the belgian pharmacologist bacq discovered the existence of toxins in nemertines during the mid- 1930s while searching for invertebrate neurotransmitters .
an aqueous homogenate of a small intertidal hoplonemertine species ( amphiporus lactifloreus ) , like acetylcholine ( ach ) , contracted isolated frog skeletal muscle and stimulated the cat cervical autonomic ganglion .
unlike ach the cholinergic activity of the homogenate was stable in highly alkaline solution and was soluble in organic solvents under basic conditions .
king   showed that amphiporine acted as an organic base and attempted its further purification by crystallization with standard alkaloidal precipitants , however a crystalline salt was not obtained .
while over a 1,000 species have been described , the actual number of species in this inconspicuous phylum is likely to be several times this figure .
being soft - bodied and relatively vulnerable to predators , they contain integumentary toxins which serve as chemical defenses against predators .
the phylum is roughly divided into two large groups , the enoplans ( hoplonemertines ) bearing a mineralized proboscis stylet and the anoplans ( paleo- and heteronemertines ) lacking a stylet .
the relatively small size of most nemertines makes them more difficult to collect than many aquatic animals .
another problem is species identification , as the external morphologies of some species may be so similar that the preparation of fixed and stained tissue sections for histological examination may be necessary for unequivocal identification .
nonetheless , the phylum undoubtedly represents an unusually rich source of alkaloid , peptide and protein toxins , most of them still awaiting investigation .
approximately thirty years elapsed after bacq s discovery before another study of nemertine toxins was reported .
extracts of most species were found to be toxic to crustaceans , but only those of hoplonemertines displayed nicotinic agonist properties and contained pyridyl alkaloids .
since this article focuses on the alkaloids , those interested in the peptide toxins should consult a recent review .
this first nemertine alkaloid to be isolated and identified , anabaseine , occurs in relatively large concentrations in the intertidal pacific nemertine paranemertes peregrina .
the  peregrine  ( wandering ) designation refers to the relatively unique foraging behavior of this moderately large ( > 15 cm ) species : it glides along the exposed surface of mud flats at low tide searching for annelid worms in full view of potential predators such as seagulls , raccoons and other large predators .
several thousand worms were collected and an alkaloid fraction was obtained from the ethanolic extract , much as described by king . because more than a gram of alkaloid was isolated , it was possible to obtain a homogeneous picrate salt , even though in relatively small yield .
after conversion back to the free base , nuclear magnetic resonance and mass spectrometric analyses indicated that the alkaloid was anabaseine , a previously synthesized compound that had not been reported as a natural product .
this was corroborated by comparison of the chemical and toxicological properties of natural and synthetic samples .
anabaseine is chemically similar to the tobacco alkaloid , anabasine , but possesses an imine double bond in the otherwise saturated piperidine ring ( fig .
2 ) . imine - enamine tautomerism constrains the -carbon to lie within the same plane as the -carbon and the imine nitrogen .
this electronic conjugation strongly favors the two rings of anabaseine being approximately co - planar with respect to each other .
this contrasts with nicotine and anabasine , whose respective pyrrolidine and piperidine rings are oriented approximately at right angles in their preferred conformations .
anabaseine was first prepared as an intermediate in the synthesis of anabasine by two austrian tobacco chemists . a mixed aldol - like condensation reaction between nicotinic acid ethyl ester and n - benzoyl piperidone yielded the expected diketone , which rearranged in the presence of concentrated hydrochloride acid at high temperature to anabaseine hydrochloride .
conversion of the salt to the free base , extraction of the free base with organic solvent and purification by distillation , the method reported by spath and mamoli   or column chromatography generally provided anabaseine in relatively low yields .
