Cervical Spinal Cord Injury Upregulates Ventral Spinal 5-HT_2A Receptors

Abstract

Following chronic C2 spinal hemisection (C2HS), crossed spinal pathways to phrenic motoneurons exhibit a slow, spontaneous increase in efficacy by a serotonin (5-HT)-dependent mechanism associated with 5-HT2A receptor activation. Further, the spontaneous appearance of cross-phrenic activity following C2HS is accelerated and enhanced by exposure to chronic intermittent hypoxia (CIH). We hypothesized that chronic C2HS would increase 5-HT and 5-HT2A receptor expression in ventral cervical spinal segments containing phrenic motoneurons. In addition, we hypothesized that CIH exposure would further increase 5-HT and 5-HT2A receptor density in this region. Control, sham-operated, and C2HS Sprague-Dawley rats were studied following normoxia or CIH (11% O2-air; 5-min intervals; nights 7-14 post-surgery). At 2 weeks post-surgery, ventral spinal gray matter extending from C4 and C5 was isolated ipsilateral and contralateral to C2HS. Neither C2HS nor CIH altered 5-HT concentration measured with an ELISA on either side of the spinal cord. However, 5-HT2A receptor expression assessed with immunoblots increased in ipsilateral gray matter following C2HS, an effect independent of CIH. Immunocytochemistry revealed increased 5-HT2A receptor expression on identified phrenic motoneurons (p<0.05), as well as in the surrounding gray matter. Contralateral to injury, 5-HT2A receptor expression was elevated in CIH, but not normoxic C2HS rats (p<0.05). Our data are consistent with the hypothesis that spontaneous increase in 5-HT2A receptor expression on or near phrenic motoneurons contributes to strengthened crossed-spinal synaptic pathways to phrenic motoneurons following C2HS.

Up-regulation of 5-HT2 receptors is involved in the increased H-reflex amplitude after contusive spinal cord injury

“…Compared to uninjured controls, a significant 35% increase was seen in the lumbar spinal cord of transected rats (Kim et al, 1999). As expected, no 5-HT-IR was seen distal to the transection site but application of serotonergic agonists demonstrated a super sensitivity of lumbar motor neuron activity associated with treadmill locomotion, consistent with functional stimulation of the increased 5-HT 2 R. Chronically, after a C2 hemisection, there is up-regulation of ipsilateral 5-HT 2A R density in phrenic motor neurons distal to the injury site (Fuller et al, 2005). In this case, the density of 5-HT-IR is not different from that in uninjured controls, similar to what we observed in the rDLn chronically after incomplete contusion injury.…”

“…Further, in our SCI model, as in the C2 hemisection model (Fuller et al, 2005), the increased receptor expression is present chronically along with an apparently normal level of serotonergic (re)innervation thus should lead to increased responsiveness of the motoneurons to endogenous serotonergic stimulation. One clinical consequence after SCI may be spasticity, as suggested by the effects of treatment of patients with the 5-HT2R antagonist, cyproheptadine (Barbeau et al, 1982;Nance, 1994;Wainberg et al, 1990) A theoretically possible alternative explanation for the increased ID 50 for serotonin antagonists observed in our experiments is an increased release of 5-HT at serotonergic terminals after SCI without any changes in receptor number.…”

“…The increased expression of 5-HT 2 R has previously been reported after SCI in a transection and in a hemisection SCI model (Fuller, et al, 2005, Kim, et al, 1999. In the first of these studies, neonatal rats that were subjected to complete transection were examined 8 weeks later for 5-HT 2C receptor binding activity in ventral horn tissue rostral and caudal to the injury site.…”

Respiratory Abnormalities Resulting from Midcervical Spinal Cord Injury and their Reversal by Serotonin 1A Agonists in Conscious Rats

“…It might be postulated that our observed respiratory recovery could have been augmented through 5-HT 2 receptor activation and expression, as reported after C2 hemisection (Zhou et al, 2001;Fuller et al, 2005) and cervical dorsal rhizotomy (CDR) (Kinkead et al, 1998). Post-CDR rats displayed increased serotonin-immunoreactive terminals on phrenic motor neurons, and the consequent long-lasting enhancement of phrenic motor output triggered by hypoxia was blocked by pretreatment with a 5-HT 2 -specific antagonist.…”

Administration of phosphodiesterase inhibitors and an adenosine A1 receptor antagonist induces phrenic nerve recovery in high cervical spinal cord injured rats

“…The crossed phrenic pathway can be activated either by increasing respiratory drive or via the administration of a variety of chemical compounds such as serotonin receptor agonists, NMDA receptor antagonists and theophylline (Alilain and Goshgarian, 2007;Ling et al 1994;Zhou et al 2001;Basura et al 2002;Fuller et al 2005;Nantwi et al 1996;Nantwi and Goshgarian 1998). The various methods mentioned above activate the latent respiratory pathway transiently; however, only the chronic administration of theophylline induces plasticity in the respiratory pathways which results in recovery that persists days after the animal has been weaned from the drug (Nantwi et al 2003).…”