Figure 1.
Substance P evokes long-term metaplastic facilitation of EIN-evoked EPSPs. A, An example of the metaplastic facilitation evoked by substance P at a single EIN to motor neuron connection. The EIN input depressed in control (black circles; average of 9 spike trains) but was facilitated in substance P (white squares; average of 9 spike trains). In all cases, metaplasticity was examined using trains of 20 presynaptic spikes evoked at 20 Hz. B, Traces from another EIN to motor neuron connection showing the potentiation of the initial EPSP in the train and the metaplastic facilitation 10 min after the start of substance P (subs P) wash-off. Note that the potentiation of the initial EPSP amplitude, but not the metaplastic facilitation, had recovered after washing for 45 min. All traces are averages of 10 sweeps. Gray lines show the input after substance P and black lines the control response. C, Comparison of the initial EIN-evoked EPSP amplitude in different EIN to motor neuron connections before and 4 hr after substance P application. D, Graph showing the proportion of facilitating and depressing EIN connections before (n = 225) and 4 hr after (n = 35) substance P application. E, Schematic diagram showing the technique for stimulating identified EINS extracellularly. The traces show the synaptic input evoked by an EIN when a spike was evoked intracellularly on rebound from a hyperpolarizing current pulse, or when it was evoked by stimulating the cord (gray line; the stimulus artifact has been removed). The PSP in both cases was the same, suggesting that only the EIN recorded from had been stimulated. Fi, Graph showing the time course of the metaplastic facilitation (measured as the average of the 2nd to 5th EPSPs in the train) and the potentiation of the initial EPSP in experiments that used extracellular EIN stimulation (n = 4). Fii, Traces showing the initial EPSP amplitude and the metaplasticity in control and at different times after substance P application from a single experiment using extracellular EIN stimulation. The metaplastic facilitation is shown by the increase in the average of the second to fifth EPSPs in the train 3 hr after substance P application. Gi, Gii, Substance P did not evoke metaplastic facilitation of reticulospinal-evoked inputs. Instead, depression was usually enhanced. This may be because of the increased amplitude of the initial EPSP in the spike train. The black trace shows the control response and the gray trace the response in substance P. The bottom voltage value on the scale bar refers to action potential amplitudes and the top value to EPSP amplitudes.