite for a striatal cholinergic interneuron is also lower than the published 1.22-3.3 synapses per microm of dendrite for neurons in the mammalian cerebral cortex. Such anatomical data are important for the construction of new generation computer models that are better able to emulate the operation of striatal cholinergic interneurons.

Localisation of pre- and postsynaptic cholinergic markers in the human brain

The cholinergic neurotransmission in the central nervous system plays an important role in modulating cognitive processes such as learning, memory, arousal and sleep as well as in modulating locomotor activity. Dysfunction of the central cholinergic system is involved in numerous neuropsychiatric diseases. This review will provide a synopsis on the regional localisation of cholinergic and cholinoceptive structures within the adult human brain. On the cholinergic site data based on the distribution of choline acetyltransferase-immunoreactive structures are in the focus, complemented by data from acetylcholinesterase and vesicular acetylcholine transporter studies. On the cholinoceptive site, the distribution and localisation of receptors that transduce the acetylcholine message, i.e. the muscarinic and the nicotinic acetylcholine receptors is summarized. In addition to these data obtained on post mortem brain an overview of markers which allow for the in vivo monitoring of the cholinergic system in the brain is given. The detailed knowledge on the distribution and localisation of cholinergic markers in human brain will provide further information on the cholinergic circuits of neurotransmission - a prerequisite for the interpretation of in vivo imaging data and the development of selective diagnostic and therapeutic compounds.

Muscarinic acetylcholine receptor localization and activation effects on ganglion response properties

PURPOSE

The activation and blockade of muscarinic acetylcholine receptors (mAChRs) affects retinal ganglion cell light responses and firing rates. This study was undertaken to identify the full complement of mAChRs expressed in the rabbit retina and to assess mAChR distribution and the functional effects of mAChR activation and blockade on retinal response properties.

METHODS

RT-PCR, Western blot analysis, and immunohistochemistry were used to identify the complement and distribution of mAChRs in the rabbit retina. Extracellular electrophysiology was used to determine the effects of the activation or blockade of mAChRs on ganglion cell response properties.

RESULTS

RT-PCR of whole neural retina resulted in the amplification of mRNA transcripts for the m1 to m5 mAChR subtypes. Western blot and immunohistochemical analyses confirmed that all five mAChR subtypes were expressed by subpopulations of bipolar, amacrine, and ganglion cells in the rabbit retina, including subsets of cells in cholinergic and glycinergic circuits. Nonspecific muscarinic activation and blockade resulted in the class-specific modulation of maintained ganglion cell firing rates and light responses.

CONCLUSIONS

The expression of mAChR subtypes on subsets of bipolar, amacrine, and ganglion cells provides a substrate for both enhancement and suppression of retinal responses via activation by cholinergic agents. Thus, the muscarinic cholinergic system in the retina may contribute to the modulation of complex stimuli. Understanding the distribution and function of mAChRs in the retina has the potential to provide important insights into the visual changes that are caused by decreased ACh in the retinas of Alzheimer's patients and the potential visual effects of anticholinergic treatments for ocular diseases.

Sleep, activity, temperature and arousal responses of mice deficient for muscarinic receptor M2 or M4

AIMS

The type 2 muscarinic receptor (M2R) differs from the other G-protein-coupled muscarinic receptor (type 4, or M4R) in tissue distribution and physiologic effects. We studied the impact of these receptors on sleep and arousal by using M2R and M4R knock-out (KO) mice.

MAIN METHODS

M2R and M4R KO and genetically intact mice were compared in terms of normal patterns of sleep, responses to sleep loss, infectious challenge and acoustic startle, and acoustic prepulse inhibition of startle (PPI).

KEY FINDINGS

Under basal conditions, M2R and M4R KO mice do not differ from the background strain or each other in the amount or diurnal pattern of sleep, locomotor activity, and body temperature. After enforced sleep loss, M2R KO mice, in contrast to the other two strains, show no rebound in slow-wave sleep (SWS) time, although their SWS is consolidated, and they show a greater rebound in time spent in REMS (rapid-eye-movement sleep) and REMS consolidation. During influenza infection, M2R KO mice, as compared with the other strains, show marked hypothermia and a less robust increase in SWS. During Candida albicans infection, M2R KO mice show a greater increase in SWS and a greater inflammatory response than do the other strains. M2R KO mice also show greater acoustic startle amplitude than does the background strain, although PPI was not different across the 3 strains over a range of stimulus intensities.

