Uridine release during aminopyridine-induced epilepsy.

Uridine, like adenosine, is released under sustained depolarization and it can inhibit hippocampal neuronal activity, suggesting that uridine may be released during seizures and can be involved in epileptic mechanisms. In an in vivo microdialysis study, we measured the extracellular changes of nucleoside and amino acid levels and recorded cortical EEG during 3-aminopyridine-induced epilepsy. Applying silver impregnation and immunohistochemistry, we examined the degree of hippocampal cell loss. We found that extracellular concentration of uridine, adenosine, inosine, and glutamate increased significantly, while glutamine level decreased during seizures. The release of uridine correlated with seizure activity. Systemic and local uridine application was ineffective. The number of parvalbumin- and calretinin-containing interneurons of dorsal hippocampi decreased. We conclude that uridine is released during epileptic activity, and suggest that as a neuromodulator, uridine may contribute to epilepsy-related neuronal activity changes, but uridine analogues having slower turnover would be needed for further investigation of physiological role of uridine.

Uridine is released by depolarization and inhibits unit activity in the rat hippocampus.

Perfusion of 5 microM kainate through microdialysis probes induced >2-fold elevation of extracellular uridine and adenosine concentrations in the hippocampus and in the thalamus of anaesthetized rats. Administration of uridine via this route produced an estimated uridine concentration of 50-100 microM around the electrode surface. This markedly decreased the average firing rate of neurones in the hippocampus, but not in the thalamus. Activity of separated single hippocampal pyramidal cells was completely inhibited by uridine. The same amount of adenosine completely blocked neuronal activity in both hippocampus and thalamus. Uridine administration had no effect on extracellular adenosine concentration. These findings suggest an important neuromodulatory role for depolarization-released uridine in the CNS.

Analysis of purine and pyrimidine bases, nucleosides and deoxynucleosides in brain microsamples (microdialysates and micropunches) and cerebrospinal fluid.

A new chromatographic method is reported for the synchronous analysis of endogenous purine and pyrimidine bases, ribonucleosides, and deoxyribonucleosides in brain samples. An optimized gradient chromatography system with a cooled reversed-phase column allows the detection of these compounds in very low concentrations in microsamples (microdialysates and micropunches). Chromatographic peaks were identified via the retention times of known standards, with detection at two wavelengths, and also by electrospray tandem mass spectrometry, which permits the identification of certain compounds at extremely low concentrations. The method was tested on in vivo brain microdialysis samples, micropunch tissue sample and cerebrospinal fluid of rats. Extracellular concentrations of pyrimidine metabolites in brain samples and of various purine metabolites in thalamic samples are reported here first. A comparison of the results on microdialysis and cerebrospinal fluid samples suggests that the analysis of cerebrospinal fluid provides limited information on the local extracellular concentrations of these compounds. Basic dialysis experiments revealed temporarily stable baseline levels one hour after implantation of the microdialysis probes. An elevated potassium concentration in the perfusion solution caused increases in the extracellular levels of adenosine and its metabolites, and of guanosine and the pyrimidine nucleoside uridine.

Natural sleep modifies the rat electroretinogram.

We show here electroretinograms (ERGs) recorded from freely moving rats during sleep and wakefulness. Bilateral ERGs were evoked by flashes delivered through a light-emitting diode implanted under the skin above one eye and recorded through electrodes inside each orbit near the optic nerve. Additional electrodes over each visual cortex monitored the brain waves and collected flash-evoked cortical potentials to compare with the ERGs. Connections to the stimulating and recording instruments through a plug on the head made data collection possible at any time without physically disturbing the animal. The three major findings are (i) the ERG amplitude during slow-wave sleep can be 2 or more times that of the waking response; (ii) the ERG patterns in slow-wave and REM sleep are different; and (iii) the sleep-related ERG changes closely mimic those taking place at the same time in the responses evoked from the visual cortex. We conclude that the mechanisms that alter the visual cortical-evoked responses during sleep operate also and similarly at the retinal level.