Multiple patterns and components of persistent inward current with serotonergic modulation in locomotor activity-related neurons in Cfos-EGFP mice

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 μm from T ₁₃-L₄). 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 μM TTX or sodium dependent (Na-PIC) by administration of 20-30 μM 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 μM) 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.