Ma. Perillo et al., Tagetone modulates the coupling of flunitrazepam and GABA binding sites atGABA(A) receptor from chick brain membranes, MOL MEMBR B, 16(2), 1999, pp. 189-194
The effects of tagetone on flunitrazepam (FNTZ) binding to synaptosomal mem
branes from chick brains in the presence and absence of allosteric modulati
ons induced by gamma-aminobutyric acid (GABA) were investigated. Tagetone,
at 50 mu g/ml (final concentration), decreased the binding affinity of [H-3
]FNTZ to synaptosomal membranes form chick brain (K-d = 3.34 +/- 0.36 nM wi
thout tagetone and K-d,K-t = 5.86 +/- 0.86 nM with tagetone; p < 0.05, two
tailed Student's t-test) without affecting maximal binding (B-max = 488 +/-
24 fmoles/mg protein, and B-max,B-t = 500 +/- 25 fmoles/ mg protein in the
absence and in the presence of tagetone respectively). The potency of GABA
to stimulate [H-3]FNTZ binding increased in the presence of tagetone (EC50
values were 2.78 and 1.12 mu M with and without tagetone respectively). GA
BA was able to decrease merocyanine Delta A(570-610) values in a concentrat
ion dependent manner; half maximal effect was attained at a GABA concentrat
ion of 34 +/- 13 mu M. Tagetone, at a concentration of 50 mu g/ml and in th
e presence of GABA 30 mu M or 60 mu M, enhanced the ability of GABA alone o
n decreasing Delta A(570-610).Tagetone alone did not change Delta A570-610
values. FNTZ, a well known GABA modulator, could also potentiate the effect
of GABA. Theoretical calculations indicate that the effects on merocyanine
Delta A(570-610) Value are mainly exerted at the membrane potential level
(Delta Psi(m)). The present results strongly suggest that tagetone affected
the function of GABA, receptor in a complex way: on the one hand it impair
ed FNTZ binding; on the other hand tagetone improved both the coupling betw
een FNTZ and GABA binding sites and it enhanced GABA-induced chloride perme
ability. Changes in the geometrical and electrostatic properties of the sel
f-organized membrane structure may account for these effects of tagetone.