I. Shibuya et al., EFFECTIVENESS OF GABA-B ANTAGONISTS IN INHIBITING BACLOFEN-INDUCED REDUCTIONS IN CYTOSOLIC FREE CA CONCENTRATION IN ISOLATED MELANOTROPHS OF RAT, British Journal of Pharmacology, 105(4), 1992, pp. 893-898
1 The purpose of the present experiments was to assess the activities
of GABA(B) receptor antagonists in mammalian isolated melanotrophs. 2
Cytosolic free Ca concentration ([Ca2+]i) in rat melanotrophs in prima
ry culture was monitored with the fluorescent probe, fura-2. 3 (-)-Bac
lofen lowered [Ca2+]i in a concentration-dependent manner with an EC50
of 0.96-mu-M. The reduction in [Ca2+]i produced by (-)-baclofen at a
maximally effective concentration (100-mu-M) was similar to that produ
ced by the classic transmitter inhibitory to melanotroph secretion, do
pamine, at a corresponding concentration (100 nM), or by perifusion wi
th a nominally Ca-free solution. 4 The GABA(B) receptor antagonists, 3
-aminopropyl(diethoxymethyl)phosphinic acid (CGP 35348), 2-hydroxy sac
lofen, phaclofen and 4-amino-3-(5-methoxybenzo[b]furan-2-yl) butanoic
acid (9H), had inhibitory effects on the reduction in [Ca2+]i produced
by (-)-baclofen (3-mu-M). Of the antagonists tested, CGP 35348 was th
e most potent with an IC50 of 60-mu-M, compared to 120 to 400-mu-M for
the others. CGP 35348 acted competitively. 5 CGP 35348 alone had no e
ffect on basal [Ca2+]i, or on the changes in [Ca2+]i produced by dopam
ine (10 nM) or the specific GABA(A) receptor agonist, muscimol (10-mu-
M). 6 The evidence indicates that of the antagonists tested, CGP 35348
offers the greatest promise for pharmacological analysis of the funct
ional significance of the GABA(B) receptors in melanotrophs.