Nitric-oxide synthase mediates the ability of darbepoetin alfa to attenuate pre-existing spatial working memory deficits in rats subjected to transient global ischemia

Erythropoietin has been reported to improve the behavioral performance of healthy mice in tests thought to depend on synaptic plasticity in the CA1 region of the hippocampus. We show here for the first time that a single injection of the erythropoietin analog darbepoetin alfa reverses pre-existing cognitive deficits in adult rats that had been subjected to transient global ischemia produced by four-vessel occlusion (4-VO). Quantification of neuronal density demonstrated that 12 min of 4-VO selectively killed more than 90% of CA1 neurons in the dorsal hippocampus. Rats that had sustained a bilateral loss of hippocampal CA1 neurons in this range (4-VO rats) displayed more errors and longer escape latencies in the Barnes maze compared with sham-operated controls. A single injection of darbepoetin alfa (5000 U/kg i.p.) 4 h before behavioral testing decreased deficits in escape latency for 4-VO rats but not sham-operated controls. This improvement in spatial working memory performance was correlated with increased levels of nitric-oxide metabolites in the ventral hippocampus. Systemic administration of the nitric-oxide synthase inhibitor N(G)-nitro-L-nitro-arginine methyl ester reversed the increase in nitricoxide metabolites and improvements in spatial working memory produced by darbepoetin alfa (5000 U/kg, i.p.) at a dose (10 mg/kg, i.p.) that did not impair the spatial working memory performance of intact rats. Taken together, these findings suggest that darbepoetin alfa reverses pre-existing spatial working memory deficits resulting from transient global ischemia by increasing the activity of nitric-oxide synthase, an enzyme implicated in synaptic plasticity.