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

Abstract

1 We have studied the effects of selective GABA_A and GABA_B 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 GABA_B 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 GABA_A 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 GABA_A and GABA_B receptors are present on the endocrine cells of the intermediate lobe.