Exploring the functional domain and the target of the tetanus toxin light chain in neurohypophysial terminals

The tetanus toxin light chain blocks calcium induced vasopressin release from neurohypophysial nerve terminals. Here we show that histidine residue 233 within the putative zinc binding motif of the tetanus toxin light chain is essential for the inhibition of exocytosis, in the rat. The zinc chelating agent dipicolinic acid as well as captopril, an inhibitor of zinc-dependent peptidases, counteract the effect of the neurotoxin. Synthetic peptides, the sequences of which correspond to motifs present in the cytoplasmic domain of the synaptic vesicle membrane protein synaptobrevin 1 and 2, prevent the effect of the tetanus toxin light chain. Our results indicate that zinc bound to the zinc binding motif constitutes the active site of the tetanus toxin light chain. Moreover they suggest that cleavage of synaptobrevin by the neurotoxin causes the inhibition of exocytotic release of vasopressin from secretory granules.

Botulinum A like type B and tetanus toxins fulfils criteria for being a zinc-dependent protease

Although botulinum neurotoxin (BoNT) types A and B and tetanus toxin (TeTx) are specific inhibitors of transmitter release whose light chains contain a zinc-binding motif characteristic of metalloendoproteases, only the latter two proteolyse synaptobrevin. Chelation of zinc or its readdition at high concentration hindered blockade of neuromuscular transmission by BoNT/A and B, indicating that type A also acts via a zinc-dependent mechanism. Such treatments prevented proteolysis of synaptobrevin II in rat brain synaptic vesicles by BoNT/B and TeTx but only the activity of the latter was antagonised appreciably by ASQFETS, a peptide spanning their cleavage site. The toxin's neuroparalytic activities were attenuated by phosphoramidon or captopril, inhibitors of certain zinc requiring proteases. However, these agents were ineffective in reducing the toxins' degradation of synaptobrevin except that a high concentration of captopril partially blocked the activity of TeTx but not BoNT/B, as also found for these drugs when tested on synaptosomal noradrenaline release. These various criteria establish that a zinc-dependent protease activity underlies the neurotoxicity of BoNT/A, a finding confirmed at motor nerve endings for type B and TeTx. Moreover, the low potencies of captopril and phosphoramidon in counteracting the toxins' effects necessitate the design of improved inhibitors for possible use in the clinical treatment of tetanus or botulism.

Disulfide formation in reduced tetanus toxin by thioredoxin: the pharmacological role of interchain covalent and noncovalent bonds

The interchain disulfide bond of tetanus toxin is known to be cleaved by reduced thioredoxin and by rat brain homogenate. We now show that this bond, but not the disulfide loop in the heavy chain of the toxin, can be restored quickly and completely by oxidized thioredoxin. Oxidized glutathione was at least 100 times less potent and less specific. Reduced tetanus toxin did not measurably (KD below 50 nM) dissociate into its chains, as revealed by HPLC gel chromatography under nondenaturing conditions. Accordingly, when the reduced toxin or its recombined chains were injected into mice, general toxicity was diminished but not abolished, as compared with the native form. Inhibition of Ca(2+)-evoked [3H]noradrenaline release was assayed in cultured adrenomedullary cells after permeabilization with digitonin. Reduced two-chain tetanus toxin was as active as the isolated light chain in this system, and the action of the light chain was only slightly diminished by the addition of excess heavy chain. The results show that thioredoxin can both open and close the covalent bond between the chains of tetanus toxin, and that the reduced chains remain linked by noncovalent forces. The role of the thioredoxin system for reversible activation of tetanus toxin in vivo remains to be established.

