Effects of nefiracetam on deficits in active avoidance response and hippocampal cholinergic and monoaminergic dysfunctions induced by AF64A in mice

The effects of nefiracetam [DM-9384; N-(2,6-dimethyl-phenyl)-2-(2-oxo-pyrrolidinyl)acetamide] and of phosphatidylcholine on a step-up active avoidance response, locomotor activities and regional brain cholinergic and monoaminergic neurotransmitters in AF64A-treated mice were investigated. Intracerebroventricular (i.c.v.) injection of AF64A (ethylcholine mustard aziridinium ion; 8 nmol/ventricle) impaired acquisition and retention of the avoidance task, and increased vertical and horizontal locomotor activities. Regional levels of acetylcholine, noradrenaline, 3-methoxy-4-hydroxyphenylglycol (MHPG), 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were significantly decreased and homovanillic acid (HVA) levels were increased in the hippocampus but not in the septum, cerebral cortex or striatum of AF64A-treated animals. Administration of nefiracetam (3 mg/kg, p.o.) twice daily for 9 days to AF64A-treated animals ameliorated the deficit in active avoidance response in addition to attenuating the increase in locomotor activities. In parallel with these behavioural effects, nefiracetam reversed AF64A-induced alterations in the hippocampal profiles of cholinergic and monoaminergic neurotransmitters and their metabolites. In contrast, administration of phosphatidylcholine (30 mg/kg, p.o.) twice daily for 9 days had no significant effect on the deficit in active avoidance response, despite significantly reversing the decrease in acetylcholine levels in the hippocampus. These results indicate that the effects of nefiracetam on AF64A-induced behavioural deficits are probably due to its ability to facilitate both cholinergic and monoaminergic neurotransmitter systems.

Is the A-ring lactone of brevetoxin PbTx-3 required for sodium channel orphan receptor binding and activity?

Brevetoxin PbTx-3 and non-toxic derivative 4 were investigated for their abilities to bind to the specific brevetoxin receptor site on rat brain synaptosomes and to modulate the normal function of voltage-gated sodium channels as determined by patch clamping of cultured neurons. Compounds 4 and 5 are produced from PbTx-3 by opening of the A-ring lactone to the saturated and unsaturated diols using sodium borohydride in ethanol. Natural PbTx-3 exhibited tighter binding to rat brain synaptosomes by at least 3 orders of magnitude as determined by competitive radioligand binding experiments, and was also more effective at activating voltage-gated channels. Patch clamping revealed the 3 orders of magnitude greater potency of PbTx-3 toxin over 5, although each produced delayed sodium channel opening and a pronounced delay in inactivation. Conformational modeling of the Brevetoxin B backbone indicates that the two molecules are identical except for the region of the A-Ring lactone. Thus, we conclude that the brevetoxin PbTx-3 backbone requires electrophilic functionality in the region of the lactone in PbTx-3, and that opening of the ring in 5 is sufficient to substantially reduce both binding and activity.

AF64A-induced brain damage and its relation to dementia

Several data obtained in the AF64A-model are of particular relevance for our understanding of the pathogenesis and progression of Alzheimer's disease. The AF64A-induced withdrawal of cholinergic function in the rat hippocampus was associated with reversible functional changes in other neurotransmitters, including noradrenaline, serotonin, somatostatin and glutamate, thereby mimicking changes in Alzheimer's disease. Identical changes in markers for synaptic vesicles were found in Alzheimer's disease and AF64A-model. A study on the role of gender revealed a higher susceptibility to the neurotoxic action of AF64A in female rats. The cholinergic deficit was also responsible for a disinhibition of the negative feedback regulation of glucocorticoids. Increased exposure to glucocorticoids, however, enhanced the vulnerability of hippocampal cholinergic neurons to AF64A. These data indicate that the AF64A-induced cholinergic deficit in the rat brain represents a reliable tool to study several mechanisms possibly involved in Alzheimer's disease.

