Dietary polyamines promote the growth of azoxymethane-induced aberrant crypt foci in rat colon

We have examined whether dietary polyamines influence the formation and initial growth of azoxymethane (AOM)-induced aberrant crypt foci (ACF) in rat colon. Effects of a combination of dietary polyamines at three dose levels (putrescine: 50, 280, 740 nmol/g; spermidine: 10, 261, 763 nmol/g; spermine: 1, 31, 91 nmol/g) in the polyamine-poor AIN-76A diet were studied in animals in two different experimental situations: animals treated with AOM alone and animals treated with AOM + difluoromethylornithine (DFMO), a specific inhibitor of endogenous polyamine synthesis. In both experimental situations, dietary polyamines enhanced the growth of ACF, expressed as the number of large ACF (foci with three or more aberrant crypts, ACF > or = 3), whereas the formation of ACF, expressed as the number of ACF, was apparently not altered. In animals treated with AOM alone, maximal growth enhancing effect on ACF was nearly obtained with the median level of dietary polyamine. In rats fed a low polyamine diet, basic AIN-76A, DFMO reduced the growth of AOM-induced ACF by 83%. This inhibitory effect of DFMO was counteracted by dietary polyamines in a dose- dependent manner, and it was abolished at the highest level of polyamines. In conclusion, it was demonstrated that dietary polyamines are able to enhance the growth of AOM-induced ACF. Further, dietary polyamines reversed the DFMO-caused inhibition of ACF growth, probably by compensating for the DFMO-reduced endogenous polyamine synthesis.      

Single Audiogenic Seizures Induce Increases in Calcineurin and Ca<Superscript>2+</Superscript>-Calmodulin-Dependent Protein Kinase II in the Rat Brain

Ca²⁺-calmodulin-dependent protein kinase II (CaMKII) and protein phosphatase 2B (calcineurin) play critical roles in the formation of nerve cell responses to incoming Ca²⁺ signals. The studies report here addressed how single seizures might affect the functioning of these enzymes. Sounds (80 dB, 12–15 kHz) induced single convulsions in male Krushinskii–Molodkina rats, which have an inherited predisposition to audiogenic seizures. Biochemical investigations were performed two days after seizures. Immunoblotting studies with specific monoclonal antibodies demonstrated increases in the level of the neurospecific α subunit of CaMKII in the sensorimotor cortex and hippocampus of the rats of the experimental group, as compared with controls. The hippocampus of rats which had had seizures also showed increases in the catalytic subunit of calcineurin. CaMKII activity in the hippocampus and cortex of postseizure rats increased, though that of the Ca²⁺-calmodulin-independent functional form of the enzyme did not change. It is suggested that the long-term changes seen here may represent an adaptive mechanism induced by convulsive activity and directed to increasing the threshold of neuron excitability in the brain.     

Neuronal hyperexcitability induced by repeated brief episodes of hypoxia in rat hippocampal slices: involvement of ionotropic glutamate receptors and L-type Ca(2+) channels

Repeated exposures of rat hippocampal slices to short episodes of hypoxia induce a sustained decrease in the threshold of the development of stimulus-evoked epileptiform discharges in CA1 pyramidal neurons. We have previously demonstrated that the K(+)(o)-induced hyperexcitability required functional L-type voltage-dependent Ca(2+) channels and NMDA-receptors, but was independent of AMPA/kainate-receptor activation. As hypoxia/ischaemia can lead to increased K(+)(o), the epileptiform activity observed after exposure to these challenges could also result from high K(+)(o). The purpose of this study was: (i) to determine whether ionotropic glutamate receptors and L-type Ca(2+) channels are involved in the development of epileptiform activity induced by repeated exposures of hippocampal slices to hypoxia; and (ii) to compare the properties of hypoxia- and high K(+)(o)-induced hyperexcitability. Population spike of presynaptic fibres with field excitatory postsynaptic potential from the stratum radiatum, and population spike of CA1 pyramidal neurons from the stratum pyramidale, were recorded simultaneously in the CA1 area of rat hippocampal slices in response to electrical stimulation of the Schaffer collateral/commissural fibres. Repeated, brief hypoxic episodes induced a sustained decrease in the threshold for development of evoked epileptiform discharges that was associated with long-term potentiation of the CA3-CA1 synapses, but without EPSP-spike potentiation (i.e. in contrast to high K(+)(o)-induced hyperexcitability). The selective antagonist of NMDA receptors, D-APV (25 microM), and the selective blocker of L-type Ca(2+) channels, nifedipine (10 microM) depressed the development of hypoxia-induced hyperexcitability. However, in contrast to high K(+)(o)-induced hyperexcitability, hypoxia-induced hyperexcitability was also blocked by the AMPA/kainite-receptor antagonist, CNQX (5 microM). The present findings confirm that repeated, brief episodes of hypoxia, like exposure to high extracellular K(+), can induce a pro-epileptic state in the CA1 neuronal network, but that the mechanisms leading to hyperexcitability are different for the two stimuli.     

