A peptide muscarinic toxin from the Green Mamba venom shows agonist-like action in an inhibitory avoidance learning task

Memory Recovery Effect of a New Bioactive Innovative Combination in Rats with Experimental Dementia

Alzheimer's disease manifests as a complex pathological condition, with neuroinflammation, oxidative stress and cholinergic dysfunction being a few of the many pathological changes. Due to the complexity of the disease, current therapeutic strategies aim at a multitargeted approach, often relying on a combination of substances with versatile and complementary effects. In the present study, a unique combination of α-lipoic acid, citicoline, extracts of leaves from olive tree and green tea, vitamin D3, selenium and an immune-supporting complex was tested in scopolamine-induced dementia in rats. Using behavioral and biochemical methods, we assessed the effects of the combination on learning and memory, and elucidated the mechanisms of these effects. Our results showed that, compared to its components, the experimental combination was most efficient in improving short- and long-term memory as assessed by the step-through method as well as spatial memory as assessed by T-maze and Barnes maze underlined by decreases in AChE activity (p < 0.05) and LPO (p < 0.001), increases in SOD activity in the cortex (p < 0.05) and increases in catalase (p < 0.05) and GPx (p < 0.01) activities and BDNF (p < 0.001) and pCREB (p < 0.05) levels in the hippocampus. No significant histopathological changes or blood parameter changes were detected, making the experimental combination an effective and safe candidate in a multitargeted treatment of AD.

Hippocampal cholinergic receptors and the mTOR participation in fear-motivated inhibitory avoidance extinction memory

Evidence has demonstrated the hippocampal cholinergic system and the mammalian target of rapamycin (mTOR) participation during the memory formation of aversive events. This study assessed the role of these systems in the hippocampus for the extinction memory process by submitting male Wistar rats to fear-motivated step-down inhibitory avoidance (IA). The post-extinction session administration of the nicotinic and muscarinic cholinergic receptor antagonists, mecamylamine and scopolamine, respectively, both at doses of 2 µg/µl/side, and rapamycin, an mTOR inhibitor (0.02 µg/µl/side), into the CA1 region of the dorsal hippocampus, impaired the IA extinction memory. Furthermore, the nicotinic and muscarinic cholinergic receptor agonists, nicotine and muscarine, respectively, had a dose-dependent effect on the IA extinction memory when administered intra-CA1, immediately after the extinction session. Nicotine (0.6 µg/µl/side) and muscarine (0.02 µg/µl/side), respectively, had no effect, while the higher doses (6 and 2 µg/µl/side, respectively) impaired the IA extinction memory. Interestingly, the co-administration of muscarine at the lower dose blocked the impairment that was induced by rapamycin. This effect was not observed when nicotine at the lower dose was co-administered. These results have demonstrated the participation of the cholinergic receptors and mTOR in the hippocampus for IA extinction, and that the cholinergic agonists had a dose-dependent effect on the IA extinction memory. This study provides insights related to the behavioural aspects and the neurobiological properties underlying the early stage of fear-motivated IA extinction memory consolidation and suggests that there is hippocampal muscarinic receptor participation independent of mTOR in this memory process.

Peptides isolated from animal venom as a platform for new therapeutics for the treatment of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease that deeply affects patients, their family and society. Although scientists have made intense efforts in seeking the cure for AD, no drug available today is able to stop AD progression. In this context, compounds isolated from animal venom are potentially successful drugs for neuroprotection, since they selectively bind to nervous system targets. In this review, we presented different studies using peptides isolated from animal venom for the treatment of AD. This is a growing field that will be very helpful in understanding and even curing neurodegenerative diseases, especially AD.

Critical role of CA1 muscarinic receptors on memory acquisition deficit induced by total (TSD) and REM sleep deprivation (RSD)

AIM

Despite different theories regarding sleep physiological function, an overall census indicates that sleep is useful for neural plasticity which eventually strengthens cognition and brain performance. Different studies show that sleep deprivation (SD) leads to impaired learning and hippocampus dependent memory. According to some studies, cholinergic system plays an important role in sleep (particularly REM sleep), learning, memory, and its retrieval. So this study has been designed to investigate the effect of CA1 Cholinergic Muscarinic Receptors on memory acquisition deficit induced by total sleep deprivation (TSD) and REM sleep deprivation (RSD).

METHOD

A modified water box (locomotor activity may be provide a limiting factor in this method of SD) or multiple platforms were used for induction of TSD or RSD, respectively. Inhibitory passive avoidance apparatus has been used to determine the effects of SD and its changes by physostigmine (as cholinesterase inhibitor) or scopolamine (muscarinic receptor antagonist) on memory formation. Because locomotor activity and pain perception induce critical roles in passive avoidance memory formation, we also measured these factors by open field and hot-plate instruments, respectively.

