Multiple effects of serotonin and acetylcholine on hyperpolarization- activated inward current in locomotor activity-related neurons in Cfos-EGFP mice

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 (T₁₃-L₄, 200-250 μm). 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 μM) 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 μM) and ACh (20-30 μM) 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.