[Voltametric detection of cerebral NO in rats. Variations of the signal throughout the sleep-wakefulness cycle].

Abstract

Nitric oxide (NO) is synthesized in the neurons by constitutive NO synthase (NOS). Within given neuronal sets, this enzyme is colocalized with different other neurotransmitters such as, for example, GABA, acethylcholine or serotonin. Our attention has been focused on the fact that serotoninergic neurons, well known for their involvement in sleep triggering and maintenance, synthesize also NO. In order to evaluate the modalities of release of this compound throughout the rat sleep-waking cycle, we prepared a sensor allowing its specific detection in freely moving animals. The active part of this sensor is a carbon fiber (phi = 30 microns) successively coated with porphyrin nickel and nafion. In vitro, together with differential normal pulse voltammetric measurements, it allows the detection of a 650 mV signal varying linearly in NO solutions ranging from 5.10(-7) to 10(-4) M. At physiological concentrations, L-arginine, L-citrulline, nitrites and nitrates do not yield a signal at 650 mV. Similarly, the compounds administered to the animals, hydroxylamine, L-arginine p-nitroanilide (L-ANA) and L-N omega-nitro arginine methyl ester (L-NAME) are not electroactive at 650 mV. L-ANA and L-NAME, also appear to be trapping agents for NO while leaving the electrochemical properties of the sensor untouched. In vivo, in the frontal cortex of the anesthetized rat, a signal is measured at 650 mV. The administration of hydroxylamine (40 mg/kg, i.p.) induces a 100% increase in its height. The administration of L-ANA (100 mg/kg, i.p.) produces its complete disappearance within 50 min. Finally, the administration of L-NAME (100 mg/kg, i.p.) is without effect. This last aspect might be dependent upon the inability of L-NAME to cross the blood brain barrier. On the contrary, the increase in the signal height obtained with hydroxylamine and its disappearance with L-ANA support that it might depend upon NO. In vivo, and in animals also equipped with polygraphic electrodes, the signal measured in the same area of the cortex exhibits the highest height during the waking state and decreases during either slow-wave sleep (-6%) or paradoxical sleep (-9%). These mild variations might represent the mean of several NO sources (cortical GABAergic interneurons, cholinergic and serotoninergic axonal nerve endings), each of them varying differently throughout the sleep-waking cycle.