RESULTS

The results showed that TSD and RSD for 24 hours impaired memory formation but they did not alter locomotor activity. TSD also induced analgesia effect, but RSD did not alter it. Intra-CA1 injection of physostigmine (0.0001μg/rat) and scopolamine (0.01μg/rat) did not alter memory acquisition in the sham-TSD or sham-RSD, by themselves. Moreover, intra-CA1 injection of sub-threshold dose of physostigmine (0.0001μg/rat) and scopolamine (0.01μg/rat) could restore the memory acquisition deficit induced by RSD, while scopolamine could restore TSD-induced amnesia. Both drugs reversed analgesia induced by TSD. None of previous interventions altered locomotor activity.

CONCLUSION

According to this study, CA1 cholinergic muscarinic receptors play an important role in amnesia induced by both TSD and RSD. However further studies are needed for showing cellular and molecular mechanisms of surprising result of similar pharmacological effects using compounds with opposite profiles.

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.

Muscarinic toxins

Muscarinic toxins isolated from the venom of Dendroaspis snakes may interact with a high affinity, large selectivity and various functional properties with muscarinic receptors. Therefore, these toxins are invaluable tools for studying the physiological role, molecular functioning and structural organization of the five subtypes of these G-Protein Coupled Receptors. We review the data on the most relevant results dealing with the isolation/identification, mode of action, structure/function and exploitation of these toxins and finally highlight the unresolved issues related to their pharmacological studies.

Involvement of the cholinergic system in conditioning and perceptual memory

The cholinergic systems play a pivotal role in learning and memory, and have been the centre of attention when it comes to diseases containing cognitive deficits. It is therefore not surprising, that the cholinergic transmitter system has experienced detailed examination of its role in numerous behavioural situations not least with the perspective that cognition may be rescued with appropriate cholinergic 'boosters'. Here we reviewed the literature on (i) cholinergic lesions, (ii) pharmacological intervention of muscarinic or nicotinic system, or (iii) genetic deletion of selective receptor subtypes with respect to sensory discrimination and conditioning procedures. We consider visual, auditory, olfactory and somatosensory processing first before discussing more complex tasks such as startle responses, latent inhibition, negative patterning, eye blink and fear conditioning, and passive avoidance paradigms. An overarching reoccurring theme is that lesions of the cholinergic projection neurones of the basal forebrain impact negatively on acquisition learning in these paradigms and blockade of muscarinic (and to a lesser extent nicotinic) receptors in the target structures produce similar behavioural deficits. While these pertain mainly to impairments in acquisition learning, some rare cases extend to memory consolidation. Such single case observations warranted replication and more in-depth studies. Intriguingly, receptor blockade or receptor gene knockout repeatedly produced contradictory results (for example in fear conditioning) and combined studies, in which genetically altered mice are pharmacological manipulated, are so far missing. However, they are desperately needed to clarify underlying reasons for these contradictions. Consistently, stimulation of either muscarinic (mainly M(1)) or nicotinic (predominantly α7) receptors was beneficial for learning and memory formation across all paradigms supporting the notion that research into the development and mechanisms of novel and better cholinomimetics may prove useful in the treatment of neurodegenerative or psychiatric disorders with cognitive endophenotypes.

Muscarinic toxins: tools for the study of the pharmacological and functional properties of muscarinic receptors

Muscarinic receptors mediate metabotropic actions of acetylcholine in the CNS and PNS and autocrine functions of acetylcholine in non-neuronal systems. Because of the lack of highly selective muscarinic ligands, the precise location, functional role, and roles in various diseases of the five muscarinic receptor subtypes remain unclear. Muscarinic toxins isolated from the venom of Dendroaspis snakes have a natural high affinity and selectivity, associated with roles as competitive antagonists, allosteric modulators, and potential agonists. These toxins may therefore be invaluable tools for studying muscarinic receptors. We review data on the structural and pharmacological characterization of the muscarinic toxins, focusing on recent structure-function studies on toxin-receptor interactions. We discuss the potential benefits of using these toxins for investigating muscarinic function in vivo.

Muscarinic inhibition of hippocampal and striatal adenylyl cyclase is mainly due to the M(4) receptor

The five muscarinic acetylcholine receptors (M(1)-M(5)) are differentially expressed in the brain. M(2) and M(4) are coupled to inhibition of stimulated adenylyl cyclase, while M(1), M(3) and M(5) are mainly coupled to the phosphoinositide pathway. We studied the muscarinic receptor regulation of adenylyl cyclase activity in the rat hippocampus, compared to the striatum and amygdala. Basal and forskolin-stimulated adenylyl cyclase activity was higher in the striatum but the muscarinic inhibition was much lower. Highly selective muscarinic toxins MT1 and MT2-affinity order M(1) > or = M(4) >> others-and MT3-highly selective M(4) antagonist-did not show significant effects on basal or forskolin-stimulated cyclic AMP production but, like scopolamine, counteracted oxotremorine inhibition. Since MTs have negligible affinity for M(2), M(4) would be the main subtype responsible for muscarinic inhibition of forskolin-stimulated enzyme. Dopamine stimulated a small fraction of the enzyme (3.1% in striatum, 1.3% in the hippocampus). Since MT3 fully blocked muscarinic inhibition of dopamine-stimulated enzyme, M(4) receptor would be responsible for this regulation.

