Control of cell function by neuronal calcium-activated nonselective (CAN) cation channels

Ionic mechanism of long-lasting discharges of action potentials triggered by membrane hyperpolarization in the medial lateral habenula

The activation of inhibitory synapses typically suppresses the generation of action potentials by hyperpolarizing the membrane of postsynaptic cells. In contrast to such conventional action of inhibitory synapses, we report here the ionic mechanism through which hyperpolarizing synapses trigger long-lasting discharges of action potentials that persist up to several tens of seconds. By using extracellular and intracellular recordings in slice preparations, we demonstrate that the activation of synaptic input from the limbic forebrain generates transient hyperpolarizing postsynaptic potentials in neurons of the medial part of the lateral habenular nucleus of the epithalamus. The synaptic hyperpolarization then sets off the coordinated activation of a distinct set of membrane ion channels and intracellular Ca2+ mobilization by internal stores. The activation of these cellular events in distinct temporal order drives a persistent depolarization of habenular cells and promotes long-lasting discharges of tonic action potentials. The cells in the medial division of the lateral habenula project to dopamine and serotonin cells in the midbrain. We suggest that these habenular cells, by generating persistent action potentials in response to a transient increase in the activity of the limbic forebrain, may contribute to the regulation of the serotonergic and dopaminergic activity in the brain.

Regional differences in distribution and functional expression of small-conductance Ca2+-activated K+ channels in rat brain

Small-conductance Ca2+-activated K+ (SK) channels are important for excitability control and afterhyperpolarizations in vertebrate neurons and have been implicated in regulation of the functional state of the forebrain. We have examined the distribution, functional expression, and subunit composition of SK channels in rat brain. Immunoprecipitation detected solely homotetrameric SK2 and SK3 channels in native tissue and their constitutive association with calmodulin. Immunohistochemistry revealed a restricted distribution of SK1 and SK2 protein with highest densities in subregions of the hippocampus and neocortex. In contrast, SK3 protein was distributed more diffusely in these brain regions and predominantly expressed in phylogenetically older brain regions. Whole-cell recording showed a sharp segregation of apamin-sensitive SK current within the hippocampal formation, in agreement with the SK2 distribution, suggesting that SK2 homotetramers underlie the apamin-sensitive medium afterhyperpolarizations in rat hippocampus.