Subthreshold oscillations generated by TTX-sensitive sodium currents in dorsal cochlear nucleus pyramidal cells

During intracellular recordings in rodent brainstem slice preparations, dorsal cochlear nucleus (DCN) pyramidal cells (PCs) exhibit characteristic discharge patterns to depolarizing current injection that depend on the membrane potential from which the responses are evoked. When depolarized from hyperpolarized potentials, PCs can respond with a short-latency action potential followed by a long silent interval (pauser) or a train of action potentials with a long latency (buildup). During the silent intervals in a pauser or a buildup response, the membrane potential slowly depolarizes towards spike threshold, often exhibiting distinct voltage oscillations of 1–2 mV before the first spike. The subthreshold voltage oscillations were investigated using whole cell recordings from DCN PCs in rat pup (P10–14) brainstem slices. The oscillations were unaffected by excitatory and inhibitory neurotransmitter antagonists, and were not temporally locked to the onset of the depolarization. The oscillations typically became larger as spike threshold was approached, and had a characteristic frequency between 40 and 100 Hz. In the presence of tetrodotoxin (TTX, 500 nM), the oscillations were significantly suppressed, and could not be evoked at any voltage below or above spike threshold. The oscillations were not blocked by phenytoin or Cd²⁺, but they were affected by prior activity in the neuron for approximately 1 s. We conclude that voltage-gated Na⁺ channels are required to generate membrane oscillations during the buildup phase. We suggest that the subthreshold oscillations play a role in controlling spike timing in PCs when the membrane potential slowly approaches, or hovers near, spike threshold.     

The ventral hippocampus and fear conditioning in rats. Different anterograde amnesias of fear after tetrodotoxin inactivation and infusion of the GABA(A) agonist muscimol

Studies on the involvement of the rat hippocampus in classical fear conditioning have focused mainly on the dorsal hippocampus and conditioning to a context. However, the ventral hippocampus has intimate connections with the amygdala and the nucleus accumbens, which are involved in classical fear conditioning to explicit and contextual cues. Consistently, a few recent lesion studies have indicated a role for the ventral hippocampus in classical fear conditioning to explicit and contextual cues. The present study examined whether neuronal activity within the ventral hippocampus is important for the formation of fear memory to explicit and contextual cues by classical fear conditioning. Tetrodotoxin (TTX; 10 ng/side), which completely blocks neuronal activity, or muscimol (1 microg/side), which increases GABA(A) receptor-mediated inhibition, were bilaterally infused into the ventral hippocampus of Wistar rats before the conditioning session of a classical fear-conditioning experiment. Conditioning to a tone and the context were assessed using freezing as a measure of conditioned fear. TTX blocked fear conditioning to both tone and context. Muscimol only blocked fear conditioning to the context. The data of the present study indicate that activity of neurons in the ventral hippocampus is necessary for the formation of fear memory to both explicit and contextual cues and that neurons in the ventral hippocampus that bear the GABA(A) receptor are important for the formation of fear conditioning to a context. In addition, both bilateral muscimol (0.5 microg/side and 1 microg/side) and TTX (5 ng/side and 10 ng/side) infusion into the ventral hippocampus dose-dependently decreased locomotor activity in an open-field experiment.     

Enhanced inhibitory avoidance learning prevents the memory-impairing effects of post-training hippocampal inactivation

Rats were trained on an inhibitory avoidance task to study the effects of post-training administration of tetrodotoxin (TTX, which temporarily inactivates neural activity) on memory consolidation. During training, independent groups of rats received either a mild foot shock (0.8 mA) or a stronger (1.0 mA) foot shock. TTX was administered bilaterally into the dorsal hippocampus immediately after training, and memory of the task was measured 48 h later. We corroborated the typical amnesic effect of intrahippocampal infusions of TTX in those rats trained with the mild-intensity foot shock. More importantly, with the stronger foot shock, the same treatment was ineffective in producing amnesia. These results suggest that, after an enhanced learning experience, other brain regions are also activated, which may compensate for the amnesic effect of TTX infusions into the hippocampus.Due to an error in the citation line, this revised PDF (published in December 2003) deviates from the printed version, and is the correct and authoritative version of the paper.     

Sodium channels in ocular epithelia

Voltage-gated, tetrodotoxin(TTX)-blockable sodium channels are found in most excitable cells and are the primary contributors to action potentials generated by many of these cells. To date, there has only been one report of a non-cultured vertebrate epithelial cell type containing TTX-blockable Na⁺ channels: rabbit non-pigmented ciliary body epithelial cells [Cilluffo MC et al. (1991) Invest Opthalmol Vis Sci 32:1619–1629], and three reports of cultured epithelial cells containing TTX-blockable Na⁺ channels: rabbit non-pigmented and pigmented ciliary body epithelium [Ciluffo MC et al. (1991) Invest Opthalmol Vis Sci 32:1619–1629; Fain GL, Farahbakhsh (1989) J Physiol (Lond) 417:83–103] and human lens epithelium [Cooper K et al. (1990) J Membr Biol 117:285–298]. We report here the presence of sodium currents in two different non-cultured, freshly dissociated transporting epithelial cell types: the rabbit corneal endothelium and the frog lens epithelium. We also report the occurrence of sodium currents in six additional cultured ocular epithelial cell types from three different species. These currents have a current/voltage (I/V) relationship consistent with traditional voltage-gated Na⁺ currents, are quinidine- and TTX-blockable (of the low-affinity TTX-sensitive type), and disappear following bath substitution of Na⁺ with Cs⁺ or K⁺.     