Messenger RNA levels of chromogranin B, secretogranin II, and VGF in rat brain after AF64A-induced septohippocampal cholinergic lesions

The mRNA levels of secretogranin II, chromogranin B, and VGF were compared in brains of control and AF64A-treated rats. This toxin induces specific lesions of the septohippocampal cholinergic pathway. As a consequence of this treatment, the chromogranin B message was elevated in the dentate gyrus granule cells of the hippocampus. In the paraventricular nucleus of the hypothalamus, a concomitant elevation of the messages of secretogranin II and corticotropin-releasing factor occurred in the parvocellular neurons, and an increase of those of secretogranin II and VGF occurred in a subgroup of magnocellular neurons. Further increases for secretogranin II were seen in the amygdaloid nuclei and the reticular thalamic nuclei and increases for chromogranin B in the temporal cortex, substantia nigra compacta, and ventral tegmental area. These results indicate that the toxin-induced lesion of the cholinergic pathway innervating the hippocampus apparently leads to the stimulation of several defined groups of neurons that react with an increase in the mRNA levels of their secretory peptides. We suggest that changes in mRNA expression of these peptides are useful parameters for defining neurons under chronic stimulation.

Reduction in striatal D2 dopamine receptor mRNA and binding following AF64A lesions

Unilateral lesions by a cholinotoxin, receptor autoradiography, and in situ hybridization techniques were employed to determine if dopaminergic receptors are located on cholinergic interneurons in the caudate-putamen (CPu). Lesion of the CPu with small amounts of the cholinotoxin AF64A resulted in a significant decrease in D2 receptor mRNA and D2 receptor binding. The loss was more pronounced in lateral and central portions of the CPu. Results obtained using [3H] SCH23390 binding to D1 receptors indicated that there was no change in this dopamine receptor subtype in the AF64A-lesioned CPu. A decrease in D2 receptor mRNA and receptor binding in AF64A-lesioned animals indicates that a population of postsynaptic D2 receptors is associated with the cholinergic interneurons. Lack of any change in [3H]SCH23390 binding in the AF64A-lesioned animals suggests that D1 receptors are not located on cholinergic neurons. These results provide evidence to support the selectivity of the lesion when used as indicated.

Effects of sustained release formulation of thyrotropin-releasing hormone on learning impairments caused by scopolamine and AF64A in rodents

The effects of a sustained-release formulation of thyrotropin-releasing hormone (TRH-SR) on learning impairments induced by scopolamine and a cholinergic neurotoxin, ethylcholine aziridinium ion (AF64A), were examined in rodents. Subcutaneous injection of TRH-SR (2.8 mg/kg as free TRH) produced a sustained increase in immunoreactive plasma TRH levels up to about 2 weeks after dosing in rats. TRH-SR (0.56 and 2.8 mg/kg) given subcutaneously 7 days before the acquisition trial markedly ameliorated scopolamine-induced amnesia in mice, as evaluated with a passive avoidance task. Repeated administration of TRH for 7 days at doses of 0.2-5 mg/kg s.c. elicited a dose-dependent recovery from amnesia induced by scopolamine, whereas only the group treated with 5 mg/kg/day showed a significant improvement. The rats with bilateral intracerebroventricular injection of AF64A (3.75 nmol/brain) showed a significant impairment in the water maze task 2 weeks after surgery. TRH-SR (0.56 and 2.8 mg/kg) also exhibited a dose-dependent ameliorating action on the deficit. These findings indicate that TRH-SR ameliorates learning impairments produced by scopolamine and AF64A, and suggest that continuous infusion of TRH may have a potent learning and memory improving action at low doses.

Morphological assessment of ethyl choline mustard aziridinium-induced neurotoxicity in rat brain reaggregate cultures

Foetal rat brain reaggregate cultures have been employed to investigate the morphological changes associated with the neurotoxic action of ethylcholine mustard aziridinium (ECMA). In a companion study we provided evidence for apparent selective cholinergic neurotoxicity. Exposure of 9-day-old cultures to 12.5 microM ECMA for 3 days produced dilatation of selected axon preterminals and terminals in the outer core tissue layer. Axoplasm in these dilated terminals was electron lucent and contained a flocculent, plasma-like material with remnants of the smooth endoplasmic reticulum. Their synaptic vesicle content was much reduced or, absent. Microglial cells were engaged in phagocytosis of these effete structures and a few necrotic neurons were enveloped by glial processes. Exposure to 50 microM ECMA produced widespread necrosis with some surviving neurons, surrounded by the still-persisting capsular layer. Treatment with 100 microM ECMA generated a greater extent of tissue necrosis, with only a few surviving neurons and glial cells being contained within the necrotic tissue mass. Reaggregates frequently disintegrated following capsule loss. Our results indicate that the initial morphological manifestation of ECMA-induced toxicity is dilatation of axon terminals, that are probably of cholinergic origin and are targeted due to their possession of the high-affinity choline transport system which is unique to these neurons.

