Voltage‐dependent block of native AMPA receptor channels by dicationic compounds

Abstract

1The kinetics of open channel block of GluR2-containing and GluR2-lacking AMPA receptors (AMPAR) by dicationic compounds (IEM-1460, IEM-1754, and IEM-1925 have been studied in rat hippocampal neurones using whole-cell patch clamp recording and concentration-jump techniques. Neurones were isolated from hippocampal slices by vibrodissociation. 2 The dicationic compounds were approximately 100 ± 200 times more potent as blockers of GluR2-lacking AMPAR than as blockers of GluR2-containing AMPAR. The subunit speci®city of channel block is determined by the blocking rate constant of a dicationic compound, whereas di erences in unblocking rate constants account for di erences in potency. 3 Hyperpolarization may decrease the block produced by IEM-1460 and IEM-1754 block due to the voltage-dependence of the unblocking rate constants for these compounds. This suggests that dicationic compounds permeate the AMPAR channel at negative membrane potentials. The e ect was particularly apparent for GluR2-lacking AMPAR. These ®ndings indicate that the presence of GluR2-subunit(s) in AMPAR hinders the binding of the cationic compounds and their permeation through the channel.4 The most potent compound tested was IEM-1925. The presence of a phenylcyclohexyl moiety instead of an adamantane moiety, as in IEM-1460 and IEM1754, is probably responsible for the higher potency of IEM-1925. Dicationic compounds are important not only as pharmacological tools, but also as templates for the synthesis of new selective AMPAR blockers which may be potential therapeutic agents.

AMPA receptor ligands: Synthetic and pharmacological studies of polyamines and polyamine toxins

“…For example, compounds with an adamantane head group could permeate GluR2-lacking AMPAR channels at holding potentials more negative than À80 mV, while those with a phenylcyclohexyl head-group could not. 103 The two head-groups result in a maximum width of 6.6 and 11.2 Å , respectively (Tikhonov, Personal Communication), suggesting a pore dimension a little greater than 6.6 Å , but less than 11.2 Å , at its narrowest point. Other studies have provided an estimate of 7.8 Å for GluR2 lacking AMPAR.…”

“…183 The voltage-dependence of AMPAR antagonism by externally applied polyamines sometimes takes on an unusual profile. 103,184 Instead of the antagonism simply increasing with cell hyperpolarization, there is often a peak and decline in antagonism at more negative holding potentials. The explanation for this is that when the voltage gradient is sufficiently high, charged blocking molecules may start to permeate the channels providing they are not too large to do so.…”

“…The explanation for this is that when the voltage gradient is sufficiently high, charged blocking molecules may start to permeate the channels providing they are not too large to do so. 103,184 This is like the relief of block by intracellular spermine (1) at V H more positive than þ50 mV. It is thus possible to use carefully designed channel blockers to determine the largest permeable molecule and estimate the channel dimensions.…”

“…Voltage-dependent block of inward current from the extracellular side has been shown for the spermine (1), 171 [164][165][166] and diamino derivatives of adamantane and phenylcyclohexyl (e.g., IEM-1460, 22). 103 In addition, many of these polyamines exhibit usedependence meaning that the AMPAR ion channel needs to be opened before it can be blocked and the more it is opened the greater the probability that it becomes blocked. This conclusion is drawn from the fact that preapplication of PCCs does not increase the initial antagonism when agonist is applied and antagonism develops relatively slowly.…”

“…The template of polyamine toxins has led to other derivatives with polyamine moieties such as 1-trimethylamino-5-(1-adamantane-methylaminopentane) (IEM-1460, 22, Fig. 7) 102,103 and N 1 -naphthylacetylspermine (NAS, 23), [104][105][106] both of which are potent non-competitive antagonists of Ca 2þ -permeable AMPAR, but also show a high degree of antagonistic effect at NMDAR. The specific potential of PCCs as drug candidates, lies in their function as selective inhibitors of Ca 2þ -permeable AMPAR (see ''Mechanisms of Action''), combined with increasing evidence suggesting that Ca 2þ -permeable AMPAR are important for the pathogenesis of neurological disorders.…”