Calcium homeostasis of acutely denervated and lesioned dentate gyrus in organotypic entorhino-hippocampal co-cultures

Denervation of neurons, e.g. upon traumatic injury or neuronal degeneration, induces transneuronal degenerative events, such as spine loss, dendritic pruning, and even cell loss. We studied one possible mechanism proposed to trigger such events, i.e. excess glutamate release from severed axons conveyed transsynaptically via postsynaptic calcium influx. Using 2-photon microscopical calcium imaging in organotypic entorhino-hippocampal co-cultures, we show that acute transection of the perforant path elicits two independent effects on calcium homeostasis in the dentate gyrus: a brief, short-latency elevation of postsynaptic calcium levels in denervated granule cells, which can be blocked by preincubation with tetrodotoxin, and a long-latency astroglial calcium wave, not blocked by tetrodotoxin and propagating slowly through the hippocampus. While neuronal calcium elevations upon axonal transection placed remote from the target area were similar to those elicited by brief trains of electrical stimulation of the perforant path, large-scale calcium signals were observed upon lesions placed close to or within the dendritic field of granule cells. Concordantly, induction of c-fos in denervated neurons coincided spatially with cell populations showing prolonged calcium elevations upon concomitant dendritic damage. Since denervation of dentate granule cells by remote transection of the perforant path induces transsynaptic dendritic reorganization in the utilized organotypic cultures, a generalized breakdown of the cellular calcium homeostasis is unlikely to underlie these transneuronal changes.