Induction of tolerance to crotoxin in mice

Crotoxin, the major toxic component from the venom of Crotalus durissus terrificus is a potent neurotoxin (LD50, i.p., mice, 0.09 mg/kg) which possesses phospholipase A2 activity and causes a blockade of neuromuscular transmission. In this article, we show that mice injected daily with progressively increasing doses of crotoxin develop tolerance to the lethal action of this toxin. Treated mice tolerated daily doses of crotoxin 20- to 35-fold higher than the original LD50 without the characteristic signs of toxicity. Studies on the isolated phrenic nerve-diaphragm preparation in vitro from control (crotoxin-naive) mice showed that the exposure to 2 to 10 micrograms/ml crotoxin in the bath produced complete transmission blockade in 120 to 150 min. Conversely, the preparations from crotoxin-treated mice required crotoxin concentrations in the range of 17.5 to 100 micrograms/ml to produce complete neuromuscular block, being virtually insensitive during 200 min of exposure to 5 to 10 micrograms/ml crotoxin. Phrenic nerve-diaphragm preparations of control (crotoxin-naive) and crotoxin-treated mice did not show significant differences in sensitivity to the blocking action of carbamylcholine, suggesting that induction of tolerance to crotoxin is likely a presynaptic event.

Identification of the site at which phospholipase A2 neurotoxins localize to produce their neuromuscular blocking effects

Experiments were conducted on mouse hemidiaphragm preparations using five phospholipase A2 neurotoxins of differing chain structures and antigenicities [notexin (one chain); crotoxin (two chains not covalently bound), beta-bungarotoxin (two chains covalently bound); taipoxin (three chains), and textilotoxin (five chains; one copy each of three chains and two copies of a fourth chain)]. Three clostridial neurotoxins (botulinum neurotoxin types A and B, and tetanus toxin) were used in comparison experiments. Phospholipase A2 neurotoxins produced concentration-dependent blockade of neuromuscular transmission. There was no obvious relationship between chain structure and potency, but there was an indication of a relationship between chain structure and binding. The binding of notexin was substantially reversible, the binding of crotoxin was slightly reversible, and the binding of beta-bungarotoxin, taipoxin and textilotoxin was poorly reversible. Experiments with neutralizing antibodies indicated that phospholipase A2 neurotoxins became associated with binding sites on or near the cell surface. This binding did not produce neuromuscular blockade. When exposed to physiological temperatures and nerve stimulation, bound toxin disappeared from accessibility to neutralizing antibody. This finding suggests that there was some form of molecular rearrangement. The two most likely possibilities are: (1) there was a change in the conformation of the toxin molecule, or (2) there was a change in the relationship between the toxin and the membrane. The molecular rearrangement step did not produce neuromuscular blockade. At a later time there was onset of paralysis; the amount of time necessary for onset of blockade was a function of toxin concentration. Phospholipase A2 neurotoxins were not antagonized by drugs that inhibit receptor-mediated endocytosis. In addition, phospholipase A2 neurotoxins did not display the pH-induced conformational changes that are typical of other endocytosed proteins, such as clostridial neurotoxins. However, phospholipase A2 neurotoxins were antagonized by strontium, and this antagonism was expressed against toxins that were free in solution and toxins that were bound to the cell surface. Limited antagonism was expressed after toxins had undergone molecular rearrangement, and no antagonism was expressed after toxin-induced neuromuscular blockade. The cumulative data suggest that phospholipase A2 neurotoxins are not internalized to produce their poisoning effects. These toxins appear to act on the plasma membrane, and this is the site at which they initiate the events that culminate in neuromuscular blockade.

