Transport mechanisms governing serotonin clearance in vivo revealed by high-speed chronoamperometry.
High-speed chronoamperometry was used to determine the kinetics of clearance of exogenously applied serotonin (5-HT) in the dorsal raphe nucleus (DRN), dentate gyrus, CA3 region of the hippocampus or corpus callosum of anesthetized rats. Maximal velocity (Vmax) for 5-HT clearance was greatest in the DRN > dentate gyrus > CA3 > corpus callosum. Apparent affinity (K(T)) of the serotonin transporter (5-HTT) was similar in DRN and CA3 but greater in dentate gyrus and corpus callosum. A 90% loss of norepinephrine transporters (NET) produced by 6-hydroxydopamine pretreatment, resulted in a two-fold reduction in Vmax and a 30% decrease in K(T) in the dentate gyrus, but no change in kinetic parameters in the CA3 region. Pretreatment with 5,7-dihydroxytryptamine that resulted in a 90% reduction in 5-HTT density, modestly reduced Vmax in dentate gyrus but not in CA3. The same treatment had no effect on K(T) in the dentate gyrus but increased K(T) two-fold in the CA3. Neurotoxin treatments had no effect on 5-HT clearance in the corpus callosum. In hippocampal regions of intact rats, local application of the selective serotonin reuptake inhibitor, fluvoxamine, inhibited 5-HT clearance most robustly when the extracellular concentration of 5-HT was less than the K(T) value. By contrast, the NET antagonist, desipramine, significantly inhibited 5-HT clearance when extracellular concentrations of 5-HT were greater than the K(T) value. These data indicate that hippocampal uptake of 5-HT may be mediated by two processes, one with high affinity but low capacity (i.e. the 5-HTT) and the other with low affinity but a high capacity (i.e. the NET). These data show for the first time in the whole animal that 5-HT clearance in brain is regionally distinct with regard to rate and affinity.
Daws
Lynette C
LC
Department of Psychiatry, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, TX 78229-3900, USA. daws@uthscsa.edu
Montañez
Sylvia
S
Owens
W Anthony
WA
Gould
Georgianna G
GG
Frazer
Alan
A
Toney
Glenn M
GM
Gerhardt
Greg A
GA
D015116:5,7-Dihydroxytryptamine / Q000494:pharmacology; D018663:Adrenergic Agents / Q000494:pharmacology; D018759:Adrenergic Uptake Inhibitors / Q000494:pharmacology; D000818:Animals; D001665:Binding Sites / Q000187:drug effects / Q000502:physiology; D001692:Biological Transport / Q000502:physiology; D001921:Brain / Q000378:metabolism*; D001923:Brain Chemistry / Q000502:physiology*; D003337:Corpus Callosum / Q000187:drug effects / Q000378:metabolism; D015536:Down-Regulation / Q000187:drug effects / Q000502:physiology; D004563:Electrochemistry; D006624:Hippocampus / Q000187:drug effects / Q000378:metabolism; D007700:Kinetics; D008297:Male; D008562:Membrane Glycoproteins / Q000378:metabolism*; D026901:Membrane Transport Proteins / Q000378:metabolism*; D008657:Metabolic Clearance Rate / Q000502:physiology; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D009448:Neurochemistry / Q000379:methods*; D050484:Norepinephrine Plasma Membrane Transport Proteins; D016627:Oxidopamine / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012701:Serotonin / Q000378:metabolism / Q000493:pharmacokinetics*; D018490:Serotonin Agents / Q000494:pharmacology; D050486:Serotonin Plasma Membrane Transport Proteins; D017367:Selective Serotonin Reuptake Inhibitors / Q000494:pharmacology; D027981:Symporters / Q000037:antagonists & inhibitors / Q000378:metabolism; D013997:Time Factors
D015116:5,7-Dihydroxytryptamine; D018663:Adrenergic Agents; D018759:Adrenergic Uptake Inhibitors; D008562:Membrane Glycoproteins; D026901:Membrane Transport Proteins; D009419:Nerve Tissue Proteins; D050484:Norepinephrine Plasma Membrane Transport Proteins; D016627:Oxidopamine; D012701:Serotonin; D018490:Serotonin Agents; D050486:Serotonin Plasma Membrane Transport Proteins; D017367:Selective Serotonin Reuptake Inhibitors; D027981:Symporters; C493226:Slc6a2 protein, rat; C493232:Slc6a4 protein, rat