Acute and subacute exposure to malathion impairs aversive but not non-associative memory in rats

Malathion [S-(1,2-dicarbethoxy) ethyl-0,0-dimethyl-phosphorodithioate] is an organophosphorus compound that is widely used as pesticide especially in developing countries. This pesticide affects the central nervous system by inhibiting acetylcholinesterase, leading to an increase of acetylcholine in the synaptic cleft, and subsequent activation of cholinergic muscarinic and nicotinic receptors. In humans, intoxication with organophosphates causes a wide range of neurological symptoms, including memory deficits. The present study was aimed to investigate the effects of the acute (1 h prior the test) and subacute (once a day for 28 days) exposure to malathion at doses of 25, 50, 100 and 150 mg/kg in rats tested in the step-down inhibitory avoidance task, open-field habituation and elevated plus-maze tests. Interestingly, the acute and subacute treatment with malathion impaired aversive-memory in the step-down inhibitory avoidance task, but did not alter the animal performance in the elevated plus-maze and in the habituation to the open-field tests, and neither modified spontaneous locomotion. The activity of acetylcholinesterase enzyme was significantly reduced after subacute, but not acute, treatment with malathion (25, 100 and 150 mg/kg). Our results suggest that malathion impairs aversive-memory retention but not non-associative memory, without affecting anxiety-related behaviors. These findings support the view that the inhibition of acetylcholinesterase enzyme is not correlated with cognitive deficits observed in acute and subacute malathion-treated rats.

M1 Muscarinic Acetylcholine Receptor Agonism Alters Sleep without Affecting Memory Consolidation

Preclinical studies have implicated cholinergic neurotransmission, specifically M1 muscarinic acetylcholine receptor (mAChR) activation, in sleep-associated memory consolidation. In the present study, we investigated the effects of administering the direct M1 mAChR agonist RS-86 on pre-post sleep memory consolidation. Twenty healthy human participants were tested in a declarative word-list task and a procedural mirror-tracing task. RS-86 significantly reduced rapid eye movement (REM) sleep latency and slow wave sleep (SWS) duration in comparison with placebo. Presleep acquisition and postsleep recall rates were within the expected ranges. However, recall rates in both tasks were almost identical for the RS-86 and placebo conditions. These results indicate that selective M1 mAChR activation in healthy humans has no clinically relevant effect on pre-post sleep consolidation of declarative or procedural memories at a dose that reduces REM sleep latency and SWS duration.

Identification of Various Allosteric Interaction Sites on M1Muscarinic Receptor Using125I-Met35-Oxidized Muscarinic Toxin 7

Monoiodinated, Met35-oxidized muscarinic toxin 7 (MT7ox) was synthesized, and its affinity constants for free or N-methyl scopolamine (NMS)-occupied hM1 receptor were measured directly by equilibrium and kinetic binding experiments. Identical values were obtained with the two types of assay methods, 14 pM and 0.9 nM in free or NMS-liganded receptor states, respectively, highlighting a strong negative cooperativity between this allosteric toxin and NMS. Identical results were obtained with indirect binding experiments with [3H]NMS using the ternary complex model, clearly demonstrating the reciprocal nature of this cooperativity. Furthermore, the effects of various orthosteric and allosteric agents on the dissociation kinetic of 125I-MT7ox were measured and show that, except for the MT1 toxin, all of the ligands studied [NMS, atropine, gallamine, brucine, tacrine, staurosporine, and (9S,10S,12R)-2,3,9,10,11-hexahydro-10-hydroxy-9-methyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid hexyl ester (KT5720)] interact allosterically with muscarinic toxin 7. Equilibrium binding experiments with 125I-MT7ox and [3H]NMS were conducted to reveal the effects of these ligands on the free receptor, and affinity constants (pKx values) were calculated using the allosteric ternary complex model. Our results suggest that MT7 toxin interacts with hM1 receptor at a specific allosteric site, which may partially overlap those identified previously for "classic" or "atypical" allosteric agents and highlight the potential of this new allosteric tracer in studying allosterism at muscarinic receptors.

Effects of muscarinic toxins MT2 and MT7, from green mamba venom, on m1, m3 and m5 muscarinic receptors expressed in Chinese Hamster Ovary cells

Several small proteins called muscarinic toxins (MTs) have been isolated from venom of green mamba (Dendroaspis angusticeps). They have previously been shown in radioligand binding studies to have high selectivity and affinity for individual muscarinic receptor subtypes, but less is known of their functional effects. This study has examined the actions of two of these MTs, MT2 and MT7, using changes in cytosolic Ca(2+) ([Ca(2+)](i)) measured using the fluorescent indicator fura-2 in Chinese Hamster Ovary (CHO) cells stably transfected with individual muscarinic receptor subtypes, m1, m3 and m5. MT2 activated the m1 receptor: at concentrations above 100 nM it caused significant and concentration-dependent increases in [Ca(2+)](i). From 25 to 800 nM MT2 also produced increases in [Ca(2+)](i) by activating m3 receptors, although these increases in [Ca(2+)](i) were not strictly concentration-dependent with only intermittent responses being recorded (i.e. it was not always possible to obtain a response to the agonist with each application of the compound). MT2 (800-1600 nM) also caused significant increases in [Ca(2+)](i) in CHO cells expressing the m5 muscarinic receptor subtype. MT7 (1 microM) displayed no agonist activity at any of the muscarinic receptors but was a potent non-competitive antagonist (at 20 nM) at the m1 muscarinic receptor subtype. It had no antagonist activity at the m3 or m5 subtypes. These results indicate that MT7 is a highly specific antagonist at the m1 muscarinic receptor subtype as suggested by results from radioligand binding studies. However, MT2 is less selective for the m1 muscarinic receptor than previously described as it also exhibits agonist activity at the m3 and m5 muscarinic receptors, which was not detected in radioligand binding studies.

