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1 Centre for Neuroscience, University of Alberta, Edmonton, Canada
2 Department of Physiology, Northwest University, Medical School, Chicago, Illinois, United States
* To whom correspondence should be addressed. E-mail: bennettd{at}ualberta.ca.
In the months after spinal cord transection, motoneurons in the rat spinal cord develop large persistent inward currents (PICs), which lead to spasticity. These PICs are mediated by low threshold TTX-sensitive sodium currents (Na PIC) and L-type calcium currents (Ca PIC). Recently, the Na PIC has been 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 two months 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 µM) 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 (~30 µM) 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.
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