Basal and stress-induced release of noradrenaline in hypothalamus of spontaneously hypertensive rats at different ages.

Basal and stress-induced noradrenaline (NA) release was studied by intracerebral microdialysis in the hypothalamic paraventricular nucleus of spontaneously hypertensive rats (SHR) at different ages (9 weeks, 6, 18 and 24 months). NA was measured in 20-min dialysate samples by high performance liquid chromatography with electrochemical detection. Microdialysis sampling was done at baseline, during a 20-min immobilization stress and for the next 100 min. Basal NA levels decreased with age, showing a highly significant correlation. Immobilization stress raised NA similarly in the four age groups (respectively 281%, 235%, 243%, 251% of baseline at 9 weeks, 6, 18, 24 months), indicating that the response to stress is maintained at all these ages and is not affected by the development of hypertension or by aging.

Catecholamine metabolism in the rat locus coeruleus as studied by in vivo differential pulse voltammetry. I. Nature and origin of contributors to the oxidation current at +0.1 V.

Differential pulse voltammetry was used together with treated carbon fiber microelectrodes to study the in vivo catecholamine (CA) metabolism in the locus coeruleus (LC), a brain region densely packed with noradrenergic neurons. In chronically implanted rats, an in vivo oxidation current that peaks at +0.1 V has been detected inside the LC complex. This current whose potential is characteristic of the oxidation of the catechols, had the same anatomical localization as the noradrenergic cells. Pharmacological experiments have been made to ascertain which catechols contribute to this in vivo current. Monoamine oxidase inhibition by pargyline was followed by a total and rapid suppression of the in vivo signal. Blockade of dopamine-beta-hydroxylase by FLA-63 induced a significant increase in the electrochemical signal. Post-mortem analysis of LC catechol levels after administration of this drug revealed a considerable decrease in NA and its major catechol metabolite, 3,4-dihydroxyphenylglycol (DOPEG) although DA and its metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) were significantly increased. Comparison of these results led us to conclude that DOPAC is probably the most important contributor to the in vivo oxidation current. This assertion is corroborated by results obtained after tyrosine hydroxylase inhibition with alpha-methyl-p-tyrosine: the in vivo catechol current was rapidly suppressed and post-mortem levels of DOPAC were significantly reduced while DOPEG remained almost normal. An attempt was made to selectively destroy the LC cell bodies by a unilateral injection of ibotenic acid (10 micrograms). Eight to 15 days after injection, no current was detectable in the injected side although it was still present in the contralateral intact side. Post-mortem levels of DOPAC and DOPEG levels of the lesioned side were 29% and 17%, respectively, of those in the intact side. Thus, we assumed that the in vivo catechol current in the LC comes from the oxidation of DOPAC most probably synthesized by the noradrenergic cell bodies.

Catecholamine metabolism in the rat locus coeruleus as studied by in vivo differential pulse voltammetry. II. Pharmacological and behavioral study.

Differential pulse voltammetry used in combination with electrochemically treated carbon fiber electrodes allowed us to detect catechols in the locus coeruleus (LC) of conscious freely moving rats. A micromanipulator cemented on the rat skull was designed in order to implant carbon fiber electrodes without anesthesia. Voltammograms were recorded every 2 min for 5 h. After the in vivo experiments electrodes were tested in 3,4-dihydroxyphenylacetic acid (DOPAC), 3,4-dihydroxyphenylglycol (DOPEG) and noradrenaline (NA) solutions. The catechol peak recorded from LC was suppressed by pargyline treatment and slightly reappeared after inhibition of the NA reuptake by desipramine (DMI). This reappearing signal was attributed to NA and estimated at a concentration 50 nM NA. Various drug treatments (piperoxane, haloperidol, clonidine, DMI and reserpine) allowed us to further support the conclusion of part I of this study: the catechol peak recorded from LC is mainly due to DOPAC synthesized by LC noradrenergic neurons. This DOPAC signal corresponded to a DOPAC concentration which reached 23 microM when the whole active part of the electrode was implanted in the LC. In addition to this pharmacological study, data from stress experiments pointed out a striking parallel between the variations of the DOPAC signal and those of the activity of LC noradrenergic neurons as revealed by reported electrophysiological studies.