Role of hippocampal M1 and M4 muscarinic receptor subtypes in memory consolidation in the rat

Muscarinic receptors in the hippocampus are relevant to learning and memory, but the role of each subtype is poorly understood. Muscarinic toxins (MTs) from Dendroaspis snakes venom are selective for muscarinic receptor subtypes. MT2, a selective agonist for M(1) receptors, given into the hippocampus immediately after training, improved memory consolidation of an inhibitory avoidance task in rats, whereas the antagonist pirenzepine was amnestic, supporting a facilitatory role of M(1) receptors. Instead, MT3, a selective antagonist at M(4) receptors, caused amnesia. Neither M(1) nor M(4) receptor appeared involved in habituation to a new environment. Thus, our results suggest that memory consolidation of an inhibitory avoidance task in the rat involves the participation of both M(1) and M(4) hippocampal receptors, with a positive modulatory role.

Effects of muscarinic toxins MT1 and MT2 from green mamba on different muscarinic cholinoceptors

MT1 and MT2, polypeptides from green mamba venom, known to bind to muscarinic cholinoceptors, behave like muscarinic agonists in an inhibitory avoidance task in rats. We have further characterised their functional effects using different preparations. MT1 and MT2 behaved like relatively selective muscarinic M1 receptor agonists in rabbit vas deferens, but their effects were not reversed by washing or prevented by muscarinic antagonists, although allosteric modulators altered responses to MT1. Radioligand binding experiments indicated that both toxins irreversibly inhibited [3H]N-methylscopolamine binding to cloned muscarinic M1 and M4 receptors, and reduced binding to M5 subtype with lower affinity, while they reversibly inhibited the binding of [3H]prazosin to rat cerebral cortex and vas deferens, with 20 fold lower affinity. High concentrations of MT1 reversibly blocked responses of vas deferens to noradrenaline. MT1 and MT2 appear to irreversibly activate muscarinic M1 receptors at a site distinct from the classical one, and to have affinity for some alpha-adrenoceptors.

Inhibition of acetylcholine muscarinic M(1) receptor function by the M(1)-selective ligand muscarinic toxin 7 (MT-7)

MT-7 (1 - 30 nM), a peptide toxin isolated from the venom of the green mamba Dendroaspis angusticeps and previously found to bind selectively to the muscarinic M(1) receptor, inhibited the acetylcholine (ACh)-stimulated [(35)S]-guanosine-5'-O-(3-thio)triphosphate ([(35)S]-GTPgammaS) binding to membranes of Chinese hamster ovary (CHO) cells stably expressing the cloned human muscarinic M(1) receptor subtype. MT-7 failed to affect the ACh-stimulated [(35)S]-GTPgammaS binding in membranes of CHO cells expressing either the M(2), M(3) or M(4) receptor subtype. In N1E-115 neuroblastoma cells endogenously expressing the M(1) and M(4) receptor subtypes, MT-7 (0.3 - 3.0 nM) inhibited the carbachol (CCh)-stimulated inositol phosphates accumulation, but failed to affect the CCh-induced inhibition of pituitary adenylate cyclase activating polypeptide (PACAP) 38-stimulated cyclic AMP accumulation. In both CHO/M(1) and N1E-115 cells the MT-7 inhibition consisted in a decrease of the maximal agonist effect with minimal changes in the agonist EC(50) value. In CHO/M(1) cell membranes, MT-7 (0.05 - 25 nM) reduced the specific binding of 0.05, 1.0 and 15 nM [(3)H]-N-methylscopolamine ([(3)H]-NMS) in a concentration-dependent manner, but failed to cause a complete displacement of the radioligand. Moreover, MT-7 (3 nM) decreased the dissociation rate of [(3)H]-NMS by about 5 fold. CHO/M(1) cell membranes preincubated with MT-7 (10 nM) and washed by centrifugation and resuspension did not recover control [(3)H]-NMS binding for at least 8 h at 30 degrees C. It is concluded that MT-7 acts as a selective noncompetitive antagonist of the muscarinic M(1) receptors by binding stably to an allosteric site.