251 ) obtained in this manner exists as the ammonium - ketone form and contains one molecule of water .
while stable as the dried salt , aqueous solutions of anabaseine hydrochloride should be refrigerated when not in use and replaced after several weeks .
the cationic forms of anabaseine are quite soluble in protic solvents such as water , methanol and ethanol , but the more lipophilic free base is best dissolved in non - aqueous solvents such as alcohols , acetone , or ethyl acetate .
although anabaseine appears to be chemically simple , it actually occurs in several different forms under physiological conditions . at neutral ph
there are three forms in roughly equal concentrations : the unprotonated cyclic imine , the monocationic cyclic iminium and the monocationic ammonium - ketone ( fig .
this multiplicity complicated our initial attempts to determine which forms interact with achrs based upon the ph dependence of anabaseine potency , so stable analogs of each form were prepared so that the pharmacological properties of the different forms could be inferred .
2,3-bipyridyl , which can be prepared by oxidation of anabaseine or anabasine , is predicted to possess a chemical conformation similar to the cyclic imine , so it was selected as an analog of the unprotonated form , while 2-(3,4,5,6-tetrahydropyrimidinyl)-3-pyridine ( pthp ) was selected as a stable permanently ionized analog of the cyclic iminium form .
two stable analogs of the open - chain ammonium - ketone form were prepared by di- or tri - methylation of the ammonium group . amongst the various stable analogs ,
only pthp potently stimulated skeletal muscle and brain nicotinic acetylcholine receptor ( achr)-expressing cells ( kem et al . , in preparation ) .
thus , we conclude that only the cyclic iminium form of anabaseine is active on these mammalian achrs .
achrs are a family of receptors that belong to the cys - loop ligand - gated ion channel superfamily that includes types a and c -amminobutyric acid , glycine , and type 3 serotonin ( 5-hydroxytryptamine ; 5-ht ) receptors ( reviewed in ) . in the peripheral nervous system
, achrs can be subdivided into muscle - type , that have the stoichiometry 121 ( embryonic or torpedo ) or 121 ( adult ) , and ganglionic achrs ( e.g. , 34 ) . in the central nervous system ( cns ) , achrs are of two main subclasses : receptors that bind the competitive antagonist -bungarotoxin ( -btx ) with high affinity but the agonist nicotine with low affinity ( e.g. , 7-containing receptors ) , and achrs that bind nicotine with high affinity but -btx with low affinity ( e.g. , 42-containing receptors ) .
the physiological and pharmacological effects of anabaseine on a variety of vertebrate achrs were previously reported . like nicotine
, anabaseine stimulates all achrs to some degree and thus must be classified as a non - selective nicotinic agonist .
however , it preferentially stimulates the same achrs ( e.g. , skeletal muscle and brain 7 subtypes ) that display high affinities for the snake toxin -btx .
in contrast , nicotine preferentially and almost fully stimulates 42 ( brain ) and 34 ( predominantly autonomic ) receptors .
anabaseine is a full agonist at the 7 receptor but only a very weak ( low efficacy ) agonist at the 42 subtype .
the maximal effect of nicotine on the latter receptor is much greater than its maximal effect on the 7 receptor . since nicotine also binds to 42 receptors at much lower ( about 100-fold ) concentrations than to the 7 receptor , its in vivo effects at  smoking  concentrations are most likely mediated through 2 subunit - containing receptors .
anabaseine stimulates pc12 cell and guinea pig ileum achrs thought to contain 34 ( and probably other ) autonomic receptors .
a more recent study of anabaseine action on rat achrs expressed in xenopus oocytes indicates that anabaseine is a rather weak partial agonist on the 34 receptor subtype .
the whole animal ( mouse ) toxicity of anabaseine is very similar to that of nicotine and is significantly higher than for anabasine .
in contrast , anabaseine has very weak partial agonist activity at this receptor and probably exerts its toxicity by causing peripheral neuromuscular block and respiratory arrest . because of its high toxicity and relative lack of receptor selectivity , few in vivo studies have been carried out with anabaseine
. the significantly higher potency of nicotine relative to anabaseine in causing prostration is consistent with the notion that 42 receptors primarily mediate this characteristic behavior .
marine annelids which are the usual prey for paranemertes are paralyzed , as are crustaceans and insects .
nicotinic cholinergic receptors primarily reside on central neurons in arthropods , but are also found in their cardiac pacemaker ganglion .
