I_hBlock Reveals Separation of Timescales in Pyloric Rhythm Response to Temperature Changes inCancer borealis

Motor systems operate over a range of frequencies and relative timing (phase). We studied the contribution of the hyperpolarization-activated inward current (I_h) to frequency and phase in the pyloric rhythm of the stomatogastric ganglion (STG) of the crab,Cancer borealisas temperature was altered from 11°C to 21°C. Under control conditions, the frequency of the rhythm increased monotonically with temperature, while the phases of the pyloric dilator (PD), lateral pyloric (LP), and pyloric (PY) neurons remained constant. When we blocked I_hwith cesium (Cs⁺) PD offset, LP onset, and LP offset were all phase advanced in Cs⁺at 11°C, and the latter two further advanced as temperature increased. In Cs⁺the steady state increase in pyloric frequency with temperature diminished and the Q₁₀of the pyloric frequency dropped from ∼1.75 to ∼1.35. Unexpectedly in Cs⁺, the frequency displayed non-monotonic dynamics during temperature transitions; the frequency initially dropped as temperature increased, then rose once temperature stabilized, creating a characteristic “jag”. Interestingly, these jags were still present during temperature transitions in Cs⁺when the pacemaker was isolated by picrotoxin, although the temperature-induced change in frequency recovered to control levels. Overall, these data suggest that I_hplays an important role in the ability of this circuit to produce smooth transitory responses and persistent frequency increases by different mechanisms during temperature fluctuations.