Muscarinic toxins: novel pharmacological tools for the muscarinic cholinergic system

Muscarinic receptors are widely spread throughout the body, and are involved in the regulation of fundamental physiological processes, like the modulation of the heart rate, control of motor systems and modulation of learning and memory. In the central nervous system the cholinergic transmission is mainly mediated by muscarinic receptors; there are five subtypes that are all expressed in the brain of mammals (m1-m5). There are regional differences in their concentrations in the brain and more than one subtype is expressed in the same cell. It has been difficult to study their localization and function in vivo due to the lack of ligands that exclusively act on one subtype of the receptor. We studied the action of the muscarinic toxins MT1, MT2 and MT3, from the venom of the snake Dendroaspis angusticeps, on muscarinic receptors, by using the classical muscarinic radioligand 3H-NMS as reporter of the inhibition of its own binding, to either native or cloned receptors. We have also studied the in vivo effects on memory retention of the injection of the toxins into discrete brain regions. The muscarinic toxins appear to be invaluable tools to study receptor pharmacology, physiology and structure/function relationships. They would enable the design of new, more selective, pharmacological agents.

Time-dependent enhancement of inhibitory avoidance retention and MAPK activation by post-training infusion of nerve growth factor into CA1 region of hippocampus of adult rats

Several studies have demonstrated that chronic intracerebroventricular nerve growth factor (NGF) infusion has a beneficial effect on cognitive performance of lesioned as well as old and developing animals. Here we investigate: (i) the effect of post-training infusion of NGF into the CA1 region of hippocampus on inhibitory avoidance (IA) retention in rats; (ii) the extension of the effect, in time and space, of NGF infusion into CA1 on the activity of mitogen-activated protein kinase (MAPK, syn: ERK1/2, p42/p44 MAPK). NGF was bilaterally injected into the CA1 regions of the dorsal hippocampus (0.05, 0.5 or 5.0 ng diluted in 0.5 microL of saline per side ) at 0, 120 or 360 min after IA training in rats. Retention testing was carried out 24 h after training. The injection of 5.0 and 0.5, but not 0.05, ng per side of NGF at 0 and 120 min after IA training enhanced IA retention. The highest dose used was ineffective when injected 360 min after training. The infusion of 0. 5 microL of NGF (5.0 ng) induced a significant enhancement of MAPK activity in hippocampal microslices; this enhancement was restricted to a volume with 0.8 mm radius at 30 min after injection. The MAPK activation was still seen 180 min after NGF infusion, although this value showed only a tendency. In conclusion, localized infusion of NGF into the CA1 region enhanced MAPK activity, restricted in time and space, and enhanced IA retention in a time- and dose-dependent manner.

Combined solid-phase and solution approach for the synthesis of large peptides or proteins

In the synthesis of large peptides or proteins, highly homogeneous segments are indispensable for a convergent strategy either on a solid-phase resin or in solution. Employing Boc/Bzl chemistry to prepare fully protected segments with a free alpha-carboxyl group from the solid support, base-labile linkers are profitable for practical peptide synthesis since they require no special equipment. For this purpose, an N-[9-(hydroxymethyl)-2-fluorenyl]succinamic acid (HMFS) linker was adopted. Consequently, there must be high compatibility between the protecting groups of the segment and the anchoring group which is cleavable by treatment with morpholine or piperidine in DMF. Instead of using the 2-bromobenzyloxycarbonyl (BrZ) group for the Tyr residue and the formyl (For) group for the Trp residue, both of which are the most susceptible protecting groups under these base-catalysed conditions, the base-resistant 3-pentyl (Pen) and cyclohexyloxycarbonyl (Hoc) groups were introduced to the respective side-chain functional groups. By applying the present strategy, the authors were able to rapidly synthesize homogeneous protected segments for use in the subsequent segment coupling in solution. In the present paper, the utility of the combined solid-phase and solution approach is demonstrated by synthesizing muscarinic toxin 1 (MTX1) which binds to the muscarinic acetylcholine receptors.

Muscarinic toxins from the green mamba

Muscarinic acetylcholine receptors are involved in many important physiological processes. Discovery of different subtypes of muscarinic receptors that are responsible for modulating specific physiological events was a key development in muscarinic receptor research. However, the lack of highly selective muscarinic agonists and antagonists has made the classification of a muscarinic receptor subtype responsible for the mediation or modulation of a particular response very difficult. Toxins have previously proved to be highly useful pharmacological tools, due to their high potency and selectivity. This review looks at a new class of muscarinic ligand isolated from the venom of the Eastern green mamba (Dendroaspis angusticeps). Just over a decade ago, it was found that two toxins from the green mamba venom appeared to distinguish between different muscarinic receptor subtypes. Since then, at least 10 more muscarinic toxins (MTs) have been isolated from mamba venom. In recent years, some of the MTs have been used as pharmacological tools; for example, to determine the muscarinic receptor subtype involved in inhibition of adenylyl cyclase in rat striatum. This review looks at the progress that has been made over the past 10 years in the area of MT research and examines whether or not these new peptides are a new way forward in the field of muscarinic receptor research.

