Cyclic AMP stimulates neurite outgrowth of lamprey reticulospinal neurons without substantially altering their biophysical properties

Abstract

Reticulospinal (RS) neurons are critical for initiation of locomotor behavior, and following spinal cord injury (SCI) in the lamprey, the axons of these neurons regenerate and restore locomotor behavior within a few weeks. For lamprey RS neurons in culture, experimental induction of calcium influx, either in the growth cone or cell body, is inhibitory for neurite outgrowth. Following SCI, these neurons partially downregulate calcium channel expression, which would be expected to reduce calcium influx and possibly provide supportive conditions for axonal regeneration. In the present study, it was tested whether activation of second messenger signaling pathways stimulates neurite outgrowth of lamprey RS neurons without altering their electrical properties (e.g. spike broadening) so as to possibly increase calcium influx and compromise axonal growth. First, activation of cAMP pathways with forskolin or dbcAMP stimulated neurite outgrowth of RS neurons in culture in a PKA-dependent manner, while activation of cGMP signaling pathways with dbcGMP inhibited outgrowth. Second, neurophysiological recordings from uninjured RS neurons in isolated lamprey brain-spinal cord preparations indicated that dbcAMP or dbcGMP did not significantly affect any of the measured electrical properties. In contrast, for uninjured RS neurons, forskolin increased action potential duration, which might have increased calcium influx, but did not significantly affect most other electrical properties. Importantly, for injured RS neurons during the period of axonal regeneration, forskolin did not significantly alter their electrical properties. Taken together, these results suggest that activation of cAMP signaling by dbcAMP stimulates neurite outgrowth, but does not alter the electrical properties of lamprey RS neurons in such a way that would be expected to induce calcium influx. In conclusion, our results suggest that activation of cAMP pathways alone, without compensation for possible deleterious effects on electrical properties, is an effective approach for stimulating axonal regeneration of RS neuron following SCI.

Platelet-Rich Plasma Promotes Axon Regeneration, Wound Healing, and Pain Reduction: Fact or Fiction

“…In vitro, a cAMP analog promotes an increase in the distance axons regenerate [312,313]. In vivo, the elevation of cAMP in axotomized neurons accelerates peripheral axon outgrowth by increasing the levels of neuron-produced endogenous neurotrophic factors [314].…”

Persistent alterations in active and passive electrical membrane properties of regenerated nerve fibers of man and mice

“…). Although it cannot be excluded that changes in cAMP levels during regeneration may contribute to the increased GH (Lau et al ., ; Pale et al ., ), the simplest explanation for the increased GH remains the increased electrical access to the internode, as it was the case with GKfN. This is supported by the fact that GH showed a marked recovery with regeneration from 1 to 5 months, in parallel with recovery in passive GBB linking the nodal and internodal compartments (Barrett & Barrett, ; Fig.…”

Serotonin inhibits axonal regeneration of identifiable descending neurons after a complete spinal cord injury in lampreys

“…Previous studies have shown that a single dose of cAMP applied at the time of transection promotes axon regeneration of identifiable descending neurons following a complete spinal cord transection in larval sea lampreys [23][24][25] . Here, and to reveal a possible involvement of this second messenger in the inhibitory effect of serotonin in axonal regeneration, we carried out a rescue experiment in which animals treated with 6 serotonin as above were also treated with dibutyryl-cAMP (db-cAMP).…”

“…Several reports in mammals and fishes have shown that cAMP promotes axon regeneration following SCI [18][19][20][21][22] . Subsequent studies have also shown that cAMP promotes axon regeneration in descending neurons of lampreys after SCI [23][24][25] . The challenge now is to define the signals that control cAMP levels in descending neurons after axotomy and during regeneration.…”