2,3-bipyridyl , a largely unionized analog of anabaseine , is even more active than anabaseine in paralyzing crustaceans .
while it does not cause paralysis , nemertelline ( a tetrapyridyl found in amphiporus angulatus ) , like anabaseine and 2,3-bipyridyl , stimulates an unusual receptor in the stomatogastric muscle of the crayfish which is apparently a chloride channel . at present
this is the only known action of this alkaloid , which is the most abundant pyridine in this species of amphiporus . a variety of pyridine compounds including anabaseine and 2,3-bipyridyl stimulate chemoreceptor present in sensory neurons present at the surfaces of crayfish and lobster walking legs that influence feeding behavior .
anabaseine and 2,3-bipyridyl were found to be two of the most active compounds in stimulating similar pyridine receptors on spiny lobster sensory nerves .
the nemertine alkaloids , by acting upon these chemoreceptors , may act as repellants against certain predators .
some of these compounds are also able to inhibit the settlement of barnacle larvae to marine surfaces and thus might be useful  antifouling  additives to marine paints .
this first nemertine alkaloid to be isolated and identified , anabaseine , occurs in relatively large concentrations in the intertidal pacific nemertine paranemertes peregrina .
the  peregrine  ( wandering ) designation refers to the relatively unique foraging behavior of this moderately large ( > 15 cm ) species : it glides along the exposed surface of mud flats at low tide searching for annelid worms in full view of potential predators such as seagulls , raccoons and other large predators .
several thousand worms were collected and an alkaloid fraction was obtained from the ethanolic extract , much as described by king . because more than a gram of alkaloid was isolated , it was possible to obtain a homogeneous picrate salt , even though in relatively small yield .
after conversion back to the free base , nuclear magnetic resonance and mass spectrometric analyses indicated that the alkaloid was anabaseine , a previously synthesized compound that had not been reported as a natural product .
this was corroborated by comparison of the chemical and toxicological properties of natural and synthetic samples .
anabaseine is chemically similar to the tobacco alkaloid , anabasine , but possesses an imine double bond in the otherwise saturated piperidine ring ( fig .
2 ) . imine - enamine tautomerism constrains the -carbon to lie within the same plane as the -carbon and the imine nitrogen .
this electronic conjugation strongly favors the two rings of anabaseine being approximately co - planar with respect to each other .
this contrasts with nicotine and anabasine , whose respective pyrrolidine and piperidine rings are oriented approximately at right angles in their preferred conformations .
anabaseine was first prepared as an intermediate in the synthesis of anabasine by two austrian tobacco chemists . a mixed aldol - like condensation reaction between nicotinic acid ethyl ester and n - benzoyl piperidone yielded the expected diketone , which rearranged in the presence of concentrated hydrochloride acid at high temperature to anabaseine hydrochloride .
conversion of the salt to the free base , extraction of the free base with organic solvent and purification by distillation , the method reported by spath and mamoli   or column chromatography generally provided anabaseine in relatively low yields .
251 ) obtained in this manner exists as the ammonium - ketone form and contains one molecule of water .
while stable as the dried salt , aqueous solutions of anabaseine hydrochloride should be refrigerated when not in use and replaced after several weeks .
the cationic forms of anabaseine are quite soluble in protic solvents such as water , methanol and ethanol , but the more lipophilic free base is best dissolved in non - aqueous solvents such as alcohols , acetone , or ethyl acetate . although anabaseine appears to be chemically simple , it actually occurs in several different forms under physiological conditions . at neutral ph
there are three forms in roughly equal concentrations : the unprotonated cyclic imine , the monocationic cyclic iminium and the monocationic ammonium - ketone ( fig .
this multiplicity complicated our initial attempts to determine which forms interact with achrs based upon the ph dependence of anabaseine potency , so stable analogs of each form were prepared so that the pharmacological properties of the different forms could be inferred .
