Sheep Pineal Beta-Adrenoceptor Function – Interaction with Gamma-Aminobutyric Acid

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Abstract
To clarify the role and site of action of GABA in ovine pineal glands, the effects of aminooxyacetic acid (AOAA), an inhibitor of GABA transaminase, on endogenous GABA content and .beta.-adrenoceptor mediated pineal function were investigated 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 adenylate cyclase activity. Merino pineal glands resemble bovine pineals in that .beta.-adrenoceptor mediated melatonin biosynthesis in both species may be regulated in part by GABA. GABA may exert its effect on Merino pineal NAT activity at a locus distal to the site of action of adenylate cyclase; the detailed mechanism and physiological role of this regulation remain to be elucidated.