Astrocytic glycogenolysis energizes memory processes in neonate chicks

Abstract

In previous pharmaco-behavioural experiments, we have shown that three sequential stages can be distinguished in discrimination memory for a single trial passive avoidance experience in neonate chicks: a short-term (STM) stage, available for 10 min following learning; an intermediate (ITM) stage, operating between 20 and 50 min (ITMB) post-learning; and a long-term (LTM) stage formed by 60 min after learning. The ITM stage can be divided into two parts: a first phase (ITMA) which is susceptible to inhibition by the uncoupler of oxidative phosphorylation (and thus of oxidative metabolism), 2,4-dinitrophenol (DNP), and a second DNP-insensitive phase (ITMB). ITMA occurs between 20 and 30 min post-training and ITMB between 30 and 50 min. In the present study we have shown: (1) that day-old chicks trained in the passive avoidance task and immediately thereafter injected with the glycolytic inhibitor iodoacetate show retention deficits that are first evident 30 min post-training, and (2) that glycogenolysis, i.e. breakdown of glycogen, a high-molecular carbohydrate energy store localized in astrocytes, occurs in the forebrains of trained, but otherwise untreated birds, between 35 and 55 min after learning. These findings strongly suggest that glycolysis, including astrocytically localized glycogenolysis, is essential to provide energy for active processes occurring during ITMB and that these processes are indispensable for subsequent development of long-term memory.