MIT(1), a black mamba toxin with a new and highly potent activity on intestinal contraction

Mamba intestinal toxin (MIT(1)) isolated from Dendroaspis polylepis venom is a 81 amino acid polypeptide cross-linked by five disulphide bridges. MIT(1) has a very potent action on guinea-pig intestinal contractility. MIT(1) (1 nM) potently contracts longitudinal ileal muscle and distal colon, and this contraction is equivalent to that of 40 mM K(+). Conversely MIT(1) relaxes proximal colon again as potently as 40 mM K(+). The MIT(1)-induced effects are antagonised by tetrodotoxin (1 microM) in proximal and distal colon but not in longitudinal ileum. The MIT(1)-induced relaxation of the proximal colon is reversibly inhibited by the NO synthase inhibitor L-NAME (200 microM). (125)I-labelled MIT(1) binds with a very high affinity to both ileum and brain membranes (K(d)=1.3 pM and 0.9 pM, and B(max)=30 fmol/mg and 26 fmol/mg, respectively). MIT(1) is a very highly selective toxin for a receptor present both in the CNS and in the smooth muscle and which might be an as yet unidentified K(+) channel.

Contribution of cholinergic and gabaergic functions to memory processes in BALB/cANnCrlBR mice

Several lines of evidence indicate that glucose influences on memory depend on interactions between glucose, glucoregulation and hippocampal cholinergic function. We previously demonstrated that glucose and scopolamine differentially affected memory consolidation for an operant bar pressing task in two closely-related BALB/c mouse strains. Whereas glucose normally improves memory in several animal strains, memory consolidation was not effected by systemic glucose injections in BALB/cANnCrlBR mice. Moreover, these mice were relatively insensitive to the normally observed amnestic effects of scopolamine. We therefore sought to determine whether cholinergic mechanisms in the dorsal hippocampus were involved in such atypical drug effects on memory processing in that strain of mice. In Experiment 1, we examined whether post-training oxotremorine would also atypically influence memory consolidation for an appetitively reinforced operant bar pressing task following microinjection in the dorsal hippocampus. In Experiment 2, we examined the effects of intrahippocampal GABAA drugs on memory consolidation. The non-selective muscarinic agonist, oxotremorine, dose-dependently impaired memory and the GABAA antagonist, bicuculline, improved retention in BALB/cANnCrlBR mice. It was concluded that GABA-mediated influences on hippocampal pyramidal output in BALB/cANnCrlBR mice and other strains are similar; but the amnestic effects of oxotremorine from the dorsal hippocampus were opposite to facilitating effects normally observed in other animal strains. Results are discussed relative to possible altered septo-hippocampal cholinergic neurotransmission in BALB/cANnCrlBR mice.

What can toxins tell us for drug discovery?

Toxins are of interest in drug design because the toxins provide three-dimensional templates for creating small molecular mimics with interesting pharmacological properties. Toxins are also useful in drug discovery because they can be used as pharmacological tools to uncover potential therapeutic targets. With their high potency and selectivity, toxins are often more useful in functional experiments than standard pharmacological agents. We have used two groups of neurotoxins, the dendrotoxins and the muscarinic toxins (MTs), to explore the involvement of subtypes of potassium ion channels and muscarinic receptors, respectively, in processes involved in cognition and the changes in neuronal properties with aging. From our current work, quantitative autoradiographic studies with radiolabelled dendrotoxins reveal widespread distribution of binding sites throughout rat brain sections, but few differences exist between young adult and aged rats. However, displacement studies with toxin K, which preferentially binds to the Kv1.1 subtype of cloned potassium channel, show the selective loss of such sites in regions of the hippocampus and septohippocampal pathway with aging. MTs have been tested for effects on performance of rats in memory paradigms. MT2, which activates m1 receptors, improves performance of rats in a step-down inhibitory avoidance test, whereas MT3, which blocks m4 receptors, decreases performance when given into the hippocampus. This is the first clear demonstration of a role for m4 muscarinic receptors in cognition.

Anti-muscarinic toxins from Dendroaspis angusticeps

Toxins from the venom of the African green mamba, Dendroaspis angusticeps, fulfill a major need for selective ligands for some of the five genetically defined subtypes of muscarinic acetylcholine receptors (m1-m5). Two toxins have been found that are highly selective antagonists for m1 and m4 receptors (m1-toxin and m4-toxin, respectively). Two other toxins (MT1 and MT2) bind with high affinity to both m1 and m4 receptors, and are agonists. Components of the venom also modify the binding of radiolabeled antagonists to m2 receptors, but an m2-selective toxin has not yet been isolated, m1-Toxin can bind to m1 receptors at the same time as typical competitive antagonists, suggesting that this toxin binds to the N-terminal and outer loops of m1 receptor molecules, rather than within the receptor pocket where typical agonists and antagonists bind. The binding of toxins to the outer parts of receptor molecules probably accounts for their much higher specificity for individual receptor subtypes than is seen with smaller ligands. Toxins are useful for identifying, counting, localizing, activating and blocking m1 and m4 receptors with high specificity.

