Tetanus toxin blocks inhibition of granule cells in the dentate gyrus of the urethane-anaesthetized rat

Rapid eye movement sleep and hippocampal theta oscillations precede seizure onset in the tetanus toxin model of temporal lobe epilepsy

Improved understanding of the interaction between state of vigilance (SOV) and seizure onset has therapeutic potential. Six rats received injections of tetanus toxin (TeTX) in the ventral hippocampus that resulted in chronic spontaneous seizures. The distribution of SOV before 486 seizures was analyzed for a total of 19 d of recording. Rapid eye movement sleep (REM) and exploratory wake, both of which express prominent hippocampal theta rhythm, preceded 47 and 34%, for a total of 81%, of all seizures. Nonrapid eye movement sleep (NREM) and nonexploratory wake, neither of which expresses prominent theta, preceded 6.8 and 13% of seizures. We demonstrate that identification of SOV yields significant differentiation of seizure susceptibilities, with the instantaneous seizure rate during REM nearly 10 times higher than baseline and the rate for NREM less than half of baseline. Survival analysis indicated a shorter duration of preseizure REM bouts, with a maximum transition to seizure at ∼90 s after the onset of REM. This study provides the first analysis of a correlation between SOV and seizure onset in the TeTX model of temporal lobe epilepsy, as well as the first demonstration that hippocampal theta rhythms associated with natural behavioral states can serve a seizure-promoting role. Our findings are in contrast with previous studies suggesting that the correlations between SOV and seizures are primarily governed by circadian oscillations and the notion that hippocampal theta rhythms inhibit seizures. The documentation of significant SOV-dependent seizure susceptibilities indicates the potential utility of SOV and its time course in seizure prediction and control.

The tetanus toxin model of chronic epilepsy

In experimental models of epilepsy, single and recurrent seizures are often used in an attempt to determine the effects of the seizures themselves on mammalian brain function. These models attempt to emulate as many features as possible of their human disease counterparts without many of the confounding factors such as underlying disease processes and medication effects. Numerous models have been used in the past to address different questions. Nevertheless, the basic questions are often the same: 1. Do seizures cause long-term damage? 2. Do seizures predispose to chronic epilepsy (epileptogenesis), that is long-term spontaneous repetitive seizures? 3. Are these results developmentally regulated? 4. Are the underlying mechanisms of epileptogenesis and brain damage related? In pursuing these questions, the goal is to determine how seizures exert their effects and to minimize any side effects from the methods employed to induce the seizures themselves. This requires a detailed characterization of the methods used to induce seizures. In this chapter, we will review the literature regarding the tetanus toxin model of chronic epilepsy with regard to its mechanisms of action, clinical comparisons, how it is experimentally implemented and the results obtained thus far. These results will be compared to other models of chronic epilepsy in order to make generalizations about the effects of repetitive seizures in adult and early life. At this time, it appears that repetitive seizures cause long-term changes in learning ability and may cause a predisposition to chronic seizures at all ages. In younger animals, both features of learning impairment and epilepsy are not typically associated with cell loss as they are in adult animals. At all ages, some form of synaptic reorganization has been demonstrated to occur.

A new protocol for the transmission of physiological signals by digital telemetry

A method has been devised to transmit physiological signals from the brains of rats using a new digital telemetry transmission protocol. The chief advantage of the present system over existing systems is that the circuit only consumes power during the transmission of a brief pulse of information. This results in a very much extended battery life allowing the device to be implanted and recordings to be carried out over a longer period of time.

Increased susceptibility to tetanus toxin-induced seizures in immature rats

Studies were undertaken to compare tetanus toxin-induced electrographic and behavioral seizure activity in immature and adult rats. Multifocal epileptiform activity was induced by unilateral hippocampal injection of tetanus toxin in 10-day-old immature and in 90-120-day-old adult rats. Bilateral cortical surface electrodes and, in some animals, hippocampal depth electrodes were placed and serial video EEG was recorded for 2 h during the week following tetanus toxin injection. Tetanus toxin-induced interictal spike frequency was significantly higher and seizure duration was significantly longer in immature compared to adult rats. Immature rats tended to have more frequent seizures compared with adults, but the difference was not statistically significant. Ictal behaviors were different for the two groups with immature rats commonly demonstrating wild running seizures which were not seen in adult rats. These findings suggest that there are age-dependent differences in electrographic and behavioral seizure expression in the tetanus toxin seizure model and that the epileptic syndrome in immature rats is more severe.