SIGNIFICANCE

Taken together, these findings support different roles for M2R and M4R in the modulation of sleep and arousal during homeostatic challenge.

Suitability of muscarinic acetylcholine receptor antibodies for immunohistochemistry evaluated on tissue sections of receptor gene-deficient mice

Acetylcholine (ACh) is a major regulator of visceral function exerting pharmacologically relevant effects upon smooth muscle tone and epithelial function via five types of muscarinic receptors (M1R-M5R). In this paper, we assessed the specificity of muscarinic receptor (MR) antibodies in immunohistochemical labelling on tissue sections by analysing specimens from wild-type and respective gene-deficient mice. Of 24 antibodies evaluated in this study, 16 were tested at 18 different conditions each, and eight of them in 21 different protocols, resulting in a total number of 456 antibody/protocol combinations. Each of them was tested at four antibody dilutions at minimum, so that finally, at least 1,824 conditions were evaluated. For each of them, dorsal root ganglia, urinary bladder and cross-sections through all thoracic viscera were investigated. In all cases where the antigen was available, at least one incubation condition was identified in which only select cell types were immunolabelled in the positive control but remained unlabelled in the pre-absorption control. With two exceptions (M2R antibodies), however, all antibodies produced identical immunohistochemical labelling patterns in tissues taken from corresponding gene-deficient mice even when the pre-absorption control in wild-type mice suggested specificity. Hence, the present data demonstrate the unpleasant fact that reliable immunohistochemical localisation of MR subtypes with antibodies is the exception rather than the rule. Immunohistochemical detection of MR subtype localisation in tissue sections of peripheral organs is limited to the M2R subtype utilising the most commonly used methodological approaches.

Increasing hippocampal acetylcholine levels enhance behavioral performance in an animal model of diencephalic amnesia

Pyrithiamine-induced thiamine deficiency (PTD) was used to produce a rodent model of Wernicke-Korsakoff syndrome that results in acute neurological disturbances, thalamic lesions, and learning and memory impairments. There is also cholinergic septohippocampal dysfunction in the PTD model. Systemic (Experiment 1) and intrahippocampal (Experiment 2) injections of the acetylcholinesterase inhibitor physostigmine were administered to determine if increasing acetylcholine levels would eliminate the behavioral impairment produced by PTD. Prior to spontaneous alternation testing, rats received injections of either physostigmine (systemic=0.075 mg/kg; intrahippocampal=20, 40 ng/muL) or saline. In Experiment 2, intrahippocampal injections of physostigmine significantly enhanced alternation rates in the PTD-treated rats. In addition, although intrahippocampal infusions of 40 ng of physostigmine increased the available amount of ACh in both pair-fed (PF) and PTD rats, it did so to a greater extent in PF rats. The increase in ACh levels induced by the direct hippocampal application of physostigmine in the PTD model likely increased activation of the extended limbic system, which was dysfunctional, and therefore led to recovery of function on the spontaneous alternation task. In contrast, the lack of behavioral improvement by intrahippocampal physostigmine infusion in the PF rats, despite a greater rise in hippocampal ACh levels, supports the theory that there is an optimal range of cholinergic tone for optimal behavioral and hippocampal function.

Learning and memory impairments in a congenic C57BL/6 strain of mice that lacks the M2 muscarinic acetylcholine receptor subtype

The neurotransmitter acetylcholine is an important modulator of cognitive functions including attention, learning, and memory. The actions of acetylcholine are mediated by five distinct muscarinic acetylcholine receptor subtypes (M(1)-M(5)). The lack of drugs with a high degree of selectivity for these subtypes has impeded the determination of which subtypes mediate which components of cholinergic neurotransmission relevant to cognitive abilities. The present study examined the behavioral functions of the M(2) muscarinic receptor subtype by utilizing congenic C57BL/6 mice possessing a null-mutation in the M(2) muscarinic receptor gene (M(2)(-/-) mice). Comprehensive assessment of general health and the neurological function found no major differences between M(2)(-/-) and wild-type (M(2)(+/+)) mice. In the tests of learning and memory, M(2)(-/-) mice were impaired in the acquisition (trials to criterion), but not the retention (72h) of a passive avoidance task. In a novel open field, M(2)(-/-) mice were impaired in between-sessions, but not within-session habituation. In a holeboard test of spatial memory, M(2)(-/-) mice committed more errors in working memory than M(2)(+/+) mice. Reference memory did not differ between the genotypes. M(2)(-/-) mice showed no impairments in either cued or contextual fear conditioning. These findings replicate and extend earlier findings in a hybrid strain and solidify the interpretation that the M(2) receptor plays a critical role in specific components of cognitive abilities.