Long-term time course of regional changes in cholinergic indices following transient ischemia in the spontaneously hypertensive rat brain

Using an animal model of forebrain ischemia in spontaneously hypertensive rats (SHR) by 3-h bilateral carotid occlusion, and various indices of the cerebral cholinergic system were assessed for periods up to 24 weeks. The lesions observed histologically in the hippocampus of SHR 2 weeks after ischemia were less severe than those in the frontal cortex. Marked elevation of acetylcholine concentration was transiently observed in the frontal cortex, hippocampus and thalamus + midbrain at 2 weeks, and in the striatum at 1-4 weeks after ischemia. Choline acetyltransferase activity remained unchanged in all regions throughout the experimental period except for a minimal decrease in the frontal cortex at 4 weeks. Choline esterase (ChE) activity was slightly decreased in the frontal cortex at 2-4 weeks after ischemia but recovered by 8 weeks. A decrease in the hippocampus was seen at 8 weeks. The B(max) for the M1-receptor was significantly reduced by 2 weeks in the frontal cortex and by 4 weeks in the hippocampus. Low B(max) values in both regions persisted through week 24. These delayed hippocampal changes in the ChE activity and M1-receptor in SHR were similar to those of the very much delayed changes in M1-receptor previously reported in the gerbil model for transient ischemia. In contrast, Wistar-Kyoto rats (WKY), used as normotensive controls, exhibited no histological or biochemical changes for up to 24 weeks. The difference between SHR and WKY may depend on the more severe cerebral blood flow depletion during carotid ligation in the former. The chronic state of SHR after the transient ischemia may be a useful pathophysiological model for human cerebral infarctions with hypertension.

A particularly labile Asp-Pro bond in the green mamba muscarinic toxin MTX2. Effect of protein conformation on the rate of cleavage

The single Asp53-Pro54 bond of the MTX2 toxin from the mamba snake Dendroaspis angusticeps is rapidly and efficiently cleaved in acidic solution (pH 1.5-2.5) at 45 degrees C. Unfolding of the toxin slows down the cleavage reaction by several times. Modelling studies indicate that the native toxin conformation can catalyse the Asp53-Pro54 bond cleavage. The implications of this study are: (i) cleavage of Asp-Pro bond for sequence determination may occur better in absence than in presence of denaturant, (ii) mild acid conditions, commonly used in NMR structure determinations, may irreversibly affect the structural integrity of Asp-Pro containing peptides and proteins.

M1 receptors in blood pressure-controlled ischemic spontaneously hypertensive rats

BACKGROUND AND PURPOSE

Hypertension is a primary aggravating factor in cerebral infarction. An acute rise in blood pressure (BP) at the time of a stroke may be harmful to the brain in a hypertensive subject because both cerebral vascular structure and function are altered by hypertension. Muscarinic M1 receptors are concerned with memory and learning. We aimed to evaluate the effect of controlling BP in hypertensive subjects at the time of stroke with a biochemical index of brain damage.

METHODS

We gave a single dose of either the antihypertensive alpha-blocker phentolamine (2 mg/kg IP) or the calcium antagonist nicardipine (2 mg/kg IP) at the start of bilateral carotid artery occlusion to spontaneously hypertensive rats undergoing 3 hours of transient ischemia; we measured the time course of mean BP (MBP) and changes in the M1 receptor and its mRNA in three brain regions 2 weeks after the transient ischemia.

RESULTS

Administration of phentolamine or nicardipine not only significantly suppressed the ischemia-induced rise of MBP, it actually decreased MBP during ischemia. In an ischemic control group, M1 receptor binding decreased in the frontal cortex and M1 receptor mRNA increased in the hippocampus 2 weeks after the ischemia. In contrast, both phentolamine- and nicardipine-treated ischemic rats showed no changes in either index compared with sham-operated controls.

CONCLUSIONS

Controlling BP during an ischemic insult attenuates ischemia-induced damage of M1 receptors in the brain of spontaneously hypertensive rats. These results suggest that a rapid intensive increase of BP at the time of a stroke may exacerbate brain damage in hypertensive individuals.

A snake toxin against muscarinic acetylcholine receptors: amino acid sequence, subtype specificity and effect on guinea-pig ileum

The sequence of muscarinic toxin 1 (MT1) from Dendroaspis angusticeps (green mamba) was determined (66 amino acids, M(r) 7509). The central part, peptide 25-40, is rich in hydrophobic amino acids, which is a characteristic of muscarinic toxins. MT1 started to inhibit [3H]-NMS (N-methylscopolamine) binding to synaptosomal membranes of porcine brain (contains all five receptor subtypes) at about 1 nM and to membranes from pig heart muscle (only subtype m2) at about 1 microM. Binding of [3H]-AF-DX 384 to heart was inhibited with an IC50 of 14 microM and to brain in two steps. In the first step (IC50 = 32 nM) binding decreased by 37%, indicating that the toxin acted on m1 or m4 receptors, each accounting for about 40% of total receptor content. The second step was similar to the effect on heart. Pirenzepine inhibited binding of [125I]-MT1 to brain receptors with an IC50 of 6.5 nM, corresponding to a Ki of about 6 nM. Literature values of Ki for pirenzepine are 16-18 nM for m1 and > or = 120 mM for other subtypes. This indicates binding to m1 receptors. mM for other subtypes. This indicates binding to m1 receptors. [125I]-MT1 bound to brain with a Kd of 20 nM and a Hill coefficient of 1.0, i.e. one toxin molecule per receptor. In guinea-pig ileum, MT1 (670 nM) produced a rapid contraction, reversible by atropine. The toxin may be an agonist, but might also cause contraction by inducing acetylcholine release by a different mechanism.