alpha-Conotoxin GI produces tetanic fade at the rat neuromuscular junction

The ability of the marine snail toxin, alpha-conotoxin GI, to produce blockade of singly evoked twitches and to produce tetanic and train-of-four fade has been determined in the isolated rat hemidiaphragm preparation. Results were compared to those obtained with a reversible (vecuronium) and an irreversible (alpha-bungarotoxin) nicotinic acetylcholine antagonist and have been interpreted in terms of relative effects on post- and prejunctional nicotinic acetylcholine receptors at the neuromuscular junction. alpha-Conotoxin GI (0.5-2 microM) produced a concentration-dependent, readily reversible, decrease in the peak amplitude of single twitches and 50 Hz tetani, and an increase in tetanic and train-of-four fade. alpha-Conotoxin GI was consistently 2-3-fold more potent than vecuronium with respect to all of the measured tension parameters. Both alpha-conotoxin GI and vecuronium were approximately 2-fold more potent in producing tetanic fade and in blocking tetanic contractions than in blocking single twitches. In contrast to both alpha-conotoxin GI and vecuronium, alpha-bungarotoxin (0.13 microM) reduced the peak amplitude of both single twitches and 50 Hz tetani to the same extent without the appearance of a large degree of tetanic or train-of-four fade. Based on a comparison of the in vitro time course of neuromuscular block and of the relative effects of vecuronium, alpha-conotoxin GI and alpha-bungarotoxin on twitches, tetani and trains-of-four, we conclude that alpha-conotoxin GI has both pre- and postjunctional activity at the neuromuscular junction. In this respect, alpha-conotoxin GI resembles the clinically used competitive neuromuscular blocking drugs rather than the irreversible snake alpha-neurotoxins.

Alterations of nerve-muscle interaction during postnatal development influence motoneurone survival in rats

The effect of temporary paralysis of rat soleus muscles shortly after birth on motoneurone survival was studied using retrograde labelling with HRP. Following a single application of alpha-bungarotoxin (alpha-BTX) at birth the muscles were paralysed for 24-48 h. The number of HRP-labelled motoneurones in the treated ventral horn of the spinal cord in 10-week-old rats decreased to 63.7% (+/- 2.7 S.E.M.) of the control ventral horn. This motoneurone loss occurs relatively late after alpha-BTX application, for in animals examined at 3-4 weeks of age, the number of labelled motoneurones in the treated ventral horn was not reduced. When paralysis of the soleus muscles was extended to 6-8 days by application of an additional alpha-BTX implant, then by 10 weeks of age only 34.7% (+/- 1.5 S.E.M.) of soleus motoneurones were present. Thus, prolonging the duration of paralysis reduced the number of surviving motoneurones. Furthermore, the mean area of motoneurones that survived alpha-BTX treatment was smaller than that of controls. There was also a shift in the size distribution of the motoneurones in that there was a relative increase in the proportion of small motoneurones. This finding is similar to observations on motoneurone sizes after neonatal nerve injury. Thus, interruption of neuromuscular interaction during early postnatal development causes many motoneurones to die, and in addition alters the size distribution of the remaining cells.

Reversal by 3,3?,5-triido-l-thyronine of the working memory deficit, and the decrease in acetylcholine, glutamate and ?-aminobutyric acid induced by ethylcholine aziridinium ion in mice

The effect of 3,3',5-triiodo-L-thyronine (T3) on working memory in ethylcholine aziridinium ion (AF64A)-treated mice was studied in a delayed non-matching to sample task using a T-maze. After behavioural testing was completed, mice were killed by microwave irradiation and regional brain levels of acetylcholine, aspartate, glutamate, glutamine, glycine, taurine, and gamma-aminobutyric acid (GABA) were measured by high-performance liquid chromatography with electrochemical detection. Treatment with AF64A (7 nmol, i.c.v.) produced a deficit in working memory performance in the non-matching to sample task at 30 s delay, and decreased acetylcholine, glutamate, and GABA levels in the hippocampus, but not in the septum and cerebral cortex. Administration of T3 (0.3 mg/kg, p.o., once daily for 6 days) to AF64A-treated animals improved the deficit in working memory performance and reversed the decrease in acetylcholine, glutamate, and GABA levels in the hippocampus. These results indicate that the deficit in performance induced by AF64A can be improved by T3 administration.

