Fig. 4.

D₁-like receptor activation increases rhythm stability with no effect on frequency. A and B: spontaneous activity from single ventral root neurograms of the isolated spinal cord (A) were used to determine the EC₅₀ for D₁-mediated excitation of motor output (B). The EC₅₀ for SKF-81297 was calculated as 10 μM by generating a response ratio between the root mean square of activity in a 10–20 min window after drug application compared with a 10-min baseline. Data are presented as mean ± SE response ratio as a function of SKF-81297 concentration. Bath application of the D₁-like receptor agonist SKF-81297 during a 5-HT-, NMA-evoked rhythm (Ci) increased stability (Cii and Eii) but had no effect on frequency (Cii and Ei). Similarly, the D₁-like antagonist LE 300 (1–4 μM) destabilized the locomotor rhythm (Dii and Eii) but had no effect on frequency (Dii and Ei) of a rhythm evoked by 5-HT, NMA, and dopamine (Di). Spectrograms depict cross-wavelet analysis of neurogram activity from left and right L₂ ventral roots over time, with the rhythm frequency on the y-axis and rhythm stability indicated by the power as color bands. A more stable rhythm is displayed as warmer colors (C and D). Data are presented as mean ± SE normalized rhythm frequency (Ei) and power (Eii) over a 10-min window between 10 and 20 min after dopamine application and are normalized to a 10-min window of the control rhythm evoked by 5-HT and NMA. *Significant differences from baseline control rhythm (repeated-measures ANOVA, Tukey post hoc) with P < 0.05.