Serotonin-induced enhancement of vasopressin and oxytocin secretion in rat neurohypophyseal tissue culture.

The effects of serotonin (5-hydroxytryptamine; 5-HT) on vasopressin (VP) and oxytocin (OT) secretion were studied in 13-14-day cultures of isolated rat neurohypophyseal (NH) tissue. The VP and OT contents of the supernatant were determined by radioimmunoassay after a 1 or 2 h incubation. Significantly increased levels of VP and OT production were detected in the tissue culture media following 5-HT administration, depending on the 5-HT dose. The elevation of NH hormone secretion could be partially blocked by previous administration of the 5-HT antagonist ketanserin or metergoline. WAY-100635 did not influence the increased VP secretion induced by 5-HT, but the elevated OT production was prevented by WAY-100635 before 5-HT administration. The application of WAY-100635, ketanserin or metergoline, after 5-HT administration proved ineffective. The results indicate that NH hormone release is influenced directly by the serotonergic system. The serotonergic control of VP and OT secretion from the NH tissue in rats can occur at the level of the posterior pituitary.

Characterization and quantitative autoradiography of [3H]tryptamine binding sites in rat brain.

[3H]Tryptamine binds with high affinity (Kd = 9.1 nM, Bmax = 54 fmol/mg wet wt.) to tissue sections of rat brain. The binding occurs rapidly and is reversible. Low concentrations of the beta-carbolines harmaline (IC50 = 25 nM) and tetrahydronorharman (tetrahydro-beta-carboline, IC50 = 50 nM) inhibit [3H]tryptamine binding. Serotonin (5-HT, IC50 = 2600 nM) as well as the 5-HT receptor antagonists methysergide and metergoline displace [3H]tryptamine at much higher concentrations from brain slices. The distribution of [3H]tryptamine binding sites in sections of rat brain has been analyzed by quantitative autoradiography. The highest density of binding sites is found in the nucleus (n.) interpeduncularis, a slightly lower one in the locus coeruleus. Moderately labelled are the n. accumbens septi, n. septi lateralis, n. medialis habenulae, n. tractus olfactorii lateralis, the central region of the amygdala, n. caudatus/putamen, n. reuniens and the hippocampal formation. A low density of binding sites is detected in the cerebral cortex and the subiculum. Even less binding sites are found in the n. dorsalis raphe and the substantia nigra. The pattern of distribution of [3H]tryptamine binding sites differs from that of [3H]5-HT (5-HT1), [3H]ketanserin (5-HT2) as well as [3H]imipramine binding sites. These data suggest unique tryptamine binding sites.

The affinity of metergoline for 3H-serotonin (5HT) binding sites is regulated by guanine nucleotide.

100 microM guanine nucleotide Gpp (NH)p reduces the affinity of the serotonergic antagonist metergoline for 3H-5HT binding sites in rat cerebral cortex. This effect is present both in inhibition binding and in saturation experiments. The hypothesis that the interaction of some serotonergic antagonists with 3H-5HT binding sites is regulated by guanine nucleotides is discussed.

Pharmacokinetics and bioavailability of metergoline in healthy volunteers after single i.v. and oral administration.

Concentrations of unchanged metergoline and its main metabolite, 1-demethylmetergoline, were measured by HPLC and fluorescence detector in the plasma of 13 healthy male volunteers. The subjects received on various occasions the following single-dose metergoline treatments: 4 mg by i.v. infusion (n = 7), 8 mg orally as aqueous solution (n = 7) and 8 mg orally as two different formulations of film-coated tablets (Formulation A, n = 12; Formulation B, n = 12). The mean plasma t 1/2 of metergoline and of 1-demethylmetergoline were about 50 min and 100 min, respectively, independent of the route of administration. A considerable first-pass effect was evident from the data, with about 75% of metergoline being metabolized by the liver before reaching the systemic circulation. However, the availability of the drug in terms of 1-demethylmetergoline was similar for the i.v. and oral routes of administration indicating a complete absorption of the solution from the gastrointestinal tract. Very low plasma levels of another metabolite (12-hydroxymetergoline) were detected in some patients. The bioavailability of film-coated tablets in Formulation B was slightly better than for Formulation A with regard to both relative absorption (A vs B = 82%) and lower interpatient variation. Compared with oral solution, the absorption of Formulation B was slightly slower but practically complete.

Metabolism of metergoline in the rat.

[19,10-3H] Metergoline was administered orally to female rats and radioactive metabolites were recovered form urine and bile. Besides unchanged drug, the presence of 1-demethylmetergoline (II) was confirmed both in urine and bile. Two other urinary metabolites were identified, namely 8 beta-aminomethyl-6-methylergoline (IX) and 8 beta-acetylaminomethyl-6-methylergoline (VI). A third metabolite, 1,6-dimethyl-8 beta-aminomethylergoline (VIII), was identified only in bile. Beside these metabolites, further polar and so far unidentified biotransformation products are present both in bile and urine, accounting for about half of the radioactivity present in these excreta.

[3H]Metergoline: a new ligand of serotonin receptors in the rat brain.

A specific binding site for [3H]metergoline characterized by a KD of 0.5-1.0 nM was detected in microsomal and synaptic plasma membranes from various areas of the adult rat brain. Experiments with 5,7-dihydroxytryptamine- and kainic acid-induced lesions indicated that this specific binding site was localized post-synaptically with respect to serotoninergic neurons. The pharmacological characteristics of [3H]metergoline binding to microsomal membranes from the whole forebrain strongly suggest that this ligand labels a class of serotonin receptors. This was particularly obvious in the hippocampus in which serotonin was about 400 times more potent than dopamine and noradrenaline for displacing bound [3H]metergoline. In the striatum, serotonin was only 10 times as potent as dopamine in inhibiting [3H]metergoline binding, suggesting that this ligand may also bind to dopamine receptors. Striking similarities between the binding sites for [3H]metergoline and [3H]serotonin were observed in the hippocampus. Thus, not only the total numbers of binding sites for these two ligands in control rats but also their respective increases following intracerebral 5,7-dihydroxytryptamine treatment were very similar. Therefore, at least in the hippocampus, [3H]metergoline might well be the appropriate ligand for studying the characteristics of the 'antagonist form' of serotonin receptors postulated by Bennett and Snyder.

Exploratory hypoactivity induced by m-trifluoromethylphenylpiperazine (TFMPP) and m-chlorophenylpiperazine (m-CPP).

The action of m-trifluoromethylphenylpiperazine (TFMPP) and m-chlorophenylpiperazine (m-CPP), inhibiting the exploratory activity (ambulation and peeping) of the rat was studied in the open field test. The effects of both these drugs were antagonized by mesulergine, metergoline and mianserin, and partly by methysergide. Spiperone showed an antagonistic action in one (mean) dose only. The effects of TFMPP and m-CPP were not antagonized by ipsapirone, gepirone, cyanopindolol, compound 21009, cyproheptadine, ritanserine, ICS 205930, idazoxan or atropine. The lesion produced by p-chloramphetamine attenuated the effects of TFMPP and abolished those of m-CPP. The obtained results permit an assumption that the TFMPP- and m-CPP-induced decrease in the exploratory activity is mediated probably by 5-HT1C receptors.