2,3-bipyridyl , which can be prepared by oxidation of anabaseine or anabasine , is predicted to possess a chemical conformation similar to the cyclic imine , so it was selected as an analog of the unprotonated form , while 2-(3,4,5,6-tetrahydropyrimidinyl)-3-pyridine ( pthp ) was selected as a stable permanently ionized analog of the cyclic iminium form .
two stable analogs of the open - chain ammonium - ketone form were prepared by di- or tri - methylation of the ammonium group . amongst the various stable analogs ,
only pthp potently stimulated skeletal muscle and brain nicotinic acetylcholine receptor ( achr)-expressing cells ( kem et al . , in preparation ) .
thus , we conclude that only the cyclic iminium form of anabaseine is active on these mammalian achrs .
achrs are a family of receptors that belong to the cys - loop ligand - gated ion channel superfamily that includes types a and c -amminobutyric acid , glycine , and type 3 serotonin ( 5-hydroxytryptamine ; 5-ht ) receptors ( reviewed in ) . in the peripheral nervous system , achrs can be subdivided into muscle - type , that have the stoichiometry 121 ( embryonic or torpedo ) or 121 ( adult ) , and ganglionic achrs ( e.g. , 34 ) . in the central nervous system ( cns ) , achrs are of two main subclasses : receptors that bind the competitive antagonist -bungarotoxin ( -btx ) with high affinity but the agonist nicotine with low affinity ( e.g. , 7-containing receptors ) , and achrs that bind nicotine with high affinity but -btx with low affinity ( e.g. , 42-containing receptors ) .
the physiological and pharmacological effects of anabaseine on a variety of vertebrate achrs were previously reported .
like nicotine , anabaseine stimulates all achrs to some degree and thus must be classified as a non - selective nicotinic agonist .
however , it preferentially stimulates the same achrs ( e.g. , skeletal muscle and brain 7 subtypes ) that display high affinities for the snake toxin -btx .
in contrast , nicotine preferentially and almost fully stimulates 42 ( brain ) and 34 ( predominantly autonomic ) receptors .
anabaseine is a full agonist at the 7 receptor but only a very weak ( low efficacy ) agonist at the 42 subtype .
the maximal effect of nicotine on the latter receptor is much greater than its maximal effect on the 7 receptor .
since nicotine also binds to 42 receptors at much lower ( about 100-fold ) concentrations than to the 7 receptor , its in vivo effects at  smoking  concentrations are most likely mediated through 2 subunit - containing receptors .
anabaseine stimulates pc12 cell and guinea pig ileum achrs thought to contain 34 ( and probably other ) autonomic receptors .
a more recent study of anabaseine action on rat achrs expressed in xenopus oocytes indicates that anabaseine is a rather weak partial agonist on the 34 receptor subtype .
the whole animal ( mouse ) toxicity of anabaseine is very similar to that of nicotine and is significantly higher than for anabasine .
in contrast , anabaseine has very weak partial agonist activity at this receptor and probably exerts its toxicity by causing peripheral neuromuscular block and respiratory arrest . because of its high toxicity and relative lack of receptor selectivity , few in vivo studies have been carried out with anabaseine
. the significantly higher potency of nicotine relative to anabaseine in causing prostration is consistent with the notion that 42 receptors primarily mediate this characteristic behavior .
marine annelids which are the usual prey for paranemertes are paralyzed , as are crustaceans and insects .
nicotinic cholinergic receptors primarily reside on central neurons in arthropods , but are also found in their cardiac pacemaker ganglion .
2,3-bipyridyl , a largely unionized analog of anabaseine , is even more active than anabaseine in paralyzing crustaceans .
while it does not cause paralysis , nemertelline ( a tetrapyridyl found in amphiporus angulatus ) , like anabaseine and 2,3-bipyridyl , stimulates an unusual receptor in the stomatogastric muscle of the crayfish which is apparently a chloride channel . at present
this is the only known action of this alkaloid , which is the most abundant pyridine in this species of amphiporus .
a variety of pyridine compounds including anabaseine and 2,3-bipyridyl stimulate chemoreceptor present in sensory neurons present at the surfaces of crayfish and lobster walking legs that influence feeding behavior .
anabaseine and 2,3-bipyridyl were found to be two of the most active compounds in stimulating similar pyridine receptors on spiny lobster sensory nerves .
the nemertine alkaloids , by acting upon these chemoreceptors , may act as repellants against certain predators .
