Passive avoidance training results in increased responsiveness of voltage- and ligand-gated calcium channels in chick brain synaptoneurosomes

A temporal cascade of events has been described from a number of biochemical investigations of passive avoidance training in day-old chicks. Among these, within minutes of training, there is a transient, enhanced release of glutamate and increased agonist and antagonist binding to N-methyl-D-aspartate-sensitive glutamate receptors in the intermediate medial hyperstriatum ventrale of the forebrain. Some 6.5h later, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate binding to glutamate receptors is also increased in the same region. These processes might be predicted to affect the uptake of calcium via voltage-sensitive calcium channels or glutamate receptor-associated channels, thereby changing the intracellular calcium concentration. To test this possibility, we have measured the calcium concentration in synaptoneurosomes, containing both pre- and postsynaptic elements, prepared from left and right intermediate medial hyperstriatum ventrale at various times following training, using Fura 2-AM as the indicator of intracellular calcium concentration. Synaptoneurosomes, prepared immediately and 5min after training, were stimulated with 70mM potassium chloride in the presence of 2mM calcium, resulting in a significantly enhanced increase in calcium concentration in synaptoneurosomes from the left hemisphere of trained chicks. This effect was absent in samples obtained at later times after training. N-Methyl-D-aspartate (0.5mM) induced a significant enhancement in the increase in calcium concentration in intermediate medial hyperstriatum ventrale from both left and right hemispheres 10min and 30min after training. At 3h and 6h after training, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (0.5mM) induced a significantly enhanced increase in calcium concentration in samples from either hemisphere.These results suggest that immediately after training there is an engagement of both pre- and postsynaptic voltage-sensitive calcium channels, followed by an increased reponse to N-methyl-D-aspartate receptor stimulation, and coinciding with the enhanced calcium-dependent glutamate release and an increase in N-methyl-D-aspartate-sensitive glutamate receptor binding that has been reported previously. The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-sensitive mechanisms are activated at a later stage of memory formation, when increased α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate binding to glutamate receptors has been reported. Thus, responsiveness of calcium channels to agonist stimulation is implicated in temporally diverse stages in the cascade of events involved in memory formation following passive avoidance training in the chick. Copyright (C) 1999 IBRO.