Spontaneous network activity transiently depresses synaptic transmission in the embryonic chick spinal cord

We examined the effects of spontaneous or evoked episodes of rhythmic activ ity on synaptic transmission in several spinal pathways of embryonic day 9- 12 chick embryos. We compared the amplitude of synaptic potentials evoked b y stimulation of the ventrolateral funiculus (VLF), the dorsal or ventral r oots, before and after episodes of activity. With the exception of the shor t-latency responses evoked by dorsal root stimulation. the potentials were briefly potentiated and then reduced for several minutes after an episode o f rhythmic activity. Their amplitude progressively recovered in the interva l between successive episodes. The lack of post-episode depression in the s hort-latency component of the dorsal root evoked responses is probably attr ibutable to the absence of firing in cut muscle efferents during an episode of activity. The post-episode depression of VLF-evoked potentials was mimicked by prolon ged stimulation of the VLF, subthreshold for an episode of activity. By con trast, antidromically induced motoneuron firing and the accompanying calciu m entry did not depress VLF-evoked potentials recorded from the stimulated ventral root. In addition, post-episode depression of VLF-evoked synaptic c urrents was observed in voltage-clamped spinal neurons. Collectively, these findings suggest that somatic postsynaptic activity and calcium entry are not required for the depression. We propose instead that the mechanism may involve a form of long-lasting activity-induced synaptic depression, possib ly a combination of transmitter depletion and ligand-induced changes in the postsynaptic current accompanying transmitter release. This activity-depen dent depression appears to be an important mechanism underlying the occurre nce of spontaneous activity in developing spinal networks.