Sheep pineal beta-adrenoceptor function--interaction with gamma-aminobutyric acid.
To clarify the role and site of action of gamma-aminobutyric acid (GABA) in ovine pineal glands, we have investigated the effects of aminooxyacetic acid (AOAA), an inhibitor of GABA transaminase, on endogenous GABA content and beta-adrenoceptor mediated pineal function in Merino sheep. A significant elevation of endogenous GABA levels was noted in the glands, but no effect was observed on radioligand binding in vitro to pineal beta-adrenoceptors following in vivo administration of AOAA. Incubation of washed pineal membranes with GABA or AOAA had no effect on ligand binding to beta-adrenoceptors. Incubation of Merino pineal slices with GABA inhibited isoprenaline-stimulated but not basal serotonin N-acetyltransferase (NAT) activity. Incubation of whole pineal homogenates with GABA was without effect on either isoprenaline-stimulated or basal adenyl cyclase activity. Thus, Merino pineal glands resemble bovine pineals in that beta-adrenoceptor mediated melatonin biosynthesis in both species may be regulated in part by GABA. Our results indicate that GABA may exert its effect on Merino pineal NAT activity at a locus distal to the site of action of adenyl cyclase; however, the detailed mechanism and physiological role of this regulation remain to be elucidated.
D000123:Acetyltransferases / Q000378:metabolism; D000262:Adenylyl Cyclases / Q000378:metabolism; D000625:Aminooxyacetic Acid / Q000494:pharmacology; D000818:Animals; D005260:Female; D066298:In Vitro Techniques; D007545:Isoproterenol / Q000494:pharmacology; D008297:Male; D010870:Pineal Gland / Q000187:drug effects / Q000378:metabolism*; D011943:Receptors, Adrenergic, beta / Q000378:metabolism*; D012756:Sheep; D005680:gamma-Aminobutyric Acid / Q000378:metabolism / Q000502:physiology*
D011943:Receptors, Adrenergic, beta; D000625:Aminooxyacetic Acid; D005680:gamma-Aminobutyric Acid; D000123:Acetyltransferases; D000262:Adenylyl Cyclases; D007545:Isoproterenol