Papers by Andrew McClellan
Journal of Comparative Neurology, 2004
In larval lamprey, with increasing recovery times after a transection of the rostral spinal cord,... more In larval lamprey, with increasing recovery times after a transection of the rostral spinal cord, there is a gradual recovery of locomotor behavior, and descending brain neurons regenerate their axons for progressively greater distances below the transection site. In the present study, spinal cord “conditioning lesions” (i.e., transections) were performed in the spinal cord at 30% body length (BL; normalized distance from the head) or 50% BL. After various “lesion delay times” (D), a more proximal spinal cord “test lesion” (i.e., transection) was performed at 10% BL, and then, after various recovery times (R), horseradish peroxidase was applied to the spinal cord at 20% BL to determine the extent of axonal regeneration of descending brain neurons. Conditioning lesions at 30% BL, lesion delay times of 2 weeks, and recovery times of 4 weeks (D-R = 2-4 group) resulted in a significant enhancement of axonal regeneration for the total numbers of descending brain neurons as well as neurons in certain brain cell groups compared to control animals without conditioning lesions. Experiments with hemiconditioning lesions, which reduce interanimal variability, confirmed that conditioning lesions do significantly enhance axonal regeneration and indicate that axotomy rather than diffusible factors released at the injury site is primarily involved in this enhancement. Results from the present study suggest that conditioning lesions “prime” descending brain neurons via cell body responses and enhance subsequent axonal regeneration, probably by reducing the initial delay and/or increasing the initial rate of axonal outgrowth. J. Comp. Neurol. 478:395–404, 2004. © 2004 Wiley-Liss, Inc.
Journal of Comparative Neurology, 1999
In larval lamprey, the large, identified descending brain neurons (Müller and Mauthner cells) are... more In larval lamprey, the large, identified descending brain neurons (Müller and Mauthner cells) are capable of axonal regeneration. However, smaller, unidentified descending brain neurons, such as many of the reticulospinal (RS) neurons, probably initiate locomotion, and it is not known whether the majority of these neurons regenerate their axons after spinal cord transection. In the present study, this issue was addressed by using double labeling of descending brain neurons. In double-label control animals, in which Fluoro-Gold (FG) was applied to the spinal cord at 40% body length (BL; measured from anterior to posterior from tip of head) and Texas red dextran amine (TRDA) was applied later to the spinal cord at 20% BL, an average of 98% of descending brain neurons were double labeled. In double-label experimental animals, FG was applied to the spinal cord at 40% BL; two weeks later the spinal cord was transected at 10% BL; and, eight weeks or 16 weeks after spinal cord transection, TRDA was applied to the spinal cord at 20% BL. At eight weeks and 16 weeks after spinal cord transection, an average of 49% and 68%, respectively, of descending brain neurons, including many unidentified RS neurons, were double labeled. These results in larval lamprey are the first to demonstrate that the majority of descending brain neurons, including small, unidentified RS neurons, regenerate their axons after spinal cord transection. Therefore, in spinal cord-transected lamprey, axonal regeneration of descending brain neurons probably contributes significantly to the recovery of locomotor function.
Journal of Comparative Neurology, 2002
The purpose of this study was to determine whether new descending brain-spinal cord projections a... more The purpose of this study was to determine whether new descending brain-spinal cord projections are added with age in larval lamprey and might contribute substantially to restoration of these projections following spinal cord injury. Retrograde horseradish peroxidase (HRP) labeling of descending brain neurons was performed in “young” and “old” larval lamprey that differed in age by at least one year. In old larval lamprey, significantly more descending brain neurons projected to specific rostral levels of the spinal cord than in young animals. Furthermore, in young and old lamprey, the main morphological change in Müller and Mauthner cells was an increase in soma size. The major conclusion from the present study is that in larval lamprey, some new brain-spinal cord projections are added with age that could be due to axonal elongation by preexisting brain neurons and/or descending projections from new neurons (i.e., neurogenesis or maturation of incompletely differentiated neurons). Following spinal cord transections, the numbers of descending projections were not significantly different than those in normal, unlesioned animals. Thus, some new descending projections are added with age, but at a relatively slow rate, and the rate does not appear to be affected significantly by spinal cord transections. Together, the present results and those from our recent double-labeling study suggest that following spinal cord transection in larval lamprey, axonal regeneration by descending brain neurons, rather than the relatively slow addition of new brain-spinal cord projections with age, probably accounts for the majority of restored projections and recovery of locomotor function. J. Comp. Neurol. 447:128–137, 2002. © 2002 Wiley-Liss, Inc.
