The hypothesis that minimizing energy expenditure for a given functional task is an important biological optimization goal in the nervous system (Science 301:1870, 2003) has recently been substantiated for axonal and somatic action potentials (APs) in mammalian cortical neurons (Science 325:1405, 2009; Neuron 64:898, 2009; PNAS 107:12329, 2010). In hippocampal mossy fibers, fast Na⁺ current decay and delayed K⁺ current onset during APs minimize overlap of their respective ion fluxes. This results in total Na⁺ influx and associated energy demand per AP of only 1.3 times the theoretical minimum, in contrast to previous assumptions of a factor of 4. Minimization of metabolic AP costs as a result of optimized ionic conductance kinetics, however, might not provide a comprehensive insight into an organism's adaptation to energy constraints. In view of protein turnover - making up considerable parts of the energy budget for the mammalian cortex (Curr Biol 13:493, 2003) - usage of highly specialized ion channel proteins may contribute to limit energy expenditures. In glutamatergic hippocampal mossy fiber boutons, AP repolarization is not exclusively mediated by K_v1 as previously suggested, but in fact is dominated by K_v3. Their recruitment by presynaptic APs is four times, single channel conductance two times that of K_v1; overall, K_v3 have a ten times higher repolarization potency per channel. In consequence, specialized K_v3 combined with multi-purpose K_v1 half the density of K⁺ channels necessary to perform presynaptic AP repolarization compared to K_v1 alone, indicating cost minimization processes also on the structural level.