some of these compounds are also able to inhibit the settlement of barnacle larvae to marine surfaces and thus might be useful  antifouling  additives to marine paints .
while anabaseine is a broad spectrum nicotinic agonist , a large variety ( > 200 ) of substituted anabaseines that have been synthesized over the past two decades displayed selective agonistic effects on the 7 achr .
the 3-arylidene - anabaseines are of special potential therapeutic interest because they have been shown to possess neuroprotective as well as cognition enhancing properties . here
we shall only consider 3-(2,4-dimethoxybenzylidene)-anabaseine ( dmxba ) , whose pharmaceutical code name gts-21 refers to its origination as the 21 compound generated in a joint project by gainesville ( university of florida ) and tokushima ( taiho pharmaceuticals ) scientists .
dmxba was the first nicotinic agonist reported to selectively stimulate 7 achrs ; it was also the first 7 agonist to enter clinical tests for possible treatment of cognition problems such as occur in schizophrenia , parkinsonism and alzheimer s disease ( ad ) .
dmxba is prepared by reaction of 2,4-dimethoxybenzaldehyde with anabaseine in acidic alcohol at approximately 70c , in a manner similar to the preparation of 3-(4-dimethylaminobenzylidene)- anabaseine .
the resulting product can be precipitated and recrystallized using less polar solvents . whereas the two rings of anabaseine have been shown to be electronically conjugated and
thus nearly coplanar , all three rings of dmxba are predicted to lie in different planes . unlike anabaseine , 3-arylidene - anabaseines do not readily hydrolyze to open - chain forms at physiological phs like anabaseine . in principle , these compounds can adopt two possible conformations with respect to the vinyl portion of the arylidene ring , namely e- or z. by nmr we have shown that only the e form occurs in aqueous solution when the synthetic dmxba dihydrochloride is dissolved in water .
only after intense or maintained light exposure does the e to z conversion become significant ( kem et al .
thus , the synthetic compound solid and stock solutions of the compound must be stored in containers that exclude light and stock . while photosensitivity of dmxba was observed in the laboratory , when plasma samples from animal and human tests were prepared in an unlighted fume hood and subsequently determined by hplc with a photodiode array detector , no z - isomeric product was observed .
dmxba is a lipophilic compound which readily passes across biological membranes including the gastrointestinal wall and the blood - brain barrier and reaches peak concentrations in the blood and brain within a very short time .
it is o - demethylated primarily at the p - position of the benzylidene ring , but demethylation at the o - methoxy group also occurs to a much lesser extent . while the resulting hydroxy metabolites are actually more efficacious at the 7 receptor in vitro , their peak brain concentrations are much less than for dmxba .
based on the crystal structure of acetylcholine - binding protein ( achbp ) , monomer , homodimer , and homopentamer models of the 7 achr were derived . since the agonist binding sites are located at the subunit interface ( reviewed in ) ,
a detailed analysis about the interface , as well as its interaction with the hepes molecule that has been observed in the achbp crystal , was performed .
furthermore , a ligand - binding pocket was defined providing useful information for conducting various mutagenesis studies to get clues for drug design . although computer - predicted protein structures are still not as accurate as x - ray structures , the three modeled structures can at least serve as a basis for designing new ligands .
theoretical and molecular modeling studies were done to better understand the details of how dmxba and its two primary o - demethylated metabolites ( 2oh- and 4oh - mba ) might bind to 7 achrs .
figure 4 depicts a model of the binding pocket of 2oh - mba at the extracellular domain of the 7 achr .
there was rather good accordance of the calculated preferred energies with the observed binding affinities .
van der waals repulsions made the dominated contribution to the predicted binding energy for the receptor .
dmxba and its metabolites seemed larger than the optimal size for fitting into the agonist binding site .
thus , one possible approach to improving the effectiveness of benzylidene - anabaseine binding might be to reduce the molecular volume while retaining the active groups .
we are optimistic that molecular modeling , in combination with experimental studies on model proteins such as achbp , may provide a useful basis for rational design of nicotinic drug candidates for treating neurodegenerative and possibly other diseases .
