Ag. Zhang et Ra. Nicholson, PRESYNAPTIC INHIBITION OF [H-3] NITRENDIPINE BINDING AND MODIFICATIONOF MEMBRANE-PROPERTIES BY INSECTICIDAL DIHYDROPYRAZOLES IN MAMMALIAN BRAIN, Pesticide biochemistry and physiology, 55(3), 1996, pp. 200-209
RH-3421 and RH-5529 displace specifically bound [H-3]nitrendipine in m
ouse brain synaptosomes, suggesting that dihydropyrazoles act at presy
naptic L-type calcium channels in mammalian brain. Scatchard analysis
revealed that both compounds interfere with binding of [H-3]nitrendipi
ne through a mechanism which reduces the number of available radioliga
nd binding sites (B-max) without altering the affinity (K-d) for remai
ning sites. In addition, there was no discernible change in either the
rate of association of [H-3]nitrendipine or in the rate of radioligan
d dissociation when the binding preparation was exposed to dihydropyra
zoles, Such a binding profile indicates that dihydropyrazoles operate
as noncompetitive blockers of [H-3]nitrendipine binding without allost
eric involvement. The effects of RH-3421 and RH-5529 on the physical p
roperties of synaptic plasma membranes prepared from mouse brain were
also evaluated using the fluorescence probes 1,6-diphenyl-1,3,5-hexatr
iene (DPH) and 1-(4-trimethyl-ammonium phenyl)-6-phenyl-1,3,5-hexatrie
ne (TMA-DPH), which monitor the physical state of lipids in the membra
ne core and membrane surface, respectively. Exposure of synaptic membr
anes to RH-3421 and RH-5529 resulted in a reduction in fluorescence po
larization of both probes, confirming that these lipid domains are acc
essible to dihydropyrazoles. Estimates of static and dynamic parameter
s show that lipid order and fluidity are modified within surface and c
ore regions by dihydropyrazoles; however, the changes to regions probe
d by TMA-DPH appear more closely correlated with binding data. Overall
, our results are consistent with the proposal that dihydropyrazoles r
educe the availability of [H-3]nitrendipine binding sites, possibly by
modifying the physicostructural properties of the surface membrane. H
owever, we do not exclude the possibility that dihydropyrazoles are ir
reversible inhibitors of [H-3]nitrendipine binding or cause more subtl
e interference with regulatory biomolecules in the channel environment
. (C) 1996 Academic Press