Role of 5-HT1A receptors in the effects of acute chronic fluoxetine on extracellular serotonin in the frontal cortex.
Fluoxetine 10 mg/kg i.p. significantly increased the extracellular concentrations of serotonin (5-HT) in the frontal cortex as assessed by in vivo microdialysis. This effect was significantly potentiated when 0.3 mg/kg s.c. WAY-100635, a 5-HT1A receptor antagonist, was administered 30 min before. WAY-100635 by itself had no effect on extracellular 5-HT. Twenty-four hours after chronic fluoxetine schedule (10 mg/kg/day i.p. x 14 days), basal extracellular 5-HT concentrations in the frontal cortex were higher than those of animals that had received the vehicle chronically. At 24 h after the last dose, a challenge dose of fluoxetine (10 mg/kg i.p.) raised extracellular 5-HT similarly in chronically vehicle or fluoxetine treated rats. At this same interval 25 micrograms/kg s.c. 8-OH-DPAT, a 5-HT1A receptor agonist, significantly reduced extracellular 5-HT only in the frontal cortex of rats treated chronically with the vehicle. Examining basal extracellular 5-HT, the effect of a challenge dose of fluoxetine and the effect of 25 micrograms/kg 8-OH-DPAT after 96 h washout, no differences were found between chronically fluoxetine and vehicle-treated rats. The results confirm that the ability of fluoxetine to stimulate 5-HT1A autoreceptors through an increase of endogenous 5-HT attenuates its effect on cortical dialysate 5-HT. Chronic fluoxetine increased the basal concentrations of extracellular 5-HT only when a substantial amount of its metabolite was present in the brain and during the desensitization of presynaptic 5-HT1A autoreceptors (24 h after the last dose). These effects, in fact, disappeared after 96 h washout. The continuous presence of the drug may, therefore, be necessary to maintain extracellular 5-HT at concentrations high enough to produce a therapeutic effect.
Invernizzi
R
R
Istituto di Ricerche Farmacologiche Mario Negri, Milano, Italy.
Bramante
M
M
Samanin
R
R
D017371:8-Hydroxy-2-(di-n-propylamino)tetralin / Q000494:pharmacology; D000818:Animals; D001835:Body Weight / Q000187:drug effects; D005110:Extracellular Space / Q000187:drug effects / Q000378:metabolism; D005473:Fluoxetine / Q000494:pharmacology*; D005625:Frontal Lobe / Q000033:anatomy & histology / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D017551:Microdialysis; D010879:Piperazines / Q000494:pharmacology; D011725:Pyridines / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D011985:Receptors, Serotonin / Q000187:drug effects / Q000378:metabolism*; D012701:Serotonin / Q000378:metabolism*; D012702:Serotonin Antagonists / Q000494:pharmacology; D017366:Serotonin Receptor Agonists / Q000494:pharmacology; D017367:Selective Serotonin Reuptake Inhibitors / Q000494:pharmacology*
D017371:8-Hydroxy-2-(di-n-propylamino)tetralin; D005473:Fluoxetine; D010879:Piperazines; D011725:Pyridines; D011985:Receptors, Serotonin; D012701:Serotonin; D012702:Serotonin Antagonists; D017366:Serotonin Receptor Agonists; D017367:Selective Serotonin Reuptake Inhibitors; C090413:N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridinyl)cyclohexanecarboxamide