Pharmacology of ampakine modulators: From AMPA receptors to synapses and behavior

Ampakines are drugs structurally derived from aniracetam that potentiate currents mediated by AMPA type glutamate receptors. These drugs slow deactivation and attenuate desensitization of AMPA receptor currents, increase synaptic responses and enhance long-term potentiation. This review focuses mainly on recent physiological studies and on evidence for two distinct subfamilies. Type I compounds like CX546 are very effective in prolonging synaptic responses while type II compounds like CX516 mainly increase response amplitude. Type I and II drugs do not compete in binding assays and thus presumably act through separate sites. Their differences are likely to have consequences also for synaptic plasticity and behavior. Thus, while all ampakines facilitated long-term potentiation, only CX546 enhanced long-term depression. Further discussed are studies showing that ampakine effects vary substantially between neurons, with increases in EPSCs being larger in CAI pyramidal cells than in thalamus and in hippocampal interneurons. In behavioral tests, ampakines facilitate learning in many paradigms including odor discrimination, spatial mazes, and conditioning, and they improved short-term memory in a non-matching-to-sample task. Positive results were also obtained in various psychological tests with human subjects. The drugs were effective in correcting behaviors in various animal models of schizophrenia and depression. Lastly, evidence is discussed that ampakines have few adverse effects at therapeutically relevant concentrations and that they protect neurons against neurotoxic insults, in part by mobilizing growth factors like BDNF. Type II drugs like CX516 in particular appear to be inherently safe since their ability to prolong responses is kinetically limited.