The most common consequence of SCI is the paralysis due to neural circuit interruptions. Although there is some degree of spontaneous recovery, lost connections are hardly restored. It is an option to consider the use of neurotrophic factors to induce nerve regeneration. Neurotrophic factors have a key role in the modulation of neuronal survival, neurite outgrowth, synaptic plasticity and neurotransmission in both intact and injured nervous system.
In this study, we found that LA, a GnRH agonist, significantly regained locomotor activity of SCI rats. Calderon-Vallejo and Quintanar (2012) reported that GnRH treatment recovered locomotor activity with similar findings to ours. Both results are strongly tied, because these effects are probably mediated through activation of GnRH receptors, which have been described in the spinal cord motoneurons (Dolan et al., 2003; Quintanar et al., 2007). In an experimental autoimmune encephalomyelitis model, LA administration produces an increase in the axonal growth and in neurofilaments and myelin basic protein expression (Guzmαn-Soto et al., 2012), which could explain the improvement in locomotor activity found in our experiments. The degree of locomotor activity recovery in rats treated with LA was higher than in those treated with GnRH (Calderon-Vallejo and Quintanar, 2012), which suggests that LA has greater potential for clinical application than GnRH.
In kinematic analysis, we observed that LA treatment in SCI rats decreased step cycle, stance phase, and swing phase durations, but it resulted in increases in the stride distance and speed. LA rats spent less time in each step phase, traveled greater distance, and performed faster steps. These results can be considered as a sign of locomotor activity recovery, which were not observed in rats with physiological saline solution treatment. These results are similar to the outcomes by Hamers et al. (2001) who studied gait parameters in two different SCI types, i.e
., transection of the dorsal half of the spinal cord and spinal cord contusion.
The locomotor activity recovery in LA-treated animals is due to different mechanisms including remyelination (Guzman-Soto et al., 2012) or increases in microfilament protein expression, neurite outgrowth (Calderon-Vallejo and Quintanar, 2012), and axonal diameter (Quintanar et al., 2011)
. These results can be explained by reorganization of local networks via propriospinal circuitry. These local networks control the involuntary movements, which do not require cortico- or rubrospinal tracts, proposed as an explanation for the ability of spinal cord injured animals to produce walking movements (Ek et al., 2010). However, supraspinal tracts were likely involved in the control of voluntary movements observed in animals treated with LA. It could be related to reconnections going through injured area originated from supraspinal neurons.
Propiospinal circuits provide an explanation for the restorations of micturition reflex in LA animals, in which the number of days that they need assistance of manual bladder emptying was reduced. This is possible due to the reorganization in reflex pathways inside the spinal cord, as reported by Groat and Yoshimura (2012).
In this study, LA administration modified the gray and white matter areas, decreasing scar area and promoting thte recovery of spared tissue in SCI rats
. High BBB scores were positively correlated with spared tissue in the spinal cord lesion (Basso et al., 1995). Structural conformation of the spinal cord is kept by both sufficient numbers of neurons and axons including their myelin. In traumatized spinal cord, only remnants of both gray and white matter exist due to the presence of hypercellularity, inflammation, cavitation, increased extracellular space and a loose fibrous matrix (Totoiu and Keirstead, 2005). Our results showed that in the LA group, spared tissue was increased, BBB scores were higher, faster stride speed and longer stride distance were observed in the LA group than in the sham group
. These results reflect the new integration of spared descending and afferent-driven signals, as recovery after contusive SCI has been reported to be identified by changes in gait biomechanics and muscle activation patterns (Hansen et al., 2012).
In the LA group, significantly higher BBB score and improved gait were observed at the 1 st
and 2 nd
weeks during the evaluation period compared to the sham group. As a consequence of SCI, microglia considerably increased independently of spared tissue. An early infiltration of macrophages/microglia in traumatic lesions of spinal cords has been observed in a rat model of SCI. The immune cell cascade involves infiltration of neutrophils and activation of resident microglia, followed by subsequent accumulation of monocyte-derived macrophages and the later entry of lymphocytes into the lesion site. The monocytes that infiltrate the injured area acquire a pro-inflammatory/classical profile (Raposo and Schartz, 2014). The reduction in the density of microglial cells observed in the LA group is probably associated with the locomotor activity in these rats.
In conclusion, administration of LA partially improves lotomotor activity, gait, micturition reflex, spinal cord morphology and decreases microglial area in a rat model of SCI. Promotion of neuronal survival by administration of neurotrophic factors and a possible immunomodulation to counteract secondary injury are a promising approach to repair of SCI. Additionally, even LA administered via intramuscular injection is able to cross the blood-spinal cord barrier as observed in previous studies on patients with prostate cancer. LA is a potential alternative treatment of SCI because of its safety and ease in use as well as few side effects.