Multiple Patterns and Components of Persistent Inward Current With Serotonergic Modulation in Locomotor Activity-Related Neurons in Cfos-EGFP Mice

Dai Y, Jordan LM. Multiple patterns and components of persistent inward current with serotonergic modulation in locomotor activity-related neurons in cfos-EGFP mice. J Neurophysiol 103: 1712-1727, 2010. First published February 17, 2010; doi:10.1152/jn.01111.2009. Using CFos-EGFP transgenic mice (P6-P12), we targeted persistent inward current (PIC) in the spinal interneurons activated by locomotion. Following a locomotor task, whole cell patch-clamp recordings were obtained from ventral EGFP(+) neurons in spinal cord slices (200-250 mu m from T-13-L-4). PIC was recorded by a family of 10 s voltage bi-ramps starting from -70 mV with 30 mV steps. PIC could be classified as ascending and descending forms based on the rising and falling phases of the bi-ramps. Multiple patterns of PIC with various hystereses were found in EGFP(+) neurons. A novel form of PIC, single PIC crossing both phases of the bi-ramps, was described in this study. PIC was found in 82% of EGFP(+) neurons (n=129) with no significant difference in laminar distribution. PIC activated at -56.7 +/- 8 mV with an amplitude of 85.3 +/- 59 pA and time constant of 657.0 +/- 272 ms (n=63). PIC in lamina VIII neurons activated significantly lower (-60.2 +/- 7 mV) than in lamina VII (-54.8 +/- 6 mV) and lamina X (-55.8 +/- 9 mV) neurons. PIC could be differentiated as calcium dependent (Ca-PIC) by bath application of 1-5 mu M TTX or sodium dependent (Na-PIC) by administration of 20-30 mu M dihydropyridine. Ca-PIC activated at -40.2 +/- 19 mV (n=49), whereas Na-PIC activated at -46.8 +/- 16 mV (n=17). Composite-, Ca-, and Na-PICs were significantly different in activation but not amplitude and time constant. Bath application of 5-HT (10-30 mu M) enhanced PIC (n=32) by hyperpolarizing onset (4.2 +/- 6 mV) and increasing amplitude (16%). 5-HT-increased amplitude seemed to be significantly larger in lamina VII neurons (32%) than VIII (6%) and X (14%) neurons. 5-HT enhancement of Ca-PIC (n=6) and Na-PICs (n=4) was also observed in EGFP(+) neurons. This study unveiled unique properties of PICs in EGFP(+) neurons. The lamina-related PIC activation and variable effects of 5-HT on PIC amplitude provides insight into the ionic basis on which locomotion could be generated.