Toxic effects of tetanus toxin on GG2EE macrophages: prevention of gamma interferon-mediated upregulation of lysozyme-specific mRNA levels

By using a nonneuronal cell system, evidence has previously been provided that tetanus toxin (TT) intoxication occurs in macrophages, impairing their secretory activity as well as their antitumoral activity. In particular, both secreted and total lysozyme (LZM) activities are reduced by TT treatment, provided that GG2EE macrophages have been preexposed to gamma interferon (IFN-gamma). In an attempt to provide insight into the molecular mechanisms underlying this phenomenon, we focused our attention on the levels of LZM-specific transcripts. GG2EE macrophages preexposed to IFN-gamma exhibited augmented levels of LZM-specific mRNA. Such an effect was detected 1 h after removal of IFN-gamma, peaked at 3 h, and gradually decreased with time in culture. Exposure of IFN-gamma-pretreated GG2EE macrophages to TT resulted in the prevention of the IFN-gamma-mediated upregulation of LZM mRNA levels. The phenomenon was mediated by the holotoxin (> or = 1 micrograms/ml) and abrogated by preexposure of the macrophages to the C fragment of TT. Protein kinase C (PKC) and Ca(2+)-calmodulin-dependent PK were likely involved in the IFN-gamma-mediated upregulation of LZM mRNA levels and biological activity, as assessed by PK inhibitors. Furthermore, PK inhibitors mimicked TT in impairing LZM activity of GG2EE macrophages, thus suggesting that impairment of PKC and/or the Ca(2+)-calmodulin-dependent PK pathway(s) may be one of the events involved in TT intoxication of macrophages.

Production of biologically active light chain of tetanus toxin in Escherichia coli. Evidence for the importance of the C-terminal 16 amino acids for full biological activity

The activity of the light (L) chain of tetanus toxin, and of mutants constructed by site-directed mutagenesis, was studied by expression and purification of the proteins from E. coli. Wild-type recombinant L chain (pTet87) was active in the inhibition of exocytosis from cultured bovine adrenal chromaffin cells, although at a level 5-15% of that of L chain purified from tetanus toxin. L chain mutants which terminated at Leu-438 (pTet89), or which contained a Cys-to-Ser mutation at residue 439 (pTet88) were equally as active as the full-length recombinant protein. The reduced activity of pTet87 L chain correlated with C-terminal proteolysis of the protein upon purification. A tryptic fragment derived from native light chain and which terminated at Leu-434 also showed reduced activity in the exocytosis assay, consistent with a requirement of the C-terminal region of the L chain for maximal activity. pTet87 L chain, but neither of the mutants, could be associated with purified H (heavy) chain to form a covalent dimer which induced the symptoms of tetanus in mice. The ability to form biologically active toxin using recombinant L chain will be of great value in structure-function studies of tetanus toxin.

Modulation of T lymphocyte function by the angiogenesis inhibitor AGM-1470

The angiogenesis inhibitor AGM-1470 has recently been reported to inhibit collagen-induced arthritis in rats. To determine if the anti-arthritic effects of AGM-1470 might be due to T cell inhibition, we have studied its effects on T cell responses in vitro. Responses of human cells to tetanus toxoid (TT), and those of murine splenocytes to staphylococcal enterotoxin (SE), mitogens or a mls difference were inhibited by AGM-1470. Responses of human cells to SE, OKT3 and PHA were all partially inhibited on day 2 (d2) but not d3, and in fact were augmented on d6-8. The amount of IL-2 in SEA cultures was augmented on d4 and d5. There were no differences in the expression of CD3, CD4, CD8, CD25, CD45RA, CD45RO, LFA-1, VLA-4 or VLA-6 in inhibited cultures, except for slight decreases in CD25 and CD45RO in TT cultures. These results indicated that the angiogenesis inhibitor AGM-1470 also modulates human and murine lymphocyte function.

