A positive allosteric modulator of α7 nAChRs augments neuroprotective effects of endogenous nicotinic agonists in cerebral ischaemia

BACKGROUND AND PURPOSE Activation of alpha 7 nicotinic acetylcholine receptors (nAChRs) can be neuroprotective. However, endogenous choline and ACh have not been regarded as potent neuroprotective agents because physiological levels of choline/ACh do not produce neuroprotective levels of alpha 7 activation. This limitation may be overcome by the use of type-II positive allosteric modulators (PAMs-II) of alpha 7 nAChRs, such as 1-(5-chloro-2,4-dimethoxyphenyl)-3-(5-methylisoxazol-3-yl)-urea (PNU-120596). This proof-of-concept study presents a novel neuroprotective paradigm that converts endogenous choline/ACh into potent neuroprotective agents in cerebral ischaemia by inhibiting alpha 7 nAChR desensitization using PNU-120596. EXPERIMENTAL APPROACH An electrophysiological ex vivo cell injury assay (to quantify the susceptibility of hippocampal neurons to acute injury by complete oxygen and glucose deprivation; COGD) and an in vivo middle cerebral artery occlusion model of ischaemia were used in rats. KEY RESULTS Choline (20-200 mu M) in the presence, but not absence of 1 mu M PNU-120596 significantly delayed anoxic depolarization/injury of hippocampal CA1 pyramidal neurons, but not CA1 stratum radiatum interneurons, subjected to COGD in acute hippocampal slices and these effects were blocked by 20 nM methyllycaconitine, a selective alpha 7 antagonist, thus, activation of alpha 7 nAChRs was required. PNU-120596 alone was ineffective ex vivo. In in vivo experiments, both pre- and post-ischaemia treatments with PNU-120596 (30 mg.kg(-1), s.c. and 1 mg.kg(-1), i.v., respectively) significantly reduced the cortical/subcortical infarct volume caused by transient focal cerebral ischaemia. PNU-120596 (1 mg.kg(-1), i.v., 30 min post-ischaemia) remained neuroprotective in rats subjected to a choline-deficient diet for 14 days prior to experiments. CONCLUSIONS AND IMPLICATIONS PNU-120596 and possibly other PAMs-II significantly improved neuronal survival in cerebral ischaemia by augmenting neuroprotective effects of endogenous choline/ACh.