An allosteric potentiator of M4 mAChR modulates hippocampal synaptic transmission

Muscarinic acetylcholine receptors (mAChRs) provide viable targets for the treatment of multiple central nervous system disorders. We have used cheminformatics and medicinal chemistry to develop new, highly selective M4 allosteric potentiators. VU10010, the lead compound, potentiates the M4 response to acetylcholine 47-fold while having no activity at other mAChR subtypes. This compound binds to an allosteric site on the receptor and increases affinity for acetylcholine and coupling to G proteins. Whole-cell patch clamp recordings revealed that selective potentiation of M4 with VU10010 increases carbachol-induced depression of transmission at excitatory but not inhibitory synapses in the hippocampus. The effect was not mimicked by an inactive analog of VU10010 and was absent in M4 knockout mice. Selective regulation of excitatory transmission by M4 suggests that targeting of individual mAChR subtypes could be used to differentially regulate specific aspects of mAChR modulation of function in this important forebrain structure.

Effects of beta-amyloid protein on M1 and M2 subtypes of muscarinic acetylcholine receptors in the medial septum-diagonal band complex of the rat: relationship with cholinergic, GABAergic, and calcium-binding protein perikarya

Cortical cholinergic dysfunction has been correlated with the expression and processing of beta-amyloid precursor protein. However, it remains unclear as to how cholinergic dysfunction and beta-amyloid (Abeta) formation and deposition might be related to one another. Since the M1- and M2 subtypes of muscarinic acetylcholine receptors (mAChRs) are considered key molecules that transduce the cholinergic message, the purpose of the present study was to assess the effects of the injected Abeta peptide on the number of M1mAchR- and M2mAChR-immunoreactive cells in the medial septum-diagonal band (MS-nDBB) complex of the rat. Injections of Abeta protein into the retrosplenial cortex resulted in a decrease in M1mAChR and M2mAChR immunoreactivity in the MS-nDBB complex. Quantitative analysis revealed a significant reduction in the number of M1mAChR- and M2mAChR-immunoreactive cells in the medial septum nucleus (MS) and in the horizontal nucleus of the diagonal band of Broca (HDB) as compared to the corresponding hemisphere in control animals and with that seen in the contralateral hemisphere, which corresponds to the PBS-injected side. Co-localization studies showed that the M1mAChR protein is localized in GABA-immunoreactive cells of the MS-nDBB complex, in particular those of the MS nucleus, while M2mAChR protein is localized in both the cholinergic and GABAergic cells. Moreover, GABAergic cells containing M2mAChR are mainly localized in the MS nucleus, while cholinergic cells containing M2mAChR are localized in the MS and the HDB nuclei. Our findings suggest that Abeta induces a reduction in M1mAChR- and M2mAChR-containing cells, which may contribute to impairments of cholinergic and GABAergic transmission in the MS-nDBB complex.

Olanzapine treatment decreases the density of muscarinic M2 receptors in the dorsal vagal complex of rats

In this study, we investigated the effects of antipsychotic drugs, olanzapine and haloperidol, on the density of the muscarinic M2 receptors in the dorsal vagal complex (DVC) and hypoglossal nucleus (HN). Female Sprague Dawley rats were treated with olanzapine, haloperidol or vehicle (control) for 1 (short-term) or 12 weeks (long-term). Quantitative autoradiography was used to investigate the M2 receptor density in the DVC and HN using a muscarinic antagonist [(3)H] AF-DX384. Olanzapine, but not haloperidol, treatment induced a significant decrease in the binding density of M2 receptors in the DVC compared to control groups. Although the HN showed a higher density of [(3)H] AF-DX384 binding than the DVC, treatment with both olanzapine and haloperidol did not induce any significant changes in [(3)H] AF-DX384 binding in the HN. These results suggest that olanzapine-induced body weight gain may be associated with functional changes in the muscarinic neurotransmission in the DVC.

