CHARACTERIZATION OF GAMMA-AMINOBUTYRIC ACID RECEPTORS ON MELANOTROPHS OF THE ANOLE PITUITARY

Atrium

Objective. To determine the gamma-aminobutyric acid (GABA) receptor types found on melanotrophs of the intermediate lobe of the pituitary in the lizard Anolis carolinensis. Background. Previous studies have provided evidence for an inhibitory role of GABA in control of MSH secretion in the anole. Methods. Anole neurointermediate lobes (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 used affected the assay. Results. GABA (100 ¼M) reversibly inhibited MSH secretion stimulated by high (50 mM) [K+]o but did not alter basal secretion. The inhibition of Kstimulated MSH secretion by GABA was reversibly blocked by the GABAA receptor antagonist bicuculline (100 ¼M). Furthermore, the inhibitory effect of GABA on K-stimulated MSH secretion was mimicked by the GABAA agonist muscimol (100 ¼M) and the inhibitory effect of muscimol was also blocked by bucuculline (100 ¼M). The GABAB receptor agonist baclofen (100 ¼M) had no effect on either basal or K-stimulated secretion. Conclusion. The results indicate that the inhibition of K-stimulated MSH secretion by GABA is mediated by GABAA receptor activation. Grants. This study was funded by an HPD research award.