while dmxba selectively stimulates 7 achrs , at significantly higher concentrations it also is an antagonist of 42 achrs and related type 3 5-ht receptors . at even higher concentrations dmxba
also is a weak antagonist at other achrs . in at least some achr subtypes , dmxba and its metabolites
for instance , it has been found that dmxba at micromolar concentrations [ inhibition constant ( ki ) = 13  1 m ] displaces the binding of [ h]thienylcyclohexylpiperidine ( [ h]tcp ) within the channel of the neuromuscular - type achr .
while the 4-hydroxy metabolite also displayed this inhibitory binding , it occurred at a higher concentration ( ki = 48  5 m ) .
schild - type analyses of these experiments indicated that these competitions are mediated by a steric mechanism .
thus , considering that [ h]tcp is a structural and functional analog of the dissociative general anesthetic and potent noncompetitive antagonist phencyclidine ( pcp ) , we suggest that the anabaseine analog binding site overlaps the pcp locus in the desensitized ion channel . in this regard , photoaffinity labeling studies using [ h]ethidium diazide , which binds with high affinity to the pcp locus , helped to determine the structural components of this site in the desensitized state   ( reviewed in ) .
the results indicated that residues leu at position 9 ( e.g. , the leucine ring ) and ser at position 10 from the 1-m2 transmembrane segment as well as other unknown amino acids located in the m1 and m2 transmembrane segments from the  subunit are structurally involved in the pcp binding site . in the resting state
( in the presence of -btx ) , anabaseine analogs modulate either [ h]tcp , [ h]tetracaine , or [ c]amobarbital binding to the torpedo achr in an allosteric fashion .
these results indicate that the anabaseine analog binding site overlaps neither the pcp , the tetracaine , nor the amobarbital binding domain in the resting ion channel .
we suggested that the pcp binding site in the resting state is located more extracellularly than that in the desensitized state , probably close to the mouth of the external vestibule ( probably after position 13 and closer to position 20 ) , whereas the barbiturate locus is located practically in the middle of the resting ion channel ( between position 9 and 13 )   ( reviewed in ) .
in addition , the tetracaine binding domain bridges both the pcp and the amobarbital loci in the resting ion channel ( probably between position 5 and 20 ) .
interestingly , anabaseine analogs enhance [ h]tcp binding to the torpedo achr when the receptor is in the resting but activatable state ( in the absence of -btx ) .
we consider that this enhancement is due to an anabaseine analog - induced achr desensitization process .
this hypothesis was supported by the fact that anabaseine analogs also increase the binding of the agonist [ h]cytisine to the resting but activatable achr . in this
regard , achr desensitization seems to be another mechanism by which anabaseine analogs produce the noncompetitive inhibition of achrs , which in turn , might account for the partial agonistic effect of these compounds in 7 achrs . considering this new experimental evidence
it is plausible that the maximal channel activation observed in conventional voltage - clamp electrophysiological recordings might be influenced by the propensity of anabaseine analogs for causing channel block and/or desensitization as well as the probability of the bound agonist to trigger the conformational changes associated with moving from a resting ( closed but activatable ) to an activated ( open ) channel state , to finally a desensitized ( closed ) conformation .
what makes dmxba of considerable scientific as well as potential clinical interest is its selective stimulation of 7 achrs .
the physiological function of this receptor had been very difficult to investigate in the past due to its propensity to rapidly desensitize when high concentrations of agonist are applied .
later , after cloning and expression in cultured cells , it was found to be physiologically active as a ligand - gated ion channel with unusually high permeability for calcium ions .
