title: GABAA and GABAB receptor-mediated inhibition of release of 5-hydroxytryptamine in the intermediate lobe of the rat pituitary gland.
abstract: Isolated neurointermediate lobes or neural lobes of the rat pituitary gland attached to the pituitary stalk were incubated in vitro and the spontaneous or electrically (pituitary stalk stimulation, 5 Hz, 1,500 pulses) evoked release of 5-hydroxytryptamine was determined. The evoked release of 5-hydroxytryptamine from the neurointermediate lobe was increased fivefold in the presence of the dopamine receptor antagonist (-)-sulpiride (1 microM). The evoked release of 5-hydroxytryptamine from the isolated neural lobe was not altered by (-)-sulpiride. The evoked release of 5-hydroxytryptamine from the isolated neural lobe in the presence of (-)-sulpiride was less than 5% of that from the combined neurointermediate lobe showing that most of the 5-hydroxytryptamine released from the combined neurointermediate lobe originated in the intermediate lobe. The evoked release of 5-hydroxytryptamine from the neurointermediate lobe in the presence of (-)-sulpiride showed a diurnal variation. It was three to five times higher between 9.30 and 14.00 h than between 8.30 and 9.30 h or between 14.00 and 16.00 h. The 5-hydroxytryptamine tissue content at the end of the incubation experiments also showed similar variations which were, however, less pronounced. The evoked release of 5-hydroxytryptamine from the neurointermediate lobe, in the presence of (-)-sulpiride, was reduced by the preferential GABAA receptor agonist muscimol or the selective GABAB receptor agonist (-)-baclofen in a concentration-dependent manner. Bicuculline, a selective GABAA receptor antagonist inhibited the effect of muscimol, but not that of (-)-baclofen. Bicuculline alone did not affect the release of 5-hydroxytryptamine from the gland. It is concluded that the release of 5-hydroxytryptamine in the intermediate lobe is influenced by both dopaminergic and GABAergic mechanisms.
authors.affiliation: Department of Pharmacology, University of Mainz, F.R.G.
authors.forename: K
authors.initials: K
authors.lastname: Racké
authors.forename: M
authors.initials: M
authors.lastname: Holzbauer
authors.forename: D F
authors.initials: DF
authors.lastname: Sharman
authors.forename: T R
authors.initials: TR
authors.lastname: Cooper
mesh_terms: D000818:Animals; D001418:Baclofen / Q000494:pharmacology; D001640:Bicuculline / Q000494:pharmacology; D002940:Circadian Rhythm; D004298:Dopamine / Q000502:physiology; D004558:Electric Stimulation; D066298:In Vitro Techniques; D008297:Male; D009118:Muscimol / Q000494:pharmacology; D010902:Pituitary Gland / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D011963:Receptors, GABA-A / Q000502:physiology*; D012701:Serotonin / Q000378:metabolism*; D013469:Sulpiride / Q000494:pharmacology
chemical_list: D011963:Receptors, GABA-A; D009118:Muscimol; D012701:Serotonin; D013469:Sulpiride; D001418:Baclofen; D004298:Dopamine; D001640:Bicuculline