Dopaminergic innervation of the rat globus pallidus characterized by microdialysis and immunohistochemistry

The present study examined the dopaminergic innervation of the rat globus pallidus by in vivo microdialysis and immunohistochemistry in more detail. Using tyrosine hydroxylase immunohistochemistry, two classes of dopaminergic fibers were distinguished morphologically in the globus pallidus. Unilateral infusion of 6-hydroxydopamine into the substantia nigra produced a loss of dopaminergic fiber density in the globus pallidus which was correlated with the nigral extent of the lesion. These findings are in line with the notion that a degenerative loss of nigral dopaminergic cell bodies might also affect the dopamine input of extrastriatal structures such as the globus pallidus. Using in vivo microdialysis, we tested whether dopamine measured in the globus pallidus is of neuronal origin. Perfusion of tetrodotoxin induced a strong and transient decrease of pallidal dopamine. The tetrodotoxin-sensitivity of pallidal dopamine demonstrates the functional significance of the nigropallidal dopaminergic innervation.     

Action potentials of the rat diaphragm and their sensitivity to tetrodotoxin during postnatal development and old age

Summary Using the microelectrode technique, parameters of action potentials obtained from the diaphragm muscle fibres were studied at 3, 7, 14, 30 and 90 days of age and on 28–30 month old rats. The maximum rate of rise was always greater at the end-plate zone than at the extrajunctional parts of muscle fibre at all ages examined. A maximum difference of 40% was found in animals aged 7–30 days. Sensitivity to tetrodotoxin (TTX) was maximal 3 days after birth at the extrajunctiona zone (5×10^–8 M) and minimal at the end-plate zone (5×10^–6 M). This difference declined during the postnatal life until it had disappeared in old rats. The greater resistance of the end-plate zone action potentials to TTX and their greater maximum rate of rise is not apparently connected with the presence of acetylcholine sensitivity.     

河豚毒素对小鼠镇痛作用的实验研究

为探讨河豚毒素（ＴＴＸ）镇痛作用与剂量的效应关系,应用化学刺激和热刺激法研究了ＴＴＸ对小鼠的镇痛作用。结果表明,腹腔注射ＴＴＸ０．５、１、２μｇ／ｋｇ,小鼠扭体抑制率分别为２９．５７％、４３．４７％和７６．１２％;在小鼠甩尾试验中,ＴＴＸ１μｇ／ｋｇ腹腔给药后３０～６０ｍｉｎ镇痛作用明显,与对照组比较,Ｐ＜０．０１;２ｇ／ｋｇ剂量组给药后１５ｍｉｎ痛阈达高峰,９０ｍｉｎ内镇痛作用显著（Ｐ＜０．０１）,２ｈ内痛阈仍高于对照组;小鼠热板试验也显示随剂量增加镇痛作用加强,但总体敏感性较差,提示ＴＴＸ确有镇痛作用并有剂量－效应关系。     

P物质对变应性鼻炎动物模型鼻粘膜上皮短路电流的影响

目的观察Ｐ物质（ＳＰ）作用于变应性鼻炎动物模型鼻粘膜引起的离子分泌改变导致的上皮表面短路电流（Ｓｈｏｒｔ－ｃｉｒｃｕｉｔｃｕｒｒｅｎｔ，Ｉｓｃ）的变化。方法以鸡蛋清蛋白致敏大鼠后，用Ｕｓｓ－ｉｎｇ室技术测定鼻粘膜上皮表面短路电流。并观察神经激肽受体（ＮＫ１）拮抗剂ＣＰ９６３４５、组胺Ｈ１受体拮抗剂吡拉明、Ｈ２受体拮抗剂雷尼替丁和河豚毒素对ＳＰ的阻断作用。结果经ＳＰ刺激，致敏大鼠鼻粘膜表面Ｉｓｃ显著增高。经四种物质预处理均能显著阻断ＳＰ引起的Ｉｓｃ变化。结论ＳＰ经感觉神经末梢释放后，可引起上皮表面离子电位变化、上皮通透性增高等一系列鼻粘膜病理变化。     

Occurrence of saxitoxins as a major toxin in the ovary of a marine puffer Arothron firmamentum.

Eleven male and 14 female specimens of a marine puffer Arothron firmamentum were collected from Oita and Iwate Prefectures, Japan. The toxicity assay using mouse showed that only ovary and skin of the female specimens were toxic, the toxicity scores being 5-740 as paralytic shellfish poison and <5-30 MU/g as tetrodotoxin (TTX), respectively. The toxin extracts from the both tissues were then treated with cartridge columns, and subjected to high performance liquid chromatography and liquid chromatography-mass spectral analyses. In the analyses, saxitoxin (STX) and decarbamoylSTX (dcSTX) were identified as the major toxins in the ovary, while the skin contained only TTX.     

Tetrodotoxin poisoning caused by Goby fish consumption in southeast China: a retrospective case series analysis

OBJECTIVES:

To investigate an unusual outbreak of tetrodotoxin poisoning in Leizhou, southeast China, a case series analysis was conducted to identify the source of illness.

METHODS:

A total of 22 individuals experienced symptoms of poisoning, including tongue numbness, dizziness, nausea and limb numbness and weakness. Two toxic species, Amoya caninus and Yongeichthys nebulosus, were morphologically identified from the batches of gobies consumed by the patients. Tetrodotoxin levels in the blood and Goby fish samples were detected using liquid chromatography-tandem mass spectrometry.

RESULTS:

The tetrodotoxin levels in the remaining cooked Goby fish were determined to be 2090.12 µg/kg. For Amoya caninus, the toxicity levels were 1858.29 µg/kg in the muscle and 1997.19 µg/kg in the viscera and for Yongeichthys nebulosus, they were 2783.00 µg/kg in the muscle and 2966.21 µg/kg in the viscera.

CONCLUSION:

This outbreak demonstrates an underestimation of the risk of Goby fish poisoning. Furthermore, the relationships among the toxic species, climates and marine algae present should be clarified in the future.