Alterations in cortical muscarinic receptors following cholinotoxin (AF64A) lesion of the rat nucleus basalis magnocellularis

Cortical choline acetyltransferase (ChAT), tyrosine hydroxylase (TH), tryptophan hydroxylase (TPH), muscarinic receptors and sodium-dependent, high-affinity, choline uptake (SDHACU) sites were examined in the rat brain following unilateral stereotaxic injection of the cholinotoxin, AF64A, into the nucleus basalis magnocellularis (NBM). Injection of AF64A resulted in a significant loss of presynaptic cholinergic markers in the cortex without alteration in TH and TPH activity. The binding to SDHACU sites was reduced to background values in the NBM and increased in the central amygdala (Ce) and cortex. The increase in cortical [3H]QNB binding was the result of a change in muscarinic receptor number (BMAX) and not a change in receptor affinity (KD). Examination of muscarinic receptor subtypes demonstrated a reduction of M1 receptor binding in the cortex and NBM without any alteration in the Ce. Non-M1 binding was significantly increased in all the laminae of the cortex and in the Ce, but decreased in the NBM. These data suggest that there exists a population of M1 receptors on NBM projections to the cortex and that NBM projections influence a population of postsynaptic receptors in the cortex and Ce which are not of the M1 subtype.

The histopathological, behavioral and neurochemical effects of intraventricular injection of ethylcholine mustard aziridinium (AF64A) in the neonatal rat

This study investigated the histopathological, behavioral and neurochemical effects of bilateral injection of 2.0, 0.5 and 0.1 nmol/ventricle ethylcholine aziridinium (AF64A) on postnatal day (PND) 2. The rats showed a significant, but non-dose-related reduction of choline acetyltransferase (ChAT) in the hippocampus but not the cerebral cortex or the caudate nucleus when sacrificed on PND 16. No effect on ChAT was found in any region at PND 58. The group given 2 nmol/ventricle were hyperactive and showed a deficit in spatial learning when tested on the Morris water maze at PND 38-43. No such differences were observed for the rats injected with 0.1 or 0.5 nmol/ventricle AF64A. This spatial learning impairment in the 2 nmol group was associated with non-specific tissue damage seen only in animals from this group that were sacrificed at PND 40. This tissue damage was most evident in the left medial frontal cortex, the caudate nuclei and the anterior dorsal hippocampus.

The biochemical and ultrastructural examinations in central cholinergic damage of the rat induced by the intraperitoneal administration of AF64A

Ethylcholine mustard aziridinium ion (AF64A), a synthesized cholinergic neurotoxin, was administered via intraperitoneal injection to the rat to study its effect on the central cholinergic nervous system. A single or consecutive daily injection of AF64A for 10 days resulted in a persistent reduction of acetylcholine (ACh) content in the several tested regions of the brain in the following order: hippocampus greater than cerebral cortex = striatum, the degree was the greatest in the hippocampus. Both resting and K(+)-stimulated release of ACh from the hippocampus were also significantly reduced 24 hr after a single injection of AF64A. Furthermore, daily injection of AF64A for 10 days induced a significant reduction of choline acetyltransferase (ChAT) activity in the homogenate obtained from the hippocampus but not from the cerebral cortex and striatum. ChAT activity in the crude synaptosomal fraction of the cerebral cortex was also significantly decreased. These results suggest that intraperitoneal administration of AF64A could induce cholinergic hypofunction more selectively in the nerve terminals. The high affinity choline uptake, which is located mainly on cholinergic nerve terminals, was not affected by the administration of AF64A. Any notable changes of ultrastructure in the cholinergic nerve terminals after the administration were not observed in all three regions examined. The present findings suggested that intraperitoneal administration of AF64A induces a specific damage of cholinergic nerve terminals by inhibiting ChAT activity. The cholinergic damage was most prominent in the hippocampus.

Sector-dependent neurotoxicity of ethylcholine aziridinium (AF64A) in the rat hippocampus

The present study was aimed at measuring the distribution of ethylcholine aziridinium (AF64A)-induced cholinotoxicity within the hippocampus 6 days after bilateral (icv) administration of 1, 2 or 3 nmol, or vehicle. The dissected hippocampus was sectioned with a vibratome into 5 parallel sectors distributed along its long axis from its thalamic surface (medial) to its cortical surface (lateral). In vehicle-treated rats, the high affinity cholinergic transport (HAChT), choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) activities were distributed according to a gradient of increasing activity, extending from the lateral to the medial surface of the hippocampus. After treatment with AF64A, the normal gradient of enzyme activity was profoundly disrupted at all doses of AF64A and the core sectors of the hippocampus were significantly more affected than the superficial sectors. The HAChT gradient was progressively abolished with increasing doses of toxin, and the effect was maximal at 2 nmol.