Cytoarchitectonic and chemoarchitectonic subdivisions of the perirhinal and parahippocampal cortices in macaque monkeys

Although the perirhinal and parahippocampal cortices have been shown to be critically involved in memory processing, the boundaries and extent of these areas have been controversial. To produce a more objective and reproducible description, the architectonic boundaries and structure of the perirhinal (areas 35 and 36) and parahippocampal (areas TF and TH) cortices were analyzed in three macaque species, with four different staining methods [Nissl and immunohistochemistry for parvalbumin, nonphosphorylated neurofilaments (with SMI-32), and the m2 muscarinic acetylcholine receptor]. We further correlated the architectonic boundary of the parahippocampal cortex with connections to and from different subregions of anterior area TE and with previously published connections with the prefrontal cortex and temporal pole (Kondo et al. [2005] J. Comp. Neurol. 493:479-509). Together, these data provided a clear delineation of the perirhinal and parahippocampal areas, although it differs from previous descriptions. In particular, we did not extend the perirhinal cortex into the temporal pole, and the lateral boundaries of areas 36 and TF with area TE were placed more medially than in other studies. The lateral boundary of area TF in Macaca fuscata was located more laterally than in Macaca fascicularis or Macaca mulatta, although there was no difference in architectonic structure. We recognized a caudal, granular part of the parahippocampal cortex that we termed "area TFO." This area closely resembles the laterally adjacent area TE and the caudally adjacent area V4 but is clearly different from the more rostral area TF. These areas are likely to have distinct functions.

Distribution of RET immunoreactivity in the rodent spinal cord and changes after nerve injury

RET (for "rearranged during transfection") is a transmembrane tyrosine kinase signaling receptor for members of the glial cell line-derived neurotrophic factor (GDNF) family of ligands. We used RET immunohistochemistry (IHC), double-labeling immunofluorescence (IF), and in situ hybridization (ISH) in adult naïve and nerve-injured rats to study the distribution of RET in the spinal cord. In the dorsal horn, strong RET-immunoreactive (-ir) fibers were abundant in lamina II-inner (II(i)), although this labeling was preferentially observed after an antigen-unmasking procedure. After dorsal rhizotomy, RET-ir fibers in lamina II(i) completely disappeared from the dorsal horn, indicating that they were all primary afferents. After peripheral axotomy, RET-ir in primary afferents decreased in lamina II(i) and appeared to increase slightly in laminae III and IV. RET-ir was also observed in neurons and dendrites throughout the dorsal horn. Some RET-ir neurons in lamina I had the morphological appearance of nociceptive projection neurons, which was confirmed by the finding that 53% of RET-ir neurons in lamina I colocalized with neurokinin-1. GDNF-ir terminals were in close proximity to RET-ir neurons in the superficial dorsal horn. In the ventral horn, RET-ir was strongly expressed by motoneurons, with the strongest staining in small, presumably gamma-motoneurons. Increased RET expression following peripheral axotomy was most pronounced in alpha-motoneurons. The expression and regulation pattern of RET in the spinal cord are in line with its involvement in regenerative processes following nerve injury. The presence of RET in dorsal horn neurons, including nociceptive projection neurons, suggests that RET also has a role in signal transduction at the spinal level. This role may include mediating the effects of GDNF released from nociceptive afferent fibers.

Motor cortex inhibitory circuits in dementia with Lewy bodies and in Alzheimer's disease

To determine whether a peculiar neurophysiological profile may contribute to characterize dementia with Lewy bodies (DLB) vs. Alzheimer disease (AD), we used transcranial magnetic stimulation to examine the excitability of two different inhibitory systems of the motor cortex, short latency intracortical inhibition (SICI) and short latency afferent inhibition (SAI) in 10 patients with DLB, in 13 patients with AD and in 15 healthy subjects. SICI and SAI were significantly reduced in AD patients, while both were not significantly different from the controls in DLB patients. The differential pattern of SICI and SAI exhibited by AD vs. DLB may have diagnostic significance in discriminating DLB from AD. Furthermore, this technique may help to clarify the pathophysiological entity of DLB; since SAI is a cortical phenomenon that depends on central cholinergic activity, our findings suggest that the mechanisms of cholinergic depletion in DLB may be different from that in AD, while normal SICI may reflect a less pronounced dysregulation of the intracortical GABAergic inhibitory circuitries in DLB.

