Multiple Effects of Serotonin and Acetylcholine on Hyperpolarization-Activated Inward Current in Locomotor Activity-Related Neurons in Cfos-EGFP Mice

Dai Y, Jordan LM. Multiple effects of serotonin and acetylcholine on hyperpolarization-activated inward current in locomotor activity-related neurons in Cfos-EGFP mice. J Neurophysiol 104: 366-381, 2010. First published April 14, 2010; doi:10.1152/jn.01110.2009. Hyperpolarization-activated inward current (I-h) has been shown to be involved in production of bursting during various forms of rhythmic activity. However, details of I-h in spinal interneurons related to locomotion remain unknown. Using Cfos-EGFP transgenic mice (P6-P12) we are able to target 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 (T-13-L-4, 200-250 mu m). I-h was found in 51% of EGFP + neurons (n = 149) with almost even distribution in lamina VII (51%), VIII (47%), and X (55%). I-h could be blocked by ZD7288 (10-20 mu M) or cesium (1-1.5 mM) but was insensitive to barium (2-2.5 mM). I-h activated at -80.1 +/- 9.2 mV with half-maximal activation -95.5 +/- 13.3 mV, activation rate 10.0 +/- 3.2 mV, time constant 745 +/- 501 ms, maximal conductance 1.0 +/- 0.7 nS, and reversal potential -34.3 +/- 3.6 mV. 5-HT (15-20 mu M) and ACh (20-30 mu M) produced variable effects on I-h. 5-HT increased I-h in 43% of EGFP + neurons (n = 37), decreased I-h in 24%, and had no effect on I-h in 33% of the neurons. ACh decreased I-h in 67% of EGFP + neurons (n = 18) with unchanged I-h in 33% of the neurons. This study characterizes the I-h in locomotor-related interneurons and is the first to demonstrate the variable effects of 5-HT and ACh on I-h in rodent spinal interneurons. The finding of 5-HT and ACh-induced reduction of I-h in EGFP + neurons suggests a novel mechanism that the motor system could use to limit the participation of certain neurons in locomotion.