Effects of fenfluramine on free-operant timing behaviour: evidence for involvement of 5-HT2A receptors.
BACKGROUND
Temporal differentiation in the free-operant psychophysical procedure is sensitive to the 5-hydroxytryptamine (5-HT)1A receptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) and the 5-HT2 receptor agonist 2,5-dimethoxy-4-iodo-amphetamine (DOI); both drugs shift the psychophysical curve leftwards, reducing the indifference point, T50. We have examined the effect of the 5-HT releasing agent fenfluramine on temporal differentiation.


OBJECTIVE
We examined whether fenfluramine's effect on temporal differentiation can be antagonised by the 5-HT1A receptor antagonist N-[2-(4-[2-methoxy-phenyl]-1-piperazinyl)ethyl]-N-2-pyridinylcyclohexane-carboxamide (WAY-100635) and the 5-HT2A receptor antagonist ketanserin, and compared the effects of fenfluramine, DOI and 8-OH-DPAT in intact rats and rats whose 5-HTergic pathways had been destroyed by 5,7-dihydroxytryptamine.


METHODS
Rats were trained under the free-operant psychophysical procedure to press levers A and B in 50-s trials in which reinforcers were provided intermittently for responding on A in the first half, and B in the second half of the trial. Percent responding on B (%B) was recorded in successive 5-s epochs of the trials; logistic psychophysical curves were fitted to the data for derivation of timing indices (T50, time corresponding to %B=50%, and Weber fraction). Experiment 1 examined the effects of acute treatment with fenfluramine, and the interaction between fenfluramine and the 5-HT1A and 5-HT2A receptor antagonists WAY-100635 and ketanserin; experiment 2 compared the effects of fenfluramine, 8-OH-DPAT and DOI in intact rats and rats whose 5-HTergic pathways had been destroyed by intra-raphe injection of 5,7-dihydroxytryptamine. Concentrations of 5-HT and catecholamines in the brain were measured by high-performance liquid chromatography.


RESULTS
Experiment 1: fenfluramine (2 mg/kg) reduced T50; this effect was attenuated by ketanserin (1.0 mg/kg) but not by WAY-100635 (100 microg/kg). Experiment 2: 8-OH-DPAT (100 microg/kg) and DOI (250 microg/kg) reduced T50 in both groups; fenfluramine reduced T50 only in the sham-lesioned group. Levels of 5-HT were reduced by 80% in the lesioned group; catecholamine levels were not affected.


CONCLUSIONS
The results suggest that fenfluramine affects temporal differentiation via the release of endogenous 5-HT which acts mainly on postsynaptic 5-HT2A receptors.
Body
S
S
Psychopharmacology Section, Division of Psychiatry, University of Nottingham, NG7 2UH, Nottingham, UK.
Kheramin
S
S
Ho
M-Y
MY
Miranda Herrera
F
F
Bradshaw
C M
CM
Szabadi
E
E
D000818:Animals; D003216:Conditioning, Operant / Q000187:drug effects* / Q000502:physiology; D005260:Female; D005277:Fenfluramine / Q000494:pharmacology*; D051381:Rats; D017208:Rats, Wistar; D011930:Reaction Time / Q000187:drug effects / Q000502:physiology; D044402:Receptor, Serotonin, 5-HT2A / Q000502:physiology*; D058830:Serotonin 5-HT2 Receptor Antagonists; D012702:Serotonin Antagonists / Q000494:pharmacology; D013997:Time Factors
D044402:Receptor, Serotonin, 5-HT2A; D058830:Serotonin 5-HT2 Receptor Antagonists; D012702:Serotonin Antagonists; D005277:Fenfluramine