Transient effects of nerve injury on estimates of sensory neuron number in juvenile bullfrog

Abstract

The effect of lumbar spinal nerve (SN) transection on estimates of neuron number was investigated in the dorsal root ganglia (DRGs) of juvenile bullfrogs (Rana catesbeiana). SN8 and SN10 were transected on one side, and SN9 was left intact. Two weeks after nerve injury, estimates of neuron number in DRG8 and DRG10 on both the operated and unoperated sides were more than twice the estimates obtained from control animals. Neuron number in the uninjured DRG9 was also elevated relative to that of control animals. Eight weeks after axotomy, differences in neuron number were less apparent. The mean cross-sectional area of DRG neurons was reduced 2 weeks after nerve injury in all DRGs. The decrease in mean area was the result of the addition of neurons to the smallest size classes. These data are discussed in the context of previous results showing that neurons are added as juvenile frogs grow to adult size (St. Wecker and Farel [1994] J. Comp. Neurol. 342:430-438). This addition results from the maturation of a population of incompletely differentiated neurons (Meeker and Farel [1997] J. Comp. Neurol. 389:569-576). The present results suggest that axotomy precipitates differentiation of these incompletely differentiated neurons, perhaps as a compensatory response to nerve injury.

Sensory neuron addition in juvenile rat: Time course and specificity

“…In P100 rats, 55% of identified neurons fall below the mean, whereas only 45% of the identified neurons in P1 rats fall below the mean ( P < 0.01). These results are consistent with the hypothesis that, in rat as in frog (Boyer and Farel, 1999), neuron addition occurs disproportionately among smaller neurons.…”

“…In frog, the number of small neurons increases as juveniles grow to adult size, whereas the number of large neurons remains unchanged (Farel and Boyer, 1999). In mammals, small‐diameter DRG cells have unmyelinated axons and are principally nociceptors and thermoceptors (Averill et al, 1995; Bennett et al, 1998).…”

Increase in descending brain–spinal cord projections with age in larval lamprey: Implications for spinal cord injury

“…Although data from the present study indicate that the rate of addition of new descending brain‐spinal cord projections is relatively slow, in theory, spinal cord transections might enhance this rate (see Materials and Methods; Farel and Boyer, 1999). Therefore, in normal, unlesioned lamprey, the numbers of descending brain neurons projecting to 10% BL or 20% BL (Fig.…”

“…Following spinal cord transections, axons from descending brain neurons grow across the lesion site at a much higher rate than the relatively slow addition of projections with age found in the present study (see Discussion), suggesting that axonal regeneration is the main mechanism for restoration of these projections. However, it is possible that spinal cord transections enhance the rate of addition of new descending projections (Farel and Boyer, 1999). To determine the likelihood of this possibility, spinal cord pretransections were made at 15% BL (n = 13) or 25% BL (n = 11), and following a recovery period (4 or 8 weeks), HRP was applied to the spinal cord at 10% BL or 20% BL, respectively (see Fig.…”

SCG postnatal remodelling – hypertrophy and neuron number stability – in Spix's Yellow‐toothed Cavies (Galea spixii)

“…It is widely known that the increase in neuron number is not always attributable to neurogenesis, but may also be the outcome of late maturation or incomplete differentiation of nerve cells (Miolan and Niel, 1996; Ciaronia et al, 2000; Farel, 2003). In juvenile bullfrogs, DRG houses a population of immature, incompletely differentiated neurons that can be induced to differentiate into a mature form in response to injury (Farel and Boyer, 1999). Indeed, new DRG neurons derive mostly from differentiation rather than from proliferation of precursor cells (Meeker and Farel, 1997).…”