Dual population of GABAA and GABAB receptors in rat pars intermedia demonstrated by release of αMSH caused by barium ions

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Abstract
1 We have studied the effects of selective GABAA and GABAB agonists on α-melanophore stimulating hormone (αMSH) release from intact rat neurointermediate lobes (NIL) in vitro. Agonist effects were tested against either basal αMSH output or BaCl2 (5 mM)-evoked release. 2 GABA (50 μM) produced a biphasic effect on basal release, with an enhancement followed by inhibition of release. The enhancement but not the inhibition was blocked by bicuculline methiodide (100 μM). 3 Baclofen (10 μM), a specific GABAB agonist, reduced the basal and Ba2+-evoked hormonal release in a stereospecific manner. (−)−Baclofen (5 μM) was active whereas the (+)-isomer was inactive at the same concentration. 4 Isoguvacine (50 μM) a specific GABAA agonist, potentiated the Ba2+-evoked release of αMSH. GABA (50 μM) mimicked this effect, and its action was antagonized by bicuculline methiodide (200 μM). 5 The results suggest that both GABAA and GABAB receptors are present on the endocrine cells of the intermediate lobe.