7 achrs occur at presynaptic as well as on postsynaptic sites at densities that are sometimes as high as that of glutamate receptors . by causing an influx of calcium ions even at normal membrane resting potentials , when most voltage - gated calcium channels are closed , these achrs are able to stimulate a variety of second messenger systems responsive to elevations in intracellular calcium , including nitric oxide synthesis .
that dmxba enhances performance in cognitive tasks indicates that 7 achrs play a significant role in learning and memory . that dmxba and other nicotinic agonists primarily exert their cognition - enhancing actions through achr stimulation rather than desensitization follows from the finding that their pro - cognitive effects are inhibited by administering nicotinic antagonists like mecamylamine , -btx and methyllycaconitine .
dmxba and nicotine both enhance long - term potentiation ( ltp ) in the hippocampus .
one hypothesis is that stimulation of 7 receptors by released ach or choline ( an endogenous weak agonist ) enhances the action of synaptically liberated glutamate on nearby n - methyl - d - aspartate ( nmda)-type glutamate receptors , as the depolarization resulting from 7 channel opening would eliminate the resting block of nmda receptors by intracellular magnesium ions .
the dmxba enhancement displays less acute tolerance ( i.e. , reduced response with successive applications ) than does the nicotine effect . since the auditory gating effects of both compounds in mice are prevented by prior administration of -btx , 7 receptors are the dominant achrs mediating this action .
schizophrenics suffer from a relative inability to filter or gate repetitive sensory stimuli , particularly auditory and visual stimuli .
this gating defect probably contributes to the negative symptoms of the disease , which are not well treated by neuroleptic drugs which in general are dopamine receptor antagonists .
a recent recommendation by an expert panel of psychiatrists recommended development of new therapies for treating the cognitive problems associated with this disease , since they are particularly problematic in preventing schizophrenics from holding jobs and functioning in society .
the university of colorado and university of florida labs have recently collaborated on a phase 1 test of dmxba in schizophrenics .
the results of this study , both regarding safety and initial assessments of efficacy , encourage further tests . it has also been shown that the deleterious effects of cocaine on auditory gating can be counteracted by dmxba . thus , 7 nicotinic agonists may also be useful in treating psychoses resulting from use of these stimulants .
two decades ago a drastic decrease in achrs was first reported in alzheimer s patients and some parkinson s patients .
this finding stimulated considerable academic and pharmaceutical interest in the development of nicotinic agonists that could selectively stimulate the remaining brain achrs involved in cognitive and other critical mental functions . at that time it was already apparent that cholinesterase inhibitors and non - selective muscarinic agonists were relatively weak therapeutic agents for counteracting the neurodegeneration and dementia associated with ad .
dmxba is a relatively unique drug candidate which readily enters the brain and acts as an 7 achr partial agonist .
its effects upon cognitive behavior have been investigated by many laboratories using a variety of mammalian species ( reviewed in ) .
nucleus basalis - lesioned rats or aging rats , mice and rabbits were often used to simulate a cholinergic deficit .
initially it was observed that the compound enhanced passive avoidance performance in rats   and active avoidance in mice   and acquisition of conditioned eye - blink reflex in aging rabbits .
memory of more intricate learning tasks such as water and radial maze performance by rats   and delayed matching by monkeys   was also enhanced , suggesting that the compound may be able to enhance cognition in aging humans , particularly ad patients .
the latter paper is noteworthy in providing compelling evidence that single doses of relatively short plasma half - life ( hours ) nicotinic agonists are capable of enhancing cognition for relatively long periods of time ( days ) .
generally , cognition enhancement is more readily demonstrated under conditions where cognitive function is deficient , as in chemically lesioned or aging animals . in the case of dmxba performance on several cognitive tasks
thus , 7 nicotinic agonists may be useful in treating deficits in cognition , regardless of age .
one advantage of targeting 7 receptors for therapeutic enhancement of cognition , instead of 42 receptors , is that modulation of the former receptor does not seem to affect activities associated with nicotine dependence , namely hyperlocomotion , nicotine discrimination and nicotine self - administration , whereas the latter receptor is thought to be a major mediator of the euphoric and anxiolytic effects of nicotine .
the actions of subcutaneously administered anabaseine and dmxba upon the brain levels of ach and several biogenic amines have been investigated using cerebral ( frontoparietal location ) microdialysis methods .
anabaseine , like nicotine   and other 42 agonists , elevated ach levels ( table 2 ) .
however , an equimolar ( 3.6 mol / kg ) dose of dmxba did not affect ach levels at this cortical site or within the hippocampus . both anabaseine and dmxba elevated dopamine and norepinephrine levels , but did not significantly affect serotonin levels .
