Dendritic L-type calcium currents in mouse spinal motoneurons: implications for bistability

The intrinsic properties of mammalian spinal motoneurons provide them with the capability to produce high rates of sustained firing in response to tra nsient inputs (bistability). Even though it has been suggested that a persi stent dendritic calcium current is responsible for the depolarizing drive u nderlying this firing property, such a current has not been demonstrated in these cells. In this study, calcium currents are recorded from functionall y mature mouse spinal motoneurons using somatic whole-cell patch-clamp tech niques. Under these conditions a component of the current demonstrated kine tics consistent with a current originating at a site spatially segregated f rom the soma. In response to step commands this component was seen as a lat e-onset, low amplitude persistent current whilst in response to depolarizin g-repolarizing ramp commands a low voltage clockwise current hysteresis was recorded. Simulations using a neuromorphic motoneuron model could reproduc e these currents only if a noninactivating calcium conductance was placed i n the dendritic compartments. Pharmacological studies demonstrated that bot h the late-onset and hysteretic currents demonstrated sensitivity to both d ihydropyridines and the L-channel activator FPL-64176. Furthermore, the alp ha(1D) subunits of L-type calcium channels were immunohistochemically demon strated on motoneuronal dendrites. It is concluded that there are dendritic ally located L-type channels in mammalian motoneurons capable of mediating a persistent depolarizing drive to the soma and which probably mediate the bistable behaviour of these cells.