Loss of hilar somatostatin neurons following tetanus toxin-induced seizures

A loss of inhibitory interneurons has been reported in the hippocampus following seizure activity in various animal models of epilepsy and in human epileptic tissue. The question of whether particular populations of inhibitory neurons are similarly affected by the chronic block of inhibition that results after tetanus toxin injections directly into the brain has not previously been addressed. In the present study a unilateral intrahippocampal injection of tetanus toxin into the ventral hippocampus was used to produce a chronic epileptic syndrome characterised by brief seizures that recurred intermittently for 6-8 weeks. The results reveal, for the first time, the morphological changes in somatostatin interneurons following tetanus toxin-induced seizures in the rat. A bilateral short-term increase in immunoreactivity of somatostatin neurons is present 1 week after injection. This is accompanied by an increased intensity of somatostatin-immunoreactive axon terminals in the outer molecular layer of the dentate gyrus, which is more marked on the contralateral side. A chronic and significant loss of somatostatin-immunoreactive neurons was noted in the hilus of the dentate gyrus 2 months later. The significance of the chronic loss of the hilar somatostatin neurons in the control of excitatory activity in the dentate gyrus and whether the acute morphological changes are due to a direct action of the toxin on release mechanisms or as a result of seizure activity are discussed.

Tetanus toxin alters 5-hydroxytryptamine, dopamine, and their metabolites in rat hippocampus measured by in vivo microdialysis

Tetanus toxin is a potent neurotoxin, the mode of action of which is unknown, although it is widely considered to be mediated by loss of inhibitory neurotransmission. In the present study we have investigated the effect of a single unilateral injection of tetanus toxin in the hippocampus on extracellular concentrations of 5-hydroxytryptamine (5-HT), dopamine and their metabolites in freely moving rats. Tetanus toxin treatment caused a progressive decline in hippocampal dialysate 5-hydroxytryptamine over the 3-day period following the injection. After 3 days the level of 5-HT on the injected side was only 20% of the control side. On the second day of observation the level of dopamine was found to be elevated by more than 2-fold but this was not the case for either of the other 2 days studied. The 5-hydroxytryptamine metabolite, 5-hydroxyindoleacetic acid declined in a manner similar to that of the parent transmitter, but to a lesser extent. Of the dopamine metabolites, 3,4-dihydroxyphenylacetic acid was decreased on the second and third days of observation, whilst homovanillic acid was not significantly altered during the period of observation.

Sustained and selective block of IPSPs in brain slices from rats made epileptic by intrahippocampal tetanus toxin

A small dose of tetanus toxin (2-5 ng; 10 mouse LD50) injected into the rat hippocampus produces a chronic epileptic syndrome in which epileptic discharges recur intermittently for 6-8 weeks. Hippocampal slices prepared during this period and maintained in vitro generate both evoked and spontaneous epileptic discharges. The present study used slices prepared 8-18 days after injection of tetanus toxin or vehicle solution into both hippocampi to test whether or not synaptic inhibition was selectively impaired in this experimental epilepsy. Intracellular recordings were made from CA3 pyramidal layer neurones within the tetanus toxin focus, which was identified by field potential recordings of synchronous bursts evoked by afferent stimulation. The intrinsic properties of these neurones did not differ from comparable cells in control-injected rats. All cells generated excitatory postsynaptic potentials (EPSPs) following stimulation of stratum radiatum in CA3. In control slices EPSPs were followed by a 'fast' inhibitory postsynaptic potential (IPSP), peaking at 25-30 ms, with a mean amplitude (+/- SEM) of -6.7 mV (+/- 0.66). In the epileptic slices these were absent, and the EPSP prolonged so that the potential at 30 ms was a depolarisation of +6.6 mV (+/- 2.75). The slow IPSP at 120 ms dropped to -0.27 mV (+/- 0.18) from -3.97 mV (+/- 1.43) (11 cells in each group). The loss of IPSPs cannot be attributed to a shift in reversal potentials in the toxin-injected group because no IPSPs were unmasked by current injection (n = 11). IPSPs also occurred spontaneously in the neurones in control slices, with a mean amplitude of -1.30 mV. Their frequency decreased by a factor of 13 in cells from the chronic focus induced by tetanus toxin (P less than 0.0001, analysis of variance), but their amplitude did not change significantly (mean of -1.22 mV). Spontaneous EPSPs were significantly more frequent and slightly smaller in the toxin-injected group (mean amplitudes 1.35 and 1.13 mV respectively). Together these studies support the hypothesis that the chronically recurring seizures induced by low doses of tetanus toxin can be attributed to a substantial, persistent and selective reduction of inhibitory neurotransmission in the hippocampus.

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.