Oxidative stress induced by cumene hydroperoxide produces synaptic depression and transient hyperexcitability in rat primary motor cortex neurons.

Pyramidal neurons of the motor cortex are selectively degenerated in Amyotrophic Lateral Sclerosis (ALS). The mechanisms underlying neuronal death in ALS are not well established. In the absence of useful biomarkers, the early increased neuronal excitability seems to be the unique characteristic of ALS. Lipid peroxidation caused by oxidative stress has been postulated as one of the possible mechanisms involved in degeneration motor cortex pyramidal neurons. This paper examines the effect of lipid peroxidation on layer V pyramidal neurons induced by cumene hydroperoxide (CH) in brain slices from wild type rats. CH induces a synaptic depression of pyramidal neurons in a time dependent manner, already observable on GABAergic synaptic transmission after 5min application of the drug. Altogether, our whole-cell patch-clamp recording data suggest that the functional changes induced by CH upon pyramidal neurons are due to pre- and postsynaptic mechanisms. CH did not alter mEPSCs or mIPSCs, but decreased the frequency, amplitude, and decay rate of spontaneous EPSCs and IPSCs. These effects may be explained by a presynaptic mechanism causing a decrease in action potential-dependent neurotransmitter release. Additionally, CH induced a postsynaptic inward current that underlies a membrane depolarization. Depressing the input flow from the inhibitory premotor interneurons causes a transient hyperexcitability (higher resistance and lower rheobase) in pyramidal neurons of the motor cortex by presumably altering a tonic inhibitory current. These findings, which resemble relevant cortical pathophysiology of ALS, point to oxidative stress, presumably by lipid peroxidation, as an important contributor to the causes underlying this disease.

Time and dose dependent effects of oxidative stress induced by cumene hydroperoxide in neuronal excitability of rat motor cortex neurons.

It has been claimed that oxidative stress and the production of reactive oxygen radicals can contribute to neuron degeneration and might be one factor in the development of different neurological diseases. In our study, we have attempted to clarify how oxidative damage induces dose dependent changes in functional membrane properties of neurons by means of whole cell patch clamp techniques in brain slices from young adult rats. Our research demonstrates physiological changes in membrane properties of pyramidal motor cortex neurons exposed to 3 concentrations of cumene hydroperoxide (CH; 1, 10 and 100µM) during 30min. Results show that oxidative stress induced by CH evokes important changes, in a concentration and time dependent manner, in the neuronal excitability of motor cortex neurons of the rat: (i) Low concentration of the drug (1µM) already blocks inward rectifications (sag) and decreases action potential amplitude and gain, a drug concentration which has no effects on other neuronal populations, (ii) 10µM of CH depresses the excitability of pyramidal motor cortex neurons by decreasing input resistance, amplitude of the action potential, and gain and maximum frequency of the repetitive firing discharge, and (iii) 100µM completely blocks the capability to produce repetitive discharge of action potentials in all cells. Both larger drug concentrations and/or longer times of exposure to CH narrow the current working range. This happens because of the increase in the rheobase, and the reduction of the cancelation current. The effects caused by oxidative stress, including those produced by the level of lipid peroxidation, are practically irreversible and, this, therefore, indicates that neuroprotective agents should be administered at the first symptoms of alterations to membrane properties. In fact, the pre-treatment with melatonin, acting as an antioxidant, prevented the lipid peroxidation and the physiological changes induced by CH. Larger cells (as estimated by their cell capacitance) were also more susceptible to oxidative stress. Our results provide previously unavailable observations that large size and high sensitivity to oxidative stress (even at low concentrations) make pyramidal neurons of the motor cortex, in particular corticofugal neurons, more susceptible to cell death when compared with other neuronal populations. These results could also shed some light on explaining the causes behind diseases such as Amyotrophic Lateral Sclerosis.

Oxidative stress induced by cumene hydroperoxide evokes changes in neuronal excitability of rat motor cortex neurons.

Oxidative stress and the production of reactive oxygen radicals play a key role in neuronal cell damage. This paper describes an in vitro study that explores the neuronal responses to oxidative stress focusing on changes in neuronal excitability and functional membrane properties. This study was carried out in pyramidal cells of the motor cortex by applying whole-cell patch-clamp techniques on brain slices from young adult rats. Oxygen-derived free radical formation was induced by bath application of 10µM cumene hydroperoxide (CH) for 30min. CH produced marked changes in the electrophysiological properties of neurons (n=30). Resting membrane potential became progressively depolarized, as well as depolarization voltage, with no variations in voltage threshold. Membrane resistance showed a biphasic behavior, increasing after 5min of drug exposure and then it started to decrease, even under control values, after 15 and 30min. At the same time, changes in membrane resistance produced compensatory variations in the rheobase. The amplitude of the action potentials diminished and the duration increased progressively over time. Some of the neurons under study also lost their ability to discharge action potentials in a repetitive way. Most of the neurons, however, kept their repetitive discharge even though their maximum frequency and gain decreased. Furthermore, cancelation of the repetitive firing discharge took place at intensities that decreased with time of exposure to CH, which resulted in a narrower working range. We can conclude that oxidative stress compromises both neuronal excitability and the capability of generating action potentials, and so this type of neuronal functional failure could precede the neuronal death characteristics of many neurodegenerative diseases.