Measurement of endogenous noradrenaline release in the rat cerebral cortex in vivo by transcortical dialysis: effects of drugs affecting noradrenergic transmission.

The release of endogenous noradrenaline was measured in the cerebral cortex of the halothane-anesthetized rat by using the technique of brain dialysis coupled to a radioenzymatic assay. A thin dialysis tube was inserted transversally in the cerebral cortex (transcortical dialysis) and perfused with Ringer medium (2 microliter min-1). Under basal conditions, the cortical output of noradrenaline was stable over a period of at least 6 h and amounted to 8.7 pg/20 min (not corrected for recovery). Histological control of the perfused area revealed very little damage and normal morphology in the vicinity of the dialysis tube. Omission of calcium from the perfusion medium caused a marked drop in cortical noradrenaline output. Bilateral electrical stimulation (for 10 min) of the ascending noradrenergic pathways in the medial forebrain bundle caused a frequency-dependent increase in cortical noradrenaline output over the range 5-20 Hz. Stimulation at a higher frequency (50 Hz) resulted in a levelling off of the increase in cortical noradrenaline release. Systemic administration of the dopamine-beta-hydroxylase inhibitor bis-(4-methyl-1-homopiperazinylthiocarbonyl) disulfide (FLA 63) (25 mg/kg i.p.) markedly reduced, whereas injection of the monoamine oxidase inhibitor pargyline (75 mg/kg i.p.) resulted in a progressive increase in, cortical noradrenaline output. d-Amphetamine (2 mg/kg i.p.) provoked a sharp increase in cortical noradrenaline release (+450% over basal values within 40 min). Desmethylimipramine (10 mg/kg i.p.) produced a twofold increase of cortical noradrenaline release. Finally, idazoxan (20 mg/kg i.p.) and clonidine (0.3 mg/kg i.p.), respectively, increased and decreased the release of noradrenaline from the cerebral cortex.(ABSTRACT TRUNCATED AT 250 WORDS)