Functional Recovery of a Locomotor Network after Injury: Plasticity beyond the Central Nervous System

Abstract

Many animals depend on descending information from the brain for the initiation and proper execution of locomotion. Interestingly, after injury and the loss of such inputs, locomotor function can sometimes be regained without the regrowth of central connections. In the medicinal leech, Hirudo verbana, we have shown that crawling reemerges after removal of descending inputs. Here, we studied the mechanisms underlying this return of locomotion by asking if central pattern generators (CPGs) in crawl-recovered leeches are sufficient to produce crawl-specific intersegmental coordination. From recovered animals, we treated isolated chains of ganglia with dopamine to activate the crawl CPGs (one crawl CPG per ganglion) and observed fictive crawl-like bursting in the dorsal-longitudinal-excitor motoneuron (DE-3), an established crawl-monitor neuron. However, these preparations did not exhibit crawl-specific coordination across the CPGs. Although the crawl CPGs always generated bidirectional activation of adjacent CPGs, we never observed crawl-appropriate intersegmental phase delays. Because central circuits alone were unable to organize crawl-specific coordination, we tested the coordinating role of the peripheral nervous system. In transected leeches normally destined for recovery, we removed afferent information to the anterior-most (lead) ganglion located below the nerve-cord transection site. In these dually treated animals, overt crawling was greatly delayed or prevented. After filling the peripheral nerves with Neurobiotin tracer distal to the nerve-root lesion, we found a perfect correlation between regrowth of peripheral neuronal fibers and crawl recovery. Our study establishes that during recovery after injury, crawl-specific intersegmental coordination switches to a new dependence on afferent information.

Focused Ultrasound Neuromodulation and the Confounds of Intracellular Electrophysiological Investigation

“…The ganglion was incubated at room temperature for 45 min following iontophoretic injection to allow the dye to diffuse to distal structures. Following this incubation period, the ganglion was fixed in 4% paraformaldehyde (overnight at 4°C) and rinsed in iso-osmotic Millonig's buffer (all components from Sigma-Aldrich, recipe from Puhl et al, 2018). Cells were permeabilized in 1% Triton X-100 in iso-osmotic buffer for 2 h and incubated overnight at 4°C in a 1:50 dilution of streptavidin conjugated to Cy3 (Jackson ImmunoResearch).…”

Neuromodulation of central pattern generators and its role in the functional recovery of central pattern generator activity

“…Some level of recovery of function after injury occurs throughout the central nervous system in likely all animals (Herman et al 2018;Luther et al 2003;Martinez et al 2011;Molinari 2009;Puhl et al 2018;Sakurai and Katz 2009;Telgkamp et al 2002). Considering the grave consequences that the loss of neural activity due to injury or disease has on the behavior and quality of life in humans, a large amount of research is devoted to it.…”

“…Recovery of crawling activity in the leech (i.e., intersegmental coordination) occurs after full transection of the descending inputs. Interestingly, this involves regeneration of sensory axons that take over part of the coordination of activity between CPGs along the ventral cord (Puhl et al 2018). In adult fish, generation of spinal motor neurons seems to be greatly influenced by dopaminergic projections, which occur at the expense of interneurons both during development and in the adult (Reimer et al 2013).…”

“…In this view, rhythmic activity incorporates not only the CPG network but the key stabilizing and integrating inputs that the CPG receives from central as well as peripheral structures. This view is receiving renewed attention and includes the role of motor neurons (Diekman et al 2017;Falgairolle et al 2017;Rotstein et al 2017;Song et al 2016) and sensory feedback (Bässler 1986;Li et al 2017;Puhl et al 2018). Consistent with this more expansive view of CPGs, recent attempts to design locomotion robots have expanded the use of concepts derived from the original CPG literature, to include either multiple coupled CPGs (Kiehn 2016;Ramirez and Baertsch 2018a) or layered CPGs (Grillner 2006;Grillner and El Manira 2015) integrated with sophisticated peripheral sensors and actuators that control stable and maneuverable robots.…”

Brief Electrical Stimulation Triggers an Effective Regeneration of Leech CNS

“…Despite the differences in complexity between the vertebrate and the invertebrate nervous systems, the latter has been proven to be useful for understanding basic mechanisms related to neurophysiology, development, and regenerative biology (Baylor and Nicholls, 1971;Brenner, 1974;Ready and Nicholls, 1979;Bellen et al, 2010;Kandel, 2012). The CNS of the adult leech has been a useful model for studying these topics over 50 years (Kuffler and Nicholls, 1966;Macagno, 1980;Kristan et al, 2005;Firme et al, 2012;Tomina and Wagenaar, 2017;Puhl et al, 2018). It is finite and relatively simply interconnected.…”