Journal of Neuroscience Methods, 1998
In larval lamprey, seven fluorescent tracers were tested as potential candidates for retrograde d... more In larval lamprey, seven fluorescent tracers were tested as potential candidates for retrograde double labeling of descending brain neurons: Fluoro Gold (FG); fluorescein dextran amine (FDA); True Blue (TB); cascade blue dextran amine (CBDA); Fast Blue (FB); Texas red dextran amine (TRDA); and tetramethylrhodamine dextran amine (RDA). The first tracer (FG, TB, FB, or CBDA) was applied to the spinal cord at 40% body length (BL). In separate experiments, the second tracer (TRDA or RDA) was applied to the spinal cord at 20% BL. The tracer combination FG/TRDA was found to have the best optical properties for double labeling. However, application of FG to the spinal cord with the method used for the other tracers resulted in labeling of 'lateral cells' along the sides of the rhombencephalon that were presumed to be non-neuronal and that obscured some of the descending brain neurons. Control experiments suggested that FG was transported in the circulation to the brain, where the tracer was taken up by lateral cells. Therefore, a special technique was developed for applying FG to the spinal cord without allowing the tracer to enter the circulation. In larval lamprey, this important double-labeling technique that was developed for TRDA and FG is being used to examine axonal regeneration and projection patterns of descending brain neurons.
Neuroscience Letters, 2006
Journal of Neurobiology, 2006
In larval lamprey, descending brain neurons, which regenerate their axons following spinal cord i... more In larval lamprey, descending brain neurons, which regenerate their axons following spinal cord injury, were isolated and examined in cell culture to identify some of the factors that regulate neurite outgrowth. Focal application of 5 mM or 25 mML-glutamate to single growth cones inhibited outgrowth of the treated neurite, but other neurites from the same neuron were not inhibited, an effect that has not been well studied for neurons in other systems. Glutamate-induced inhibition of neurite outgrowth was abolished by 10 mM kynurenic acid. Application of high potassium media to growth cones inhibited neurite outgrowth, an effect that was blocked by 2 mM cobalt or 100 μM cadmium, suggesting that calcium influx via voltage-gated channels contributes to glutamate-induced regulation of neurite outgrowth. Application of glutamate to growth cones in the presence of 2 μM ω-conotoxin MVIIC (CTX) still inhibited neurite outgrowth, while CTX blocked high potassium-induced inhibition of neurite outgrowth. Thus, CTX blocked virtually all of the calcium influx resulting from depolarization. To our knowledge, this is the first direct demonstration that calcium influx via ligand-gated ion channels can contribute to regulation of neurite outgrowth. Finally, focal application of glutamate to the cell bodies of descending brain neurons inhibited outgrowth of multiple neurites from the same neuron, and this is the first demonstration that multiple neurites can be regulated in this fashion. Signaling mechanisms involving intracellular calcium, similar to those shown here, may be important for regulating axonal regeneration following spinal cord injury in the lamprey. © 2006 Wiley Periodicals, Inc. Develop Neurobiol 67: 173–188, 2007.
Developmental Neurobiology, 2007
In larval lamprey, descending brain neurons, which regenerate their axons following spinal cord i... more In larval lamprey, descending brain neurons, which regenerate their axons following spinal cord injury, were isolated and examined in cell culture to identify some of the factors that regulate neurite outgrowth. Focal application of 5 mM or 25 mML-glutamate to single growth cones inhibited outgrowth of the treated neurite, but other neurites from the same neuron were not inhibited, an effect that has not been well studied for neurons in other systems. Glutamate-induced inhibition of neurite outgrowth was abolished by 10 mM kynurenic acid. Application of high potassium media to growth cones inhibited neurite outgrowth, an effect that was blocked by 2 mM cobalt or 100 μM cadmium, suggesting that calcium influx via voltage-gated channels contributes to glutamate-induced regulation of neurite outgrowth. Application of glutamate to growth cones in the presence of 2 μM ω-conotoxin MVIIC (CTX) still inhibited neurite outgrowth, while CTX blocked high potassium-induced inhibition of neurite outgrowth. Thus, CTX blocked virtually all of the calcium influx resulting from depolarization. To our knowledge, this is the first direct demonstration that calcium influx via ligand-gated ion channels can contribute to regulation of neurite outgrowth. Finally, focal application of glutamate to the cell bodies of descending brain neurons inhibited outgrowth of multiple neurites from the same neuron, and this is the first demonstration that multiple neurites can be regulated in this fashion. Signaling mechanisms involving intracellular calcium, similar to those shown here, may be important for regulating axonal regeneration following spinal cord injury in the lamprey. © 2006 Wiley Periodicals, Inc. Develop Neurobiol 67: 173–188, 2007.
