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1 Physiology, Queen's University, Kingston, Canada
2 Anatomy and Neurobiology, Dalhousie, Halifax, Canada
3 Department of Physiology, Queens University, Kingston, Canada
* To whom correspondence should be addressed. E-mail: john{at}biomed.queensu.ca.
In the presence of monoamines, L-type Ca2+ channels on the dendrites of motoneurons contribute to persistent inward currents (PICs) which can amplify synaptic inputs two-to-six fold. However, the exact location of the L-type Ca2+ channels is controversial and the importance of the location as a means of regulating the input-output properties of motoneurons is unknown. In the present study, we have employed a computational strategy developed by Bui et al. (2006) to estimate the dendritic location of the L-type Ca2+ channels and test the hypothesis that the location of L-type Ca2+ channels varies as a function of motoneuron size. Compartmental models were constructed based on dendritic trees of five motoneurons that ranged in size from small to large. These models were constrained by known differences in PIC activation reported for low- and high-conductance motoneurons and the relationship between somatic PIC threshold and the presence or absence of tonic inhibitory or synaptic activity. Our simulations suggest that L-type Ca2+ channels are concentrated in hot-spots whose distance from the soma increases with the size of the dendritic tree. Moving the hot-spots away from these sites (e.g. using the hot-spot locations from large motoneurons on intermediate-sized motoneurons) fails to replicate the shifts in PIC threshold that occur experimentally during tonic excitatory or inhibitory synaptic activity. In models equipped with a size-dependent distribution of L-type Ca2+ channels, the amplification of synaptic current by PICs depends on motoneuron size and the location of the synaptic input on the dendritic tree.
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