Comparison of the effects of botulinum toxin in adult and neonatal rats: neuromuscular blockade and toxicity

A single dose of botulinum toxin (BoTX) was injected subcutaneously to induce neuromuscular blockade in the triceps surae muscles of the hindlimbs of neonatal and adult rats. The efficacy of the toxin in producing complete neuromuscular blockade of the lower limb muscles, assessed by blockade of (a) postural and flexor reflexes and (b) muscle contraction in response to nerve stimulation, was dose dependent at all ages over a BoTX dose range of 10-60 ng/kg. However, BoTX was dramatically more toxic in adult animals resulting in a decline in body weight and lethal consequences in 25% of adult animals 1 week after administration of BoTX doses as low as 40 ng/kg. In contrast, neonatal animals, given the same dosage, continued to grow and no mortalities were observed. The differences in toxicity of BoTX in adult and neonatal rats are readily accounted for by the short duration of effect in the younger animals, which, in turn, is probably the result of more rapid generation of new and functional nerve terminals.

[Effects of ethylcholine mustard aziridinium ion (AF64A) administered intraperitoneally on cholinergic nerves in rat brain]

Rats injected i.p. with AF64A at a dose of 1/3 to 11/25 LD50 showed transient signs of motor disturbance such as ataxia, squatting down and respiratory distress and also longer lasting signs consisting of adipsia, aphagia and loss of body weight. Twenty-four hours after a single injection with AF64A (129 mumol/kg), ACh contents in the cerebral cortex, hippocampus and striatum decreased significantly, and the content in the cerebral cortex still remained at a lowered level even 10 days after injection. When rats received AF64A i.p. with an initial dose of 129 mumol/kg and nine consecutive daily doses of 12.9 mumol/kg and thereafter they were kept with no medication for at least 20 days, the ACh contents in the three brain regions were still significantly reduced. ACh release by electrical stimulation or K+, but not spontaneous release, was significantly reduced in the AF64A treated rats. The T-maze spontaneous alternation behavior in rats treated with AF64A was impaired. The present results indicate that AF64A administered i.p. can induce a long lasting defect in central cholinergic transmission.

Comparative toxicity of atracurium and metocurine in isolated rat hepatocytes

Primary cultures of liver cells isolated from seven rats were used to study the possible toxicity of atracurium and metocurine. The muscle relaxants were separately added to the culture medium and the cells then incubated for 4 hr. The amount of lactic dehydrogenase (LDH) that leaked into the culture medium was determined at the end of incubation. The customary assumption was made that the exudation of LDH reflects the toxic effects of the relaxants. In untreated dishes, approximately 11% of the total intracellular LDH leaked out during the incubation. The net leakage of LDH produced by the relaxants was obtained by subtracting this amount from the LDH activity determined in the media of dishes with the relaxants added. On this basis, metocurine, in concentrations of 12-850 X 10(-6)M, did not cause a net leak of LDH. On the other hand, atracurium, in similar molar concentrations, caused a statistically significant and concentration-dependent leak of LDH that, at its maximum, amounted to more than one half of the intracellular LDH. The results are interpreted in terms of damage to cellular membranes produced by atracurium or its metabolites. Although the exact biochemical process was not identified, we hypothesize that acrylates--produced by Hofmann elimination from atracurium--might be the likely toxic species.

Specific toxic effects of ethylcholine nitrogen mustard on cholinergic neurons of the nucleus basalis of Meynert