Brain Research Bulletin, 1999
Experimental Brain Research, 1999
The extent and strength of long-distance coupling between locomotor networks in the rostral and ... more The extent and strength of long-distance coupling between locomotor networks in the rostral and caudal spinal cord of larval lamprey were examined with in vitro brain/spinal cord preparations, in which spinal locomotor activity was initiated by chemical microstimulation in the brain, as well as with computer modeling. When locomotor activity and short-distance coupling were blocked in the middle spinal cord for at least 40 segments, burst activity in the rostral and caudal spinal cord was still coupled 1:1, indicating that long-distance coupling is extensive. However, in the absence of short-distance coupling, intersegmental phase lags were not constant but decreased significantly with increasing cycle times, suggesting that long-distance coupling maintains a relatively constant delay rather than a constant phase lag between rostral and caudal bursts. In addition, under these conditions, intersegmental phase lags, measured between rostral and caudal burst activity, were significantly less than normal, and the decrease was greater for longer distances between rostral and caudal locomotor networks. The above result could be mimicked by a computer model consisting of pairs of oscillators in the rostral, middle, and caudal spinal cord that were connected by short- and long-distance coupling. With short-distance coupling blocked in the middle spinal cord, strychnine was applied to either the rostral or caudal spinal cord to convert the pattern locally from right-left alternation to synchronous burst activity. Synchronous burst activity in the rostral spinal cord resulted in a reduction in right-left phase values for burst activity in the caudal cord. These results also could be mimicked by the computer model. Strychnine-induced synchronous burst activity in the caudal spinal cord did not appear to alter the right-left phase values of rostral burst activity. Taken together, the experimental and modeling results suggest that the descending and ascending components of long-distance coupling, although producing qualitatively different effects, are comparatively weak. In particular, the descending component of long-distance coupling appears to become progressively weaker with increasing distance between two given regions of spinal cord. Therefore, short-distance coupling probably contributes substantially to normal rostrocaudal phase lags for locomotor activity along the spinal cord. However, short-distance coupling may be more extensive than ”nearest neighbor coupling.”
Experimental Brain Research, 1999
In the lamprey and most fish, locomotion is characterized by caudally propagating body undulation... more In the lamprey and most fish, locomotion is characterized by caudally propagating body undulations that result from a rostrocaudal phase lag for ipsilateral burst activity. One of the mechanisms that might contribute to rostrocaudal phase lags is a gradient of oscillator burst frequencies along the spinal cord that presumably are produced, in part, by descending drive from the brain. The purpose of the present study was to test whether a gradient of oscillator frequencies does exist along the lamprey spinal cord. First, during brain-initiated locomotor activity in in vitro brain/spinal cord preparations, the cycle times (=1/frequency) of locomotor activity generated by the functionally isolated rostral spinal cord (activity blocked in middle and caudal cord) were significantly shorter than control cycle times when the entire spinal cord was generating locomotor activity. Second, the cycle times of locomotor activity generated by the functionally isolated caudal cord (activity blocked in rostral and middle cord) were significantly longer than control cycle times for activity generated by the entire spinal cord. Thus, no one region of the spinal cord appears to dictate the overall cycle times of locomotor activity generated by the entire spinal cord, although overall cycle times tended to be closest to those of the isolated rostral spinal cord. Finally, although short- and long-distance coupling as well as oscillator frequency gradients probably contribute to rostrocaudal phase lags of spinal locomotor activity, the asymmetrical nature of short-distance coupling, in which the descending component dominates, appears to be the main factor.
Experimental Neurology, 2003
Journal of Comparative Neurology, 1994
The purpose of the present study was to determine the numbers of descending brainstem projections... more The purpose of the present study was to determine the numbers of descending brainstem projections to different levels of the spinal cord in normal larval sea lamprey (Petromyzon marinus) and to examine the restoration of these projections in animals 3–32 weeks after transection of the rostral spinal cord ( ∼ 10% of body length). In normal animals approximately 1, 250, 900, and 825 brainstem neurons projected to 20%, 40%, and 60% of body length, respecively. Spinal projections originated from the diencephaln, mesencephalon, three rhombencephalic reticular nuclei, Müller and Mauthner neurons, and four cell groups in the caudal rhombencephalon. In spinal cord-transected animals the number of brainstem neurons projecting to 20% of body length increased with recovery time, and at 32 weeks post-transection the total number and distribution of brainstem neurons was not significantly differnt from normal animals. Brainstem projections first appeared at 40% of body length by 8 weeks post-transection, and were present at 60% of body length by 32 weeks post-transection. There was substantial restoration of brainstem projections to 40% of body length but limited restoration to 60% of body length. The ∼ 50 brainstem neurons, including some Müller cells, that projected to 60% of body length at 32 weeks post-transection indicate that restoration of descending projections in excess of 50 mm can occur within the central nervous system of this vertebrate. These anatomical results are discussed in relation to the time course of recovery of locomotor functions in spinal cord-transected lampreys. © 1994 Wiley-Liss, Inc.
Experimental Neurology, 2010
Experimental Neurology, 1997
The organization and distribution of propriospinal neurons with descending axons were determined ... more The organization and distribution of propriospinal neurons with descending axons were determined via retrograde HRP labeling in normal lamprey and in animals that had behaviorally recovered for various times (4, 8, 16, and 32 weeks) following transection of the rostral spinal cord. In normal animals, descending propriospinal neurons were found in the rostral, middle, and caudal spinal cord. Theoretical analysis
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Papers by Andrew McClellan