Presynaptic homeostasis at CNS nerve terminals compensates for lack of a key Ca²⁺ entry pathway

At central synapses, P/Q-type Ca²⁺ channels normally provide a critical Ca²⁺ entry pathway for neurotransmission. Nevertheless, we found that nerve terminals lacking α_1A (Cav2.1), the pore-forming subunit of P/Q-type channels, displayed a remarkable preservation of synaptic function. Two consistent physiological changes reflective of synaptic homeostasis were observed in cultured hippocampal neurons derived from α_1A (-/-) mice. First, the presynaptic response to an ionophore-mediated Ca²⁺ elevation was 50% greater, indicating an enhanced Ca²⁺ sensitivity of the release machinery. Second, basal miniature excitatory postsynaptic current frequency in α_1A (-/-) neurons was increased 2-fold compared with WT neurons and occluded the normal response of presynaptic terminals to cAMP elevation, suggesting that the compensatory mechanism in α_1A (-/-) synapses and the modulation of presynaptic function by PKA might share a final common pathway. We used cDNA microarray analysis to identify molecular changes underlying homeostatic regulation in the α1A (-/-) hippocampus. The 40,000 entries in our custom-made array included likely targets of presynaptic homeostasis, along with many other transcripts, allowing a wide-ranging examination of gene expression. The developmental pattern of changes in transcript levels relative to WT was striking; mRNAs at 5 and 11 days postnatal showed little deviation, but clear differences emerged by 22 days. Many of the transcripts that differed significantly in abundance corresponded to known genes that could be incorporated within a logical pattern consitent with the modulation of presynaptic function. Changes in endocytotic proteins, signal transduction kinases, and candidates for Ca²⁺-sensing molecules were consistent with implications of the direct physiological experiments.