Contrasting effects of the imidazol(in)e alpha 2-adrenoceptor agonists, medetomidine, clonidine and UK 14,304 on extraneuronal levels of noradrenaline in the rat frontal cortex: evaluation using in vivo microdialysis and synaptosomal uptake studies.
1. In vivo microdialysis in halothane-anaesthetized rats and synaptosomal [3H]-noradrenaline uptake studies in vitro were used to evaluate the effects of imidazole (medetomidine) and imidazoline (clonidine and UK 14,304) alpha 2-adrenoceptor agonists on extraneuronal levels of noradrenaline in the frontal cortex. 2. Levels of noradrenaline in the dialysate were increased by a depolarizing concentration of K+ (60 mM for 20 min) and substantially attenuated by reducing Ca2+ supply in the perfusate. These results suggest that spontaneous efflux of noradrenaline in the cortex is regulated predominantly by cation-dependent exocytotic mechanisms. 3. At a low perfusion concentration (0.5 microM), medetomidine, clonidine and UK 14,304 all reduced the level of noradrenaline in cortical dialysates. Continuous perfusion of the selective alpha 2-adrenoceptor antagonist, atipamezole (0.5 microM) caused a sustained increase in noradrenaline efflux and reversed the inhibitory effects of medetomidine. All these changes are consistent with drug actions at presynaptic alpha 2-adrenoceptors. 4. Higher concentrations of medetomidine (5-50 microM), but not clonidine or UK 14,304, evoked a non-desensitizing increase in noradrenaline efflux. This effect was not antagonized by 0.5 microM atipamezole. 5. The tricyclic noradrenaline reuptake inhibitor, desmethylimipramine (0.5-50 microM), increased noradrenaline efflux in a concentration-dependent manner. 6. The specific uptake of [3H]-noradrenaline into cortical synaptosomes was inhibited by medetomidine and desmethylimipramine with IC50 values of approximately 7 microM and 8 microM respectively. Neither clonidine nor UK 14,304 inhibited [3H]-noradrenaline uptake. 7. These results indicate that micromolar concentrations of the selective alpha2-adrenoceptor agonist,medetomidine, can augment extraneuronal levels of noradrenaline in the rat frontal cortex; this effect seems to involve an inhibition of noradrenaline reuptake rather than an action at alpha2-adrenoceptors.
Dalley
J W
JW
Department of Pharmacology, University College London.
Stanford
S C
SC
D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000818:Animals; D000068438:Brimonidine Tartrate; D003000:Clonidine / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D005625:Frontal Lobe / Q000187:drug effects* / Q000378:metabolism; D007093:Imidazoles / Q000494:pharmacology*; D008297:Male; D020926:Medetomidine; D017551:Microdialysis; D009638:Norepinephrine / Q000378:metabolism*; D011810:Quinoxalines / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism; D013997:Time Factors
D000316:Adrenergic alpha-Agonists; D007093:Imidazoles; D011810:Quinoxalines; C050701:atipamezole; D000068438:Brimonidine Tartrate; D003000:Clonidine; D020926:Medetomidine; D009638:Norepinephrine