however , when mecamylamine , a noncompetitive achr antagonist , was administered thirty minutes before administration of either compound , significant increases in ach and 5-ht levels were observed .
explanations for these extraordinary mecamylamine effects are not yet at hand ; one possible interpretation would be that at the mecamylamine dose administered , the more sensitive 42 receptors expressed on inhibitory ( gabaergic ) neurons innervating the basalis ( cholinergic ) and raphe ( serotonergic ) nuclei were preferentially inhibited , leaving the excitatory effects of anabaseine and dmxba on the most resistant achrs ( 7 ) to be expressed without opposition from these inhibitory effects .
further investigation of the effects of 7 agonists upon brain neurotransmitter levels and their sensitivity to block by achr antagonists is clearly warranted .
several laboratories have recently reported that -amyloid ( a ) binds to 7 receptors at very low concentrations .
the peptide inhibits -btx binding to its ach - binding site and has been reported to activate   or inhibit this achr .
it was hypothesized that 7 receptors are a target of a action on brain neurons .
the initially reported selective inhibition of 7 receptors was followed by reports from other laboratories that non-7 achrs are also affected by similarly low concentrations of the peptide . if the 7 achr were a major target for a in generating ad , then one would predict that neurons bearing high concentrations of this receptor would be particularly susceptible and this would lead to a decrease in brain 7 receptors as the disease progresses .
however , the loss of 7 receptors in ad brains is much smaller than the loss of 42 achrs . 7
receptor levels in transgenic mice overexpressing a may be reduced   although this may occur very early in life .
further investigations of the interaction of a with achrs are needed to determine whether 7 and/or other achrs are directly involved in mediating neuronal destruction in ad .
besides affecting cognitive functions , dmxba and other nicotinic agonists also display neuroprotective properties such as inhibiton of the excitotoxic effects of a   and high concentrations of ethanol . in a stroke model ,
at very high concentrations ( > 10 m ) , approximately 50 times higher than would occur under clinical conditions ( 200 nm ) , rapid addition of this compound to cultured neurons dmxba was also excitotoxic .
if the compound was allowed to reach the cells gradually these high concentrations were not toxic .
the neuroprotective effects of dmxba were inhibited by reducing extracellular and intracellular calcium levels , and thus seem to be a consequence of calcium influx into the neuron .
these are interesting times for the investigation of achrs and their roles in health and disease .
besides the brain achr targets discussed above , several peripherally expressed achrs may also be useful therapeutic targets for treating other disease states .
examples which readily come to mind are acute inflammation   and controlling the growth ( angiogenesis ) of new blood vessels . in these two examples considerable evidence
we hope to have convinced the reader that naturally occurring toxins acting on achrs , besides being useful probes for particular nicotinic receptors , can also serve as molecular models for the design of nicotinic agonists and antagonists of possible therapeutic utility .
 nemertines are a phylum of carnivorous marine worms that possess a variety of alkaloidal , peptidic or proteinaceous toxins that serve as chemical defenses against potential predators . 
 the hoplonemertines additionally envenomate their prey with a mixture of proboscis alkaloids delivered with the help of a calcareous stylet that punctures the skin of the victim . 
 anabaseine , the first of these alkaloids to be identified , stimulates a wide variety of animal nicotinic acetylcholine receptors ( achrs ) , especially the neuromuscular [ e.g. , 121 ( embryogenic ) or 121 ( adult ) ] and 7 achrs that are inhibited by the snake peptide -bungarotoxin . 
 a synthetic derivative , 3-(2,4-dimethoxybenzylidene)-anabaseine ( dmxba ; also called gts-21 ) , improves memory in experimental animals and humans and is currently in clinical trials to determine whether it can ameliorate cognitive problems associated with schizophrenia . 
 here we summarize present knowledge concerning the chemistry and mechanisms of action of these two substances ( anabaseine and dmxba ) on achrs , especially those found in the mammalian brain . 
Introduction
Anabaseine
DMXB-Anabaseine (DMXBA), A Synthetic Anabaseine Derivative
Future Directions of Research