Glutamate transmission is involved in the mechanisms of neuronal degeneration produced by intrahippocampal tetanus toxin in rats

Tetanus toxin (TT) blocks GABA-mediated inhibitory neurotransmission in the mammalian CNS via selective inhibition of transmitter release. The loss of central inhibition produces an excitatory focus resembling human limbic epilepsy. We now report that the net excitation caused by an unopposed action of glutamic acid may also produce neuronal degeneration in the rat brain. Anaesthetized rats were placed in a stereotaxic frame and TT (1 microliter dissolved in phosphate buffer, pH 7.0) was injected unilaterally into the dorsal hippocampus. Injection of TT (1000 mouse minimum lethal doses, MLDs; n = 3-6 rats per group) produced time-dependent neuronal loss in the CA1 pyramidal cell layer which was significant (p < 0.05) 7 and 10 days, but not 1 day, after the injection. Systemic treatment with competitive (CGP 37849, 3 mg/kg i.p) or non-competitive (MK801, 0.3 mg/kg i.p.) antagonists at the N-methyl D-aspartate (NMDA) receptor complex 1 h before and 1 h after TT and then once daily for 10 days protected rats from the hippocampal damage produced by TT (1000 MLDs). In addition, in rats bearing a monolateral surgical lesion of the Schaffer collaterals, through which CA1 neurones receive a robust excitatory input from CA3 pyramids, the bilateral injection of TT (1000 MLDs/side) produced significant neuronal loss in the unlesioned hippocampus whereas the contralateral appeared to be preserved. In conclusion, these results demonstrate that excitatory neurotransmission may be involved in the neuropathology elicited by intrahippocampal TT in rats.

Tetanus toxin is a zinc protein and its inhibition of neurotransmitter release and protease activity depend on zinc

Tetanus and botulinum neurotoxins are the most potent toxins known. They bind to nerve cells, penetrate the cytosol and block neurotransmitter release. Comparison of their predicted amino acid sequences reveals a highly conserved segment that contains the HexxH zinc binding motif of metalloendopeptidases. The metal content of tetanus toxin was then measured and it was found that one atom of zinc is bound to the light chain of tetanus toxin. Zinc could be reversibly removed by incubation with heavy metal chelators. Zn2+ is coordinated by two histidines with no involvement in cysteines, suggesting that it plays a catalytic rather than a structural role. Bound Zn2+ was found to be essential for the tetanus toxin inhibition of neurotransmitter release in Aplysia neurons injected with the light chain. The intracellular activity of the toxin was blocked by phosphoramidon, a very specific inhibitor of zinc endopeptidases. Purified preparations of light chain showed a highly specific proteolytic activity against synaptobrevin, an integral membrane protein of small synaptic vesicles. The present findings indicate that tetanus toxin, and possibly also the botulinum neurotoxins, are metalloproteases and that they block neurotransmitter release via this protease activity.

Minimal essential domains specifying toxicity of the light chains of tetanus toxin and botulinum neurotoxin type A

To define conserved domains within the light (L) chains of clostridial neurotoxins, we determined the sequence of botulinum neurotoxin type B (BoNT/B) and aligned it with those of tetanus toxin (TeTx) and BoNT/A, BoNT/C1, BoNT/D, and BoNT/E. The L chains of BoNT/B and TeTx share 51.6% identical amino acid residues whereas the degree of identity to other clostridial neurotoxins does not exceed 36.5%. Each of the L chains contains a conserved motif, HExxHxxH, characteristic for metalloproteases. We then generated specific 5'- and 3'-deletion mutants of the L chain genes of TeTx and BoNT/A and tested the biological properties of the gene products by microinjection of the corresponding mRNAs into identified presynaptic cholinergic neurons of the buccal ganglia of Aplysia californica. Toxicity was determined by measurement of neurotransmitter release, as detected by depression of postsynaptic responses to presynaptic stimuli (Mochida, S., Poulain, B., Eisel, U., Binz, T., Kurazono, H., Niemann, H., and Tauc, L. (1990) Proc. Natl. Acad. Sci. U. S. A. 87, 7844-7848). Our studies allow the following conclusions. 1) Residues Cys439 of TeTx and Cys430 of BoNT/A, both of which participate in the interchain disulfide bond, play no role in the toxification reaction. 2) Derivatives of TeTx that lacked either 8 amino- or 65 carboxyl-terminal residues are still toxic, whereas those lacking 10 amino- or 68 carboxyl-terminal residues are nontoxic. 3) For BoNT/A, toxicity could be demonstrated only in the presence of added nontoxic heavy (H) chain. A deletion of 8 amino-terminal or 32 carboxyl-terminal residues from the L chain had no effect on toxicity, whereas a removal of 10 amino-terminal or 57 carboxyl-terminal amino acids abolished toxicity. 4) The synergistic effect mediated by the H chain is linked to the carboxyl-terminal portion of the H chain, as demonstrated by injection of HC-specific mRNA into neurons containing the L chain. This finding suggests that the HC domain of the H chain becomes exposed to the cytosol during or after the putative translocation step of the L chain.