Vesicular glutamate (VGlut), GABA (VGAT), and acetylcholine (VACht) transporters in basal forebrain axon terminals innervating the lateral hypothalamus

The basal forebrain (BF) is known to play important roles in cortical activation and sleep, which are likely mediated by chemically differentiated cell groups including cholinergic, gamma-aminobutyric acid (GABA)ergic and other unidentified neurons. One important target of these cells is the lateral hypothalamus (LH), which is critical for arousal and the maintenance of wakefulness. To determine whether chemically specific BF neurons provide an innervation to the LH, we employed anterograde transport of 10,000 MW biotinylated dextran amine (BDA) together with immunohistochemical staining of the vesicular transporter proteins (VTPs) for glutamate (VGluT1, -2, and -3), GABA (VGAT), or acetylcholine (ACh, VAChT). In addition, we applied triple staining for the postsynaptic proteins (PSPs), PSD-95 with VGluT or Gephyrin (Geph) with VGAT, to examine whether the BDA-labeled varicosities may form excitatory or inhibitory synapses in the LH. Axons originating from BDA-labeled neurons in the magnocellular preoptic nucleus (MCPO) and substantia innominata (SI) descended within the medial forebrain bundle and extended collateral varicose fibers to contact LH neurons. In the LH, the BDA-labeled varicosities were immunopositive (+) for VAChT ( approximately 10%), VGluT2 ( approximately 25%), or VGAT ( approximately 50%), revealing an important influence of newly identified glutamatergic together with GABAergic BF inputs. Moreover, in confocal microscopy, VGluT2+ and VGAT+ terminals were apposed to PSD-95+ and Geph+ profiles respectively, indicating that they formed synaptic contacts with LH neurons. The important inputs from glutamatergic and GABAergic BF cells could thus regulate LH neurons in an opposing manner to stimulate vs. suppress cortical activation and behavioral arousal reciprocally.

Muscarinic M(2) acetylcholine receptor distribution in the guinea-pig gastrointestinal tract

In the enteric nervous system, acetylcholine is the most common neurotransmitter to induce gastrointestinal smooth muscle contractions. Cholinergic signaling is mediated by muscarinic acetylcholine receptors on the surface of smooth muscle cells. Five different muscarinic receptor subtypes (M(1)-M(5)) have been identified and characterized, all of which belong to the superfamily of the G-protein-coupled receptor. The muscarinic M(2) acetylcholine receptor is the major muscarinic receptor subtype expressed by smooth muscle tissues in the gastrointestinal tract, where it is coexpressed with a smaller population of M(3) receptor. In this study, we examined the immunohistochemical distribution of the M(2) receptor using a specific antibody in the guinea-pig gastrointestinal tract. M(2) receptor-like immunoreactivity was mainly observed as associated with smooth muscle cells in the gastrointestinal tract. M(2) receptor-like immunoreactivity in smooth muscle cells was distributed throughout the cell membrane associated with caveolae. In the proximal colon, M(2) receptor-like immunoreactivity in the smooth muscle cells was weak. In the small intestine, interstitial cells of Cajal that possessed neurokinin 1 receptor-like immunoreactivity had intense M(2) receptor-like immunoreactivity. In the proximal colon, intramuscular and myenteric interstitial cells of Cajal exhibited M(2) receptor-like immunoreactivity. These findings indicate that, in the gastrointestinal musculature, M(2) receptors are distributed both in the smooth muscle cells and interstitial cells of Cajal, suggesting that the M(2) receptor elicits smooth muscle cell contraction and the interstitial cells of Cajal are the sites of innervation by enteric cholinergic neurons.

Aging and subcellular localization of m2 muscarinic autoreceptor in basalocortical neurons in vivo

By using immunohistochemical approaches at the light and electron microscopic levels, we have shown that aging modifies the subcellular distribution of the m2 muscarinic autoreceptor (m2R) differentially at somato-dendritic postsynaptic sites and at axonal presynaptic sites in cholinergic basalocortical neurons, in vivo. In cholinergic perikarya and dendrites of the nucleus basalis magnocellularis (NBM), aging is associated with a decrease of the density of m2R at the plasma membrane and in the cytoplasm, suggesting a decrease of the total number of m2R in the somato-dendritic field. In contrast, the number of substance P receptors per somato-dendritic surface was not affected. In the frontal cortex (FC), we have shown a decrease of cytoplasmic m2R density also leading to a decrease of the number of m2R per surface of varicosities but with no change of the density of m2R at the membrane. Our results suggest that the decrease of m2R in the somato-dendritic field of the NBM, but not a modification of the number of presynaptic m2 autoreceptors at the plasma membrane in the FC, could contribute to the decrease of the efficacy of cholinergic transmission observed with aging in the rat.

