Evidence for Inhibitory Control of Melanophore-Stimulating Hormone Secretion from the Neuro-intermediate Lobe of the Lizard Pituitary by Gamma-aminobutyric Acid

Objective. To determine the involvement of gamma-aminobutyric acid (GABA) in the control of Melanophore-Stimulating Hormone (MSH) secretion from the neuro-intermediate lobe (NIL) of the lizard Anolis carolinensis. Background. Previous studies have provided strong evidence for a stimulant control of MSH secretion in this species exerted by serotonin. Evidence for the involvement of inhibitory factors in the control of MSH secretion in the lizard is weak and the studies have focused exclusively on dopamine. Methods. Anole NILs were placed in a perifusion chamber and the MSH content of the perifusate was measured using the Anolis skin bioassay. None of the drugs affected the assay. The presence of glutamic acid decarboxylase (GAD) was determined using Western Blot analysis. Results. GABA (100 μM) reversibly inhibited MSH secretion stimulated by high (50 mM) [K+]O. The inhibition was due to activation of GABAA receptors since it was blocked by the GABAA antagonist bicuculline (100 μM) and mimicked by the GABAA agonist muscimol (100 μM). Western blot analysis revealed the presence of glutamic acid decarboxylase, the enzyme necessary for the synthesis of GABA, in the NILs. Conclusion. The results indicate that GABA, acting through, GABAA receptors, inhibits MSH secretion and that the tissue has the capability of synthesizing GABA. Both these results suggest that GABA exerts an inhibitory control over MSH secretion in the anole in addition to the well known stimulant control provided by serotonin. Grants. This study was funded by an HPD research award.