Tetanus toxin potently stimulates tissue transglutaminase. A possible mechanism of neurotoxicity

The observation that tetanus toxin (TT) contains two sequences that show homology to known transglutaminase (TGase) substrate sites suggested that the toxin and TGase might interact. This prediction was confirmed by two pieces of evidence. First, TT potently stimulated the enzymatic activity of TGase. The effect was maximal at physiological (micromolar) concentrations of the endogenous TGase regulators calcium and GTP. Second, TT and TGase displayed marked variations of their intrinsic fluorescence properties when they were coincubated, indicating the occurrence of binding between them. TT-TGase binding and TGase activation occurred at similar concentrations of TT and are probably causally related. The activation of TGase, an enzyme present in nerve endings that, when activated, can irreversibly cross-link cellular proteins, might mediate the neurotoxic action of TT.

Neuropathology of the chronic epileptic syndrome induced by intrahippocampal tetanus toxin in rat: preservation of pyramidal cells and incidence of dark cells

A few nanograms of tetanus toxin injected into a rat hippocampus causes a chronic epileptic syndrome characterized by brief seizures that recur intermittently for about 6 weeks. Cognitive and other behavioural impairments persist after the seizures and other epileptic electrographic activity have remitted, and may be permanent. Our previous studies suggested that the behavioural changes following seizure remission were an indication of functional impairment associated with decreased neuronal excitability rather than with neuronal loss. The conclusion that neurons were preserved relied on qualitative histological observations and, indirectly, on electrophysiological measurements of the amplitudes of antidromic population spikes. Recently, gross histopathology has been described in a quantitative histological study of rats 7-10 days after they had received rather higher doses of intrahippocampal tetanus toxin. Here we report a quantitative histological study of hippocampi from rats which had gained remission from seizures induced by low doses of tetanus toxin. Adult Sprague Dawley rats received unilateral injections of 3-4 ng (about 6-8 mouse LD50) tetanus toxin, or vehicle, into the dorsal hippocampus. The first experiment confirmed that postsynaptic evoked responses recorded from pyramidal cells were depressed 10-19 weeks after injection. Unexpectedly, there also was a decrease of 20% in the antidromic response from CA3a contralateral to the injection. However, cell counts in these hippocampi revealed no change in pyramidal cell numbers. The second experiment used rats from two breeding colonies, prepared for histology 7 weeks after injection. Hippocampal pyramidal cell numbers were within the normal range in all but three of the 24 rats that had received tetanus toxin. These three had lesions of the CA1 pyramidal layer contralateral to the injection. The lesions were of the order of 2 mm in diameter, and were associated with glial proliferation. When these three cases were excluded, there remained a small increase in glial density in CA1 of the toxin-injected rats. In addition, toxin-injected rats from one of the colonies were susceptible to a pathology known as acidophylic or dark cell change. These occurred in 11 of 18 toxin-injected rats from this colony, in all divisions of the pyramidal layer, in both the injected and the contralateral hippocampus (where parallel studies revealed independent secondary epileptic foci). We conclude that loss of pyramidal neurons is not necessary for the persistent behavioural changes in this model.(ABSTRACT TRUNCATED AT 400 WORDS)

Distinct targets for tetanus and botulinum A neurotoxins within the signal transducing pathway in chromaffin cells