Muscarinic toxins from the venom of Dendroaspis snakes with agonist-like actions

The venom of some Dendroaspis snakes contains small proteins (7500 mol. wt) that inhibit the binding of radiolabelled muscarinic antagonist to brain synaptomal membranes. There were no peptides described among muscarinic ligands until Adem et al. (Biochim. biophys. Acta 968, 340-345, 1988) reported that muscarinic toxins (MTxs), MTx1 and 2 were able to inhibit 3H-QNB binding to rat brain membranes. Since MTxs inhibit around half of specific binding of 3H-quinuclidinyl benzilate (3H-QNB) and 3H-N-methyl-scopolamine (3H-NMS), which do not discriminate between subtypes of muscarinic receptors, it has been proposed that MTxs might selectively bind to some subtype. MTx1 and 2 from Dendroaspis angusticeps almost completely inhibit the binding of 3H-pirenzepine (3H-PZ), a preferential M1 muscarinic receptor subtype ligand to cerebral cortex synaptosomal membranes. A much higher concentration was needed to inhibit partially 3H-PZ binding to atrial muscarinic receptors. These results support the hypothesis that MTx1 and 2 may be M1 selective muscarinic ligands. Similar activities have been found in Dendroaspis polylepis and D. viridis venoms, but with lower affinities. The Ki obtained from inhibition curves of the binding of 3H-PZ showed that MTx1 has higher affinity for the putative M1 muscarinic receptor subtype, followed by MTx2. DpMTx has lower affinity, while DvMTx seems to have the lowest affinity. All these peptides are devoid of anticholinesterase activity. Dendrotoxin and fasciculin from D. angusticeps venom do not inhibit the binding of muscarinic radioligands to cerebral cortex membranes. The injection of MTxs into dorsal hippocampus of rats immediately after training in an inhibitory avoidance task improves memory consolidation, as does oxotremorine.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding of muscarinic toxins MTx1 and MTx2 from the venom of the green mamba Dendroaspis angusticeps to cloned human muscarinic cholinoceptors

Muscarinic toxins MTx1 and MTx2 are 7500 mol. wt polypeptides isolated from the venom of the green mamba snake Dendroaspis angusticeps. Previous competition binding studies indicate that the MTxs may be selective for the M1 subtype of muscarinic acetylcholine receptors. The present work was undertaken in order to clarify the muscarinic subtype specificity and functional effects of MTx1 and MTx2. Binding interactions were determined using 3H-N-methyl scopolamine (NMS) and cloned human muscarinic receptor subtypes m1, m2, m3 and m4. Some preliminary functional studies were performed on rabbit vas deferens preparations, which contain M1 cholinoceptors. MTx1 and MTx2 inhibited 3H-NMS binding to m1 and m3 receptors, with little effect on binding to m2 and m4 receptors. Affinity was higher for m1 receptors: Ki for MTx1 were 48 nM at m1 receptors and 72 nM at m3 receptors, and Ki for MTx2 were 364 nM at m1 and 1.2 microM at m3 receptors. At m1 receptors, about 90% of the binding of MTx1 and MTx2 appears to be irreversible. On rabbit vas deferens preparations, MTx1 and MTx2 at concentrations above 50 nM behaved in a similar way to the relatively selective M1-agonists McN-A-343 and CPCP (4-[N-(chlorophenyl)carbamoyloxy]-4-20-ynyl-trimethylammoniu m iodide) by reducing responses to nerve stimulation. The results confirm that MTx1 and MTx2 bind to m1 receptors rather than to m2 or m4 receptors, but they also reveal a slightly weaker effect at m3 receptors. The interaction at m1 receptors appears to be essentially irreversible, implying that the toxins could be useful tools in studies of the functional role of m1 muscarinic receptors.

Toxins from mamba venoms: small proteins with selectivities for different subtypes of muscarinic acetylcholine receptors

Muscarinic acetylcholine receptors exist as five subtypes that are widely distributed throughout the body. Conventional pharmacological agents are not highly selective for particular subtypes, making investigations on the functional significance of the subtypes difficult. Recent findings indicate that mamba snake venoms contain several small proteins ('muscarinic toxins') that are highly specific for muscarinic receptors, and are discussed in this review by Diana Jerusalinsky and Alan Harvey. Some of these toxins act selectively and irreversibly on individual subtypes of receptor, and some are antagonists, while others activate muscarinic receptors. The toxins should be useful tools in studies of the functions of individual receptor subtypes, and comparisons of their three-dimensional structures should give clues about how selective binding to muscarinic receptor subtypes can be obtained.