Locomotor central pattern generator excitability states and serotonin sensitivity after spontaneous recovery from a neonatal lumbar spinal cord injury.

The spinal locomotor central pattern generator (CPG) in neonatal mice exhibits diverse output patterns, ranging from sub-rhythmic to multi-rhythmic to fictive locomotion, depending on its general level of excitation and neuromodulatory status. We have recently reported that the locomotor CPG in neonatal mice rapidly recovers the ability to produce neurochemically induced fictive locomotion following an upper lumbar spinal cord compression injury. Here we address the question of recovery of multi-rhythmic activity and the serotonin-sensitivity of the CPG. In isolated spinal cords from control and 3 days post-injury mice, application of dopamine and NMDA elicited multi-rhythmic activity with slow and fast components. The slow component comprised 10-20 s episodes of activity that were synchronous in ipsilateral or all lumbar ventral roots, and the fast components involved bursts within these episodes that displayed coordinated patterns of alternation between ipsilateral roots. Rhythm strength was the same in control and injured spinal cords. However, power spectral analysis of signal within episodes showed a reduced peak frequency after recovery. In control spinal cords, serotonin triggered fictive locomotion only when applied at high concentration (30 µM, constant NMDA). By contrast, in about 50% of injured preparations fictive locomotion was evoked by 2-3 times lower serotonin concentrations (10-15 µM). This increased serotonin sensitivity was correlated with post-injury changes in the expression of specific serotonin receptor transcripts, but not of dopamine receptor transcripts.

Maturation of peptide-positive synaptic arbors in the medicinal leech requires rhythmic target activity.

The formation and refinement of synaptic connections are dependent on the activity that emerges from nascent synaptic connections. Such activity has the effect of regulating the production and release of specific neurotransmitters. To determine the role of activity in regulating the production of peptide-positive synapses, we used antibodies against Phe-Met-Arg-Phe-NH2 and acetylated α-tubulin as well as intracellular injections of Neurobiotin to examine varicosities belonging to heart excitor (HE) neurons on the heart tubes of medicinal leeches, Hirudo spp. We found that the production of peptide-positive varicosities increased considerably during the last week of embryogenesis, which coincided with the emergence of rhythmic activity of the heart tube. When we compromised central input to HE neurons with bicuculline or by surgical ablation of the central pattern generator during early embryogenesis, we found that activity in the heart tubes and its rhythmicity were greatly diminished. Furthermore, the activity of HE neurons had also lost its rhythmicity and appeared tonic, and production of peptide-positive varicosities was substantially reduced as well. Partial surgical ablations that preserved rhythmic activity in the heart tube while disrupting heart tube innervation by some HE neurons still resulted in peptide-positive varicosity production. Taken together, our results suggest that postsynaptic rhythmic activity of the heart tube is necessary and sufficient for the development and maturation of peptide-positive synapses.