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Figurę 20-3. Enhanetd GARA jynapric trans-mission.

In ihc presence of GADA, ilic GABA_a recepto; tsiruc-ture on ieft) is opened, allowing an influx of Cl*, which in tum, inefeases membranę polarization [ser also Chapter 17). Sonie amiseizure drugs (shown in larger blue text) act by reducing the metabolism of GABA. Olhcrs act at the GABA_a receptor, cnhancing Cl* influx in response to GABA. As outlined in the text, gabapentin acts presynaptically to promoie GABA rclcase; its molecular target is currcmly under investigation. GABA-T, GABA transaminasr.

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Conlormational

equilibrium

Fig. 33.3 Model of benzodiazepine/GABA receptor interaction. Benzodiazepine agonists (e.g. diazeparn) and antagonists (e.g. flumazenil) are believed to bind to a site on the GABA receptor distinct from Ihe GABA binding site. A conformational equilibrium exists between States in which the benzodiazepine receptor exisls in its agonist-binding conformation (above) and in its aniagonist-binding conformalion (below). In the latter siatę, the GABA receptor has a much reduced attinity (or GABA, so that Ihe chloride channel remains closed.