Release and effect of gamma-aminobutyric acid (GABA) on rat pineal melatonin production in vitro.
1. 3H-gamma-Aminobutyric acid (GABA) release elicited by a depolarizing K+ stimulus or by noradrenergic transmitter was examined in rat pineals in vitro. 2. The release of 3H-GABA was detectable at a 20 mM K+ concentration in medium and increased steadily up to 80 mM K+. 3. In a Ca2+-free medium 3H-GABA release elicited by 30 mM K+, but not that elicited by 50 mM K+, became blunted. 4. Norepinephrine (NE; 10(-6)-10(-4) M) stimulated 3H-GABA release from rat pineal explants in a dose-dependent manner. 5. The activity of 10(-5) M NE on pineal GABA release was suppressed by equimolecular amounts of prazosin or phentolamine (alpha 1- and alpha 1/alpha 2-adrenoceptor blockers, respectively) and was unaffected by propranolol (beta-adrenoceptor blocker). 6. The alpha 1-adrenoceptor agonist phenylephrine (10(-7)-10(-5) M) and the beta-adrenoceptor agonist isoproterenol (10(-5) M) mimicked the GABA releasing activity of NE, while 10(-7) M isoproterenol failed to affect it; the alpha 2-adrenoceptor agonist clonidine (10(-7)-10(-5) M) did not modify 3H-GABA release. 7. The addition of 10(-4) M GABA or of the GABA transaminase inhibitor gamma-acetylenic GABA or aminooxyacetic acid inhibited the melatonin content and/or release to the medium in rat pineal organotypic cultures. 8. GABA at concentrations of 10(-5) M or greater partially inhibited the NE-induced increase in melatonin production by pineal explants. 9. The depressant effect of GABA on melatonin production was inhibited by the GABA type A receptor antagonist bicuculline; bicuculline alone increased the pineal melatonin content. Baclofen, a GABA type B receptor agonist, did not affect the pineal melatonin content or release. 10. The decrease in serotonin (5-HT) content of rat pineal explants brought about by NE was not modified by GABA; GABA by itself increased 5-HT levels. 11. These results indicate that (a) GABA is released from rat pineals by a depolarizing stimulus of K+ through a mechanism which is partially Ca2+ dependent; (b) NE releases rat pineal GABA via interaction with alpha 1-adrenoceptors; (c) GABA inhibits melatonin production in vitro via interaction with GABA type A receptor sites; and (d) GABA's effect on NE-induced melatonin release does not correlate with the lack of effect on the NE-induced decrease in pineal 5-HT content.
Rosenstein
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RE
Departamento de Fisiología, Facultad de Medicina, Universidad de Buenos Aires, Argentina.
Chuluyan
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HE
Pereyra
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EN
Cardinali
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DP
D000818:Animals; D002118:Calcium / Q000378:metabolism; D006897:Hydroxyindoleacetic Acid / Q000378:metabolism; D008297:Male; D008550:Melatonin / Q000378:metabolism*; D009638:Norepinephrine / Q000378:metabolism; D009924:Organ Culture Techniques; D010870:Pineal Gland / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D012701:Serotonin / Q000378:metabolism; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
D012701:Serotonin; D006897:Hydroxyindoleacetic Acid; D005680:gamma-Aminobutyric Acid; D008550:Melatonin; D011188:Potassium; D002118:Calcium; D009638:Norepinephrine