Serotonin facilitates a persistent calcium current in motoneurons of rats with and without chronic spinal cord injury

In the months after spinal cord transection, motoneurons in the rat spinal cord develop large persistent inward currents ( PICs) that are responsible for muscle spasticity. These PICs are mediated by lowthreshold TTX- sensitive sodium currents ( Na PIC) and L- type calcium currents ( Ca PIC). Recently, the Na PIC was shown to become supersensitive to serotonin (5- HT) after chronic injury. In the present paper, a similar change in the sensitivity of the Ca PIC to 5- HT was investigated after injury. The whole sacrocaudal spinal cord from acute spinal rats and spastic chronic spinal rats (S2 level transection 2 mo previously) was studied in vitro. Intracellular recordings were made from motoneurons and slow voltages ramps were applied to measure PICs. TTX was used to block the Na PIC. For motoneurons of chronic spinal rats, a low dose of 5- HT ( 1 mu M) significantly lowered the threshold of the Ca PIC from -56.7 +/- 6.0 to -63.1 +/- 7.1 mV and increased the amplitude of the Ca PIC from 2.4 +/- 1.0 to 3.0 +/- 0.73 nA. Higher doses of 5- HT acted similarly. For motoneurons of acute spinal rats, low doses of 5- HT had no significant effects, whereas a high dose ( about 30 mu M) significantly lowered the threshold of the L- Ca PIC from - 58.5 +/- 14.8 to - 62.5 +/- 3.6 mV and increased the amplitude of the Ca PIC from 0.69 +/- 1.05 to 1.27 +/- 1.1 nA. Thus Ca PICs in motoneurons are about 30- fold supersensitive to 5- HT in chronic spinal rats. The 5- HT - induced facilitation of the Ca PIC was blocked by nimodipine, not by the 1(h) current blocker Cs+ ( 3 mM) or the SK current blocker apamin ( 0.15 mu M), and it lasted for hours after the removal of 5- HT from the nCSF, even increasing initially after removing 5- HT. The effects of 5- HT make motoneurons more excitable and ultimately lead to larger, more easily activated plateaus and self- sustained firing. The supersensitivity to 5- HT suggests the small amounts of endogenous 5- HT below the injury in a chronic spinal rat may act on supersensitive receptors to produce large Ca PICs and ultimately enable muscle spasms.