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 6.5 h 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 5 min after training, were stimulated with 70 mM potassium chloride in the presence of 2 mM 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 (0.5 mM) induced a significant enhancement in the increase in calcium concentration in intermediate medial hyperstriatum ventrale from both left and right hemispheres 10 min and 30 min after training. At 3 h and 6 h after training, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (0.5 mM) 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, responsivenes