The putative cholinergic neurotoxin, ethylcholine aziridinium ion (AF64A), was injected unilaterally into the nucleus basalis of Meynert (nbM) in order to determine whether it would produce specific damage to the cholinergic cell bodies of this nucleus. Injections of small amounts of AF64A (0.01 nmol in 1 microliter) or of its vehicle had little effect on the appearance of the nbM or on the levels of choline acetyltransferase (ChAT) in the cortex. Injections of larger amounts of AF64A (0.02 and 0.05 nmol in 1 microliter and 0.02 nmol in 10 microliters) produced a loss of diffuse acetylcholinesterase staining in the nbM and a loss of large positively staining neurons. Furthermore, these injections produced a significant reduction of ChAT activity in the central portion of the cortex. However, non-cholinergic neurons in the area of the nbM were not affected by these AF64A injections. In addition, cortical uptake of monoamines was not affected by these lesions. Further increases in the amount of AF64A injected (0.1 nmol in 1 microliter and 0.035 nmol in 10 microliters) caused damage at the site of the injection which was not limited to the cholinergic elements of the nbM. These results suggest that AF64A can be used to produce specific lesions of cholinergic neurons, and therefore may be useful in developing animal models of human disorders involving cholinergic hypofunction, such as senile dementia of the Alzheimer type. However, there is a narrow dose range for producing these specific effects.

The screening of atracurium in MHS swine

The potential role of atracurium besylate as a trigger or attenuator of the malignant hyperthermia syndrome was tested in six MHS Landrace swine. Animals were tested for susceptibility and then exposed to atracurium given as an i.v. bolus both alone and concomitantly with 2% halothane. In no instance could the syndrome be triggered by atracurium nor did it convincingly attenuate the syndrome when triggered by halothane.

Regional differences in ethylcholine mustard aziridinium ion (AF64A)-induced deficits in presynaptic cholinergic markers for the rat central nervous system

Several highly selective biochemical markers were used to assess the persistent central cholinergic dysfunction which accompanies administration of the cholinergic neurotoxin ethylcholine mustard aziridinium ion (AF64A). Rats received a single bilateral intracerebroventricular injection of AF64A (3 nmol/3 microliter/side) or vehicle and measurements were carried out in the cerebral cortices, hippocampi and corpora striata at 7 and 21 days postinjection. The drug binding sites of muscarinic cholinergic receptors, as revealed by high-affinity binding of (-)-[3H]quinuclidinyl benzilate (a classical muscarinic antagonist), [3H]pirenzepine (a selective antagonist of the putative M1 muscarinic receptor subclass) and (+)-[3H]cis-methyldioxolane (a potent muscarinic agonist), were not significantly affected by AF64A treatment. As reported previously, activity of the cholinergic synthetic enzyme choline acetyltransferase was reduced markedly (60-65%) in the hippocampi of AF64A-treated rats. A similar reduction was noted in high-affinity binding of [3H]hemicholinium-3 (a putative radioligand for sodium-dependent high-affinity choline uptake sites on cholinergic nerve terminals) in hippocampal membranes (59-65%). However, in the cerebral cortex, these presynaptic cholinergic markers were differentially altered by AF64A pretreatment (choline acetyltransferase, unchanged; [3H]hemicholinium-3 binding, reduced by 59-65%). These results indicate that a single intracerebroventricular injection of AF64A promotes biochemical and possibly functional deficits in presynaptic cholinergic nerve terminals distal from the injection site while having minimal influences upon muscarinic cholinergic receptor populations.

Structure-activity studies of homologues of short chain neurotoxins from Elapid snake venoms

Three neurotoxin homologues (CM10 and CM12 from Naja haje annulifera and S5C10 from Dendroaspis jamesoni kaimosae) and two short neurotoxins (CM14 from Naja haje annulifera and erabutoxin b from Laticauda semifasciata) were examined by circular dichroism (c.d.) and tested for neuromuscular activity on chick biventer cervicis nerve-muscle preparations. All three homologues had acetylcholine receptor blocking activity, as they abolished responses to indirect stimulation, acetylcholine and carbachol but had no effect on responses to direct muscle stimulation. CM10 was only about 5 times less potent than the short neurotoxin CM14; S5C10 and CM12 were respectively 30 and 300 times less active. The block induced by the three homologues, but not by the neurotoxins, was readily reversed by washing. CM10 and CM12 had virtually identical c.d. spectra which were closely similar to those of the neurotoxins. The spectrum of S5C10 indicated changes in the environment of tyrosine-25 and in the position of tryptophan-29. These alterations could distort the 3-dimensional arrangement of the residues postulated to form the receptor binding site. The results with CM10 and CM12 highlight a role for the first loop (residues 6-16) in the binding of neurotoxins to acetylcholine receptors, in addition to the previously postulated reactive site.