Somato-dendritic nicotinic receptor responses recorded in vitro from the medial septal diagonal band complex of the rodent

The medial septal diagonal band area (MS/DB), made up of GABAergic and cholinergic neurones, plays an essential role in the generation and modulation of the hippocampal theta rhythm. To understand the part that the cholinergic neurones might play in this activity, we sought to determine whether postsynaptic nicotinic receptor responses can be detected in slices of the rodent MS/DB by puffing on acetylcholine (ACh). Neurones were characterized electrophysiologically into GABAergic and cholinergic neurones according to previous criteria. Responses of the MS/SB neurones to ACh were various combinations of fast depolarizations (1.5-2.5 s), fast hyperpolarizations (3-4 s) and slow depolarizations (20-30 s), the latter two being blocked by atropine. The fast depolarizations were partially or not blocked with cadmium and low calcium, tetrodotoxin, and antagonists of other ionotropic receptors, and were antagonized with 25 microm mecamylamine. Pharmacological investigation of the responses showed that the alpha 7* nicotinic receptor type is associated with cholinergic neurones and 10% of the GABAergic neurones, and that non alpha 7* nicotinic receptor subtypes are associated with 50% of the GABAergic neurones. Pharmacological dissection of evoked and spontaneous postsynaptic responses, however, did not provide evidence for synaptic nicotinic receptor transmission in the MS/DB. It was concluded that nicotinic receptors, although prevalent on the somatic and/or dendritic membrane compartments of neurones in the MS/DB, are on extrasynaptic sites where they presumably play a neuromodulatory role. The presence of alpha 7* nicotinic receptors on cholinergic neurones may also render these cells specifically vulnerable to degeneration in Alzheimer's disease.

Degeneration of the basalocortical pathway from the cortex induces a functional increase in galaninergic markers in the nucleus basalis magnocellularis of the rat

The present work aimed 1) to evaluate whether an increase in galanin or galanin receptors could be induced in the nucleus basalis magnocellularis (nbm) by degeneration of the basalocortical neurons from the cortex and 2) to analyze the consequences of such an increase on cortical activity. First, a mild ischemic insult to the frontoparietal cortex was performed to induce the degeneration of the basalocortical system; galanin immunoreactivity, galanin binding sites, and cholinergic muscarinic receptors were quantified through immunocytochemistry and autoradiography. Second, galanin infusions in the nbm were undertaken to mimic a local increase of the galaninergic innervation; cortical acetylcholine release, cerebral glucose use, and cerebral blood flow were then measured as indices of cortical activity. As a result of the cortical ischemic lesion, the postsynaptic M1 and presynaptic M2 muscarinic receptors were found to be reduced in the altered cortex. In contrast, galaninergic binding capacity and fiber density were found to be increased in the ipsilateral nbm in parallel with a local decrease in the cholinergic markers such as the muscarinic M1 receptor density. Galanin infusion into the nbm inhibited the cortical acetylcholine release and cerebral blood flow increases elicited by the activation of the cholinergic basalocortical system but failed to affect acetylcholine release, cerebral blood flow, and cerebral glucose use when injected alone in the nbm. These results demonstrate that degeneration of the basalocortical system from the cortex induces an increase in galaninergic markers in the nbm, a result that might suggest that the galaninergic overexpression described in the basal forebrain of patients with Alzheimer's disease can result from a degeneration of the cholinergic basalocortical system from the cortex. Because galanin was found to reduce the activity of the basalocortical cholinergic system only when this one is activated, galanin might exert its role rather during activation deficits than under resting conditions such as the resting cortical hypometabolism, which is characteristic of Alzheimer's disease.

Septohippocampal acetylcholine: involved in but not necessary for learning and memory?

The neurotransmitter acetylcholine (ACh) has been accorded an important role in supporting learning and memory processes in the hippocampus. Cholinergic activity in the hippocampus is correlated with memory, and restoration of ACh in the hippocampus after disruption of the septohippocampal pathway is sufficient to rescue memory. However, selective ablation of cholinergic septohippocampal projections is largely without effect on hippocampal-dependent learning and memory processes. We consider the evidence underlying each of these statements, and the contradictions they pose for understanding the functional role of hippocampal ACh in memory. We suggest that although hippocampal ACh is involved in memory in the intact brain, it is not necessary for many aspects of hippocampal memory function.

