Activity-dependent [Ca2+]i changes in guinea pig vagal motoneurons: relationship to the slow afterhyperpolarization

Vagal motoneurons in slices from the guinea-pig brain stem were injected with the fluorescent [Ca2+]i indicators fura-2, furaptra, or Calcium Green-1. Spike-induced fluorescence changes were measured in the soma and dendrites and simultaneously the long-lasting afterhyperpolarization was recorded with a sharp microelectrode in the soma. Na+ spikes or Ca2+ spikes increased [Ca2+]i (measured as a change in indicator fluorescence) in all locations in the soma and dendrites. Each spike in a train of action potentials caused a step increase in fluorescence of about equal amplitude when nonsaturating indicators were used. Peak changes at all locations occurred at the time of the last action potential. Transients measured with low concentrations of Calcium Green-1 or furaptra had a recovery time constant of approximately 500-1,500 ms in the cell body. The recovery time course was faster in the dendrites than in the soma. The norepinephrine-sensitive, slow afterhyperpolarization (sAHP) had a time to peak of approximately 800 ms and a recovery time constant of 2-5 s, much longer than the recovery time course of the fluorescence changes. Some of these experiments were repeated on pyramidal neurons from the CA1 region of the rat hippocampus with similar results. In both cell types, the data suggest that the time course of neither the rising phase nor the falling phase of the sAHP, nor the underlying conductance, directly reflects the time course of the [Ca2+]i change. The mechanism connecting the parameters remains unclear. One possibility is that an additional second messenger system is involved.