Tetanus and botulinum A neurotoxins inhibited exocytosis evoked by various secretagogues in intact and permeabilized chromaffin cells. The block of exocytosis in intact chromaffin cells due to botulinum A neurotoxin could partially be overcome by enhancing nicotine- and veratridine-induced stimulation, whereas the block due to tetanus toxin persisted under the same conditions. The receptor-mediated restoration of 3H-noradrenaline release was specific for nicotinic stimulation, because exocytosis did not occur during muscarinic stimulation. Depolarization of intact chromaffin cells with increasing concentration of K+ failed to restore exocytosis that had been blocked by either toxin. When chromaffin cells, treated with tetanus or botulinum A neurotoxins, were exposed to the Ca2(+)-ionophore A 23187 or permeabilized by staphylococcal alpha-toxin, Ca2(+)-stimulated exocytosis was also inhibited. The inhibition was unaffected by increasing concentrations of free Ca2+. Activation of proteinkinase C and of G-proteins by phorbolester and GMPPNHP, respectively, increased Ca2(+)-induced exocytosis in control cells as well as in cells treated with tetanus and botulinum A neurotoxins. The block, however, could not be relieved by these manipulations, and it could not be relieved by activating the cGMP or cAMP pathways with analoga of cyclic nucleotides, phosphodiesterases inhibitors, and forskolin either. It is concluded that nicotine and veratridine trigger a mechanism within the sequence of events leading to exocytosis that is located beyond the increase in intracellular Ca2(+)-concentration. This pathway may not be affected by botulinum A neurotoxin. The target of tetanus toxin is probably located even closer to the fusion process, i.e. beyond the step upon which botulinum A neurotoxin acts.

[The standardization of the in-vitro tetanus toxin neutralization test on Chinese hamster ovary cells]

A test for the titration of B. pertussis toxin with antisera on Chinese hamster ovary (CHO) cells has been worked out. B. pertussis protective antigenic cell-free complex containing 48-54% of B. pertussis toxin has been used as antigen. The specificity of the effect of this complex on CHO cells has been confirmed in the toxicity neutralization test with antisera. CHO cells have been adapted to reagents and culture media made in the USSR. The titration of B. pertussis toxin and antisera on CHO cells did not require the use of highly purified antigen.

Tetanus toxin and neuronal membranes: the relationship between binding and toxicity

Tetanus toxin labeled by the Bolton-Hunter technique possesses high specific activity and retains substantial biological activity. This material can be used to characterize tetanus toxin binding to receptors in brain membrane preparations. In experiments aimed at measuring the absorption of labeled toxin, the displacement of labeled toxin by unlabeled toxin and the on-rate and off-rate constants, the data revealed two binding sites. The high affinity site had a Kd of 0.033 to 0.070 nM and a Bmax of 0.26 to 0.4 pmol/mg of protein; the low affinity site had a Kd of 0.89 to 6.9 nM and a Bmax of 1.55 to 3.0 pmol/mg of protein. The binding of tetanus toxin to brain membranes was enhanced greatly by low pH and ionic strength. Similarly to tetanus toxin, botulinum neurotoxin could be labeled by the Bolton-Hunter technique, and its binding to brain membranes was also enhanced by low pH and ionic strength. In studies with a neutralizing monoclonal antibody against tetanus toxin, the antigen-antibody interaction was not significantly altered by media with low ionic strength and pH. On the other hand, the ability of the antibody to block toxin binding to brain membranes was reduced substantially in nonphysiologic media. In a bioassay aimed at determining the effect of pH and tonicity on tissue association by toxin, low pH and ionic strength did not enhance toxicity. The biological activity of tetanus toxin was unaffected and that of botulinum neurotoxin was greatly diminished. The present findings confirm the widely reported observation that low pH and ionic strength promote tissue association by tetanus toxin, but they challenge the premise that this binding is relevant to the normal process of cell poisoning.

