Acute aggregation of serotonin-immunoreactive platelets in the injured spinal cord of rat and change of serotonin content in the neural fibers

The recovery of 5-HT transporter and 5-HT immunoreactivity in injured rat spinal cord

STUDY DESIGN

Experimental spinal cord injury.

OBJECTIVE

To determine the role of serotonin (5-HT) and 5-HT transporter in recovery from spinal cord injury.

METHOD

We examined 5-HT and 5-HT transporter of spinal cord immunohistologically and assessed locomotor recovery after extradural compression at the thoracic (T8) spinal cord in 21 rats. Eighteen rats had laminectomy and spinal cord injury, while the remaining three rats received laminectomy only. All rats were evaluated every other day for 4 weeks, using a 0-14 point scale open field test.

RESULTS

Extradural compression markedly reduced mean hindlimbs scores from 14 to 1.5 +/- 2.0 (mean +/- standard error of mean). The rats recovered apparently normal walking by 4 weeks. The animals were perfused with fixative 1-3 days, 1, 2 and 4 weeks (three rats in each) after a spinal cord injury. The 5-HT transporter immunohistological study revealed a marked reduction of 5-HT transporter-containing terminals by 1 day after injury. By 4 weeks after injury, 5-HT transporter immunoreactive terminals returned to the control level. The 5-HT immunohistological study revealed a reduction of 5-HT-containing terminals by 1 week after injury. By 4 weeks after injury, 5-HT immunoreactive fibers and terminals returned to the control level.

CONCLUSION

We estimated the recovery of 5-HT transporter and 5-HT neural elements in lumbosacral ventral horn by ranking 5-HT transporter and 5-HT staining intensity and counting 5-HT and 5-HT transporter terminals. The return of 5-HT transporter and 5-HT immunoreactivity of the lumbosacral ventral horn correlated with locomotor recovery, while 5-HT transporter showed closer relationship with locomotor recovery than 5-HT. The presence of 5-HT transporter indicates that the 5-HT fibers certainly function. This study shows that return of the function of 5-HT fibers predict the time course and extent of locomotory recovery after thoracic spinal cord injury.

Steady-State Levels of Monoamines in the Rat Lumbar Spinal Cord: Spatial Mapping and the Effect of Acute Spinal Cord Injury

Monoamines in the spinal cord are important in the regulation of locomotor rhythms, nociception, and motor reflexes. To gain further insight into the control of these functions, the steady-state extracellular distribution of monoamines was mapped in the anesthetized rat's lumbar spinal cord. The effect of acute spinal cord lesions at sites selected for high resting levels was determined over ∼1 h to estimate contributions to resting levels from tonic descending activity and to delineate chemical changes that may influence the degree of pathology and recovery after spinal injury. Measurements employed fast cyclic voltammetry with carbon fiber microelectrodes to give high spatial resolution. Monoamine oxidation currents, sampled at equal vertical spacings within each segment, were displayed as contours over the boundaries delineated by histologically reconstructed electrode tracks. Monoamine oxidation currents were found in well defined foci, often confined within a single lamina. Larger currents were typically found in the dorsal or ventral horns and in the lateral aspect of the intermediate zone. Cooling of the low-thoracic spinal cord led to a decrease in the oxidation current (to 71–85% of control) in dorsal and ventral horns. Subsequent low-thoracic transection produced a transient increase in signal in some animals followed by a longer lasting decrease to levels similar to or below that with cooling (to 17–86% of control values). We conclude that descending fibers tonically release high amounts of monoamines in localized regions of the dorsal and ventral horn of the lumbar spinal cord at rest. Lower amounts of monoamines were detected in medial intermediate zone areas, where strong release may be needed for descending activation of locomotor rhythms.

Effects of serotonin 1A agonist on acute spinal cord injury

STUDY DESIGN

We evaluated the effects of serotonin (5-HT) agonists on in vitro models of spinal cord compressive injury. Evoked potentials in injured rat spinal cords (n=24) were recorded during perfusion with 5-HT agonists.

OBJECTIVES

To evaluate the therapeutic effects of 5-HT agonists on the recovery of compound action potentials in injured spinal cords.

METHODS

Rat dorsal columns were isolated, placed in a chamber, and injured by extradural compression with a clip. Conducting action potentials were activated by supramaximal constant current electrical stimuli and recorded during perfusion with 5-HT agonists and antagonists.

RESULTS

After inducing compression injuries, mean action potential amplitudes were reduced to 33.9+/-5.4% of the pre-injury level. After 120 min of perfusion with Ringer's solution, the mean amplitudes recovered to 62.8+/-8.4% of the pre-injury level. At a concentration of 100 micro M, perfusion with tandospirone (a 5-HT1A agonist) resulted in a significantly greater recovery of mean action potential amplitudes at 2 h after the injury (86.2+/-6.9% of pre-injury value) as compared with the control Ringer's solution (62.8+/-8.4% of pre-injury value, P<0.05). In contrast, quipazine (a 5-HT2A agonist) accelerated the decrease of amplitude (54.5+/-11.7% of pre-injury value). 5-HT1A and 5-HT2A agonist did not consistently alter latencies of the action potentials.

CONCLUSION

The 5-HT1A receptor agonist was effective for the recovery of spinal action potential amplitudes in a rat spinal cord injury model.

