Anticonvulsant enaminones depress excitatory synaptic transmission in the rat brain by enhancing extracellular GABA levels

Abstract

1 Enaminones are a novel group of compounds that have been shown to possess anticonvulsant activity in in vivo animal models of seizures. The cellular mechanism by which these compounds produce their anticonvulsant effects is not yet known. This study examined the effects of enaminones on excitatory synaptic transmission. 2 We studied the effects of 3-(4 0 -chlorophenyl)aminocyclohex-2-enone (E118), methyl 4-(4 0 -bromophenyl)aminocyclohex-3-en-6-methyl-2-oxo-1-oate (E139) and ethyl 4-(4 0 -hydroxyphenyl)aminocyclohex-3-en-6-methyl-2-oxo-1-oate (E169) on isolated evoked, glutamate-mediated excitatory synaptic responses by recording whole-cell currents and potentials in cells of the nucleus accumbens (NAc) contained in forebrain slices. 3 The anticonvulsant enaminones (E118 and E139), but not E169, depressed NMDA and non-NMDA receptor-mediated synaptic responses. The inhibition of the non-NMDA response was concentration-dependent (1.0-100 mM) with a maximal depression of BÀ30%. E118 and E139 had similar potencies (EC 50 ¼ 3.0 and 3.5 mM, respectively) in depressing this response but E139 was more efficacious (E max ¼ À31.373.8%) than E118 (E max ¼ À22.671.6%). 4 The excitatory postsynaptic current (EPSC) depression caused by 10 mM E139 (À27.773.8%) was blocked by 1 mM CGP55845 (6.378.1%), a potent GABA B receptor antagonist. 5 Pretreatment of slices with g-vinylGABA and 1-(2-(((diphenylmethylene)imino)oxy)ethyl)-1,2,5,6-tetrahydro-3-pyridine-carboxylic acid (NO-711), an irreversible GABA transaminase (GABA-T) inhibitor and a GABA reuptake blocker, respectively, like the anticonvulsant enaminones, also caused a depression of the evoked EPSC (À38.1714.1 and À24.178.9%, respectively). In the presence of these compounds, E139 did not cause a further depression of the EPSC. Our data suggest that anticonvulsant enaminones cause EPSC depression by enhancing extracellular GABA levels possibly through the inhibition of either GABA reuptake or GABA-T enzyme, or both.

A Substituted Anilino Enaminone Acts as a Novel Positive Allosteric Modulator of GABAA Receptors in the Mouse Brain

“…The subcutaneous pentylenetetrazol seizure model identifies compounds that inhibit the GABA antagonistic effects of pentylenetetrazol or raise the seizure threshold (Stables and Kupferberg, 1997). Studies have shown that a number of enaminone compounds display inhibition against glutamate-mediated excitatory synaptic transmission by modulation of GABAergic transmission (Kombian et al, 2005;Ananthalakshmi et al, 2006). Based on the above findings and the results we present, we hypothesize the existence of an essential pharmacophore within the enaminone structure that possibly interacts with the GABA receptor, which is significant for achieving anticonvulsant activity.…”

“…Another anticonvulsant enaminone, E139 (Fig. 2), was reported to suppress excitatory synaptic transmission by enhancing extracellular GABA levels (Kombian et al, 2005) and blocking tetrodotoxin-sensitive sodium channels and, thereby, directly inhibiting postsynaptic neuronal excitability . Meanwhile, several enaminones with chemical moieties different from KRS-5Me-4-OCF 3 were described to interact with GABA A receptors (Reitz et al, 1999;Yohannes et al, 2003;Hogenkamp et al, 2007).…”

“…Previously, nucleus accumbens and coronal hippocampal slices in the rat brain have been used to study anticonvulsant enaminone suppression of excitatory synaptic transmission and epileptiform activity (Kombian et al, 2005;Ananthalakshmi et al, 2007). The MOB is anatomically different from nucleus accumbens and hippocampus and provides three advantages to test cellular mechanisms of anilino enaminone action.…”

“…Structurally, these compounds are uniquely different from currently available antiepileptic drugs (Edafiogho et al, 1992(Edafiogho et al, , 2009Foster et al, 1999;Abdel-Hamid et al, 2002;Kombian et al, 2005). This class of enaminones has shown good to moderate protection in the traditional preclinical animal models, the subcutaneous pentylenetetrazol test and the maximal electroshock seizure test.…”

“…We reported previously the anticonvulsant activity of anilino enaminones in vivo and the possible mechanisms of action of these compounds by which they elicit their response (Edafiogho et al, 1992;Mulzac and Scott, 1993;Ananthalakshmi et al, 2007). The anilino enaminone E139 inhibited excitatory postsynaptic currents in the rat nucleus accumbens and hippocampus by enhancing extracellular GABA levels (Kombian et al, 2005;Ananthalakshmi et al, 2007) and inhibiting tetrodotoxin-sensitive sodium currents to modulate excessive firing in individual neurons . A study aimed at elucidating the essential structural parameters necessary for anticonvulsant activity found that some benzylamino enaminones, which possess a similar chemical structure to anilino enaminones with benzyl-substitution at the NH-moiety, produced anticonvulsant effects in rats and mice neurons by suppressing glutamate-mediated excitation and action potential firing .…”