Muscarinic acetylcholine receptor subtypes in cerebral cortex and hippocampus

The M1, M2 and M4 subtypes of mAChRs are the predominant receptors in the CNS. These receptors activate a multitude of signaling pathways important for modulating neuronal excitability, synaptic plasticity and feedback regulation of ACh release. In addition, novel functions mediated by mAChRs are currently being discovered. These studies are greatly facilitated by the recent development of subtype selective toxins and mice lacking individual mAChR genes. Studies in cell culture and the rodent brain demonstrate that mAChR internalization and intracellular trafficking is an important component of mAChR regulation. Characterizing mAChR intracellular trafficking could help facilitate the development of selective mAChR ligands. For example, a selective M1 agonist would cause a shift in the distribution of M1 from the cell surface to an intracellular distribution, while M2 and M4 would remain on the cell surface. Characterizing mAChR intracellular trafficking is also important for understanding the cellular mechanisms that regulate mAChR cell surface expression and signaling. Furthermore, intracellular trafficking has recently been demonstrated to play a role in the development of tolerance to drugs (Whistler et al., 1999; He et al., 2002). Because individual mAChR subtypes are novel targets for treatments of diseases such as Alzheimer's disease and schizophrenia, understanding the mechanisms that regulate mAChR signaling and intracellular trafficking following acute and chronic stimulation might lead to the development of rational strategies.

Spike timing of dendrite-targeting bistratified cells during hippocampal network oscillations in vivo

Behavior-contingent network oscillations bring about transient, functionally coherent neuronal assemblies in the cerebral cortex, including the hippocampus. Inhibitory input on and close to the soma is believed to phase intrinsic oscillations and output of pyramidal cells, but the function of GABA release to pyramidal cell dendrites remains unknown. We recorded the oscillation-locked spike timing of identified bistratified interneurons in rats. These cells mainly innervated small dendritic shafts of pyramidal cells co-aligned with the glutamatergic Schaffer collateral/commissural input. During theta oscillations, bistratified cells fired at a phase when, on average, pyramidal cell dendrites are most hyperpolarized. Interneurons targeting the perisomatic domain discharge at an earlier phase. During sharp wave-associated ripples, bistratified cells fired with high frequency and in-phase with basket cells, on average 1-2 ms after the discharges in pyramidal cell somata and dendrites. Our results indicate that bistratified cells rhythmically modulate glutamatergic input to the dendrites of pyramidal cells to actively promote the precise input/output transformation during network oscillations.

Vesicular localization and activity-dependent trafficking of presynaptic choline transporters

Presynaptic synthesis of acetylcholine (ACh) requires a steady supply of choline, acquired by a plasma membrane, hemicholinium-3-sensitive (HC-3) choline transporter (CHT). A significant fraction of synaptic choline is recovered from ACh hydrolyzed by acetylcholinesterase (AChE) after vesicular release. Although antecedent neuronal activity is known to dictate presynaptic CHT activity, the mechanisms supporting this regulation are unknown. We observe an exclusive localization of CHT to cholinergic neurons and demonstrate that the majority of CHTs reside on small vesicles within cholinergic presynaptic terminals in the rat and mouse brain. Furthermore, immunoisolation of presynaptic vesicles with multiple antibodies reveals that CHT-positive vesicles carry the vesicular acetylcholine transporter (VAChT) and synaptic vesicle markers such as synaptophysin and Rab3A and also contain acetylcholine. Depolarization of synaptosomes evokes a Ca2+-dependent botulinum neurotoxin C-sensitive increase in the Vmax for HC-3-sensitive choline uptake that is accompanied by an increase in the density of CHTs in the synaptic plasma membrane. Our study leads to the novel hypothesis that CHTs reside on a subpopulation of synaptic vesicles in cholinergic terminals that can transit to the plasma membrane in response to neuronal activity to couple levels of choline re-uptake to the rate of ACh release.