High vulnerability of dentate granule cells to the neuropathological effects induced by intrahippocampal injection of tetanus toxin

The behavioural and neuropathological effects of tetanus toxin, injected into the dentate gyrus, were studied in rats. The monolateral injection of a single dose (1000 mouse minimum lethal doses, MLDs; n = 14 rats) of tetanus toxin produced time-dependent behavioural stimulation. Wet-dog shakes and facial stereotypy were observed 3-4 days after the injection, culminating 4-5 days after treatment, in "limbic motor seizures". Ten days after injection, histological examination revealed death of dentate granule cells in the tetanus toxin-treated side but not in the contralateral, control side (treated with neutralized toxin). This effect was observed in all rats (n = 5) receiving tetanus toxin in the dentate gyrus and no damage was reported in other sectors (e.g. CA1 and CA3 pyramidal cell layers) of the hippocampus. Quantification of the neuronal damage yielded an approximately 70% reduction (P less than 0.01) in the number of granule cells in the toxin-injected dentate gyrus, compared with the control side. This was greater than that previously reported (30% reduction) in the CA1 pyramidal cell layer of rats receiving the same dose of toxin, into the regio superior of the hippocampus. In conclusion, the present experiments have shown that the focal injection of tetanus toxin into the dentate gyrus produced behavioural excitation and selective death of dentate granule cells.

Neonatal tetanus despite protective serum antitoxin concentration

Using the ELISA technique to estimate serum antibodies against tetanus toxin, seven neonates with clinical tetanus were found to have antibody levels 4-13 times higher than the presumed minimum protective level of 0.01 IU/ml. All but one of their mothers had been vaccinated with tetanus toxoid in pregnancy. In two other neonates, whose mothers had received multiple booster doses of toxoid during pregnancy, the anti-toxin concentrations were 100- and 400-times the presumed protective level. Therefore the toxin dose may overwhelm the pre-existing anti-toxin level and produce disease. Furthermore, multiple booster injections of tetanus toxoid may not only enhance serum anti-toxin titres, but could also lead to an ineffective immune response.

Hippocampal damage produced by tetanus toxin in rats can be prevented by lesioning CA1 pyramidal cell excitatory afferents

The neuropathological effects induced by tetanus toxin (TT) bilaterally microinjected into the hippocampus were studied in rats bearing a surgical unilateral lesion of the Schaffer collaterals. TT (1000 mouse minimum lethal doses, MLDs; n = 5 rats) produced neurodegeneration in the CA1 pyramidal cell layer in the unlesioned side of the hippocampus ten days after injection. By contrast, the injection of TT into the lesioned hippocampus produced no degeneration. In rats bilaterally treated with BSA (n = 3 rats) no neuropathological effects were observed in either hippocampi. In conclusion, our results have demonstrated that the lesion of the Schaffer collaterals may protect against the neuropathological effects induced by TT in rats.

Intact inhibition of the stretch reflex during general tetanus

Tetanus toxin at lethal doses (2-10 x 10(3) mouse minimal lethal doses per kg body weight, mMLD/kg) was injected i.v. into 10 cats under pentobarbital anaesthesia. After the appearance of the first sign of generalized tetanus the animal was anaesthetized by a mixture of urethane and chloralose. Experiments were performed several hr thereafter when the toxin action was anticipated to be optimal. The stretch reflexes were elicited manually, by the contraction of the antagonistic muscles or by a stretch device. In toxin treated animals the spontaneous electromyographic activity was inhibited by strong stretching of the tested muscle or by that of the antagonistic muscle. The stretch reflex of the extensor muscle elicited by a contraction of the flexor muscle was inhibited by electrical stimulation of the flexor afferent fibres. The stretch reflex elicited by a stretch device as well as the electrically elicited monosynaptic reflex were inhibited by conditioning stimulation of the antagonistic nerve. The inhibition curves were almost the same as those of healthy animals. It is concluded that the spinal inhibitions, such as antagonistic group Ia, autogenic group Ib, groups II and III and the presynaptic inhibitions, were kept intact in severe general tetanus.

