Dopaminergic modulation of spinal neuronal excitability.

It is well recognized that dopamine (DA) can modulate spinal networks and reflexes. DA fibers and receptors are present in the spinal cord, and evidence for DA release within the spinal cord has been published. A critical gap is the lack of data regarding dopaminergic modulation of intrinsic and synaptic properties of motoneurons and ventral interneurons in the mammalian spinal cord. In this paper, we address this issue by examining the cellular mechanisms underlying the excitatory effect of DA on motor systems. We examine the effects of DA on two classes of cells important for motor control, motoneurons and Hb9 interneurons, located in lamina VIII. We show that DA can boost excitability in spinal motoneurons by decreasing the first spike latency and the afterhyperpolarization. Collectively, this leads to an increase in the frequency-current slope likely attributable to modulation of I(A) and SK(Ca) (small-conductance calcium-activated K+ channel) currents. We also demonstrate that DA increases glutamatergic transmission onto motoneurons. Our data also suggest that DA stabilizes the rhythmic output of conditionally bursting interneurons. Collectively, these data indicate that DA has widespread actions on intrinsic and synaptic properties of ventral spinal neurons.