The membrane potential of striatal medium spiny neurons (MSNs) fluctuates between down- and up-states. In ventral striatum, inward rectifying potassium (KIR) currents in 40% of MSNs inactivate . KIR current inactivation appears to alter spike frequency and onset during up-states . However, it is not known whether these translate into significant changes in calcium dynamics in the dendrites. We describe a computational study investigating how this inactivation influences dendritic calcium transients.
Two MSNs were modeled using NEURON, one equipped with non-inactivating KIR currents (henceforth, "non-inKIR") and the other with inactivating KIR currents (henceforth "inKIR") and their dendritic calcium transients were compared in response to injected current and synaptic inputs. Measurements were made from the distal dendrites.
It was observed that dendritic calcium transients were significantly enhanced by KIR current inactivation (Figure 1). For instance, inKIR cell when compared with non-inKIR cell had dendritic calcium transient peaks higher by as much as 93% in response to an injected current of 0.25 nA in the distal dendrite (Figure 1A). Though this difference decreased with higher currents, still the calcium peak remained higher for the inKIR cell by at least 10% (Figure 1B). The enhanced calcium influx (by up to 51%) was present even when the injected currents were matched for firing frequency. Similar results were obtained with synaptic inputs.
Figure 1. Calcium transients in non-inKIR and inKIR cell dendrites.
The facilitatory effect of KIR current inactivation on dendritic calcium influx appears to be mediated through action potential firing frequency and their timing. In view of the reports that dendritic intracellular calcium levels influence cortico-striatal input plasticity , our findings suggest that KIR current inactivation may significantly modulate synaptic plasticity.
BMC Neurosci 2007, 8:116. BioMed Central Full Text