An intact interchain disulfide bond is required for the neurotoxicity of tetanus toxin

Tetanus toxin is composed of a heavy chain (100 kDa) and a light chain (50 kDa) held together by a single interchain disulfide bridge. An additional intrachain disulfide is present in the carboxy-terminal part of the heavy chain. Reduction of the two disulfide bonds in tetanus toxin with both chemical and proteinaceous reducing agents was studied. Dithiothreitol and 2-mercaptoethanol cleaved both the inter- and intrachain disulfide bridges of the toxin, while glutathione and cysteine were ineffective. Specific reduction of the single interchain disulfide link was achieved with the thioredoxin-thioredoxin reductase system, thus indicating that this bond is exposed at the protein surface. Also, dead or permeabilized cells were able to reduce the toxin. Such reduced toxin bound to neuronal membranes as well as the native toxin but was not neurotoxic. These findings open the possibility that reduction by cytoplasmic agents released by dead cells contributes to detoxification of tetanus toxin. Moreover, together with the notion that the light chain is the active form of the toxin in the cytoplasm, these results suggest that the interchain disulfide bond of tetanus toxin plays a role in nerve cell penetration.

Prevention by the NMDA receptor antagonist, MK801 of neuronal loss produced by tetanus toxin in the rat hippocampus

1. The behavioural and neuropathological effects of tetanus toxin, microinjected directly into the hippocampus, were studied in rats. 2. A single dose (1000 minimum lethal doses, MLDs) of tetanus toxin, injected unilaterally into the hippocampus produced a time-dependent neuronal loss in the CA1 pyramidal cell layer. In comparison with the contralateral, untreated side these effects became statistically significant (P less than 0.05) 7 days (22.0 +/- 1.1% reduction) and 10 days (29.2 +/- 1.7% reduction) after the injection. No significant changes were observed 7 days after treatment with 500 MLDs whereas a reduction of 37.5 +/- 3.1% in the CA1 area cell number was produced 4 days after the injection of 2000 MLDs. 3. Behavioral stimulatory effects were also induced by tetanus toxin (1000 MLDs) within 48 h of the injection and these culminated in generalized convulsions 5-7 days later. Convulsions were observed after a shorter period of latency in rats receiving 2000 MLDs tetanus toxin whereas 500 MLDs were ineffective. 4. No behavioural and neuropathological effects were observed in rats treated with neutralized tetanus toxin (1000 MLDs), bovine serum albumin or phosphate buffer. 5. Pretreatment with MK801 (0.3 mg kg-1, i.p., given 1 h before and after the injection with tetanus toxin and then once daily for 4 or 7 days) prevented the behavioural and neuropathological effects induced by tetanus toxin (1000-2000 MLDs). In addition, such treatment fully protected the animals from the lethal effects induced by 1000 MLDs tetanus toxin. In addition, such treatment fully protected the animals from the lethal effects induced by 1000MLDs tetanus toxin. By contrast, pretreatment with diazepam (3.Omgkg-1, i.p.) using the same schedule as for MK801 did not antagonize the effects of tetanus toxin (1000-2000 MLDs). 6. In conclusion, the present experiments have demonstrated that the intrahippocampal injection of tetanus toxin produces in rats a dose- and time-dependent behavioural stimulation and neuronal loss in the CAl pyramidal cell layer which can be prevented by the non-competitive NMDA antagonist, MK801.

Tetanus toxin-induced seizures cause microglial activation in rat hippocampus

Tetanus toxin (about 20 mouse LD50) injected into the ventral hippocampus of rats leads to brief seizures occurring intermittently over a period of weeks. Toxin injection leads to the appearance of activated microglia (detected with OX42 immunohistochemistry) in the hippocampus. After 7-14 days, many activated microglia are visible in CA1 area of dorsal hippocampus aligned with the pyramidal cell dendrites and having the morphology characteristic of 'rod cells'. Extensive cell loss is found in dorsal CA1, but not at the injection site, in about one third of injected rats.