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Primary afferent synapses on developing and adult Renshaw cells
George Z Mentis1, Valerie C Siembab, Ricardo Zerda
1Laboratory of Neural Control, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20892, USA.
Summary
Renshaw cells initially integrate sensory and motor inputs but mature to primarily receive motor signals. This developmental shift involves selective weakening of sensory synapses after postnatal day 15.
Area of Science:
- Neuroscience
- Developmental Biology
- Spinal Cord Circuitry
Background:
- Adult mammalian spinal interneurons, including Renshaw cells and Ia inhibitory interneurons (IaINs), originate from common embryonic V1 progenitors.
- Unlike other V1-derived interneurons, adult Renshaw cells exhibit motor axon synapses and lack proprioceptive inputs, suggesting a unique developmental trajectory.
Purpose of the Study:
- To elucidate the developmental mechanisms underlying the specific connectivity pattern of Renshaw cells.
- To investigate how Renshaw cells transition from integrating sensory and motor inputs in neonates to predominantly motor inputs in adults.
Main Methods:
- Utilized tract tracing and immunocytochemical markers (parvalbumin, VGLUT1, vesicular acetylcholine transporter) to analyze synaptic inputs.
- Performed electrophysiological recordings in neonatal mice to assess functional synaptic transmission.
- Quantified bouton density and postsynaptic density size at different developmental stages (embryonic day 18, postnatal day 0, P10-P15, adult).
Main Results:
- Embryonic Renshaw cells initially lack dorsal root inputs but acquire them by postnatal day 0, with widespread innervation by P10-P15.
- Sensory (VGLUT1-IR) synapse density on Renshaw cells peaks at P15 and subsequently decreases, accompanied by smaller postsynaptic densities.
- Motor (vesicular acetylcholine transporter-IR) synapses are present embryonically and proliferate postnatally, matching Renshaw cell growth.
Conclusions:
- Renshaw cells are developmentally competent to receive sensory inputs, but these are selectively reduced after P15 through synapse weakening and arrested proliferation.
- This synaptic plasticity results in a shift from mixed sensory-motor integration in neonates to predominantly motor input in adults.
- Similar synaptic weight modifications in interneurons may be crucial for the maturation of motor reflexes and locomotor circuits.