In vivo hippocampal kindling occludes the development of in vitro kindling-like state in CA1 area of rat hippocampal slices

In our previous work (Semyanov and Godukhin, 1997), we showed that the repeated short-term extracellular K+ (K0+) increases induced long-lasting reduction of the threshold of evoked epileptiform discharges in CA1 hippocampal slices isolated from normal (nonkindled) rats. This state had some features characteristic of traditional in vivo kindling and was described as in vitro kindling-like state (VKLS). The aim of the present investigations was to determine the features of the VKLS development in CA1 slices isolated from electrical hippocampal kindled (nonepileptic) rats and from genetically-prone to audiogenic seizures audiokindled rats. We found that both forms of in vivo kindling occluded the VKLS development induced by the repeated K& increases in CA1 slices. These data provide more evidence that the in vivo kindling and VKLS developments in CA1 region of hippocampus are based on activation of similar cellular mechanisms. It is suggested that the described model of in vitro kindling can be useful for further studies of the cellular-molecular mechanisms of plastic alterations in neurons associated with the start of kindling induced epileptogenesis.     

Anti-inflammatory cytokines, TGF-β1 and IL-10, exert anti-hypoxic action and abolish posthypoxic hyperexcitability in hippocampal slice neurons: comparative aspects

The aim of this study was to investigate the comparative effects of transforming growth factor β1 (TGF-β1) and interleukin-10 (IL-10) on the repeated brief hypoxia-induced alterations in the activity of hippocampal slice CA1 pyramidal neurons. The method of field potentials measurement in CA1 region of hippocampal slices was used. The principal results of our work are summarized as follow. 1. TGF-β1 reduces the depressive effect of brief hypoxia on the population spike amplitude more effectively than IL-10. 2. During TGF-β1 exposure (in contrast to IL-10), three 3-min hypoxic episodes do not induce the rapid hypoxic preconditioning. 3. TGF-β1 and IL-10 equally abolish posthypoxic hyperexcitability induced by repeated brief episodes of hypoxia in CA1 pyramidal neurons. These findings indicated that TGF-β1 and IL-10 are able to evoke anti-hypoxic effect and abolish the development of posthypoxic hyperexcitability induced by repeated brief hypoxic episodes in hippocampal CA1 pyramidal neurons. Our results also demonstrated that TGF-β1 reduced the effectiveness of hypoxia to depress neuronal activity more effectively than IL-10. We suggest that the present findings allow to explain the certain neuroprotective mechanisms of IL-10 and TGF-beta1 in the early phase of hypoxia and indicate that a therapeutic anti-inflammatory approach using these substances can provide neuroprotection in the brain hypoxic conditions.     

The Role of High-Conductivity Ca<Superscript>2+</Superscript>-Activated Potassium Channels in the Preconditioning Effects of Hypoxia and Posthypoxic Hyperexcitability of Neurons in Hippocampal Field CA1 in Vitro

The effects of the specific high-conductivity Ca²⁺-activated potassium channel blocker iberiotoxin on processes induced in rat hippocampal field CA1 slices by transient episodes of hypoxia were studied, i.e., 1) the suppressing effect of hypoxia on pyramidal neuron activity during hypoxic episodes, 2) the preconditioning action of hypoxia, and 3) posthypoxic neuron hyperexcitability. These experiments showed that iberiotoxin (10–20 nM) produced a tendency to decreases in the effectiveness of hypoxic episodes in depressing the amplitudes of population spikes recorded in hippocampal field CA1 during hypoxia. The high-conductivity Ca²⁺-activated potassium channel blocker significantly decreased the preconditioning effects of the first two episodes of hypoxia on the ability of the third episode of hypoxia to suppress neuron activity. Iberiotoxin also suppressed the posthypoxic hyperexcitability of pyramidal neurons. It is suggested that high-conductivity Ca²⁺-activated potassium channels play an important role in the mechanisms of development of types of neuroplasticity induced by transient episodes of hypoxia such as rapid hypoxic preconditioning and posthypoxic hyperexcitability.