Spike frequency decoding and autonomous activation of Ca2+-calmodulin-dependent protein kinase II in dorsal root ganglion neurons

Autonomous activation of calcium-calmodulin kinase (CaMKII) has been proposed as a molecular mechanism for decoding Ca2+ spike frequencies resulting from action potential firing, but this has not been investigated in intact neurons. This was studied in mouse DRG neurons in culture using confocal measurements of [Ca2+](i) and biochemical measurements of CaMKII autophosphorylation and autonomous activity. Using electrical stimulation at different frequencies, we find that CaMKII autonomous activity reached near maximal levels after similar to 45 impulses, regardless of firing frequency (1-10 Hz), and autonomous activity declined with prolonged stimulation. Frequency-dependent activation of CaMKII was limited to spike frequencies in the range of 0.1-1 Hz, despite marked increases in [Ca2+](i) at higher frequencies (1-30 Hz). The high levels of autonomous activity measured before stimulation and the relatively long duration of Ca2+ spikes induced by action potentials (similar to 300 msec) are consistent with the lower frequency range of action potential decoding by CaMKII. The high autonomous activity under basal conditions was associated with extracellular [Ca2+], independently from changes in [Ca2+](i), and unrelated to synaptic or spontaneous impulse activity. CaMKII autonomous activity in response to brief bursts of action potentials correlated better with the frequency of Ca2+ transients than with the concentration of [Ca2+](i). In conclusion, CaMKII may decode frequency-modulated responses between 0.1 and 1 Hz in these neurons, but other mechanisms may be required to decode higher frequencies. Alternatively, CaMKII may mediate high-frequency responses in subcellular microdomains in which the enzyme is maintained at a low level of autonomous activity or the Ca2+ transients have faster kinetics.