Characterization of afterhyperpolarization (AHP) in cryopreserved brain slices
An ongoing quest in cryonics is the successful demonstration of memory sustainment after cryopreservation of the brain and rewarming from cryogenic temperatures. To that end, landmark experiments were performed by Pichugin, et al. (2006) on rat hippocampal brain slices which indicate that the hippocampus retains excellent structural integrity and viability (as measured by Na+/K+ ion pump recovery) after vitrification, rewarming, cryoprotectant removal, and exposure to 35°C for over an hour. To address the question of memory itself, investigations into the maintenance of long-term potentiation (LTP) after vitrification of the brain are currently in progress. But even successful observation of LTP after cryopreservation provides only indirect evidence for memory maintenance.
Alternatively, post-burst afterhyperpolarization (AHP) of hippocampal CA1 neurons may be characterized after cryopreservation of animals that have successfully acquired a hippocampus-dependent task. CA1 pyramidal neurons show decreased post-burst AHPs and less accommodation (i.e., increased firing frequency) following learning of such hippocampus-dependent tasks as trace eyeblink conditioning (Moyer et al., 1996, 2000; Thompson et al., 1996) and spatial watermaze training (Oh et al., 2003) with a time course appropriate to support memory consolidation. Furthermore, CA1 neurons of aging animals (i.e., animals at ages that exhibit learning deficits) show greater AHPs and more accommodation than those of young animals (Landfield & Pitler, 1984; Moyer et al., 1992, 2000), indicating an age-related decrease in neuronal excitability in the hippocampus that may underlie learning deficits related to aging.
A carefully designed experiment demonstrating reduced afterhyperpolarization and accommodation in hippocampal CA1 neurons after acquisition of a hippocampus-dependent task and subsequent cryopreservation of the brain would be a huge step in the direction of proving that memories can be cryopreserved.