Neuroscience Vol. 32, No. 3, PP. 637-645, 1989 Printed in Great Britain 0306-4522/89 $3.00 + 0.00 Pergamon Press plc 0 1989IBRO SEROTONIN RELEASE ESTIMATED BY TRANSCORTICAL DIALYSIS IN FREELY-MOVING RATS E. CARBONI and G. DI CHIARA* Institute of Experimental Pharmacology and Toxicology, University of Cagliari, Viale Diaz 182, 09125 Cagliari, Italy Abstract-The transcerebral dialysis method has been utilized for measuring extracellular brain concen- trations of serotonin and S-hydroxyindolacetic acid. Dialysis fibres were implanted transversally in the rat frontal cortex and perfused by Ringer. Serotonin and 5hydroxyindolacetic acid were quantified by reverse phase high performance liquid chromatography with electrochemical detection. Experiments were performed in freely-moving rats 20-24 h after the implant of the fibre. Basal output of serotonin and 5hydroxyindolacetic acid was 0.12 and 22.8 pmol in 20 min, respectively. The output of serotonin was calcium-dependent and tetrodotoxin-sensitive (1 pm in the Ringer) while was stimulated by veratridine (50ym) and by high concentrations of K+ (60 and 1OOmM). Serotonin output was increased in a concentration-dependent manner by chlorimipramine (l-IOpM) in the Ringer; this drug stimulated serotonin release also when administered S.C. (20mg/kg) in a tetrodotoxin-sensitive manner. The irreversible monoamine-oxidase inhibitor pargyline (75 mg/kg, i.p.) strongly stimulated serotonin output while reduced 5-hydroxyindolacetic acid output. A proposed serotonin releaser, fenfluramine (25 mg/kg, SC.), stimulated serotonin release and this effect was strongly potentiated by local application of tetrodotoxin (I PM). Agonists of serotonin receptors such as lisuride (0.03 mg/kg, s.c.), 8hydroxy-2- (di-n-propilamino)tetraline (0.25 mg/kg, s.c.) and 5-methoxy 3(1,2,3,6-tetrahydro-4-pyridinil)-1H indole succinate (I mg/kg, s.c.) reduced serotonin release. It appears that brain dialysis is a suitable method for the study of serotonin release in the cortex of freely-moving rats. Serotonin (5-HT)S,9,‘4is a central neurotransmitter- neuromodulator which has been implicated in vari- ous aspects of neural function such as sleep,24 sexual,3’x39 aggressive and predatory behaviour,13 as well as in psychiatric disorders such as major depres- sion.2s20 Moreover, various drugs such as tricyclic antidepressants, hallucinogens, 5-hydroxytrypto- phan, are able to modify behaviour through an action on central serotonergic mechanisms.‘J9,23*2*,33*35 In spite of such diverse functions attributed to SHT, direct evidence for a role in behaviour and in the actions of centrally acting drugs has been hampered by the difficulties inherent in the zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCB in viva estimation of serotonergic function. Thus, the recording of the firing activity of serotonergic units in freely-moving animals has been utilized in the effort to evaluate the physiological activity of serotonergic neurons;4’ this method, however, is unable to detect changes in the amount of transmitter released as a consequence of distal (presynaptic) changes circumscribed to the synaptic terminal and independent from changes in the rate of spike generation. In viuo release of 5-HT has been estimated by push-pull cannula but this *To whom correspondence should be addressed Abbreviations: ACh, acetylcholine; DA, dopamine; DOPAC, dihydroxyphenylacetic acid; 5-HIAA, 5-hydroxyindol- acetic acid; serotonin, 5-HT; MAO, monoamineoxidase; NA, noradrenaline; 8-OH-DPAT, 8-hydroxy-2-(di- n-propylaminotetralin HBr; RU24969, 5-methoxy 3( 1,2,3,6-tetrahydro-4-pyridinil)-IH indole succinate; TTX, tetrodotoxin. procedure is affected by some limitations such as tissue damage at the cannula tip due to the direct physical impact of the perfusion fluid on the tissue.“,38 Voltammetry through implanted electrodes has also been proposed for the estimation of 5-HT release in uiuoz9 but this issue is still being debated.” Brain dialysis 21,42 has been sucessfully applied to the in viuo estimation of the release of neurotransmitters such as dopamine (DA), acetylcholine (ACh) and noradren- aline (NA) (see Ref. 43 for review). In spite of this, only limited information exists on the possibility of applying brain dialysis to the estimation of serotonin neurotransmission in freely-moving animals.4,‘7,36,37 Here we report the results of a systematic study devoted to investigate this possibility. EXPERIMENTALPROCEDURES Animals Male Sprague-Dawley rats (Charles River, Italy) (280-300g) were used in these experiments. Rats were housed for at least 5 days under standard conditions before use. Rats were caged in groups of five with food and water ad libitum under an artificial light-dark cycle of 12 h (light at 6.00 a.m.). Experiments were started between 8.00 and 10.00 am. Surgery Rats were implanted under halothane anaesthesia with AN 69 (polyacrylonitrile/sodimn methalyl sulfonate co- polymer) dialysis fibres (310pm outer diameter, 220ym inner diameter), (Hospal, Bologna, Italy), inserted trans- versally in the cortex (coordinates A 0.4; V - 2.0 from temporal bone).” Dialysis was confined to the cortex by covering the dialysis fibre with Superepoxy glue through its 637