Specificity of synaptic regeneration in the spinal cord of the larval sea lamprey.

Abstract

SUMMARY1. Pairs of central neurones in large larval sea lampreys were impaled with micro-electrodes and studied for synaptic connexions in both unoperated control animals and animals which had recovered from complete spinal transection. Two identified classes of neurones served as post-synaptic targets: giant interneurones (g.i.s) and lateral cells (l.c.s). Several identified neurone types were tested as potential sources of presynaptic input.2. When synaptic potentials had short, fixed latencies they also persisted during activation of the presynaptic cell at 3 3-33 3 Hz and were not eliminated after addition of lamprey saline containing high (20 mM) Ca2+. These presumably represented monosynaptic connexions. Variable-latency responses were eliminated by faster rates of stimulation of the presynaptic cell and were mediated via polysynaptic pathways.3. In control animals, g.i.s received monosynaptic input from more caudal g.i.s in fourteen of thirty-two tested cell pairs. These excitatory post-synaptic potentials (e.p.s.p.s) were composite electrochemical responses. The amplitudes of the earlier electrical component averaged 1-66 + 0-24 mV -(mean+ s.E. of mean) and the amplitude of the later chemical component averaged 0-79 ± 0 05 mV.4. In operated larvae, eight of forty-seven g.i.-g.i. pairs separated by the transection scar were connected by monosynaptic composite e.p.s.p.s. In these pairs the electrical component averaged 0-84 + 0-17 mV (P < 0 05 vs. control) and the chemical component averaged 1 56 + 0 40 mV. The average conduction velocity between these cells was less than that in control g.i.-g.i. pairs (0-93 + 0-11 vs. 1-61 + 0-25 m/s; P<0-01).5. The l.c.s showed monosynaptic e.p.s.p.s after activation of a subset of the bulbar Muller neurones (B2<) in seven of twelve pairs. In behaviourally recovered larvae three of twenty-two similar pairs separated by the transection scar were also connected via monosynaptic e.p.s.p.s. The average conduction velocity between these experimental neurones was also less than that in control bulbar-l.c. pairs (1-03+0-03vs. 1-58+0-09 m/s; P < 0-001).6. Several types of neurones were either infrequently linked or never connected to g.i.s or to l.c.s in control larvae. In animals which had recovered from a spinal S. A. MACKLER AND M. E. SELZER transection, no synaptic connexions were found from such neurones on to g.i.s or l.c.s respectively, in 124 tested cell pairs. In addition, dorsal cells (intraspinal primary sensory neurones) received no synaptic input upon stimulation of the spinal cord before or after transection.7. These findings indicate that regenerating spinal axons in the larval sea lamprey form functioning synapses directly onto neurones positioned within 8 mm distal to the healed transection scar. These axons appear to exhibit selectivity in their choice of synaptic targets.

Magnetic resonance microimaging of intraaxonal water diffusion in live excised lamprey spinal cord

“…and perfused with cold buffer at a rate of 0.4 ml͞min. This method of excision of the live cord has been used routinely in electrophysiological studies (20) and has been shown to preserve action potentials in refrigerated pieces of spinal cord for 1 week's experiments after harvest (13). Immediately after MRI, three of the cords were fixed with 2% paraformaldehyde for 12 h. The tissues then were dehydrated in serial ethanol and embedded in paraffin, and tissue blocks were cut 4 mm caudal and rostral to the imaging site.…”

Spinal Cord Injury Induces Changes in Electrophysiological Properties and Ion Channel Expression of Reticulospinal Neurons in Larval Lamprey

“…For example, in other animals, disconnection of neurons from their synaptic targets can trigger changes in neurophysiological properties, and reconnection with these targets restores normal properties (Kuno et al, 1974;Foehring et al, 1986;Belmonte et al, 1988;Kelly et al, 1988;Pinter and Vanden Noven, 1989;Petrov et al, 2001). Unfortunately, in the lamprey, paired recordings between Müller cells and spinal neurons have a relatively low yield after spinal cord injury (Mackler and Selzer, 1987), and negative results are difficult to interpret.…”

“…At long recovery times (12-16 weeks), orthodromic responses caudal to healed hemitransections elicited by axotomized RS neurons on the right side of the brain were taken as an indication that a neuron had regenerated its axon below the lesion site, at least for a few millimeters. Previous studies (Mackler and Selzer, 1987) indicate that, 7-40 weeks after spinal cord transection, stimulation of single Müller cells elicits ϳ0.5-1.0 mV EPSPs in spinal neurons caudal to the lesion and does not elicit action potentials. In addition, the orthodromic responses are virtually identical before and after adding calcium channel blockers to the bath.…”

“…Other animals had relatively long recovery times (12-16 weeks; n ϭ 20 animals), at which time many axotomized RS neurons have regenerated their axons for at least 10 mm below the lesion (Davis and McClellan, 1994b), made synapse with spinal neurons (Mackler and Selzer, 1987), and display properties similar to those of uninjured neurons . Intracellular recordings.…”

Recovery of Neurofilament Expression Selectively in Regenerating Reticulospinal Neurons

“…By 10 weeks after spinal transection, larval lampreys recover behavioral functions (Rovainen, 1976;Selzer, 1978) that are mediated by the formation of synapses with appropriate neurons distal to the site of injury (Mackler and Selzer, 1987). Spinal-projecting neurons of the lamprey have a complex architecture, with 36 large identified reticulospinal neurons (including seven pairs of giant Müller cells and a pair of Mauthner neurons) and several nuclear groups that contain variable numbers of smaller neurons (Swain et al, 1993).…”