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Bourgoin S, Artaud F, Bockaert J, Hery F, Glowinski J, Hamon JM. 
“Paradoxical Decrease of Brain 5-HT Turnover by Metergoline, a central 5-HT Receptor Blocker”. 
Naunyn-Schmiedeberg's Arch.Pharmacol.. 1978;302(3):313-21.
Abstract
The effects of metergoline and methiothepin on brain serotonin and behavior in rats (250-300 g) were studied. Methiothepin (Roche, 20 mg/kg i.p.) produced a long lasting catalepsy ( 24 fur), while metergoline (Liserdol, Farmitalia, 0.2 mg/kg, i.p.) produced flaccid and abducted limbs. Methiothepin (20 mg/kg) or metergoline (10 mg/kg) prevented the behavioral effects of LSD (Sandoz, 1 mg/kg, i.p.). Both drugs reduced body temperature by 1.5-2 in a non-additive manner. In-vitro, 0,1 mM metergoline and methiothepin increased rat collicular adenylate cyclase activity by 40% and 230%, respectively. The methiothepin effect occurred in the cerebellum but not the cortex or striatum. Methiothepin (2.9 mcM)-and metergoline (12 mcM) reduced the stimulant effect of serotonin on adenylate cyclase activity by 50%, without affecting basal activity. In synaptosomal membranes, the maximum capacity of the high affinity binding sites was 90.5 + 12,6 femtomoles of 3H-serotonin bound/mg protein. Both drugs produce dosedependent reductions of binding, metergoline being more potent. Non-specific serotonin binding was unaffected. Methiothepin, but not metergoline, given 20 hr before sacrifice reduced in-vitro binding. Methiothepin, but not metergoline, increased brain trytophan levels. Metergoline (10 mg/kg) significantly increased serotonin levels in the brain stem and forebrain. Metergoline (2-10 ma/ kg) significantly reduced brain 5-HIAA levels while methiothepin increased them. Methiothepin (20 mg/kg) produced a 10-13 0ecrease in serotonin levels and a 31-52 0ncrease in brain 5-HIAA levels, but the ratio of 5-HIAA to serotonin was significantly reduced after LSD (1 mg/kg, i.p ) or metergoline (10 mg/kg). Metergoline prevented the effect of methiothepin, but did not alter the effect of LSD. After benserazid (Roche, 800 mg/kg, i.p.) the metergoline (2-10 mg/kg) reduced the accumulation rate of 5-hydroxytrytophan (5-HTP) in the forebrain and brain stem by 48% and 25%, respectively. Methiothepin (10-20 rum) doubled5-HIAA synthesis in-VitrO, but metergoline had no effect. Increasing the potassium concentration of brain slices from 5.6 to 33.6 mM produced a 157 0ncrease in the medium/tissue 3H-serotonin ratio. IBID (10 M) partially reduced this effect. The inhibition was prevented by metergoline but not by methiothepin. There may be at least 2 types of postsynaptic serotonin receptor in the rat brain.
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