Calcium-mediated intracellular messengers modulate the serotonergic effects on axonal excitability

We carried out experiments to investigate the mechanisms of serotonin-induced axonal excitability changes using isolated dorsal columns from young (seven to 11-day-old) Long-Evan's hooded rats. Conducting action potentials were activated by submaximal (50%) and supramaximal constant current electrical stimuli and recorded with glass micropipette electrodes. In experiment 1, to study Ca(2+)-mediated mechanisms, we superfused the preparations with Ringer solutions containing varying Ca2+ concentrations. Following superfusion with Ca(2+)-free Ringer solution for 4 h, we tested initial responses to serotonin agonists. Studies then were repeated after preparations had been washed for 1 h with Ringer solution containing 1.5 mM Ca2+ and 1.5 mM Mg2+. After 4 h superfusion of Ca(2+)-free Ringer solution, quipazine (a serotonin2A agonist, 100 microM) did not induce significant axonal excitability changes (amplitude change of 1.4 +/- 1.3%, percentage of predrug control level, +/-S.D., n = 6). A 100 microM concentration of 8-hydroxy-dipropylaminotetralin (a serotonin1A agonist) reduced response amplitudes by 36.3 +/- 4.2% (+/-S.D., P < 0.0005, n = 7) and prolonged latencies by 22.3 +/- 4.3% (+/-S.D., P < 0.0005, n = 7). Application of serotonin (100 microM) decreased amplitudes by 6.6 +/- 5.0% (+/-S.D., P < 0.05, n = 6). Extracellular calcium concentration ([Ca2+]e) was measured at various depths in the dorsal column with ion-selective microelectrodes. Four hours' superfusion with Ca(2+)-free Ringer solution reduced [Ca2+]e to less than 0.1 mM in dorsal columns. In 1.5 mM Ca2+ Ringer solution, quipazine increased the amplitudes by 38.3 +/- 5.8% (P < 0.0005, n = 6). Likewise, serotonin increased the amplitudes by 13.8 +/- 4.9% (P < 0.005, n = 6). In contrast however, 8-hydroxy-dipropylaminotetralin still reduced amplitudes by 35.0 +/- 6.4% (P < 0.0005, n = 7) and prolonged latencies by 24.1 +/- 4.5% (P < 0.0005, n = 7). In experiment 2, we investigated calcium-dependent and cAMP-mediated protein kinase signalling pathways to evaluate their role as intracellular messengers for serotonin2A receptor activation. Two protein kinase inhibitors, 50 microM H7 (an inhibitor of protein kinase C and c-AMP dependent protein kinase) and 100 microM D-sphingosine (an inhibitor of protein kinase A and C) effectively eliminated the excitatory effects of the serotonin2A agonist. 100 microM cadmium (a Ca2+ channel blocker) also blocked the effects of quipazine. Neither these protein kinase inhibitors nor cadmium alone affected action potential amplitudes. These results suggest that replacing Ca2+ with Mg2+ blocks the excitatory effects of quipazine but does not prevent the inhibitory effects of 8-hydroxy-dipropylaminotetralin, and calcium-mediated protein kinase mechanisms modulate axonal excitability changes induced by serotonin and its agonist.

The role of GM1-ganglioside in the injured spinal cord of rats: an immunohistochemical study using GM1-antisera

The effect of GM1-ganglioside (GM1) administration was investigated in the injured spinal cord of rats. Immunohistochemistry using GM1-antisera was applied in the study of GM1 distribution, and locomotor recovery was also evaluated. A total of 86 rats, subdivided into four groups, were used in the study. The SI + GM1 group (n = 30) underwent a thoracic cord injury, and then received daily intraperitoneal injections of GM1 (10 mg/kg) from 0 to 13 days after injury. The SI group (n = 30) also underwent thoracic cord injury, but did not receive GM1 treatment. The GM1 group (n = 20) received daily injections of GM1 in the absence of any spinal cord injury. The control group (n = 6) neither underwent spinal cord injury nor received GM1 treatment. The animals were sacrificed at 1, 3, 5, 7, and 14 days after injury for immunohistochemical evaluation. GM1 immunoreactive axons, myelin sheaths, and glial cells were counted in 5 light microscopic fields of spinal white matter. Immunohistochemical studies of the spinal cord revealed that GM1 treatment significantly increased both GM1-positive axons and GM1-positive myelin sheaths in the lateral funiculus surrounding the lesion site. The exogenous GM1 was incorporated predominantly into the myelin sheath rather than the axon. This distribution was detectable by day 1 of injury and persisted until day 14, and was significantly different from that of the control group on days 1 and 7. Moreover, GM1 treatment significantly accelerated the recovery of motor function. Collectively, these results suggest that exogenous GM1 administration after a spinal cord injury results in the rapid transfer of GM1 to the lateral funiculus where the motor transmission pathway is located. Furthermore, exogenous GM1 was shown to be specifically incorporated into the myelin sheath. Thus GM1 treatment may prevent demyelination and may contribute to motor function recovery.

Evidence for a neural source of acute accumulation of serotonin in platelets in the injured spinal cord of rats. An experimental study using 5,6-dihydroxytryptamine treatment

It was found previously that large numbers of platelets showing high serotonin (5-hydroxytryptamine; 5-HT) immunoreactivity appeared in hemostatic plugs at the traumatized cord segment in the acute phase of a trauma. In order to determine the origin of 5-HT in the platelets, we investigated the 5-HT immunoreactivity of platelets accumulating in hemostatic plugs at the traumatized spinal cord segment at 5 minutes after injury. This investigation was carried out by light and electron microscopic immunohistochemistry in rat spinal cord pretreated with 5,6-dihydroxytryptamine (5,6-DHT). The hemorrhagic lesion formed at the neural 5-HT-depleted spinal cord segment was completely 5-HT immunonegative, while platelets in lesions in cord segments of control animals or rostral to the injection site of 5,6-DHT in experimental animals where neural 5-HT was not depleted were 5-HT immunoreactive. The results strongly suggest that a significant amount of 5-HT is released from neural elements at the injury site and is transiently incorporated into the platelets in situ.