Acetylcholinesterase inhibitors activate septohippocampal GABAergic neurons via muscarinic but not nicotinic receptors

Acetylcholinesterase (AChE) inhibitors, which increase synaptic levels of available acetylcholine (ACh) by preventing its degradation, are the most extensively prescribed drugs for the treatment of Alzheimer's disease. In animals, AChE inhibitors improve learning and memory, reverse scopolamine-induced amnesia, and produce hippocampal theta rhythm. The medial septum/diagonal band of Broca (MSDB), which maintains hippocampal theta rhythm and associated mnemonic functions via the septohippocampal pathway, is considered a critical locus for mediating the effects of AChE inhibitors. Using electrophysiological recordings and fluorescent labeling techniques to identify living septohippocampal neurons in rat brain slices, we report that AChE inhibitors, in the absence of exogenous ACh, produce a profound excitation in 94% of septohippocampal GABAergic neurons and an inhibition in 24% of septohippocampal cholinergic neurons. The inhibitory and excitatory effects of AChE inhibitors, presumably, occur due to accumulation of ACh that is released locally within the MSDB via axon collaterals of septohippocampal cholinergic neurons. The excitatory effects of AChE inhibitors on septohippocampal GABAergic neurons were blocked by muscarinic but not nicotinic receptor antagonists, especially by the M3 receptor antagonist, 4-diphenylacetoxy-N-methylpiperidine mustard, and not by M1 or M2/M4 muscarinic receptor antagonists. M3 muscarinic receptor mRNA colocalized with the calcium-binding protein, parvalbumin, a marker of septohippocampal GABAergic neurons. These findings may be useful in designing therapeutic strategies that do not depend on endogenous ACh and may therefore be effective in situations where AChE inhibitors cease to be effective, such as in progressive neurodegeneration.

Long-term effects of BIBN-99, a selective muscarinic M2 receptor antagonist, on improving spatial memory performance in aged cognitively impaired rats

Aged Long-Evans rats were screened for spatial memory deficits using the Morris water maze task. Rats found to have impaired performance on the task (aged-impaired, AI) were then treated with a selective muscarinic M2 receptor antagonist, 5,11-dihydro-8-chloro-11-[[4-[3-[(2,2-dimethyl-1-oxopentyl)ethylamino]propyl]-1-piperidinyl]acetyl]-6H-pyrido[2,3-b][1,4]benzodiazepin-6-one (BIBN-99; 0.5 mg/kg, s.c.), for 3 successive days while receiving additional water maze training. BIBN-99 significantly improved performance in the task during the 3 days of drug treatment. Treatment was then ceased for the remainder of the study and rats were tested again in the water maze on days 10, 17, and 24. Compared to vehicle-treated rats, enhanced performance was observed in the AI rats that had previously been treated with BIBN-99. These results indicate that BIBN-99 enhances spatial learning in AI animals and that enhanced (or long-term) memory persists in the absence of the drug. In a second experiment, a 2-month delay was imposed in between the original water maze screening and the drug treatment regime. Again, BIBN-99 significantly improved performance in AI rats. This latter study suggests that reference memory does not decay, even in an AI animal that had displayed poor learning following original water maze screening. Together, these studies help provide further insight into possible mechanism(s) of reference memory and its potential clinical usefulness.

Co-expression of alpha7 and beta2 nicotinic acetylcholine receptor subunit mRNAs within rat brain cholinergic neurons

Nicotine enhances cognitive and attentional processes through stimulation of the basal forebrain cholinergic system. Although muscarinic cholinergic autoreceptors have been well characterized, pharmacological characterization of nicotinic autoreceptors has proven more difficult. The present study used double-labeling in situ hybridization to determine expression of nicotinic acetylcholine receptor (nAChR) subunit mRNAs within basal forebrain cholinergic neurons in order to gain information about possible nAChR autoreceptor properties. Cholinergic cells of the mesopontine tegmentum and striatal interneurons were also examined, as were septohippocampal GABAergic neurons that interact with cholinergic neurons to regulate hippocampal activity. alpha7 and beta2 nAChR mRNAs were found to be co-expressed in almost all cholinergic cells and in the majority of GABAergic neurons examined. alpha4 nAChR mRNA expression was restricted to cholinergic cells of the nucleus basalis magnocellularis, and to non-cholinergic cells of the medial septum and mesopontine tegmentum. These data suggest possible regional differences in the pharmacological properties of nicotinic autoreceptors on cholinergic cells. Whereas most cholinergic cells express rapidly desensitizing alpha7 homomers or alpha7beta2 heteromers, cortical projection neurons may also express a pharmacologically distinct alpha4beta2 nAChR subtype. There may also be differential nAChR regulation of cholinergic and non-cholinergic cells within the mesopontine tegmentum that are implicated in acquisition of nicotine self-administration.